High-pressure pump–probe experiments reveal the mechanism of excited-state proton-coupled electron transfer and a shift from stepwise to concerted pathways

Chemical energy conversion and storage in natural and artificial systems rely on proton-coupled electron transfer (PCET) processes. Concerted proton-electron transfer (CPET) can provide kinetic advantages over stepwise processes (electron transfer (ET)/proton transfer (PT) or PT/ET), so understanding how to distinguish and modulate these processes is important for their associated applications. Here, we examined PCET from the excited state of a ruthenium complex under high pressures. At lower buffer or quencher concentrations, a stepwise PT/ET mechanism was observed. With increasing pressure, PT slowed and ET sped up, indicating a merging of the two steps. In contrast, CPET at higher concentrations of buffer or quencher showed no pressure dependence of the reaction rate. This is because the simultaneous transfer of electrons and protons circumvents changes in charges and, consequently, in solvent electrostriction during the transition state. Our findings demonstrate that pressure can serve as a tool to monitor charge changes along PCET pathways, aiding in the identification of its mechanisms.