Impact of Hydrogen Bonding on Proton-Coupled Electron Transfer

Proton-coupled electron transfer (PCET) reactions play a critical role in a wide range of chemical and biological processes. This talk will present insights gained from our vibronically nonadiabatic PCET theory and nonequilibrium dynamical simulations. Our general theoretical formulation for PCET treats the electrons and transferring protons quantum mechanically and includes the motions of the donor-acceptor modes as well as the solvent or protein environment. 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 dynamics 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 predicted two distinct pathways following photoexcitation of the anthracene: the electron transfers from phenol to anthracene to generate a charge separated state for PCET, whereas the electron transfers from phenol to pyridine to generate a local electron-proton transfer state for proton-coupled energy transfer (PCEnT). The novel PCEnT mechanism does not entail charge transfer to the anthracene, but rather occurs via nonadiabatic electronic energy transfer coupled to proton tunneling. This theoretically predicted mechanism was subsequently observed experimentally. Hydrogen-bonding interactions are essential for all of these PCET and PCEnT processes.