Connecting Spin Dynamics and Electron Transfer Reactions in Organic Molecules with Multiple Unpaired Electrons

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The work presented in this dissertation examines the interplay between electron transfer reactions and electron spin in photoactive organic molecules. Organic compounds that undergo electron transfer reactions after absorbing light are important in natural photosynthesis, photobiology, and synthetic photovoltaics. These electron transfer reactions depend on the spin states of the electrons involved, and in turn influence these spin states. In this thesis, the design and synthesis of several molecules to investigate these relationships are described. Electron paramagnetic resonance (EPR) and transient optical absorption (TA) spectroscopies are used to examine the spin states and movements of electrons following photoexcitation. Systems containing a single unpaired electron are investigated in Chapter 2. Upon photoexcitation, this electron moves between different sites in the molecule. We study the dependence of the electron transfer rates on solvent parameters, and perform preliminary EPR experiments towards using similar systems for spin coherence transfer. In Chapter 3, EPR spectroscopy and time-resolved microwave conductivity are used to investigate a photoactive organic framework material. We show that these guanine-quadruplex frameworks generate spin-correlated radical pairs upon photoexcitation, and the resulting radical ions are delocalized and mobile within the framework. These results suggest that this new class of materials may be useful for photovoltaics and batteries. In Chapters 4 and 5, radical-donor-acceptor molecules that form a triradical state upon photoexcitation are investigated. In Chapter 4, the additional radical is found to induce intersystem crossing on a timescale much faster than in similar systems. In Chapter 5, small differences in the rates of electron transfer processes are found to induce significant changes in the spin polarization of the stable radical. In Chapter 6, a molecule containing two stable radicals coupled to a donor-acceptor system is studied by EPR to examine spin polarization and coherence transfer between the four radicals following photoexcitation. Finally, in Chapter 7, a new experimental technique is proposed and tested for measuring the spin states of photogenerated radical pairs with high time resolution. While detection of a specific spin state remains challenging, initial experiments show the technique is feasible.

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  • 01/10/2019
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