Molecular Approaches to Solar Fuel Production

The conversion of water to hydrogen and of carbon dioxide to industrially relevant chemical precursors are examples of reactions that can be used to store renewable energy as fuels or chemical building blocks for creating sustainable chemical manufacturing cycles. Unfortunately, current industrial catalysts for these transformations are reliant on relatively expensive and/or rare materials, such as platinum in the case of hydrogen generation, or lack selectivity towards producing a desired chemical product. Such drawbacks prevent global-scale applications. Although replacing such catalysts with more efficient and earth-abundant catalysts could improve this situation, the fundamental science required for this is lacking. In the first part of this dissertation, the synthesis and characterization of a novel binuclear iron fused porphyrin designed to break traditional scaling relationships in electrocatalysis is presented. Key features of the fused porphyrin include: 1) bimetallic sites, 2) a π-extended ligand that delocalizes electrons across the multimetallic scaffold, and 3) the ability to store up to six reducing equivalents. In the second part of this thesis, the electrochemical characterization of benzimidazole-phenols as “proton wires” is described. These bioinspired assemblies model the tyrosine-histidine pair of photosystem II, which serves as a redox mediator between the light-harvesting reaction center P680 and the oxygen evolution complex that enables production of molecular oxygen from water in cyanobacteria, algae, and higher plants. Results show that as the length of the hydrogen-bond network increases across a series of benzimidazole-phenols, the midpoint potential of the phenoxyl/phenol redox couple becomes less oxidizing. However, benzimidazole-phenols containing electron-withdrawing trifluoromethyl substituents enable access to potentials that are thermodynamically sufficient for oxidative processes relevant to artificial photosynthesis, including the oxidation of water, while translocating protons over ~11 Å.