Insights into Proton Coupled Electron Transfer in the Field of Artificial Photosynthesis

Artificial photosynthesis with respect to water splitting is usually divided into water oxidation catalysis (WOC) and the hydrogen evolution reaction (HER). Though in the combined redox dissociation of water into oxygen and hydrogen no protons and electrons occur, both half reactions show photoinduced proton coupled electron transfer (PCET). Regarding a classical approach, photosensitizers (PS) deliver electrons and protons that are accepted by water reduction catalysts (WRC) containing suitable basic atoms like nitrogen working as proton relays. However, the mechanisms of PCET reactions differ, where concerted proton electron transfer (CPET) is an elementary step. In CPET simultaneous electron and proton transfer occurs in the femtoseconds range, being rapid when compared to the periods for coupled vibrations and solvent modes. This has to be distinguished from stepwise electron and proton transfer, leading to underlying thermodynamics of the intermediates. DFT calculations based on X-ray diffraction (XRD) data help to specify the different reaction pathways. A plethora of experimental procedures are used in order to verify the theoretical predictions. Among them femtosecond pump-probe spectroscopic measurements play an important role. Furthermore, cyclic voltammetry (CV) has proven to be also a powerful tool. In the case of electrochemical PCET rotating-disk electrode voltammetry, electrochemical impedance spectroscopy and spectroelectrochemistry complete the experimental tools. In this minireview a selection of examples, where PCET occurs is discussed with respect to possible mechanisms and used methods.