Proton-coupled electron transfer across benzimidazole bridges in bioinspired proton wires

EMMANUEL ODELLA, S. JIMENA MORA, BRIAN L. WADSWORTH; MIGUEL A. GERVALDO; ANA L. MOORE; THOMAS L. GROY; SHARON HAMMES-SCHIFFER; JOSHUA J. GOINGS; LEONIDES E. SERENO; DEVENS GUST, THOMAS A. MOORE, GARY F. MOORE

Chemical Science

‎Royal Society of Chemistry‎

Lugar: london; Año: 2020

2041-6539

Designing molecular platforms for controlling proton and electron movement in artificial photosyntheticsystems is crucial to efficient catalysis and solar energy conversion. The transfer of both protons andelectrons during a reaction is known as proton-coupled electron transfer (PCET) and is used by nature inmyriad ways to provide low overpotential pathways for redox reactions and redox leveling, as well as togenerate bioenergetic proton currents. Herein, we describe theoretical and electrochemical studies ofa series of bioinspired benzimidazole-phenol (BIP) derivatives and a series of dibenzimidazole-phenol(BI2P) analogs with each series bearing the same set of terminal proton-accepting (TPA) groups. The setof TPAs spans more than 6 pKa units. These compounds have been designed to explore the role of thebridging benzimidazole(s) in a one-electron oxidation process coupled to intramolecular protontranslocation across either two (the BIP series) or three (the BI2P series) acid/base sites. These molecularconstructs feature an electrochemically active phenol connected to the TPA group througha benzimidazole-based bridge, which together with the phenol and TPA group form a covalentframework supporting a Grotthuss-type hydrogen-bonded network. Infrared spectroelectrochemistrydemonstrates that upon oxidation of the phenol, protons translocate across this well-defined hydrogenbonded network to a TPA group. The experimental data show the benzimidazole bridges are noninnocent participants in the PCET process in that the addition of each benzimidazole unit lowers theredox potential of the phenoxyl radical/phenol couple by 60 mV, regardless of the nature of the TPAgroup. Using a series of hypothetical thermodynamic steps, density functional theory calculationscorrectly predicted the dependence of the redox potential of the phenoxyl radical/phenol couple on thenature of the final protonated species and provided insight into the thermodynamic role ofdibenzimidazole units in the PCET process. This information is crucial for developing molecular ?dryproton wires? with these moieties, which can transfer protons via a Grotthuss-type mechanism over longdistances without the intervention of water molecules.