Emergence of ligand-to-metal charge transfer in homogeneous photocatalysis and photosensitization

Light energy can be harnessed by photosensitizers or photocatalysts so that some chemical reactions can be carried out under milder conditions compared to the traditional heat-driven processes. To facilitate the photo-driven reactions, a large variety of chromophores that are operated via charge transfer excitations have been reported because of their typically longer excited-state lifetimes, which are the key to the downstream photochemical processes. Although both metal-to-ligand charge transfers and ligand-to-metal charge transfers are well-established light absorption pathways; the former has been widely adopted in photocatalysis, whereas the latter has recently taken on greater importance in photosensitization applications. In this article, we review the latest developments on ligand-to-metal charge transfer photosensitization by molecular complexes across the periodic table by focusing homogeneous photocatalysis and the use of photophysical measurements and computational calculations to understand the electronic structures, photochemical processes, structure–activity relationships, and reaction mechanisms. We also present our perspectives on the possible future developments in the field.