photosensitized

Photosensitized describes a process in which a substance becomes chemically reactive after absorbing light in the presence of a photosensitizer. In such systems, the photosensitizer absorbs photons and reaches an excited state, from which it can transfer energy or electrons to other molecules, enabling reactions that may not occur in the ground state of the substrate.

There are two common pathways. Type I involves electron or hydrogen transfer from the excited photosensitizer to surrounding molecules, generating reactive radicals that can initiate oxidation or fragmentation. Type II involves energy transfer to ground-state molecular oxygen, producing singlet oxygen, a highly reactive form of oxygen that can oxidize a wide range of substrates. Both pathways can lead to chemical transformations such as polymerization, degradation, or crosslinking, depending on the system.

Photosensitizers include natural pigments such as chlorophyll and porphyrins, as well as synthetic dyes like methylene blue and Rose Bengal. In medicine, photosensitizers are used in photodynamic therapy and antimicrobial photodynamic therapy, where light activation generates reactive oxygen species to damage targeted cells or microbes. In environmental and materials science, photosensitized processes drive the degradation of pollutants, the modification of polymers, and the formation of radical species for various synthetic applications.

Key considerations for photosensitized systems are the light wavelength and intensity, the quantum yield for generating reactive species, oxygen availability, and the localization of the photosensitizer. After certain photosensitized treatments, retained sensitivity to light can pose safety concerns, requiring protective measures to avoid unintended tissue damage.