Mechanisms of Photoredox Catalysts: The Role of Optical Spectroscopy

Abstract

The photoinduced organic transformation has stimulated the organic chemistry community to develop light-driven renewed reaction methodologies, which in many cases are complementary to standard thermal catalysis. This revitalization of photoinduced transformations is in part due to the straightforward access to powerful photosensitizers. Among those, Ru(II) and Ir(III) polypyridyl complexes have been extensively utilized as prototypical photoredox catalyst. Despite the flourish of new organic reactivity, studies of photocatalytic cycles are still scarce. The current mechanistic proposal mostly relies on luminescence quenching studies, empirical redox potentials, and bond-dissociation energy values, which provide a partial picture of the real catalytic processes occurring. Besides, quantum efficiency and overall energy efficiency of photoredox organic transformation are not usually considered merit yet. On the other hand, during the last decades, the photochemistry community has studied the energy and electron transfer mechanism of transition metal complexes from the ground and the excited-state extensively, without fully understanding the catalytic photoredox cycles probably due to its complexity. Those studies are needed to develop new photoredox organic transformations further and make them more sustainable and energy-efficient. We outline an overview of selected basic concepts of photophysics and photochemistry encountered in the photocatalytic cycles in this context. Selected examples of studies are detailed in the review to illustrate how steady-state and time-resolved optical spectroscopy can be employed to elucidate catalytic intermediates and photocatalytic mechanisms. As such, this review aims to motivate mechanistic studies on photoredox catalysis and serve as a guide to perform them to develop more sustainable and energy-efficient chemical transformations.