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Abstract

Due to the ever-increasing global demand for energy and the looming disaster of climate change fueled by the increase in atmospheric carbon dioxide concentration, there is a pressing need for non-fossil-fuel energy sources. Among the renewable energy sources, solar energy presents the largest potential capacity, provided that it can be efficiently captured and stored for offline use. Storage of solar energy as a liquid fuel is attractive both in terms of energy density and as fuel source easily integrated into existing infrastructure. To this end, an artificial photosynthetic system must be developed that integrates the oxidation of water and the reduction of carbon dioxide. Photochemical integration of these two redox catalytic cycles is the problem at the heart of any solar fuel system. The systems presented in this dissertation are homogeneous, integrated chromophore/two-catalyst systems that are thermodynamically capable of photochemically driving the energy-storing reverse water-gas shift reaction (CO2 + H2 → CO + H2O), where the reducing equivalents are provided catalytic oxidation of renewable H2, a proxy for water. In this thesis, zinc tetraarylporphyrin, a broadly absorbing chromophore, is used as the photosensitizer. In Chapters 2 and 3, the systems described are freely dispersed in solution, while the system in Chapter 4 is based on self-assembly using rigid spacers with soft contacts. Using time-resolved spectroscopic methods, a comprehensive mechanistic and kinetic picture of the photoinitiated reactions of these systems has been developed. It has been found that absorption of a single photon by a broadly absorbing zinc porphyrin sensitizes intercatalyst electron transfer to produce the substrate-active forms of each catalyst. The initial photochemical step in Chapters 2 and 3 is the heretofore unobserved reductive quenching of the low-energy T1 state of ZnTPP. In Chapter 3, reductive binding of CO2 by the sensitized form of the CO2 reduction catalyst is observed under conditions that are catalytic for H2 oxidation. Insights regarding the ability to tune the rates of charge separation and charge recombination for each system are discussed.

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