Making Molecules for Light-Driven Water Splitting

Norris, Michael Ryan

Making Molecules for Light-Driven Water Splitting Public Deposited

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March 22, 2019

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Norris, Michael Ryan
- Affiliation: College of Arts and Sciences, Department of Chemistry

Abstract

To meet growing global energy demand, methods to transform solar energy into chemical fuels are necessary. Photosynthesis provides a blueprint: using electrons from the photo-driven oxidation of water to generate reductive equivalents, which ultimately convert CO2 to carbohydrates. Key to any artificial photosynthetic device that attempts to mimic this process is the light-driven water oxidation reaction, 2H2O + 4hv → O2 + 4e-. Incorporation of a chromophoric material for absorption of light and a catalyst for water oxidation are therefore required. Several monomeric, single-site Ru polypryidyl catalysts have been investigated for this transformation. Electrochemical or Ce?IV? oxidation results in homogeneous catalytic water oxidation. Catalytic rates and onset potentials were found to be highly tunable based on the ligand environment. Water oxidation catalysts built with first-row transition metals were also investigated. Electrochemical studies of these complexes indicated complexes of Fe and Cu were catalysts for water oxidation at elevated pH. In addition to water oxidation catalysts, chromophore/redox mediator (CRM) complexes are necessary for light-driven water oxidation. A series of nine Ru polypyridyl complexes as well as four organic derivatives of perylene diimide (PDI) were synthesized, and their electrochemical and photophysical properties were investigated for possible use in an artificial photosynthetic device. Control of conjugation and heteroatoms in the polypyridyl ligands led to highly tunable redox potentials, UV/vis absorptions, and emission energies for the Ru complexes, while the PDI derivatives were found to undergo rapid charge separation upon excitation. For artificial photosynthetic devices to operate in aqueous media, it is necessary to form stable anchors to metal oxide surfaces. New synthetic procedures for making phosphonic acid-derivatized bipyridine ligands and their Ru polypyridyl complexes were devised, and these routes allow easy access to important ligands and to complexes that can bind to metal oxide surfaces. Finally, CRM complexes and catalysts were combined in a molecular Ru-choromphore-Ru-catalyst assembly. Solution studies led to the discovery of a redox mediator effect in water oxidation driven by CeIV. Detailed electrochemical and photophysical studies of the assembly anchored to metal oxide surfaces were performed, and the assembly was shown to be the first molecular chromophore-catalyst assembly capable of light-driven water oxidation.

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