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

J Am Chem Soc. 2022 Aug 31;144(34):15672-15679. doi: 10.1021/jacs.2c05820. Epub 2022 Aug 22.

Abstract

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.

Publication types

  • Research Support, Non-U.S. Gov't
  • Research Support, U.S. Gov't, Non-P.H.S.

MeSH terms

  • Benzimidazoles / chemistry
  • Electron Transport
  • Hydrogen / chemistry
  • Oxidation-Reduction
  • Phenol / chemistry
  • Phenols* / chemistry
  • Protons*

Substances

  • Benzimidazoles
  • Phenols
  • Protons
  • Phenol
  • Hydrogen