The design of bioinspired systems for artificial photosynthesis and energy conversion processes is pivotal to the development of alternative renewable energy sources. The groups of Gust, Moore, and Moore have designed benzimidazole-phenol (BiP) model systems, which are inspired by the TyrZ-His190 redox relay found in photosystem II. Combined experimental and theoretical analyses have provided mechanistic insights into the proton-coupled electron transfer (PCET) processes occurring in these systems. Kinetic isotope effects (KIEs) of around two were measured electrochemically for substituted BiP-X (X = COOH and COOCH3) complexes. In the case of the BiP-porphyrin dyad, in which the phenol moiety is attached at the meso-position of the porphyrin, the experimentally measured KIE was unity. The KIEs calculated with a vibronically nonadiabatic rate constant expression for electrochemical PCET and density functional theory are in good agreement with the experimental values. The KIEs are found to be strongly influenced by the overlap between the reactant and product proton vibrational wavefunctions. Moreover, the calculations indicate that the KIE of unity for the BiP-porphyrin system may be attributed to the non-innocent porphyrin, in which the unpaired electron of the phenoxyl radical can delocalize onto. These insights are guiding the design of artificial photosynthetic systems capable of PCET processes involving multiple proton transfers.

Theoretical analysis of proton-coupled electron transfer in benzimidazole-phenol complexes

251st National Meeting (2016)

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