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Concerted proton-electron transfer reactions in the Marcus inverted region

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Science  03 May 2019:
Vol. 364, Issue 6439, pp. 471-475
DOI: 10.1126/science.aaw4675

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Protons venture into the inverted region

One of the most counterintuitive features of electron transfer kinetics is the inverted region. As Marcus theory predicts and experiments have borne out, electron transfer slows down once the driving force for it becomes especially favorable. Parada et al. now offer evidence for such inverted behavior in proton-coupled electron transfer (see the Perspective by Dempsey). Specifically, they examined a series of compounds with phenol bridging anthracene (electron acceptor) and pyridine (proton acceptor) derivatives. Time-resolved spectroscopy and accompanying theory revealed slower rates at higher driving forces in the back reaction that follows light-induced intramolecular proton and electron transfer.

Science, this issue p. 471; see also p. 436

Abstract

Electron transfer reactions slow down when they become very thermodynamically favorable, a counterintuitive interplay of kinetics and thermodynamics termed the inverted region in Marcus theory. Here we report inverted region behavior for proton-coupled electron transfer (PCET). Photochemical studies of anthracene-phenol-pyridine triads give rate constants for PCET charge recombination that are slower for the more thermodynamically favorable reactions. Photoexcitation forms an anthracene excited state that undergoes PCET to create a charge-separated state. The rate constants for return charge recombination show an inverted dependence on the driving force upon changing pyridine substituents and the solvent. Calculations using vibronically nonadiabatic PCET theory yield rate constants for simultaneous tunneling of the electron and proton that account for the results.

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