Molecular Design for Electron-Driven Double-Proton Transfer: A New Scenario for Excited-State Proton-Coupled Electron Transfer

Proton-coupled electron transfer (PCET) reactions play important roles in solar energy conversion processes. Designing efficient artificial photosystems with PCET mechanisms is a promising solution for the growing demands of energy resources. Compared to ground states, inducing the PCET reactions directly from electronically excited states, named excited-state PCET (ES-PCET) reactions, is a more direct and efficient avenue to the formation of solar fuels. Here, based on benzimidazole phenolic derivatives, we have designed and studied some molecular structures that can undergo the electron-driven double-proton transfer (EDDPT) reactions within the ES-PCET framework. According to our DFT/TDDFT calculation results, the two protons transfer in a stepwise manner in the EDDPT process, and compared to the common way of electron-driven single-proton transfer (EDSPT) reactions, the proton transfer in the EDDPT process not only has a smaller energy barrier but also experiences a longer transferring distance, which has beneficial effects on producing solar fuels. The study of ES-PCET reactions under the mechanism of EDDPT may cast light on the regulation of proton transfer at defined distances and time scales, which is important in energy conversion processes.