Polymorphism of 1,2-bis(2-methyl-5-p-methoxyphenyl-3-thienyl)perfluoro-cyclopentene and photochromic reactivity of the single crystals

Four polymorphic crystals were obtained by recrystallization of 1,2-bis(2-methyl-5-p-methoxyphenyl-3-thienyl)perfluorocyclopentene (1a) from hexane. All crystals underwent photochromic reactions upon alternate irradiation with ultraviolet (lambda = 370 nm) and visible light (lambda > 500 nm). The hotocyclization quantum yields were found to be close to unity irrespective of the crystal types, while the photocycloreversion quantum yields were different as much as four times depending on the conformation of the closed-ring isomers in the crystals.