The effects of 3,5-substitutions on the photochromism and photoisomerization of some 2,3,4,4,5,6-hexasubstituted 4H-thiopyrans

Abstract

Some new derivatives of 4-aryl- and 4-methyl-2,3,4,5,6-pentaphenyl-4H-thiopyrans ( 6a–6d) and 4-aryl- and 4-methyl-2,4,6-triphenyl3,5-dimethyl-4H-thiopyrans ( 7a–7d) were synthesized, and their photochromism and photoisomerization reactions were compared with those of 2,4,4,6-tetraphenyl-4H-thiopyran ( 1a) (Ar 1-Ar 2-Ph) and 2,4,6-triphenyl-4-methyl-4H-thiopyran ( 2a) (R - Me) model compounds having no substituents at the 3,5-positions. The results show that, unlike the photochromic model compound 1, the 3,5-diphenyl-substituted 4H-thiopyrans 6a–6c, on exposure to sunlight or UV radiation in the solid state, do not exhibit any photochromic properties, whereas photochromic behaviour is retained in the 3,5-dimethyl-substituted 4H-thiopyran analogues 7a–7c with a blue shift in the absorption in the visible region. Moreover, on irradiation of 6a–6c with UV light in methanol or benzene solution, no photoisomerization to isomeric 2H-thiopyrans is detected, where as the 3,5-dimethyl-substituted 4H-thiopyrans 7a–7d, as well as the 3,5-diphenyl analogue 6d, give the isomeric 2H-thiopyrans 10, 11 and 9d respectively with relative yields lower than the corresponding model compounds 1a and 2a.