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

Abstract

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.

Light-driven hydrogen evolutionviaa novel pincer/no pincer mechanism including a possible concerted proton electron transfer

“…By contrast, for EPT or PET the electron transfer is assumed to be the rate determining step, where none or a small KIE is measurable. 89 to nickel catalysts is in contrast to previous results, which likely arise from different irradiation conditions like the duration (5.5 h vs. 60/120 min), the light source (150 W Hg lamp vs. LED) and the WRC/PS ratio (10 vs. 5).…”

Density functional theory and machine learning for electrochemical square-scheme prediction: an application to quinone-type molecules relevant to redox flow batteries

“…Proton–electron transfer (PET) is a fundamental reaction in electrochemistry, 1–3 biochemistry, 4,5 material science, 6,7 and in some other fields. 8,9 To give one example, we can consider the charging and discharging processes in aqueous redox flow batteries, 10–14 in which the two elementary steps, i .…”

The influence of trinuclear complexes on light-induced hydrogen production

“…23 Inserted proton relays in the form of an amine function are intended to facilitate proton binding, the proton transfer and hydride binding to the catalytic metal centre. 9,[25][26][27] Please do not adjust margins Please do not adjust margins It has also been reported that pendant amines support rapid hydride formation and proton-hydride coupling in the immediate vicinity of the metal centre, leading to increased hydrogen formation rates. Beyond that, it is assumed that the basicity of the neighbouring amines in interaction with the hydride acceptor capability of the metal centre favours thermodynamically favourable processes, which leads to higher and faster hydrogen evolution.…”

“…23 Inserted proton relays in the form of an amine function are intended to facilitate proton binding, proton transfer and hydride binding to the catalytic metal centre. 9,25–27…”