Photochemistry of Polyenes. IX. Excitation, Relaxation, and Deactivation of Dienes, Trienes, and Higher Polyenes in the Vitamin A Series in the Sensitized Isomerization Reaction

In a detailed study of photosensitized isomerization of dienes, trienes, and higher polyenes in the vitamin A series it was found that dienes and trienes behave photochemically differently from the higher members of the series. Not only are the sterically hindered 7-cis isomers present in the photostationary states of the lower members but also under selective sensitization conversion to such isomers becomes quantitative, while for the longer polyenes 7-cis isomers are absent under all conditions of sensitized irradiation. This different photochemical behavior is explained in terms of the expected different structures of relaxed triplets, i.e., perpendicular ones in dienes and trienes and planar ones in higher polyenes. Direct irradiation was also reexamined with emphasis on the nature of the spin states where chemical reactions originate.