photoreactions

Photoreactions are chemical reactions driven by the absorption of light. In a photoreaction, a molecule absorbs a photon, promoting electrons to excited electronic states. The absorbed energy provides a pathway that may be inaccessible thermally, allowing bond breaking, bond forming, or rearrangements that would not occur at room temperature.

Upon absorption, molecules reach excited states, typically singlet or triplet. From these states, reactions can proceed via energy transfer, electron transfer, or direct bond-making and breaking. The outcome depends on the excited-state lifetime, the reactants, and the surrounding environment, including oxygen and solvent.

Direct photochemistry occurs when the reacting species absorbs light directly; sensitized photochemistry uses a photosensitizer that harvests light and transfers energy or electrons to the substrate, enabling photoreactions that the substrate would not undergo efficiently on its own.

Common photoreactions include cis–trans isomerization (as in stilbene), [2+2] photocycloadditions, and Norrish reactions of excited carbonyls (type I fragmentation and type II hydrogen abstraction). Photocatalytic and photoredox processes, using metal complexes or organic dyes, enable a wide range of transformations under light.

Biological photoreactions, such as the formation of thymine dimers in DNA under ultraviolet irradiation, illustrate the role of light in chemistry and biology. In materials science, photochromic compounds, spiropyrans, and azobenzenes exhibit light-induced switching.

Applications include synthetic photochemistry, polymerization and curing, photolithography, and solar-energy conversion. Important considerations are light wavelength and intensity, quantum yield, photostationary states, oxygen sensitivity, and solvent effects.