This process starts upon an electron transfer event as rushed by the high-energy content of an electronically excited state (formed upon light absorption) localized on a component; a cascade of reduction/oxidation steps follows. As a consequence, the electron and the hole (the positive charge) are finally found at a certain distance each other, which amounts to yielding a relatively long-lived state available for further chemical usage. Within this scheme, the choice of the photoactive and electroactive components is of key relevance; among the range of available components, we use metal-transition complexes, porphyrins, fullerenes, aromatic amines, and so forth.