Manganese catalysts and photosensitized polyoxotitanates as synthetic models for light-driven water oxidation

The harvest of solar energy is a promising strategy to meet global energy demands. The development of a system for converting sunlight into storable chemical fuels is inspired by natural photosynthesis. In natural photosynthesis, solar energy is used to convert water to oxygen at the CaMn 4Ox center of the enzyme Photosystem II. In one design of a synthetic photoanode for light-driven water oxidation, light energy is absorbed by a chromophore, and the excited chromophore transfers an electron to the conduction band of a semiconductor oxide. The hole remaining on the chromophore oxidizes a water-oxidation catalyst to advance the catalytic cycle. After the accumulation of four light-induced oxidation steps, the catalyst converts two water molecules into dioxygen and four protons. These protons and electrons can be combined to form a reduced chemical fuel. The photoanode design relies on the synchrony of several components, each of which must be developed and optimized. In order to understand the properties of each component individually, simpler model systems may be employed. In this dissertation, model systems for each of the photoanode components were examined.;The requirements for active water-oxidation catalysts were explored using manganese catalysts as models of the oxygen-evolving center in Photosystem II. The addition of a strongly electron-donating carboxamide ligand trans to the water-binding site in a manganese coordination complex was observed to promote water oxidation in the presence of the chemical oxidant Oxone. In an alternative approach, two manganese tetramers were encapsulated in Nafion proton exchange membrane on an electrode surface. Photocurrent arising from water oxidation was observed when the samples were illuminated under oxidizing conditions, but the coordination complexes were observed to decompose to active manganese oxides under the catalytic conditions. The instability of coordination complexes under the highly oxidizing conditions required for water oxidation was avoided by studying manganese oxide materials. A series of manganese oxide materials was prepared by electrodeposition in the presence of oxidizable organic molecules. The presence of oxidizable hydroxyl groups lowered the overpotential for electrocatalytic water oxidation.;The interface between a chromophore and a semiconductor oxide is modeled using the polyoxotitanate [Ti17O24(OiPr) 20] (Ti17) sensitized with simple chromophores, 4-nitrophenyl acetylacetonate (NPA) or coumarin 343 (C343). The sensitized Ti17 clusters were structurally characterized using single-crystal X-ray diffraction, revealing titanium atoms in very similar coordination environments as are found in nanoparticulate TiO2. Using EPR spectroscopy, the properties of the light-induced charge-separated state were examined, and the Ti3+ centers, oxygen-centered radicals, and organic radicals produced during illumination were assigned to specific centers within the Ti17 cluster. The difference between a hole-injection and electron-injection mechanism was distinguished by assigning the types of components produced in the EPR spectra of the sensitized polyoxotitanates, and this analysis was supported by computational modeling.;In all of these studies, the use of model systems provided insight into the development of photoanodes for light-driven water oxidation.