Managing the Redox Potential of PCET in Grotthuss-Type Proton Wires

• DOI: 10.1021/jacs.2c05820
• Publication Date: 2022-08-22T13:52:47Z
• Authors: Emmanuel Odella; Maxim Secor; Edgar A. Reyes Cruz; Walter D. Guerra; María N. Urrutia; Paul A. Liddell; Thomas A. Moore; Gary F. Moore; Sharon Hammes-Sch...; Ana L. Moore
• Keywords: 0301 basic medicine; Phenol; REDOX POTENTIAL; Electron Transport; 03 medical and health sciences; Phenols; https://purl.org/becyt/ford/1.4; BENZIMIDAZOLE PHENOL (BIP); Benzimidazoles; Protons; https://purl.org/becyt/ford/1; Oxidation-Reduction; ELECTRON WITHDRAWING GROUP; Hydrogen

Expanding proton-coupled electron transfer to multiproton translocations (MPCET) provides a bioinspired mechanism to transport protons away from the redox site. This expansion has been accomplished by separating the initial phenolic proton donor from the pyridine-based terminal proton acceptor by a Grotthuss-type proton wire made up of concatenated benzimidazoles that form a hydrogen-bonded network. However, it was found that the midpoint potential of the phenol oxidation that launched the Grotthuss-type proton translocations is a function of the number of benzimidazoles in the hydrogen-bonded network; it becomes less positive (i.e., a weaker oxidant) as the number of bridging benzimidazoles increases. Herein, we report a strategy to maintain the high redox potential necessary for oxidative processes relevant to artificial photosynthesis, e.g., water oxidation and long-range MPCET processes for managing protons. The integrated structural and functional roles of the benzimidazole-based bridge provide sites for substitution of the benzimidazoles with electron-withdrawing groups (e.g., trifluoromethyl groups). Such substitution increases the midpoint potential of the phenoxyl radical/phenol couple so that proton translocations over ∼11 Å become thermodynamically comparable to that of an unsubstituted system where one proton is transferred over ∼2.5 Å. The extended, substituted system maintains the hydrogen-bonded network; infrared spectroelectrochemistry confirms reversible proton translocations from the phenol to the pyridyl terminal proton acceptor upon oxidation and reduction. Theory supports the change in driving force with added electron-withdrawing groups and provides insight into the role of electron density and electrostatic potential in MPCET processes associated with these Grotthuss-type proton translocations.