ROLE OF EXCIPLEX AND ION-PAIR IN THE PHOTOSENSITIZED OXYGENATION OF 1,4-DIPHENYL-1,3-BUTADIENE

9,10‐Dicyanoanthracene (DCA) sensitized the photooxygenation of 1,4‐diphenyl‐1,3‐butadiene(1a) and its 4,4′‐dimethoxy derivative (1b) to afford the corresponding endoperoxides 2 and the other oxidized products such as aldehydes and epoxides. The mechanism of the DCA‐sensitized photooxygenation of 1 was diversified by solvent polarity. In non‐polar solvents the reaction involves an exciplex intermediate, which leads to formation of triplet DCA (3DCA*) with an efficiency of 0·64 in the case of 1a. The resulting 3DCA* acts as a sensitizer for the generation of singlet oxygen (1O2) and thus 1O2 reaction occurs exclusively to give 2. On the other hand, single electron‐transfer quenching of 1DCA* by 1 is feasible in polar acetonitrile and a primary ion pair can be formed. Competing with fast back electron‐transfer deactivation, the primary ion pair produces free ions in limited yield to furnish electron transfer oxygenation. In the case of the DCA–1a system, free ions were produced with an efficiency of ca 4%. Thus, in acetonitrile, electron‐transfer oxygenation partly took place to give the other oxidized products, whereas the 1O2 pathway was still valid and responsible for the formation of 2. The effect of solvent polarity was apparently less pronounced in the case of 1b because the reactivity of 1b toward 1O2 is about five times higher than tht of 1a. For such 1O2‐reactive substrates, the electron‐transfer pathway would become dominant only when the substrate concentration is impractically high. Copyright © 1990 John Wiley & Sons Ltd.