Proton-coupled electron transfer (PCET) reactions play a critical role in a wide range of chemical processes in molecules and materials. This talk will present recent insights gained from a vibronically nonadiabatic PCET theory that treats the electrons and transferring protons quantum mechanically and includes the motions of the donor-acceptor modes as well as the solvent and solid-state environment. Photoinduced PCET in anthracene-phenol-pyridine triads exhibits inverted region behavior, where the more thermodynamically favorable process is slower. This behavior is explained in terms of significant contributions from vibrationally excited states with appreciable vibrational wave function overlap integrals but negligible contributions from higher oscillatory states with near-zero overlap integrals. Nonequilibrium dynamics simulations identified two distinct PCET pathways following photoexcitation of the anthracene: the electron transfers from phenol to anthracene to generate a charge separated state for the triads exhibiting inverted region behavior, whereas the electron transfers from phenol to pyridine to generate a local electron-proton transfer (LEPT) state for the other triads. Subsequent experiments identified the LEPT state, and the associated mechanism was revealed to be proton-coupled energy transfer (PCEnT) because it does not entail charge transfer to the anthracene, but rather occurs via nonadiabatic electronic energy transfer coupled to proton tunneling. Application of the vibronically nonadiabatic PCET theory to a TEMPO radical at a TiO<sub>2</sub> interface elucidates the impact of different types of defects in TiO<sub>2</sub> on the PCET rate constant.

Proton-coupled electron and energy transfer processes

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ACS Spring 2023

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