Sensitizers absorb light to give redox-active excited states.  For many metal-based sensitizers, excitation is realized as a metal-to-ligand charge transfer, whereby an electron moves from the metal (e.g., a d orbital) to an orbital localized on the ligands (e.g. the π* orbital of an aromatic ligand). The initial excited electronic state relaxes to the lowest energy singlet excited state through internal conversion, a process where energy is dissipated as vibrational energy rather than as electromagnetic radiation. This singlet excited state can relax further by two distinct processes: the catalyst may fluoresce, radiating a photon and returning to the singlet ground state, or it can move to the lowest energy triplet excited state (a state where two unpaired electrons have the same spin) by a second non-radiative process termed intersystem crossing.
Direct relaxation of the excited triplet to the ground state, termed phosphorescence, requires both emission of a photon and inversion of the spin of the excited electron. This pathway is slow because it is spin-forbidden so the triplet excited state has a substantial average lifetime. For the common photosensitizer, tris-(2,2’-bipyridyl)ruthenium (abbreviated as [Ru(bipy)] or [Ru(bpy)]), the lifetime of the triplet excited state is approximately 1100 ns. This lifetime is sufficient for other relaxation pathways (specifically, electron-transfer pathways) to occur before decay of the catalyst to its ground state.
The long-lived triplet excited state accessible by photoexcitation is both a more potent reducing agent and a more potent oxidizing agent than the ground state of the catalyst. Since sensitizer is coordinatively saturated, electron transfer must occur by an outer sphere process, where the electron tunnels between the catalyst and the substrate.