Design and Properties of Porphyrin-based Singlet Oxygen Generator

In this review, we address recent studies of porphyrin-based molecular systems for efficient singlet oxygen (O-1(2)) generation. Porphyrins have a highly conjugated, delocalized 18-pi electron system (for the shortest cyclic path) that is responsible for the strong absorption and emission characteristics in the visible region. Singlet oxygen is useful for applications in photodynamic cancer therapy (PDT), photooxidation of toxic molecules, and photoproduction of important intermediates for various chemicals owing to its high oxidation ability. Porphyrin and its analogues have been investigated as photosensitizers for O-1(2) generation. The production of O-1(2) is successfully regulated by photophysical parameters, such as intersystem crossing, triplet state (T-1) lifetime, and photosensitizer to O-3(2) energy transfer in molecular systems. Introduction of substituents with heavy atoms (halogen, metal) and radicals onto the photosensitizer (PS) exerts a strongly positive impact on intersystem crossing of the singlet excited state of the PS, promoting generation of the triplet excited state. Environmental conditions, such as solvent and pH, may also influence O-1(2) generation. As a means to limit cellular photodamage, two-photon absorption and DNA switches for O-1(2) generation have been proposed and developed. Achieving control of singlet oxygen generation is pertinent to many arenas at the cutting edge of modern science, ranging from the chemical industry to biomedical applications.