The coupling between electrons and protons and the long-range transport of protons play important roles throughout chemistry and biology. Simulating the nonequilibrium dynamics of proton-coupled electron transfer (PCET) provides insight into the reaction pathways, mechanisms, and rates of such processes. Biomimetic systems derived from benzimidazole-phenol constructs undergo PCET upon electrochemical or photochemical oxidation and can transport protons along hydrogen-bonded networks or proton wires through multiproton PCET. Nonequilibrium dynamical simulations predict that these proton wires are concerted in that no thermodynamically stable intermediate is observed but are asynchronous on the ultrafast time scale. As a second example, 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 provide an explanation as to why inverted region behavior is only observed experimentally for certain triads. These simulations identified two distinct PCET pathways following photoexcitation to the locally excited state of anthracene: the electron transfers from phenol to anthracene to generate a long-lived charge-separated state for the triads exhibiting inverted region behavior, whereas the electron transfers to the pyridine, collinear with the proton, to provide an alternative relaxation pathway for the other triads. For both of these examples, the mechanistic pathways, as well as their time scales, can be controlled by tuning the substituents on the molecular constructs.

Controlling the pathways for protons and electrons

ACS Fall 2021

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