conversion and storage, including artificial photosynthesis. In nature, the active sites of
enzymes are typically earth-abundant metal centers and the protein provides a unique
three-dimensional environment for effecting catalytic transformations. Inspired by this
biological architecture, a synthetic methodology using surface-grafted polymers with
discrete chemical recognition sites for assembling human-engineered catalysts in three-dimensional
environments is presented. The use of polymeric coatings to interface cobalt-containing
catalysts with semiconductors for solar fuel production is introduced in
Chapter 1. The following three chapters demonstrate the versatility of this modular
approach to interface cobalt-containing catalysts with semiconductors for solar fuel
production. The catalyst-containing coatings are characterized through a suite of
spectroscopic techniques, including ellipsometry, grazing angle attenuated total reflection
Fourier transform infrared spectroscopy (GATR-FTIR) and x-ray photoelectron (XP)
spectroscopy. It is demonstrated that the polymeric interface can be varied to control the
surface chemistry and photoelectrochemical response of gallium phosphide (GaP) (100)
electrodes by using thin-film coatings comprising surface-immobilized pyridyl or
imidazole ligands to coordinate cobaloximes, known catalysts for hydrogen evolution.
The polymer grafting chemistry and subsequent cobaloxime attachment is applicable to
both the (111)A and (111)B crystal face of the gallium phosphide (GaP) semiconductor,
providing insights into the surface connectivity of the hard/soft matter interface and
demonstrating the applicability of the UV-induced immobilization of vinyl monomers to
a range of GaP crystal indices. Finally, thin-film polypyridine surface coatings provide a
molecular interface to assemble cobalt porphyrin catalysts for hydrogen evolution onto
GaP. In all constructs, photoelectrochemical measurements confirm the hybrid
photocathode uses solar energy to power reductive fuel-forming transformations in
aqueous solutions without the use of organic acids, sacrificial chemical reductants, or
electrochemical forward biasing.
In Photosystem II of plants, the proton motive force that is essential for life is generated partly by the water oxidation process where the tyrosine and histidine 190 (hydrogen bonded) amino acids play an important role. The proton-coupled electron transfer (PCET) process involving these two molecules has been replicated using a benzimidazole-phenol (BIP) construct as an artificial model of both the intramolecular hydrogen bond interaction and the associated PCET process. BIP is a nearly planar molecule and features a strong intramolecular hydrogen bond between the phenol and the nitrogen of the benzimidazole. When the molecule is oxidized electrochemically, the phenolic proton is transferred to the nitrogen of the benzimidazole moiety in a PCET mechanism. Herein the design, synthesis, and physicochemical characterization of a new BIP derivative is described. By introducing a methyl group in the new design, we intentionally increase the dihedral angle between the benzimidazole and phenol rings. The presence of the methyl group affects the ground-state PCET and the excited-state intramolecular proton transfer processes as well. The break in the coplanarity weakens the strength of the intramolecular hydrogen bond, decreases the chemical reversibility, and quenches the emission from the excited-state intramolecular proton transfer state. The findings contribute to understanding the importance of having a nearly planar structure in bioinspired artificial photosynthetic systems.