Conversion of solar energy to a fuel occurs requires a series of photo-induced charge separation steps that lead up to proton-coupled electron transfer (PCET) reactions at the catalysts for water oxidation and fuel formation. In natural photosynthesis and respiration, PCET is involved in a similar manner. PCET is required for levelling the potentials of the sequential catalyst redox steps. It also has substantial impact on the reaction rate, due to modulations of the reaction energy barrier as well as the strong dependence on the distance the proton has to tunnel during the reaction. For the design of efficient solar fuels production, and understanding of biological PCET, it is therefore necessary to understand and control PCET reactions. Model systems play an important role in these efforts. Biomimetic systems showing PCET oxidation of tyrosine and tryptophan derivatives will be presented in this presentation, following two topics. 1) Rate dependence on proton tunnelling distance: Different series of phenol-base complexes have been made to study the rate dependence on the proton tunneling distance. In a recent example, we were able to keep other parameters that affect the rate (driving force, reorganization energy, electronic coupling) essentially constant, in contrast to previously published series. This allowed for a more direct analysis of the effects of distance on the PCET rate constant. 2) The competition between step-wise and concerted PCET: The competition between step-wise and concerted transfer of electron and proton is one of the most fundamentally important aspects of PCET. We have made and studies several model systems where the mechanism can be switched, and the rationale and significance of these findings will be discussed. In a recent study, we could switch between all three major mechanisms of PCET (ET-first, PT-first and concerted), as evidenced by their distinctly different kinetic behavior. We have also found evidence for a concerted PCET in tryptophan systems with water as primary proton acceptor, in contrast to theoretical predictions that this is impossible.

Mechanisms and distance-dependence of proton-coupled electron transfer

255th National Meeting (2018)

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