Redox photocatalysis

The past decade has seen a massive resurgence of photoredox catalysis-themed research, a field that uses photon-induced electron and energy transfer processes for driving synthetic transformations. Transition metal-based photocatalysts have played a key role in this revival, given their photochemical and electrochemical robustness, and, most importantly, their ability to form charge-transfer excited states and consequently act as strong photooxidants and/or photoreductants. In addition, some of these photocatalysts can participate in inner-sphere processes, leading to previously unattainable mechanistic pathways and products. While the initial exploration phase involved the utilization of ruthenium(II) polypyridine and cyclometalated iridium(III)-based photocatalysts, recent years have witnessed an influx of earth-abundant metal-based compounds, with tremendous progress being made for copper(I/II), chromium(III), iron(II/III), zirconium(IV) and tungsten(VI)-based chromophores. This chapter assesses the advances made in the field of transition metal-based photoredox catalysis, with emphasis on mononuclear photocatalysts. An overview of the photophysical and mechanistic aspects of transition metal photocatalysts is provided, followed by a discussion of current examples of photo-driven synthetic reactions. Multinuclear or cooperative transition-metal based catalytic systems are also briefly reviewed. This new era of photoredox catalysis has embraced emerging concepts like the use of multiphoton excitation, enantioselective photocatalysts, high-throughput photocatalyst synthesis and screening, and has also been used for the development of pharmaceutically relevant molecules. Examples from literature utilizing these concepts will also be presented.