4.8 Article

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

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AMER CHEMICAL SOC
DOI: 10.1021/jacs.2c05820

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  1. U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences [DE-FG02-03ER15393, DE-SC0021186]
  2. Camille Dreyfus Teacher- Scholar Awards Program
  3. U.S. Department of Energy, Office of Science, Basic Energy Sciences
  4. U.S. Department of Energy (DOE) [DE-SC0021186] Funding Source: U.S. Department of Energy (DOE)

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By substituting electron-withdrawing groups onto the benzimidazole-based bridge, the oxidation potential in proton-coupled electron transfer processes can be increased, enabling long-range proton translocation. This strategy of altering the driving force provides a new solution for oxidative processes, such as artificial photosynthesis.
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 similar to 11 & Aring; become thermodynamically comparable to that of an unsubstituted system where one proton is transferred over similar to 2.5 & Aring;. 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.

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