Mechanism of Molecular Triplet Excited State Generation in Quantum Dot-Molecule Complex Open Access
Jin, Tao (Spring 2022)
Abstract
Efficient generation of molecular triplet excited states has been extensively studied as it is a critical step in systems of photon-upconversion, photodynamic therapy and photocatalysis. Recently, quantum dot (QD) has been developed to sensitize or enhance molecular triplet excited state generation (TESG) because of its unique properties compared to traditional molecular sensitizers. Despite the emerging progress achieved in QD sensitized/assisted TESG, thorough understanding of the TESG mechanism in QD-molecule complexes have not been accomplished. In this dissertation, we investigate the TESG mechanism in QD-molecule complexes with transient absorption spectroscopy (TA) and time-resolved photoluminescence (TRPL).
First, we tested whether QD sensitized triplet energy transfer (TET) is mediated by charge transfer virtual state by studying the shell-thickness dependent TET coupling strength in CdSe/CdS core/shell QD attached with 9-anthracene carboxylic acid (ACA). The measured TET coupling strength decreases exponentially with CdS shell thickness, and the exponential decay factor β is smaller than the sum of βs for electron transfer and hole transfer from QD. We propose that core/shell QD sensitized TET is mediated by both higher-energy virtual exciton states in CdSe/CdS QD and the charge transfer virtual state.
Second, we studied the correlation between QD bright/dark states and QD sensitized TET with CdSe/CdS core/shell QD-oligothiophene as a model system. We tuned the equilibrium between QD bright and dark states by varying temperature, and TET dynamics were measured with TA. We demonstrated the significant contribution of both bright and dark states to TET, which was attributed to the components with triplet-state-like spin characters in bright/dark state wavefunctions.
Furthermore, we examined the TET from trap states in CdSe QDs to adsorbed ACA with TA and TRPL and showed that both band edge and trap excitons can be transferred from QDs to generate ACA triplet excited states. The rate of TET decreases with decreasing trap exciton energies and is much slower than the TET rate from band edge excitons.
Finally, we demonstrated the TESG in CdSe QD-modified boron dipyrromethene (BODIPY) with charge separated intermediate state under excitation of either QD or BODIPY, and this TESG pathway kinetically competes the direct TET pathway.
Table of Contents
Chapter 1. Introduction 1
1.1 Molecular triplet excited state sensitized by quantum dot through triplet energy transfer 1
1.1.1 Quantum dot sensitized triplet energy transfer in triplet-triplet annihilation based upconversion 1
1.1.2 Theoretical model for triplet energy transfer 6
1.1.3 Mechanism studies of quantum dot sensitized triplet energy transfer 10
1.2 Quantum dot exciton properties and quantum dot sensitized triplet energy transfer 15
1.2.1 Exciton fine structures of quantum dot 15
1.2.2 Triplet energy transfer from trap states of quantum dot 19
1.3 Molecular triplet excited state generation in quantum dot-molecule complex through charge transfer intermediate 20
1.5 Reference 23
Chapter 2. Experimental Methods 32
2.1 Sample preparation 32
2.1.1 Reagents: 32
2.1.2 Synthesis of modified boron dipyrromethene (BODIPY) 32
2.1.3 Synthesis of phosphonic acid capped CdSe quantum dot (QD) and preparation of CdSe-ACA and CdSe-BQ complex 36
2.1.4 Synthesis of carboxylic acid capped CdSe QD and preparation of CdSe-BODIPY complex 36
2.1.5 Synthesis of CdSe/CdS QDs and preparation of CdSe/CdS QD-ACA, CdSe/CdS QD-MV2+, CdSe/CdS QD-PTZ, CdSe/CdS QD-functionalized oligothiophene (T6) complexes 37
2.2 Steady state and time-resolved spectroscopy setups 39
2.2.1 Transient absorption spectroscopy (TA) setups 39
2.2.2 Temperature dependent TA setups 41
2.2.3 Time-resolved photoluminescence (TRPL) setups 42
2.2.4 Steady state photoluminescence setups 43
2.2.5 Cyclic voltammetry (CV) measurement 43
2.3 Reference 44
Chapter 3. On the Coupling Strength of Core-shell Quantum Dot Sensitized Triplet Energy Transfer 45
3.1 Introduction 45
3.2 Results and discussion 48
3.2.1 Characterization of CdSe/CdS Core-shell QDs 48
3.2.2 Core-shell QD sensitized TET 52
3.2.3 Shell thickness dependence of TET rate 58
3.2.4 Coupling strengths of TET, electron transfer and hole transfer 61
3.3 Conclusion 69
Appendix 3.1 70
Appendix 3.2 75
Appendix 3.3 82
Appendix 3.4 86
Appendix 3.5 92
3.4 Reference 101
Chapter 4. Bright State Sensitized Triplet Energy Transfer from Quantum Dot to Molecular Acceptor Revealed by Temperature Dependent Energy Transfer Dynamics 107
4.1 Introduction 107
4.2 Results and discussion 109
4.2.1 Bright and dark state equilibrium in CdSe/CdS QDs 109
4.2.2 Temperature dependent TET from CdSe/CdS QD 116
4.2.3 Discussion 122
4.3 Conclusion 125
Appendix 4.1 127
Appendix 4.2 131
Appendix 4.3 134
Appendix 4.4 136
4.4 Reference 140
Chapter 5. Trap State Mediated Triplet Energy Transfer from CdSe Quantum Dots to Molecular Acceptors 145
5.1 Introduction 145
5.2 Results and discussion 147
5.2.1 Steady state absorption and photoluminescence spectra of CdSe QD and CdSe QD-ACA 147
5.2.2 Transient absorption spectra of CdSe QD and CdSe QD-ACA 149
5.2.3 Wavelength dependent time-resolved photoluminescence of CdSe QD and CdSe QD-ACA 153
5.2.4 Triplet energy transfer rate and efficiency from band edge exciton and trap states 158
5.3 Conclusion 160
5.4 Reference 161
Chapter 6. Enhanced Triplet State Generation through Radical Pair Intermediates in BODIPY-Quantum Dot Complexes 166
6.1 Introduction 166
6.2 Results and discussion 168
6.2.1 Sample preparation and optical properties 168
6.2.2 Transient absorption spectra of free BODIPY 168
6.2.3 Transient absorption spectra of BODIPY-QD complex 170
6.2.4 Spectra and kinetics analysis 175
6.2.5 Mechanism of triplet state formation 182
6.3 Conclusion 184
6.4 Reference 184
Chapter 7. Competition of Dexter, Förster and Charge Transfer Pathways for Quantum Dot Sensitized Triplet Generation 190
7.1 Introduction 190
7.2 Results and discussion 192
7.2.1 Sample preparation and optical properties 192
7.2.2 Transient absorption spectra of QD and QD-BODIPY complex 192
7.2.3 Spectra and kinetics analysis 198
7.2.4 Discussion 208
7.3 Conclusion 210
7.4 Reference 211
Chapter 8. Conclusion and Outlook 216
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