photocycle

A photocycle is a sequence of light-induced chemical and physical transformations within a molecule or molecular system, progressing from an initial state through one or more photogenerated intermediates and returning to the original state or to a new stable state. In many cases the cycle is initiated by absorption of photons that triggers a reaction such as isomerization or electron transfer, followed by subsequent steps that proceed through metastable intermediates before thermal relaxation or additional photochemical events complete the cycle.

Mechanistically, a photon excites a chromophore to a higher electronic state, often inducing bond rearrangements or charge movements. This creates intermediate species with distinct structures and spectroscopic signatures. The system may involve proteins, cofactors, or a solid-state matrix that stabilizes these intermediates. Completion of the cycle typically requires either thermal relaxation back to the starting configuration or further photochemical steps that regenerate the initial state or an equivalent photoactive state.

Examples occur in biology and materials science. In vision, rhodopsin contains 11-cis retinal; photon absorption converts it to all-trans retinal, triggering a signaling cascade. The photocycle passes through several photointermediates and ends with regeneration of 11-cis retinal in the retinal pigment epithelium, enabling repeated light sensing. In microbial rhodopsins such as bacteriorhodopsin, a light-driven cycle involving K, L, M, N, and O intermediates drives proton pumping across membranes. Similar cycles are exploited in photoswitchable proteins and synthetic photoresponsive systems for optical control of structure, reactivity, or signaling.