Pressure effects on thermal and photochemical proton-coupled electron transfer reactions with metal complexes
Final Report Abstract
Since thermal, electrochemical, and/or photochemical proton-coupled electron transfer (PCET) governs the chemical energy conversion in natural and artificial systems, the differentiation and manipulation of PCET processes are of biological, environmental and industrial relevance. In this context, the major objective of this project was the implementation of pressure approach as a novel, valuable tool for kinetic/thermodynamic elucidations and mechanistic distinction of different classes of PCET as well as the modulation of the latter to earn the chemical systems showing desired reactivity and high catalytic efficiency. The results are the outcome of combined work between the research groups of Dirk M. Guldi and Ivana Ivanović-Burmazović. They represent a synergy between mechanistic/kinetic expertise and the application of a variety of special high-pressure (HP) techniques as well as pump probe spectroscopy. Besides using temperature- and pressure-dependent NMR spectroscopy and HP-electrochemistry, within this project we newly developed a unique methodology for the variable temperature and pressure, femto- and nanosecond resolved pump probe spectroscopy with home-made high-pressure cell for absorption detection. This is a major methodological result of this project that for the first time in the literature allowed us to study the excited-state PCET chemistry under high-pressures, opening new avenues in the fields of PCET and photoinduced processes as well as in transient absorption spectroscopy (TAS). More importantly, this development enabled us to confirm our hypothesis that pressure variation i) can modulate PCET by turning a stepwise into beneficial concerted mechanism and ii) can serve as a kinetic tool to give an answer on critical question about a nature of PCET mechanisms. Just, recently the group of Leif Hammarström at Uppsala University published a work in the same direction. However, they could not study exited state kinetics, which is the unique feature of our project and have resulted in the second major outcome, namely a time resolved visualization of merging of a proton and an electron transfer steps into a concerted process upon concentration or pressure increase, respectively. To the best of our knowledge, this is the most direct experimental evidence for a simultaneous movement of a proton and an electron within photoinduced redox processes.
