photoreactivity

Photoreactivity is the propensity of a chemical species to undergo chemical transformation upon absorption of light. When a molecule absorbs a photon, it is promoted from the ground state to an excited electronic state, which can open reaction pathways that are inaccessible at thermal energies. Photochemical reactions include bond cleavage, isomerization, cycloadditions, electron transfer, and catalyzed processes by a photosensitizer. The outcomes depend on the wavelength and intensity of light, the molecule’s absorption spectrum, and the surrounding medium. The efficiency of a photoreaction is often described by a quantum yield, the fraction of absorbed photons that produce the chemical event.

Mechanisms and factors include the role of photosensitizers that transfer energy or electrons; excited states may undergo intersystem crossing to triplet states; reactions can be concerted or stepwise. Oxygen can quench excited states or participate to form reactive oxygen species. Solvent polarity, temperature, and concentration influence rates and pathways. Photoreactivity is studied using spectroscopic monitoring and quantum yield measurements.

Applications and safety: Photoreactivity underlies photochemistry, polymerization, photocatalysis, and solar energy conversion. It enables photochemical synthesis, patterning, and photodynamic therapy. In materials science, photoresponsive or photoswitchable systems change properties under light. Risks include unwanted degradation of dyes, plastics, and biological tissues, and the formation of reactive species. Researchers control outcomes by selecting light wavelengths and intensities, employing sensitizers, and tuning environmental conditions.