Visible Light Photoredox Catalysts

A photocatalyst working for both one-electron oxidation and reduction under visible light irradiation, the so-called “visible light photoredox catalyst”, receives much attention, because there is the possibility for use as a solar energy source.¹⁾ A reaction mediated by a photoredox catalyst particularly works simply, whereas a thermal reaction under coexistence of oxidizing and reducing reagents is normally hard to make work. The reaction cycle using a photoredox catalyst involves both oxidative and reductive pathways, thus it shows a ‘redox-neutral’ mechanism overall.

Some ruthenium(II) polypyridyl complexes and iridium(III) phenylpyridyl complexes (1, 2 and 3) work as photoredox catalysts under irradiation of visible light.^2,3) These transition metal complexes are useful photocatalysts, because they can form a long-lived triplet-excited state under photoirradiation. A chemical modification of the coordinating ligands controls the redox potentials of the transition metal complexes.⁴⁾