Excited-State Proton-Coupled Electron Transfer: Different Avenues for Promoting Proton/Electron Movement with Solar Photons

Excited-state proton-coupled electron transfer (ES-PCET) is a promising avenue for solar fuel production and small molecule activation. Although less is known about ES-PCET reactions than related transformations involving exclusively thermal processes, ES-PCET holds promise as a simple and efficient reaction scheme for the formation of solar fuels, and it may provide access to new reactivity not accessible from ground electronic states. This review classifies ES-PCET into six categories based on the identity of the photoexcited reactant: excited-state H+/e– donors, excited-state H+/e– acceptors, photo-oxidants, photoreductants, photoacids, and photobases. A brief overview of each class of ES-PCET is presented. Recent advances and key discoveries within the six classes of ES-PCET are examined, and underexplored reaction systems and promising paths for future research are discussed.