The role of intersystem crossing steps in singlet oxygen chemistry and photo-oxidations

Singlet oxygen chemistry and photo-oxidation reactions, in general, often require one or more critical reaction steps that involve an intersystem crossing from a singlet state to a triplet state or vice versa. This paper considers two important intersystem crossing mechanisms, electron spin-electron orbit (spin-orbit) coupling and electron spin-nuclear spin (spin-spin) coupling, and how they may be involved : (1) in the deactivation of ¹O₂ to ³O₂ ; (2) in the thermal catalytic conversion of ³O₂ to ¹O₂; and (3) in the fragmentation of aromatic endoperoxides to yield O₂ and an aromatic substrate.