Photooxidation

Photooxidation refers to oxidative processes driven by light, typically sunlight or artificial UV/visible radiation. It occurs when light-absorbing molecules, chromophores, reach excited electronic states and interact with molecular oxygen or other substrates, generating reactive oxygen species such as singlet oxygen, superoxide, and hydroxyl radicals. These species initiate chain reactions that convert substrates into oxidized products.

Two main pathways are: photosensitized oxidation, where an excited photosensitizer transfers energy or electrons to oxygen to form reactive oxygen species; and direct photolytic oxidation, where the substrate itself absorbs light and forms radicals that react with O2. In photosensitized oxidation, energy transfer from a triplet-state sensitizer to ground-state O2 yields singlet oxygen, which reacts with double bonds or allylic positions. In radical chain processes, substrate radicals react with O2 to form peroxyl radicals that propagate oxidation.

Photooxidation affects lipids, pigments, polymers, dyes, pharmaceuticals, and foods. In biology, it contributes to photooxidative stress, damaging lipids (lipid peroxidation), proteins, and DNA. In foods, it leads to rancidity and color loss; in polymers, it causes embrittlement, discoloration, and loss of mechanical properties.

Understanding photooxidation informs aging of materials, photodegradation of pollutants, and photoinitiated polymerization. Mitigation strategies include minimizing light exposure, incorporating UV absorbers and hindered amine light stabilizers, using antioxidants, and controlling oxygen levels. Packaging and storage conditions can greatly influence rate.