The inverted region for outer-sphere electron transfer – where reaction rates become slower at higher driving forces – was predicted by Marcus and has emerged as a key feature of natural processes and technologies. We describe here the experimental evidence for the inverted region for proton-coupled electron transfer (PCET) reactions. Anthracene-phenol-pyridine have been designed and synthesized to undergo photo-induced intramolecular PCET. In a single concerted kinetic step, the phenol unit transfers an electron to the photoexcited anthracene and a proton to the pendent pyridine. Concerted PCET recombination then regenerates the ground-state triad. The effects of driving force on the rate constants for PCET charge separation and recombination reactions have been investigated with a combination of femtosecond time-resolved spectroscopic techniques, electronic structure modeling and vibronically nonadiabatic theory. Using derivatives with substituted pyridines and substituted anthracenes, recombination has been observed to be slower for reactions that are more exoergic. These studies are the first example of the Marcus inverted region for CPET.

Concerted proton-electron transfer reactions in the Marcus inverted region

2019 Spring National Meeting

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