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 ......................................................................................................................