Photoredox Catalysis

Single-electron transfer is employed in the photoredox catalysis subfield of photochemistry. Transition-metal complexes, organic dyes, and semiconductors are the three main classes of materials used as photoredox catalysts. Soluble transition-metal complexes are more frequently used today, whereas organic photoredox catalysts dominated in the 1990s and the early 2000s. Sensitizers absorb light to create excited states that are redox-active. For many metal-based sensitizers, excitation is realised as a metal-to-ligand charge transfer, whereby an electron moves from the metal (for example, a d orbital) to an orbital localised on the ligands (for example, the * orbital of an aromatic ligand). Internal conversion, where energy is lost as vibrational energy rather than as electromagnetic radiation, causes the initial excited electronic state to relax to the lowest energy singlet excited state