Primary mechanisms of photoactivation of molecular oxygen. History of development and the modern status of research.

This review starts from a brief historical account devoted to the principles of the Bach-Engler peroxidation theory and experiments and ideas which led A. N. Bach to its creation. Then, the discovery of photodynamic action is described, which was shown to result from pigment photosensitized activation of molecular oxygen. The dramatic history of mechanistic studies of oxygen photoactivation is reviewed starting from the Bach-Engler peroxidation theory to the hypothesis of moloxide, discovery of singlet oxygen and free radicals and, then, to modern views on the primary photoactivation processes. The origin of widely used division of photodynamic processes into type I and type II and the relation of these processes to the nature of the primary photochemical reactions of photosensitizers are discussed. New definitions of these reactions are proposed on the basis of the mechanisms of oxygen photoactivation. Photographs of the scientists who greatly contributed to the development of this field of research are presented.