Electron transfer within tetrathiafulvalene calixpyrrole supramolecular ensembles
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Over the last decade, the ecological need for clean and renewable energy sources has resulted in considerable resources being directed toward the development of systems capable of converting light energy into chemical energy. This has led to a focus on artificial photosynthetic systems and solar cell devices. These types of devices are desirable since they do not contribute to greenhouse gas emissions, as compared with fossil fuels. Organic solar cells (OSCs) are able to convert solar energy into chemical energy via photon absorption that creates a potential difference in the medium and results in electron transfer. The evolution and lifetime of the charge-separated state produced upon this electron transfer has proved difficult to mimic with synthetic materials. One well recognized problem is that to achieve efficient electron transfer, the rate of back electron transfer must be slower than that of forward electron transfer. Creating a molecular dyad that undergoes rapid electron transfer and results in a long lifetime is a key step in the creation of an organic solar cell that permits efficient solar energy conversion. One such way to achieve these systems is to employ supramolecular interactions to pre-organize the donors and acceptors in solution. The goal of the studies depicted in this dissertation was to explore whether tetrathiafulvalene substituted calixpyrroles (TTF–C4Ps) paired with suitable electron acceptors would lead to systems that undergo electron transfer, either thermal or photoinduced, followed by the formation of stable charge-separated states. We chose to employ fullerenes (Chapter 2) and a porphyrin substituted with a carboxylate functional group (Chapter 3) as the acceptors in the putative electron transfer complexes since both are well known as photoabsorbers that have been extensively studied in photosynthetic model systems. The electron transfer from a TTF moiety of the calixpyrrole to either fullerene or porphyrin was studied via UV-Vis-NIR, fluorescence, and electron spin resonance spectroscopies as well as with laser flash photolysis measurements and theoretical calculations. Chapter 4 details work in which TTF oxidation states were used to create a stable TTF mixed-valence dimer as well as a redox switched “on—off—on” fluorescent system.