Chapter 5: Photosensitization

Photosensitization is the most common process producing singlet oxygen. The photosensitized production of singlet oxygen is important both in nature and in synthetic chemistry. Some natural products, such as cercosporin and hypericin, are potent singlet-oxygen-generating photosensitizers. Many singlet oxygen-generating photosensitizers have also been synthesized in the laboratory and form the basis of several practical uses for singlet oxygen. These uses include synthesis of fine chemicals, photoactivated herbicides, photoactivated insecticides, photodynamic therapy of cancer, photosterilization of blood products and wastewater treatment. Much is now known about the primary photochemical reactions generating singlet oxygen. Singlet oxygen is most commonly produced by oxygen quenching of a photosensitizer in an excited triplet state. Less commonly, singlet oxygen is produced by quenching of a photosensitizer in an excited singlet state. Photosensitized singlet-oxygen production always competes with other photochemical pathways. The novel electronic structure of the oxygen molecule and the high concentration of oxygen in the environment often favor singlet-oxygen generation over other pathways.