One area of photochemistry that makes advantage of single-electron transfer is photo redox catalysis. Transition-metal complexes, organic dyes, and semiconductors are the three main categories of materials used as photo redox catalysts. Today, soluble transition-metal complexes are increasingly widely utilised, whereas in the 1990s and the early 2000s, organic photo redox catalysts dominated. Sensitizers absorb light to produce excited states that are redox-active. Many metal-based sensitizers achieve excitation by a metal-to-ligand charge transfer, in which an electron is transferred from an orbital localised on the metal (such as a d orbital) to an orbital localised on the ligands (such as the * orbital of an aromatic ligand). Internal conversion, where energy is lost as vibrational energy rather than as electromagnetic radiation, causes the initially excited electronic state to relax to the lowest energy singlet excited state.
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