PhotoChemical

Photochemical refers to chemical processes initiated by light. Photochemistry is the branch of chemistry that studies reactions and processes driven by light absorption. When a molecule absorbs a photon, an electron is promoted to an excited state. In this state, the molecule may return to the ground state by emitting light (fluorescence or phosphorescence) or by nonradiative decay, or it may undergo chemical transformations such as bond cleavage, bond formation, or isomerization. The outcome depends on the wavelength, intensity of light, the presence of photosensitizers, and the surrounding environment.

Photochemical reactions can proceed via direct photolysis, where the substrate itself absorbs light and reacts, or via photosensitized processes, where a sensitizer absorbs light and transfers energy or electrons to the substrate. Quantum yield expresses efficiency: the number of molecules transformed per absorbed photon. Reactions commonly observed include photoisomerization (changing configuration), photocycloadditions, and oxidative or reductive cleavages. Photocatalysis uses catalysts that are activated by light to drive transformations under milder conditions and with selective control.

Applications span organic synthesis, polymer chemistry (photopolymerization), and materials science, as well as environmental and atmospheric chemistry, where sunlight initiates the breakdown of pollutants and the formation or destruction of reactive species. In solar energy contexts, photochemical processes underpin certain dye-sensitized solar cells and light-driven water splitting schemes. Safety and design considerations include controlling light exposure, avoiding unwanted photodegradation, and ensuring that photosensitizers or reactive intermediates do not pose hazards.