Photochemically

Photochemically is an adverb used to describe a process that occurs by means of photochemistry, i.e., chemical reactions initiated or driven by light. In photochemical processes, molecules absorb photons, promoting electrons to excited states. Depending on the system, this can lead to bond cleavage, bond formation, isomerization, electron transfer, or radical generation. The outcome and rate depend on the light’s wavelength and intensity, the absorbing properties of the reactants, and the presence of sensitizers or catalysts.

Common examples and applications include historical and modern photography (exposure of light-sensitive emulsions), atmospheric chemistry (photolysis of pollutants and ozone formation), polymer chemistry (UV curing and photopolymerization), and semiconductor fabrication (photolithography). In energy research, photochemical steps underpin artificial photosynthesis and dye-sensitized solar cells, where light absorption drives charge separation and chemical transformations.

Key concepts in photochemical processes include quantum yield, which measures efficiency of photon use; action spectrum, which maps effectiveness across wavelengths; and the roles of excited singlet and triplet states, intersystem crossing, and radical intermediates. Photochemical reactions can be initiated by natural light or artificial light sources and are often influenced by oxygen, solvents, and temperature.

Photochemical versus photophysical or thermal processes: photochemical reactions require absorption of light to alter the chemical pathway, whereas purely thermal or photophysical processes may occur without chemical bond changes. The term photochemically describes the manner of action in a given reaction, as in "the compound degrades photochemically under sunlight." Etymology derives from Greek photos (light) and chemia (alchemy, chemistry).