Pressure effects on thermal and photochemical proton-coupled electron transfer reactions with metal complexes

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Thermal, electrochemical, and/or photochemical proton-coupled electron transfer (PCET) governs the chemical energy conversion in natural and artificial systems. Thermodynamic and kinetic benefits of PCET correlate with the fact that neither charge generation, nor charge shift/increase impact product or transition state. A leading example is a concerted proton-electron transfer (CPET). As such, understanding PCET and ways to modulate them by favoring CPET rather than stepwise electron (ET) and proton transfer (PT) is of utmost importance toward increasing efficiency of the energy conversion. Our project aims at pioneering fundamental studies that should reveal how pressure as an important physical parameter can be used as a tool to: (i) assess and distinguish between different PCET mechanisms, (ii) understand the factors that affect them, and (iii) induce mechanistic changeovers and, in turn, maximize reaction rates. A pressure effect on kinetics is characterized by a volume of activation, which is highly sensitive to changes in overall charge or intramolecular charge distribution en route to the transition state. Thus, it evolves as a key factor for differentiating and manipulating PCETs. In addition, pressure modulation is a unique way to impact bond breaking / formation in inner-sphere (PC)ET and reorganization energy in outer sphere (PC)ET, on one hand, and to tune electron-donor acceptor distance / alignment/electronic coupling in long-range (PC)ET, on the other hand. Here in our model studies regarding variably pressure kinetics of a series of newly synthesized molecular systems, based on redox-active metal centers and hydrogen bonded assemblies, we will gather a comprehensive understanding of thermal, electrochemical, and photochemical PCETs. To this end, the most prominent examples are homogeneous vs. heterogeneous self-exchange PCETs, uni- vs. bi-directional cross PCETs, as well as PCET processes within covalent or hydrogen bond molecular assemblies and their combinations. By systematically increasing the complexity of the investigated systems, we will be able to correlate the effects between different, but complementary processes such as between inter- and intramolecular reactions or between self exchange and cross PCET reactions. Methodologically, we will use a synergy between syntheses and a variety of experimental techniques for kinetic elucidations. The respective choices will depend on the molecular system and will range from, for example, absorption, fluorescence, IR, and Raman to electrochemistry and NMR in high pressure cells either in a steady-state or in a time-resolved modus, complemented by general analytical techniques such as ESI-MS, EPR, X-ray, etc.. In addition to pressure variation, temperature will also be an important variable both as synergistic tools to corroborate reaction mechanisms and to fine-tune reaction kinetics.