Arylamines as More Strongly Reducing Organic Photoredox Catalysts than fac-[Ir(ppy)3]

Organic photoredox catalysts (OPCs) have the potential to replace precious-metal-based photoredox catalysts (PMPCs). Compared with strongly oxidizing OPCs, such as the representative acridinium salts, however, the recent development of strongly reducing OPCs has been relatively sluggish. In this Perspective, strongly reducing OPCs bearing arylamine motifs are introduced. One of the advantages of OPCs is their versatility in catalyst design, which makes it easier to develop catalysts with a reducing capability superior to that of fac-[Ir(ppy)3], which is the strongest reductant among the commonly used PMPCs. Easy access to structural diversity also contributes to the rapid development of appropriate catalysts for various applications, for instance, not only simple organo-radical reactions but also precise control of polymer synthesis and properties through photocatalytic (organocatalyzed) atom-transfer radical polymerization. While light with a shorter wavelength (higher energy), such as near-ultraviolet light, is typically involved in conferring OPCs with their strongly reducing natures, strategies to develop strongly reducing catalytic systems using a longer wavelength (lower energy) of visible light, including consecutive photoinduced electron transfer, are emerging as a defacto standard. These strategies for the design of OPC systems, which allow them to achieve otherwise inaccessible reactions using visible light, are also described.