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A photoreaction undergoes a particular molecular conversion to form a product that is not obtained by a thermal reaction. Recently, visible light induced photoreactions were intensively developed. A photoreaction under visible light does not require high energy UV light and the reactions occur under mild conditions to avoid forming unexpected by-products.
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 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.
The formation of radical species normally requires a photoirradiation, redox reactions, and thermal activation by supplying high energy. The research field of photocatalysis developed transition metal complex catalysts and organic catalysts to form radical species under mild conditions, such as irradiating with visible light.
Some ruthenium(II) polypyridyl complexes and iridium(III) phenylpyridyl complexes work as photoredox catalysts under irradiation of visible light.2) 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.3) Metal-free organic catalysts have also been developed. Some acridinium compounds with a donor-acceptor structure can be photoredox catalysts, because the excited state exhibits a long-lived charge separation by irradiating with visible light.4) In addition, one reported that eosin and xanthene dyes are photoredox catalysts as well.5)