The chemical review highlights the early work on the use of transition metal complexes as photoredox catalysts to promote reactions of organic compounds. Studies on the use of transition metal complexes as visible light photocatalysts for organic synthesis have significantly benefited from advances in the related fields of organic and semiconductor photocatalysis. It is better to consider the photochemistry of the prototypical photoredox catalyst Ru(bpy)2+3 before discussing organic transformations enabled by photoredox catalysis. An electron in one of the photocatalyst's metal-centered t2g orbitals is excited to a ligand-centered π* orbital on absorption of a photon in the visible region. This transition is termed a metal to ligand charge transfer (MLCT) and results in a species in which the metal has been effectively oxidized to a Ru(III) oxidation state and the ligand framework has undergone a single-electron reduction.
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