Spectroscopic Studies of Singlet Fission and Triplet Excited States in Assemblies of Conjugated Organic Molecules

UNIVERSITY OF CALIFONIA, SAN DIEGO Spectroscopic studies of singlet fission and triplet excited states in assemblies of conjugated organic molecules A dissertation submitted in partial satisfaction of the requirements for the degree Doctor of Philosophy in Chemistry by Chen Wang Committee in Charge: Professor Michael Tauber, Chair Professor Michael Galperin Professor Clifford Kubiak Professor Lu Sham Professor Amitabha Sinha 2014 i ii The dissertation of Chen Wang is approved and it is acceptable in quality and form for publication on microfilm and electronically: Chair University of California, San Diego 2014 iii DEDICATION This dissertation is dedicated to my parents Xunsheng Wang and Jinghua Chen iv TABLE OF CONTENTS Signature Page ............................................................................................................... iii Dedication ...................................................................................................................... iv Table of Contents ............................................................................................................ v List of Figures ............................................................................................................. viii List of Schemes ........................................................................................................... xiii List of Tables................................................................................................................ xvi Acknowledgements ....................................................................................................... xv Vita ............................................................................................................................. xvii Abstract of the Dissertation........................................................................................... xix Chapter 1 Introduction ..................................................................................................... 1 Chapter 2 Characterization of Carotenoid Aggregates by Steady-State Optical Spectroscopy ................................................................................................................. 24 2.1 Introduction .................................................................................................. 24 2.2 Experimental ................................................................................................ 26 2.3 Results ......................................................................................................... 33 2.4 Discussion .................................................................................................... 47 2.5 Conclusions .................................................................................................. 64 Appendix ........................................................................................................... 75 Chapter 3 High Yield Singlet Fission in a Zeaxanthin Aggregate Observed by Picosecond Time-Resolved Resonance Raman Spectroscopy ........................................................... 92 3.1 Introduction .................................................................................................. 92 3.2 Result and discussion ................................................................................... 93 3.3 Conclusions ................................................................................................ 101 v Appendix ......................................................................................................... 106 Chapter 4 Triplet Exciton of Carotenoids Formed by Singlet Fission in a Membrane ... 123 4.1 Introduction ................................................................................................ 123 4.2 Experimental .............................................................................................. 124 4.3 Results and discussion ................................................................................ 125 4.4 Conclusions ................................................................................................ 132 Chapter 5 Singlet fission dynamics of carotenoid aggregates: insights from transient absorption spectroscopy ............................................................................................... 138 5.1 Introduction ................................................................................................ 138 5.2 Experimental .............................................................................................. 139 5.3 Results ....................................................................................................... 144 5.3.1 Fs-ns-TA spectroscopy of zeaxanthin aggregates ......................... 144 5.3.2 Ns-TA spectroscopy of zeaxanthin aggregates ............................. 150 5.4 Discussion .................................................................................................. 152 5.5 Conclusions ................................................................................................ 159 Chapter 6 Singlet Fission Process in Zeaxanthin Aggregates: Insights from Transient Resonance Raman and Absorption Spectroscopy ......................................................... 166 6.1 Introduction ................................................................................................ 166 6.2 Materials and methods ................................................................................ 169 6.2.1 Preparation of samples ................................................................. 169 6.2.2 Picosecond time-resolved Raman spectroscopy (TRRR) .............. 170 6.2.3Transient microscopic resonance Raman spectroscopy .................. 174 6.3 Results ....................................................................................................... 175 6.3.1 The ground state absorption and terminology ............................... 175 vi 6.3.2 Transient absorption results and the selection of TRRR probe wavelengths .......................................................................................... 176 6.3.3 TRRR Stokes spectra ................................................................... 178 6.3.4 Reconstructed excited state absorption (ESA) and Raman excitation profile (REP) ....................................................................................... 187 6.3.5 TRRR anti-Stokes spectroscopy ................................................... 190 6.4 Discussion .................................................................................................. 192 6.4.1 Exciton coupling of ground state transition in aggregates ............. 192 6.4.2 The excited states in aggregates ................................................... 195 6.4.3 Singlet fission in different aggregates........................................... 199 6.4.4 The influence of exciton coupling on excited state spectra ........... 201 6.4.5 Vibrational relaxation in singlet fission and TT annihilation ........ 206 Appendix ......................................................................................................... 221 Chapter 7 Resonance Raman Spectroscopy of the Triplet Excited State of Oligothiophenes........................................................................................................... 233 Appendix ......................................................................................................... 252 vii LIST OF FIGURES Figure 1.1 The spin diagram of singlet and triplet state .................................................... 1 Figure 1.2 Electronic state transitions of zeaxanthin monomer ......................................... 9 Figure 2.1. TEM images of zeaxanthin J1, J2 and H-aggregates ..................................... 34 Figure 2.2. UV-vis absorption spectra and circular dichroism spectra of zeaxanthin monomer in EtOH, H, J1, and J2-aggregate ................................................................... 35 Figure 2.3. Fits to absorption and emission spectra of the zeaxanthin monomer and aggregates...................................................................................................................... 38 Figure 2.4. Emission spectra of monomeric zeaxanthin (1.0 µM in EtOH) and aggregates, with 488.0 nm excitation ............................................................................................... 41 Figure 2.5. Fluorescence excitation spectra and fitted emission spectra of zeaxanthin monomer and aggregates .......................................................................................................................... 44 Figure 2.6. Raman spectra of monomer and J1-aggregate of zeaxanthin with 488.0 nm excitation, and the DFT calculated spectrum of zeaxanthin ............................................ 46 Figure 2.7. Model for the absorption, emission, and non-radiative relaxation processes of zeaxanthin aggregates .................................................................................................... 60 Figure A2.1. H-aggregate spectra resulting from varied rates of H2O addition to 25-75 μM solutions of zeaxanthin in ethanol. ........................................................................... 76 Figure A2.2A. Temperature-dependent UV-Vis spectra for the H- and J1-aggregates ... 77 Figure A2.2B. Temperature dependent UV-Vis spectra for the monomer and J2- aggregate ......................................................................................................................

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