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Prepared for the U.S. Department of Energy under Contract No. DE-AC03-76SF00098. DISCLAIMER
This document was prepared as an account of work sponsored by the United States Government. While this document is believed to contain correct information, neither the United States Government nor any agency thereof, nor the Regents of the University of California, nor any of their employees, makes any warranty, express or implied, or assumes any legal responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by its trade name, trademark, manufacturer, or otherwise, does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof, or the Regents of the University of California. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof or the Regents of the University of California. LBL-35647 CONF-9406158 UC-900
Proceedings of the Eighteenth DOE Solar Photochemistry .Research Conference
Held at Granlibakken Conference Center Tahoe City, California June 5-9, 1994
Hosted by Lawrence Berkeley Laboratory University of California Berkeley, California 94720
This work was prepared as an account of a meeting supported by the Division of Chemical Sciences, Office of Basic Energy Sciences, U.S. Department of Energy under Contract No. DE-AC03-76SF00098. FOREWORD
The Eighteenth DOE Solar Photochemistry Research Conference, sponsored by the Division of Chemical Sciences, Office of Basic Energy Sciences, is being held June 5-9, 1994, at the Granlibakken Conference Center, in Tahoe City, California. The meeting is hosted by the Structural Biology Division of Lawrence Berkeley Laboratory.
This annual conference brings together grantees and contractors of the DOE Division of Chemical Sciences engaged in fundamental research on solar photochemical energy conversion. The conference provides a focus for a wide spectrum of activities which contribute to providing the knowledge base and concepts needed for the capture and chemical conversion of solar energy. The research will provide the foundations for solar technologies of the distant future, in which light-induc~d charge separation processes will be applied to conversion of light energy to chemical energy, e.g., production of alcohols from· carbon dioxide, hydrogen from water, ammonia from atmospheric nitrogen, or other needed chemicals at lower cost by using sunlight.
Our special guest plenary lecturer is Professor Michael Gratzel of the Ecole Polytechnique Federale, Lausanne, who will speak on photoconversion by nanocrystalline films of oxide semiconductors. The topical se~sions which follow will feature presentations on charge transfer at semiconductor-liquid electrolyte junctions, long-range vectorial electron transfer in macromolecular arrays, transition metal photophysics, electronic structure and solvent effects on electron transfer processes, artificial assemblies for photosynthesis, and the photosynthetic bacterial reaction center.
This volume contains the agenda for the meeting, abstracts of the 30 formal presentations and 56 posters, as well as an addre~s listing for the 107 attendees.
I would like to express my sincere appreciation to the Calvin and Frei research groups, especially John Otvos and Marilyn Taylor for their efforts, far beyond the call of duty, in organizing and running the conference. A special word of thanks is also due to Michael Kasha, for his banquet lecture on the art and science of musical string instruments. The success of the conference, notwithstanding, will be due in large measure to the willingness of all participants to engage in candid discussion, and to share views and opinions to the furtherance of common research goals.
Mary E. Gress Fundamental Interactions Branch Division of Chemical Sciences Office of Basic Energy Sciences
1 TABLE OF CONTENTS
FOREWORD ...... i
PROGRAM ...... xi
Session I - Mary E. Gress. Chairman
Plenary Lecture: Nanocrystalline Electronic Junctions for Solar Light Harvesting and Conversion Michael Gratzel ...... 1
Session II - David H. Waldeck. Chairman
Investigations of High Quantum Yield Dye Sensitization Processes on Two Dimensional Semiconductors Xiumei Dou, Laura Sharp and Bruce A. Parkinson ...... 3
Picosecond Dynamic Studies of Electron Transfer Rates at TII-V Semiconductor/Liquid Interfaces Marcel Sturzenegger, C. N. Kenyon and Nathan S. Lewis ...... 5
Ultrafast Optical Studies of Surface Reaction Processes at Semiconductor Interfaces R. J. Dwayne Miller ...... 7
Session III - OI2a L. Micic, Chairman
Interfacial Photochemistry A. J. Frank and D. Mao ...... 11
Mechanistic Aspects of Photoconversion at Semiconductor-Liquid Junctions and in Facilitated Transport Membranes Carl A. Koval, Marcus Groner, Jason Howard, Robert Torres, Thanh To and David Watts ...... 14
Session IV- Stephen E. Webber, Chairman
Asymmetric Polymeric Porphyrin Films for Solar Energy Conversion Carl C. Wamser ...... 17
Pho.toconversion Processes in Molecular Semiconductor Films Brian A. Gregg, Yeong II Kim and Robert Torres ...... 20
Session V - Stephan E. Isied, Chairman
Directional Electron Transfer Through Macromolecular Arrays Marye Anne Fox, Pei-Wei Wang and Diana M. Watkins ...... 23
iii Photoinduced Charge-Shift Reactions of Donor-Substituted Acridinium Ions: Effects of Electronic Coupling, Medium, and Polymer/Peptide Binding Guilford Jones. II ...... 26
Theoretical Studies of Electron Transfer and Optical Spectroscopy in Complex Molecules · Richard A. Friesner ...... 29
Intramolecular Electron Transfers in Linked Donor-Acceptor-Chromophore Assemblies C. Michael Elliott, Suzanne Ferrere, Steven L. Larson, Daniel L. Derr and D. F. Kelley ...... 32
Energy Conversion Based on Molecular Excited States. Redox Splitting in Soluble Polymers Thomas J. Meyer ...... 34
Strategies for Efficient Production and Access to Photochemically Generated Redox Species in Zeolites Michael B. Ledney and Prabir K. Dutta...... 37
Session VI - Glenn A. Crosby. Chairman
The Influence of Bridging Ligand Structure on Intramolecular Energy Transfer in Bimetallic Ru(II) Diimine Sensitizers Aaron I. Baba, Yongwu Liang, Won Kim and Russell H. Schmehl ...... 41
The Photochemistry and Spectroscopy of Platinum Dithiolate Complexes R. Eisenberg, S. Cummings, D. J. Anderson, G. Suardi, J. E. Hill and J. M. Bevilacqua ...... ,...... 44
Electroabsorption Spectra of Metal-to-Ligand Charge Transfer Transitions of (NH3)5 Rull L Complexes Yeung-gyo K. Shin, Bruce S. Brunschwig, Carol Creutz and Norman Sutin ...... 47
Magnetokinetic Effects in Reactions of Inorganic Compounds G. Ferraudi ...... 49
Session VII - .James H. Espenson. Chairman
Mechanistic Analysis of Electron Transfer Kinetics: Electron Coupling and Medium Reorganization M.D. Newton ...... 53
Long Distance Electronic Couplings for Intramolecular Electron Transfer John R. Miller ...... 56
iv The Role of Ion Pairing and Mode-Specific Vibronic Coupling in Intramolecular Charge and Energy Transfer Processes Piotr Piotrowiak ...... · ...... 59
Charge Separation in the Redox Quenching of the Excited States of Ruthenium(II)-Diimine Photosensitizers Hai Sun and Morton Z. Hoffman ...... 62
Radio lytic Studies on the Reactions of Porphyrins with Organic Radicals P. Neta, S. Marguet and J. Grodkowski ...... 65
Photoinduced Electron Spectroscopic and Electrochemical Studies of Transfer Reactions at Silver Surfaces Therese M. Cotton, George Chumanov, Morgan Sibbald, Hong Tian and Robert A. Uphaus ...... · ...... 69
Fundamental Studies in Photosynthesis for the Production of Fuels and Chemicals from Renewable Resources E. Greenbaum, J. W. Lee, I. Lee, C. V. Tevault, C. Y. Ma, D. P. Allison and R. J. Warmack ...... 72
Session VIII- .Jack Fajer, Chairman
Theoretical Studies of Electron Transfer in Complex Media David Chandler ...... 75
From ET to ESE Jau Tang, Marion C. Thumauer and James R. Norris ...... 77
Probing Reaction Center Structure Using Photochemically Generated Coherent Quantum Beats James R. Norris, Marion Thumauer, Stefan Weber, Ernst Ohmes and Gerd Kothe ...... 81
Primary Charge Separation in the Bacterial RC: A View from Hole Burning Gerald Small, Steven Kolaczkowski, Narla Reddy, Ryszard Jankowiak, Hai-Chou Chang, John Hayes, James Norris, Deborah Hansen and Michael Seibert ...... 86
Ionic Relaxation Following Quinone Reduction in Photosynthetic Reaction Centers David M. Tiede ...... 89
v Posters
1. XPS Study of a Valence-Averaged Mixed-Valence Di-Ruthenium Compound Larry Spreer, Christian B. Allan, D. Brent MacQueen, John W. Otvos and Melvin Calvin ...... 93
2. Direct Transient Absorption Spectroscopy of the Solvated Electron in Water and Alcohols and Ultrafast Studies on Intermolecular Electron Transfer in Contact and Solvent Separated Ion Pairs Paul F. Barbara ...... 94
3. Models of the Chlorophyll C Antenna Chromophores. Molecular Structure of a Zinc Pheoporphyrin K. M. Barkigia, M. W. Renner, K. M. Smith and J.Fajer ...... 95
4. Selective Visible Light Photooxidation of Propylene and Isobutane by 02 in Zeolite Y Fritz Blatter, Hai Sun and Heinz Frei ...... 96
5. Dielectric Friction Studied by Solute Rotational Diffusion and Femtosecond Solvent Dynamics Experiments, and ab initio Molcular Orbital Calculations Edward J. Castner. Jr., Yong J. Chang, William Konitsky and David H. Waldeck ...... 97
6. X-Ray Absorption Studies on Electrtonic Spin State Transitions of [Fe(a Picolylamine)3]Ch•EtOH in Different Media Lin X. Chen, Zhiyu Wang, Pedro A. Montano and James R. Norris ...... 98
7. Structures and Fundamental Vibrations of p-Benzosemiquinone- and Phenoxyl Related Radicals Daniel M.Chipman ...... 99
8. Redox Reactions at Chemically Modified Surfaces of Laser Ablated Silver Colloids ' George Chumanov, Morgan Sibbald and Therese M.Cotton ...... 100
9. Nature of the Emitting 3MLCT Manifold ofRhenium(I)(Diimine)(C0)3Cl Complexes D. R. Striplin and G.A.Crosby ..... ~ ...... 101
10. Ultrafast Isomerization Dynamics at Interfaces by Time-Resolved Second Harmonic Generation E. Borguet, X.Shi and K.B.Eisenthal ...... : 102
11. Observations on Electronic Coupling in Covalently Linked Transition Metal Donor-Acceptor Complexes Murielle A. Watzky, V. Swayambunathan and John F. Endicott ...... 103
12. Conformational Effects on Porphyrin Excited Singlet and Triplet States J. Fajer, A. Regev, H. Levanon, K. M.Smith, S. Gentemann and D. Holten ..... 104
vi 13. Excluded Volume in Calculation of Electron Transfer in Restricted Geometries and Infinite Solutions: Comparison of Simulations and Analytical Theory S. F. Swallen, K. Weidemaier and M.D. Fayer ...... 105
14. An Indirect Laser-Induced Temperature Jump Study of Dipole Effects in Mixed Thiol!Ferrocene-Terminated Alkanethiol Monolayers on Gold John F. Smalley, Stephen F. Feldberg, Christopher E. D. Chidsey, Marshall D. Newton and Yi-Ping Liu ...... 106
15. Mechanism and Kinetics of Photoreduction of C02 withp-Terphenyl and Cobalt Macrocycles Tomoyuki Ogata, Shozo Yanagida, Bruce S. Brunschwig and Etsuko Fuiita ...... ;...... 107
16. Increased Yield of Long-Lived Charge Separation Resulting from Protonation of an Electron Transfer Intermediate Su-Churi Hung, Alisdair Macpherson, Ana L. Moore, Thomas A. Moore and Devens Gust ...... 108
17. Photoinduced Electron Transfer between Sulfur-Containing Carboxylic Acids and the 4-Carboxybenzophenone Triplet State in Aqueous Solution Bronislaw Marciniak, Krzysztof, Bobrowski, Gordon L. Hug and J aroslaw Rozwadowski ...... 109
18. Light-Induced Redox Reactivity Involving Strongly Interacting Acceptor Systems: Precursors to Real Molecular Wires? Joseph T. Hupp, Laba Karki and VladimirPetrov ...... llO-
19. Structural and Dynamical Investigations of the Catalytic Mechanisms of Water Oxidation by the [(byph Ru(OHz)]z04+ Ion Yabin Lei and James K. Hurst ...... 111
20. Spectroscopic Studies of Donor-Acceptor Systems Linked via Hydrogen Bonds T. T. Chin, StephenS. Isied and Isabelle LeLouche ...... 112
21. Novel Photon-Antenna and Energy-Trap Systems. The Interplay Between R-3 and R-6 Energy Transfer Michael Kasha ...... 113
22. Photoelectrochemical Behavior of Naked and Surface-Modified Nanocrystalline SnOz Semiconductor Films · Idriss Bedja, Surat Hotchandani, Di Liu, Richard W. Fessenden and Prashant V. Kamat ...... 114
23. Photooxidation of Phenothiazine Derivatives in Silicas of Different Pore Sizes Bosong Xiang and Lan:y Kevan ...... 115
24. Dramatic Prolongation of 3MLCT State Lifetimes for Zeolite Entrapped Polypyridine Complexes of Ruthenium(II) Krzysztof Maruszewski and James R. Kincaid ...... 116
vii 25. Carotenoids: Optical, Electrochemical and Magnetic Resonance Studies A.S. Jeevarajan, C. C. Wei, J. A. Jeevarajan, G. Gao and Lowell D. Kispert .... 117
26. Formation of a Possible Precursor to Bimetallic and Hetero-Bimetallic Redox Catalysts Larry Spreer, Christian B. Allan, Chris Lange, D. Brent MacQueen, John W. Otvos and Melvin Calvin ...... 118
27. Photophysics and Photochemistry of 1-Nitronaphthalene: Triplet and Radical Yields, Quenching Rates and Solvents Effects in Reductions by Simple Anions and Aryl Donors · Laszlo Biczok and Hemy Linschitz ...... 119
28. Photochemical Hydrogen Evolution from Non-Sacrificial Electron Donors in Supramolecular Systems Based on Oxide Semiconductors Steven W. Keller, Yeong ll Kim, Edward H. Yonemoto, Elaine S. Brigham and Thomas E. Mallouk ...... '...... 120
29. Improved Efficiency and Stability of Si Photocathodes by Electrochemical Etching D. Mao, K. J. Kim, Y. S. Tsuo and Arthur J. Frank ...... 121
30. Simulation Studies of Rotational Dielectric Friction Arno Papazyan, P. Vijaya Kumar and Mark Maroncelli ...... 122
31. Photophysics of Surface Modified Small Particles C. Luangdilok, S. Kartha, S. Kapoor, D. Lawless and Dan Meisel...... 123
32. Synthesis and Characterization of GaP, InP, and GalnP2 Quantum Dots Olga R. Micic, Calvin J. Curtis, Julian R. SpragJ.Ie and Arthur J. Nozik ...... 124
33. Electric Field Effects on Ultrafast Photoinduced Electron Transfer Across Semiconductor-Liquid Interfaces Y. Rosenwaks, B. R. Thacker and A. J. Nozik ...... 125
34. Electron Transfer from Lipid Functionalized Pyrene in Langmuir Films to Lipid Bound and Bulk Phase Acceptors Li Li and L. K. Patterson ...... 126
35. Analytic Quantum Theory of Electron Transfer with a Reaction Mode Strongly Coupled to the Electron and Weakly Coupled to the Bath Katja Lindenberg, Emilio Cortes and Robert M. Pearlstein·····················~········· 127 36. Excited State Electron Transfer Reactions in Polymetallic Complexes John D. Petersen, Sean L.Gahan, Seth C. Rasmussen and Nuh-Tome Huang ...... 128
37. Axial Ligand Control of Oxidation States in Nonplanar Porphyrins: Conversion of Ni(ID 1t Cation Radicals to "Ni(ill)" Cations M. W. Renner, K. M. Barkigia, Y. Zhang, K. M.Smith andJ. Fajer ...... l29
viii 38. The Chemical and Physical Properties of a Platinum(ll) Biphenyl Dicarbonyl Complex D. Paul Rillema, Y. Chen and Jon Merkert ...... 130
39. Femtosecond Photodichroism Studies of Isolated Photosystem IT Reaction Centers Michael Seibert, Gary P. Wiederrecht and Michael Wasielewski ...... 131
40. 13-Desmethyl-13-Acetoxyretinal: A NewChromophore for Bacteriorhodopsin Stanley Seltzer ...... 132
41 An Indirect Laser-Induced Temperature Jump Study of the Kinetics of Electron Transfer Through Mixed Thiol!Ferrocene-Terminated Alkanethiol Monolayers on Gold John F. Smalley, Stephen W. Feldberg, Christopher E. D. Chidsey, Marshall D. Newton and Yi-Ping Liu ...... 133
42. Vibrational Dynamics in Photoinduced Electron Transfer Kenneth G. Spears and Xiaoning Wen ...... 134
43. Vibrational and Electronic Structure of the Excited States of ~-Substituted Metalloporphyrins from Time-Resolved Resonance Raman Spectroscopy Ranjit Kumble, Glen R. Loppnow, Songzhou Hu and Thomas J. Spiro ...... 135
44. Charge-Transfer Interactions in meso-Linked Porphyrin-Quinone Molecules JohnS. Connolly, Russell A. Cormier, Tina L. Palmer and Julian R. Sprague ...... " ...... :...... 136
45. Femtosecond Spectroscopy of Chlorosome Antennae from the Green Photosynthetic Bacterium Chloroflexus aurantiacus Sergie Savikhin, Yinwen Zhu, Su Lin, Robert E. Blankenship and WalterS. Struve ...... 137
46. Highly Selective Oxidation of Toluene and Cyclohexane by 02 with Visible Light in Zeolite Y Hai Sun, Fritz Blatter and Heinz Frei ...... 138
47. Quinone Binding in Photosynthetic Reaction Centers Aaron Barkoff, Seth W. Snyder, Yuenian Zhang, Daisy Zhang, Agnes Ostafin, James R. Norris and Marion C. Thumauer ...... 139
48. Photochemical Water-Splitting Using Organometallic Oxides as Sensitizers David R. Tyler, Tim Auketvand Greg Baxley ...... 140
49. Fourier Transform EPR Study of C6o Triplets Carlos A. Steren and Hans van Willigen ...... 141
IX 50. Photocapacitance and Time-Resolved Fluorescence Studies of Chemically Sensitized Ti02 Electrodes K. H. Liao and D. H. Waldeck ...... 142
51. The Effect of Photo-Generated, Oriented Electric Fields on Carotenoids: A Model for the Carotenoid Bandshift in Photosynthesis ' Michael R. Wasielewski, David Gosztola and Hiroko Yamada ...... 143
52. Transients Formed in Laser Flash Photolysis of Ir-Si Bonded Diastereomeric <;omplexes Peter I. Djurovich, Radhika Joshi, Wendy Cook and R. J. Watts ...... 144
53. Photophysics and Photoredox Processes at Polymer-Water Interfaces Jiunn-Shyong Hsiao, Andrew Eckert, Ti Cao, Weiping Yin and S. E. Webber ...... 145
54. Evidence for Solid State Photooxidative C-C Bond Cleavage in 1,2-Diamines Jeffrey W. Leon and David W. Whitten ...... 146
55. Photochemical lnterconversions of Organic Radical Cations by Hydrogen Transfer Using Visible Light Thomas Bally, Leo Truttmann, Jib Tzong Wang and Ffrancon Williams ...... 147
56. Effect of Pressure on the Thermodynamics and Kinetics of Electron Transfer in Ruthenium-Modified Cytochromes c Ji Sun, James F. Wishart, Martin Meier, Rudi van Eldik, Richard D. Shalders and Thomas W. Swaddle ...... 148
LIST OF PARTICIPANTS ...... 149
AUTHOR INDEX ...... 157
X EIGHTEENTH DOE SOLAR PHOTOCHEMISTRY RESEARCH CONFERENCE
June 5-9, 1994
Granlibakken Conference Center Tahoe City, .california
PROGRAM
Sunday, June 5
12:00 noon - 8:00 PM Registration
8:00PM Welcoming Reception
Monday Morning, June 6
7:00 AM - 8:30AM Registration
SESSION I
Mary E. Gress, Chairman
8:30AM Welcome to the Conference Sung-Hou Kim, Director, Structural Biology Division, Lawrence Berkeley Laboratory
Introductory Comments John W. Otvos, Structural Biology Division, Lawrence Berkeley Laboratory Bill Parson, General Manager, Granlibakken Conference Center Mary E. Gress, U.S. Department of Energy
9:00AM Plenary Lecture: Nanocrystalline Electronic Junctions for Solar Light Harvesting and Conversion Michael Gratzel, Ecole Polytechnique Federale, Lausanne, Switzerland
10:00 AM Coffee Break
xi SESSION ll
David H. Waldeck, Chairman
10:30 AM Investigations of High Quantum Yield Dye Sensitization Processes on Two-Dimensional Semiconductors Bruce A. Parkinson, Colorado State University
11:00 AM Picosecond Dynamic Studies of Electron Transfer Rates at III-V Semiconductor /Liquid Interfaces Nathan S. Lewis, California Institute of Technology
11:30 AM , Ultrafast Optical Studies of Surface Reaction Processes at Semiconductor Interfaces R. J. Dwayne Miller, University of Rochester
Monday Afternoon, June 6
SESSION ID
Olga I. Micic, Chairman
1:30PM Interfacial Photochemistry Arthur J. Frank, National Renewable Energy Laboratory
2:00PM Mechanistic Aspects of Photoconversion at Semiconductor Liquid Junctions and in Facilitated Transport Membranes Carl A. Koval, University of Colorado
Monday Evening, June 6
SESSION IV
Stephen E. Webber, Chairman
7:30PM Asymmetric Polymeric Porphyrin Films for Solar Energy Conversion Carl C. Wamser, Portland State University
8:00PM Photoconversion Processes in Molecular Semiconductor Films Brian A. Gregg, National Renewable Energy Laboratory
8:30PM Posters (odd numbers)
xii Tuesday Morning, I une 7
SESSION V
Stephan S. Isied, Chairman
8:30 AM Directional Electron Transfer through Macromolecular Arrays Marye Anne Fox, University of Texas
9:00 AM Photoinduced Charge-Shift Reactions of Donor-Substituted Acridinium Ions: Effects of Electronic Coupling, Medium, and Polymer /Peptide Binding Guilford Jones II, Boston University
9:30 AM Theoretical Studies of Electron Transfer and Optical Spectroscopy in Complex Molecules Richard A. Friesner, Columbia University
10:00 AM Coffee Break
10:30 AM Intramolecular Electron Transfers in Linked Donor-Acceptor Chromophore Assemblies C. Michael Elliott, Colorado State University
11:00AM Energy Conversion Based on Molecular Excited States. Redox Splitting in Soluble Polymers Thomas J. Meyer, University of North Carolina at Chapel Hill
11:30 AM Strategies for Efficient Production and Access to Photochemically Generated Redox Species in Zeolites Prabir K. Dutta, Ohio State University
Tuesday Evening, June 7
SESSION VI
Glenn A. Crosby, Chairman
7:30PM The Influence of Bridging Ligand Structure on Intramolecular Energy Transfer in Bimetallic Ru(II) Diimine Sensitizers Russell H. Schmehl,. Tulane University
8:00PM The Photochemistry and Spectroscopy of Platinum Dithiolate Complexes Richard Eisenberg , University of Rochester
xiii 8:30PM Break
8:45PM Electroabsorption Spectra of Metal-to-Ligand Charge Transfer Transitions of (NH3)5 RuliL Complexes Bruce S. Brunschwig, Brookhaven National Laboratory
9:15PM Magnetokinetic Effects in Reactions of Inorganic Compounds Guillermo Ferraudi, Radiation Laboratory, University of Notre Dame
Wednesday Morning, June 8
SESSION VII
James H. Espenson, Chairman
8:00 AM Mechanistic Analysis of Electron Transfer Kinetics: Electronic Coupling and Medium Reorganization Marshall D. Newton, Brookhaven National Laboratory
8:30 AM Long Distance Electronic Couplings for Intramolecular Electron Transfer John R. Miller, Argonne National Laboratory
9:00 AM The Role of Ion Pairing and Mode-Specific Vibronic Coupling in Intramolecular Charge and Energy Transfer Processes Piotr Piotrowiak, University of New Orleans
9:30 AM Coffee Break
10:00 AM Charge Separation in the Redox Quenching of the Excited States of Ruthenium(II)-Diimine Photosensitizers Morton Z. Hoffman, Boston University
10:30 AM Radiolytic Studies on the Reactions of Porphyrins with Organic Radicals Pedatsur Neta, National Institute of Standards and Technology 11:00 AM Coffee Break
xiv 11:20 AM Spectroscopic and Electrochemical Studies of Photoinduced Electron Transfer Reactions at Silver Surfaces Therese M. Cotton, Ames Laboratory
11:50 AM Fundamental Studies in Photosynthesis for the Production of Fuels and Chemicals from Renewable Resources Elias Greenbaum, Oak Ridge National Laboratory
Wednesday Evenin~, June 8
8:30PM Posters (even numbers)
Thursday Momin~. June 9
SESSION VIIT
Jack Fajer, Chairman
8:00 AM Theoretical Studies of Electron Transfer in Complex Media David Chandler, University of California, Berkeley
8:30 AM From ET to ESE Jau Tang, Argonne National Laboratory
9:00 AM Coffee Break
9:30AM Probing Reaction Center Structure Using Photochemically Generated Coherent Quantum Beats James R. Norris, Argonne National Laboratory
10:00 AM Primary Charge Separation in the Bacterial RC: A View from Hole Burning Gerald Small, Ames Laboratory
10:30 AM Ionic Relaxation Following Quinone Reduction in Photosynthetic Reaction Centers David M. Tiede, Argonne National Laboratory
11:00 AM Closing Remarks Mary E. Gress, U.S. Department of Energy
XV Monday Morning
Session I
Mary E. Gress, Chairman NANOCRYSTALLINE ELECTRONIC JUNCTIONS FOR SOLAR LIGHT HARVESTING AND CONVERSION
Michael Gratzel
Institut de Chimie Physique, Ecole Polytechnique Federate, CH-1015 Lausanne, Switzerland
Transparent nanocrystalline films of oxide semiconductors such as Ti021
ZnO Nb20 5, Sn02 and Fe20 3 have been prepared on a conducting glass support employing a sol-gel procedure. The films are composed of nanometer-sized particles sintered together to allow for charge carrier migration. The internal surface of these films is very high, roughness factors of the order of 1000 being readily obtained. The unique optical and electronic properties of the nanocrystalline films have been examined in detail. Their surface can be contacted with conducting or semiconducting materials to produce Schottky or nip- type junctions, respectively. Polarization was applied for forward and reverse biasing of the films and the resulting absorption changes in the visible were used to monitor optically the dynamics of electron transfer events within the film and to determine their flat band potential. Surprisingly, charge percolation across the network of interconnected nanocrystalline particles can occur rapidly, allowing for facile and quantitative collection of photo-generated carriers on the conducting glass support.
Surface derivatization of nanocrystalline oxides, such as Ti02• with a monolayer of a charge transfer sensitizer, e.g. cis-(SCN)2bis(2,2'-bipyridyl-4,4' dicarboxylate)ruthenium(II), produces intensely colored films harvesting visible light very efficiently. MLCT excitation of the ruthenium complex results in electron injection into the oxide with a qua~tum yield of practically unity. The bipyridyl ligand is endowed with interlocking groups, i.e. carboxylates, whose role is to graft the ruthenium complex to the oxide surface and to enhance the electronic coupling of the 1t* wave function of the bipyridyl with the conduction band manifold of the semiconductor. Strikingly high efficiencies for conversion of incident photons into electric current exceeding 90% over a broad spectral range have been obtained with such dye derivatized nanocrystalline films.
1 Time resolved laser photolysis studies have been applied to analyze the dynamics of the charge transfer events involved in the sensitization process. The electron injection into the nanocrystalline film is very rapid occuring in less than 10 ps, while the electron recapture by the oxidized dye is several orders of magnitude slower. This explains why light induced charge separation is so efficient in these sensitized semiconductor systems. The large difference in rate arises from the fact that the reverse electron transfer, due to its high exergonicity, is in the inverted Marcus region. The effect of the nature of the medium contacting the surface of the dye derivatized nanocrystalline film on the back electron transfer dynamics was scrutinized. The nanopores present in the film were either exposed to high vauum or filled with an organic liquid. No significant effect of the nature of the contacting medium on the back reaction rate was observed. The temperature effect on the reverse electron transfer kinetics was also examined. The interfacial charge transfer was found to be activationless in the range of 70 to 280 K. The lack of a temperature effect has been interpreted in terms of a quantum mechanical model taking into account the participation of high frequency vibrational modes in the heterogeneous redox process.
Based on the encouraging results obtained from these investigations, a nanocrystalline photovoltaic device was developed whose overall light to electric energy conversion yield exceeds 10% in AM 1.5 solar light. The current voltage characteristics and spectral response of the system have been analyzed. Room temperature molten salt based electrolytes have been developed in order to ensure output stability under long term operation. Apart from photovoltaic applications, nanocrystalline devices offer interesting perspectives for the direct conversion of sunlight into chemical fuels.
2 Monday Morning
Session II
David H. Waldeck, Chairman INVESTIGATIONS OF IDGH QUANTUM YIELD DYE SENSITIZATION PROCESSES ON TWO-DIMENSIONAL SEMICONDUCTORS
Xiumei Dou, Laura Sharp and Bruce A. Parkinson Department of Chemistry Colorado State University Fort Collins, CO 80523 Dye sensitization has the potential to increase the light utilization of large band gap semiconductors. The excited state of a dye molecule adsorbed onto the surface of the semiconductor electrode can inject electrons into the conduction band of an n-type semiconductor. These electrons can then be detected as a photocurrent generated at photon energies less than the band gap of the semiconductor. The quantum yield for electrons collected per photon absorbed by the dye at single crystal oxide electrodes was always less than a few percent. We have discovered that when two dimensional chalcogenide materials are used as photoelectrodes the quantum yield per absorbed photon can approach 100%. SnSz, which has the layered Cdlz structure and a bandgap of 2.2 eV, has been used as the prototypical material for our studies. Large crystals of this material can be grown with the Bridgeman technique. Doping of the material with chlorine (as a replacement for sulfur) results in n-type materials with doping levels suitable for sensitization studies. A large variety of dyes with absorption maximum at energies less than 2.2 eV can be used to sensitize this material. We have recently been concentrating on two dyes, methylene blue and cresyl violet. We have studied the competitive adsorption and adsorption rates of these dyes on the SnSz surfaces with varying degrees of perfection. We have also been able to selectively photooxidized the methylene blue from the surface by using laser light at wavelengths corresponding to either the methylene blue or cresyl violet absorption maxima. Recently methods for increasing the surface area of SnSz photoelectrodes have been developed. By photoelectrochemically etching the surface in either acid or basic solutions, increases in the quantum yield for electron flow per incident photon have been obtained. The adsorption isotherm for methylene blue on etched and unetched surfaces has the same shape but shows an increase of 20 times or more in the quantum yield. Cresyl violet does not sensitize nearly defect free van der Waals surfaces of SnSz photoanodes but does show substantial sensitization current on photoetched surfaces.
The contrasting behavior of the two dyes, methylene blue and cresyl violet, on nearly perfect and photoetched surfaces is explained by a new model for sensitization of these surfaces. In the new model a majority of adsorbed dyes act as antennae while a few at special sites are efficient at electron injection. The long-lived triplet state of methylene blue allows for sampling many surface sites via energy transfer during the lifetime of the dye whereas the short lived singlet state of cresyl violet permits sampling only a small area of the surface with a lower probability of finding a special injection site. We are also developing an in situ scanning tunneling microscopy method for detecting the position and energy levels of dye molecules adsorbed on these surfaces with molecular resolution. This is accomplished by modulating a light source at the wavelength of· the dye absorption maximum and extracting the photo-induced contribution to the tunneling current via a lock-in amplifier. A simultaneous picture of
3 photocurrent response and topography can then be obtained. Questions such as the state of dye aggregation on the surface and whether dye molecules are adsorbed on special sites could then be answered. The photo-STM technique has already been applied to semiconductor surfaces illuminated with bandgap light Simultaneous images of topography and photocurrent can be obtained for recombination centers, quantum wells and heterojunction structures. We. are also continuing a quest for other high quantum yield semiconductor substrates for sensitization studies.
DOE Sponsored Publications 1992-1994 "Increasing the Dye Sensitization Efficiency of SnS2 Photoanodes by Photoelectrochemical Etching", Xiumei Dou, Laura Sharp, P. A. Morris and B. A. Parkinson, Langmuir, submitted
"Studies of the Sensitization of SnS2 Photoanodes with a Singlet and Triplet Dyes", Xiumei Dou, P. A. Morris and B. A. Parkinson, Langmuir, submitted
"Fundamental Review: Scanning Probe Microscopies", D. Louder aiid B. A. P?-fkinson Analytical Chemistry, in press
C. A. Foss, D. L. Feldheim, D. R. Lawson, P. K. Dorhout, C. M. Elliott, C. R. Martin and B. A. Parkinson, "Transition Metal Chelate - Fulleride Compounds: Electrocrystal lization of Semiconducting [Ru(ll)(bpy)32+](C60-1)2", J. Electrochem. Soc., 140, L84, (1993)
4 PICOSECOND DYNAMIC STUDIES OF ELECTRON TRANSFER RATES AT III-V SEMICONDUCTOR/LIQUID INTERFACES
Marcel Sturzenegger, C.N. Kenyon and Nathan S. Lewis
Division of Chemistry and Chemical Engineering, California Institute of Technology Pasadena CA 91125
In the past, a key obstacle to progress in the fundamental understanding of photoelectrochemical kinetics has been the use of a variety of fundamentally different physical quantities to describe the rate constant for heterogeneous charge transfer at the semiconductor/liquid interface. The apparently large dicscrepancies in the reported rate data are often simply a result of this confusion. To address this problem, a coherent theoretical framework has been developed which allows meaningful comparisons to be made between values obtained from steady state currents and branching ratios measured at semiconductor/liquid junctions, from experiments monitoring charge injection into semiconductor electrodes from adsorbed.dyes, and those extracted from semiconductor carrier decays following pulsed excitation. Furthermore, a straightforward method has been presented for relating these kinetic measurements to the extensive body of heterogeneous rate constant data available for metal electrodes. This model has been applieCl to the InP /liquid interface which we have been investigating in the past DOE grant period.
We have also performed a series of real-time dynamic experiments aimed at elucidating the nature of electron transfer at the semiconductor/electrolyte interface. In addtion to our efforts with naked serninconductor electrodes in contact with dissolved redox couples, we have also been interested in covalently linking alkyl chains terminated by an acceptor molecule to the crystal face so that electron transfer at fixed distances may be examined. To reach this goal, we have investigated the modification of the <111> face oflnP with alkylating agents and have characterized both the inital and final surfaces by high resolution x-ray photoelectron spectroscopy (XPS) and real-time surface recombination velocity measurments. As we are interested in knowing the nature of the bonding to the semiconductor, the ability to prepare clean electrode surfaces is essential. In agreement with earlier reports, it was found that the standard Br2fMeOH//NH40H etch for InP, while producing electrically well behaved InP interfaces, yields a substantially oxidized surface. We have found however that essentially oxide-free surfaces can be prepared using entirely non-aqueous Br2fMeOH/INH3 etches. Reaction of these clean surfaces with BrCH2PhCF3 has been followed by monitoring the ratio ofF to In signals in the XPS. The ratio has been observed to saturate after approximately 1 hour and remain even after heating the modified surface in clean solvent . Extension of this reactivity to other organic species is currently being performed. This approach looks promising in that it will allow us to functionalize the surface with electroactive reagents at a fixed, known distance. This method will then provide us with the desired method to examine the time and distance dependence of charge transfer from a photoexcited, delocalized charge carrier in the solid into a localized, redox-active molecular state in the liquid phase. Progress in this area, as well as in the real-time dynamic decay processes at etched and chemically modified GaAs and InP single crystal surfaces, will be discussed at this DOE-sponsored conference.
5 Publications 1992-1994
Ming X. Tan, Paul E. Laibinis, Sonbinh T. Nguyen, Janet M. Kesselman, Colby E. Staton, and Nathan S. Lewis, "Principles and Applications of Semiconductor Photoelectrochemistry," in Kenneth D. Karlin, Ed., Progress in Inorganic Chemistry, Wiley, New York, 41, 21 (1994)
Amit Kumar, Wolf Christian A. Wilisch, and Nathan S. Lewis, "The Electrical Properties of Semiconductor/Metal, Semiconductor/Liquid and Semiconductor/Conducting Polymer Contacts", CRC Critical Reviews., 18, 327 (1993).
Ashish Bansal, Ming X. Tan, Bruce J. Tufts, and Nathan S. Lewis, "Distinguishing Between Buried Semiconductor/Metal Contacts and Hybrid Semiconductor/Metal/Liquid Contacts at n-GaAs/KOH-Se·2·(aq) Junctions", J. Phys. Chem., 97,7309 (1993).
C. N. Kenyon, Gail N. Ryba, and Nathan S. Lewis, "An Analysis of Time Resolved Photocurrent Transients at Semiconductor/Liquid Interfaces", J. Phys. Chem., 97, 12928 (1993).
Gail N. Ryba, C. M. Kenyon, and Nathan S. Lewis, "The Effects of Metal Ion Chemisorption on GaAs Surface Recombination: Picosecond Luminescence Decay Measurements", J. Phys. Chem., 97, 13814 (1993).
Colby E. Stanton, SonBinh T. Nguyen, Janet M. Kesselman, Paul E. Laibinis, and Nathan S. Lewis, "Semiconductor", Encyclopedia ofInorganic Chemistry, J. Wiley & Sons, in press.
Ming X. Tan, Ashish Bansal, Iver Lauermann, Gary A. Shreve, and Nathan S. Lewis, "Semiconductor Interfaces", Encyclopedia ofInorganic Chemistry, J. Wiley & Sons, in press.
6 ULTRAFAST OPTICAL STUDIES OF SURFACE REACTION PROCESSES AT SEMICONDUCTOR INTERFACES
R. J. Dwayne Miller Department of Chemistry, University of Rochester, Rochester, NY 14627-0216
The level of fundamental understanding of the various dynamical processes controlling surface photochemistry is much less well developed than the analogous processes involved in molecular photochemistry. There is little information on the non-radiative relaxation channels at surfaces, the relative energetics of coupled states, and there is a wide spread in observed reaction rates that makes it difficult to ascertain general features in the laws governing the surface reaction dynamics. The discrepancy in experimental details documenting homogeneous versus heterogeneous processes is largely due to the inherent difficulty in studying surfaces. The DOE funded research has focused on the development of non-linear optical methods for the in situ study of surface reaction dynamics to address some of these issues. In particular, the work has concentrated on interfacial charge transfer processes as this is the simplest of all surface reactions, i.e., no bonds are broken and the reaction is derived from nuclear repolarization. For this reason, we have the greatest chance of coming to detailed understanding of this reaction coordinate.
This work exploited semiconductor liquid junctions as an optical switch to turn on the surface photochemistry. These systems are extremely efficient at charge separation (photocurrent quantum yield -1) and can exhibit solar energy conversion efficiencies approaching those of the best solid state devices. The most tantalizing feature of these systems is that theoretically they are capable of attaining nearly twice the solar energy conversion efficiencies of all solid state semiconductor systems. The higher potential efficiency of semiconductor liquid junctions is related to the molecular states (rather than a band) that act as the charge acceptors. Due to the discrete nature of these states, it is possible to avoid energy conversion losses in the junction through lattice heating. However, to achieve this goal, interfacial charge transfer must be made sufficiently fast to compete with carrier thermalization. This prospect is related to the "hot carrier" model initially proposed by Nozik et al. for fully optimized solar cell performance based on a threshold absorber (e.g., semiconductors). The key question is whether or not interfacial electron transfer can fundamentally approach time scales comparable to electron thermalization in the solid state. This aspect of the model requires that the charge transfer process occur in the strong coupling or adiabatic limit for electron transfer. This is an important issue as it determines the level of theoretical modeling needed to properly treat the electronic coupling between the highly delocalized states of the solid state and the molecular acceptors across the interface.
To this end, a number of non-linear optical methods have been conducted to arrive at a real time view of interfacial charge transfer processes at single crystal surfaces. In particular, these studies have focused on GaAs(100)/(Se-2J-1) aqueous liquid junctions as this system is expected to show some of the fastest interfacial charge transfer dynamics and serves as a test case for the hot carrier model. The experimental work has been involved in the development of two new surface spectroscopies (surface electro-optic sampling and grating spectroscopies) which provide combination of high time resolution and sensitivity to follow the carrier transport and population dynamics at the surface. This work, along with the dynamic studies of the water response function, is beginning to give a complete picture for the operating dynamics at GaAs interfaces which is summarized in Figure 1. Assuming bulk studies of carrier relaxation hold at surfaces, this work illustrates that a significant fraction of the carriers are undergoing interfacial transitions prior to thermalization. A direct observation of the carrier thermalization at the surface is needed to solidify this statement and ongoing femtosecond photoemission experiments along this line will be
7 discussed at the meeting. The most important observation from this work is that interfacial charge transfer processes can occur in the picosecond range which is consistent with the electronic coupling between molecular states within the Helmoltz layer and the electrode band states approaching adiabatic limits. Given the highly damped nature of the molecular wavefunctions (f3 - 0.9 A-1), this condition was not obvious for a solvated state at an electrode interface. This work needs to be extended to physisorbed outer sphere reagents such as the ongoing studies of InP to determine whether this phenomena holds in general (in the absence of a nuclear barrier, i.e., for optimally poised redox couples).
--0
-2 EV------T Se - ---a -1 -----""'
Fi~ure 1. Summary. The transport of the minority carrier distribution to the surface from within the space charge region occurs within 200 fs as determined from electro-optic sampling. Surface grating studies have found 1-2 picosecond hole depletion components at the surface attributable to interfacial hole transfer. The solvent barrier crossing dynamics have sub 100 fs to ps components as determined from OKE studies. The hole transfer dynamics are comparable to the diffusive relaxation components in the water relaxation.
8 Publications - 1992-1994
1. L.A. Gomez-Jahn and R. J.D. Miller, "Picosecond Studies of Carrier Reaction Dynamics at n-GaAs(lOO) Interfaces," J. Chem. Phys. 1992, 3892.
2. J. Lanzafame and R. J. D. Miller, "Surface Electron Transfer Dynamics," in Understanding Chemical Reactivity, J. R. Simon (ed.), Academic Press, 1993, pp. 163- 204. 3. R. J.D. Miller, L.A. Gomez-Jahn, J. Lanzafame, L. Min, and S. Palese, "Ultrafast Non Linear Optical Studies of Surface Reaction Processes," S.P.l.E. Proceedings 1993, 1858, 354-366.
4. R. J. D. Miller, G. McLendon, A. Nozik, W. Schmickler, and F. Willig, Chapter 4 in Surface Electron Transfer Processes, Verlag, 1994, book reviewed and accepted for publication. 5. S. Palese, L. Schilling, R. J.D. Miller, P.R. Staver, and W. T. Lotshaw, "Femtosecond Optical Kerr Studies of Water," J. Phys. Chem., submitted. 6. R. J.D. Miller and J. Lanzafame, "Ultrafast Surface Reaction Dynamics. Mapping the Electron Trajectory," J. Phys. Chem., submitted. 7. D. Wang, C. Bonner, J. Buontempo, and R. J. D. Miller, "Subpicosecond Surface Grating Studies in Interfacial Hole Transfer Dynamics at n-GaAs(lOO) Interfaces," Chem. Phys. Lett., submitted.
9 Monday Afternoon
Session III
Olga Io Micic, Chairman . INTERFACIAL PHOTOCHEMISTRY
A. J. Frank and D. Mao
Basic Sciences Division, National Renewable Energy Laboratory, Golden, Colorado 80401
Fundamental issues are being addressed relating to semiconductor electrodes and particle systems whose surface properties have been chemically altered for improving their photostability and conversion efficiency. During this reporting period, the research has focused on junction modification of semiconductor electrodes, charge recombination at semiconductor-liquid interfaces, photoinduced-charge generation mechanisms of organic semiconducting polymers and oligomers, molecular water-oxidation and water-reduction catalysts for derivatizing semiconductors, and molecular solids as photosensitizers. Highlights of some of these studies are given below.
Junction Modification and Charge Recombination Studies: During photoelectro chemical (PEC) studies of n-CdTe electrodes, we have discovered a promising approach for significantly augmenting the open-circuit photovoltages (V0 J and photoconversion efficiencies of Au:n-CdTe Schottky barrier-type cells. The improved characteristics can be traced to the formation of a thin p-type layer, resulting from the photoelectrochemical oxidation of the n-CdTe surface. X-ray photoelectron spectroscopy, Auger electron spectroscopy depth profiling, and electron-beam-induced-current measurements provide insight into the underlying causes for the formation of the p-type layer. Current-voltage (J-V) and capacitance-voltage measurements are consistent with a Au:p-n-CdTe type structure and show that the thin p-layer enhances the effective barrier height relative to that of traditional Au:n-CdTe Schottky-barrier cells. From temperature-dependence studies of the J-V characteristics, detailed information on the charge transport mechanism of the junction was obtained. Photocurrent spectra of Au:p-n-CdTe as a function of temperature reveal that exciton excitation in CdTe contributes to the photocurrent.
The Voc of semiconductor/liquid junction solar cells is a critical parameter in determining the energy conversion efficiency. The fundamental process controlling Voc is the recombination of photoexcited electrons and holes. The lower the recombination rate, the larger is Voc· Of the various recombination processes (e.g., recombination in the bulk semiconductor, recombination in solution via redox species, and recombination at the interface via surface trapping levels), surface recombination is the least understood. To obtain a more detailed understanding, we have derived a simple quantitative expression, based on semiconductor solid state theory, that directly relates the surface recombination velocity (Sr) to Voc· The effectiveness of this expression to account for PEC behavior of n-Si in acetone with FeCJ>2 +to (ferrocenium ion/ferrocene) was investigated. Based on J-V data and the.dependence ofVoc on both the temperature and the concentration of FeCP2 +, we were able to exclude other possible recombination channels and identify surface recombination as the dominant recombination process in determining Voc· The V0 c-Sr expression yielded values of the barrier height and the
11 surface recombination velocity that were in very good agreement with published data. Application of the equation to other semiconductor/liquid junction cells suggests that the expression may be useful for evaluating and understanding the behavior of a wide variety of PEC systems.
Organic Semiconductor Films: A fundamental issue of importance to the development of conducting polymer films for solar photoelectrochemical conversion relates to the identity of charge carriers generated by light. Controversy surrounds both the experimental data and their interpretation, regarding the assignment of photogenerated charge carriers in the undoped polythiophene family of conjugated polymers. In collaboration with Prof. A. Ehrenfreund of Technion-Israel Institute of Technology, both light-induced electron spin resonance and photoinduced absorption measurements as a function of excitation energy and temperature were applied to address this issue. The results suggest that the photogenerated electron-hole pairs rapidly decay, via structural relaxation of the polymer chains, into pairs of negative and positive polarons (P, cation or anion radicals; spin 1/2) e--h+ ... p+ + p-. Most of these polarons recombine quickly except for single polarons left on different chains. These remnant long-lived polarons eventually form bipolarons (B, dication or dianion, spin 0), which have a lower total energy than the separate polarons, via, for example, p+ + p+ - B++. In a complementary study, we have investigated with M. Ibrahim (NREL) semiconductor thin films of sexithiophene; sexithiophene (6 T) is a molecule comprising six thiophene units. Oligomers, such as 6 T, are attractive from the perspective of solar energy applications because of their well-defined structure, low density of structural defects, and the possibility of synthetically altering their structure or conjugation length to improve their optical and optoelectronic properties. Also, because of these characteristics, oligomers present an opportunity to correlate more rigorously their observed properties with their molecular structure and can serve as model compounds for comparison with polymer analogues. We have studied 6 T to characterize the nature of the photogenerated species and to determine how these species compare with those observed in the neutral polymer. Photoinduced absorption data of sexithiophene show that in marked contrast to the polymer, polarons are the principal stable photogenerated species in the millisecond-time domain. Moreover, only positive polarons (radical cations) are observed, suggesting that photogenerated negative charges are trapped outside the oligomer molecules and do not contribute to the photoinduced absorption spectrum. Theoretical energy level and absorption calculations are in very good agreement with the observed polaron spectrum and confirm that the polaron spectrum of 6 T consists of two electronic transitions; bipolarons in 6 T exhibit only one band. The number of electronic transitions observed for polarons and bipolarons in the oligomer differs from that seen in the polymer. Polarons and bipolarons in polythiophene give rise, respectively, to three and two optical transitions below the T-r* interband transition threshold.
12 DOE/OER-SPONSORED PUBLICATIONS (1992-1994)
"Characterization of Rose Bengal-N,N'-Dimethyl-4,4'-Bipyridinium Complexes and their Separation in Aqueous Si02 Colloids: Photophysical Properties of Rose Bengal in the Microheterogeneous System," Willner, I.; Eichen,Y; Joselevich, E.; Frank, A. J. J. Phys. Chem. 1992, 96, 6061.
"Photophysics and Photosensitization Behavior of Microcrystalline [Pt(bpy):zh~(pop)J • nH20," Palmans, R.; Frank, A. J.; Houlding, V. H.; Miskowski, V. M. J. Mol. Catal. 1993, 80, 327.
"Photoinduced Electron-Transfer Processes Using Organized Redox -Functionalized Bipyridinium Polyethylenimine- Ti02 Colloids and Particulate Assemblies," Willner, I.; Eichen, Y.; Frank, A. J.; Fox, M. A. J. Phys. Chem. 1993, 97, 7264.
"A Metal:p-n-CdTe Schottky-Barrier Solar Cell: Photoelectrochemical Generation of a Shallow p-Type Region in n-CdTe," Pugh, J. R.; Mao, D.; Zhang, J.-G.; Heben, M. J.; Nelson, A. J.; Frank, A. J. J. Appl. Phys. 1993, 74, 2619.
"Molecular Water Oxidation Catalyst," Gratzel, M.; Munavalli, S.; Pem, F.-J.; Frank, A. J. U.S. Patent Serial No. 5,223,634, issued June 29, 1993.
"Open-Circuit Photovoltage and Charge Recombination at Semiconductor/Liquid Interfaces," Mao, D.; Kim, K.-J.; Frank, A. J. J. Electrochem. Soc. In press.
"Photoinduced Absorption Study of Sexithiophene: Evidence for Photogeneration of Polarons," Poplawski, J.; Ehrenfreund, E.; Cornil, J.; Bredas, J. L.; Pugh, R.; Ibrahim, M.; Frank, A. J. Submitted.
13 MECHANISTIC ASPECTS OF PHOTOCONVERSION AT SEMICONDUCTOR -LIQUID JUNCTIONS AND IN FACILITATED TRANSPORT MEMBRANES
Carl A. Koyal, Marcus Groner, Jason Howard, Robert Torres, Thanh To, David Watts Department of Chemistry and Biochemistry University of Colorado, Boulder, CO 80309-0215
Electron Transfer Rate Constants at Semiconductor Electrodes In the 1960's, H. Gerischer first formulated a model for electron transfer (ET) at semiconductor electrodes. Several modifications have been added, but the fundamental aspects of the model remain unchanged. Since this model is applied to virtually all semiconductor-liquid redox processes, experimentally demonstrating its validity and obtaining values of rate constants has remained an important yet elusive goal. An electrochemical minicell was used to investigate the interfacial properties and redox kinetics at nonilluminated regions (2-3 mm) of n-WSez ~rystals in acetonitrile. Interfacial capacitance measurements together with the shape of current potential curves were used to identify regions of n-WSez crystals with properties that approached an ideal semiconductor-solution interface. Exchange current densities for reduction of dimethylferricenium ion at these nearly regions were independent of concentration between 0.05 and 250 mM. This lack of concentration dependence is not consistent with the predictions of the Gerisher model for direct electron transfer between the conduction band of n-WSez and dimethylferricenium ion and suggests that the rate constant for direct electron transfer must be less than lQ-17 cm4jsec. The results can be explained by postulating the existence of a low level of surface states with energies several hundred millivolts positive of the conduction band. Superlattice and Multiple Quantum Well Rotating Ring Disk Electrodes Although recent reports by our group and by others indicate that hot electron processes are observable in PEC's, these results remain highly controversial. The basic difficulty with photoelectrochemical utilization of hot electrons is that thermalization times are expected to be quite rapid (psec) while interfacial electron transfer times are expected to be slower (JlS- ns). Our approach to the study of hot electron photoelectrochemistry has emphasized the identification of stable semiconductor-solution interfaces where the products formed indicate that some fraction of photogenerated electrons cross the interface prior to complete thermalization. The data that we have obtained on p-InP are consistent with enhanced photoreduction rates due to a hot carrier
14 mechanism involving quantization in the space charge region of highly-doped electrodes. This mechanism assumes that photogenerated electrons in the bulk relax to the bottom of the conduction band, therefore, no wavelength dependence is anticipated or observed. In contrast, photoreduction via a mechanism involving reaction directly from higher conduction band states would be expected to yield a wavelength dependence. While this mechanism is unlikely for bulk semiconductors, the possibilities are enhanced for size quantized materials such as superlattices. Our initial studies have utilized GaAs/AlxGat-xAs superlattices grown onto a GaAs buffer layer which have been generously provided by Nozik and coworkers at NREL. These materials have been successfully fabricated into Pt/superlattice rotating ring-disk electrodes. Finding conditions (solvent, redox couples, applied potentials, etc.) where stable photoreduction of both a thermal and a hot electron acceptor occurs has been problematic. We had hoped to use similar redox systems as were used with p-InP, however, stable currents with the superlattices are only obtained with very reducing thermal acceptors which tend to react homogeneously with the hot electron acceptors. Recent attempts to circumvent these problems will be described. Photochemically Controlled Mass Transport Within the DOE Solar Photochemistry Program, numerous studies have attempted to mimic photosynthetic membranes with the goal of separating electron-hole pairs and eventually producing stored electrical energy or chemical free energy. In contrast, relatively few studies have explored the use of photochemistry to separate molecules or ions and, in particular, to pump these chemical species uphill. The ability to selectively and efficiently photopump chemical species uphill could, in principle, be useful in a variety of contexts ranging from environmental restoration to energy storage. The overall goal of this research project is to identify, characterize and understand photochemical systems that not only selectively separate ions or molecules, but also allow these species to be transported against their concentration gradients. We have observed photomodulation of the rates of transport of Cu(II) salts across hydrophobic organic liquid membranes containing indolino spiropyrans, which act as carriers. The spiropyrans can be photoisomerized between a neutral form and a zwitterionic form, which associates with the Cu(ll) salts through ion-pairing. Photopumping of Cu(ll) salts across liquid and polymer membranes containing has been observed using these dyes.
15 DOE Sponsored Publications (1992-94)
1. Carl A. Koval and Robert Torres, "Detection of Hot Electrons at a p-InP /Pt Rotating Ring/Disk Photoelectrode," J. Am· Chern. Soc., (1993) 115, 8368-75.
2. Robert Torres, "Hot Electron Reactions in p-InP Photoelectrochemical Cells," Ph.D. Thesis, University of Colorado, 1993.
3. J.N. Howard and Carl A. Koval, "Kinetics of the Reduction of Dimethylferricenium Ion at Nearly Ideal Regions of n-Tungsten Diselenide Electrodes," Anal. Chern., submitted.
4. J.N. Howard, "Electron Transfer at Semiconducting Metal Dichalcogenide/Liquid Electrolyte Interfaces," Ph.D. Thesis, University of Colorado, 1992.
5. C.A. Koval and J.N. Howard, "Electron Transfer at Semiconductor Electrode I Liquid Electrolyte Interfaces," Chern. Rev., (1992) .2b 411-33.
6. G.N. Brown, J.W. Birks, C.A. Koval," Development and Characterization of a Titanium Dioxide-Based Semiconductor Photoelectrochemical Detector," Anal. Chern., (1992) M, 427-34.
16 Monday Evening
Session IV
Stephen E. Webber, Chairman ASYMMETRIC POLYMERIC PORPHYRIN FILMS FOR SOLAR ENERGY CONVERSION Carl C. Wamser Department of Chemistry, Portland State University Portland, Oregon 97207-0751 Two different methods have been used to create polymeric films of substituted tetraphenylporphyrins on transparent electrodes: a) interfacial polymerization of a pair of reactive monomers into a thin film, later deposited onto an electrode, or b) oxidative electropolymerization of electron-rich porphyrins directly onto an electrode. a) Interfacial polymerization of two different porphyrin monomers
NH2
( T APP in H20 , pH - 3 )
CIOC ------'_ #------== >--< ~ crosslinked polyamide >----<. Q network porphyrin \:) N ~ 0 polymer film TCCPP CCCI (TCCPP in CH 2C12) ©(
For interfacial polymerization, an aqueous solution of either tetra(4-hydroxyphenyl)porphyrin (THPP) at pH 11 or tetra(4-aminophenyl) porphyrin (TAPP) at pH 3 is layered atop a CH2Cl2 solution of tetra(4-chloro carbonylphenyl)porphyrin (TCCPP), creating a thin polyester or polyamide film at the interface. Such films are highly crosslinked, with a distinctive asymmetry of functional groups that creates a gradient of porphyrin redox potentials across the film. 1 Observed photopotentials are directional, with the charge separation following the predicted direction of the redox potential gradient.2 Electron flow moves from the side with excess electron-rich porphyrin (TAPP or THPP) towards the side with excess electron-deficient porphyrin (TCPP). Under simulated solar irradiation, the interfacially-
17 polymerized porphyrin films give photopotentials up to 0.25 V, but low and irreproducible photocurrents.3 Continued work in this area is aimed at~ characterizing the unique structural asymmetry and developing a model of the directional charge transport processes within the film. b) Oxidative electropolym.erization of electron-rich porphyrins
(]) "0 .....0 t5 (]) + 0.6 v VS Ag/AgCI (]) .,.. -c ~ ctS 0.en c -.....ctS
©NH ~lyaniline-like H N;Jf}J- network porphyrin 2 polymer film
In order to improve electronic contact between the porphyrins and the electrode, electropolymerizations have been studied. Electropolymerized films are observed on oxidation of solutions ofTAPP or THPP, consistent with earlier reports. 4•5 Electropolymerized porphyrin films are also being studied as a possible conductive mediator for high-surface-area colloidal Ti02 solar cells. 6•7 We have shown that TCPP can be adsorbed on colloidal Ti02 electrodes and acts as an effective photosensitizer. We have also shown that this system can photosensitize the electropolymerization of TAPP. Further studies are aimed at. developing a continuous, asymmetric polymeric porphyrin film on colloidal Ti02 electrodes. 1) Wamser, C. C., et al. J. Am. Chem. Soc. 1989, 111,8485-8491. 2) Ransdell, R. ~.;Wamser, C. C. J. Phys. Chem. 1992, 96, 10572-10575. 3) Wamser, C. C.; Senthilathipan, V.; Li, W. SPIE Proc. 1991, 1436, 114-124. 4) Bettelheim, A.; White, B. A.; Raybuck, S. A.; Murray, R. W. Inorg. Chem. 1987,26, 1009-1017. 5) White, B. A.; Murray, R. W. J. Electroanal. Chem. 1985, 189, 345-352. 6) Kay, A.; Gratzel, M. J. Phys. Chem. 1993,97,6272-6277. 7) Nazeeruddin, M. K.; Kay, A.; Rodicio, I.; Humphry-Baker, R.; MUller, E.; . Liska, P.; Vlachopoulos, N.; Gratzel, M. J. Am. Chem. Soc. 1993, 115, 6382:-90.
18 DOE-SPONSORED PUBLICATIONS (1992 - 94)
Contact Angle Titrations Detect Surface Functional Group Asymmetry in lnterfacially-Polymerized Films, C. C. Wamser and M. I. Gilbert, Langmuir 1992, .8., 1608-1614.
Substituent, Solvent, and Ionization Effects on the Redox Potentials of Free-Base Tetraphenylporphyrins, R. A. Ransdell and C. C. Wamser, J. Phys Chem. 1992, 96, 10572-10575.
Syntheses of a Series of Electron Donor and Electron Acceptor Derivatives, C. Hoefl~r, N. A. Kizilbash, and C. C. Wamser, Synth. Comm., 1993, 23 (9), 1339-1349.
Synthesis and Characterization of Inierfacially Polymerized Films of Tetraphenylporphyrin Derivatives, W. Li and C. C. Wamser, Langmuir, manuscript in preparation.
Aqueous Complexation Equilibria of Tetra(4-carboxyphenyl)porphyrin with Viologens: Evidence for 1:1 and 1:2 Complexes and Induced Porphyrin Dimerization, S. E. Clarke, C. C. Wamser, H. E. Bell, and M. E. Wong, J. Phys. Chem. , manuscript in preparation.
19 PHOTOCONVERSION PROCESSES IN MOLECULAR SEMICONDUCTOR FILMS
Brian A. Gre~~. Yeong ll Kim and Robert Torres National Renewable Energy Laboratory 1617 Cole Blvd., Golden, Colorado 80401-3393
We are studying the mechanisms of photoconversion processes in molecular semiconductor films, primarily in highly ordered films of a liquid crystal (LC) porphyrin. Although the molecular semiconductors (e.g., porphyrins, perylenes, phthalocyanines) are routinely described in the same theoretical framework as inorganic semiconductors (e.g., Si, GaAs), we find some marked deviations from the behavior expected from this theory. Neither the "n-type" or "p-type" character of the molecular semiconductor, nor the difference in work functions between the semiconductor and the electrodes, seems to determine the photoconversion properties of highly ordered films. Rather, the interfacial exciton dissociation processes appear to control the magnitude and polarity of the observed photoeffects. Thus, some fraction of the excitons photogenerated in the bulk diffuse to the surface where they tend to dissociate asymmetrically, e.g., by preferential injection of an electron into the electrode and a hole into the semiconductor. Subsequent migration of the hole through the bulk results in a photocurrent. This surface-sensitized charge generation and separation process can occur in the absence ·of an internal electric field (band bending) in the ftlms, or even in a direction opposite to that expected from the internal electric field. Junction formation is not required to affect charge separation. In some respects, the bulk of the LC porphyrin may be viewed as "antenna" pigment that functions primarily to absorb light and funnel excitons to the interface where charge separation occurs. The charge separation process is controlled primarily by the kinetics of the interfacial donor-acceptor interactions, rather than by an electric field. Thus, our system somewhat resembles a solid state analog of a molecular ET system with an attached light-harvesting array. The photoconversion processes occurring in these films have some mechanistic similarities to those occurring in natural photosynthetic systems, and are quite distinct from those occurring in inorganic semiconductors. The energetics of the interfacial exciton dissociation and electron transfer (ET) processes are being varied by interposing thin ftlms of materials having discrete electronic energy levels between the molecular semiconductor and the electrodes. Redox polymers and evaporated -films of aromatic anhydrides are examples of the interfacial ftlms employed. Since both the LC porphyrin and the interfacial ftlms have discrete energy levels, the kinetics of the interfacial ET processes are expected to be governed by the same factors that govern ET reactions between solution species and in donor-acceptor molecules. The reorganization energies are expected to be quite low in solid state systems of low dielectric constant, thus substantial changes in ET rates with reaction free energy are anticipated. Ultimately we hope to employ a series of such interfacial films as a tool to map out the rate constants of the interfacial ET reactions as a function of the thermodynamic driving force in these solid state systems. Our results so far are primarily measurements of macroscopic variables such as voltage, current and capacitance. Varying the energy levels of the interfacial films results in variation of the magnitude and polarity of the photo voltage and photocurrent. However, dark current measurements show
20 no evidence for diode-like behavior. Furthermore, capacitance measurements at low frequency and high temperature show that there is essentially no internal electric field in these cells in the dark, i.e., there is no junction formation, yet photovoltages ranging from V oc:::::: - 1.0 V to V oc = + 0.6 V are observed with different interfacial films. An average electric field of up to E = V0 cld::::::- 3.3 x 1Q3 V/cm must be applied before the interfacial charge separation process can be reversed. Thus, charge separation under illumination can proceed spontaneously against the applied internal electric field for 0 > E > - 3.3 x 103 V/cm. The interfacial kinetic effects can thus overwhelm the effects of the internal electric fields. Similarly, the asymmetric exciton dissociation can result in injection of a charge carrier density into the semiconductor that is many orders of magnitude greater than the equilibrium carrier density, thus overwhelming the equilibrium "p-type" or "n-type" characteristics of the materials. For these reasons, some of the fundamental concepts employed in standard semiconductor physics are not of comparable importance in the description of molecular semiconductors.
Photovoltages increase rapidly in these cells at low light intensity (reaching e.g., 0.4 V at 1 JlW/cm2) before saturating at less than solar intensities. Capacitance measurements of illuminated cells, as well as a simple kinetic model, shows that band-bending is induced by illumination and tends to counteract the charge separation process, quite the opposite of what occurs in a typical inorganic semiconductor electrode. We are presently attempting to develop a theoretical model to describe the essential aspects of the photoconversion processes in these materials. We are also experimenting with the dissolution of electrolytes into the LC porphyrin f:tlms as a means of eliminating the photo-induced internal electric fields; this would greatly simplify the theoretical model.
Our results suggest that the usual theoretical description of photoconversion processes in molecular semiconductors, using concepts developed for inorganic semiconductors, may be unwarranted in a number of cases. However, a complete theoretical description of photoconversion processes based on the surface-sensitized charge generation mechanism is not yet available. These issues will be discussed as well as a number of results pertaining to the quenching of excitons in the LC porphyrin by different surfaces, the effects of the structure and orientation of the LC porphyrin fllms, photoeffects in perylene films, charge carrier mobilities and trapping effects.
Publications 1993- 1994
Gregg, B. A.; Nozik, A. J. "Existence of a Light Intensity Threshold for Photoconversion Processes," J. Phys. Chern., 1993, 97, 13441-13443.
Gregg, B. A.; Kim, Y.l. "Redox Polymer Contacts to Molecular Semiconductor Films: Toward Kinetic Control of Interfacial Exciton Dissociation and Electron Transfer Processes," J. Phys. Chern. 1994, 98, 2412-2417.
Gregg, B. A. "Effects of Long Range Order, Temperature and Phase on the Photoconversion Properties of Liquid Crystal Porphyrin Films," in press, Mol. Cryst. Liq. Cryst. 1994.
21 Tuesday Morning
Session V
StephenS. Isied, Chairman · DIRECTIONAL ELECTRON TRANSFER THROUGH MACROMOLECULAR ARRAYS
Mazye Anne Fox, Pei-Wei Wang, and Diana M. Watkins Department of Chemistry and Biochemistry, University ofTexas, Austin, TX 78712
Macroscopic photoelectrochemical charge separation requires carefully designed materials with controlled placement of light absorbers, relays, and multielectron catalysts at defmed sites from a fixed semiconductor or metal surface. We are exploring the use of synthetic polymers as efficient light collection devices. that can provide attractive substrates for defming the fundamental factors that govern the utility of long distance electron transfer and energy migration. We have synthesized integrated, macromolecular chemical systems as soluble arrays, as solid thin films, and as adsorbates on solid electrodes. We have also synthesized and physically characterized semiconductor containing composites capable of controlling interfacial electron transfer. Electron and energy transport in these materials is being evaluated by photophysical and electrochemical techniques. This work has several goals: a) the synthesis of new materials for directional electron transfer; b) the preparation and characterization of anisotropic composites containing organic and inorganic components; c) the elaboration of mechanisms of photoconductivity and electrocatalysis; and d) the development of new methods for surface modification of metals and semiconductors. We have recently explored new synthetic routes for the spatial definition of these catalytic arrays on the nanometer to micrometer scale. The resulting chemically modified surfaces are envisioned as important vehicles for conversion of incident photons to chemical or electrical energy. Many of the most exciting applications of photoinduced charge transfer involve multicomponent systems with a large number of discrete absorbing (light harvesting) units coupled with an extended array through which electrons or energy can migrate over macroscopically observable distances, e.g., through polymeric thin films. In these polymers, as in other artificial photosynthetic units, several key issues influence the efficiency of photodriven processes: (1) energetics of the primary electron transfer step; 2) unidirectionality of the forward electron transfer and the occurrence of a single pathway; 3) possible intervention of superexchange and/or sequential hops as a mechanism for electron migration; 4) the source of the high observed quantum efficiencies; and 5) the temperature dependence of the charge separation. Structural variation of the component polymers has been undertaken as a means to probe the effect of these variable on long distance charge separation. Chemical modification of an electrode surface by attachment of such supramolecular or macromolecular arrays to manipulate surface states is then undertaken in order to control the kinetics of interfacial electron transfer and to inhibit surface corrosion.
23 Synthetic Routes to New Multiblock Copolymers: Several routes to di- tri- and rerzrporZ}f(Z)gti::~7::~~==~:··IDg polymers of controlled molecular weight and rigidity. These include anionic polymerization, group transfer polymerization, ring-opening metathesis polymerization, solid state polymerization of functionalized a-amino acids, and metal-induced polymerization of functionalized isocyanides. Each of these techniques provide living polymers in which composition is controlled by the relative kinetics of initiation and propagation. Attainable polydispersities vary with the identity of the initiating catalyst, but narrow molecular weight distributions (PD < 1.2) can be routinely obtained with optimized synthetic routes. Backbone rigidity is directly affected by the structure of the component monomer, and photophysical measurements in polymers heavily loaded with organic arenes can be used to defme the distance for nearest neighbor approach along a single rigid chain. Light Harvesting in Rigid Chromophore-Appended Polymers: Light- harvesting polynorbornene homopolymers and block copolymers bearing appended naphthalene, phenanthrene, and anthracene units, with and without attached quenchers, exhibit steady-state fluorescence spectra indicative of isolated absorbing units. The lack of excimer emission in any of these monodisperse homopolymers is indicative of a rigid polymer chain. Enhanced intramolecular fluorescence quenching of the naphthalene. emission in the naphthalene-dimethylaniline diblock copolymer and phenanthrene emission in the phenanthrene-dimethylaniline diblock copolymer takes place through vectorial energy migration to the interface. A statistical mechanical analysis of energy hopping to the interface (where quenching takes place) suggests that optimal design of block length for energy migration over long distances depends on excited state lifetime, driving force for electron transfer quenching, and polymer rigidity. Semiconductive Organometallic Coordination Polymers: Intermetallic electronic coupling in 1,2,4,5-tetrakis(dimethylphosphino)benzene bridged bimetallic Ni, Pd, and Pt complexes has been studied by electrochemical and spectroelectrochemical methods. These bimetallic complexes show minimal cross-ring metal-metal interactions. The observed oxidation potential for the monometallic Ni II complex is similar to that of its bimolecular analog, but substantial shifts from the monometallic complexes are observed for the Pdii and Ptll complexes. The very low conductivities observed in the metal-phosphine coordination polymers (M = Nill, Pdll, and Ptll) and their partially oxidized (p-doped) analogs are consistent with the minimal coupling observed in the bimetallic complexes. Photoinduced charge transfer between Os and Pt centers in monomeric model complexes shows enhanced interaction along the excited state surface compared with the ground state. Future Directions: Work now underway seeks to employ these synthetic polymers in the construction of integrated systems on metal and semiconductor surfaces, with emphasis on photophysical principles controlling vectorial energy and electron migration over distances that are large on a molecular scale. We also hope to expand our synthetic goals for spatial defmition of these catalytic arrays to the nanometer to micrometer scale. Chemically modified surfaces are envisioned as important vehicles for conversion of incident photons to chemical or electrical energy.
24 Scientific Publications Acknowledging DOE Support (1992-94)
"Predicting the Effect of Oxidative Doping on the Conductivity of Metal-Tetrakis (dimethylphosphino )benzene Coordination Polymers from the Electrochemical Properties of Their Related Bimetallic Complexes," Pei-wei Wang and Marye Anne Fox, Jnorg. Chim. Acta 1994, submitted. "Rigid, Well-defined Block Copolymers for Efficient Light Harvesting," Diana M. Watkins and Marye Anne Fox, J. Am. Chem. Soc. 1994, submitted. "Metal-Metal Interactions in Tetrakis(diphenylphosphino)benzene-bridged Bimetallic Complexes and Their Related Coordination Polymers," Pei-Wei Wang and Marye Anne Fox, I norg. Chem. 1994, submitted.
"Light Harvesting Polymers," Diana M. Watkins and Marye Anne Fox, Adv. Chem. Ser. 1994, in press. "Interblock Electron Transfer in 2-Vinylnaphthalene-N,N-Dimethaminomethylstyrene Block Copolymers," Jun Lin and Marye Anne Fox, Macromol. 1994,27, 902. "A Picosecond Study of the Photophysics of CdS Clusters Grown in Situ in Reversed Micelles," A.V. Barzykin and Marye Anne Fox, lsr. J. Chem. 1993, 33, 21. "Graded Spatial Distribution in Conducting Co-polymers of Pyrrole and Thiophene," Walter Torres and Marye Anne Fox, Chem. Mater. 1992, 4, 146.
"Covalently-linked Cyclooctatetraenyl Dianions as Sensitizers in Photoelectrochemical Cells," Marye Anne Fox, Kay A. Campbell, John R. Hurst, Rene Maldonado, Luis Echegoyen, and Robert L. Soulen, J. Org. Chem. 1992, 57, 3728. "Influence of Sterle Bulk on the Relative Stability and Interchangeability of the Various Oxidation States for Nickel Complexed with 1,2-bis(d.ialkylphosphino) benzene," Marye Anne Fox, Daniel A. Chandler, and Evan P. Kyba, J. Coord. Chem. 1992, 25, 1. "Photosensitization of Semiconductor Electrodes by Adsorbed Polymeric Thin Films,"Marye Anne Fox and Phillip F. Britt, New J. Chem. 1992, 16, 113. "Polymeric and Supramolecular Arrays for Directional Energy and Electron Transport over Macroscopic Distances," Marye Anne Fox, Accts. Chem. Res.1992, 25, 569. · "Electronic Excitation Transport and Trapping in Micellar Systems: Monte Carlo Simulations and Density Expansion Approximation," A.V. Barzykin, N.S. Barzykina, and M.A. Fox, Chem. Phys.1992, 163, 1. "Electrosynthesis of Polypyrrole in a Nematic Liquid Crystal," Walter Torres and Marye Anne Fox, Chem. Mater. 1992, 4, 583.
25 PHOTOINDUCED CHARGE-SHIFf REACTIONS OF DONOR-SUBSTITUTED ACRIDINIUM IONS: EFFECTS OF ELECTRONIC COUPLING, MEDIUM, AND POL YMERIPEPTIDE BINDING
Guilford Jones, II
Department of Chemistry, Boston University Boston, Massachusetts 02215
New systems for study of intramolecular electron transfer have been prepared based on the 9-arylacridinium ion. This chromophore displays convenient bands for laser excitation at 355 and 420 nm. When the acridinium ring is substituted with electron donor groups in the 9-position (e.g., structures I-X), efficient photoinduced shift of charge can be demonstrated in flourescence and in flash photolysis studies. The acridinium systems produce reactive intermediates with a wide range of lifetimes, suitable for the further entrainment of charge within a matrix, or the promotion of secondary redox reactions. The acridinium chromophores have "rod-like" arrangements of linked aryl rings that can be "tuned" to control the rate of charge shift and charge return processes. Electron donor and acceptor groups are hindered from planarity so that electronic coupling depends on the geometric variable of twist angle for the single bonds that link aromatic rings. An efficient general method of synthesis of the 9- arylacridiniums has been developed, based on addition of organolithium reagents to 10-methylacridone.
The charge-shift fluorescence for the donor-substituted acridiniums is characterized by a red-shifted band (560-680 nm) that is sensitive to solvent polarity, (a solvent series ranging from chloroform--> water may be generally used). The balance of quantum yield of emission from the charge-shift state vs flourescence from the local acridinium chromophore is strongly dependent on substitution at the 9-position. Ultra-fast transient absorption measurements have been carried out at the Argonne National Laboratory through a collaboration with the group of Dr. M. R. Wasielewski. Charge shift states have been identified as radical pair species that appear and decay in the 100 fs - 10 ns time domain. For the series, the kinetics for forward and return charge-shift processes depend on length and geometry (planarity, overlap) of aniline or carbazole donors (1-V). Other variations in design include incorporation of substituted phenol (IX, X), thiophene (VIII) and thianthrene (VI-VII) donors, moieties that provide a range of internal reorganization energies and driving forces for electron transfer to the acridinium chromophore.
26 The acridiniums bind through hydrophobic and electrostatic interaction to electrolyte (bio)polymers [e.g., poly(methacrylic acid) or poly (L glutamic acid)]. Flourescence and phototransient data provide valuable signatures of binding within the domains of polymers, peptides, and proteins, and a measure of microenvironmental control on electron transfer kinetics. The expectation is that systematic adjustment of the length and electronic coupling for these linkages will provide a means for demonstrating charge separation with highly variable lifetime for electron transfers over relatively short distances (5-10 A).
~J QP ceo ~·~ C~ I I ,. c&CH3 CH"· 3 I n Ill IV v
VI VII VIII IX X
27 Publications (1992-94)
1. G. Jones, II and B. Huang, "Photoactive Peptides: Development of a Redox Catalytic Triad for Sulfide Oxidation Based on Poly(L)-histidine," J. Phys. Chern., 96, 9603 (1992).
2. G. Jones, II and M.S. Farahat, "Photoinduced Electron Transfer in Flexible Biaryl Donor-acceptor Molecules," in "Advances in Electron Transfer Chemistry," vol 3, P. S. Mariano, Ed., JAI Press, Inc., Greenwich, Connecticut, 1993.
3. G. Jones, II and B. Huang, "Photoinduced Electron Transfer for High Potential Quinone Sensitizers and Thianthrenes. The Rate of Trapping of Thianthrene Radical-cation by Water," Tetrahedron Lett., 34, 269 (1993).
4. G. Jones, II, B. Huang, and S. F. Griffin, "Electron Transfer Photochemistry of Thianthrene. Nucleophile-Assisted Photooxidation to Sulfoxide," J. Org. Chern., 58, 2035 (1993).
5. G. Jones, II and C. Oh, "Photophysical and Electron Transfer Properties of Pseudoisocyanine in the Hydrophobic Domain of an Aqueous Polyelectrolyte," J. Phys. Chern., 98, 2367 (1994).
6. G. Jones, II and C. W. Farahat, "Photoinduced Electron and Hole Transfer Involving Eosin Conjugates of Tryptophan Derivatives," Reviews of Chemical Intermediates, in press.
7. G. Jones, II, M. S. Farahat, S. R. Greenfield, D. J. Gosztola, and M. R. Wasielewski, "Tuning of the Photoinduced Charge-shift Reaction in Donor-acceptor 9-Arylacridinium Ions Having Twisted Geometries, J. Am. Chern. Soc., submitted.
8. G. Jones, II, C. W. Farahat and C. Oh, "Photoinduced Electron Transfer Involving Eosin-Tryptophan Conjugates. Long-lived Radical Pair States for Systems Incorporating Aromatic Amino Side Chains," J. Phys. Chern., submitted.
9. G. Jones, II and Z. Feng, "Photoinduced Electron Transfer for an Eosin Tyrosine Conjugate in a Polyvinyl-2-pyrrolidinone Matrix. Activity of the Tyros ina te Anion in Long Distance Electron Transfer," in preparation.
10. G. Jones, II and C. Oh, "Styryl-7 Bound to Poly(glutamic acid) in Aqueous Solution. Cooperative Binding, Induced Circular Dichroism and Superhelical Dye Aggregation," in preparation.
28 THEORETICAL STUDIES OF ELECTRON TRANSFER AND OPTICAL SPECTROSCOPY IN COMPLEX MOLECULES
Richard A. Friesner Department of Chemistry Columbia University New York, NY 10027
We have been developing theoretical models and methods to study important dynamical processes such as electron transfer, optical excitation, and relaxation in complex molecular systems. At present we are studying two such systems in detail; optical spectroscopy and electron dynamics in small semiconductor particles of well characterized size, and electron transfer between donors and acceptors mediated by DNA. Each of these systems has interesting qualitative questions about the basic physics as well as the challenge of handling the complexity of the macromolecular structure. New results from both projects will be described below.
Semiconductor Clusters: Using a simple one electron empirical pseudopotential model (EPM), we have successfully modeled the bandgap in small semiconductor particles as a function of cluster size for CdS clusters, the system for which the best experimental data exists; agreement with experiment is at the level of 0.1 ev with no adjustable parameters. Predictions have been made for GaP, GaAs, and Si particles prior to experiment Recent results from several groups for Si are in good agreement with our theoretical predictions, although the experimental uncertainty is as yet too great to make a quantitative evaluation at the same precision level as for CdS.
Over the past year, reliable experiments for CdSe have been carried out in the groups of Alivisatos at Berkeley and Bawendi at MIT. We have computed CdSe spectra and discovered that for this material, the energy as a function of direction ink-space m~st be taken into account to determine the bandgap. The CdS particles have a zinc blende crystal structure and for this structure the use of an isotropic approximation for the direction of the valence and conduction orbitals is acceptable. However, CdSe has a wurtzite structure and the approximation breaks down in this case. Calculations which properly incorporate anisotropy yield agreement with experiment in the 0.1 ev range, again without major parameter adjustments. They also make an interesting prediction that there are levels of particular symmetry with very low oscillator strengths below the bandgap, i.e. that in some sense CdSe is an "indirect gap" material. This may contribute to an explanation of some of the luminescence properties, although participation of surface states, hypothesized by other workers as important, is likely to still play a role and the actual situation probably involves an interaction of both types of states.
We are presently calculating the absorption spectrum of both CdSe and Si, and developing the machinery to include multiple electronic transitions, phonons, and inhomogeneous broadening as contributors to the lineshape. For CdSe there are a large number of sophisticated chemical physics experiments (Stark measurements, holeburning, ultrafast pump-probe studies) which can be modeled by such calculations. These comparisons will be essential in validating the more complicated models beyond the EPM (e.g. force fields) necessary to understand the dynamical behavior of electrons in semiconductor particles. Once this validation process is completed, we will move on to understanding the excited electron dynamics; this will require modeling of surface states, for which we will carry out quantum chemical calculations.
Electron Transfer Through DNA: Several years ago, Turro, Barton, and coworkers reponed the apparent observation of an extraordinarily weak distance
29 dependence of electron transfer for donor and acceptor metal complexes intercalated into DNA. While were, and remain, questions about the validity of the experiments and the subsequent analysis of the data, the question of whether a strongly coupled intermediate bridge like the DNA bases is capable of yielding abnormally weak distance dependence (an exponential falloff of0.2A-1 as compared with the usual range of0.6-1.5 A-1 observed in a wide range of other systems) is an interesting question. The delocalization of electronic states in the bridge may render conventional Marcus theory invalid. Consquently, we have developed a quantum mechanical method capable of obtaining accurate results for strongly coupled multistate systems.
The method is based upon Redfield relaxation theory. The electronic states and a single reaction coordinate are treated exactly via matrix diagonalization; the remaining degrees of freedom are treated perturbatively, essentially as a harmonic bath. We have developed novel numerical methods which reduce the computational scaling for large Redfield calculations from a naive Nl2 (where N is the number of quantum levels in the system) to N3. This permits treatment of systems with hundreds or even thousands of qunatum levels. An efficient timestepping algorithms allows us to carry out the simulations to long timescales in reasonable CPU times. Finally, we have developed methods utilizing canonical transformations which allow recovery of the correct quantum dynamical results in the adiabatic and diabatic limits; this permits construction of an accurate perturbation theory from an initial microscopic Hamiltonian.
Our initial calculation have focused on a simple Ruckel model of the DNA electronic states, investigating the rate as a function of the donor-acceptor distance for various parameter regimes. We fmd that when the bridge is delocalized there is a new mechanism for electron transfer in addition to the usual Golden Rule process. In this mechanism, an electron is thermally excited to the bridge, dephases, and then can undergo wavelike transport which is distance-independent. The process requires sufficiently high temperature to activate an electron into the bridge orbitals; the importance of the effect thus critically depends upon the size of the energy gap between donor and bridge.
Based on this analysis, we predict that at low temperature the long range transfer in the Turro-Barton system should be shut off. Preliminary experiments in the Turro group confirm this prediction. This does not prove that the mechanism is operative or that the experiments are correct, only that the results are consistent with the picture we have hypothhesized. More extensive experiments will be required to actually verify the mechanism.
30 Publications Supported by DOE Grant DE-FG02-90ER14162
1. M. V. Ramakrishna and R. A. Friesner, Prediction of Anomalous Red Shift iri
Semiconductor Clusters", J. Chern. Phys. 96, 873 (1992)
2. J. M. Jean, G. Fleming, and R. A. Friesner," Application of a Multilevel Redfield Theory to Electron Transfer in Condensed Phases", J. Chern. Phys. 96, 5827 (1992)
3. T. Pollard and R. Friesner, " Solutions of the Redfield-Liouville-von Neumann
equations for the Interactions of a Quantum Mechanical System with a Bath", J.
Chern. Phys., in press.
4. M. V. Ramakrishna and R. A. Friesner," Quantum Chemistry of Semiconductor ' Clusters", Isr. J. Chern. 33, 3 (1993).
5. F. Webster, E. Wang, R. Friesner, and P. Rossky," Stationary Phase Surface
Hopping for Nonadiabatic Dynamics; Two State Systems", J. Chern. Phys., in
. press.
31 INTRAMOLECULAR ELECTRON TRANSFERS IN LINKED DONOR-ACCEPTOR-CHROMOPHORE ASSEMBLIES
C. Michael Elliott, Suzanne Ferrere, Steven L. Larson, Daniel L. Derr and D.F. Kelley
Department of Chemistry· Colorado State University Ft. Collins, CO 80523 The general focus of much of our recent work has been to probe the structure-function relationships that exist in intramolecular electron transfer processes. Two general types of molecular assemblies will be discussed. In the first, the light harvesting chromophore is a tris bipyridineruthenium moiety which is linked via polymethylene chains to organic donors and/or acceptors. While the various linkages in these systems are flexible and, thus, the electron trans fer distances are not well defined, they have the advantage of being synthetically very accessible and, therefore, easily modifiable. The second type of system to be discussed consists of a series of triply-bridged heterodinuclear trisbipyridinemetal complexes (vide infra). In these systems one metal center acts as the donor (of either energy or an electron) and the other serves as an acceptor. Unlike the first systems above, the distance and orientation in these complexes is well defined.
Charge Separation in Flexibly-Linked Donor/Acceptor/Chromophore Assemblies. The figure below gives a representative example of the type of systems under study. The three arrows point to three different polymethylene chain sights where synthetic modifications are possible.
N 1~. I vi 0 i .{J M &· The chain labeled P determines the fully-extended distance between the chromophore and the phenothiazine (PTZ) electron donor. The chain labeled M, likewise, determines the fully ex tended distance for the "diquat" (DQ 2+) 1Iectron acceptor. Finally, the chain labeled N deter mines the reduction potential of the DQ + acceP.tor and, thus, the driving force for processes involving electron transfers to and from the DQ2+.
In addition to the above D-C-A systems we have prepared and studied analogous C-A and C-D for comparison. Through numerous collaborative efforts we have examined the elec tron transfer kinetic of various processes following the photoexcitation of the ruthenium chro-
32 mophore in these systems. While space does not allow for discussion here, in solvents of low polarity, the above class of D-C-A assemblies form, with generally good quantum yield, a long lived (>I 00 ns) charge separated state (CSS), D+ -C-A-. The influence of N, M. and P on the relative quantum yield for CSS formation will be the subject of discussion.
Triply-Bridged Trisbipyridine Metal Complexes and the Question of Sigma Framework Super exchange. We have developed a general synthetic procedure for the preparation of Q,w-bisbi pyridinealkane ligands containing two or more methylenes in the alkane chain. Under appropri ate conditions these ligands can be used to prepare hetero- and homo-dinuclear trisbipyridine metal complexes that are triply linked by the alkyl chains connecting each pair of bipyridines. The figure below shows the superposition of the crystal structures for the homodinuclear diiron complexes of ethyl- and propyl-linked bipyridines.
Curiously, within experimental error the metal-metal distances are the same in each complex. Furthermore, the relative orientations of the bipyridines are very nearly the same, as well. Molecular mechanics studies, (Biograph) indicate that these structures should be similar to what is expected in solution and that they are only weakly dependent on the metals coordinated (for Ru, Fe and Co). The only significant difference between the two structures is the nature of the three alkyl bridges connecting the two trisbipyridine metal complexes. Consequently, this series of dinuclear complexes present an especially good opportunity to experimentally probe the importance of superexchange through the polymethylene linkages in the processes of electron and energy transfer.
To date we have conducted a number of preliminary experiments on several complexes in this series. The most definitive results so far have come from spectroscopic studies of the mixed valent Fe(II)/Fe(III) complexes. Both complexes are extremely weakly coupled, but each exhibits a very weak intervalence charge transfer band in the near infrare~. The extinction coefficient of the IVCT band for ythyl-pridged complex is ca. 2-3 1 mol- cm-1 while that for the propyl-bridged is ca. 7 I mol- em- . These values would argue, then, that the propyl bridged complex is nominally more strongly coupled of the two.
PUBLICATIONS ACKNOWLEDGING DOE-BES SUPPORT
Elliott, C. M., Larson, S. L., and Kelley, D. F., "Electron Transfers in Linked Donor/Acceptor/Chromophore Systems: Linkage and Driving Force Dependence on Charge Separated State Formation", J. Am. Chern. Soc., (submitted).
Elliott, C. M. and Ferrere, S., "Electrochemical and Spectroscopic Studies of Triply-Linked Dinuclear Trisbipyridineiron Complexes", Inorg. Chern. (in preparation).
33 ENERGY CONVERSION BASED ON MOLECULAR EXCITED STATES. REDOX SPLITTING IN SOLUBLE POLYMERS.
Thomas J. Meyer
Department of Chemistry University ofNorth Carolina CB# 3290, Venable Hall Chapel Hill, NC 27599-3290
Our DOE supported work has focused on the rational design ofpolypyridyl complexes of ruthenium(II), osmium(II), and rhenium(!) that exhibit interesting photochemical, photophysical, and redox properties. We have gained a large degree of control over both the ground and excited state properties of such complexes and have established protocols for the elucidation of excited state dynamics utilizing time resolved resonance raman and transient infrared techniques. Our exploration of molecular design has moved along three fronts. Firstly, we have developed a synthetic strategy for the preparation of heteroleptic complexes based on ruthenium(Il) that contain three different bidentate ligands. Secondly, we are utilizing this synthetic procedure to prepare complexes with broad band absorbance throughout the UV and visible regions of the spectrum, so called "black MLCT absorbers." Finally, the controlled immobilization of the d6 chromophores on soluble styrenic polymers is under investigation in an attempt to study long range electron and energy transfer.
Tris-heteroleptic compounds. The bis-chloro bridged oligomer [Ru(C0)2Cl21n can be prepared from RuC13·xH20 in refluxing formic acid by utilizing formaldehyde as reductant. One equivalent of a derivatized bipyridylligand can be inserted into the oligomer to form [Ru(bpya)(C0)2Cl2]. Substitution of the chloride with triflate followed by addition of a second, bipyridylligand forms [Ru(bpya)(bpyb)(C0)2J2+. Trimethylamine N-oxide oxidatively decarbonylates the dicarbonyls in the presence of a third ligand to produce either [Ru(bpya)(bpyb)(bpyc)1 2 + or [Ru(bpya)(bpyb)(XX)]2 + where XX represents two monodentate anions (e.g. Cl·) or one bidentate anion such as dithiocarbamate anion (Et2dtc·). We have been able to prepare a wide variety of very pure complexes of all three kinds in reasonable yields. Black Absorbers. An example of our efforts to design useful ground state properties with rutheniurn(II) polypyridyls is the preparation of complexes that have appreciable visible light absorption throughout the near UV and visible regions of the spectrum. Since separate MLCT transitions occur to each of the ligands in mixed chelates, it is possible to choose ligands so that the MLCT bands associated with each ligand overlap to form a nearly continuous absorption plateau from the UV into the low energy visible. Delocalized ligands with low lying acceptor orbitals such as dipyridobenzoquinoxaline (dpb) used in conjunction with strong sigma donating ligands such as Et2dtc· have helped to stretch the composite absorption farther into the low energy region of the spectrum so that significant absorption (e > 3000) is extended well past 700 nm in [Ru(bpy(C00Et)2} (dpb)(Et2dtc)]2+ where bpy(C00Et)2 is 4,4'-bis(carboxyethyl)bipyridine. Members of this class are exceptionally photoinert, near-IR emitters having reasonable lifetimes (30-100 ns). The lifetimes are remarkably long given their energy gaps and we are currently investigating the role that delocalization of the excited electron plays in extending lifetimes. Soluble polymers. Our investigations with soluble polymers started with the derivatization of styrene-chloromethylstyrene copolymers by nucleophilic displacement of chloride under basic conditions. A series of mixed polymers containing osmium and ruthenium were prepared for the purpose of understanding long-range photoinduced electron and energy transfer. Laser flash photolysis ofPS-[Rull22Qsll5](PF6)54 in which of the -:-27 available sites, 22 were
34 occupied by [Ru(bpy)2(bpyCH20-)](PF6)2 and 5 by [0s(bpy)2(bpyCH20-))(PF6)2, leads to rapid energy transfer (t < 5 ns) from Rull*to Osll. The remaining Run* emission was relatively unperturbed 11 11 compared to [PS-Ruii27](PF6)54 • From these observations, it was concluded that Ru * -~ Os energy transfer does occur but only to Os 11 sites that are adjacent to Ru 11 *. In the presence of an irreversible, oxidative diazonium quencher, 95% of the emission was rapidly quenched and subsequent electron transfer from Osll to Ru III was observed. An analysis of the data revealed that the rate of electron transfer is limited by site to site Ru II ~ Ru III self exchange. In order to increase synthetic efficiency, we have turned our efforts to amide links between the polymer and attached chromophores and quenchers. The styrene-p-chloromethylstyrene polymer was converted first into its amino analog and then derivatized by reaction with various chromophores and/or quenchers bearing a carboxylic acid functionality in the presence of a coupling reagent (below). 1) K+pht- .. R-a>OH 2) NHzNHz BOP/HOPT/ NMM
pht = phthalimide 1Ao,o BOP= benzotriazo-1-yloxytris (dirnethylamino )phosphonium hellafluorophosphate HOPT = 1-hydroxybenzotriazole hydrate NMM = N-methylrnorpholine Ac2o =acetic anhydride R-COOH = [Ru(bpy)z(bpy-COOH)l (Pr6>z,
/012 ~,etc. NH\. a>OH C=O HC/ 3 A series of samples containing different loadings of ruthenium and osmium have been prepared and characterized spectroscopically and photophysically. It was found that efficient intramolecular energy transfer occurs from Ru11 to Osll following laser flash excitation ofPS NHCO-[Rull4 0sll4 Me8](PF6 )16 where the unmetallated amine sites are coupled to CH3C02H (Me). We are currently preparing some mixed systems which will be of interest for the study oflong-range electron transfer. · Vibrational Spectroscopy. We have developed protocols for the use of both transient raman and IR spectroscopy in the study ofMLCT excited states. In mixed chelates, time-resolved resonance raman measurements have allowed us to identitify the ligand on which the excited electron is localized. Additionally, selective use of resonance enhancement has allowed us to probe individual units in extended assemblies of chromophores. Transient IR measurements, made in collaboration with the groups of Dyer and Woodruff at Los Alamos National Laboratories, have proven to be particularly useful with complexes ofrhenium(l) that contain carbonyl groups and to ruthenium complexes that contain cyano ligands. We have been able to observe transient shifts of CO stretchingfrequencies in the excited state 1Re 11 (bpy-)(py)(C0lai+ of50-80 cm-1. In llphen' 1 11 (C0)3Re(NC)Ru(CN)(bpy)2]+, Re ~ phen excitation gives the Re (phen-) excited state on the -1 ps timescale. This state decays on the 5-10 ps timescale to give a Ru 111 Cbpy·).based excited state due to energy transfer a·cross the cyano bridge.
35 Publications (1992-1994).
MLCT-dd Energy Gap in Pyridyl-Pyrimidine and Bis(pyridine) Complexes ofRuthenium(ll). D.P. Rillema, C.B. Blanton, R.J. Shaver, D.C. Jackman, M. Boldaji, S. Bundy, L.A. Worland T.J. Meyer. Inorg. Chem. 311600-1606 (1992).
Synthesis of Polypyridyl Complexes of Ruthenium (II) Containing Three Different Bidentate Ligands. G.F. Strouse, P.A Anderson, J.F. Schoonover and T.J. Meyer, Inorg. Chern. 31 3004-3006 (1992).
Long-Range Energy Transfer in Oligomeric Metal Complexes Assemblies. C.A Bignozzi, R. Argazzi, C.G. Garcia, F. Scandola, J.R. Schoonover and T.J. Meyer, J. Am. Chem. Soc. 114, 8727-8729(1992).
Energy Relationships in Optical and Thermal Electron Transfer. Temperature Dependence of lntervalence Transfer Absorption Band. J.T. Hupp, E.M. Kober and T.J. Meyer, J.Phys. Chem.96, 10820-10830( 1992).
Photophysical and Photochemical Behavior ofNitro Complexes ofRuthenium(II). C.A Bignozzi, C. Chiorboli, Z. Murtaza, W.E. Jones, T.J. Meyer, lnorg. Chern. 32, 1036-1038 (1993). lntrastrand Electron and Energy Transfer between Polypyridyl Complexes on a Soluble Polymer. W.E. Jones, S.M. Baxter, G.F. Strouse and T.J. Meyer, J. Am. Chern. Soc. 115, 7363-7373 ( 1993).
Application of Transient Infrared Spectroscopy to Intramolecular Energy Transfer in [(phen)(C0)3Rei(NC)Ruii(CN)(bpy)2]+. J.R. Schoonover, K.C. Gordon, R. Argazzi, W.H. Woodruff. KA Peterson, C.A. Bignozzi, R.B. Dyer, T.J. Meyer, J. Am. Chern. Soc. 115, 10996-10997 ll993!.
Application of Transient Resonance Raman Spectroscopy to the Structure of a Photoinduced Electron Transfer Intermediate. J.R. Schoonover, P. Chen, W.D. Bates, R.B. Dyer, T.J. Meyer, Inorg. Chem. 33, 793-797 (1994).
Submitted or in press.
Vibrational and Electronic Spectroscopy of Electronically Excited Polychromophoric Ruthenium (II) Complexes. C.A Bignozzi, R. Argazzi, C. Chioboli, F. Scandala, R.B. Dyer, J.R. Schoonover, T.J. Meyer. Submitted to lnorg. Chern., May 10, 1993.
The Influence of Electronic Delocalization on Metal-to-Ligand Charge Transfer Excited States. G.F. Strouse, J. Schoonover, R. Duesing, S. Boyde, W.E. Jones, T.J. Meyer. Submitted to J. Am. Chern. Soc., November 19, 1993.
Synthesis and Photophysical Properties ofMono-(2,2':6',2"-Terpyridine) Complexes of Ruthenium(II). B.J. Coe, D.W. Thompson, C.T. Culberston, J.R. Schoonover, T.J. Meyer. Submitted to Inorg. Chern., March 10, 1994.
Black MLCT Absorbers. P.A. Anderson, G.F. Strouse, J.A. Treadway, T.J. Meyer, F.R. Keene. Submitted to J. Am. Chem. Soc., April 3, 1994.
36 STRATEGIES FOR EFFICIENT PRODUCTION AND ACCESS TO PHOTOCHEMICALLY GENERATED REDOX SPECIES IN ZEOLITES
Michael B. Ledney and Prabir K Dutta, The Ohio State University, Department of Chemistry, 120 West 18th Avenue, Columbus, Ohio 43210
The possibility of using sunlight to convert readily available chemicals to fuels is very attractive. For example, the splitting of H20 to H2 and 0 2 can occur via the scheme sunlight • D ------> D n· +A -----> n• + A· 4D+ + 40H· ---- > 0 2 + 4D + 2H20 2A- + 2H+ ----> ~2 + 2A However, there are several problems that need to be circumvented in order to make this strategy work. First, the back electron transfer between n• and A· needs to be prevented and secondly, n• and A· have to be accessible for the H20 splitting reaction.
Over the past several years, we have been investigating the possibility of using zeolite cages as a partitioning medium for D and A and thereby enhancing photochemical charge separation. Our choice of D and A has been Ru(bpy~+ and viologens, respectively. Their redox forms are suitable for H20 splitting, and they satisfy the special architectural requirements for assembly within the zeolitic System.
Zeolite Y consists of 13 A supercages, separated by 7 A windows. Thus, Ru(bpy~+, a molecule of -12 A dimension, once synthesized within the cages is trapped, but can communicate with molecules in neighboring supercages through the 7 A windows. We assembled a system with 1 molecule of Ru(bpy~+ per 15 supercages, with the remaining supercages containing 2 molecules of ~· per supercage. Quenching of Ru(bpy~+· by~· was readily accomplished through the 7 A windows, followed by a back electron transfer (half-life= 15 J.ISec). Prolonged visible photolysis led to slow formation of MV .., which was stable for period of hours. An electron hopping mechanism was proposed to explain these observations.
In order to further improve the efficiency of charge separation, a ternary system was assembled with the strategy of promoting directional electron transfer from the z.eolite to a neutral viologen in solution (PVS), as shown below. In this case, the Ru(bpy)j+ inside the zeolite is surrounded by N,N'-tetramethylene-2,2'-bipyridinium 2 (DQ •), prepared via ion-exchange. The neutral propyl viologen sulfonate (PVS) cannot replace the D(i+ ions and remains in solution.
37 Zeafote
·e· n.N-: o-o 1 PVS . E"-~.65v ~~ Ru(BPY},.. ,'--~ ...?
This led to considerable improvement in yields, and the photogenerated electron resided on the PVS in solution, thus resulting in easy access. We have confirmed that the electron transfer is primarily interfacial, and that the Ru(bpy)j+ in the interior of the zeolite does not contribute to the photochemical yield. This necessitated investigation into synthesis of smaller zeolite crystals, where most of the Ru(bpy); would be at the surface. Using a sucrose medium to nucleate zeolites, we have been able to synthesize 300 A zeolite particles. Preliminary studies indicate that there is considerable improvement in photochemical yields. This system is presently being characterized.
In order to complete the photochemical cycle, the Ru(bpy»+ entrapped in the zeolite needs to be regenerated. The chemistry of Ru(bpy)~+ in aqueous solution has been studied and involves a multimolecular ligand degradation process resulting in C02 formation. Since the zeolite cages can isolate Ru(bpy»+ from each other, significantly different results are expected, even though in an aqueous environment. For these studies, we have generated intrazeolitic Ru(bpy)~+ by oxidation of Ru(bpy)l+ with Cl2 gas. We find that the degradation pathways are eliminated at low loadings and slow 0 2 formation by oxidation of water is observed. Intermediates • OH and <>2 are observed in the EPR spectrum, as shown below.
Ru(bpyh•3 I u / - 2200 3000 3800 Present effort is focussed on finding ways to catalyze this oxidation reaction and integrate the system with the viologens.
38 Publications
P.K Dutta and M. Borja, Pulsed Laser Photolysis of Fe(C0)5 in Na-Y Zeolite, · Zeolites, 12, 142-145 (1992).
P.K Dutta, M. Borja, "Fatty Acids in Layered Metal Hydroxides: Membrane-like Structure and Dynamics, J. Phys. Chern., 96, 5434-5444, (1992).
P.K. Dutta, W. Turbeville, "Zeolite Entrapped Ru(bpy~+: Intermolecular Structural and Dynamic Effects, J. Phys. Chern., 2Q, 5024-5029, (1992).
P.K Dutta, W. Turbeville, "lntrazeolitic Photoinduced Reactions Between Ru(bpyJ'3+ and Methylviologen" J. Phys. Chern., 2Q, 9410-9416, (1992).
P.K Dutta, M. Borja, "Storage of light Via Photoelectron Transfer Across A Sensitized Zeolite- Solution Interface" Nature, 362, 43-45, (1993).
P.K Dutta, M. Borja, "Separation of Photogenerated Redox Species in Zeolites via Ion-exchange" J. Chern. Soc., Chern. Commun., 2Q, 1568-1569, (1993).
P.K Dutta, D.S. Robins, "Pyrene Sorption in Organic-Layered Double Metal Hydroxides" accepted in Langmuir.
39 Tuesday Evening
Session VI
Glenn A. Crosby, Chairman THE INFLUENCE OF BRIDGING LIGAND STRUCTURE ON INTRAMOLECULAR ENERGY TRANSFER IN BIMETALLIC Ru(II) DIIMINE SENSITIZERS
Aaron I. Baba, Yongwu Liang, Won Kim, and Russell H. Schmehl Department of Chemistry, Tulane University New Orleans, Louisiana 70118
The wealth of synthetic, photophysical and photochemical results available for group VIII transition metal complexes provides and excellent resource for designing specific linear chain oligomers which contain covalently linked chromophores having controlled redox and photophysical properties. For instance oligomers may be prepared such that electronic energy migration will proceed in one direction and electron migration following photoinduced electron transfer from any chromophore in the chain will proceed in the opposite direction. The objective in ligand design is to develop ligands for which (a) energy migration between linked metal centers is efficient (optimized by moderate electronic coupling through the bridge) and (b) the individual linked metal centers retain their spectroscopic and redox characteristics. The work presented here summarizes synthetic results and reports photophysical properties of bichromophoric complexes containing Ru(II) and Re(I) diimines.
Ligand Synthesis: Our efforts have focused on developing relatively simple procedures for preparing a broad range ofbis-bipyridine, bis-terpyridine and mixed bipyridine-pyridine bridging ligands. In earlier work we had developed a method for preparing mb_ph_mb by a 7 step procedure which also yielded mb_bd_mb and mb_ch_mb in the course of the synthesis. Recently we elaborated on the work of Krohnke (Synthesis, (1976)1. ) to prepare b_ph_b in a very simple four step sequence. In addition, we used the same methodology to prepare b _phph_ b and t _ph_t.
'~' 'M'No oN No oN ·n·No oN mb_bd_mb mb_ph_mb mb_ch_mb '~ •0 No oN ~'ON 'M'
mb_dvb_mb b_ph_b t_ph_t
41 Complexes Having Ligand Localized Excited States : Spectropic characterization of symmetrical complexes of the type [(bpy) 2Ru(b-b)Ru(bpy)~(PF6)4 and [(AN)(C0)3Re(b-b)Re(C0)3((AN)](PF6) 2 where b-b is one of the bridging ligands above, bpy 1 is 2,2' -bipyridine and AN is CH3CN indicates that exciting the lowest energy MLCT state leads to formation of both bridging ligand localized triplet states and the 3MLCT states of the complex 3 when b-b is mb ----bd mb or mb dvb mb. Only the MLCT state is observed in bimetallic Ru(II) complexes of mb_ph_mb, b_ph_b and b_phph_b. However, the Re(I) complexes of these 3 3 ligands exhibit luminescence from both MLCT and IL states in CH3CN solutions at room temperature. 3 The results suggest the coordinated bridging ligands have localized triplet ( 1r-1r*) states of nearly the same energy as simple aromatic hydrocarbons having nearly the same framework. For example coordinated b bd b has a triplet energy within 200 cm-1 of that of 1 ,4- diphenyI butadiene.
Intramolecular Electronic Energy Transfer : Symmetric bimetallic Ru(II) complexes of bphb and b_phph _ b have 3MLCT excited states which have much higher energy ligand localized states and energy transfer between 3MLCT states in neighboring metal centers of unsymmetric bimetallic complexes can be investigated. Our efforts have been focused on examining the free energy dependence of the energy transfer rate constant and in determining the degree of electronic coupling for different bridging ligands. The electronic coupling can be evaluated from (1) the temperature and free energy dependence of energy transfer rate constants and (2) analysis of intervalence transfer absorption spectra in mixed valence complexes of the type
[(bpy)2Ru(II)(b-b)Ru(III)(bpy)2](PF6) 5• We have examined energy transfer in a series of complexes with the general formula [(bpy)2Ru(b-b)Ru(tpy)(Ln-)]<4-nJ+ where b-b is b_ph_b or b_phph_b, tpy is 2,2' ,6' ,2"-terpyridine and Lis pyridine, eN-, or CL The terpyridine complex has a 3MLCT state lower in energy than the bpy complex and the energy transfer can be represented as shown in eq. 1. Luminescence from the tpy complex
[(bpy) Ru(Ill)(b-b -)Ru(Il)(tpy)(CN)]3-* .... [(bpy) Ru(I/)(b-b -)Ru(/l/)(tpy)(CN)]3-* (1) 2 2 is very weak and short lived (approx. 10 ns) making the system well suited for examination of energy transfer from the bpy complex to tpy complex by determination of luminescence intensity and lifetime quenching of the bpy complex. The energy gap can be varied by changing L in the tpy complex or by substituting other bidentate ligands for bpy. We have also prepared trimetallic complexes of the type [(bpy)2Ru(b_ph_b)(bpy) Ru(b_ph_ b)Ru(tpy)(Ln-)]<6-n)+ and examined the redox and luminescence behavior. Very efficient sensitization of the tpy complex occurs from both of the attached chromophores. We are attempting to separate and characterize the geometric isomers of the trimetallic complex in order to examine distance and/or coordination geometry effects on the intramolecular energy transfer process.
42 DOE Sponsored Research (1993- present)
1. A. I. Baba, W. Wang, W. Y. Kim, L.A. Strong, R. H. Schmehl "Convenient Synthesis of Bis-Bipyridines and Bis-terpyridines Bridging by Phenyl and Biphenyl Tethers", Synthetic Communications, in press.
2. A. I. Baba, H. E. Ensley and R. H. Schmehl "Influences of Bridging Ligand Unsaturation on Excited State Behavior in Mono and Bimetallic Ru(/l) Diimine Complexes", submitted.
3. R. Wang, Y. Liang, R. H. Schmehl "Light Induced Exchange Energy Transfer Reactions ofMicelle Forming Ru(II) Diimine Complexes", Inorg. Chim. Acta, submitted.
4. Y. Liang, A. I. Baba, R. H. Schmehl "Direct Measurement of Self-Exchange Energy Transfer in Covalently Linked Bimetallic and Trimetallic Donor-Acceptor Complexes" , manuscript in preparation.
5. W. Y. Kim, R. Thummel, E. Taffarel, S. Chirayil and R. H. Schmehl "Photophysical Behavior ofRu(Il) Complexes of 2-(2'-pyridyl)benzo[g]quinoline: Coexistence of Ligand Localized and Charge Transfer Excited States ", manuscript in preparation.
6. A. I. Baba, W. Y. Kim, and R. H. Schmehl, "Ligand Localized Excited States of Bimetallic Re(I) Complexes Bridged by Bis-Diimine Ligands", manuscript in preparation.
43 THE PHOTOCHEMISTRY AND SPECTROSCOPY OF PLATINUM DITHIOLATE COMPLEXES
R. Eisenberg, S. Cummings, D. J. Anderson, G. Suardi, J. E. Hill and J. M. Bevilacqua
Department of Chemistry University of Rochester Rochester, NY 14627-0216
The design, synthesis and characterization of new emissive complexes is fundamental to the development of transition metal photochemistry. Over the past several years, efforts in this regard have focused on square planar complexes of platinum. These complexes contain a dithiolate ligand and have the general formula PtLL'(S-S) where L and L' are neutral monodentate ligands or parts of a bidentate ligand such as 1,5-cyclooctadiene (COD), 1,2-bis(diphenylphosphino)ethane (dppe) or a heteroaromatic moiety (bpy, phen and their derivatives). All of the complexes synthesized are emissive in rigid media at 77 K, and for the diimine derivatives, the complexes luminesce in fluid solution at ambient temperature. Prior to the onset of the present investigation, existence of solution luminescence for square planar complexes was extremely rare. However, studies over the past decade, both at Rochester and elsewhere (Scandola, Balzani, von Zelewsky, Rillema, Che), have shown solution luminescence to exist more commonly in square planar Pt(II) diimine complexes and related systems. This property, in conjunction with the intrinsic coordinative unsaturation of square planar complexes, makes these Pt(ll) systems of interest for their potential photochemistry. Previous studies have focused on the spectroscopy of PtLL'(S-S) and elucidation of the excited state structure of these systems. The nature of the emission varies with the dithiolate ligand, being 3MLCT involving 7t*s-s for complexes where Land L' are phosphine or olefm donors and/or the dithiolate is maleonitriledithiolate (mnt- a 1,2-dithiolate), and multiple emitting states for Pt(diimine)(ecda) where ecda = ethyl-2-cyano-3,3-dithioacrylate (a 1,1-dithiolate). For the complexes containing diimine ligands, a strongly solvatochromic absorption band was observed that was assigned to a mixed metal/ligand-to-ligand' charge transfer (MMLL'CT). The Pt(diimine)(S-S) complexes were also found to undergo electron transfer quenching. Based on the reduction potential E112 and excited state energy E00 for Pt(Ph2phen)(ecda), its excited state reduction potential has been estimated as ca. 1.0 v vs SCE, a value confmned through an emission quantum yield study using a graded series of amine quenchers. The range of luminescent platinum complexes has been extended during the past year through the synthesis and characterization of new systems containing the dithiolate ligands met (1), qdt (2) and tdt(3). All of the new complexes containing diimines are solution emissive and exhibit the same negative solvatochromism characteristic of a MMLL'CT absorption and a highly polar ground state. For the tdt complexes, systematic variation of the diimine ligand yields a correlation consistent with the experimental
44 and theoretical conclusion that the LUMO in these complexes is strictly of 1t* diimine character. The complex Pt(Me2bpy)(met) was characterized structurally and its emission is similar to that seen for the closely related mnt complexes. The diolefin complex Pt(COD)(qdt) exhibits a Q.ighly structured emission at 77 K similar to that of other 1,2-dithiolate systems. The qdt complexes (TBA)2[Pt(qdt)2] and Pt(phen)(qdt) are solution emissive (
P.IPh3 ..., N P.l Ph3 ""'N oc...... s c:;::; hv. CaDs oc, ,...... sXc- / t.( X ...... , I + C21-4 Et 1 s c~N Smin H 1 s c~N PPh 3 PPh3 Additionally, recent efforts on the chemistry of d7 odd electron systems will be discussed. The chemistry of inorganic radicals is of interest in the context of substrate activation and these systems may be accessible photochemically. Recent work has focused on a rhodium Schiff base complex that undergoes photochemical reaction with H2 to form a hydride species which appears to insert CO to generate a formyl complex. These transformations take place via the intermediacy of d7 Rh(ll) radicals. ~s~~
45 PUBLICATIONS (1992-1994)
Solvatochromic and Emissive Properties ofPt(II) Complexes with 1,1- and 1,2-Dithiolates, Zuleta, J. A.; Bevilacqua, J. M.; Eisenberg, R. Coord. Chem. Rev. 1992, 111, 237.
Multiple State Emission from Platinum(II) Diimine Dithiolate Complexes. Solvent and Temperature Effects, Zuleta, J. A.; Rehm, J. M.; Bevilacqua, J. M.; Eisenberg, R. Inorg. Chem. 1992, 31, 1332.
Spectroscopic and Theoretical Studies on the Excited State in Diimine Dithiolate Complexes of Platinum(ll), Zuleta, J. A.; Bevilacqua, J. M.; Proserpio, D. M.; Harvey, P. D.; Eisenberg, R. Inorg. Chem. 1992,31, 2396.
Synthesis, Structure, and Emission Spectroscopy of Luminescent Pt(COD)(dithiolate) Complexes, Bevilacqua, J. M.; Zuleta, J. A.; Eisenberg, R. Inorg. Chem. 1993,32, 3689-3693.
Luminescent Diphosphine Dithiolate Complexes of Platinum(II): Synthesis, Characterization, and Structure, Bevilacqua, J. M.; Zuleta, J. A., Eisenberg, R. Inorg. Chem. 1994, in press.
Quenching Studies of the Excited State of (4,7-Diphenylphenanthioline)-(1-(ethoxycarbonyl)-1- cyanoethylene-2,2-dithiolato)platinum(ll), Pt(Ph2phen)(ecda), by Aromatic Amines and Metallocenes and Determination of its Excited State Reduction Potential, Bevilacqua, J. M.; Eisenberg, R. Inorg. Chem. 1994, in press.
Synthesis and Characterization of Luminescent Square-Planar Platinum(II) Complexes Containing Dithiolate or Dithiocarbamate Ligands, Bevilacqua, J. M.; Eisenberg, R. Inorg. Chem. 1994, in press.
Photochemistry of the Luminescent Alkyl Complexes Alkyl(carbonyl)bis(triaryl phosphine)(maleonitriledithiolato)iridium, IrR(CO)(PAr3)2(mnt), Bradley, P.; Suardi, G.; Eisenberg, R. J. Am. Chem. Soc. 1994, in press. Luminescent Pt(II) Complexes of Quinoxaline-2,3-dithiolate, Cummings, S.D.; Eisenberg, R. Inorg. Chem. 1994, submitted.
Acid-Base Behavior of the Ground and Excited States of Pt(II) Complexes of Quinoxaline-2,3- dithiolate, Cummings, S.D.; Eisenberg, R. Inorg. Chem. 1994, submitted.
46 ELECTROABSORPTION SPECTRA OF METAL-TO-LIGAND CHARGE
. TRANSFER TRANSITIONS OF {NH3) 5RuiiL COMPLEXES
Yeung-gyo K. Shin, Bruce S. Brunschwig, Carol Creutz and Norman Sutin
Department of Chemistry, Brookhaven National Laboratory, Upton, New York 11973-5000 One aspect of our program addresses both the experimental and theoretical investigation of electron-transfer reactions, charge-transfer (CT) transitions, and solvatochromism. Theoretical models for these phenomena require detailed knowledge of the location of the transferring charge within the molecular or supra-molecular complex. Unfortunately it is difficult to experimentally "determine the location of the charges in many systems and naive assumptions must often be used to assign their location. Electroabsorption spectroscopy, the study of the changes in absorption spectra produced when samples are placed in strong electric fields, provides a method of determining the changes in dipole moment and polarizability that accompany the optical transition. In order to obtain data on these changes for CT transitions in transition-metal complexes, we have built a new electroabsorption spectrometer that allows the collection of both absorption and electroabsorption spectra simultaneously. The instrument utilizes a quarter-meter monochromator and pairs of detectors matched to the spectral range. Detectors used include Si and InGaAs photodiodes with hybrid amplifiers and photomultiplier tubes. Glassy samples are held at 77 Kin cells constructed of transparent indium-tin-oxide (ITO) coated glass slides with optical path lengths of -30 J.lm. An electric field is developed between the ITO slides by applying a high voltage to the windows. A polarized probe beam is used to measure changes in the absorption profile of less than I0-6 AU. Spectra can be collected as a function of the electric field strength and of the angle between the applied electric field and of the polarization of the light, X· Electroabsorption spectra of mono- and binuclear (NHs)sRuiiL complexes were first reported by Oh, Sano and Boxer; however, those studies involved only a limited number of systems and the spectral changes observed were not systematically analyzed. Our current research focuses on understanding the 1v1LCT spectra of a whole family of (NHs)sRuliL complexes. The MLCT transition for these complexes occurs between 400 and 800 nm. Spectra for many of the complexes can be interpreted as arising primarily from a single absorption band. Typical spectra are shown below for the (NHs)sRullisonicotinamide complex in glycerol/water glass at 77 K. The two spectra were taken at different angles. Dipole-moment changes have been extracted from these data by fitting the spectra to a model proposed by Liptay and coworkers. The obs.erved dipole-moment changes in these complexes are significantly smaller than those predicted by simple theory. The consequences of the small dipole-moment changes will be considered and compared with the changes obtained from molecular orbital calculations. ·
47 8~------~--~------~ 7 6 5.88 X 10 VIm - :x=54.7° 10 4 ----- :X= 90~ s 2 Xo ~ -2 -4
-6
-~~4--~16~--~.8~~2~0--~2~2--~2~4--~26 Wavenumber I kK
Future work will include the development of a database of electroabsorption results for both MLCT and LMCT transitions in transition-metal complexes and for · . MMCT transitions in bridged metal dimers. We hope to develop a simple interpretation of the electroabsorption spectra of these complexes that will allow us to understand in detail the distance of the photo-induced charge transfers in these complexes. Recent Publications Milder, S. J., and Brunschwig, B. S. Factors Mfecting Nonradiative Decay: Temperature Dependence of the Picosecond Fluorescence Lifetime ofPt2(pop)44-. J. Phys. Chern. 96, 2189-2196 (1992). Fujita, E., Milder, S. J., and Brunschwig, B.S. Photophysical Properties of Covalently Attached Ru(bpy)g2+ and Mcyclam2+ (M = Ni, H2) Complexes. Inorg. Chern. 31, 2079-2085 (1992) Chou, M. H., Brunschwig, B. S., Creutz, C., Sutin, N., Yeh, A, Chang, R. C., and Lin, C.-T. Facile Amido to Pyridyl Isomerization: Pentaammineruthenium(II) Walks the Nicotinamide and Isnicotinamide Rings. Inorg. Chern. 31, 5347-5348 (1992) Fujita, E., Creutz, C., Sutin, N., and Brunschwig, B.S. Carbon Dioxide Activation by Cobalt Macrocycles. Evidence of Hydrogen Bonding between Bound C02 and the Macrocycle in Solution. Inorg. Chern. 32, 2657-2662 (1993). Fujita, E., Brunschwig, B. S., Ogata, T. and Yanagida, S. Toward Photochemical Carbon Dioxide Activation by Transition Metal Complexes. Coord. Chern. Rev. (in press). Ogata, T., Yanagida, S., Brunschwig, B., and Fujita, E. Mechanism and Kinetics of Photoreduction of C02 with p-Terphenyl and Cobalt Macrocycles (in preparation). Shin, Y. G., Brunschwig, B. S., Creutz, C., and Sutin, N. Electric Field Effects on Metal-To-Ligand Charge Transfer Bands of(NHg)sRuiiL Complexes (in preparation).
48 MAGNETOKINETIC EFFECTS IN REACTIONS OF INORGANIC COMPOUNDS
G. Ferraudi
Radiation Laboratory University of Notre Dame, Notre Dame, IN 46556
One goal of the present studies is to learn about the nature of magnetic interactions which influence the reactivity and spectroscopy of inorganic compounds. A second goal is to apply such knowledge to learn about spin-related effects in the mechanisms of electron and energy transfer reactions involving coordination complexes. In these regards the investigated reactions are either electron transfers, energy transfers or the relaxation of an excited state to the ground state. The basis for this selection is that they are tractable from a theoretical standpoint and because they provide different viewpoints for the examination of magnetokinetic effects in chemical reactions.
Magnetic field effects on the rates of ground- or excited-state reactions have been investigated at a constant field with intensities between 0 and 9 Tesla (90,000 Gauss). Flash photochemical and stopped-flow techniques were used for the measurements of the reaction rate and changes in the absorption and emission spectra under such fields. For example, in our initial work we explored effects on outer-sphere electron transfer between coordination complexes. These experimental observations are related to several contributions to the electronic matrix element. In a more recent development we have been studying field-induced changes in the inner-sphere reactions between a coordination complex and an inorganic radical, e.g., the reactions of Co(II) and Mn(II) complexes with c1;, Br; and C03H·. We selected these reactions because they can be modeled in terms of the radical pair mechanism and because they can be related to some of the reactions appearing in photochemical processes. The dependence of the rate with field can be related to simultaneous contributions from the suppression of HFC-induced transitions in the multiplet and to the ~g mechanism. Indeed, such combined contributions in reactions of high-spin Co{ll) complexes result in an additional extremum in plots of k vs. Bat fields of several tens of Teslas. The sense of the effect (increase or decrease with field) is in' some reactions opposite to that in reactions between organic radicals. This is related to the symmetry of the orbitals involved in the electron transfer. In some reactions, magnetokinetic effects decrease monotonically with field at field intensities above 3 Tesla, i.e., where literature models predict quenching of the magnetokinetic effect by anisotropic terms in the Hamiltonian. Magnetic field dependencies of the rates for outer-sphere oxidations of complexes by radicals approach those of outer-sphere electron transfers between coordination
49 complexes. Possible relationships between these two types of processes will be explored.
The experimental observations on electron transfer reactions have been extended to the interpretation of field effects on the charge transfer photochemistry of Co(ITI) 2 complexes, e.g., Co(NH3)5x + where X is halide. Quantum yields of radical photogeneration depend on the magnetic induction in a manner that is near the mirror image of the k vs. B curve for the fusion reaction of low-spin Co(ll) with radicals. A large part of these magnetokinetic effects appears to be related to recombination processes of radical-ion pairs before spin relaxation in the metallo fragment.
Future Work: Magnetokinetic effects are to be studied in intramolecular electron 2 transfer reactions of Co(ID) complexes with ligand radicals, e.g., [Co(NH3)5(L·-)] + where L(·-) = nitrophenyl radical. Similar studies will be carried out on outer-sphere electron transfer reactions between inorganic radicals and cryptate complexes of transition metal ions. Reaction kinetic studies will be complemented with time-resolved MCD spectroscopy of paramagnetic intermediates.
Publications (1992-1994)
Reduction of Hexaazacyclophane Complexes of Co(ll) and Cu(ll) by CH20H and e ;01; A Pulse Radiolysis Study. J. Vargas, G. Ferraudi, J. Canale and J. Costamagna, submitted, Inorg. Chim. Acta, 1994.
Temperature Effects on the Luminescence of Re(I) Complexes: On the Competitive Relaxations of Two Electronic States. G. Ferraudi, M. Feliz, E. Wolcan, I. Hsu, S. A. Moya and J. Guerrero, submitted, J. Phys. Chern., 1994.
Photochemical and Photophysical Properties of fac-Re(C0)3L2Cl, L =Quinoline or Isoquinoline. Neyde Murakami Iha and G. Ferraudi, J. Chern. Soc. Dalton, 0000 (1994).
Redox Reactions of an Mo(V) Tetrasulfophthalocyanine. G. Ferraudi, T. Nyokong, M. Feliz, M. Perkovic and D.P. Rillema, Inorg. Chim. Acta, 215,27-32 (1994).
Mechanism of C03H Generation in the Charge-Transfer Photochemistry of Co(NH3)4C03 +. A Picosecond to Microsecond Flash Photochemical Investigation of the Reaction Intermediates. G.Ferraudi and M. Perkovic, Inorg. Chern. 32, 2587-2590 (1993).
50 Magnetic Field Effects on the Rates of c1; and Br; Reactions: The Dependence of the Rates of Radical Disproportionation and Oxidation of Mn(II) Complexes on Field Intensity. G. Ferraudi, J. Phys. Chern., 97,2793-2797 (1993).
Magnetic Field Effects in Rates of Reactions between Radicals and Transition Metal Coordination Complexes of Co(II) and Mn(II): Probes for Spin Polarization. G. Ferraudi, J. Phys. Chern., 97,11929-11936 (1993).
Charge-transfer Spectroscopy and Photochemistry of Alkylamine Cobalt(III) Complexes. Scott K. Weit, Guillermo Ferraudi, Paul A. Grutsch and Charles Kutal, Coord. Chern. Rev., 128, 225-243 (1993).
Photosensitive Metal-Organic Systems: Mechanistic Principles and Applications. G. Ferraudi, Chapter 5, ACS Advances in Chemistry Series No. 238, American Chemical Society, Washington, DC, 1993.
Magnetic Field Effects in the Charge Transfer Photochemistry of Co(III) Complexes. G. Ferraudi, Chern. Phys. Lett., 203, 487-489 (1993).
Magnetic Field Perturbation of the Doublet states in Cr(ill) Complexes with Quadratic 2 and Cubic Symmetries. A Study on the Role of Levels Having T 1 Parentage. S. Ronco, M. Perkovic, G. Ferraudi and M. Cozzi, Chern. Phys.162,95-106 (f992).
Primary Photochemical Processes in Jac-C1Re(CO>JL2 (L = 4-Phenylpyridine and 4-Cyanopyridine): A Steady-State and Flash Photochemical Study of Reaction Products and Intermediates. M. Feliz, G. Ferraudi and H. Altmiller, J. Phys. Chern., 96, 257 (1992).
51 Wednesday Morning
Session VII
James H. Espenson, Chairman MECHANISTIC ANALYSIS OF ELECTRON TRANSFER KINETICS: ELECTRONIC COUPLING AND MEDIUM REORGANIZATION
M.D. Newton
Chemistry Department, Brookhaven National Laboratory, Upton, NY 11973
Electron transfer is an important subset of a larger class of electronic processes, of both the l-and 2-electron type, which involve long-range electronic coupling of local donor and acceptor sites in chemical systems. These include ionization and electron attachment processes, charge transfer spectral transitions, excitation transfer, and magnetic exchange, as well as thermal and photoinitiated electron transfer kinetics. In this research project, the mechanisms of electron-transfer reactions and related phenomena in polar media are modeled and analyzed, with particular focus on the role of electronic structure and the manner in which its influence may be affected by energetic and dynamical properties of the medium in which the reaction occurs. Increasing emphasis is also placed on elucidating the role of medium reorganization in controlling the kinetics and on the combined influence of electronic coupling and medium reorganization in controlling the dependence of the kinetics on the spatial separation of donor and acceptor sites. Theoretical techniques are developed both for calculating and interpreting the extent of donor-acceptor coupling in terms of superexchange models. The superexchange model is also used to provide a unified understanding of related l and 2-electron processes. Exploiting appropriate electronic structural techniques, both ab initio and semiempirical, we are analyzing a variety of inter-molecular processes in inorganic, organic, and organometallic systems in both ground and excited states. The role of solvent is studied in several ways, including the use of (1) supermolecular clusters containing "inner-shell" solvent, (2) continuum reaction-field models, and (3) molecular level solvation models implemented by molecular dynamics computer simulation or by integral equation techniques. The degree of importance of non-linear response and electrostatic image effects in solvent reorganization is inferred from the detailed calculations. The extent to which donor/acceptor electronic coupling (i.e., as manifested in transfer integrals) exhibits a simple exponential fall-off with increasing separation has been evaluated for a number of homologous organic spacer systems, including acyclic, mono and bicyclic units. For the cyclic spacer units the calculations are employed in assessing the degree of additivity among the multiple covalently bonded pathways, which may interfere constructively or destructively. The distance-dependence of coupling found in the covalently-linked intramolecular systems is similar to that exhibited by non-covalently-linked spacers of methane molecules in van der Waals contact, and also by hydrogen bonded spacer systems. However, in all cases, the transfer integrals are very sensitive to conformational degrees of freedom. While nearest-neighbor (NN) McConnell-type pathways based on local bonding or anti-bonding orbitals are found to be unimportant on the basis of superexchange analysis, a
53 McConnell-type model was recovered at a more coarse-grained level based on larger spacer units and representable in a matrix form isomorphic to the standard scalar McConnell expression. Several approaches to medium reorganization energy (Er) are being explored. Detailed calculations employing molecular electronic structure techniques (primarily ab initio and semi-empirical Hartree-Fock (HF) and configuration interaction (CI) levels) are being carried out in the context of medium environments represented by realistic dielectric continuum zones and cavities (e.g., as based on superposed atomic spheres) for a variety of electron transfer processes, and the resulting Er values are compared with those based on simpler models such as the Marcus 2-sphere model, especially with regard to the predicted dependence on donor/acceptor separation. Interfacial electron transfer between the ferrocence/ferrocenium redox pair and a gold electrode, separated by an alkane thiol chain of variable length, has been modelled by a 3-zone dielectric model consisting of semi-infinite aqueous solution and metal electrode zones, separated by a central low-dielectric zone (the alkyl chains) of variable, finite width. The exact Er results from this model provide an important perspective for assessing the quantitative utility of simpler limiting cases based on the Marcus model for Er. The distance dependence of Er obtained from the 3-zone dielectric model is being employed in the analysis of ongoing kinetic studies at BNL (Feldberg and Smalley) and Stanford (Chidsey). A generalized dielectric model has been developed (with F. Raineri and H. L. Friedman) to account for the substantial Er values observed for non-dipolar solvent molecules with higher-order multipole moments (e.g., quadrupolar molecules like benzene and 1,4-dioxane) or very weakly dipolar molecules (like toluene). In this new theory, which emerges from a more detailed, molecular-level treatment implemented by integral equation techniques, the role of the higher multipoles is manifested by the spatial dispersion (i.e., non-local behavior) of the medium. In addition to Er values, the theory gives a reasonable quantitative account of related probes of solvation, such as the ET(30) "solvent polarity" scale, thus providing a unified account of dipolar and non-dipolar media. Future work along these lines will include efforts to provide a more detailed account of the quantum mechanical nature of initial and final charge transfer states, both ground and excited.
54 Publications (1992-1994)
Newton, M.D. Cluster Models for Condensed.,.Phase Electron Transfer Processes. in Cluster Models for Surface and Bulk Phenomenon, NATO AS! Series, G. Pacchioni and P. S. Bagus Eds., pp. 551-563, Plenum Press, New York, NY, 1992
Liang, C., and Newton, M. D. Ab Initio Studies of Electron Transfer: Pathway Analysis of Effective Transfer Integrals. J. Phys. Chem. 96, 2855-2866 (1992) Wang, Y. J., Newton, M. D., and Davenport, J. W. Electronic Structure and Magnetic Coupling in Copper Oxide Superconductors. Phys. Rev. B46, 935-951 (1992)
Liang, C., and Newton, M.D. Ab Initio Studies of Electron Transfer. 2. Pathway Analysis for Homologous Organic Spacers. J. Phys. Chem. 97, 3199-3211 (1993)
Marchi, M., Gehlen, J. N., Chandler, D., and Newton, M. Diabatic Surfaces and the Pathway to Primary Electron Transfer in a Photosynthetic Reaction Center. J. Am. Chem. Soc. 115, 4178-4190 (1993)
Basilevsky, M. V., Chudinov, G. E., and Newton, M.D. The Multi-Configurational Adiabatic Electron Transfer Theory and Its Invariance Under Transformations of Charge Density Basis Functions. Chem. Phys.179, 263-78 (1994)
Kneifel, C. L., Newton, M. D., and Friedman, H. L. Simulation of Solvent Isotope Effects on Aqueous Ferrous and Ferric Ions. J. Mol. Liquids (in press)
Creutz, C., Newton, M. D., and Sutin, N. Metal-Ligand and Metal-Metal Coupling El~ments. J. Photochem. Photobiol. A: Chemistry (in press)
Liu, Y.-P., and Newton, M.D. Reorganization Energy for Electron Transfer at Film-Modified Electrode Surfaces: A Dielectric Continuum Model. J. Phys. Chem. (submitted) Raineri, F.O., Perng, B.-C, Newton, M.D. Solvent Reorganization Energy for Charge Transfer in Dipolar and Nonpolar Polar Solvents. J. Phys. Chem. (submitted)
55 LONG DISTANCE ELECTRONIC COUPLINGS FOR ~OLECULARELECTRONTRANSFER
John R. Miller
Chemistry Division, Argonne National Laboratory, Argonne, IL 60439
Electron transfer (ET) theory casts the rate constant for long distance ET as the product of an electronic coupling and a Franck Condon weighted density of states. 2 kET = ~1t IVI FCWD An important goal is to quantitatively separate these two factors. The ET rates reflect the strong, exponential dependence of the electronic coupling on distance, but the distance dependence of the solvent reorganization energy, As, can influence the FCWD, as can distance dependence of 0 the free energy change (LlG ). Pulse radiolysis has been an especially valuable tool because it can measure the rates of charge shift reactions for which LlG 0 is independent of distance. In this laboratory we have previously measured distance dependence of ET rates in molecules of the formula BSN, where B =biphenyl, N =naphthyl, and the spacer groups, S, vary from 1,3- cyclohexane (four bonds) to 3,16-androstane (ten bonds). In those molecules it was possible to measure the rates of electron transfer in radical anions and hole transfer in radical cations, but electronic couplings deduced from these rates are uncertain because the distance dependence of As is uncertain. Current studies make use of molecules with either biphenyl or 2-(9,9-dimethyl)fluorene (2F) as the donor groups and benzoquinone or naphthalene as the acceptor groups for the longest spacer of the series (3,16-androstane) and the second shortest member (1,4-cyclohexane).
ET rates for these molecules will establish the distance dependence of the electronic couplings in two ways: (1) In THF both a weakly and a strongly exoergic reaction, which are affected oppositely by As, can be measured at both distances. This enables us to simultaneously obtain the distance dependence of the electronic coupling and the solvent reorganization energy. (2) In a nonpolar solvent ET rates to quinone groups are measurable for both the long and short compounds. The FCWD is expected to be independent of distance because there is no appreciable solvent reorganization energy. Preliminary results indicate that (i) the distance dependence of the electronic coupling is weaker than that previously estimated, (ii) distance dependence of the solvent reorganization energy is stronger than estimated from dielectric continuum theories, and (iii) the distance dependence of the electronic coupling is slightly weaker in the nonpolar solvent than in the polar solvent. Similar steroid molecules have been used to measure the dependence of electronic coupling on the presence of 1t material in the spacer group. The four molecules shown have a completely saturated 3,15-androstane spacer, one containing a single 1t bond, a conjugated pair of 1t bonds,
56 BSN= 8
and an aromatic ring. Previous studies have measured electron transfer through 1t systems of dif ferent lengths. The approach of the present study differs by keeping the distance between the donor and the acceptor constant and changing the fraction of that distance, which is spanned by 1t material. The single 1t bond is found to barely enhance the ET rate in anions and the conjugated pair of 1t bonds or the aromatic ring increase the ET rate by a factor of about four. Hole transfer rates in radical cations are affected more dramatically by the presence of 1t material, but, at least in the case of the conjugated pair of bonds in the aromatic ring, most of this increase can be explained by a two-step mechanism in which the hole first transfers to the 1t system in the spacer. The results suggest but do not prove that 1t material can have a larger effect on electronic couplings for the hole transfer reactions than the electron transfer reactions. Calculated electronic couplings through long saturated spacers such as the steroid, decalins or hypothetical straight-chain saturated hydrocarbons are increasingly dominated by "skipping" pathways as the spacer becomes longer. Couplings for hole transfer in radical cations, however, . can have significant contributions from McConnell-like pathways that visit several adjacent bonds in succession. This difference is due to the known difference in coupling between adjacent cr bonds as compared to adjacent cr* bonds and to the fact that bond-to-bond couplings for the cr* case actually increase with separation for pairs of cr* bonds separated by zero, one or two intermediate bonds. Calculation of the bond-to-bond interactions needed to compute long-distance through-bond couplings are enhanced by the use of "ghost orbitals"- basis functions that lie in-between the two groups for which long-distance electronic couplings are being calculated. Intentional inclusion of ghost orbitals into a basis set provides a method to check the efficacy of calculated long distance electronic couplings and provide confidence that these couplings are correctly calcu lated. It is then possible to establish with greater confidence the difference between the very steep distance dependence for direct "through space" couplings as compared to the longer range through-bond couplings. Reorganization energies from low-frequency solvent modes can generally be treated classically and, conversely, most classical reorganization energy usually comes from the solvent. The biphenyl group used in many of these studies possesses an internal twisting mode that gives rise to a classical reorganization energy that is not dependence upon distance. By substitution of biphenyl with the biphenyl-like 2-fluorenyl group (2F), it is possible to measure the internal twisting reorganization energy to be 0.13 e V. This contribution to the classical reorganization energy reduces that formerly attributed to solvent and indicates that the calculations of dielectric continuum formulas for solvent reorganization energy are even more severely in error than previously thought. Future Plans. New molecules will be used to examine the effects of 1t groups in the spacer without intederence from two-step mechanisms and to investigate intramolecular ET to fullerenes. Electronic couplings will be calculated by ab initio methods for large spacers like steroids and the results will be compared with experiment. Experiments combining linac and laser excitation will be used to investigate light-driven ET.
57 , Publications:
1. SUPEREXCHANGE PATHWAY CALCULATION OF LONG-DISTANCE ELECTRONIC COUPLING IN H2C(CH2)m-2Cfb CHAINS L.A. Curtiss, C. A. Naleway, and J. R. Miller Chern. Phys. 176,387-405 (1993)
2. THEORETICAL STUDY OF LONG-DISTANCEELECTRONIC COUPLING IN H2C(CH2)n-2CH2 CHAINS L.A. Curtiss, C. A. Naleway, and J. R. Miller J. Phys. Chern. 97,4050-4058 (1993)
3. DISTANCE DEPENDENCE OF INTRAMOLECULAR ELECTRON TRANSFER ACROSS OLIGOPROLINES IN [(bpy)2RuiiL•-(Pro) 0 -Coill(NH3)s]3+, n = 1-6: DIFFERENT EFFECTS FOR HELICAL AND NONHELICAL POLYPROLINE II STRUCTURES M. Y. Ogawa, J. F. Wishart, Z. Young, J. R. Miller, and S. S. Isied J. Phys. Chern. 97, 11456-11463 (1993)
4. LONG DISTANCE ELECTRON TRANSFER THROUGH ROD-LIKE MOLECULES WITH CUBYL SPACERS B. Paulson, P. Kakurnanu, P. Eaton, G. L. Closs, and J. R. Miller J. Phys. Chern. 97, 13042-13045 (1993)
5. COUNTER-ION EFFECTS IN INTRAMOLECULAR CHARGE TRANSFER IN RADICAL ANIONS P. Piotrowiak and J. R. Miller J. Phys. Chern. 97, 13052-13060 (1993)
58 THE ROLE OF ION PAIRING AND MODE-SPECIFIC VIBRONIC COUPLING IN INTRAMOLECULAR CHARGE AND ENERGY TRANSFER PROCESSES.
Piotr Piotrowiak
Department of Chemistry, University ofNew Orleans, New Orleans, Louisiana 70I48
During the last two years the work of our group has been concentrating on the understanding of two effects which can be potentially utilized to control the efficiency of charge separation reactions: (I) ion-pairing and electrolyte effects, and (2} the role of molecular orbital symmetry. Different model compounds have been prepared and different methods are being used in order to study the two areas. A brief outline of the recent progress is given below. Dynamics of Ion Pairing in Intramolecular Electron Transfer: Two convenient probes of the dynamics of association involving photoexcited charge-separated species and chemically inert ions have been identified: H,N--o---©-No, H,N-o----©-Q>--No,. Upon photoexcitation both the p-aminonitrobiphenyl (p-ANBP) and p-aminonitroterphenyl (p-ANTP) form a long lived (several J..I.S) CS triplet state, which is characterized by a NIR CT-band corresponding to the CT~LE state transition (Fig. Ia). The position of this band is extremely sensitive to solvent polarity and to the presence of electrolytes, e.g., upon addition of I mM of TBA~r- to THF, the Amax of p-ANTP shifts from 990 nm to 7I 0 nm, i.e. by 4000 cm-I. The presence of LiCl induces an even larger blue-shift to 570 nm, i.e. 7500 cm-I. The magnitude ofthe shift is determined by the size of the ions, while the rate of shift depends on the dissociation constant and ion mobility. Conceptually these shifts are strictly analogous to the time dependent fluorescence shifts (TDFS) observed in salt solutions by other workers, however, they offer important advantages: very dilute solutions can be used thanks to the long lifetime of the CT state, enabling one to study weakly polar media; the low polarity of the medium is translated into the much larger magnitude of the shift. Unlike in the known TDFS results, we can clearly identify the transition from the free solution spectrum to the ion-paired spectrum in all solvent/salt combinations which have been investigated up to now.
03 0.6 3
0.2 0.4
0.2 0.1
0 0 300 500 700 900 1100 1300 1500 O.OE-iO 2SE-7 S.OE-7 7SE-7 l.OE-6
Figure 1. (a) Time evolution of the transient CT spectrum in 0.1 mM solution of p-ANTP in THF containing 1 mM TBABr: 1- 10 ns after the 355 nm laser pulse; 2-200 ns; 3-800 ns. (b) Transient absorption profiles in 0.1 mM solution p-ANTP: 1- in neat THF at 990 nm; 2- in neat THF at 570 nm; 3- with 75 mM LiCl at 990 nm; 4- with 75 mM LiCl at 770 nm; 5- with 75 mM LiCl at 570 nm. The fast part of the decay 3 and 4 results from the dynamic blue-shift, the slow component is due to the charge-recombination process (1 J.1S full scale).
59 Interestingly, the early results indicate that in p-ANBP the presence of electrolyte extends the lifetime of the charge-separated state, while in the case of p-ANTP it leads to sh01tening of the lifetime. The interplay between the ion-pairing dynamics and the rates of the intramolecular charge separation and recombination is being carefully investigated. It is expected that with proper tuning the additional driving force and the reorganization mode resulting from ion-pairing, can be used to simultaneously improve the yield and extend the lifetime of a CT state. Mode-Specific Vibronic Coupling in Rigidly Linked Bichromophores: The role of molecular geometry and MO symmetry in determining the electronic coupling is being studied in a series of degenerate and nearly degenerate bifluorenes. In order to remove the solvent influence the measurements are performed in a supersonic jet (collaboration with D. Levy, University of Chicago). In three of the studied molecules, spirobifluorene (SBF), daha-spirobifluorene (daha SBF) and 1-methyl-spirobifluorene (lM-SBF), the local transition moments are perpendicular to one another. The excitation spectrum of the degenerate SBF exhibits one origin transition, while non-degenerate daha-SBF and lM-SBF give rise to two origins, assigned to transitions localized at individual chromophores. No exciton splitting was detected down to the resolution of0.05 cm- 1. The undetectably small splitting leads one to conclude that not only the first order dipole dipole term, but also the higher terms of the multipole expansion, and the exchange term, are zero, and that the chromophores in these molecules are indeed electronically decoupled from one another. In 9,9'-bifluorene (BF) the transition moments of the two moieties are not perpendicular to one another and an exciton splitting of 57 cm-1 is observed. The most interesting findings in this work come from the measurements of dispersed fluorescence spectra of daha-SBF and lM-SBF as a function of excess excitation energy (i.e., energy above the origin transition). At very low energies one observes a pure, or nearly pure, emission from the excited chromophore. At higher energies dual emission from both chromophores is observed, regardless of which chromophore has been excited. However, the intensity ratio of the two emissions varies seemingly erratically with the increasing excitation energy (Figure 2). At still higher excitation energy a plateau in the intensity ratio is achieved. What we are observing is a gradual turning on of the vibronic coupling between the localized electronic states. In the low energy region the states are sparse and mode-specific coupling, depending on energy matching and symmetry of the mode, is clearly manifested. At higher energies the density of states increases and the system follows the familiar picture in which each state is coupled to a dense manifold of states. The energy matching, as well as the symmetry, of individual modes are no longer of critical importance . .... -.. ! . & u .. "' ..
Figure 2. Emission partitioning. in .the dshs-SBF- - plotted- against- the- excitation- energy,- Eex- Eo-a• in cm-1.
60 It is thanks to the high symmetry and rigidity of the spiranes that the mode-specific vibronic coupling and energy transfer could be so clearly monitored up to as far as 600 cm-1 above the vibrational ground state. This behavior is typical for very small molecules, and it is unusual that it is found in our relatively large systems consisting of more than 20 carbon atoms. The importance of the symmetry of a particular mode in determining its ability to couple the localized electronic states is being analyzed by computation. The interpretation of the double photon ionization measurements on the same molecules is also in progress.
Publications 1993-1994
Counterion Effects in Intramolecular Charge Transfer in Radical Anions. P. Piotrowiak and J. R. Miller, J. Phys. Chem., 1993, 97, 13052-13060.
Exciton Interaction and Vibronic Coupling in Bichromophoric Molecules. N. A. van Dantzig, D. H. Levy, C. Vigo and P. P~otrowiak, J. Chem. Phys., submitted.
Vander Waals Complexes ofBichromophore Spirobifluorene. N. A. van Dantzig, P. Piotrowiak and D. H. Levy, Chem. Phys. Lett., submitted.
Ion-Pairing Effects in Intramolecular Electron Transfer Reactions: Distance and the Driving Force Dependence in the Weakly Exoergic Region. P. Piotrowiak, Solar Energy Materials and Solar Cells, accepted.
Vibronic Coupling and Electronic Energy Transfer in Bichromophoric Molecules. P. Piotrowiak, I D. H. Levy, N. A. van Dantzig and C. Vigo, Solar Energy Materials and Solar Cells, accepted.
Specific Ion-Pairing Effects in Weakly Exoergic Intramolecular Electron Transfer. P. Piotrowiak, Inorg. Chim. Acta, invited, submitted.
61 CHARGE SEPARATION IN THE REDOX QUENCHING OF THE EXCITED STATES OF RUTHENIUM(II)-DITMINE PHOTOSENSITIZERS
Hai Sun and Morton Z. Hoffman
Department of Chemistry, Boston University, Boston, Massachusetts 02215
Scope of the Work. The factors that determine the yields of redox products in bulk solution upon the bimolecular quenching of the excited states of metal complex photosensitizers must be known in order for the efficiency of photoinduced electron-transfer reactions to be controlled. According to the conventional model, the diffusional encounter of an excited state by an electron-transfer quencher results in the formation of a geminate redox pair within the solvent cage; escape of the redox species out of the cage into bulk solution (kce) competes kinetically with back electron-transfer within the cage (Jq,t) to reform the original ground-state materials. The geminate recombination reaction can be viewed as approximating intramolecular electron transfer between closely aligned entities; kbt would be expected to exhibit normal and inverted Marcus dependencies on the driving force (dGbt0 ). The efficiency of cage escape of the redox products (Tlce) can be expressed as kcef(kce + kbt); kce can be computed from the standard equations of diffusion. The conventional model holds that a plot of log(Tlce-1- 1) vs. dGbt0 should describe a "bell-shaped" curve, from which electron-transfer parameters could be extracted. There are a number of earlier reports in the literature that support this prediction. Recently, however, results have been presented that show a very weak dependence, or none at all, rather than a marked "bell shaped" curve. These conflicting results raise the question of the general applicability of the simple mechanistic model.
Our recent work has focused on the determination of Tlce for the oxidative and reductive quenching of Ru(II)-diimine complexes as a function of thermodynamic, kinetic, and solution medium parameters; the work has required, as well, the investigation of excited-state and redox properties of the photosensitizers under the same experimental conditions. We have chosen to use a well-known and well-studied series of ten complexes (Ru2+) of the general formula Ru(bpy)3-m-z(bpm)m(bpz)z2+ (bpy = 2,2'-bipyridine, bpm = 2,2'-bipyrimidine, bpz = 2,2' bipyrazine, m and z = 0,1,2,3 and m + z::;; 3) in order to maintain the charges, structures, and electronic nature of the systems as nearly alike as possible so as to minimize influences due to those factors. Q-0 () () bpy bpz bpm
Re-evaluation of the Cage Escape Model. In order to test the conventional model further, we determined Tlce for the reductive quenching of *Ru2+ by triethanolamine (TEOA) in aqueous, acetonitrile, and propylene carbonate solution; we found the dependencies of Tlce on d~to to be very weak, with no "bell-shaped" curves evident.. The accumulated evidence suggests very strongly that Marcus-like dependencies may be the exception, not the rule. In addition, we determined Tlce for the quenching of *Ru(bpy)32+ by methylviologen in H20-CH3CN mixtures as
62 a function of temperature (10-65 °C) in order to calculate the apparent activation energies of the back electron transfer process at different solvent compositions, assuming the simple model to be operative. Plots of (Tlce-1 - 1) vs. Iff were linear; the slopes would be *M + Q Quenching by Phenols. *Ru(bpz)32+ is reductively quenched by phenol and its substituted derivatives, generating Ru(bpz)3+ and the corresponding phenoxy radical; values of Tlce have been measured, but they correlate only to a limited extent with AGt,t0 • In the presence of air, a wide range of organic products are generated, undoubtedly from the reactions of the redox products with 02 and the subsequent reactions of secondary reactive species such as 102, D2-, and H202. When Ru(bpy)32+ is the photosensitizer in the presence of air, 102 is formed with a quantum yield of 0.5, leading to reaction with phenol and the very efficient production of 1,4- benzoquinone as the only organic product detectable with HPLC; *Ru(bpy)32+ is not quenched by phenol. The presence of phenol (~.1 M), however, results in an increase in the lifetime ('t) of the emitting species, a blue shift in its emission spectrum, and an increase in the luminescence quantum yield; there is no change in the absorption spectrum of the ground state. The magnitude of changes are dependent on [phenol], from which the equilibrium constant for the interaction between phenol and the complex can be estimated. The increased luminescence lifetime at room temperature with increase in [phenol] arises from a decreased rate of radiationless decay due to the increase in the energy level of the MLCT excited state. However, the overall effect of temperature (3-70 OC) and [phenol] on 't is complex: at low temperatures, where nonradiative decay is the dominant process, 't increases with increasing [phenol]; at high temperatures the opposite effect is seen, reflecting the dominance of the thermal population of the d-d state. The behavior of phenol as a solute in the presence of Ru(bpy)32+ is very similar to that observed for mixed solvents in aqueous solution, which can be interpreted in terms of preferential solvation. Future Plans. Work this coming year will include the examination of electrolyte and temperature effects in photoinduced intermolecular electron-transfer reactions in aqueous solution, the further testing of the solvent cage model of bimolecular quenching with homologous series of phenols, amines, methoxybenzenes, and viologens, and a broad investigation of Tlce from the reductive quenching of Cr(III)-polypyridyl complexes as a function of solvent composition and temperature. 63 Publications (1992-1994) 1. C. Pizzocaro, M. Bolte, and M.Z. Hoffman, "Cr(bpy)33+-Sensitized Photooxidation of Phenol in Aqueous Solution," J. Photochem. Photobiol. A: Chern., .6.8,, 115-119 (1992). 2. A. Yoshimura, M.Z. Hoffman, and H. Sun, "An Evaluation of the Excited-State Absorption Spectrum of Ru(bpy)32+ in Aqueous and Acetonitrile Solutions," J. Photochem. Photobiol. A: Chern., lQ, 29-33 (1993). 3. H. Sun and M.Z. Hoffman, "Protonation of the Excited States of Ruthenium (In Complexes Containing 2,2'-Bipyridine, 2,2'-Bipyrazine, and 2,2'-Bipyrimidine Ligands in Aqueous Solution," J. Phys. Chem., 9J.., 5014-5018 (1993). 4. C. Pizzocaro, M. Bolte, and M.Z. Hoffman, "Polymerization of Acrylamide Photosensitized by Tris(2,2'-bipyridine)chromium(ill) Ion in Aqueous Solution," Polyhedron, 12, 855-858 (1993). 5. H. Sun and M.Z. Hoffman, "Photophysics of Ruthenium(II)-Diimine Complexes in H20- CH3CN Mixed Solvents," J. Phys. Chem., 21., 11956-11959 '(1993). 6. Q.G. Mulazzani, H. Sun, M.Z. Hoffman, W.E. Ford, and M.A.J. Rodgers, "Quenching of the Excited States of Ru(II)-Diimine Complexes by Oxygen," J. Phys. Chern., 98, 1145-1150 (1994). 7. H. Sun and M.Z. Hoffman, "Photoinduced Charge Separation by Ruthenium(II) Photo sensitizers," Proc. Indian Acad. Sci., in press. 8. H. Sun, M.Z. Hoffman, and Q.G. Mulazzani, "Properties of the Excited States and One Electron Reduced Forms ofRuthenium(II)-Diimine Photosensitizers," Res. Chern. Intermed., in press. 9. H. Sun and M.Z. Hoffman, "Re-evaluation of the Solvent Cage Model for Electron Transfer Quenching of the MLCT Excited States of Ru(In Complexes," J. Photochem. Photobiol. A: Chem., in press. 10. H. Sun, A. Yoshimura, and M.Z. Hoffman, "Oxidative Quenching of the Excited State of Tris(2,2'-bipyridine)ruthenium(2+) Ion by Methylviologen. Variation of Solution Medium and Temperature," J. Phys. Chern., in press. 11. C.·Pizzocaro, M. Bolte, H. Sun, and M.Z. Hoffman, "Metal Complex Sensitized Photo oxidation of Phenol in Aqueous Solution," New J. Chern., in press. 64 RADIOLYfiC STUDIES ON THE REACTIONS OF PORPHYRINS WITH ORGANIC RADICALS P. Neta, S. Marguet, and J. Grodkowski National Institute of Standards and Technology, Gaithersburg, Maryland 20899 Porphyrins and metalloporphyrins react with a number of organic radicals by electron transfer to form 7T-radical anions or cations. In addition, certain transition metal porphyrins react with organic radicals by electron transfer at the metal center or by formation of metal carbon bonds. Such processes are studied by radiolytic techniques to obtain kinetic and mechanistic information to explore the role of metalloporphyrins in various catalytic processes. Recent studies involved examination of the solvent effect on the rate of oxidation of porphyrins and investigation of the reactions of carbon-centered radicals with several transition metal porphyrins. Solvent effects on rate constants for one-electron oxidation of porphyrins were examined for two radicals, a peroxyl radical, CC130 2·, and the radical cation derived from N-methylindole. CCI302· + P - CCI302_ + p.+ (1) Meln·+ + P - Meln + p.+ (2) These studies have been carried out with free base porphyrins and not metalloporphyrins, because the latter can bind solvent molecules as axial ligands, which will change their redox potential and the driving force for the reaction and will mask the other effect of the solvent. The advantage of CC130 2· is that it is readily produced by radiolysis of aerated solutions of CC14 in practically any solvent and can serve as a precursor for other radicals, e.g. for oxidation of MeIn to Meln .+. The rate constant for reaction 1 was measured for 7 8 1 1 hematoporphyrin in 14 solvents and found to vary between 3x10 and 2xl0 L mol" s· • The rate constants correlate with the solvent cohesive energy density but not with the dielectric constant, as we found before for reactions of this radical with simpler organic compounds. The rate constant for reaction 2 in the different solvents varied between 1.5xl08 and 1.8x109 1 L moi· s·t, but there was no correlation between the values of k1 and k2• By measuring the one-electron oxidation potentials of Meln and P and the rate constants for reaction 2 in the same solvents, it was possible to correlate the two sets of values with the solvent properties according to the Marcus equations. The results show a good correlation and are in accord with reaction 2 being an outer sphere electron transfer. Reaction 1, in contrast, is an inner sphere process that also involves intimate participation of solvent molecules. 65 During the past year we have also continued to study the re~ox and organometallic . chemistry of metalloporphyrins, which may be of potential importance in homogeneous catalysis. Previously, we have studied the reactions of alkyl and fluoroalkyl radicals with several metalloporphyrins. Alkyl radicals (R) react with Co11P and Fe11P to form R-ComP and R-Fe111P with stable metal-carbon bonds. The oxidized metalloporphyrins ComPand Fe111P do not form stable products upon reaction with alkyl radicals. In contrast, both Cr1P and Cr11P react with carbon-centered radicals to form relatively stable R-Cr11P and R-CriVP. Although Ni1P and NiuP also react rapidly with the radicals to form Ni-C bonds, only the reactions of fluoroalkyl radicals with Ni1P yield stable products. Reactions of alkyl radicals with Ni1P result in unstable R-Ni11P products that eventually yield Ni11P. Reactions of alkyl radicals with Ni11P also are reversible and the short lived R-NimP products disappear through radical reactions, with recovery of NiuP. We are now examining the case of rhodium porphyrins. RhnP react with alkyl radicals to form stable R-Rh111P. Further reaction of this product with alkyl radicals was found to yield chlorin and another stable product, which has not been reported before and is currently in the process of being identified by various techniques. The Rh11 state is unique among reduced metalloporphyrins in that it is a radical-like species, unstable in monomeric form, and can be stabilized only by steric hindrance (e.g. in tetramesityl-porphyrin) in pure aprotic solvents. Preliminary radiolytic results suggest that this unstable form, Rh11P, reacts with 2-PrOH under alkaline conditions by hydrogen abstraction and thus propagates a chain reaction in which the porphyrin is reduced to the Rh1P state and the alcohol is oxidized to acetone. We are also examining the reactions of rhodium- and alkylrhodium-porphyrins with C02 and with C02- to explore the possibility of C02 reduction by such metal complexes or insertion into alkyl metal complexes. Preliminary experiments indicate that insertion of C02 into R-RhmP (and several other alkyl-metalloporphyrins) is a very slow and inefficient process at best. The hydridorhodium complex, H-RhmP, however, appears to react more rapidly with C02, presumably to yield formic acid. Also, a product formed upon reduction of RhP appears to catalyze the reduction of C02• Further studies are underway to verify these preliminary results and to unravel their mechanisms. 66 PUBLICATIONS (1992-1994) Temperature dependence of the rate constants for oxidation of organic compounds by peroxyl radicals in aqueous alcohol solutions. Z.B. Alfassi, R.E. Huie, M. Kumar, and P. Neta, J. Phys. Chern., .2Q, 767 (1992). Radiolytic reductions and oxidations in dimethyl sulfoxide solutions. Solvent effects on the reactivity of halogen atom complexes. M. Kumar and P. Neta, J. Phys. Chern., .2Q, 3350 (1992). Rate constants and temperature effects for reaction of CI{ radicals with unsaturated alcohols and hydrocarbons in aqueous and acetonitrile/water solutions. S. Padmaja, P. Neta, and R.E. Huie, J. Phys. Chern., .2Q, 3354 (1992). Dynamics of proton transfer from cation radicals. Kinetic and thermodynamic acidities of cation radicalsofNADH analogues. A. Anne, P. Hapiot, J. Moiroux, P. Neta, andJ.-M. Saveant, J. Am. Chern. Soc., 114, 4694 (1992). One-electron reduction of chromium(III) porphyrins. Formation of chromium(II) porphyrins or chromium(III) porphyrin 7T-radical anions. D.M. Guidi, P. Hambright, D. Lexa, P. Neta, and J.-M. Saveant, J. Phys. Chern., .2Q, 4459 (1992). Oxidation of chromium(III) porphyrins to their 7T-radical cations or to oxo-chromium(IV) porphyrins. D.M. Guidi, P. Neta, and P. Hambright, J. Chern. Soc. Faraday Trans.,~ 2013 (1992). Reaction of alkyl radicals with chromium porphyrins. D.M. Guidi, P. Neta, and P. Hambright, J. Chern. Soc. Faraday Trans., ~ 2337 (19.92). Ring reduction of N-methyl-tetrakis( 4-sulfonatophenyl)- porphinato-Co(II), -Ni(II), and -Cu(II) and subsequent methyl group migration. Reversible reaction between methyl radicals and Ni(II)TSPP. D.M. Guidi, P. Neta, P. Hambright, and R. Rahimi, Inorg. Chern., 11, 4849 (1992). One-electron reduction and demetallation of copper porphyrins. M. Kumar, P. Neta, T.P.G. Sutter, and P. Hambright, J. Phys. Chern., .2Q, 9571 (1992). Reactions of alkyl and fluoroalkyl radicals with nickel-, iron-, and manganese-porphyrins. D.M. Guidi, M. Kumar, P. Neta, and P. Hambright, J. Phys. Chern., .2Q, 9576 (1992). Electrochemical reduction of some a-substituted nitroalkanes. J.C. Ruhl, D.H. Evans, and P. Neta, J. Electroanal. Chern., 340, 257 (1992). Solvent effects on the rate of reaction of CI2·• and SO/ radicals with unsaturated alcohols. Z.B. Alfassi, S. Padmaja, P. Neta, and R.E. Huie, Int. J. Chern. Kinet., ~ 151 (1993). Steric and inductive effects on the basicity of porphyrins and on the site of protonation of porphyrin dianions. Radiolytic reduction of porphyrins and metalloporphyrins to chlorins or 67 phlorins. T.P.G. Sutter, R. Rahimi, P. Hambright, J.C. Bommer, M. Kumar, and P. Neta, J. Chern. Soc. Faraday Trans., ~ 495 (1993). Rate constants for reactions of S04 •• radicals in acetonitrile. S. Padmaja, Z.B. Alfassi, P. Neta, and R.E. Huie, Int. J. Chern. Kinet, ~ 193 (1993). Electrochemical and radiolytic mechanistic studies on the primacy step of phenol coupling involved in lignification. P. Hapiot, P. Neta, J. Pinson, C. Rolando, and S. Schneider, New J. Chern., 11, 211 (1993). Rate constants for some reactions of inorganic radicals with inorganic ions. Temperature and solvent dependence. S. Padmaja, P. Neta, and R.E. Huie, Int. J. Chern. Kinet., ~ 445 (1993). Electrochemical behavior of syringaldazine, a colorimetric redox reagent. P. Hapiot, J. Pinson, P. Neta, and C. Rolando, J. Electroanal. Chern., 353, 225 (1993). Rate constants for reactions of N03· radicals with organic compounds in water and acetonitrile. Z.B. Alfassi, S. Padmaja, P. Neta, and R.E. Huie, J. Phys. Chern., 97, 3780 (1993). Charge transfer complexes of bromine atoms with haloalkanes and alkanes. Z.B. Alfassi, R.E. Huie, J.P. Mittal, P. Neta, and L.C.T. Shoute, J. Phys. Chern., .21, 9120 (1993). Rate constants for reactions of perhaloalkylperoxyl radicals with alkenes. Z.B. Alfassi, R.E. Huie, and P. Neta, J. Phys. Chern., .21, 6835 (1993). Solvent effects on the rate constants for reaction of trichlomethylperoxyl radicals with organic reductants. Z.B. Alfassi, R.E. Huie, and P. Ne~, J. Phys. Chern., .21, 7253 (1993). Oxidative dimerization of phenolic aldehydes related to lignin formation. P. Hapiot, J. Pinson, C. Francesch, F. Mhamdi, C. Rolando, and P. Neta, J. Phys. Chern., in press. Electron transfer reactions between C60 and radical ions of metalloporphyrins and arenes. D.M. Guidi, P. Neta, and K.-D. Asmus, J. Phys. Chern., in press. Solvent effects on rate constants for one-electron oxidation of porphyrins. S. Marguet, P. Hapiot, and P. Neta, J. Phys. Chern., submitted. Formation and reactivity of phenylperoxyl radicals in aqueous solutions. Z.B. Alfassi, S. Marguet, and P. Neta, J. Phys. Chern., submitted. Electron transfer and alkyl transfer with cobaloximes in aqueous solutions. Kinetic studies by pulse radiolysis. M. Kumar, E. Natarajan, and P. Neta, J. Phys. Chern., submitted. Rate constants for reactions of iodine atoms in solution. Z. B. Alfassi, R. E. Huie, S. Marguet, E. Natarajan, and P. Neta, Int. J. Chern. Kinet., submitted. 68 SPECTROSCOPIC AND ELECTROCHEMICAL STUDIES OF PHOTOINDUCED ELECTRON TRANSFER REACTIONS AT SILVER SURFACES Therese M. Cotton, George Chumanov, Morgan Sibbald, Hong Tian and Robert A. Uphaus Ames Laboratory and the Department of Chemistry Iowa State University, Ames, Iowa 50011 Surface enhanced Raman scattering (SERS) provides a highly sensitive method for monitoring photoinduced electron transfer reactions between silver surfaces and adsorbed analytes. The formation of submonolayer quantities of intermediates and subsequent products can be monitored under CW and pulsed laser illumination. The SERS effect arises from excitation of plasmon resonances within small metal particles or roughened metal films, resulting in an enhancement of the electromagnetic field of the incident light. In some systems, strong interactions between the adsorbate and the metal can give rise to an enhancement of the molecular polariza.bility of the adsorbate (chemical enhancement) as well. These combined effects can produce as much as 106 fold increase in Raman scattering. Excitation of plasmon resonances in the metal particles or surfaces can also produce enhancement of other optical phenomena, including absorption, fluorescence and photochemical reactions. The latter is of particular interest because it may be possible to improve the efficiency of photochemical reactions by employing this effect. Our recent investigations of several different chemical systems have been directed towards this possibility. Photochemical Reduction of Viologens. Methyl viologen or asymmetric viologens containing a methyl and a long chain alkyl (hexadecyl (C16)) substituent adsorbed on Ag film electrodes were found to undergo one-electron reduction to the cation radical species when irradiated at liquid nitrogen temperatures. The SERS spectrum indicated that the radicals were stable indefinitely at this temperature, but oxidation to the dication occurred upon warming the electrode to room temperature, probably as a result of exposure to oxygen. The dependence of the photoreduction on excitation wavelength supported the role of surface plasmon excitation in this ,process. Maximum radical formation was observed with excitation between 413 and 520 nm, which is coincident with the metal plasmon resonance. The viologens have no absorbance in this region. The mechanism for the photoreduction of viologens at Ag surfaces may involve the formation of an electron hole pair at the interface, as proposed in chemical enhancement theories of the SERS effect. However, if this is the case, rapid recombination is expected and the SERS spectrum should be that of the dication. A possible explanation for the stability of the viologen cation radical is that its halogen counterion acts as a secondary electron donor in the photoprocess. Halide ions are known to adsorb strongly to Ag surfaces and rapid electron transfer 69 from the halide to the Ag may prevent the back electron transfer from the viologen radical to the Ag. To test this possibility, a number of other counterions are under investigation. Photochemical Reduction of Viologen Monolayers. It is known from solution studies that Chi a can function as electron donor to viologen under irradiation. Our recent results have shown that the photoreduction ofviologen by Chi a can also be accomplished in Langmuir Blodgett multilayers constructed on Ag film electrodes. A number of different monolayer configurations were prepared from Chi a, C1c;MV arachidic acid (as a spacer layer to determine the role of Chi a. In the simplest system composed of a Chi a monolayer deposited directly on the Ag surface followed by a monolayer ofCtc;MV, the photoreduction ofviologen was observed by monitoring the SERS spectrum. As in the MV studies, the radical was stable indefinitely at liquid nitrogen temperature. However, no evidence for the Chi cation radical was obtained from the SERS spectrum, suggesting that Chi is rapidly reduced by the Ag surface following electron transfer. Other multilayer constructions employing arachidic acid as a spacer layer configurations are currently under study to verify the role of Chi as a sensitizer. Photoreduction of C01 at Ag Surfaces. As noted above, excitation of plasmon resonances in small metal particles can produce enhancement of the absorption cross section of adsorbates and increased photochemical efficiencies. Silver film electrodes were used to test the possibility that excitation within the plasmon resonance can facilitate the reduction of C02• The photoelectrochemical response of smooth and electrochemically roughened thin film electrodes were compared as a function of excitation wavelength. In the case of the roughened films, significantly greater current was produced at the roughened surface and the wavelength response corresponded with the plasmon absorption band. Modification of the roughened Ag surface with viologens resulted in an even greater photocurrent, suggesting that viologen acts as a mediator. Experiments in progress are directed toward identifying the photoproduct and determining the photochemical efficiency of these systems. Ag Colloids as Microelectrodes. A new method, based upon laser ablation of metal films using high energy laser pulses from a Nd:Yag laser, was developed for preparing very pure and uniform metal colloids. An important advantage of this method is that the colloids have no extraneous ions or chemicals, as required in previous procedures which are based upon chemical reduction of metal salts. A study of the effect of halide ions (Cl-, Brand I-) on the rest potential of these nanoparticles was recently completed. By monitoring the redox state of cytochrome c as a probe molecule, it was determined that their rest potential was dependent upon the specific halide ion and its concentration. SERS was used to monitor the reduction of cytochrome c as a function of halide ion concentration. These studies clearly demonstrated the ability of the metal particles to function as microelectrodes and store charge for subsequent redox reactions. Future Studies. The results summarized above strongly support the potential of surface plasmons for enhancing photochemical reactions at small metal particles. Complete characterization of the systems described above in terms of quantum yields and product identification will be continued. In addition, time resolved Raman spectroscopy and SERS will be utilized in an attempt to obtain mechanistic information. 70 Pubtications(1992-1994) J. C. Rubim, J.-H. Kim, E. Henderson, and T. M. Cotton, "Surface Enhanced Raman Scattering and Atomic Force Microscopy of Brass Electrodes in Sulfuric Acid Solution Containing Benzotriazole and Chloride Ion", Appl. Spectrosc., £l, 80-84 (1993). K. Sokolov, P. Khodorchenko, A. Petukhov, I. Nabiev, G. Chumanov, and T. M. Cotton, "Contributions of Short Range and Classical Electromagnetic Mechanisms to Surface Enhanced Raman Scattering from Several Types ofBiomolecules Adsorbed on Cold-Deposited Island Films", Applied Spectrosc., 47(4), 515-522 (1993). C. J. Eckhardt, N. M. Peachey, D. R Swanson, J. Y. Wang, R A. Uphaus, G. P. Lutz and P. Beak, "Crystal Engineering in Two Dimensions: Spontaneous Optical Resolution of Monolayer Single Crystals", Langmuir, .t 2591 (1992). C. J. Eckhardt, N. M. Peachey, D. R Swanson, J. M. Tackas, M.A. Khan, X. Gung, J.-H. Kim, J. Y. Wang and R A. Uphaus, "Separation ofChiral Phases in Monolayer Crystals ofRacemic Amphiphiles", Nature, 362,614 (1993). J. Neddersen, G. Chumanov, and T. M. Cotton, "Laser Ablation of Metals: A New Method for Preparing SERS Active Colloids", Appl. Spectrosc., 47, 1959-1964 (1993). D. Vaknin, J. Wang, R A. Uphaus, M. LOsche and K. Kjaer, "C6o Fullerene-Amine Adduct Monolayers at Air Water Interfaces, Studied by Neutron and X-Ray Reflection and X-Ray Diffraction", Thin Solid Films, (in press). J. Y. Wang, S. Ameenuddin, D. A. Rintoul and R A. Uphaus, "Formation ofNanoscale Cadmium Sulfide within Channel Protein Monolayers", Thin Solid Films, (in press). C. J. Eckhardt, N. M. Peachy, D. R Swanson, P. Dussault, J. Tackas, P. Beak and R A. Uphaus, "Crystal Engineering in Two Dimensions: Control of Planar Packing by Design ofNew Amphiphiles", Thin Solid Films, (in press). J. Y. Wang, J. Wang, D. A. Jaeger and R. A. Uphaus, "Monolayer Study of Cleavable Phospholipids", Thin Solid Films, (in press). J. Wang, G. Chumanov, T. M. Cotton and R A. Uphaus, "Mode of Transfer ofChorophyll Monolayers and Resultant Enhanced Raman Spectra", Thin Solid Films, (in press). J. Y. Fang and R A. Uphaus, "Surface Pressure-Induced Layer Growth of a Monolayer at the Air-Water Interface", Langmuir, (in press). E. S. Brandt, and T. M. Cotton, "Surface-Enhanced Raman Scattering", in Physical Methods in Organic Chemistry, Vol. IXB, Investigations ofSurfaces and Interfaces-Part B, B. W. Rossiter and R C. Baetzold, editors, John Willey & Sons, Inc., New York, New York, 1993, pp. 633-718. T. M. Cotton, G. Chumanov, J.-Y. Wang, and R A. Uphaus, "Probing Electron Transfer Reactions in Model Photosynthetic Systems by Raman Spectroscopy", in Fifth International Conforence on the Spectroscopy of Biological Molecules T. Theophanides, J. Anastassopoulou, and N. Fotopoulos, editors, Kluwer Academic Publishers, Dordrecht, The Netherlands, 1993, pp. 279-284. 71 FUNDAMENTAL STUDIES IN PHOTOSYNTIIESIS FOR THE PRODUCITON OF FUELS AND CHEMICALS FROM RENEWABLE RESOURCES E. Greenbaum, J. W. Lee, I. Lee, C. V. Tevault, C. Y. Ma, D.P. Allison, and R. J. Warmack Oak Ridge National Laboratory P. 0. Box 2008 Oak Ridge, TN 37831-6194 Scope of Project Photosynthesis research at Oak Ridge National Laboratory is focused on the physico-chemical mechanisms of conversion of light energy into stored chemical energy. The key object of this work is a basic understanding of the physics and chemistry of the reaction centers of photosynthesis and their incorporation into assembled arrays for energy conversion and optical signal processing. In addition, a new photosynthetic mimetic reaction has been discovered in which real-world lignocellulosic substrates such as poplar, pine, and balsa woods are capable of photosensitized direct UV-photoconversion to molecular oxygen and volatile hydrocarbons. Molecular Electronics of a Single Photosystem I Reaction Center Photosystem I reaction centers (PSI) were imaged by scanning tunneling microscopy (STM) (Fig. 1). The thylakoids were isolated from spinach chloroplasts, and PSis were extracted from thylakoid membranes. Because thylakoids are relatively thick nonconductors, they were sputter coated with Pd/Au before imaging by STM. Well-defined, images were obtained of both single and aggregated thylakoids. PSI photosynthetic centers and chemically platinized PSI were investigated without sputter-coating. They were mounted on flat gold substrates that had been treated with mercaptoacetic acid to help bind the proteins. Using tunneling spectroscopy, the PSis displayed a semiconductor-like response with a band gap of 1.8 eV (Fig. 2). Lightly platinized (platinized for one hour) PSis displayed diode-like conduction that resulted in dramatic contrast changes between images taken with opposite bias voltages. The electronic properties of this system were stable under long term storage. 1500 4 -<( -PSI Q. 1000 -dl/dV 3 -c:· ...~ 500 :J 2 Q. () 0 a: 0> < .5 (i) ·500 c: c: :J ·1000 0 1- ·1500 ·1 ·1000 ·500 0 500 1000 Bias Voltage (mV) Fig. 1. STM images of isolated Photosystem I Fig. 2. I-V curve of bare PSI compared with the differential reaction centers. conductance, dl/dV, versus bias voltage from the same PSI data. 72 Direct Photoconversion of Biomass to Molecular Oxygen and Volatile Hydrocarbons The simultaneous photoevolution of molecular oxygen and volatile hydrocarbons was observed when ferric ions and other photosensitizers such as semiconducting oxides were implanted in cellulose and wood under high pressure and irradiated with near-UV and visible light. Control experiments with purified microcrystalline cellulose and lignin indicated that both of these components of wood could undergo phototransformation. However, in the case of purified lignin, only volatile hydrocarbons were observed. Although UV-induced structural degradation of lignocellulosic substrates is well-known, the present studies reveal that in an inert atmosphere a novel photochemistry occurs that is qualitatively different from that which occurs in air. In an inert atmosphere the photochemistry bears a formal analogy to normal photosynthesis in that molecular oxygen is photoevolved and a reduced photoproduct, more reduced than the consumed substrate, is produced. This analogy is discussed in the context of oxidation-reduction levels of the photoproducts, the source of reductant for the photoredox reactions, and elementary theories of the photophysics and photochemistry in reaction cavities of the structured matrices in which the light-induced reactions occur. The Role of Carbon Dioxide in light-Activated Hydrogen Production by Chlo.mydomonas reinhardtii Light-activated hydrogen and oxygen evolution as a function of C02 concentration in helium were measured for the unicellular green alga Chlamydomonas reinhardtii. The concentrations were 58, 30, 0.8, and 0 ppm C02• The objective of these experiments was to study the differential affinity of CO:/HC03- for their respective Photosystem II and Calvin cycle binding sites vis-a-vis photoevolution of molecular oxygen and the competitive pathways of hydrogen photoevolution and C02 photoassimilation. The maximum rate of hydrogen evolution occurred at 0.8 ppm H 2, whereas the maximum rate of oxygen evolution occurred at 58 ppm C02• The key result of this work is that the rate of photosynthetic hydrogen evolution can be increased by, at least partially, satisfying the Photosystem II C02/HC03- binding site requirement without fully activating the Calvin-Benson C02 reduction pathway. Data are presented which plot the rates of hydrogen and oxygen evolution as functions of atmospheric C02 concentration in helium and light intensity. The stoichiometric ratio of hydrogen to oxygen changed from 0.1 at 58 ppm to approximately 2.5 at 0.8 ppm. A discussion of partitioning of photosynthetic reductant between the hydrogen/hydrogenase and Calvin-Benson cycle pathways is presented. Future Work Future work is a logical extension of the promising results that have been obtained thus far. In particular, additional studies on the molecular optoelectronic properties of platinized and native Photosystem I reaction centers will be performed. In addition, further studies on the sensitized direct photoconversion of lignocellulosics will also be performed. The objective of this work will be to determine the effects of specific photocatalysts on yield and product distribution of the photoconversion process. 73 Publications (1992-1994) E. Greenbaum, "Kinetic Studies of Interfacial Photocurrents in Platinized Chloroplasts," J. Phys. Chem. 96, 514-516 (1992). C. A. Woodward, J. M. Macinnis, S. N. Lewis, and E. Greenbaum, "Chemical Interaction of Flue Gas Components with the Growth of Cyanidium caldarium," AppL Biochem. Biotechnol 34/35,819- 826 (1992). R. M. Cinco, J. M. Macinnis and E. Greenbaum, "The Role of Carbon Dioxide in Light-Activated Hydrogen Production by Chlamydomonas reinhardtii," Photosyn. Res. 38, 27-33 (1993). J. M. Macinnis, K. A. McDonald, M. E. Sigman, E. Greenbaum, "Simultaneous Measurement of Photosynthetic Oxygen Evolution and Carbon Dioxide Assimilation under Anaerobiosis," Photochem. PhotobioL, 58, 718-723 (1993). T. Matsunaga and E. Greenbaum, "Introduction to Applied Biological Research," Appl Biochem. Biotechnol. 39/40. 133 (1993). E. V. Mielczarek, E. Greenbaum, and R. S. Knox (Eds. ), Biological Physics, American Institute of Physics Press, New York (1993). J. W. Lee, C. V. Tevault, S. L. Blankinship, R. T. Collins and E. Greenbaum, "Photosynthetic Water Splitting: In Situ Photoprecipitation of Metallocatalysts for Simultaneous Photoevolution of Molecular Hydrogen and Oxygen," in press, Energy and Fuels (1994). E. Greenbaum, "Application of Photosynthesis to the Production of Molecular Hydrogen by Water Splitting." Proc. Ind. Acad. Sci. in press, (1994). E. Greenbaum, "Biomolecular Electronics and Applications," Encyclopedia ofMolecular Biology and Biotechnology, VCH Publishers, in press (1994). I. Lee, J. W. Lee, R. J. Warmack, D.P. Allison, and E. Greenbaum, "Molecular Electronics of a Single Photosystem Reaction Center Studied by Scanning Tunneling Microscopy and Spectroscopy," Proc. Natl Acad. U.SA, submitted (1994). E. Greenbaum, C. V. Tevault, and C. Y. Ma, "New Photosynthesis: Direct Photoconversion of Biomass to Molecular Oxygen and Volatile Hydrocarbons," Energy & Fuels, submitted (1994). J. W. Lee, T. G. Owens and E. Greenbaum, "Chemical Platinization and its Effect on Excitation Transfer Dynamics and P700 Photooxidation Kinetics in Isolated Photosystem I," Biophys. J., submitted (1994). J. W. Lee, I. Lee. R., and, E. Greenbaum, "Platinization: A Novel Technique to Anchor Photosystem I Reaction Centers onto a Metal Surface at Biological Temperature and pH, Biophs. J., submitted (1994). E. Greenbaum, "Biomolecular Electronics and Applications," article for the Encyclopedia of Molecular Biology (in 6 Volumes + Index), VCH Publishers, in preparation (1995). 74 Thursday Morning Session VIII Jack Fajer, Chairman THEORETICAL STUDIES OF ELECTRON TRANSFER IN COMPLEX :MEDIA David Chandler Department of Chemistry, University of California Berkeley, CA 94 720 The primary electron transfer in a photosynthetic reaction center presents a set of fundamental and interesting questions about the chemical physics of electron transfer in complex systems. For this reason, and spurred by the experimental interests among many of our colleagues in the Division of Chemical Sciences, we have embarked on a theoretical program to understand the mechanism for this and related processes. Our first step was a molecular dynamics simulation of the nuclear motions in the reaction center Rps. viridis. The Marchi-Gehlen-Chandler-Newton (MGCN) model arose out of this work [1]. It makes the unambiguous prediction that the primary charge transfer occurs as a nearly activationless super-exchange process. The simulation reveals a variety of motions that remain correlated beyond a picosecond [2], as to be expected in a proteic system. These motions include slow energy gap fluctuations. Within the MGCN model, we have shown how these fluctuations provide an explanation of experimentally observed non exponential primary charge transfer kinetics [3]. Several experimental facts are explained by the MGCN model. These include the temperature dependence of the primary transfer and the relative back transfer rates. Nevertheless, many questions remain concerning its validity and predictive power. For example, the positioning of diabatic surfaces in the model is at odds with suggestions made by some other theorists. There is concern that this positioning is sensitive to amino acid charge states, and these states are poorly understood. There is also uncertainty about the importance of the membrane and aqueous solution that surrounds the proteic system. The quantitative implications of the MGCN model to other reaction centers like those examined in mutant studies is also not known. This lecture will address several of these questions introducing the results of several new calculations. In doing so, the electrostatics of the reaction center will be discussed with a statistical description that captures the essential details of the system. Concerning dynamics, the significance of coherence will be discussed. The nature of long time scale fluctuations will be described, and a stochastic model will be introduced to treat the effects of these fluctuations on charge transfer kinetics. This stochastic model should be useful in a number of circumstances, many of which could involve electron transfer occurring in complex systems distinct from proteins. Finally, this lecture will attempt to highlight important topics that remain poorly understood and are therefore natural targets for future research. 1. M. Marchi, J.N. Gehlen, D. Chandler and M. Newton, J. Am. Chern. Soc. 115, 4178 (1993). 2. D. Chandler, J.N. Gehlen and M. Marchi, "On the Mechanism of the Primary Charge Transfer in Photosynthesis," in AlP Conference Proceedings 298. Ultrafast Reaction Dynamics and Solvent Effects. Royaumont. France. 1993, ed. by Y. Gauduel and P.J. Rossky [American Institute of Physics, New York, 1994], p. 50. 3. J.N. Gehlen, M. Marchi and D. Chandler, Science 263,499 (1994). 75 Publications ( 1992-1994) 1. J.S. Bader and D. Chandler, "Computer Simulation Study of the Mean Forces between Ferrous and Ferric Ions in Water," J. Phys. Chern. 96, 6423 (1992). 2. J.N. Gehlen, D. Chandler, H.J. Kim and J. T. Hynes, "Free Energies of Electron Transfer," J. Phys. Chern. 96, 1748 (1992). 3. J.N. Gehlen and D. Chandler, "Quantum theory for free energies of electron transfer," J. Chern. Phys. 97, 4958 (1992). 4. M. Marchi, J.N. Gehlen, D. Chandler and M. Newton, "Diabatic Surfaces and the Pathway to Primary Electron Transfer in a Photosynthetic Reaction Center," J. Am. Chern. Soc., 115, 4178 (1993). 5. D. Chandler, J.N. Gehlen and M. Marchi, "On the Mechanism of the Primary Charge Transfer in Photosynthesis," in AlP Conference Proceedings 298. Ultrafast Reaction Dynamics and Solvent Effects. Royaumont. France. 1993, ed. by Y. Gauduel and P.J. Rossky [American Institute of Physics, New York, 1994], p. 50. 6. J.N. Gehlen, M. Marchi and D. Chandler, "Dynamics Affecting the Primary Charge Transfer in Photosynthesis," Science 263, 499 (1994). 76 FROM ET TO ESE Jau Tang, Marion C. Thumauer and James R. Norris Chemistry Division, Argonne National Laboratory Argonne, IL 60439 In our recent theoretical work, we have examined some regimes beyond the classical Marcus description of electron transfer (ET) reactions.· According to the superexchange mechanism of McConnell, the spacer between donor and acceptor or the intermediate medium can provide virtual states for the long range electronic coupling and therefore can assist ET over a long distance. In long range intramol~ular or intermolecular ET reactions; the Condon approximation is ideal in systems with a rigid spacer or with the donor and the acceptor firmly held to rigid matrices. As pointed out by Closs, this approximation is questionable in intramolecular ET with a floppy spacer and in intermolecular ET in liquid solution. Using the spin-boson model, assuming a fluctuating electronic coupling matrix element, we have shown that ET reactions generally diminish due to a fluctuating intermediate medium. However, if the correlation time rv for the fluctuation is so short that fzlrv is comparable to the solvent reorganization energy or free energy, ET reactions can be enhanced. To study the effects of quantum modes on the energy gap law for electron-transfer reaction, the spin-boson model was used to derive a closed-form rate formula using the steepest descent method. In addition, we have extended the ordinary linear spin-boson model to include the effects of anharmonic potentials and the case of two harmonic potentials for the donor and the acceptor having a different force constant. Interesting asymmetry between the normal and the inverted Marcus regimes in the rate plot will be illustrated. We have also shown that the simple Marcus concept of using only one "collapsed" reaction coordinate to describe many solvent modes in a multi-dimensional reaction coordinate is valid only for Gaussian processes. This is true for reactions at high temperature and for systems with a linear spin-boson coupling. ForET reactions at low temperature as well as systems involving a non-linear spin-boson coupling, the simple description using only one "collapsed" reaction coordinate fails. We have applied the stochastic Liouville approach to investigate the superexchange and the sequential ET reactions involved in a three-component system. All three regimes, the non degenerate, the quasi-degenerate and the degenerate cases, were examined. In the last two cases where the direct free energy gap is comparable to the electronic coupling strength, we have predicted interesting resonant effects on both superexchange and sequential rate constants due to energy level crossing. We have also generalized the spin-boson model to a three-component system, involving a donor, a bridging intermediate, and a distant acceptor as a model for the primary electron transfer in a photosynthetic reaction center. In the ordinary spin-boson model a fictitious spinor (S = 112 for SU(2) group) represents a two-level system of the electronic states of the donor and the acceptor. In this work, a vector spin for SU(3) group with S = 1, which was first introduced in the quark model by Gell-Mann, is used. We have derived an analytical rate expression for the superexchange process, considering the nuclear quantum tunneling effects due to high-frequency intramolecular motion. We have examined the relationship between the degree of correlation among the spin-boson couplings to three electronic states and the reorganization energy for the superexchange and the 77 sequential processes. The condition for a small reorganization energy for the superexchange mechanism requires a large degree of correlation among the energy fluctuations on the donor and the acceptor sites due to solvent motion. For a better understanding of the connection between the correlation in energy fluctuation and the reorganization energy, we have introduced a simple triangle diagram with the length for each side of the triangle representing a scaled reorganization energy for two sequential processes from the donor to the intermediate (side 12), from the intermediate to the acceptor (side 23), and the superexchange process directly from the donor to the acceptor (side 13). We have shown that if the superexchange reorganization energy is small, the triangle has to be an acute triangle with the side 13 shortest. Our analyses provide a direction of how to design a photochemical apparatus with efficient superexchange electron-transfer reactions. In our recent work on electron spin echo (ESE) spectroscopy of a photo-induced spin correlated radical pair system, we have examined the effects of electronic exchange and electron electron dipolar interactions on electron spin echo envelope modulation and free induction decay. We have shown that by using a Hahn echo pulse sequence (90°x - T - 180°x) with a strong microwave field, the normal "in-phase" echo along y-axis vanishes. Instead, an echo appears along the x-axis, which we call the "out-of-phase" echo. This abnormal phenomenon is shown to be a peculiar result of the zero-quantum coherence due to singlet-triplet mixing in the spin-correlated radical pair system. In addition, we have demonstrated that the electronic exchange and dipolar interactions can induce deep modulation or oscillation on the out-of-phase echo envelope. Using computer simulation, we have shown how to use the envelope modulation of the abnormal echo to measure small exchange and dipolar couplings. These couplings are not readily obtainable from the conventional unresolved, polarized continuous wave EPR spectrum which are often broadened by hyperfine inhomogeneity. In addition to the above out-of-phase ESE modulation technique, we have also developed a novel method to generate and detect multiple quantum signals by using a phase-cycled pulse sequence to examine a photo-induced radical pair system. By a special arrangement of the phase cycling for the microwave pulses, one can specifically excite and detect the zero, single and double quantum signals. These zero and double quantum signals are normally forbidden transitions but can now be measured with our coherence transfer technique. This new multiple quantum technique can be used to obtain useful structural and dynamic parameters of the radical pair system in photo-induced electron transfer reactions. In the future, we will extend the spin-boson model to include the field effects and to consider the regime where the direct free energy gap is small and comparable to the electronic couplings. This resonance regime is attainable through application of a strong electric field, and thus a clear theoretical understanding should provide more information about the superexchange mechanism in photosynthesis. For a better understanding of the coherent rate processes as observed for reaction centers at low temperatures, the master equation approach will be used in our future investigation. In our future work on electron spin polarization, we will include possible electron transfer or diffusive process for the radical pair during the time of microwave irradiation or pulse sequence so that the intermediate or final species and their kinetics can be determined. 78 PUBLICATIONS (1992-1994) TIIE GENERAL TREATMENT OF DYNAMIC SOLVENT EFFECTS IN ELECTRON TRANSFER AT lllGH TEMPERATURE Z. Wang, J. Tang and J. R. Norris J. Chem. Phys. 97, 7251 (1992) TIIE TIME DEVELOPMENT OF TIIE MAGNETIC MOMENT OF CORRELATED RADICAL PAIRS Z. Wang, J. Tang and J. R. Norris J. Mag. Reson. 97, 322 (1992) A SHUTTER DESIGN FOR TIME DOMAIN STUDIES USING SYNCHROTRON RADIATION AT TIIE ADVANCED PHOTON SOURCE J. R. Norris, M. K. Bowman, L. Chen, J. Tang, M. C. Thumauer, G. S. Knapp and P. A. Montano Rev. Sci. Instrum. 63, 1172 (1992) TIIE GENERAL TREATMENT OF SUPEREXCHANGE VS. SEQUENTIAL ELECTRON TRANSFER IN A THREE-COMPONENT SYSTEM J. Tang, Z. Wang and J. R. Norris J. Chem. Phys. 99, 979 (1993) ON SUPEREXCHANGE ELECTRON-TRANSFER COUPUNG FOR A THREE COMPONENT SYSTEM J. Tang and. J. R. Norris Chem. Phys. 175, 337 (1993) EFFECTS OF A FLUCTUATING ELECTRONIC COUPUNG MATRIX ELEMENT ON ELECTRON TRANSFER RATE J. Tang J. Chem. Phys. 98, 6263 (1993) TIIE EFFECTS OF QUANTUM MODES ON TIIE ENERGY GAP LAW FOR ELECTRON-TRANSFER REACTIONS J. Tang J. Chem. Phys. 99, 5828 (1993) TIIE EFFECTS OF ANHARMONICITY ON ELECTRON-TRANSFER REACTIONS J. Tang , Chem. Phys., 179, 105 (1994) 79 ( ON SUPEREXCHANGE ELECTRON-TRANSFER REACTIONS IN A 1HREE COMPONENT SYSTEM: TilE SPIN-BOSON MODEL FOR PHOTOSYNTHESIS J. Tang Chem. Phys. Lett. 217, 55 (1994) ELECTRON SPIN ECHO ENVELOPE MODULATION DUE TO EXCHANGE AND DIPOLAR INTERACTIONS IN A SPIN-CORRELATED RADICAL PAIR J. Tang, M. C. Thumauer and J. R. Norris Chem. Phys. Lett. 219, 283 (1994) SEQUENTIAL AND SUPEREXCHANGE ELECTRON-TRANSFER REACTIONS IN A THREE-COMPONENT SYSTEM J. Tang Chem. Phys. (in press) RESONANCE EFFECTS ON SUPEREXCHANGE AND SEQUENTIAL ELECTRON TRANSFER REACTIONS DUE TO ENERGY-LEVEL CROSSING J. Tang Chem. Phys. (submitted) ELECTRON-TRANSFER REACTIONS. INVOLVING HARMONIC POTENTIALS WI1H A DIFFERENT FORCE CONSTANT: TilE ZUSMAN APPROACH VERSUS TilE SPIN BOSON MODEL J. Tang Chem. Phys. (submitted) ON SUPEREXCHANGE ELECTRON-TRANSFER REACTIONS INVOLVING 1HREE PARABOLOIDAL POTENTIAL SURFACES IN A TWO-DIMENSIONAL REACTION COORDINATE J. Tang and J. R. Norris J. Chem. Phys. (submitted). MULTIPLE QUANTUM COHERENCE IN A SPIN-CORRELATED RADICAL PAIR SYSTEM J. Tang and J. R. Norris Chem. Phys. Lett. (submitted) 80 PROBING REACTION CENTER STRUCTURE USING PHOTOCHEMICALLY GENERATED COHERENT QUANTUM BEATS' James R. Nonisa, Marion Thumaue~, Stefan Weberb, Ernst Ohmesb and Gerd Kotheb achemistry Division, Argonne National Laboratory, Argonne, IL 60439, USA hrostitu~ fiir Physikalische Chemie, Universitat Stuttgart, 70550 Stuttgart, Germany Oscillatory behavior in the stimulated emission from photosynthetic bacterial reaction centers has been observed by Martin et al. and by Fleming et al. immediately after femtosecond laser excitation. These oscillations, known as quantum beats, reflect coherence in the excited state and/ or in the ground state wave functions of the reaction center donor acceptor complex. Likewise the irradiation of photosynthetic reaction centers with a short pulse of visible light creates coherence in the pair of free radicals that are formed during the initial charge separation process of photosynthesis. The coherence of these radical pairs can be probed by time domain EPR and serves as a structural probe requiring no special single crystals. The rapid generation of the radical pair, formed between the primary donor special pair and the first quinone acceptor, results in a characteristic EPR spectrum that exhibits both emissive and enhanced absorption lines. These EPR spectra are highly time dependent as a result of these coherent zero quantum beats. For straightforward technical reasons these zero quantum EPR beats have been observed only in fully deuterated systems. Both the characteristic emissive-absorptive EPR signal and the time domain oscillations contain structural information on the relative orientations between the special pair donor and the secondary quinone acceptor. We have examined both fully deuterated green plant photosystem I preparations from Synechococcus lividus as well as fully deuterated, bacterial reaction centers from R-26 Rhodobacter sphaeroides. By calculating the zero quantum beats and comparing with experiment certain structural features of the reaction center can be established. Using the structural information provided by the calculated and experimental EPR spectra of Figure 1 and especially the oscillations of the quantum beats, the structures ofphotosystem I and bacterial reaction centers can be compared. Using this EPR data the relative orientation between the special pair donor and the secondary quinone acceptor has been predicted for photosystem I in advance of the high resolution x-ray structure. Future efforts will focus on determining the structure in bacterial reaction centers, with and without Fe, and in determining the structure of artificial donor acceptor complexes using zero quantum beats and other advanced time domain EPR techniques. 81 25 20 ml2n:/ MHZ 15 10 5 0.._...... ,__ 350.4 350.8 351.2 351.6 352 352.4 8 /mT 0 25 20 ml2x I MHz 15 10 5 a ...... --~ 350.4 350.8 351.2 351.6 352 352.4 8 /mT 0 Figure 1. Comparison of experimental and calculated zero quantum beats in photosystem I of fully deuterated Synechococcus lividus. Top: experimental; Bottom: calculated. Vertical axis represents the frequency of the quantum beats. Horizontal axis represents the external magnetic field. 82 PUBLICATIONS (1992-1994) PRIMARY CHARGE SEPARATION IN PHOTOSYNTHETIC BACTERIAL REACTION CENTERS AND FEMTOSECOND SOLVATION DYNAMICS C.-K. Chan, S. J. Rosenthal, T. J. DiMagno, L. X-Q. Chen, X. Xie, J. R. Norris and G. R. Fleming Dynamics and Mechanisms of Photoinduced Electron Transfer and Related Phenomena N. Mataga, T. Okada and H. Masuhara, Eds., Elsevier Publishers, 1992, pp. 105-122 CHARACTERIZATION OF CHARGE SEPARATION IN MEMBRANE SPANNING PROTEIN REACTION CENTERS OF BACTERIAL PHOTOSYNTIIESIS T. J. DiMagno, C-K. Chan, D. K. Hanson, M. Schiffer, G. R. Fleming and J. R. Norris Charge and Field Effects in Biosystems - Ill, M. Allen, Ed., Birkhauser, Boston, 1992, pp. 333-340 RECENT EXPERIMENTAL RESULTS FOR TIIE INITIAL STEP OF BACTERIAL PHOTOSYNTHESIS T. J. DiMagno, S. J. Rosenthal, X. Xie, M. Du, C.-K. Chan, D. Hanson, M. Schiffer, J. R. Norris and G. R. Fleming The Photosynthetic Bacterial Reaction Center II: Structure, Spectroscopy, and Dynamics, J. Breton and A. Vermeglio, Eds., Plenum Press, London, NATO ASI Series, 1992, pp. 209-217 STUDY OF REACTION CENTER FUNCTION BY ANALYSIS OF TIIE EFFECTS OF SITE-SPECIFIC AND COMPENSATORY MUTATIONS M. Schiffer, C.-K. Chan, C-H. Chang, T. J. DiMagno, G. R. Fleming, S. Nance, J. Norris, S. Snyder, M. Thumauer, D. M. Tiede and D. K. Hanson The Photosynthetic Bacterial Reaction Center II: Structure, Spectroscopy, and Dynamics, J. Breton and A. Vermeglio, Eds., Plenum Press, London, NATO ASI Series, 1992, pp. 351-361 TIME-RESOLVED EPR AND FOURIER TRANSFORM EPR STUDY OF TRIPLET C60. 13 DETERMINATIONS OF T1 AND TIIE C HYPERFINE COUPliNG CONSTANT D. Zhang, J. R. Norris, P. J. Krusic, E. Wasserman, C. C. Chen and C. M. Lieber J. Phys. Chern. 97, 5886-5889 (1993) MEASUREMENT OF TIIE g-TENSOR OF TIIE P700+ SIGNAL FROM DEUTERATED CYANOBACTERIAL PS-I PARTICLES T. F. Prisner, A. E. McDermott, S. Un, J. R. Norris, M. C. Thumauer and R. G. Griffm Proc. Natl. Acad. Sci., USA, 90, 9485-9488 (1993) 83 FREE ENERGY AND ENTROPY CHANGES IN VERTICAL AND NONVERTICAL TRIPLET ENERGY TRANSFER PROCESSES BETWEEN RIGID AND NONRIGID MOLECULES. A LASER PHOTOLYSIS STUDY D. Zhang, G. L. Closs, D. D. Chung and J. R. Norris J. Am. Chern. Soc. 115, 3670-3673 (1993) ELECTRON SPIN POLARIZATION IN SEQUENTIAL ELECTRON TRANSFER. AN EXAMPLE FROM IRON-CONTAINING PHOTOSYNTIIETIC BACTERIAL REACTION CENTER PROTEINS S. W. Snyder, A. L. Morris, S. R. Bondeson, J. R. Norris and M. C. Thumauer J. Am. Chern. Soc. 115, 3774-3775 (1993) PRIMARY CHARGE SEPARATION IN MUTANT REACTION CENTERS OF Rb. capsulatus Y. Jia, T. J. DiMagno, C.-K. Chan, Z. Wang, M. S. Popov, M. Du, D. K. Hanson, M. Schiffer, J. Norris and G. R. Fleming J. Phys. Chern. 97 13180-13191 (1993) TIIE PHOTOSYNTIIETIC REACTION CENTER J. Deisenhofer and J. R. Norris, Eds. Academic Press, Vols. I and II, 1993 STRUCTURE AND FUNCTION OF TIIE PHOTOSYNTIIETIC REACTION CENTER OF Rhodobacter sphaeroides · M. Schiffer and J. R. Norris The Plwtosynthetic Reaction Center, J. Deisenhofer and J. R. Norris, Eds., Vol. I, Chapter 1, Academic Press, 1993, pp. 1-12 ELECTRON TRANSFER IN PHOTOSYNTIIETIC BACTERIA T. J. DiMagno and J. R. Norris The Plwtosynthetic Reaction Center, J. Deisenhofer and J. R. Norris, Eds., Vol. II, Chapter 6, Academic Press, 1993, pp. 105-132 MODEL PHOTO REACTION CENTERS VIA GENETIC ENGINEERING Z. Wang, T. J. DiMagno, C-K. Chan, M. Popov, J. Tang, D. Hanson, M. Schiffer, G. R. Fleming and J. R. Norris Photochemical and Photoelectrochemical Conversion and Storage of Solar Energy, Z. W. Tian andY. Cao, Eds., Inti. Academic Publishers, Beijing, 1993, pp. 1-10. TIIE GENERAL TREA1MENT OF SUPEREXCHANGE VERSUS SEQUENTIAL ELECTRON TRANSFER IN A TIIREE-COMPONENT SYSTEM J. Tang, z. Wang and J. R. Norris J. Chern. Phys. 99, 979-984 (1993) 84 ON SUPEREXCHANGE ELECTRON-TRANSFER COUPUNG FOR A THREE COMPONENT SYSTEM J. Tang and J. R. Norris Chemical Physics, 175, 337-342 (1993) INHOMOGENEOUS ELECTRON TRANSFER KINETICS IN REACTION CENTERS OF BACTERIAL PHOTOSYNTHESIS Z. Wang, R. M. Pearlstein, Y. Jia, G. R. Fleming and J. R. Norris Chemical Physics 176, 421-425 (1993) IDGH TIME RESOLUTION ELECTRON PARAMAGNETIC RESONANCE OF UGHT INDUCED RADICAL PAIRS IN PHOTOSYNTHETIC BACTERIAL REACTION CENTERS: OBSERVATION OF QUANTUM BEATS G. Kothe, S. Weber, E. Ohmes, M. C. Thumauer and J. R. Norris J. Am. Chern. Soc. (In press) SPECTRAL DIFFUSION AND 14N QUADRUPOLE SPUTTINGS IN ADMR HOLE BURNING SPECTRA OF PHOTOSYNTHETIC REACTION CENTERS J. W. Greis, A. Angerhofer, J. U. von Schutz, A. Struck, H. Scheer and J. R. Norris J. Chern. Phys. (In press) ESE MODULATION DUE TO EXCHANGE AND DIPOLAR INTERACTIONS IN A SPIN-CORRELATED RADICAL PAIR J. Tang, J. R. Norris and M. C. Thumauer Chern. Phys. Lett. (In press) TRANSIENT EPR OF UGHT-INDUCED SPIN-CORRELATED RADICAL PAIRS: MANIFESTATION OF ZERO QUANTUM COHERENCE G. Kothe, S. Weber, E. Ohmes, M. C. Thumauer and J. R. Norris J. Phys. Chern. (In press) MULTIPLE-QUANTUM COHERENCE IN A SPIN-CORRELATED RADICAL PAIR SYSTEM J. Tang and J. R. Norris Chern. Phys. Lett. (submitted) ON SUPEREXCHANGE ELECTRON-TRANSFER REACTIONS INVOLVING THREE PARABOLOIDAL POTENTIAL SURFACES IN A TWO-DIMENSIONAL REACTION COORDINATE J. Tang and J. R. Norris J. Chern. Phys. (submitted) 85 PRIMARY CHARGE SEPARATION IN THE BACTERIAL RC: A VIEW FROM HOLE BURNING Gerald Small,0 Steven Kolaczkowski,0 Narla Reddy, a Ryszard Jankowiak, a Hai-Chou Chang,a John Hayes,a James Norris,h Deborah Hansen,h and Michael Seibenc 0 Ames Laboratory-USDOE, Iowa State University, Ames, lA 50011 hArgonne National Laboratory, Argonne, IL 60439 CNational Renewable Energy Laboratory, Golden, CO 80401 Despite the fact that the crystal structure of the bacterial reaction center (RC) has been known for one decade, several important aspects of its primary charge separation are not well understood. Included here is the symmetry (C2)-breaking electron transfer that occurs exclusively along the "right" branch even in mutants which were designed to be more symmetrical than the native RC, the quite recently discovered non-single exponential decay of p*, the primary electron donor state normally associated with the special pair, and the temperature dependence of the kinetics of charge separation. It is now apparent that the initial phases of primary charge separation are more complicated and interesting than originally envisaged. It is also apparent that an understanding of the Qy and charge transfer states, low frequency modes (phonons) and their associated electron-phonon coupling and the glass-like structural heterogeneity of proteins is important to the solution of the problem of primary charge separation. In these regards, spectral hole burning spectroscopies have proven to be particularly useful. Of primary concern here are the following questions: (i) are the unique characteristics of the P* f-P absorption band of bacterial RC due to the interaction of the zero-order excitonic state of P* with an essentially degenerate charge transfer state such as p+B- (where B =bacterio chlorophyll on the active side of the RC); (ii) if so, should this interaction be considered to be strong; and (iii) if the interaction is strong, can it be used to explain the aforementioned non-single exponential decay of P* in bacterial RC. In order to explore these questions we undertook detailed hole burning studies on the RC of the purple bacteria Rb. sphaeroides (P* =P870*) and Rps. viridis (P* =P960*) and the RC of photosystem II (P* = P680*) of green plants. We had reported earlier on the results of comparable studies on the RC of PS I where P* =P700*. In addition, we have completed experiments on the His-His and Tyr-Tyrmutants associated with L181-M208 of Rb. capsulatus. Four antenna complexes of PS II were also studied. (The comparison of the properties of the Qy states of antenna complexes with significant excitonic interactions with those of P* and other Qy states of the RC turns out to be highly informative.) Taken as a whole the data suggest the following scenario for P* and the initial phases of primary charge separation in the bacterial RC: 86 A. The charge-transfer character of P* is not due to charge-resonance states of the special pair, B. The charge-transfer character of P* is due to stron& coupling of the zero-order neutral excitonic state of P* with a quasi-degenerate manifold of vibronic (phononic) levels ofp+B-, and C. The "right" sided electron transfer of the bacterial RC is already determined by optical excitation of the so-called P* +-P absorption band since the absorption is primarily to vibronic (phononic) levels ofp+B:... which steal intensity from the neutral excitonic P* state. In other words, the conventional picture that views P* as a state localized on the special pair is incorrect Although our data do not offer definitive proof that the above model is correct they do demand that the model be given serious consideration. The data may also serve as a benchmark for molecular dynamics simulations of primary charge separation. At present it is not clear whether this model, together with the intrinsic glass-like structural heterogeneity of the protein, can explain the non-exponential decay ofP*. Work on this question is currently in progress. The above experimental studies led to the following fmdings and conclusions, some of which suggested the above model: the electron-phonon coupling for antenna Qy-states is very weak; the electron-phonon coupling for P960*, P870* and P700* is strong while that for P680* is moderately strong; the electron-phonon coupling for the upper special pair component of the bacterial RC is weak; the inhomogeneous broadenings of the P* +-P transitions are comparable to those for the Qy-states of antenna protein complexes, -100 cm-1; the inhomogeneous broadenings of different states of the same complex are uncorrelated meaning that the width of the distribution of values for a given electronic energy gap is large, -150 cm-1; the identification and characterization of the elusive sixth Qy-state of the bacterial RC and, as a consequence, an improved understanding of the extent of excitonic coupling between the six cofactors; and the correct number of interior Chi a molecules belonging to the RC of PS II is four (Jlllt six). Future work over the next two years will focus on variable pressure (up to -1 GPa) and variable temperature studies of the excited state electronic structure and transport dynamics of antenna and RC protein complexes. The state-of-the-art apparatus for these studies has been successfully tested and will be briefly described. 87 Publications (1992-1994) N. R. S. Reddy, R. Picorel and G. J. Small, "The B896 and B870 Components of the Rhodobacter sphaeroides Antenna: A Hole Burning Study.. , J. Phys. Chern. 2Q, 6458 (1992). N. R. S. Reddy, P. A. Lyle and G. J. Small, "Applications of Spectral Hole Burning Spectroscopies to Antenna and Reaction Center Complexes... Photosyn. Res. (invited article) 3.1. 167 (1992). R. Jankowiak and G. J. Small, "Spectral Hole Burning: A Window on Excited State Electronic Structure, Heterogeneity, Electron-Phonon Coupling and Transport Dynamics of Photosynthetic Units.. , in Photosynthetic Reaction Centers. J. Deisenhofer and J. Norris, Eds., Academic Press, 1993. G. J. Small, J. M. Hayes and R. J. Silbey, "The Question of Dispersive Kinetics for the Initial Phase of Charge Separation in Bacterial Reaction Centers.. , J. Phys. Chern. 22. 7499 (1992). N. R. S. Reddy, R. J. Cogdell, L. Zhao and G. J. Small, "Nonphotochemical Hole Burning of the B800-B850 Complex of Rhodopseudomonas acidophila .. , Photochem. Photobiol. ~. 35 (1993). S. V. Kolaczkowski, P. A. Lyle and G. J. Small, "Is Dispersive Kinetics from Structural Heterogeneity Responsible for the Nonexponential Decay of P870* in Bacterial Reaction Centers.. , in Bacterial Photosynthetic Reaction Centers, J. Breton, Ed., Plenum Publ. Co., 1992. N. R. S. Reddy, S. V. Kolaczkowski and G. J. Small, "A Significant, Persistent Photoinduced Structural Transfonnation of the Special Pair of a Bacterial Reaction Center.. , Science 200. 68 (1993). N. R. S. Reddy, S. V. Kolaczkowski and G. J. Small, "Nonphotochemical Hole Burnign of the Reaction Center of Rhodopseudomonas viridis.. , J. Phys. Chern . .21. 6934 (1993). P. A. Lyle, S. V. Kolaczkowski and G. J. Small, "Photochemical Hole-Burned Spectra ofProtonated and Deuterated Reaction Centers of Rhodobacter sphaeroides.. , J. Pbys. Chern . .21. 6926 (1993). N. R. S. Reddy, H. van Amerongen, S. L. S. Kwa, R. van Grondelle and G. J. Small, "Low Energy Exciton Level Structure and Dynamics in LHC-11 Trimers from the Chi alb Antenna Complex of Photosystem n... J. Phys. Chern., accepted. H.-C. Chang, R. Jankowiak, C. F. Yocum, R. Picorel, M. Alfonso, M. Seibert and G. J. Small, "Exciton Level Structure and Dynamics in the CP47 Antenna Complex of Photosystem II'', J. Phys. Chern., submitted. H. C. Chang, R. Jankowiak, N. R. S. Reddy, C. F. Yocum, R. Picorel, M. Seibert and G. J. Small, "On the Question of the Chlorophyll a Content of the Photosystem II Reaction Center.. , J. Phys. Chern., submitted. S. V. Kolaczkowski, J. M. Hayes and G. J. Small, "A Theory for the Temperature Dependence of Dispersive Energy Transfer Kinetics in Antenna Protein Complexes", J. Chern. Phys., submitted. 88 IONIC RELAXATION FOLWWING QUINONE REDUCTION IN PHOTOSYNTHETIC REACTION CENTERS David M. Tiede Chemistry Division, Argonne National Laboratory Argonne, II 60439 This project examines how protein environments contribute to high reaction yields and selectivity in photosynthesis. Recent work has focused on the electron transfer between two quinones molecules, termed QA and Qs , in bacterial reaction centers. This electron transfer differs markedly from earlier photochemistry in two significant ways. First, the electron transfer between QA and QB is thermally activated, having an activation energy of approximately 10 kcal/mole. This implies that significant nuclear reorganization is required for electron transfer. Second, the lifetimes of the P+QA- and P+Qs- states, where P+ represents the cation state of the bacteriochlorophyll dimer, are sufficiently long (microsecond to seconds) that there is time for significant protein and solvent reorganization. Proton movements from the aqueous phase to sites near the quinones are known to occur following QA to QB electron transfer, although the mechanisms, intermediates and pathways for these proton movements are not known. Previous work had shown that the absorption bands of the bacteriochlorophyll and bacteriopheophytin pigments are sensitive, in characteristic ways, to the internal electric fields associated with the QA and QB anions, and to externally applied fields (Stark spectroscopy). This program sought to determine whether the absorption bands of the pigments could be used to monitor dielectric relaxation and secondary charge movement in response to quinone reduction. Spectra of the P+Q- states were monitored as a function of time following laser excitation. The overwhelming contribution by P+ to these spectra was removed by subtraction of the P+QA- spectrum from P+Q- spectra measured at subsequent times. This subtraction yielded time-resolved Q(t) --QA- difference spectra, fig la. These experiments clearly showed time-evolution in the pigment absorption changes associated with the QA- and QB- states. The spectral changes were kinetically complex, and involved bacteriochlorophyll and bacteriopheophytin absorption bands. At 270 K the different kinetic components could be resolved, fig lb. In the time before 10 p.s, relaxation of the P+QA- state was seen. Subsequently, multiphasic QA to QB electron transfer and relaxation were seen during the 20 p.s to 100 p.s time scale. Relaxation events that included proton and other charge movements were seen to continue to the seconds time scale. These measurements showed that the nominally simple electron transfer between quinones involves electron transfer and relaxation events that proceed through 6 orders of time. The electrochromic shifts of the bacteriopheophytins were found to respond promptly to electron transfer between the quinones, while slower electrochromic shifts, particularly of the bacteriochlorophylls, were found to track relaxation processes, including proton transfer. 89 0.045 0.06 0.05 0.018 Q) 0.04 Gl <.> 0 c c 0 0 ....c -o.oos -e 0.03 0 0 c "' "'0 .g 0.02 -o.035 0.01 0.00 -o.061 -6 -5 -4 -3 -2 -1 700 750 800 850 900 950 wavelength (nm) log( time, sec) Figure I. Part a: Q(t)-minus-QA- difference spectra measured at times between lOfls and 70 ms following a laser flash. Part b: Comparison of absorbance changes measured at 760 nm (open squares) and 770 nm (filled squares). The different electrochromic responses of the reaction center pigments to QA and QB anion generation, and the wavelength dependent responses to relaxation events suggest that the electrochromic responses of the pigments are sensitive to the local dielectric properties of the protein. This interpretation is supported by observation of variations in the quinone anion induced electrochromic shifts in reaction center preparations containing carotenoid, high ionic strength and reaction center aggregation. In conjunction with these spectroscopic measurements of relaxation processes associated with reaction center electron transfer, this program has also initiated work to probe structural features of the reaction centers in solution using neutron and x-ray scattering. This work has found that reaction centers exist in an equilibrium between a monodispersed state and an unusual one-dimensional aggregate. The equilibrium was found to be shifted by ionic strength and nature of the detergent. These fmdings have consequences for reaction center crystallization and on the ionic relaxation processes associated with electron transfer. Current work is using these scattering techniques to determine the dimensions of antenna complexes and reaction center antenna complexes in solution. This work will determine mechanisms for assembly of supramolecular photosynthetic complexes. Future DirectionS: Electrochromic shifts associated with QA- and QB- states will be characterized as a function of single amino acid mutants in collaboration with Debbie Hanson (ANL). This will determine how the chemical character of individual amino acids residues at sites around the quinones affect dielectric relaxation in the reaction center, and the movement of secondary charges. Neutron and x-ray scattering studies will be done to measure counterion distribution around the reaction center using anomalous scattering techniques, and to characterize extents of temperature dependent structural changes in reaction center and antenna complexes between 20 K and 300 K. These experiments ·will provide a comparison between spectroscopically detected variations in photochemistry with structural measurements. 90 DOE SPONSORED PUBLICATIONS 1992-1994 Hanson, D.K., Baciou, L., Tiede, D.M., Nance, S.L., Schiffer, M. and Sebban, P. (1992) In Bacterial Reaction Centers Protons can Diffuse to the Secondary Quinone by Alternative Pathways, Biochim. Biophys. Acta, 1102, 260-265. Tiede, D. M. and Hanson, D. K., Protein Relaxation Following Quinone Reduction in Rhodobacter capsulatus: Detection of Likely Protonation-Linked Optical Absorbance . Changes of the Chromophores, in "The Photosynthetic Bacterial Reaction Center: Structure, Spectroscopy, and Dynamics", J. Breton and A. Vermeglio, Eds., Plenum Press, London, NATO ASI Series, 1992, pp. 341-350 Schiffer, M., Chan, C.-K., Chang, C.-H., DiMagno, T. J., Fleming, G. R., Nance, S., Norris, J., Snyder, S., Thurnauer, M., Tiede, D. M. and Hanson, D. K., Study of Reaction Center Function by Analysis of the Effects of Site-Specific and Compensatory Mutations, in "The Photosynthetic Bacterial Reaction Center: Structure, Spectroscopy, and Dynamics", J. Breton and A. Vermeglio, Eds., Plenum Press, London, NATO ASI Series, 1992, pp. 351-361 Tiede, D. M. and Dutton, P. L., Electron Transfer Between Bacterial Reaction Centers and Mobile c-Type Cytochromes, in "The Photosynthetic Reaction Center", J. Deisenhofer and J. R. Norris, Eds.; Vol. I, Chapter 9, Academic: New York, 1993, pp. 257-288 Tiede, D. M., Vashishta, A.-C. and Gunner, M. R. (1993) Electron-Transfer Kinetics and Electrostatic Properties of the Rhodobacter sphaeroides Reaction Center and Soluble c-Cytochromes, Biochem. 32, 4515-4531 Hanson, D. K., Tiede, D. M., Nance, S. L., Chang, C.-H. and Schiffer, M. (1993) Site-Specific and Compensatory Mutants Imply Unexpected Pathways for Proton Delivery to the QB Binding Site of the Photosynthetic Reaction Center, Proc. Natl. Acad. Sci. 90, 8929-8933 91 POSTER II XPS STUDY OF A VALENCE-AVERAGED MIXED-VALENCE DI- RUTHENIUM COMPOUND Larry Spreer, Christian B. Allan University of the Pacific, Stockton, CA and D. Brent MacQueen, John W. Otvos and Melvin Calvin Lawrence Berkeley Laboratory, Berkeley, CA 1 A new mixed-valence ion [Ru2(C 2JI36N8)Cl4]+ (1) has been prepared which 1 1 exhibits a narrow, intense NIR band (Axnax = 805 run E = 68,000 M- cm- ) and has a large 920 m V separation between its first oxidation and first reduction potential. XPS studies of the hexafluorophosphate salt of this ion have been carried out to understand its electronic structure. The XPS spectrum of (1) exhibits singlet photopeaks in the Ru(3p) and Ru(3d) binding energy regions. This provides unambiguous evidence that there is only one type of Ru and the compound is valence-delocalized even on the very short time scale of the XPS experiment (lQ-17 s). The XPS results on (1) are in contrast to the photoelectron spectrum of 5 [(H3N)5Ru(pyrazine)Ru(NH3) 5] +, the prototype mixed-valence Creutz-Taube ion. While the consensus from a great deal of experimental data on the Creutz-Taube ion is that it is Class III (valenc_e-delocalized), its XPS spectrum shows doublets for the Ru(3p) and Ru(3d) photopeaks. Models suggest that the two Ru centers in the Creutz-Taube ion are not sufficiently coupled to prevent polarization in the excited state. The failure of XPS to distinguish between Class II and Class III mixed-valence species for the Creutz-Taube ion have led other workers to ignore this technique. Our results show that XPS studies are useful in characterizing strongly-coupled valence-delocalized mixed-valence species. 93 POSTER #2 DIRECT TRANSIENT ABSORPTION SPECTROSCOPY OF THE SOLVATED ELECTRON IN WATER AND ALCOHOLS AND ULTRAFAST STUDIES ON INTERMOLECULAR ELECTRON TRANSFER IN CONTACT AND SOLVENT SEPARATED ION PAIRS Paul F. Barbara Department of Chemistry, University of Minnesota Minneapolis, Minnesota 55455 We report the first direct IR-pump/variable-wavelength-probe time-resolved transient absorption spectroscopy on the solvated electron in alcohols. We also describe extensive new results on the solvated electron in water. The experiments employ a three laser pulse sequence, where an ultraviolet (UV) synthesis pulse (390 nm, 25-30 mJ, 2kHz) generates excess electrons by two photon photodetatchment of I-. After a delay of -4 ns, the resultant equilibrated electrons are promoted to the electronically excited p-state by an IR-pump pulse (780 nm, 3-25 mJ, 2kHz), with the resulting spectral dynamics interrogated by a variable wavelength (500 - 1000 nm) probe pulse. Transients obtained probing on the blue edge of the ground state absorption band (600 -740 nm) show a fast, instrument-limited reduction in optical density (bleach) followed by a bleach recovery that occurs on two timescales, having both a fast, -D.5 ps component and a slower 5-10 ps component. Transients probing on the red edge of the ground state absorption band (950 nm) show a transient increase in optical density, which decays on a 20 ps timescale. Finally, transients probing in the intermediate spectral region (860 nm) show complicated non-monotonic behavior, where the recovery of the initial bleach "overshoots" to yield a transient increased absorption, which decays on a 5 - 10 ps timescale. The initial bleach at blue wavelengths and increased absorption at red wavelengths result from promotion of the ground state electron into the excited p-state, which has been demonstrated in water to have an absorption band peaking near 1100-1300 nm. Experiments have been performed in methanol, ethanol, propanol, and butanol. All solvents display a fast -D.5 ps component due to electronic relaxation and a slower 5-25 ps component' resulting from ground-state solvation. Additionally, a permanent bleach caused by a reaction of the p-state electron with solvent is observed. We have made extensive femtosecond transient pump-probe spectroscopy and relaxation for various Br/benzene-derivative complexes, especially the Br atom - mesitylene hv Br/Bz -~----~~ Br/ Bz+ et charge transfer complex. This class of reaction is a promising prototype for studying intermolecular et at the molecular level. The major results of these studies are as follows: 1. Direct real-time vibration motion in charge transfer complexes have been observed by spectroscopy, demonstrating that inter nuclear motion in the complex is strongly coupled to the electron transfer process; 2. The dynamics of electron transfer have been observed to involve a distribution of geometries of the ion pair and a relaxation of these geometries during the electron transfer process. A more complex picture of electron transfer than previously proposed is observed in our studies; 3. Various aromatic molecules are observed to greatly accelerate the electron transfer process. Acceleration is observed for sandwich DAD ion pair structures, in contrast to the more conventional ADD structure. This is a significant revision of the usual model for rate acceleration at high donor concentration. 94 POSTER 113 MODELS OF THE CHLOROPHYLL C ANTENNA CHROMOPHORES MOLECULAR STRUCTURE OF A ZINC PHEOPORPHYRIN K.M. Barkigia, M.W. Renner, K.M. Smith and J. Fajer Department of Applied Science Brookhaven National Laboratory Upton, New York 11973-5000 The importance of algae in global carbon fixation is focusing increased attention on marine algae and their photosynthetic pathways. These algae contain, besides the ubiquitous chlorophyll a (Chl), a fucoxanthin carotenoid, and up to three slightly different Chl c chromophores in their light harvesting complexes. Unlike Chl a (or b), the Chls care porphyrins rather than chlorins but retain the exocyclic ring V found in all photosynthetic Chl derivatives (see structural formulas below). The chromophores are thus classified as pheoporphyrins. As part of an investigation of the physical and chemical properties of pheoporphyrins, we have determined the molecular structure of a zinc pheoporphyrin complexed by pyridine, that serves as a simple model of Chl c and of its possible histidine axial ligand. Besides the obvious change of ring IV to a pyrrole ring that establishes the chromophore as a porphyrin rather than a chlorin, the molecule retains many of the structural features due to the exocyclic ring V found in chlorophylls and bacteriochlorophylls. A direct consequence of the porphinoid electronic structure is a blue-shift of the low energy absorption bands of the Chls c (and the model), and the loss of the high oscillator strengths of these Qy transitions that enhance the light harvesting properties of (bacterio)chlorophylls. Nonetheless, the absorption properties of the Chls c appear to fall Within the windows of light energy available in aquatic environments, and the protein complexes containing Chls c show efficient energy coupling to Chl a. Zn pheoporphyrin 95 POSTER #4 SELECTIVE VISIBLE LIGHT PHOTOOXIDATION OF PROPYLENE AND ISOBUTANE BY 02 IN ZEOLITE Y Fritz Blatter, Hai Sun, and Heinz Frei Lawrence Berkeley Laboratory, University of California Berkeley, Berkeley, CA 94720 Thus far, photochemical methods in hydrocarbon oxidation have not been competitive with conventional catalysis. Therefore, selective oxidation by photo-assisted processes has not yet had an impact on practical chemical synthesis. This may change, particularly when visible or near infrared light can be employed and high selectivities and high quantum efficiencies are obtained. Under these circumstances the use of solar photons seems reasonable. Here, we present very promising photosynthetic chemistry that may initiate the development of environmentally benign production of valuable chemicals, such as aldehydes, ketones, epoxides and alcohols. We have recently reported that small alkenes such as 2,3-dimethyl-2-butene and trans-butene are selectively oxidized with red light and molecular oxygen in zeolite NaY [1,2]. Very strong stabilization of an excited alkene•02 charge-transfer state in the environment of the strong electrostatic fields of the zeolite cavities is responsible for the visible absorption. The charge transfer absorption could be detected by diffuse reflectance spectroscopy. Photooxygenation quantum yields estimated from diffuse reflectance measurements and photoproduct accumulation are on the order of 10-20% [3]. We anticipate that alkaline-earth cation exchanged zeolites exhibit stronger electrostatic fields inside the cavities. Therefore, the result would be an even stronger stabilization of the excited alkene-G2 CT state. This implies that less reactive alkenes or even alkanes can be photooxidized with visible light. Here, we present highly selective photooxidation of propylene in BaY and isobutane inCaY. Propylene and isobutane are starting materials for the production of important chemical intermediates and reagents. Photooxidation of propylene in zeolite BaY with 488 nm light of an Ar-ion laser or the emission of a filtered tungsten source gave allylhydroperoxide (AHP) with greater than 99% selectivity. The quantum efficiency is around 20%. The initial step of the reaction mechanism is a deprotonation of the excited alkene radical cation to form allyl radical and ·ooH which combine to yield the hydroperoxide. This mechanism is strongly supported by the observed deuterium isotope effect. When fully deuterated propylene is used as reactant, the yields are different, which we attribute to inhibited deprotonation of the deuterated alkene radical cation. Kinetic analysis of photooxidation of propylene-&> and propylene-D6 will be presented. The photoproduct (AHP) decomposes into acrolein and H20, or reacts with excess propylene to form propylene oxide and allylalcohol. The branching between decomposition of the AHP into acrolein and thermal oxidation of propylene to propylene oxide depends on temperature and the amount of excess propylene present in the zeolite. Isobutane is selectively oxidized to t-butylhydroperoxide in CaY with blue light. Photo chemistry of this system has been studied between -50°C and 25°C. Yields are lower than in the case of alkene + 02 reactions, but may be improved by using zeolites that stabilize more strongly the isobutane•02 charge-transfer state. [1] Blatter, F. and Frei, H., J. Am. Chem. Soc. 115 (1993) 7501-02. [2] Blatter, F. and Frei, H., J. Am. Chern. Soc. 116 (1994) 000. [3] Blatter, F., Moreau, F. and Frei, H., Chern. Phys. Lett., submitted for publication. 96 POSTER 15 DIELECTRIC FRICTION STUDIED BY SOLUTE ROTATIONAL DIFFUSION AND FEMTOSECOND SOLVENT DYNAMICS EXPERIMENTS, AND ab initio MOLECULAR ORBITAL CALCULATIONS EdwardW. Castner. Jr. and YongJ. Chang Brookhaven National Laboratory Chemistry Department Building 555A Upton, NY 11973-5000 William Konitsky and David H. Waldeck University of Pittsburgh Chemistry Department Pittsburgh, P A 15260 Ultrafast laser polarization spectroscopy studies have been done on a series of six 9,10-R,R'-anthracenes (R,R'=methyl, chloro, and cyano) in a several solvents, and on the neat solvents themselves. The solvents are DMSO, methylcyclohexane, toluene, benzyl alcohol, and benzonitrile. The anthracene series was chosen be cause the molecules have a high degree of symmetry, and all have roughly the same molecular volume, yet the different substituted anthracenes have greatly different electrostatic properties. The solvents were selected to be of similar molecular vol umes, but greatly different dielectric properties. By measuring the rotational corre lation times of the anthracenes in the different solvents, we will measure the differ ences in mechanical versus dielectric frictional coupling between the solute and sol vent. In order to calculate the dielectric friction, ab initio molecular orbital calcula tions were undertaken to provide the atom-centered charges that result from the fit of the molecular electrostatic field. This extended and delocalized charge distribu tion will provide a more accurate means to calculate a zero-frequency dielectric fric tion within a solvent dielectric-continuum model. To progress beyond dielectric continuum models, we have measured the neat solvent dynamics on a femtosecond time scale for each of the five solvents, using the optical-heterodyne detected, Raman-induced Kerr effect technique. Ultimately, we hope to be able to construct a new and more accurate frequency-dependent dielectric friction model for solute-sol vent interactions, combining the quantum calculations, femtosecond solvent dynam-· ics, and molecular dynamics simulations. 97 POSTER 116 X-RAY ABSORYfiON STUDIES ON ELECTRONIC SPIN STATE TRANSITIONS OF [FE(a-PICOLYLAl\flNE)J1CI2• EtOH IN DIFFERENT MEDIA Lin X. Chen, Zhiyu Wang, Pedro A. Montanoa and James R. Norris Chemistry Division, Argonne National Laboratory aMaterial Science Division, Argonne National Laboratory Argonne, Dlinois60439 Recently we have initiated a program designed to investigate the structural changes that accompany photoinduced intramolecular electron transfer reactions in model complexes 6 containing Fe(ll). The electronic spin state transitions in Fe(TI) (d ) complexes in an octahedral ligand field involve transfer of electron density between Fe and the ligands. The molecular structural changes associated with electronic spin state transitions for the Fe(ll) (d 6) complex, [Fe(a-picolylamineh]Cl2· EtOH, in solution are studied by XANES and EXAFS. At lOK, most of the molecules are in the low spin (LS) state, whereas at room temperature, the high spin (HS) state is dominant. The LS state to the HS state transition can be thermally activated and can also be light induced. The light induced HS state can be effectively trapped at low temperature. Because previous structural studies were performed on the crystalline form, structural changes associated in the same transition in solution are unknown. We have investigated the structural changes of [Fe(a-picolylamine)3]Cl2· EtOH in ethanol solution using X-ray absotption techniques. In contrast to previous results on the neat solid, we hope to address the role of the solvent and the influence of the molecular surrounding on the structural changes associated with the spin state transition. X-ray and optical absotption measurements were conducted on solid [Fe(a picolylamineh]Cl2· EtOH as well as when dissolved in ethanol. The structural changes of the thermally activated spin state transition in solution are very similar to previous measurements on the solid state, where a 0.2 A Fe-N bond elongation was observed as the molecules go from the LS to the HS state. The light illumination on the frozen solution sample at lOK generated a long-lived metastable state instead of the HS state as in the previous solid state studies. This new metastable state has a very different structure from that of either the LS or the HS state, showing a severe distortion from regular octahedral geometry. Two of the six Fe-N bonds become shorter than the others. Such distortion is very likely due to the Jahn-Teller effect arising from the asymmetric distribution of the electrons in the two orbitals with eg character. This metastable electronic configuration can result from electron transfer with the solvent molecules or from an intermediate spin state stabilized by the solvation. The discovery of this metastable state by X-ray absorption again demonstrates the potential of this technique in monitoring metastable structures and the importance of knowing such structures for understanding chemical reaction mechanisms. Our future studies will iriclude the time-domain structural determination of other photoinduced electron transfer reaction intermediates using a pulsed laser as the excitation source and energy dispersive X-ray absorption. 98 POSTER #7 STRUCTURES AND FUNDAMENTAL VIBRATIONS OF p-BENZOSEMIQUINONE- AND PHENOXYL-RELATED RADICALS Daniel M. Chipman Radiation Laboratory, University of Notre Dame Notre Dame, Indiana 46556 Para benzosemiquinone radical anion C6H402·- is a prototype for systems involved in electron transport, while the related phenoxy! radical C6HsO· is an intermediate in the oxidation of phenol. The chemistries of their sulphur analogs, p-thiobenzosemiquinone radical anion and thiophenoxyl radical, are less well understood and are subjects of current research interest. Methods of ab initio electronic structure theory are used to characterize and contrast the structures and fundamental vibrations of these related species. The semiquinones and thiophenoxyl are reasonably described by low level methods such as Hartree-Fock with small basis sets. However, phenoxy! radical has strong nondynamical electron correlations among all the 1t electrons that require a multiconfigurational treatment together with a large polarized basis set for proper description. Lower levels of theory consistently give an incorrect electronic distribution for phenoxy! that leads to a CO bond that is too long and too weak, resulting in misassignment of the fundamental vibrations observed in the resonance Raman spectrum. The unpaired electron is delocalized somewhat in the p-benzosemiquinone and p-thiobenzosemiquinone anions, leading to partial quinoid structure within the rings and to partial double bond character of the CO and CS bonds. In phenoxy! radical there is considerable delocaliza tion of the unpaired electron, leading to a complex ring structure and a CO bond that is nearly as short as a typical double bond. Interestingly, however, the phenoxy! CO stretching force constant still remains intermediate between those typical of single and double bonds. In marked contrast to these systems, the unpaired electron in thiophenoxyl radical is largely localized on the S atom, leading to a simple benzenoid ring structure and an essentially single CS bond. This information is useful in interpreting the different chemical reactivities of these closely related radical systems. 99 POSTER #8 REDOX REACTIONS AT CHEMICALLY MODIFIED SURFACES OF LASER ABLATED SILVER COLLOIDS George Chumanov, Morgan Sibbald and Therese M. Cotton Ames Laboratory and the Department of Chemistry, Iowa State University, Ames Iowa 50011 The primary focus of this work is the study of heterogeneous electron transfer reactions at surfaces of silver colloids modified by adsorption of halide ions. Colloidal suspensions of noble metals such as Pt, Au and Ag have been shown to catalyze reduction of different molecules. When the reduction of the donor is more negative than that of the acceptor, meaning that the process is thermodynamically allowed in homogeneous solution, the metal particle serves as an "electron wire." The metal particle can also lower the activation energy of the reaction by stabilizing the intermediate species or function as a "storage cell" for electrons or holes. Because the size of the metal particles in colloidal suspensions is smaller than the mean free path of electrons, plasmon resonances can be optically excited. Excitation of plasmon resonance results in an increase in the local field near the particle surface which can cause enhancement of photochemical reactions. Silver colloids were prepared by ablation of the metal in aqueous solution using high energy laser pulses. This method yields a highly reproducible size distribution of colloidal particles. Because no chemicals are used in the procedure, the colloid is free of extraneous ions or other chemicals. Different .halogen ions were added to the suspension in order to control the potential of the particles. Each halide ion donates an electron to the particle upon adsorption on the surface. Cytochrome c was employed as a probe molecule for observing the redox reaction. The redox state of cytochrome was monitored using surface enhanced Raman scattering. It was found that cytochrome becomes rapidly reduced after reaching a certain concentration of halide ions in the colloidal suspension. This halide concentration depends strongly on the concentration of the Ag colloid. The total amount of cytochrome which can be reduced in such a system is equal to two to three times the concentration of halide ions. Moreover, the amount of halide ion required to initiate the reduction process is approximately five times larger than the amount needed to coat the surface of the metal particles with a monolayer. An extensive study of cytochrome reduction in silver colloids was performed with different halogen ions employing Raman and light absorption spectroscopies, as well as electron microscopy. To rationalize all observations, a model is proposed in which the halide ions adsorb on the metal surface as a monolayer and donate electrons to the particle, thereby changing its potential. The role of the excess of halide ions in this model will be discussed. The halide ions also cause a shift in the plasmon resonance of the single Ag particle which supports the contention that the adsorbed halide ions donate charge to the metal. Future experiments will involve a study of the plasmon resonance on photochemically driven electron transfer reactions. The goal of these experiments is to utilize plasmon resonances for enhancing photochemical efficiency by nonradiative coupling with redox pairs on or near the metal surface. 100 POSTER #9 NATURE OF THE EMITIING 3MLCT MANIFOLD OF RHENIUM(I)(DIIMINE)(C0)3Cl COMPLEXES D. R. Striplin and G. A. Crosby Department of Chemistry Washington State University Pullman, WA 99164-4630 Spectroscopic investigations of four Re(NN)(C0)3Cl complexes (NN = 2,2' bipyridine, 4,4'-dimethyl-2,2'-bipyridine, 1,10-phenanthroline, or 2,9-dimethyl-1,10- phenanthroline) in the 77- 4 K range reveal a single 3:MLCf emitting configuration. Temperature dependences of the phosphorescence lifetimes have been analyzed in terms of a multiple state model that yields rate constants and level splittings. The lowest (kt - 0.01 ~s-1) is followed by a second one (k2- 0.05 ~s-1) lying nearby (~£- 5-10 cm-1), which is separated (~£ -102 cm-1) from the highest, and most emissive (k3- 1 ~s-1) level. A model based on C2v symmetry that depends on second-order spin-orbit coupling between d1t* configurations is proposed to 2+ rationalize the results. Implications for the well-studied Ru(NN)3 and 2+ Os(II)(NN)3 manifolds are discussed. 101 POSTER 110 Ultrafast Isomerization Dynamics at Interfaces by Time-Resolved Second Harmonic Generation E. Borguet, X. Shi and K. B. Eisenthal Department of Chemistry, COlumbia University, New York NY 10027, USA Ultrafast studies of molecular relaxation in the bulk have been of great value in revealing the dynamics of fundamental chemical and physical events. However, our knowledge of these processes at interfaces is still in its infancy. The present contribution focuses on the interfacial nature of one of the simplest chemical reactions, molecular isomerization. Tune resolved second hannonic generation, by avoiding the complications of bulk contributions, enables surface species to be probed directly and on an ultrafast timescale. Our results on the banierless isomerization of the organic molecule Malachite Green indicate novel and unanticipated behavior: a very large change in relaxation rates, differing by an order of magnitude from those in the bulk and surprisingly similar rates at very different interfaces, namely the air/liquid, liquid/liquid and liquid/solid interfaces. An amplified CPM laser providing 130 fs pulses, at 625 run at a repetition rate of 10Hz was used as the excitation and probe source. The generally accepted model for the photoisomerization of triphenylmethane dyes such as Malachite Green in bulk solution involves the intramolecular rotations (twisting) of the three aromatic groups about their respective bonds with the central carbon atom. This change in the twist angle of each of the aromatics from their ground state equilibrium angles is initiated by the change in the charge distribution of the molecule following excitation to the first excited singlet state. The frictional resistance exerted by the solvent against the twisting motions of the aromatic moieties in the bulk isomerization dynamics is seen in the viscosity dependence of the kinetics. We expect that the isomerization at the interface will also be sensitive to the friction experienced along the twisting coordinates. At the air/aqueous and alkane/aqueous interfaces the phenyl group would project into the air and alkane phases, respectively. The twisting motions about the carbon-phenyl group bond are therefore expected to be very sensitive to the frictional properties of the phase into which the phenyl group is projecting. 1be air vs. the octane vs. the hexadecane phase should offer very different frictional resistances to the twisting of phenyl~· f Air ~ ~ Water~ ¥x (CH3~P ~fcH3h Contrary to the expectations based on these considerations, we found that the isomerization dynamics were the same at these interfaces having the values of 4.5±0. 7 ps, 4.3±0.3 ps and 4.5±0.3 ps at the air/aqueous, octane/aqueous, and hexadecane/aqueous interfaces, respectively. At the silica/aqueous interface the isomerization occurs in 7 .0±0.3 ps. We therefore repon that the dynamics are not only slower than in the bulk solution (0. 7 ps) but surprisingly are the same at the air/aqueous and alkane/aqueous interfaces, contrary to the current models of isomerization of triphenylmethane dyes. We suggest on the basis of the SH experiments that rotation about the central carbon-phenyl bond is not important in the structure change dynamics. The twisting motions of the d.irnethylaniline groups control the photoisomerization dynamics. The isomerization dynamics at the silica/aqueous interface suggests the idea that the friction and therefore the structure of water in the neighborhood of the solid (silica)/aqueous interface differs from that of bulk water. 102 POSTER /Ill OBSERVATIONS ON ELECTRONIC COUPLING IN COVALENTLY LINKED TRANSITION M;ETAL DONOR-ACCEPTOR COMPLEXES Murielle A. Watzky, V. Swayambunathan and John F. Endicott Department of Chemistry, Wayne State University Detroit, MI 48202-3489 Contrasts in the photoinduced electron transfer behavior of complexes with covalently linked transition metal donors and acceptors have led us to make some systematic comparisons of the metal-to-metal charge transfer (MM'CT) spectra and the correlated variations in electrochemical behavior in several interrelated series of complexes. The families of complexes being examined are: (a) (PP)2(CN)M(CNRuL)n+; (b) (PP)2M(CNRuL)2m+; (c) trans-M(MCL)(CNRuL)2l+; and (d) cis-M(MCL)(CNRuL)2l+ (where M = Ru, Fe, Co, Rh, Cr; RuL = Ru(NH3)5; MCL = tetraazamacrocyclic ligand; PP = bpy or phen). In order to generate some direct comparisons we are also examining LRu(BL)ML'j+ complexes (where (BL) = dpp, etc.' L = (NH3)4 or (bpy)2; ML' = Ru(NH3)4, Ru(bpy)2, Crlll(MCL) or Colll(amine)4). The observed MM'CT absorption can be used with a simple perturbational argument to estimate the amount of ground state stabilization, Es op, which originates from the mixing of MM'CT character into the ground state. Related perturbational arguments indicate that Ef~sd = E~~~ ± Es0 P (where the sign depends on which component of the couple is stabilized by CT mixing with the ground state). For the eN- bridged complexes with M(PP) centers we observe that E 112 a Es 0 P, as predicted, but the slope of the dependence is much larger than expected. We are currently interpreting this to imply that the donor-acceptor coupling is CN- mediated, and that this mediation involves both the 1t(CN) and 1t*(CN) orbitals; thus that the observed vales of Es 0 P depend on different combinations of the same perturbational parameters. A different perspective on this issue is provided by the contrasts between the RuiLRuiii CT transitions in some [Ru(NH3)5]2BL complexes in which there are intense Ruii -t BL CT absorptions. When BL = pz, dpp, etc., the Ruii -t Rulli absorbance is very intense, but when BL = trans-M(MCL)CN2 the Ruii -t Rulli absorbance is weak. This contrast demonstrates the importance of the spatial locus of the orbitals involved with perturbational coupling between the donor and acceptor. 103 POSTER /112 CONFORMATIONAL EFFECTS ON PORPHYRIN EXCITED SINGLET AND TRIPLET STATES J. Fajer, A. Regev; H. Levanon; K.M. Smith,t S. Gentemann .. and D. Holten·· Brookhaven National Laboratory, Upton, New York 11973 *Hebrew University of Jerusalem, Jerusalem 91904, Israel tUniversity of California, Davis, California 95616 **Washington University, St. Louis, Missouri. 63130 Recent crystallographic determinations of antenna and reaction center bacteriochlorophyll protein complexes have established that the chromophores assume multiple nonplanar conformations. Conformational substates of the chromophore-protein complexes or of the chromophores themselves have been invoked to rationalize the inhomogeneous kinetics of primary charge separation in reaction centers. A series of "conformational designed" porphyrins has been synthesized recently wherein the introduction of multiple peripheral substituents induces large deformations of the macrocycles that minimize steric interactions between the substituents (e.g. 2, 3, 7, 8, 12, 13, 17, 18 - octaethyl- 5~ 10, 15, 20- tetraphenylporphyrin, OETPP). NMR, resonance Raman and EXAFS results confirm that the nonplanarity of the crowded porphyrins, determined crystallographically, is maintained in solution. We report here time-resolved and steady-state optical data for a series of planar and nonplanar free-base porphyrins. The distorted porphyrins exhibit unusual optical properties and enhanced decays of the first excited singlet states. Specifically, the molecules display large Stokes shifts and increased rates of decay to the ground state and of intersystem crossing to the triplet state, attributable to structural reorganization in the excited state and to enhanced spin orbit coupling caused by the distortions of the chromophores. As well, time-resolved EPR spectra of H20ETPP and ZnOETPP in uniaxial liquid crystal matrices suggest that the nonplanar porphyrins undergo fluxional conformational excursions from the canonical x-ray structures of the molecules in their ground states. These results demonstrate ·that structural perturbations, imposed by peripheral _ substitutions in vitro and, presumably, by protein environments in vivo, significantly alter the rates and yields of porphyrinic excited states, properties fundamental to their function in photosynthetic energy migration and electron transfer. 104 POSTER /113 EXCLUDED VOLUME IN CALCULATION OF ELECTRON TRANSFER IN RESTRICTED GEOMETRJES AND INFINITE SOLUTIONS: COMPARISON OF SIMULATIONS AND ANALYTICAL THEORY S.F. Swallen, K. Weidemaier, and M.D. Fayer Department of Chemistry Stanford University, Stanford, CA 94305 Photo-induced intermolecular electron transfer is made possible by very short range intermolecular interactions. Because of the short range nature of electron transfer, the role of excluded volume (EV) is very important. There are two types ofEV, donor-acceptor (DA) and acceptor-acceptor (AA). DA EV arises because there is a distance of closest approach for a donor and acceptor pair. If the molecules are modeled as hard spheres, the distance of closest approach, rm, is the sum of the radii ofthe donor and acceptor. It is straightforward to include DA EV in an analytical calculation. A cut off at rm is used in the ensemble average. However, in performing the ensemble average it is necessary to eliminate those configurations for which two acceptors are occupying the same space. Blumen developed a lattice model for dealing with AA EV at moderate concentration. Fayer and co-workers have employed this approach in a theory of electron transfer and geminate recombination. Recently, in excitation transport calculations for concentrated micelle systems, Finger, Marcus and Fayer incorporated a hard sphere radial distribution function into the ensemble average to account for EV. Weidemaier and Fayer have developed an analytical theory for electron transfer for a system of a donor and N acceptors distributed on the surfaces of spherical micelles. The analytical theory gives results that are identical to results obtained from simulations for any N. When DA EV is included, the analytical theory and simulations are identical. To include AA EV, it is necessary to know the radial distribution function for hard disks distributed on the surface of a sphere. This problem has not been solved previously. The necessary distribution function was obtained by performing a simulation in which disks were equilibrated on the surface of the sphere, and the radial distribution function determined from the average of many runs. When the analytical theory is compared to the simulations of electron transfer on the micelle surface, substantial differences are observed. This exposes a fundamental problem in doing an analytical theory of electron transfer with AA EV. For point particles, the ensemble averages can be performed rigorously because the particle positions are independent. However, once EV is taken into account, the particle locations are correlated. The radial distribution function only describes the pair correlation. The analytical theory requires the N particle correlation, an intractable theoretical problem. Comparisons of analytical theory and simulations show that this is not a serious problem in infinite three dimensional systems at concentrations of experimental interest. However, in restricted geometry systems such as micelles, AA EV based on pair the correlation function approach can lead to substantial errors in theoretical predictions. 105 POSTER #14 AN INDIRECf LASER-INDUCED TEMPERATURE JUMP STUDY OF DIPOLE EFFECfS IN MIXED TlllOU FERROCENE-TERMINATED ALKANETIDOL MONOLA YERS ON GOLD John F. Smalley! Stephen W. Feldberg! Christopher E.D. Chidsey,3 Marshall D. Newton,2 Yi-Ping Liu2 1Department of Applied Science and 2Chemistry Department Brookhaven National Laboratory Upton, New York 11973-5000 3Department of Chemistry Stanford University Stanford, California 94309 The indirect laser-induced temperature-jump (ll..IT) technique was used to study dipole effects in self-assembled mixed thiol and ferrocene-terminated alkanethiol monolayers attached to gold electrodes. The self-assembled monolayers comprise CHlCH2) 0 _1SH and 5 5 (T] C5H5)Fe(T] C5H4)-C02(CHJnSH. The electrolyte was 1 M HC104. This study focusses on the amplitude of the initial ILIT response which we interpret as a manifestation of orientation of a dipole probably associated with the ferrocene moiety and its ester linkage. The particular virtue of ILIT for these experiments is the speed with which the measurement can be made (< 10 nanoseconds) which allows us to obtain data before electron transfer occurs. The attached molecules are tilted at an approximately 30 degree angle relative to the surface normal of the electrode. A consequence of this, easily seen from a simple model, is that there are two orientations of the dipole relative to the surface: one orientation when n, the number of alkyl carbons in the tether, is even, and a different orientation when n is odd.·If one thinks of the film as a simple capacitor the initial electrochemical potential across the film (prior to the ILIT perturbation) is the sum of the dipole potential and the potential caused by charge separation across the film. A change in temperature effects a change in the open-circuit potential that is directly and sensitively related to the dipole potential. The change in that response for n = 5, 6, 7, 8, and 9 exhibits an alternation consistent with a change in the dipole orientation for odd and even values of n. The implication of these measurements for electron transfer is that the electric field in the region between the gold electrode and the plane of the redox moieties will also alternate with odd and even values of n and can subtly modify the rate of electron transfer. 106 POSTER #15 MECHANISM AND KINETICS OF PHOTOREDUCTION OF C02 WITH p TERPHENYL AND COBALT MACROCYCLES Tomoyuki Ogata, t Shozo Yanagida, :1: Bruce S. Brunschwig, t and Etsuko Fujitat Chemistry Department, Brookhaven National Laboratory, Upton, New York 11973- 5000, USAt and Chemical Process Engineering, Faculty of Engineering, Suita, Osaka University, Osaka, Japan:!: Cobalt(II) macrocycles can serve as electron transfer catalysts in the photoreduction of C02 using p-terphenyl as a photosensitizer and a tertiary amine as a sacrificial electron donor in methanolic acetonitrile.! Mediation by cobalt macrocycles suppresses the degradative and competitive photo-Birch reduction of p terphenyl, leading to efficient formation of CO and formate. Flash photolysis experiments provide spectroscopic evidence for the sequential formation of the p-terphenyl radical anion, the Co(I)L+ complex, the [Co(I)L-C02]+ complex and the [S-Co(III)L-(C022-)]+ complex (L = HMD = 5,7,7,12,14,14-hexamethyl-1,4,8,11-tetraazacyclotetradeca-4,11-diene, S = solvent) in the catalytic system. Kinetics of the formation and disappearance of the three Co(I)L + species have been measured for the macrocycles shown below. In addition to the mechanism of the photoreduction of C02, various factors controlling the kinetics of the photoreduction of C02 will be discussed. HMD OMD DMD cyclam 1 (a) S. Matsuoka, K. Yamamoto, C. Pac, S. Yanagida, Chem. Lett. 1991, 2099. (b) S. Matsuoka, K. Yamamoto, T. Ogata, M. Kusaba, N. Nakashima, E. Fujita, S. Yanagida, J. Am. Chem. Soc., 1993, 115,601. 107 POSTER #16 INCREASED YIELD OF LONG-LIVED CHARGE SEPARATION RESULTING FROM PROTONATION OF AN ELECTRON TRANSFER INTERMEDIATE Su-Chun Hung, Alisdair Macpherson, Ana L. Moore, Thomas A. Moore, and Devens Gust Department of Chemistry and Biochemistry, Center for the Study of Early Events in Photosynthesis, Arizona State University, Tempe, Arizona 85287 Achieving long-lived charge separation in molecular triads featuring a multistep electron transfer sequence necessarily involves an intermediate charge-separated state which intervenes between the electronic excited state of the primary donor (or acceptor) and the final charge separated state in which the charges are insulated by a neutral component. This intermediate state is populated by photoinduced electron transfer and typically consists of a radical anion and radi cal cation linked together in such a way that charge recombination to the ground state is neces sarily facile. If the device is to have a high yield of the final charge-separated species, the subse quent forward electron transfer process must be substantially faster than charge recombination. In previous work this has been favored by tuning electronic coupling and thermodynamic factors. Alternatively, more complex molecular devices featuring two forward electron transfer processes which compete with charge recombination have been constructed. 1 Carotenoporphyrin-quinone triad 1 illustrates a new strategy for slowing the recombina tion reaction of an intermediate charge-separated state and thus enhancing the yield of long-lived charge separation. In the ground state in aprotic solvents, 1 is expected to exist at least in part in a form in which there is an intramolecular hydrogen bond between the carboxyl proton and the nearby quinone oxygen. Excitation of 1 is followed by photoinduced electron transfer to form the quinone radical anion and the porphyrin radical cation. The quinone radical anion is postulated to rapidly accept a proton from the carboxyl group to give the semiquinone and the carboxylate an ion. This reaction is favored by the change in pK of the quinone upon one-electron reduction and the selection of an organic acid having a pK greater than that of the quinone, but less than that of the quinone radical anion. Direct charge recombination from this new state would yield the pro tonated quinone linked to the carboxylate anion, a highly energetic species. As a result, the driv ing force for charge recombination is expected to be low (in the normal Marcus region), and re combination should be relatively slow. Thus, the final charge-separated state should be formed in higher yield than that for a similar system lacking the proton transfer possibility. CH:J The efficacy of this strategy is demonstrated by spectroscopic studies of 1 in benzonitrile which reveal a two- to three-fold increase in the yield of the long-lived charge-separated species over that found for reference triad molecules in which such hydrogen bonding cannot occur. In addition to illustrating a new approach to the generation of long-lived charge separa tion in high yield, triad 1 serves as a prototype for model systems which demonstrate proton transfer processes coupled to electron transfer such as occur in energy-transducing biological membranes. D. Gust, T. A. Moore and A. L. Moore, Ace. Chem. Res., 1993, 26, 198-205. 108 POSTER #17 PHOTOINDUCED ELECTRON TRANSFER BETWEEN SULFUR-CONTAINING CARBOXYLIC ACIDS AND THE 4-CARBOXYBENZOPHENONE TRIPLET STATE IN AQUEOUS SOLUTION Bronislaw Marciniak, t' * Krzysztof Bobrowski, t Gordon L. Hug, t and J aroslaw Rozwadowski * tRadiation Laboratory, University of Notre Dame, Indiana, 46556 *Faculty of Chemistry, A. Mickiewicz University, Poznan, Poland The mechanism of photoinduced electron transfer between sulfur-containing carboxylic acids and the 4-carboxybenzophenone (CB) triplet state in aqueous solution was investigated using laser flash photolysis and steady-state photolysis techniques. Bimolecular rate constants for quenching of the CB triplet state by six sulfur-containing acids, with varying numbers of coo- groups and varying locations with respect to the sulfur atom, were found to be in the range (0.3 - 2.1) x 109 ~ 1 s-1 depending on the charge of the acid molecule. The observation of ketyl radical anions and intermolecular (S:.S)-bonded radical cations of some of the acids was direct evidence for the participation of electron transfer in the mechanism of quenching. An additional absorption band at approximately 410 nm in the transient absorption spectra for some of the acids was assigned to an intramolecularly (S.".O)-bonded species (for acids where a five member ring structure was sterically favorable). Quantum yields of formation of intermediates from flash photolysis experiments and quantum yields of C02 formation and CB disappearance from the steady-state measurements were determined. The values of these quantum yields clearly indicated that the diffusion apart (escape of the radical ions) of the charge-transfer com plex, formed as a primary photochemical step, is the main photochemical pathway (contribution of -90%). Competing processes of proton transfer and back electron transfer within the CT complex gave only minor contributions to these yields. A detailed mechanism of the CB sensitized photooxidation of sulfur-containing carboxylic acids is proposed, discussed, and com pared with that for sulfur-containing amino acids in aqueous solution. 109 POSTER /118 LIGHT-INDUCED REDOX REACTIVITY INVOLVING STRONGLY INTERACTING ACCEPTOR SYSTEMS: PRECURSORS TO REAL MOLECULAR WIRES? Joseph T. Hupp, Laba Karki and Vladimir Petrov Dept. of Chemistry, Northwestern University, Evanston, IL 60208 We have initiated studies of linear donor/primary-acceptor/secondary acceptor systems, in which the primary acceptor is an electronically delocalized oligomeric assembly. Our expectation is that the primary acceptors may be able to function as primitive, real (as opposed to virtual or superexchange type) molecular wires, displaying essentially zero charge-transfer transit times. Preliminary· studies have involved only the donor/delocalized-primary-acceptor portion of the . assembly. A prototypical design motif is the following: ··· where the metallocyanide comprises the (localized) donor and the ruthenium oligomer (a Creutz-Taube variant) comprises the acceptor. We have previously shown (collaboratively with the Barbara group) that back electron transfer oc~s in 85 fs in the localized acceptor limit (n = 0). We now find, with n = 1, that back ET is slowed by some six orders of magnitude ('t = 60 ns)- despite virtually identical forward ET energetics. The mechanism for the remarkable rate attenuation (i.e. charge separation enhancement) will be discussed in light of ancillary time-dependent scattering studies and electronic Stark effect studies. 110 POSTER #19 STRUCTURAL AND DYNAMICAL INVESTIGATIONS OF THE CATALYTIC MECHANISMS OF WATER OXIDATION BY THE [(bpy)2Ru(OH2)]204+ ION Yabin Lei and James K. Hurst Department of Chemistry, Washington State University Pullman, Washington 99164-4630 Oxo-bridged ruthenium dimers of the type (cis-L2Ru0H2) 20, where L is 2,2' bipyridine or a ring-substituted analog, exhibit remarkable catalytic capabilities toward the oxidation of water. These reactions are fundamentally interesting from the point of view of understanding mechanisms by which redox metal clusters can obviate reactant noncomplementarity and in relation to biological water· oxidation, particularly since manganese clusters that effectively mimic the structural features of the oxygen evolving complex (OEC) of photosystem II are catalytically inactive. Although the ruthenium J.l-oxo dimers have been fairly extensively studied as catalysts, there is as yet no clear understanding of their reaction mechanisms. The work presented here summarizes results from our recent structural and kinetic studies that were designed to characterize the higher oxidation states of the catalyst formed in the presence of strong oxidants, and to identify from among them the 0 2-evolving species. Specifically, we have used resonance Raman and electron paramagnetic resonance spectroscopies to identify complexes formed during progressive oxidation 4 3 with Ce + and Co + ions in acidic solutions and have measured initial rates of 0 2 evolution under catalytic conditions to obtain the steady-state rate law for the reaction. Our new findings are as follows: (1) redox titrations using RR spectroscopy indicate that oxidation states beyond the V-V ion are reached under highly oxidizing conditions; (2) cryogenic epr measurements indicate that, in addition to the III-IV and IV-V ions, a paramagnetic species with the properties of a ligand radical ion is formed; (3) the kinetic studies indicate that catalysis is unimolecular, i.e., involves a single J.l-oxo ion, and that the V-V or a higher oxidation state is the reactive species. Our tentative suggestion is that the higher oxidation state is a V-V dimer containing a bipyridine radical 1t-cation. If so, this may be relevant to reactivity within the OEC, since the penultimate oxidation state before 0 2 release (the S3 state of the manganese cluster) is thought to also include a (histidyl) radical ion. 111 POSTER 1120 SPECTROSCOPIC STUDIES OF DONOR-ACCEPTOR SYSTEMS LINKED VIA HYDROGEN BONDS T. T. Chin, StephanS. Isied, and Isabelle LeLouche Deparnnent of Chemistry, Rutgers, The State University of New Jersey P.O. Box 939, Piscataway, NJ 08855 The synthesis, electrochemical and spectroscopic characterization of a series of tris (polypyridine) ruthenium(II) complexes where one of the polypyridine ligands is linked to a barbiturate group (Guest) and their acyl derivatives (Host) will be described. The association of these compounds with the multifunctional receptors, 3,5-bis[[6-aminopyrid- 2-yl)amino] carbonyl] pyridine and the 3,5-bis£[6-aminopyrid-2-yl)amino] carbonyl] pyridine ruthenium(m pentaammine has been examined in non-aqueous solvents. The binding properties of these host-guest complexes were studied by fluorescence spectroscopy. The above results will be presented with special emphasis on the hydrogen bonding of these donor-acceptor complexes. HOST GUEST 112 POSTER #21 NOVEL PHOTON-ANTENNA AND ENERGY-TRAP SYSTEMS. THE INTERPLAY BETWEEN R - 3 AND R - 6 ENERGY TRANSFER. Michael Kasha. Department of Chemistry and Institute of Molecular Biophysics, Florida State University, Tallahassee, FL 32306-3015. Photon-antenna and energy-trap systems have many applications, from the photosynthetic reaction center, to solar energy conversion systems, radiation screen- and fibre~ optic-scintillators, to fluorescent probes in protein structure studies, immunology, and fluorescence-scanning imaging in medicine. Currently under development in our laboratory is a novel molecular system in which the R-3 dependent Davydov molecular exciton model (excitation delocalization) as a photon an tenna, is coupled to the R - 6 dependent EXCITA1TON-RESONANCE OIPOLE-DIPOLE 1RANSFER Forster excitation transfer model, as an energy trap. These systems are capable MOLECULA/-tUi>>O£ of greatly enhancing the efficiency of EXCITONS STRONG (DAVYOOV} COUPUNG 1 fluorescence probes. I· I I (Simpson-Pete:son A diagram originated by Th. criteria) Forster attempted to display the dependence of rates of transfer over the WEAK COUPUNG whole range from femtosecond to micro A£>>iUJ>>M second transfer, for mechanisms of strong coupling exciton, to weak coupl ing, to Forster donor acceptor coupling. A new interpretation is given of the mode of interplay of these divergent mechanisms. A novel pro-posal is made for a coupling system, and spectroscopic examples are given of the effectiveness of the coupling model. 10' ,.. 10' 100 The systems developed involve u. -fnlrW""' the novel approach of using molecular excitonic coupling, with· the antenna molecules dispersed in a rigid mechanism, and with antenna-trap pairs as the Forster transfer excitation energy localization site. REFERENCES (1) Th. Forster, Ann. Physik., 2, 55 (1948). (2) A S. Davydov, Theory of Molecular Excitons (transl by M. Kasha and M. Oppenheimer, Jr.), McGraw-Hill Book Company, Inc., New york, 1962, 174 pp. (3) Cf. M. Kasha, "Molecular Excitons in Small Aggregates," in Spectroscopy of the Excited State (ed. by B. DiBartolo), Plenum Publishing Co., New york, 1976, p. 337-363. (4) R M. Hochstrasser and M. Kasha, Photochem. PhotobioL,3, 317-331 (1964). (5) Th. Forster, "Excitation Transfer," in Comparative Effects of Radiation (ed. by Burton, Kirby-Smith and Magee), John Wiley and Sons, Inc., New York, p. 300-319 (1960). (6) W. T. Simpson and D. L. Peterson, J. Chern. Phys., 26, 588-593 (1957). (7) M. Kasha, Radiation Research, 20, 55-11 (1963).] (8) Th. Forster, "Delocalized Excitation and Excitation Transfer," in Modern Quantum Chemistry (ed. by 0. Sinanogtu), Academic Press, New York, p. 93-137 (1965). (9) A El-Bayoumi and M. Kasha, J. Chern. Phys., 34, 2181 (1961). (10) J. C. Baum, R L. Christiansen, M. Kasha, et aL (Manuscript in preparation). (11) A Sytnik and M. Kasha (Manuscript in preparation). 113 POSTER /122 PHOTOELECI'ROCHEMICAL BEHAVIOR OF NAKED AND SURFACE-MODIFIED NANOCRYSTALLINE Sn02 SEMICONDUCTOR FILMS Idriss Bedja,+ Surat Hotchandani,+ Di Uu, Richard W. Fessenden and Prashant V. Kamat Radiation Laboratory, University of Notre Dame, Notre Dame, Indiana 46556 Development of thin semiconductor films is of great interest because of their applications in electronics, electrooptics, imaging science, photoelectrochemical cells, and photocatalysis. We have now succeeded in developing thin metal oxide semiconductor films of thickness 0.1 to 1 micron from colloidal suspensions of ZnO, Sn02 and W03. Such methods provide a convenient way to tailor the semiconductor properties by controlling the preparative conditions of colloid precursors. Thin, transparent films of nanocrystalline Sn~ semiconductor have been prepared from 30 A diameter colloids. The electron trapping process in Sn02 particles has been investigated by both spectroelectrochemical and laser flash photolysis techniques. With increasing negative potentials (0.0 - -0.5 V vs. Ag/ AgCl) an increase in the IR absorption band with an intense bleaching in the UV is observed. The absorption spectra recorded during the charging cycle are easily restored in the discharge cycle. These electrodes are photoelectrochemically active in the UV region with incident photon-to-photocurrent conversion efficiency of 20% at 280 nm. The photocurrent increases with increasing film thickness, but attains a limiting value at thickness greater than 0.75 IJ.m. The migration of charge across the grain boundaries is a limiting factor for the photocurrent generation in thicker films. These Sn02 films are highly porous and exhibit strong affinity for adsorption of sensitizer molecules such as bis{2,2' bipyridine)(2,2'-bipyridine-4,4'-dicarboxylic acid) Ru(II)perchlorate (Ru(bpy)2(dcbpy)2+), chlorophyll a and b, squaraine and oxazine dyes. Sn02 films 2 OTE Nanocryatallltea Electrolyte modified with Ru(bpy)2(dcbpy) + exhibit excellent Mechanism of sensitized photocurrent photoelectrochemical response in the visible with a generation power conversion efficiency of -1% at 470 nm. Both microwave absorption and luminescence decay measurements have been carried out to obtain the rate constant for the charge injection process. The luminescence decay showed at least two components and the faster component gave rate constants -108 s-1 which matched well with the rate constant obtained from the growth of the microwave absorption. + Permanent Address: Centre de Recherche en Photobiophysique, Universite du Quebec a Trois Rivieres, Trois Rivieres, Quebec, Canada G9A 5H7 114 POSTER #23 PHOTOOXIDATION OF . PHENOTHIAZINE DERIVATIVES IN SILICAS OF. DIFFERENT PORE SIZES Bosong Xiang and Larry Kevan Department of Chemistry, University of Houston Houston, Texas 77204-5641 Methylphenothiazine and other N-alkylphenothiazines were introduced into silica gel pores by impregnation and sol-gel synthesis. The alkylphenothiazines were photooxidized at room temperature by 320 nm irradiation to form stable alkylphenothiazine cation radicals detected by electron spin resonance and diffuse reflectance spectroscopies. The silica gel framework is suggested to be the electron acceptor. The photoyield and stability of the methylphenothiazine cation radical depend on the silica gel pore size. In a small pore silica gel the methylphenothiazine cation radical has a larger photoyield and is more stable with a longer lifetime than in a large pore silica gel. Increasing the alkyl chain length up to hexadecyl does not affect the alkylphenothiazine cation photoyield or stability. However, addition of a anionic sulfate group to the alkyl chain of an alkylphenothiazine doubles its photoyield and increases the stability of the photoproduced radical. It is suggested that the mobility of the alkylphenothiazine cation radical in the silica gel pores controls its photoyield and stability. 115 POSTER #24 DRAMATIC PROLONGATION OF 3MLCT STATE LIFETIMES FOR ZEOLITE ENTRAPPED POLYPYRIDINE COMPLEXES OF RUTHENIUM (II) Krzysztof Mamszewski and James R. Kincaid Department of Chemistry, Marquette University Milwaukee, Wisconsin 53233 Recently, much attention has been focussed on the photophysical and photoredox properties of zeolite-entrapped photosensitizers such as tris-bipyridine ruthenium (II), Ru(bpy)/+, and related complexes. An intensive study of the temperature-dependent lifetimes of such entrapped complexes had shown that one of the important effects of the entrapment is to increase the energy of the so-called ligand-field excited state, thereby eliminating an energy wasting decay pathway and, more importantly, abolishing photo-induced Jigand loss. 1 MLCT LF 3 ---- MLCT GS daf In an effort to provide an experimentally determined estimate of the magnitude of this ligand-field state destabilization, attention has been directed toward complexes which, owing to the weak field strength of the ligands, have inherently low energy ligand-field states; eg., the 2 heteroleptic complex, Ru(bpy)2(daf) + (where daf =diazafluorene). In solution, this complex possesses a very short (sub-nanosecond) 3MLCT state lifetime and exhibits weak emission, both effects being directly ascribable to the low energy of the ligand-field excited state. The zeolite-entrapped complex was prepared via a route previously developed in our laboratory. Thus, the zeolite-entrapped bis-bipyridine ruthenium (II) precursor was synthesized, purified and spectroscopically characterized. This material was then treated with excess diazafluorene at 180°C to yield zeolite-entrapped Ru(bpy)idaf)2 +. The electronic absorption and resonance Raman spectra of both the zeolite-entrapped complex and its soluble extracts, following acid dissolution of the zeolite framework, are entirely consistent with those obtained 2 for aqueous solutions of independently prepared Ru(bpy)2(daf) +. The zeolite-entrapped complex emits relatively strongly at An~ax =608 nm and exhibits a 3MLCT state lifetime of - 260 ns at room temperature. This complex thus serves as an iiiustrative example of the utility of Y -zeolite entrapment for dramatically altering the fundamental photophysical properties of such sensitizers. 116 POSTER #25 CAROTENOIDS : OPTICAL, ELECTROCHEMICAL AND MAGNETIC RESONANCE STUDIES A. S. Jeevarajan, C. C. Wei, J. A. Jeevarajan, G. Gao and Lowell. D. Ki~, Dept. of Chemistry, The University of Alabama, Tuscaloosa, AL 35487. Although the importance of carotenoids in photosynthesis as a photoprotector and an auxiliary antenna pigment is well established, the role of the carotenoid cation radical in the photosynthetic apparatus is not well understood. Further, in order to efficiently utilize the photoprotect properties of carotenoids in artificial solar devices, it is important to understand the decomposition processes. As part of this research effort, we investigated the effect of terminal electron withdrawing and donating substituents, keto and hydroxy groups, and the change of the 1t-bond structure on the properties of carotenoid cation radicals and dications making use of electrochemical and simultaneous electrochemical electron paramagnetic resonance (SEEPR) methods. The results were compared with the electrochemical properties of 6-carotene(l). The DigiSim Cyclic Voltammogram (CV) program was used to simulate the CVs based on an electrochemical mechanism involving three electrode reactions and two homogeneous reactions. The formal potentials (El o, E2o and E30), the comproportionation (Kcom) and the deprotonation (Kelp) equilibrium constants were also deduced. ++ Kcom Car + Car 2 ear·+ Car++ K Dl transition was found for all the carotenoid cation radicals. HPLC techniques were applied to separate the cis isomers formed by a small amount of acid (0.1 to 0.2 mM) present in a dichloromethane solvent and the isomers were characterized by optical and NMR methods. The cation radical of 7 ,7'-diphenyl- 7,7'-diapocarotene (II) was found to be strongly adsorbed on the platinum electrode. The adsorption behavior of the cation radical of II on gold and glassy carbon electrodes were explored. Cation radicals of 7' ,7'-dicyano-7'-apo-B-carotene (III) and 7'-phenyl-7'-apo-B carotene (IV) were photochemically prepared on the silica gel matrix and the hyperfine coupling constants of the freely rotating methyl group protons in the cation radicals of ill and IV were measured by ENDOR spectroscopy. It was also found that the hyperfine coupling constants calculated from an RHF INDO/SP calculation -are in good agreement with the experimentally found values. The geometry for the above calculation was optimized by the AMI method. 117 POSTER /126 FORMATION OF A POSSffiLE PRECURSOR TO BIMETALLIC AND HETERO-BIMETALLIC REDOX CATALYSTS Larry Spreer, Christian B. Allan University of the Pacific, Stockton, CA and Chris Lange, D. Brent MacQueen, John W. Otvos and Melvin Calvin Lawrence Berkeley Laboratory, Berkeley, CA Aerial oxidation of the dimeric complex [Fe2 (~0 H36 N8 )(CH3CN)4](Cl04)4, (1) in the solid state or in solution results in the high-yield formation of the carbonyl containing compound [Fe(C10H18N40)(CH3CN)2](Cl04)2 , (2). Structural characterization of (2) is strongly supported by magnetic resonance and mass spectrometry. The complex is found to be unusually rigid on the time scale of the 1H NMR experiments and all ten protons can be unambiguously assigned. The Mossbauer spectrum of (2) indicates the keto-macrocycle is an exceptionally good 1r-acceptor ligand. Presumably this is due to the electron-withdrawing properties of the oxygen in conjugation with the (j-diimine moiety. The carbonyl group of (2) can be selectively reduced by triethyl silane and BF3- etherate. The resulting .8-diimine macrocyclic complex can be oxidatively coupled by dioxygen to reform the original bimetallic species (1). This result suggests that (2) in conjunction with other .8-diimine containing species can be used to obtain novel bimetallic and heterobimetallic compounds. Our synthetic efforts will be discussed. 118 POSTER #27 PHOTOPHYSICS AND PHOTOCHEMISTRY OF 1-NITRONAPHTHALENE: TRIPLET AND RADICAL YIELDS, QUENCHING RATES AND SOLVENT EFFECTS IN REDUCTIONS BY SIMPLE ANIONS AND ARYL DONORS. Laszlo Biczok and Henry Linschitz Dept. of Chemistry, Brandeis University, Waltham, MA, 02254-9110, USA and Avner Treinin Dept. of Chemistry, The Hebrew University, Jerusalem, Israel Energy flow patterns and electron-transfer quenching have been studied in 1-nitronaphthalene (NN). The triplet yield (T = 0.64, measured by energy transfer relative to benzophenone) and low fluorescence yield {p - 0) establish that NN does not obey the popular dogma, that p + T = 1. Evidence is given that this large discrepancy involves no competing chemistrY, but is due to direct internal conversion from S1 to S0• Such exceptionally fast radiationless processes may be a general phenomenon, related to charge transfer character in excited singlet or triplet states. With respect to their 3NN quenching properties, simple anions fall into two groups. Group I includes halides and pseudohalides, for which radical formation (NN-; and X2-;) occurs only at anion concentrations much higher than that required to quench the triplet. Group II (N02 -; SO/-; HC02-) form radicals efficiently in the primary quenching reaction, at low concentration. This behavior, seen also in our studies on carbonyl compounds and dyes, may be rationalized in terms of the degree of spin-orbit coupling in primary (bimolecular) and secondary (trimolecular) exciplexes. For quenching rates lower than diffusion-controlled (N02- and Br-), the results agree with previous analyses of anion quenching reactions, using spectroscopic data to estimate Marcus reorganization energies. Values of triplet and radical extinction coefficients were measured, permitting 1 determination of radical yields, ¢R. Linear double-reciprocal plots (R -t vs [quencherr ) lead to ratios of rate constants for back-electron transfer to radical separation in various solvents. These will be discussed in terms of solvent viscosity and the degree of spin restriction in the back reaction. Hydroquinone and dimethoxybenzene react differently with 3NN in acetonitrile, but similarly in water. The difference, related to the p~ of NNH • ~dical, is assigned to secondary proton movement in the former case. 119 POSTER #28 PHOTOCHEMICAL HYDROOEN EVOLUTION FROM NON-SACRIFICIAL ELECIRON DONORS IN SUPRAMOLECULAR SYSTEMS BASED ON OXIDE SEMICONDUCTORS Steven W. Keller, Yeong ll Kim, Edward H. Yonemoto, Elaine S. Brigham, and Thomas E. Mallouk Department of Chemistry, The Pennsylvania State University, University Park, PA 16802 The construction of molecular systems that contain photosynthetic electron transport chains is interesting for both fundamental and practical reasons. An important feature of these systems is the spatial organization of photoredox components, which helps to control forward and reverse electron transfer rates and to prevent charge recombination between intermediate or final products. While there are now many examples of supramolecular redox chains that undergo efficient photoinduced electron transfer to yield long-lived charge-separated states, there are very few that convert this transiently stored electrochemical energy into useful oxidized and reduced chemical products. · We are studying two photosynthetic systems which can produce molecular hydrogen from non sacrificial (i.e., electrochemically reversible) electron donors. Both of these are based on sensitization of wide bandgap oxide semiconductors with visible light-absorbing dyes, and undergo a series of light driven electron transfer reactions like that shown schematically below. Molecular sieving effects prevent the recombination of energetic products at the catalyst used for the H+fH2 interconversion. The correct choice of oxide semiconductor is essential for hydrogen evolution to occur in these systems. In particular, rE- hv th~ conduction band edge potential of the oxide must be intermediate + e between the potential of the reversible~ catalyst photoexcited sensitizer and the H+JH2 electron ___;___,:_ couple. Too positive a band edge donor sensitizer potential allows the semiconductor to mediate charge transfer in both the (e.g. x-) semiconductor 77777 forward and reverse directions, and vb results in dark recombination of hydrogen and the oxidized donor. One system of this type consists of an aluminosilicate zeolite (mordenite or zeolite L) containing Pt clusters, methylviologen, and a semiconducting metal oxide within the linear channels, together with a size-excluded photosensitizer RuL32+ (L = 4,4'-dicarboxy-2,2'-bipyridine) adsorbed at the semiconductor surface. In this system photochemical hydrogen evolution is observed from a non sacrificial donor (an N,N'-dialkyl-p-methoxyaniline derivative) when the intrazeolitic semiconductor is Nh20s, but not when it is TiD2. The latter has a conduction bandedge potential too close to the H+JH2 couple. The second system consists of a sensitized, internally platinized layered oxide semiconductor, with I- as the electron donor. The lamellar titanates Na2Ti3D7 and K2Ti4D9 mediate dark recombination of photochemically generated H2 and 13-, whereas the titanoniobates (KTiNbOs and CsTiNb07) and niobates (KNb30s and ~Nb6017) do not. Calculations of the conduction bandedge potentials of these materials at the point of zero charge, ba.sed on the electronegativities of their constituent atoms, are consistent with these observations. In the case .of the niobates, modulation of the layer spacing by ion exchange with different alkali metal cations shows that the hydrogen evolution rate decreases sharply as the average layer spacing increases. This suggests that the competition between charge recombination and electron tunneling between layers determines the efficiency of the HI photolysis reaction. 120 POSTER #29 IMPROVED EFFICIENCY AND STABILITY OF Si PHOTOCATHODES BYELECTROCHEN.UCALETCHING D. Mao, K. J. Kima, Y. S. Tsuo, and Arthur J. Frank Basic Sciences Division, National Renewable Energy Laboratory Golden, Colorado 80401 We have examined the photoelectrochemical behavior of porous Silp-Si electrodes in CH3CN containing CoCpz+/CoCpz-LiCl04. The porous Si layer is formed on the surface of crystalline Si by electrochemical etching in acid fluoride solution. A comparison is made between the electrochemically etched and chemically etched photoelectrodes. Although both methods of etching remove the insulating oxide layer and defect sites near the surface, only the electrochemical etching generates the porous surface structure on the crystalline Si substrate. The presence of this porous structure improves both the photostability and output characteristics of the Si photocathodes. Auger depth profiling studies of the photoelectrodes provide detailed information on the chemical nature of the surface and give important insight into the mechanism controlling the PEC behavior of the cell. These results and others will be discussed. a Visiting professor, on leave of absence from Korea University, Seoul, Korea. 121 POSTER 1130 SIMULATION STUDIES OF ROTATIONAL DIELECTRIC FRICTION Arno Papazyan, P. Vijaya Kumar, and Mark Mamncelli Department of Chemistry The Pennsylvania State University University Park, PA 16802 Rotational motion of a solute in condensed phases is one of the simplest probes of the sorts of dynamics involved in chemical reaction. Understanding the friction experienced by a rotating molecule can therefore be viewed as a first step in understanding the dynamical effects of solvents on reactive processes. We have undertaken a number of computer simulations which examine how electrostatic interactions affect rotational dynamics. In particular we are interested in characterizing the "extra" friction experienced by a polar compared to a nonpolar solute in a polar solvent This extra friction is termed "dielectric friction", and it can produce significant slow-down of reorientational (as well as translational) dynamics in polar solvents. Most of our results to date are based on simulations of rotational dynamics of point dipolar solutes in a Brownian dipole lattice solvent. The latter consists of a collection of point dipoles situated on a simple cubic lattice. This model is a crude representation of a real polar liquid, but its simplicity allows for a rigorous separation of dielectric and other sources of friction and lays bare the essentials of the dielectric friction effect. In addition it provides an ideal testing ground for analytical theories. A number of interesting results have emerged from these simulations. For example, there is an unexpected and marked dependence of rotational dielectric friction on the charge of the solute. We also find that none of the commonly used theories, which use as input the Solvent dielectric response, adequately describes the rotational dynamics observed in these systems. However, using the connection between solvation dynamics and time-dependent friction does provide a relatively simple route to understanding the solvent effect on rotation. In order to assess the generality of the observations made on the dipole lattice solvent, we are currently undertaking simulations of dielectric friction using more realistic solute and solvent representations. Results of these simulations will also be presented. 122 POSTER #31 PHOTOPHYSICS OF SURFACE MODIFIED SMALL PARTICLES C. Luangdilok, S. Kartha, S. Kapoor, D. Lawless and Dan Meisel Chemistry Division, Argonne National Laboratory, Argonne, IL 60439 The effect of functionalized moieties and their charge at the surface of quantum size semiconductor particles on photophysical processes of the particles was studied. Small CdS particles were synthesized and molecular species were selectively attached to their surface. Attachment of the functionalized capping molecules was achieved via binding thiolate moieties to CdS and metallic particles, binding carboxylate moieties to metal oxide (mostly TiOz) particles or adsorption of hydrophobic organic molecules to the particles. Three specific effects were sought: a) the effect of excess charge near the particle on its absorption spectrum, b) the effect of inert (alkali) ions on quenching of luminescence from the particles, and c) the effect of one particle on the photophysics of a neighboring particle. Several molecules, whose one electron redox potentials lie within the band gap of the semiconduc tor material (nitrothiophenols and several quinones), were attached to small CdS particles. This capping then is equivalent to creating a well-defmed, energetically and spatially, trap at the surface. Capping the particle with these acceptors leads to quenching of the particle emission via interfacial electron transfer. The corresponding radical anions (generated at the surface either via photoexcita tion or by injection of electrons from the solution phase) are substantially longer-lived at the surface than when free in solution. The localization of the excess electrons on the radical could be established by recording the characteristic absorption spectrum of the radical ion. Nevertheless, the absorption of the particle is also affected by the electric field that is induced by the radical ion; broadening of the exciton band is observed. The radical ions of both nitrothiophenols and benzoquinones show this effect in spite of their different spectra, pKa's, redox potentials and stability. This variant of the Stark electric field effect offers a method to control the absorption pro perties of the particle and induce nonlinear optical effects even when the excess electron localizes externally to the particle. Emission from CdS particles is quenched by attaching dithio-benz-crown ethers to the particles. However, the emission is recovered by selective inclusion of alkali metal ions in the crown ether. This emission recovery results from redistribution of electrons away from the benzene ring and into the crown ether by the included alkali ion. Wavelength selective quenching of CdS particles emission was also observed upon binding TiOz or Au colloidal particles to the former. The binding was obtained utilizing bifunctional bridges, thiol-carboxylate in the former and dithiol in the latter case. The selectivity of quenching is attributed to the wavelength dependent free energy electron-hole recombination. 123 POSTER 132 SYNTHESIS AND CHARACTERIZATION OF GaP, loP, AND GalnP2 QUANTUM DOTS Olga I. Micic, Calvin J. Curtis, Julian R. Sprague, and Arthur J. Nozik National Renewable Energy Laboratory 1617 Cole Blvd., Golden, CO 80401 Quantum dots of III-V semiconductors have not been studied widely because of numerous difficulties encountered in their pre~aration. Recently, several investigations have been 1 reported on the synthesis of GaAs - quantum dots. In the present work, colloidal dispersions of quantum dots of crystalline GaP, InP, and GalnP2 from 15 to 40 A in diameter with consistent crystal structures were synthesized. Colloidal dispersions of quantum dots were synthesized starting from chlorogallium oxalate or chloroindium oxalate and P(SiMe3h. First, a GaP or InP precursor was formed from the reactants at room temperature and then thermally decomposed in a solution of trioctylphosphine and trioctylphosphine oxide at 280-380°C for several days. Lattice fringes of GaP, InP, and GalnP2 colloidal particles were observed by transmission electron microscopy. X-ray diffraction data for powders obtained by drying colloidal dispersions of quantum dots show that the growth of GaP and GalnP2 nanocrystallites requires higher temperature (>300°C) then that for InP (>250°C). The optical absorption spectra of GaP, lnP and GalnP2 quantum dots are blue shifted with decreasing particle diameters. The emission spectra of GaP and InP QDs have broad bands with peaks that are red-shifted with respect to the position of the excitonic peaks in the 3 absorption spectra. Preparing the InP precursor with an excess of In + and then heating it in a mixture of trioctylphosphine oxide and trioctylphosphine at 270° produced a very narrow spread (<±2A) of InP QD diameters, and, for the first time, shows an exciton peak for III-V QDs. From the known InP QD size distribution and its absorption spectrum, we used a theoretical model to calculate the dependence of InP QD bandgap on QD size. 4 The increase in the InP QD bandgap with decreasing QD diameter caused by quantum confinement was found to be much less than predicted by the effective mass approximation. (1) Olshavsky, M.A.; Golstein, A.N.; Alivisatos, A.P., J. Am. Chern. Soc.~1990, 112, 9438. (2) Uchida, H.; Curtis, C.J.; Nozik, A.J., J. Phys. Chern. 1991, 95, 5383. (3) Uchida, H.; Curtis, C.J.; Kamat, P.V.; Jones, K.J.; Nozik, A.J., J. Phys. Chern. 1992, 96, 1156. (4) Micic, 0.1.; Curtis, C.J.; Kamat, P.V.; Jones, K.J.; Sprague, J.R.; Nozik, A.J. J. Phys. Chern. (In press). 124 POSTER #33 ELECTRIC FIELD EFFECTS ON ULTRAFAST PHOTOINDUCED ELECTRON TRANSFER ACROSS SEMICONDUCTOR-LIQUID INTERFACES Y. Rosenwaks, B.R. Thacker, and A.J. Nozik National Renewable Energy Laboratory Golden, Colorado 80401 A rigorous analysis of the effects of electric fields on time-resolved photoluminescence in, semiconductors is presented. We show that the effect of the field quenching of the photoluminescence decay caused by charge separation can be distinguished from the effects of field-enhanced surface recombination and electron transfer. Therefore, it becomes possible to extract the recombination and/or transfer velocity of photoinduced charge carriers from time resolved photoluminescence decay experiments in the presence of electric fields, This approach was applied to determine the rates of photoinduced electron transfer from p-InP and p-GaAs to various acceptors in liquid electrolytes as a function of the initial potential drop (i.e., band bending (V80 ) in the semiconductor space charge layer using transient PL spectroscopy (fs luminescence up-conversion and ps single photon counting). The effects of electric field on electron transfer were separated from the effects of field-enhanced charge separation and surface recombination through a rigorous numerical solution of the coupled continuity and Poisson equations using a Cray supercomputer. A very strong dependence of the electron transfer velocity (Set) on V 80 was found for some acceptors. For example, for p- InP and Fe { CN) ~-/4 -, Set reached a saturation value of 5 x 107 crn/s when the initial value of V 80 in the dark was ~ 0.5 e V; this corresponds to electron transfer times < 1 ps. When the 3 initial value of V 80 was set near zero, Set was 9 x 10 crn/s. Hot electron injection processes appear to play a role in this behavior. The values of Set for other acceptors were found to be unaffected by the space charge field. A model for the dynamics of electron transfer from semiconductors to acceptor species as a function of electric field in th~ space charge layer of the semiconductor will be presented to explain our results. 125 POSTER 134 ELECTRON TRANSFER FROM LIPID FUNCTIONALIZED PYRENE IN LANGMUIR FILMS TO LIPID BOUND AND BULK PHASE ACCEPTORS. Li Li and L. K. Patterson Radiation Laboratory, University of Notre Dame Notre Dame, Indiana 46556 Kinetics of electron transfer between excited state lipid functionalized pyrene [1-(10-(6(8)-0ctadecylpyrenyl)decanoyl)-2-hexanoylphosphatidyl choline, (OPyPC)] and alkyl viologens as well as related acceptors have been examined in spread lipid monolayers and corresponding vesicle systems. The lipids, zwitterionic dioleoylphosphatidylcholine, (DOL), negatively charged dioleoylphosphatidylglycerol, (DPG) and neutral dioleoy1glycerol, (DOG) have been used to determine the effects of headgroup on reaction rates. Using the treatment by Owen for two dimensional diffusion, it is observed that quenching by 4-(N hexadecylpyridinium-4-yl)pyridine chloride, (CB+) gives apparent diffusion coefficients similar to those obtained earlier by excimer formation A parallel measurement in vesicles gives diffusion coefficients close to those found in monolayers, suggesting that some processes in the two systems may be compared. Electron transfer to methyl viologen in the subphase is highly dependent on the lipid headgroup. Surprisingly, in the presence DOG with its neutral headgroup, dynamic transfer occurs much more rapidly than it does in DOL. Negative!( charged DPG accelerates transfer to a bulk rate constant of 1013 M-1s- due to high surface potential, giving the most efficient transfer kinetics. Measurement of back reaction in vesicles shows that for lipid functionalized acceptors, the back reactions occur in accordance with two dimensional diffusion coefficients measured in forward reactions. However, because of monolayer viscosity, the ion pair lifetime will be long. Back reaction with subphase acceptor is even slower, by a factor > 2. As expected, transfer by the triplet mechanism occurs two orders of magnitude more slowly than from singlet. 126 POSTER #35 ANALYTIC QUANTUM THEORY OF ELECTRON TRANSFER WITH A REACTION MODE STRONGLY COUPLED TO THE ELECTRON AND WEAKLY COUPLED TO THE BATH Katja Lindenbergs., Emilio Cortesb and Robert M. Pearlsteinc,d aDepartment of Chemistry and Institute for Nonlinear Science, University of California at San Diego, La Jolla, CA 92093 bDepartamento de Fisica, Universidad Autonoma Metropolitana Iztapalapa, P.O. Box 55-534, 09430 Mexico D. F., Mexico cPhysics Department, Indiana University-Purdue University at Indianapolis, Indianapolis, IN 46202 dchemistry Division, Argonne National Laboratory, Argonne, IL 60439 We reconsider the old problem of donor-acceptor electron transfer with a strongly coupled reaction mode. In the past, this problem has been treated either heuristically or numerically, but not analytically. Here, we develop an effective analytic framework built around our own extension of standard small polaron theory. We assume a Hamiltonian in which the electron operators are strongly coupled to a single linear oscillator, the reaction mode, which is in tum weakly coupled to the heat bath. The Hamiltonian is transformed so that the previously bare electron is dressed in the quanta of the reaction mode, but not in the quanta of the rest of the bath modes. The dressed electron, or "reacton", has no residual interaction with the transformed reaction mode; its remaining interactions with the bath can be treated perturbatively. Although our formalism describes the full kinetics, we present in detail here only the results of a Golden Rule calculation of the electron transfer rate constant. We fmd that in the strict high temperature limit the rate constant is of the Marcus form, but with a reorganization energy that is simply the product of the reaction mode quantum energy and the dimensionless (strong) coupling constant squared, independent of the details of the phonon spectrum. This contrasts with earlier fmdings that the reorganization energy is a weighted sum over the bath mode frequencies. We observe that our result may provide a basis for explaining the anomalously small values of the reorganization energy deduced for primary charge separation in photosynthetic reaction centers. Finally, we discuss the lowest order corrections to the high-temperature rate constant, noting the sensitivity of these to the nature and symmetry of the coupling betw~n electron and reaction mode. 127 POSTER #36 EXCITED STATE ELECTRON TRANSFER REACTIONS IN POLYMETALLIC COMPLEXES John D. Petersen, Sean L. Gahan, Seth C. Rasmussen and Nuh-Tome Huang Department of Chemistry, Clemson University Clemson, South Carolina 29634-1905 and Department of Chemistry, Wayne State University Detroit, Michigan 48202 The preparation of polymetallic complexes containing light absorber, electron-donor and electron-acceptor components involves detailed synthetic methodology. In this three component system, there is also the variability of the spacers which connect the electron donor to the light absorber and connect the light absorber to the electron acceptor. The parent system discussed will be the (NC)4Feli(BL1)Ruli(bpy)(BL2)Rhlll(tpy)(MQ+)4+, where Fell is the electron donor, the Ru(m polyazine is the light absorber and MQ+ (N-methyl-4,4'-bipyridium) is the electron acceptor. The bridging ligands in these systems are typically bidentate agine systems such I as 2,2 -bipyrimidine (bpm) and 2,3-bis(2-pyridyl)pyrazine (dpp). Transient absorbance measurements will be reported on the complex series as well as bimetallic analogs and model complexes. Current work involving the replacement of BL2 with Cu(m hexafluoro acetylacetonato polymers will be described as will the modifications of the potential as the electron acceptor. 128 POSTER #37 AXIAL LIGAND CONTROL OF OXIDATION STATES IN NONPLANAR PORPHYRINS: CONVERSION OF Ni(ll) r CATION RADICALS TO "Ni(lll)" CATIONS M.W. Renner, K.M. Barkigia, Y. Zhang, K.M. Smith and J. Fajer Department of Applied Science Brookhaven National Laboratory Upton, New York 11973-5000 Porphyrin 1r cation radicals incorporating high oxidation states of Fe, Mn, Cr and Ru catalyze epoxidations, hydroxylations and oxidations of alkenes, alkanes and other organic substrates. Ni(lll) tetraaza complexes catalyze similar oxidations, and Ni(III) tetrapyrroles (Factor 430) have been invoked in the catalytic cycle of methanogenic bacteria. We have recently investigated a series of low spin Ni(ll) meso-tetraphenylporphyrins in which the introduction of alkyl or aryl {3 pyrrole substituents forces the molecules to adapt severely distorted S4 saddle conformations (e.g. 2,3,7,8,12,13, 17,18-octaethyl-5, 10,15,20- tetraphenylporphyrin: OETPP). The distortions significantly alter the optical, redox and basicity properties of the molecules and prevent the formation of high spin Ni(II) upon addition of axial ligands such as pyridine (JACS 115, 3627, 1993). We report here that NiOETPP and related compounds in the series are oxidized to r cation radicals in CH2Cl2, a non-coordinating solvent, and that the radicals are readily converted to "Ni(lll)" cations upon addition of pyridine, imidazole, methanol or tetrahydrofuran, as evidenced by EPR spectroscopy: In neat CH2Cl2 at 113K, oxidized NiOETPP exhibits a g =2 EPR spectrum characteristic of a 1r cation radical which is quantitatively converted to a new spectrum with gJ. = 2.224 and g 0 = 2.072 upon addition of pyridine. The latter signal is typical of hexacoordinated Ni(lll) species and, indeed, hyperfine splittings due to two axial nitrogens are observed with 15N pyridine. The "Ni(lll)" species are stable, and the molecular structure of an oxidized [NiOETPPr Cl04- with one pyridine and one imidazole as axial ligands has been determined crystallographically. The structure exhibits several unusual features: i) the perchlorate counterion is hydrogen bonded to the N-H of the imidazole, ii) the porphyrin retains its S4 distortion, and iii) the distances of the Ni to the porphyrin nitrogens are longer than expected from the contractions observed on oxidation of Ni(ll) tetraaza complexes to Ni(lll). Indeed, the Ni-N distances are only slightly shorter (-0.05A) than those found in high spin hexacoordinated Ni(ll) porphyrins. The crystallographic results thus do not exclude the possibility that the oxidized Ni complex is a high spin Ni(ll) 1r cation radical in which one of the unpaired electrons of the d8 Ni is antiferromagnetically coupled to the radical leaving a single electron in the ~2 orbital, i.e. a pseudo Ni(III). Oxidation of the simpler NiTPP or NiOEP in the presence of axial ligands does not yield the equivalent Ni(lll) species. The results with the sterically encumbered NiOETPP and the related series thus further illustrate the significant effects that conformational distortions can have on the chemical properties of porphyrins. 129 POSTER 1138 THE CHEMICAL AND PHYSICAL PROPERTIES OF A PLATINUM(ll) BIPHENYL DICARBONYL COMPLEX D. Paul Rillema, Y. Chen and Jon Merkert Department of Chemistry The University ofNorth Carolina at Charlotte · Charlotte, North Carolina 28223 Complexes of platinum(ll) containing the ligand biphenyl attached in the 2,2'-position have been explored in our laboratories as potential two electron solar energy photocatalysts. Recently we isolated the dicarbonyl complex which proves to be a valuable intermediate source compound for many other derivatives. The Pt(bph)(C0)2 compound crystallized as green parallelepipeds from toluene or benzene in the space group Cmcm with a= 18.647(6)A, b = 9.566(2)A and c = 6.4060(5)A. The structure as viewed down the c axis is shown below. The Pt-C(bph) bond length is 2.04(2)A; the Pt-C(CO) bond length is 1.98(2)A. The C-Pt-C angle involving the coordinated carbon atoms of the biphenyl ligand is 80.5(8)0 which is responsible for the deviation 0 from ideal square planar geometry. The C(bph)-Pt-C(CO) bond angle is 173.0(7) • The complex displays three different types of emission behavior <"-ex= 330 nm). At 77K in a 4:1 ethanol-methanol glass, 3Lc emission is observed with Eem = 499 nm and vibrational · progressions of 1440 and 1290 cm-1. The emission spectrum in solution is solvent dependent and exhibits both 3LC and 3MLCT behavior. The spectrum in 4:1 ethanol-methanol is structured with Eem located at 509 nm. However, in CH2Cl2 the spectrum is broad and red shifted to 561 nm. At room temperature in the solid, emission is observed at 733 nm which is further red-shifted at 77K to 865 nm. The large red-shift in the solid can be rationalized on the basis of the 3.24A spacing between stacked platinum complexes which raises the energy of the filled ciz2 orbitals by interaction with the empty p orbitals of the complexes above and below it. 130 POSTER #39 FEMTOSECOND PHOTODICHROISM STUDIES OF ISOLATED PHOTOSYSTEM ll REACTION CENTERS Michael Seiberfl, Gary P. Wiederrechth, and Michael Wasielewskih D&sic Sciences Division, National Renewable Energy Laboratory, Golden, Colorado 80401 hchemistry Division, Argonne National Laboratory Argonne, Illinois 60439 Photosynthetic conversion of light energy into chemical potential begins in reaction center protein complexes, where rapid charge separation occurs with nearly unit quantum efficiency. Primary charge separation was studied in isolated photosystem II reaction centers from spinach containing 6 chlorophyll a, 2 pheophytin a, 1 cytochrome b559, and 2 {j-carotene molecules. Time-resolved pump-probe kinetic spectroscopy was carried out with 105 fs time resolution and with the pump laser polarized parallel, perpendicular, and at the magic angle (54. 7°) relative to the polarized probe beam. The time evolution of the transient absorption changes due to the formation of the oxidized primary electron donor P680+ and the. reduced primary electron acceptor Pheo- was measured at 820 nm and 545 nm, respectively. In addition, kinetics were obtained at 680 nm, the wavelength ascribed to the QY transition of the primary electron donor P680 in the reaction center. At each measured probe wavelength, the kinetics of the transient absorption changes can be fit to two major kinetic components. The relative amplitudes of these components are strongly dependent on the polarization of the pump beam relative to that of the probe. At the magic angle, where no photoselection occurs, the amplitude of the 3-ps components, which is indicative of the charge separation, dominates. When the primary electron acceptor Pheo is pre-reduced, the 3 ps component is eliminated. 131 POSTER 140 13-DESMETHYL-13-ACETOXYRETINAL: A NEW CHROMOPHORE FOR BACTERIORHODOPSIN Stanley Seltzer Chemistry Department, Brookhaven National Laboratory Upton, New York 11973 The purple membrane of Halobacterium halobium functions as a light-driven proton pump thereby converting solar to chemical energy. An obligatory part of its photocycle is the photo catalyzed all-trans -713-cis isomerization of its bound retinal and its thermal dark reisomerization of 13-cis -7 all-trans in a later step. Previous studies at BNL support the idea that the dark cis trans isomerization is catalyzed by the reversible addition of a nucleophile, aspartate-212 of bacteriorhodopsin, to C13 of retinal, bound to lysine-216 as a protonated Schiff base. 13- Desmethyl-13-acetoxy-all-trans-retinal (I) was targeted as an all-trans-retinal (II) replacement ~CHO II with the hope that it would form a pigment with the apoprotein, bacterioopsin, and that after nucleophilic addition of aspartate-212 at retinal's Cl3, followed by bond rotation, loss of the 13- acetoxy group would compete favorably with loss of the aspartate, resulting in a cross-linked denatured bacteriorhodopsin. 13-Desmethyl-13-acetoxyretinal was synthesized from ~-ionone in five steps. The 13-cis isomer of I forms a pigment with bacterioopsin with Amax 562 nm which in the dark, quickly shifts to 574 nm. Short irradiation rapidly bleaches the 574 nm absorption but unlike native bacteriorhodopsin (bR) this visible absorption maximum does not return. The 13-acetoxy-bR in the presence of retinal (II) very slowly exchanges its chromophore for retinal. Preliminary results, however, indicate that the amount of replacement by retinal is substantially reduced by prior irradiation of the 13-acetoxy-bR. The present results suggest a partial loss of bR viability due to the binding of this new chromophore and its subsequent reactions initiated by light. The synthesis and properties of 13-desmethyl-13-acetoxyretinal and its reconstituted bacteriorhodopsin will be discussed. 132 POSTER 141 AN INDIRECf LASER-INDUCED TEMPERA TIJRE JUMP STIJDY OF THE KINETICS OF ELECfRON TRANSFER THROUGH :MIXED TmOU FERROCENE TER.l\11NA TED ALKANETillOL MONOLA YERS ON GOLD John F. Smalley,1 Stephen W. Feldberg/ Christopher E.D. Chidsey,3 Marshall D. Newton,2 Yi-Ping Liu2 1Department of Applied Science and 2Chemistry Department Brookhaven National Laboratory Upton, New York 11973-5000 3Department of Chemistry Stanford University Stanford, California 94309 The indirect laser-induced temperature-jump (ILIT) technique was used to study the rate of electron transfer between a gold electrode and surface-attached ferrocene moieties. The ferrocene is part of a self-assembled monolayer composed of CH3(CH2) 0 • 1SH and 5 5 (11 C5H5)F e(rt C5H4)COiCH2) 0 SH. The kinetics of electron transfer were studied as a function of the distance between the electrode and the ferrocene moieties which is determined by n, the number ofmethylenes in the alkyl chain ofthe ferrocene compound. The electrolyte was 1M HCl04. For of n between 5 and 9 the standard electron transfer rate constants vary according to ks r(O)e·Pn where ~ r(O) is the standard rate constant for the electron transfer 1 at n=O (units: s: ) and f3=1.21±0.05 at T=25°C. These rate constants were also measured as a function of temperature, and the reorganization energies (A.) for the electron transfer process were determined as a function of n from these data. Both the variation of kr,s with distance (n) and the reorganization energy measured for n=8 and 9 are in reasonable agreement with the results reported by Chidsey et al. for n between 8 and 18. However, for n<8, the values of A. begin to decrease, and the amount of this decrease is larger than can be explained by image charge interactions alone. Possible reasons for this behavior of the reorganization energy will be discussed. 133 POSTER /142 VIBRATIONAL DYNAMICS IN PHOTOINDUCED ELECTRON TRANSFER Kenneth G. Spears and Xiaoning Wen, Department of Chemistry Northwestern University, Evanston, IL 60208-3113 Electron transfer (ET) involves vibrational states of the donor acceptor complex because molecular distortions are normally required for energy matching. For some cases efficient couplings of electronic states may be enabled by specific vibrational states. Despite the importance of vibrational states for refining our understanding of ET, only indirect inference of vibrational effects has been experimentally possible until our recent work in this program. We have provided the first measurements of ET as a function of vibrational populations, and we are now extending the initial data and developing new electron transfer models. This work is especially important for photoinduced ET, where the excitation event can leave excess vibrational energy in the molecules. Fast (sub-picosecond) intramolecular vibrational redistribution (IVR) and vibrational relaxation (VR) prevents observation of vibrational state selected ET rates in most molecules. Therefore we selected a molecular system where the intramolecular vibrational relaxation was slow enough so that electron transfer can compete; and where the innovative method of picosecond infrared (IR) absorption spectroscopy could uniquely identify vibrational levels of both the reactants and products of the electron transfer (ET). The first system we have studied is for a solution of bound ion pairs, {Cp2Co• Co(C0)4], where the two cobalts interact to form a broad charge transfer (CT) band peaked at 520 nm. Our experimental clock begins by picosecond excitation of the CT band at 600 nm to form a neutral ion pair. This neutral pair has unique IR transitions so that the populations having quantum number 0, 1, 2 in the CO stretching mode can be monitored with time. The decays of these levels are due to electron transfer to reform the ion pair complex and intramolecular vibrational redistribution. By varying the dielectric constant (exothermicity) we can make the ET rate faster than the slow IVR rate of the carbonyl stretching modes. In addition to measuring ET rates of the neutral pair, we also find that the ET populates up to 4 quanta in the ion pair. The quantum numbers and kinetics of the risetimes for the ion pair provide more tests of the vibrational model for electron transfer in this system. Furthermore, the IVR in the ion pair is a unique test of how very vibrationally excited molecules lose their energy in solution. The first level of data interpretation has required a new theoretical model for ET in competition with IVR, where IVR includes both the high frequency modes that we excite in specific quantum numbers and a second, communicating states pool of low frequency modes. We show that for state selected experiments the ratio of the ET rate in level 1 to level 0 is a very good test of electron transfer models. ET can be much faster when energy is in specific states and/or in the communicating states pool of energy. These are the first theoretical and experimental insights into such processes, and they demonstrate the importance of vibrational populations in fast ET. The next level of data collection and interpretation has begun, where we endeavor to make very specific tests of ET theory. These tests require data to define precise vibrational wavefunctions for our molecules, data in a variety of solvent polarities to define molecular energetics, and data for related molecular systems. This advanced analysis will define whether specific electronic-vibrational coupling is important for these systems or whether conventional electron transfer models are adequate. 134 POSTER #43 Vibrational and Electronic Structure of the Excited States of 13-Substituted Metalloporphyrins from Time-Resolved Resonance Raman Spectroscopy Ranjit Kumble, Glen R. Loppnow1, Songzhou Hu and Thomas G. Spiro Dept. of Chemistry, Princeton University, Princeton NJ 08544 Resonance Raman spectra of the S1 and T1excited states of zinc (II) complexes of j3-substituted metalloporphyrins have been obtained using picosecond and nanosecond two-color time-resolved techniques. Porphyrins with saturated substituents (octaalkyl porphyrins) and unsaturated substituents (protoporphyrin) have been studied: in the 1 former, vibrational features have been assigned by analyses of 5N and meso-d4 isotopomers. The S0 (ground) state and S1 state appear to have identical normal mode compositions, as judged by the magnitude of isotope shifts seen for vibrational modes in both states. All modes observed in the S 1 RR spectra exhibit frequency downshifts from their S0 values, implying an overall expansion of the macrocycle in the excited state. The maximum increase in bond length occurs at the methine bridges, with smaller accompanying expansions of the C«-CN, C«-C13 and C13-c13 bonds. The direction and relative magnitudes of these bond length changes are consistent with the description of the 1 1 S1 state as an admixture of singly-excited (a1ueg) and (~eg) configurations. Vibrational modes containing substituent contributions show downshifts smaller than 5 cm·1, indicating unaltered substituent bonds in the excited state for both saturated and unsaturated substituents and suggesting weak electronic coupling with the porphyrin x system. Assignment of vibrational modes in: the T 1 RR spectra is less straightforward, due to the lack of sensitivity of observed fearures to the available isotopic replacements. The ~-C13 stretching modes are seen to downshift in the T1 state, contrary to the expected behaviour for a pure 3(a1ueg) electronic configuration and possibly reflecting a symmetry lowering J ahn-Teller distortion. 1 Present Address: Dept. of Chemistry, University of Alberta, Edmonton, Alberta T6G 2G2, Canada 135 POSTER 144 CHARGE-TRANSFER XNTERACTIONS IN MESO-LINKED PORPHYRIN-QUINONE MOLECULES John s. Connolly, Russell A. Cormier, Tina L. Palmer and Julian R. Sprague Basic Sciences Division National Renewable Energy Laboratory Golden, Colorado 80401 USA We previously reported1 the synthesis and characterization of a zinc por phyrin-anthraquinone molecule (ZnPAQ, 1) in which the quinone is attached at a meso-position of tritolylporphyrin. The absorption spectra of 1 show only minor perturbations compared to the spectra of the non-linked reference mole cules. The emission spectra, however, display a strongly solvent-dependent band that is observed even in low~dipolar solvents such as benzene and tolu ene, which we assigned to a charge-transfer (CT) state. Both the emission decay profiles and the spectra are more complex than predicted for a simple three-state (So, s 1 and CT) model. This is attributed to a distribution of P AQ dihedral angles that give rise to a distribution of short lifetimes that, in each case, relax to a single, longer lifetime. It appears that there are also distributions of spectral components (both CT and S1), which are respon sible for the complexity of the emission spectra. We now report synthesis of an analogous structure (H2PNQ,·~) which con tains the more easily reduced naphthoquinone (NQ) in place of AQ. The ener getics for electron transfer (ET) in~ are about the same as those in 1, i.e., ca. -0. 5 eV, uncorrected for coulombic interactions. However, the sol vent dependence of the CT interactions in these two molecules is quite different. While 1 exhibits CT emission, ~ shows only porphyrin fluorescence, the inten sity and lifetime of which also decrease progressively in the same binary sol vent system (toluene, e = 2.4, and o-fluorotoluene, e = 4.2). We conclude that the reorganization energy is greater for free-base ~ than for metalated 1· Since the contribution from the solvent is the same in both cases, the differ ence (-0.1 eV) must lie in the nuclear reorganization term. Specifically, we infer that the free-base porphyrin ring is distorted either in the excited singlet state or in the cation; the former is consistent with our interpreta tion of the fluorescence behavior of free-base tetraaryl porphyrins.2 The existence of CT states in these dyad molecules indicates that triads and tetrads based on this simple architecture can be designed to maximize the fraction of excited-state energy that can be converted to stored redox energy. We have recently found evidence that a triad structure: ZnPAQ(nb)NQ [where (nb) is a norbornyl bridge] undergoes two-step ET in benzene.3 We expect that three-step ET will be detected in the analogous tetrad: ZnPAQ(nb)NQ(nb)BQ (where BQ =benzoquinone), synthesis of which is now nearly complete. REFERENCES: 1. Kamioka, K.; Cormier, R.A.; Lutton, T.W.; Connolly, J.S. J. Am. Chem. Soc. 1992, 11~, 4414-4415. 2. Fonda, H.N., Gilbert, J.V.; Cormier, R.A.; Sprague, J.R.; Kamioka; K.; Connolly, J.S. J. Phys. Chem., 1993, 97, 7024-7033. 3. Connolly, J.S.; Sprague, J.R.; Cormier, R.A.; Palmer, T.L. (to be sub mitted). 136 POSTER 145 FEMTOSECOND SPECTROSCOPY OF CHLOROSOME ANTENNAE FROM THE GREEN PHOTOSYNTHETIC BACTERIUM Chloroflexus aurantiacus Sergei Savikhinl, Yinwen Zhu2, Su Lin2, Robert E. Blankenship2, and Walter S. Strnvel 1 Ames Laboratory and Department of Chemistry, Iowa State University, Ames, lA 50011 and 2oepartment of Chemistry and Biochemistry and Center for the Study of Early Events in Photosynthesis, Arizona State University, Tempe, AZ 85287 The antenna kinetics of bacteriochlorophyll (BChl)c- and a-containing chlorosomes from the thermophilic green bacterium Ch/orojlexus aurantiacus were investigated with femtosecond pump-probe techniques. Isotropic one- and two-color absorption difference experiments using probe wavelengths between 710 and 770 nm reveal BChl c photobleaching (PB) and stimulated emission (SE) decay kinetics with major lifetimes of 50-100 fs, 1-2 ps, and 7-10 ps. Two-color PB/SE profiles pumped at 770 nm and probed at 800 nm (where BChl a is the principal absorbing pigment) exhibit no detectable risetime. However, two-color experiments using 790 and 820 pump and probe wavelengths, respectively, yields PB/SE rise components of 100 fs, 2 ps, and 10 ps. Upon excitation at 720 nm, the BChl c PB!SE spectrum observed using a broadband probe continuum displays surprisingly little spectral evolution during the first 2 ps. Upon 760 nm excitation, the BChl c PB/SE spectrum experiences a small blue shift (from 750 to 744 nm) during the first few hundred femtoseconds. The one-color anisotropies r(t) in the BChl c spectrum initialize very close to 0.4, and subsequently exhibit little decay (r(oo) - 0.36). One- and two color anisotropies probed at 800 nm are dominated by a component with lifetime - 10 ps; the final anisotropies are generally small, indicating that the energy transfers in this region are accompanied by a large reorientation of Qy transition moments. The absorption difference profiles in the BChl c (but not the BChl a) region of the chlorosome spectrum contain oscillating components at early times, which are damped within -2 ps. They likely arise from vibrational coherences in the BChl c aggregate. Our results do no appear to be consistent with subpicosecond spectral equilibration among inequivalent pigments within the main BChl c absorption envelope. The subpicosecond rise of the PB/SE absorbance changes in the 800 nm region suggests that pigments absorbing at this wavelength are strongly coupled to the main BChl c pigment aggregates. 137 POSTER #46 IDGHLY SELECTIVE OXIDATION OF TOLUENE AND CYCLOHEXANE BY 0z WITH VISffiLE UGHT IN ZEOLITE Y Hai Sun, Fritz Blatter, and Heinz Frei Laboratory of Chemical Biodynamics, Lawrence Berkeley Laboratory, University of California, Berkeley, CA 94720 The majority of organic building blocks and intermediates needed for the manufacture of plastics and other synthetic materials are made by partial oxidation of small hydrocarbons. Many current synthesizing processes in industry create problems for the environment and waste energy. Selective oxidation by molecular oxygen is particularly important and challenging in the field of environmentally benign chemical synthesis. Here we report the first selective photooxidation of toluene and cyclohexane by Oz with visible light in zeolite Y. In this work the chemistry was monitored by FT-infrared spectroscopy, and a tungsten lamp and cw lasers were used as irradiation sources. Toluene and Oz loaded into barium exchanged (BaY) and calcium exchanged zeolite Y (CaY) were found to react upon irradiation with visible light to give benzaldehyde as the sole fmal oxidation product. Diffuse reflectance spectra showed a continuous absorption tail in the visible region, which is attributed to a toluene/{}z charge-transfer transition. The visible absorption onset at 600 nm in CaY implies a stabilization of the charge-transfer state relative to {}z-saturated toluene solution by over 10,000 cm-1. We attribute it to the high electrostatic field inside the zeolite cages. In BaY the selective oxidation yielded no spectroscopically detectable intermediate, but kinetic evidence supports that benzyl hydroperoxide is produced along the reaction path. In CaY a chemical intermediate could be trapped and monitored by infrared spectroscopy even at room temperature. It reacted under continuous irradiation to form benzaldehyde. Work is in progress to elucidate its structure. This visible light-induced reaction constitutes the first selective oxidation of toluene to benzaldehyde by Oz in any phase. Photochemistry with visible light was also observed upon loading cyclohexane and Oz into CaY, BaY, and sodium exchanged zeolite Y (NaY). Cyclohexyl hydroperoxide was found to be the primary product, which undergoes slow thermal decomposition at room temperature. At -50 OC cyclohexyl hydroperoxide is very stable in CaY and can be readily generated by continuous blue light irradiation. The hydro peroxide in BaY and NaY is more stable at room temperature than that in CaY. Work is in progress to elucidate the thermal decomposition product. Small alkenes were found to react with the hydroperoxide in these zeolites to give corresponding epoxides and alcohols. The room temperature epoxidation without catalysts is an important finding that has not been achieved under any other reaction conditions. This study demonstrates the power of using zeolites as a reaction medium to carry out product specific photooxidation of aromatics and alkanes by molecular oxygen. Since only abundant oxygen and visible light were used with no added catalysts, the selective oxidation reactions described here have the potential to become efficient, inexpensive and environmentally clean alternatives to the current industrial processes. 138 POSTER #47 QUINONE BINDING IN PHOTOSYNTHETIC REACTION CENTERS Aaron Barkoffl, Seth W. Snyderb, Yuenian Zhangc, Daisy Zhang, Agnes Ostafm, James R. Norris and Marion C. Thumauer Chemistry Division, Argonne National Laboratory Argonne, lllinois 60439 A general feature of photosynthetic energy conversion in plants and bacteria is charge separation between an oxidized chlorophyll species (P+) and reduced quinone (Q-), separated spatially by the intervening acceptor(s) (X) to give p+xQ-. The formation of this state is optimized by the particular structural and energetic properties of the natural photosynthetic systems. As part of our studies directed at understanding these properties, we are interested in determining the structure of the quinone binding sites in the photosynthetic reaction centers (res). Using time-resolved electron paramagnetic resonance techniques, we have discovered that the quinone electron acceptor A1 (identified as vitamin K1 [1]) in green plant photosystem I (psI) is labile [2], i.e., a protonated vitamin K1 can be exchanged with a deuterated vitamin K1 and vice versa simply by incubating the respective quinone with the respective ps I protein. We found this process to be enhanced by light and heat. If the acceptor A1 is comparable to the first quinone acceptor, <4, in the purple bacterial rc protein, then the lability of A1 in psI is quite surprising. In the case of the purple bacterial rc, only the second quinone acceptor, ~. is labile. We are interested in determining the role of A1 in ps I electron transport, and the nature of the binding of this quinone in the rc protein. In this work we have systematically characterized the exchange process by studying the effects of temperature, incubation time, quinone structure, and source of ps I particles (spinach vs. thermophilic cyanobacteria). We found that there is a transition temperature at which the quinone exchange process· is significantly enhanced and that this temperature depends on the source of the ps I proteins. Future experiments will probe the mechanism of this specificity. In addition to providing data with which to characterize the quinone exchange process, these experiments provide the opportunity to obtain a complete set of epr spectra of p+(X)Ai, i.e., spectra at X- and Q-band microwave frequencies from the following samples: 0 (P+ Ai), Dp+HAi, Hp+ 0 Ai, H(P+ Ai), where D and H signify deuterated and protonated, respectively. We use this complete set to model the structural properties ofP+(X)Ai. The simulations allow us to assess if the quinone exchange process modifies the structural characteristics of quinone binding and to compare the Qa and Qb binding sites in purple bacterial res. [1] Snyder, S. W., Rustandi, R. R., Biggins, J., Norris, J. R., Thumauer, M. C. (1991) Proc. Natl. Acad. Sci. USA, 88, 9895-9896. [2] Rustandi, R. R., Snyder, S. W., Biggins, J., Norris, J. R., Thumauer, M. C. (1992) Biochim. Biophys. Acta, 1101, 311-320. aANL Summer Research Participant, 1993. bPresent address: Abbott Laboratories, Drug Design and Delivery, Abbott Park, Illinois. cPresent address: University of Wisconsin, Department of Medical Physics, Madison, Wisconsin. 139 POSTER #48 PHOTOCHEMICAL WATER-SPLITTING USING ORGANOMETALLIC OXIDES AS SENSITIZERS David R. Tyler, Tim Aukett, and Greg Baxley Department of Chemistry, University of Oregon Eugene, Oregon 97403 We are studying Cp2MoO (Cp = 115 -CsHs) to evaluate its performance as a sensitizer/catalyst for photochemical water splitting using visible light. Interest in the Cp2Mo0 molecule stems from our observation that irradiation of this complex gives Cp2Mo and 0 2. In ''wet" solvents, the Cp2Mo reacts with H20 to regenerate Cp2MoO. The following cycle for water-splitting is proposed: The efficiency of this cycle, the complete set of products, and the wavelength dependence of the cycle will be reported. The photochemical generation of 0 2 in homogeneous solution is still relatively uncommon, and experiments are probing the mechanism of this reaction. Irradiation of Cp2MoO solutions at low temperature gives an intermediate species that decomposes to oxygen and Cp2Mo in the dark when warmed to room temperature. NMR and infrared spectroscopic studies suggest a dimeric structure with oxygen bridges. The Cp2MoO complex was made water-soluble by attaching ammonium and carboxylate groups to the cyclopentadienyl rings, e.g.: ) The syntheses and aqueous photochemistry of these molecules will be reported. 140 POSTER /149 FOURIER TRANSFORM EPR STUDY OF C60 TRIPLETS Carlos A. Stereo and Hans van Willigen Chemistry Department University of Massachusetts at Boston Boston, Massachusetts 02125 Irradiation of C60 in liquid solution with UV or visible light generates a transient EPR signal that can be detected up to hundreds of microseconds after excitation. The resonance was first reported by Closs et a/. 1 and was assigned to the triplet state of C60 ec60). The assignment was confirmed by the results of a Fourier Transform EPR (FT-EPR) study of photoinduced electron transfer from donors 3 2 (hydroquinone and tri-p-tolylamine) to C60 • The fact that triplet state formation and decay can be monitored with EPR is of interest because it offers the opportunity to investigate excited state quenching reactions even if they do not give rise to observable free radical products. Time resolved EPR measurements can give information on chemical kinetics as well as spin state dynamics (through CIDEP effects) providing mechanistic information that may not be accessible via other spectroscopic techniques. The EPR signal from triplet state C60 also gives an insight into molecular motion which is of interest in studies of photochemical reactions in heterogeneous media. Where triplet quenching does lead to cage escape of redox products, their formation and decay also can be monitored with FT-EPR.2 Since spectral resolution is high enough to get completely resolved signals from triplet precursors and doublet radicals, this offers the opportunity to measure the fraction of triplet quenching events that gives rise to cage escape products. In this contribution we will present results of an FT-EPR study of triplet C60 quenching by electron acceptors. The investigation complements data obtained in a study using transient optical absorption spectroscopy.3 1 Closs, G.L; Gautam, P.; Zhang, D.; Krusic, P.J.; Hill, S.A.; Wasserman, E. J. Phys. Chern. 1992, 96, 5228. 2 Steren, C.A.; Levstein, P.R.; van Willigen, H.; Linschitz, H.; Biczok, L. Chern. Phys. Letters 1993, 204, 23. 3 Nadtochenko, V.A.; Denisov, N.N.; Rubtsov, I.V.; Lobach, A.S.; Moravskii, A.P.; Chern. Phys. Letters 1993,208,431-435. 141 POSTER #50 PHOTOCAPACITANCE AND TIME-RESOLVED FLUORESCENCE STUDIES OF CHEMICALLY SENSITIZED TI02 ELEC1RODES K. H. Liao and D. H. Waldeck Department of Chemistry; University of Pittsburgh Pittsburgh, PA 15260 Photoelectrochemical processes at the interface between Ti02 and aqueous electrolyte are of fundamental and practical importance. Of particular interest are chemically modified Ti02 interfaces in which visible absorbing molecules are attached in order to sensitize the material to a wider range of the solar spectrum. Such chemical modification allows the investigation of charge transfer in these systems using time-resolved fluorescence spectroscopy. The particular system discussed here consists of single crystalline Ti02 electrodes which have been treated with organometallic complexes. · This study probes the energetics of the interface between single crystalline Ti02 electrodes and aqueous electrolyte. Both the native interface and chemically modified interfaces (adsorbed organometallic complexes [Rl!L:J, Os~ and Fe~ where L = bpy(COO):zl) are studied. Using electrochemical photocapacitance spectroscopy, the energetic distribution of the interfacial states is probed. These studies indicate that the initial electronic excitation is localized on the adsorbate and that the charge injection occurs subsequent to this initial excitation. Because the initial excitation is localized on the adsorbate it is possible to follow the charge injection by monitoring the time resolved fluorescence of the adsorbate species. This study reports on . the voltage dependence of the ~ RuL3 fluorescence decay, and hence the charge injection rate. In short, a factor of three change in the excited state decay time of the adsorbate is found over a voltage VB range of less than a half of a volt. This ---- decrease in lifetime is interpreted as a quenching process which results from charge injection into the substrate. 142 POSTER #51 THE EFFECT OF PHOTO-GENERATED, ORIENTED ELECTRIC FIELDS ON CAROTENOIDS: A MODEL FOR THE CAROTENOID BANDSHIFT IN PHOTOSYNTHESIS Michael R. Wasielewski, David Gosztola, and Hiroko Yamada Chemistry Division, Argonne National Laboratory, Argonne, Illinois 60439 Observation of band shifts in the absorption spectra of carotenoids in vivo has been attributed to electric fields that result from charge separation. This electrochromism is a manifestation of the Stark effect, a term that is used to describe the effect of an applied electric field on many physical processes. Here we present a new way of studying the influence of intense, oriente·d electric fields on nearby" molecules and on the reactions of nearby molecules. This is accomplished by photo generating an ion pair in close proximity to a polarizable probe molecule, in this case a carotenoid. Neither of the photo-generated ions can oxidize or reduce the carotenoid. The molecular system that we. have utilized is shown in below. It consists of a zinc porphyrin - pyromellitic diimide electron donor-acceptor complex held rigidly proximate to an apo-~-carotene using a calix[4]arene spacer. Selective photoinduced charge separation within the ion pair results in a dipole formed by the 8.4 A separation of the ion pair product. In this study we have synthesized five calix[4]arene based molecules. These include the complete molecule as shown below, as well as four control molecules, one containing only the calix[4]arene and porphyrin-diimide, one containing only the calix[4]arene and carotenoid, one containing only the porphyrin, calix[4]arene, and carotenoid, and one in which the porphyrin and diim ide are attached to opposite sides of the calix[4]arene. Transient absorption spectra were recorded 150 ps after the formation of the charge separated state of the porphyrin diimide by a 200 fs pump pulse at 420 nm. The lifetime of the ion pair was 3.7 ns in toluene. To detect any change in the \ ground state absorption spectrum of the carotene due to the presence of the electric field, a difference spectrum was calculated using transient spectra of the full molecule and the 1£0 N molecule containing only the porphyrin-diimide donor-acceptor pair. The ground state absorption spectrum of the carotenoid exhibited significant changes in intensity, and red-shifted by about 15 nm in the presence of the electric field. The magnitude of these changes suggest that the strength of the oriented electric field at the carotenoid is about 5 x 106 V/cm. This value exceeds the magnitude of fields typically available from externally applied sources that are screened by many layers of intervening molecules, while maintaining a particular orientation of the field direction relative to the probe molecule. 143 POSTER 152 TRANSIENTS FORMED IN LASER FLASH PHOTOLYSIS. OF Ir-Si BONDED DIASTEREOMERIC COMPLEXES Peter I. Djurovich, Radhika Joshi, Wendy Cook and R.J. Watts Department of Chemistry, University of California Santa Barbara, CA 93106 Coordination of Ir(III) by three (8-quinolyl)phenyl methylsilyl ligands (pmsiqn) results in formation of several diastereomers of the type Ir(pmsiqn) 3 • Two of these have been isolated and characterized as the ~RRR, ASSS (A) and the aRRS, ASSR (B) diastereomers (see figure below). Excitation of each of these leads to an emission from a sigma-bond-to ligand charge-transfer (SBLCT) excited state; the spectral distribution and lifetime of the emission is identical for two when they are excited in glasses at 77 K. = @ (A) (B) Excitation in fluid organic solvents leads to emission lifetimes which are quite different for the two, and the lifetime of each is strongly dependent upon the nature of the solvent. Quenching is attributed to exciplex formation which is more pronounced for the less symmetric B diastereomer. Each diastereomer is subject to exciplex quenching by a wide range of solvents which may be divided into one group which is primarily electron accepting (aromatics, olefins) and a second group which is electron donating (nitriles, carbonyls, alcohols, etc). Kinetic analysis of the lifetime data in dicate that two sites in the complex are active in exciplex formation: an external site related to the quinoline ring system where electron acceptors bind, and an internal site associated with the Ir-Si bond where electron donors bind. Transient absorption analysis, which will be described in the poster, provides insights into the nature of the transients formed following excitation of each of the two diastereomers in a variety of aliphatic, aromatic, and functionalized organic solvents. 144 J POSTER #53 PHOTOPHYSICS AND PHOTOREDOX PROCESSES AT POLYMER-WATER INTERFACES Jiunn-Shyong Hsiao, Andrew Eckert, Ti Cao, Weiping Yin, S.E. Webber Department of Chemistry and Biochemistry and Center for Polymer Research The University of Texas at Austin Austin, Texas 78712 Our work has focused on the use of chromophores covalently attached to amphiphilic polymers with the underlying idea that the combination of hydrophobic compartmentalization and aqueous medium would help solvate the ion-pairs produced in photoredox reactions and enhance charge separation. Polyethylene oxide has been end-tagged with aromatic chromophores (anthracene, pyrene, referred to as PEO-A and PEO-Py) and the adsorption of these hydrophobically modified polymers onto polystyrene latexes was studied by light scattering. The exposure of the chromophore to the aqueous phase was characterized by comparing fluorescence quenching in free solution and adsorbed onto latexes. Fluorescence depolarization measurements also demonstrated a more rotationally hindered chromophore when adsorbed than in homogeneous solution. Therefore one can conclude that the hydrophobic end-group is the primary point of adsorption and that the chromophore is partially compartmentalized by the polystyrene matrix. The photoinduced electron transfer from these species has been investigated both in homogeneous aqueous solution and adsorbed at the interface of water-soluble polystyrene latexes. The electron-acceptor quencher is a zwitterionic viologen that becomes anionic upon reduction. Charge separation following singlet- and triplet-excited state quenching was found for PEO-A and PEO-Py in both H20 homogeneous solution and H20/latex biphasic systems. The efficiency of charge separation for triplet state is high (ca. 0.6-1.0) and is relatively insensitive to environment because the back electron-transfer reaction is spin-forbidden. For the singlet state, the efficiency of charge separation is more modest (ca. 0.2-0.3) and is relatively sensitive to the environment. In all cases the ion pairs have a long lifetime, in excess of 1 ms. We conclude that the aromatic moiety of PEO-A .and PEO-Py has good structural protection and is prevented from generating "tight" geminate ion pairs in electron-transfer processes. Unlike some similar amphiphilic moieties and/or polymers (Hsiao, J-S; Webber, S.E. J. Phys. Chern. 1993, 21, 8289; ibid., 8296 ) the latex did not enhance the efficiency of charge separation in the singlet state. Diblock polystyrene-blk-poly(methacrylic acid) polymers have been synthesized using anionic polymerization techniques incorporating an average of one naphthalene/polymer at either the beginning of the polystyrene block or at the junction between the polystyrene and polymethacrylic acid blocks. These polymers have been shown to form stable micelles in pure water. We have examined the ability of these polymers to adsorb on polystyrene films and have used photophysical techniques to deduce the exposure of the naphthalene group to the aqueous phase. SEM images demonstrate that the intact micelles adsorb onto the polystyrene surface and can achieve a nearly close packed monolayer coverage. The micelles adhere tenaciously to the polystyrene film and lower the contact angle of water from ca. 90° for untreated polystyrene to 30- 400 after surface adsorption. The naphthalene groups at the polystyrene-polymethacrylic junction are partially exposed to the aqueous phase. We believe this represents an interesting new strategy for modifying polymer films with photoactive groups. 145 POSTER 1154 Evidence for Solid State Photooxidative C-C Bond Cleavage in 1,2 Diamines Jeffrey W. Leon and David G. Whitten Department of Chemistry, University of Rochester Rochester, NY 14627 The development and study of bond breaking and bond forming reactions of ion radicals is a rich and vibrant area of chemistry, and research in this field has produced a wealth of useful and novel photoreactions. While many of these reactions are now well understood and the mechanisms have been thoroughly investigated, their studies have been confined almost exclusively to solution. Here we examine the feasibility of solid state ion radical reactions from both a theoretical and practical standpoint and present direct evidence for the solid state electron transfer-induced photofragm= .041), is much less efficient than the corresponding solution reaction ( =.66). The photochemistry of a similar random donor/acceptor copolymer is also discussed. 146 POSTER #55 PHOTOCHEMICAL INTERCONVERSIONS OF ORGANIC RADICAL CATIONS BY HYDROGEN TRANSFER USING VISIBLE LIGHT Thomas Bally and Leo Truttmann Institute of Physical Chemistry, University of Fribourg Perolles, CH - 1700 Fribourg, Switzerland Jih Tzong Wang and Ffrancon Williams Department of Chemistry, pniversity of Tennessee Knoxville, Tennessee 37996-1600 Electron spin resonance (ESR) and electronic absorption (EA) spectroscopy have been employed to study the photochemical interconversion of several dihydropentalene (DHP) radical cations in solid matrices. Our first entry into this set of C8H8 •+ isomers was provided by the recently characterized cation of bicyclo[3.3.0]octa-2,6-diene-4,8-diyl (r+) which can be generated by either the photoisomerization: of the cyclooctatetraene radical cation or the direct one-electron oxidation of semibullvalene.1 This radical cation (r+) can also be regarded as that of 3a,6a-dihydropentalene (3a,6a-DHP.+), its electronic structure being of considerable interest since it corresponds to a mixed-valence species in which the allylic radical and cation moieties strongly interact to produce a delocalized benzene-like structure.2 On exposure of r+ to light of J.. > 510 nm in its charge-resonance absorption band (J..max = 635 nm), it is converted to the radical cation of 1,4- dihydropentalene (2•+ ), a linear conjugated triene with a planar carbon skeleton.3 In this phototransformation, two hydrogen shifts are required from the bridgehead positions in 1•+, and it is surmised that the pathway occurs via the intermediate formation of a thermally labile 1,6a-DHP•+ isomer. Additional photochemical studies have established a cycle of transformations in which 1,4-DHP•+ is converted successively to 1,5-DHP•+ and 1,6-DHP•+, and then regenerated quantitatively from the latter species. This cycle can be repeated many times with only small losses of intensity in the ESR or EA spectra. On prolonged recycling, EA evidence was obtained for the growth of 1,2-DHP•+ which was photostable under the conditions used to bring about the other transformations. The spectroscopic assignments were validated by generating authentic samples of the DHP radical cations by the direct one-electron oxidation of the parent compounds. In cases where this approach was not feasible, spectral comparisons were made with radical cations having similar chromophores. Many of the thermal and photoinduced hydrogen shifts in this system can be rationalized by applying a simple set of rules based on the symmetry properties of frontier orbitals. 1S. Dai, J. T. Wang, and F. Williams, J. Am. Chem. Soc., 112, 2835, 2837 (1990). 2F. Williams, J. Chem. Soc., Faraday Trans., 90 (Faraday Symposium No. 29), in press (1994). 3T. Bally, L. Truttmann, S. Dai, J. T. Wang, and F. Williams, Chem. Phys. Letters, 212, 141 (1993). 147 POSTER 156 EFFECT OF PRESSURE ON THE THERMODYNAMICS AND KINETICS OF ELECTRON TRANSFER IN RUTHENIUM-MODIFIED CYTOCHROMES c Ji Sun,a James F. Wishart, a Martin Meier,b Rudi van Eldik,b Richard D. Shaldersc and Thomas W. Swaddlec anepartment of Chemistry, Brookhaven National Laboratory, Upton, NY 11973 blnstitute for Inorganic Chemistry, University' of Witten/Herdecke, Witten, Germany 58448 and cDepartment of Chemistry, The University of Calgary, Calgary, Alberta, Canada T2N IN4 In previous work, we found that the observed activation volumes for intramolecular electron transfer in pentaammineruthenium-modified cytochromes c were due primarily to electrostriction of the solvent water in the region of the pendant ruthenium complex. We have recently determined complete volume profiles for several intramolecular and intermolecular electron transfer reactions between ruthenium ammine complexes and cytochrome c, using high-pressure electrochemistry, pulse radiolysis, and stopped-flow kinetics. Redox volume changes for the metal centers in horse-heart cytochrome c and a series of ruthenium-modified cytochromes c were measured by high pressure differential pulse voltammetry in the pressure range of 5-200 MPa. Reaction volumes for inter- and intra-molecular electron transfer processes were obtained from the volume change of each redox center. Our results indicate there is !lQ. significant volume change for cytochrome c itself, consistent with our earlier findings and the literature on reactions between cytochrome c and small molecules. The observed volume changes can be largely accounted for by changes at the reference electrode or the ruthenium center. The electrochemical results agree with related data from high-pressure spectrophotometric equilibrium measurements and forward and reverse activation volumes from high-pressure stopped-flow kinetics. We have extended the earlier work on intramolecular systems by using substituted pyridine ligands to exclude some of the water from the solvation shell around ruthenium in the modified cytochromes. The effect shows up clearly in the net reaction volumes, which are +32 cc/mol in the Fe-to-Ru direction for CNHa)sRu-Cyt c, but only +20 to +23 cdmol for t-(NHa)~u{L)-Cyt c, (L = 4-ethylpyridine, 3,5-lutidine, pyridine, and isonicotinamide). Whereas the (NHg)sRu-Cyt c and intermolecular cases show symmetric volume profiles (~V* = ~V0/2), the tetraammine complexes have an early transition state (~V* = +7.4, 5.8, 7 .2, and 3.2 cdmol, respectively, considerably less than ~V 0 /2). The reasons for these differences will be discussed, and some recent results with ruthenium modified, cobalt-substituted cytochrome c will be presented for comparison. 148 List of Participants Mr. Christian B. Allan Dr. Edward W. Castner Structural Biology Division Chemistry Department Lawrence Berkeley Laboratory Brookhaven National Laboratory Berkeley, CA 94720 Upton, NY 11973 Phone: (209)~2268 Phone: (516) 282-4362 FAX: (209) 946-2607 FAX: (516)282-5815 Professor Paul F. Barbara Professor David Chandler Department of Chemistry Departrment of Chemistry University of Minnesota University of California Minneapolis, MN 55455 Berkeley, CA 94720 Phone: (612) 625-0064 Phone: (510)643-6821 FAX: (612) 626-7541 FAX: (510)642-6262 e: [email protected] Dr. Kathleen M. Barkigia Department of Applied Science, Bldg 815 Dr. Lin Chen Brookhaven National Laboratory Chemistry Department Upton, NY 11973 Argonne National Laboratory Phone: (516)282-766I 9700 South Cass Ave. FAX: (516) 282-5815 Argonne, IL 60439 e: BARKIGIA@BNLCHM Phone: (708) 252-3533 FAX: (708) 252-9289 Dr. James C. Bartholomew Structural Biology Division Dr. Daniel M.Chipman Lawrence Berkeley Laboratory Radiation Laboratory University of California University of Notre Dame, Berkeley, CA 94720 Notre Dame, IN 46556 Phone: (510) 486-4343 FAX: (5I0)486-6059 Ms. Mei Han Chou Chemistry Department Dr. Fritz Blatter Brookhaven National Laboratory Structural Biology Division Upton, NY 11973 Lawrence Berkeley Laboratory Phone: (516)282-4360 University of California FAX: (516) 282-5815 Berkeley, CA 94720 Phone: (510) 486-4307 Dr. George Chumanov FAX: (5I0)486-6059 1605 Gilman Hall e: F-BLA ITER@LBL Ames Laboratory Iowa State University Dr. Bruce S. Brunschwig Ames, lA 50011 Chemistry Department Phone: (515) 294-3443 Brookhaven National Laboratory FAX: (515)294-0105 Upton, NY 11973 Phone: (516) 282-4332 Professor Therese M.Cotton 1605 Gilman Hall Professor Melvin Calvin Ames Laboratory Structural Biology Division Iowa State University Lawrence Berkeley Laboratory Ames, lA 500 II University of California Phone: · (515) 294-9887 Berkeley, CA 94720 FAX: (515)294-0105 Phone: (510)642-0838 e: [email protected] FAX: (510) 642-8369 149 Dr. Carol Creutz Chemistry Department, Bldg 555A Professor John F. Endicott Brookhaven National Laboratory DepartmentofChemistry Upton, NY 11973 335 Chemistry Phone: (516) 282-4359 Wayne State University FAX: (516) 282-5815 Detroit, MI 48202 Phone: (313) 577-2607 Professor Glenn A.Crosby FAX: (313) 577-1377 Department of Chemistry Washington State University Dr. James H. Espenson Pullman. W A 99164-4630 Ames Labaoratory Phone: (509)33~5605 Iowa State University FAX: (509) 33~8867 Ames, lA .50011 e: GAC®WSUMI Dr. Jack Fajer Dr. Satyen Deb, Director Department of Applied Science Basic Sciences Division Brookhaven National Laboratory National Renewable Energy Laboratory Upton, NY 11973 1617 Cole Blvd. Phone: (516) 282-4521 Golden, CO 80401 FAX: (516) 282-3137 Phone: (303)231-1105 FAX: (303) 231-1271 Profesor Michael D. Fayer Department of Chemistry Professor Prabir K. Dutta Stanford University Department of Chemistry Stanford, CA 94305 Ohio State University 120 West 18th Ave. Dr. Stephen W. Feldberg Columbus, OH 43210 Department of Applied Science Phone: (614) 292-4532 Brookhaven National Laboratory .FAX: (614)292-1685 Upton, NY 11973 e: [email protected] Phone: (516) 282-4480 FAX: (516) 282-3137 Professor Richard Eisenberg e: [email protected] Department of Chemistry University of Rochester Profesor Guillermo J. Ferraudi Rochester, NY 14627 Radiation Laboratory Phone: (716) 27~5573 University of Notre Dame FAX: (716)473-6889 Notre Dame, In 46556 e: RSE7@UORCHEM Phone: (219) 631-5355 or 7676 FAX: (219) 631-8068 Professor Kenneth B. Eisenthal e: [email protected]. DepartrmentofChemistry ND.EDU Columbia University 116th St. & Broadway Professor Marye Anne Fox New York, NY 10027 Department of Chemistry Phone: (212) 854-3175 University of Texas FAX: (212) 932-1289 Austin, TX 78712 Phone: (512) 471-1811 Professor C. Michael Elliott FAX: (512) 471-7791 Department of Chemistry e: MAFOX®UTEXAS.MS.CC.EDU Colorado State University Fort Collins. CO 80523 Phone: (303) 491-5204 FAX: (303) 491-1801 150 Dr. Arthur J. Frank Dr. Mary E. Gress Basic Sciences Division Division of Chemical Sciences, ER-141 National Renewable Energy Laboratory U.S. Department of Energy 1617 Cole Blvd. Washington, DC 20585 Golden, CO 80401 Phone: (301) 903-5820 Phone: (303) 231-1962 FAX: · (301) 903-4110 FAX: (303) 231-1850 Profesor J. Devens Gust Dr. Heinz M. Frei Department of Chemistry Structural Biology Division Arizona State University Lawrence Berkeley Laboratory Tempe, AZ 85287-1604 University of California Phone: (602) 965-4.547 Berkeley, CA 94720 FAX: (602) 965-2747 Phone: (510)~325 FAX: (510)~6059 Professsor Morton Z. Hoffman Department of Chemistry Professor Richard A. Friesner Boston University Department of Chemistry· Boston, MA 02215 Havemeyer Hall Phone: (617) 353-2494 Columbia University FAX: (617)353-6466 New York, NY 10027 e: [email protected] Phone: (212) 854-7606 FAX: (212) 932-1289 Dr. Gordon Hug Radiation Laboratory Dr. Etsuko Fujita University of Notre Dame Chemistry Department Notre Dame, IN 46556 Brookhaven National Laboratory Phone: (219) 631-4504 Upton, NY 11973 FAX: (219) 631-8068 Phone: (516) 282-4356 e: HUG@NDRADLAB FAX: (516) 282-5815 Professor Michael Gratzel Professor Joseph T. Hupp Institute of Physical Chemistry Department of Chemistry Swiss Federal Institute of Technology Northwestern University CH-1015 Lausanne, SWITZERLAND Evanston, IL 60208 Phone: (708) 491-3504 Dr. Elias Greenbaum FAX: (708) 491-7713 Chemical Technology Division Bldg4500N Professor James K. Hurst I Oak Ridge National Laboratory Department of Chemical & Bio. Sciences P.O. Box 2008 Oregon Graduate Inst. of Sci. & Tech. Oak Ridge, TN 37831 19600 N.W. von Neumann Drive Phone: (615) 574-6835 Beaverton, OR 97006-1999 FAX: (615) 574-6843 Phone: (503)690-1073 e: [email protected]. FAX: (503)690-1464 e: [email protected] Dr.Brian A. Gregg Basic Sciences Division Professor StephanS. Isied National Renewable Energy Laboratory Department of Chemistry 1617 Cole Blvd. Rutgers University Golden, CO 80401 Piscataway, NJ 08855 Phone: (303) 231-1285 Phone: (908) 932-3764 FAX: (303) 231-1955 FAX: (908) 932-5312 151 Professor Guilford Jones, II Professor Carl A. Koval Department of Chemistry Department of Chemistry Boston University University of Colorado Boston, MA 02215 Boulder, CO 80309-0215 Phone: (617)353-2498 Phone: (303) 492-5564 FAX: (617) 353-6466 FAX: (303) 492-5894 e: [email protected] Dr. Christopher Lange Dr. Prashant V. Kamat Structural Biology Division Radiation Laboratory Lawrence Berkeley Laboratory University of Notre Dame University of California Notre Dame, IN 46556 Berkeley, CA 94720 Phone: (219) 631-5411 Phone: (510) 643-5239 FAX: (219) 631-8068 FAX: . (510) 486-6059 e: [email protected] Dr. Allan H. Laufer Professor Michael Kasha Division of Chemical Sciences Institute of Molecular Biophysics ER-141, GTN Aorida State University U.S. Department of Energy Tallahassee, FL32306 Washington,DC 20585 Phone: (904)~6452 Phone: (301) 903-5820 FAX: (904) 561-1406 FAX: (301) 903-4110 Professor Larry Kevan Professor Nathan S. Lewis Department of Chemistry Department of Chemistry University of Houston Mail Code:127-72 Houston, TX 77204-5641 California Institute of Technology Phone: (713) 743-3250 Pasadena, CA 91125 FAX: (713) 743-2709 Phone: (818)3~6335 FAX: (818)585-0147 Professor Sung-Hou Kim, Director Structural Biology Division Professor Edward C. Lim Lawrence Berkeley Laboratory Department of Chemistry University of California The University of Akron Berkeley, CA 94720 Akron, OH 44325 Phone: (510) 486-4333 Phone: . (216) 972-5297 FAX: (510)~5272 FAX: (216) 972-6407 e: [email protected] Professor James R. Kincaid Department of Chemistry Professor Henry Linschitz Marquette University Department of Chemistry Milwaukee, WI 53233 Brandeis University Phone: (414) 288-3539 Waltham, MA 02154 FAX: (414) 288-3300 Phone: (617) 736-2506 FAX: (617) 736-2516 Professor Lowell D. Kispert Department of Chemistry Professor Sanford Lipsky University of Alabama Department of Chemistry Tuscaloosa, AL 35487 University of Minnesota Phone: (205) 348-7134 ·Minneapolis, MN 55455 FAX: (205)348-9104 Phone: (612) 624-9581 e: LKISPERT@UA1VM FAX: (612)626-7541 152 Professor Thomas E. Mallouk Dr. John R. Miller Department of Chemistry Chemistry Division Pennsylvania State University Argonne National Laboratory University Park, PA 16802 9700 South Cass Ave. Phone: (814) 865-0898 Argonne, IL 60439 FAX: (814) 865-3314 Phone: (708) 252-3481 FAX: (708) 252-4993 Dr. DuliMao e: JRMILLER@ANLCHM Basic Sciences Division National Renewable Energy Laboaratory Professor R. J. Dwayne Miller 1617 Cole Blvd. Department of Chemistry Golden, CO 80401 University of Rochester Phone: (303) 121-1254 Rochester, NY 14627 FAX: (303) 231-1850 Phone: (716) 275-4079 e: [email protected] FAX (716) 473-8392 Dr. Robert S. Marianelli Professor Ana L. Moore Division of Chemical Sciences, ER-14 Department of Chemistry U.S. Department of Energy Arizona State University Washington, DC 20585 Tempe, AZ 85287 Phone: (602) 965-2953 Professor Mark Maroncelli FAX: (602) 965-2747 Department of Chemistry 152 Davey Laboratory Professor Thomas A. Moore Pennsylvania State University Department of Chemistry University Park, P A 16802 Arizona State University Phone: (814) 865-0898 Tempe, AZ 85287 FAX: (814) 865-3314 Phone: (602) 965-4000- e: [email protected] FAX: (602) 965-2747 Dr. Dan Meisel Dr. Venkatraman Nagarajan Chemistry Division Department of Biochemistry, SJ-70 Argonne National Laboratory University ofWashington 9700 South Cass Ave. Seattle, W A 98195 Argonne, IL 60439 Phone: (708) 252-3472 Dr. Pedatsur Neta FAX: (708)252-4993 Chemical Kinetics & Thermodynamics e: MEISEL@ANLCHM Division National Institute of Standards & Professor Thomas J. Meyer Technology Department of Chemistry · Gaithersburg, MD 20899 University of North Carolina Phone: (301) 975-5635 Chapel Hill, NC 27599-3290 FAX: (302) 926-4513 Phone: (919)962-6320 e: [email protected] FAX: (919) 962-2388 Dr. Olga I. Micic Basic Sciences Division National Renewable Energy Laboratory 1617 Cole Blvd. Golden, CO 80401-3393 Phone: (303) 231-1843 FAX: (303) 231-1850 153 Dr. Marshall D. Newton Dr. Robert Pearlstein Chemistry Department Chemistry Division Brookhaven National Laboratory Argonne National Laboratory Upton, NY 11973 9700 South Cass Ave. Phone: (516)282-4366 Argonne, IL 60439 FAX: (516) 282-5815 e: NEWTON®BNLCHM Professor John D. Petersen Department of Chemistry Clemson University Dr. James R. Norris Clemson,SC 29634-1905 Chemistry Division Phone: (803)6~5017 Argonne National Laboratory FAX: (803) 6~6613 9700 South Cass Ave. Argonne, IL 60439 Professor Piotr Piotrowiak Phone: (708) 252-3544 Department of Chemistry FAX: (708) 252-9289 University of New Orleans Lakefront Dr. Arthur J. Nozik New Orleans, LA 70148 Basic Sciences Division Phone: (504)286-6840 National Renewable Energy Laboratory FAX: (504) 286-6860 1617 Cole Blvd. Golden, CO 80401 e: [email protected] Phone: (303) 231-1953 FAX: (303) 231-1955 Dr. Mark Renner Department of Applied Science Dr. John W. Otvos Brookhaven National Labaoratory Structural Biology Division Upton, NY 11973 Lawrence Berkeley Laboratory Phone: (516) 282-4525 University of California FAX: (516) 282-3137 Berkeley, CA 94720 Phone: (510) 643-5237 Professor D. Paul Rillema FAX: (510)~6059 Department of Chemistry Univ. of North Carolina at Charlotte Professor Bruce A. Parkinson Charlotte, NC 28223 Department of Chemistry Phone: (704) 547-4445 Colorado State University FAX: (704) 547-3151 Fort Collins, CO 80523 Phone: (303) 491-0504 Professor Russell H. Schmehl FAX: (303) 491-1801 Department of Chemistry e: Tulane University [email protected]. New Orleans, LA 70125 EDU Phone: (504) 86>5579 FAX: (504) 86>5596 Dr. L. K. Patterson e: Radiation Laboratory [email protected] University of Notre Dame Notre Dame, IN 46556 Phone: (219) 239-5403 FAX: (219)239-8068 154 Dr. Robert H. Schuler, Director Dr. Julian Sprague Radiation Laboratory Basic Sciences Division University of Notre Dame National Renewable Energy Laboratoary Notre Dame, IN 46556 1617 Cole Blvd. Phone: (219) 631-7.502 Golden, CO 80401 F~: (219)631-8068 Phone: (303) 231-1018 e: RHS®NDRADlAB F~: (303) 231-18.50 Dr. Michael Seibert Professor Larry O.Spreer Basic Sciences Division Structural Biology Division National Renewable Energy Laboratory Lawrence Berkeley Laboratory 1617 Cole Blvd. Berkeley, CA 94720 Golden, CO 80401 Phone: (209)9~2268 Phone: (303) 231-1879 FAX: (209) 9~2607 F~: (303) 231-18.50 e: [email protected] Professor WalterS. Struve 1605Gilman Dr. Stanley Seltzer Ames Laboratory Chemistry Department Iowa State University Brookhaven National Laboratory Ames, lA .50011 Upton, NY 11973 Phone: (515) 294-4276 Phone: (516) 282-4390 FAX: (515)294-0105 F~: (516) 282-5815 e: WSTRUVE@ALISUV AX e: [email protected] Dr. Hai Sun Dr. Gerald J. Small Structural Biology Division 1605 Gilman Lawrence Berkeley Laboratory Ames Laboratory University of California Iowa State University Berkeley, CA 94720 Ames, lA .50011 Phone: (510)486-4307 Phone: (515) 294-3859 FAX: (510)~6059 F~: (515)294-0105 Dr. Norman Sutin, Chairman Dr. John F. Smalley Chemistry Department Department of Applied Science Brookhaven National Laboratory Brookhaven National Laboratory Upton, NY 11973 Upton, NY 11973 Phone: (516) 282-4301 FAX: (516) 282-7993 Professor Kenneth G. Spears Department of Chemistry Dr.Jau Tang Northwestern University Chemistry Division 2145 Sheridan Road Argonne National Laboratory Evanston, IL 60208 9700 South Cass Ave. Phone: (708) 491-3095 Argonne, IL 60439 F~: (708) 491-3095 or 7713 Phone: · (708) 252-3539 FAX: (708) 252-9289 Professor Thomas G. Spiro Department of Chemistry Dr. Marion Thumauer Princeton University Chemistry Division Princeton, NJ 08540 Argonne National Laboratory 9700 South Cass Ave. Argonne, IL 60439 Phone: (708) 252-3545 F~: (708) 252-9289 155 / Dr. David Tiede Professor Richard J. Watts Chemistry Division Department of Chemistry Argonne National Laboratory University of California 9700 South Cass Ave. Santa Barbara, CA 93106 Argonne, IL 60439 Phone: (805) 893-2032 FAX: (805)893-4120 Dr. John A. Turner Basic Sciences Division Professor Stephen E. Webber National Renewable Energy Laboratory Department of Chemistry 1617 Cole Blvd. University of Texas at Austin Golden, CO 80401 Austin, TX 78712 Phone: (303)231-1967 Phone: (512) 471-3633 FAX: (303) 231-1955 FAX: (512) 471-8696 Professor David R. Tyler Professor David G. Whitten Department of Chemistry Department of Chemistry University of Oregon University of Rochester Eugene, OR 97403 Rochester, NY 14627 Phone: (716)275-1858 Professor Hans van Willigen FAX: (761) 473-6889 Department of Chemistry University of Massachusetts-Boston Professor T. Ffrancon Williams Boston, MA 02125 Department of Chemistry Phone: ( 617) 287-6147 University of Tennessee FAX: (617) 265-7173 Knoxville, TN 37996-1600 e: Phone: (615)974-3468 VANWilligen%UMBSKY .DNET®NS. FAX: (615) 974-3454 UMB.EDU e: BITNET.FWILLIAM@UTKVX1 Professor David H.Waldeck Dr. James Wishart Department of Chemistry ChemistryDepartment University of Pittsburgh Brookhaven National Laboratory Pittsburgh, PA 15260 Upton, NY 11973 Phone: (412) 624-8430 Phone: (516) 282-4327 FAX: (412) 624-8552 FAX: (516) 282-5815 e: DA VE®PmVMS e: [email protected] Professor Carl Wamser Department of Chemistry Portland State University Portland, OR 97207-0751 Phone: (503) 725-4261 FAX: (503)725-3888 e: [email protected] Dr. Michael R. Wasielewski Chemistry Division Argonne National Laboratoary 9700 South Cass Ave. Argonne, IL 60439 Phone: (708) 252-3538 FAX: (708) 252-9289 156 Allan, Christian B...... 93, 118 Fessenden, Richard W ...... 114 Allison, D.P...... 72 Fox, Marye Anne ...... ~ ...... 23 Anderson, D .J...... 44 Frank, A.J...... 11, 121 Aukett, Tim ...... 140 Frei, Heinz ...... 96, 138 Baba, Aaron I...... 41 Friesner, Richard A ...... 29 Bally, Thomas ...... 147 Fujita, Etsuko ...... 107 Barbara, Paul F ...... 94 Gahan, Sean L...... 128 Barkigia, K.M ...... 95, 129 Gao, G ...... 117 Barkoff, Aaron ...... 139 Gentemann, S ...... 104 Baxley, Greg ...... 140 Gosztola, David ...... 143 Bedja, Idriss ...... 114 Gratzel, M...... 1 Bevilacqua, J.M...... 44 Greenbaum, E ...... 72 Biczok, Laszlo ...... 119 Gregg, Brian A ...... 20 Blankenship, Robert E ...... 137 Grodkowski, J ...... 65 Blatter, Fritz ...... 96, 138 Groner, Marcus ...... 14 Bobrowski, Krzysztof ...... 109 Gust, Devens ...... 108 Borguet, E ...... 102 Hansen, Deborah ...... ·86 Brigham, Elaine S ...... 120 Hayes, John ...... 86 Brunschwig, Bruce S ...... 47, 107 Hill, J.E...... 44 Calvin, Melvin ...... 93, 118 Hoffman, Morton Z ...... 62 Cao, Ti...... 145 Holten, D ...... 104 Castner, Edward W., Jr...... 97 Hotchandani, Surat ...... 114 Chandler, David ...... 75 Howard, Jason ...... 14 Chang, Hai-Chou ...... 86 Hsiao, Jiunn-Shyong ...... 145 Chang, Yong J ...... ;...... 97 Hu, Songzhou ...... 135 Chen, Lin X ...... 98 Huang, Nuh-Tome ...... 128 Chen, Y ...... 130 Hug, Gordon L ...... 109 Chidsey, Christopher E.D...... 106, 133 Hung, Su-Chun ...... 108 Chin, T.T...... 112 Hupp, Joseph T ...... 110 Chipman, Daniel M...... 99 Hurst, James K ...... 111 Chumanov, George ...... 69, 100 Isied, Stephan S ...... 112 Connolly, JohnS ...... 136 Jankowiak, Ryszard ...... 86 Cook, Wendy ...... 144 Jeevarajan, A.S ...... 117 Cormier, Russell A...... 136 Jeevarajan, J.A...... 117 Cortes, Emilio ...... 127 Jones, Guilford IT ...... 26 Cotton, Therese M ...... 69, 100 Joshi, Radhika ...... 144 Creutz, Carol ...... 47 Kamat, Prashant V ...... 114 Crosby, G.A...... 101 Kapoor, S ...... 123 Cummings, S ...... 44 Karki, Laba ...... 110 Curtis, Calvin J ...... 124 Kartha, S ...... 123 Derr, DanielL...... 32 Kasha, Michael...... 113 Djurovich, Peter I ...... 144 Keller, Steven W ...... ;...... 120 Dou, Xiumei ...... 3 Kelley, D.F...... 32 Dutta, Prabir K ...... 37 Kenyon, C.N...... 5 Eckert, Andrew ...... 145 Kevan, Larry ...... 115 Eisenberg, R ...... 44 .Kim, K.J...... 121 Eisenthal, K.B...... 102 Kim, Won ...... 41 Elliott, C. Michael ...... 32 Kim, Yeong ll ...... 20, 120 Endicott, John F ...... 103 Kincaid, James R ...... 116 Fajer, J ...... 95, 104, 129 Kispert, Lowell D ...... 117 Fayer, M.D...... 105 Kolaczkowski, Steven ...... 86 Feldberg, Stephen W ...... 106, 133 Konitsky, William ...... 97 Ferraudi, G ...... 49 Kothe, Gerd ...... 81 Ferrere, Suzanne ...... 32 Koval, Carl A ...... 14 157 Kumar, P. Vijaya ...... 122 Pearlstein, Robert M ...... 127 Kumble, Ranjit ...... 135 Petersen, John D ...... 128 Lange, Chris ...... 118 Petrov, Vladimir...... 110 Larson, Steven L ...... 32 Piotrowiak, Piotr ...... 59 Lawless, D ...... 123 Rasmussen, Seth C ...... 128 Ledney, Michael B ...... 37 Reddy, Narla ...... 86 Lee, I...... 72 Regev, A ...... 104 Lee, J.W ...... : ...... 72 Renner, M.W ...... 95, 129 Lei, Y a bin ...... Ill Rillema, D.Paul ...... 130 LeLouche, Isabelle ...... 112 Rosenwaks, Y ...... 125 Leon, Jeffrey W ...... 146 Rozwadowski, Jaroslaw ...... ,... 109 Levanon, H ...... 104 Savikhin, Sergei ...... 137 Lewis, Nathan S ...... 5 Schmehl, Russell H ...... 41 Li, Li ...... 126 Seibert, Michael ...... 86, 131 Liang, Yongwu ...... 41 Seltzer, Stanley ...... 132 Liao, K.H ...... 142 Shalders, Richard D ...... 148 Lin, Su ...... 137 Sharp, Laura ...... 3 Lindenberg, Katja ...... 127 Shi, X ...... 102 Linschitz, Henry ...... 119 Shin, Yeung-gyo K ...... 47 . Liu, Di ...... 114 Sibbald, Morgan ...... 69, 100 Liu, Yi-Ping ...... 106, 133 Small, Gerald ...... 86 Loppnow, Glen R ...... 135 Smalley, John F ...... 133, 106 Luangdilok, C ...... 123 Smith, K.M ...... 95, 104, 129 Ma, C.Y...... 72 Snyder, Seth W ...... 139 Macpherson, Alisdair ...... 108 Spears, Kenneth G ...... 134 MacQueen, D. Brent ...... 93, 118 Spiro, Thomas G ...... 135 Mallouk, Thomas E ...... 120 Sprague, Julian R...... 124, 136 Mao, D ...... 11, 121 Spreer, Larry ...... 93, 118 Marciniak, Bronislaw ...... 109 Steren, Carlos A ...... 141 Marguet, S ...... 65 Striplin, D.R...... 101 Maroncelli, Mark ...... 122 Struve, WalterS ...... 137 Maruszewski, Krzysztof...... 116 Sturzenegger, Marcel ...... 5 Meier, Martin ...... 148 Suardi, G ...... 44 Meisel, Dan ...... 123 Sun, Hai...... 62, 96, 138 Merkert, Jon ...... 130 Sun, Ji ...... 148 Meyer, Thomas J ...... 34 Sutin, Norman ...... 47 Micic, Olga I...... 124 Swaddle, Thomas W ...... 148 Miller, John R ...... 56 Swallen, S.F...... 105 Miller, R.J. Dwayne ...... 7 Swayambunathan, V ...... 103 Montano, Pedro A ...... 98 Tang, Jau ...... 77 Moore, Ana L ...... 108 Tevault, C.V...... 72 Moore, Thomas A ...... 108 Thacker, B.R...... 125 Neta, P ...... 65 Thumauer, Marion C ...... 77, 81, 139 Newton, M.D ...... 53, 106, 133 Tian, Hong ...... 69 Norris, J .R...... 77, 81, 86, 98, 139 Tiede, David M ...... 89 Nozik, A.J...... 125 To, Thanh ...... 14 Ogata, Tomoyuki ...... 107 Torres, Robert ...... 14, 20 Ohmes, Ernst ...... 81 Truttmann, Leo ...... 147 Ostafin, Agnes ...... 139 Tsuo, Y.S ...... 121 Otvos, John W ...... 93, 118 Tyler, David R...... 140 Palmer, Tina L...... 136 Uphaus, Robert A ...... 69 Papazyan, Arno ...... 122 Waldeck, D.H ...... 97, 142 Parkinson, Bruce A ...... 3 van Eldik, Rudi ...... 148 Patterson, L.K...... 126 van Willigen, Hans ...... 141 158 Wamser, Carl C ... :...... 17 Wen, Xiaoning ...... 134 Wang, Jih Tzong ...... 147 Whitten, David G ...... 146 Wang, Pei-Wei ...... 23 Wiederrecht, Gary P; ...... 131 Wang, Zhiyu ...... 98 Williams, Ffrancon ...... 147 Warmack, R.J ...... 72 Wishart, James F ...... 148 Wasielewski, Michael R...... 131, 143 Xiang, Bosong ...... 115 Watkins, Diana M ...... 23 Yamada, Hiroko ...... 143 Watts, David ...... 14 Yanagida, Shozo ...... 107 Watts, R.J.- ...... 144 Yin, Weiping ...... 145 Watzky, Murielle A ...... 103 Y onemoto, Edward H ...... 120 Webber, S.E...... 145 Zhang, Daisy ...... 139 Weber, Stefan ...... 81 Zhang, Y ...... 129 Wei, C.C ...... 117 Zhang, Yuenian ...... 139 Weidemaier, K ...... 105 Zhu, Yinwen ...... 137 159 LA~NCEBERKELEYLABORATORY UNIVERSITY OF CALIFORNIA TECHNICAL INFORMATION DEPARTMENT BERKELEY, CALIFORNIA 94720 .;>