The colored isomer produced in 1,2-bis(2,5-dimethyl-3-thienyl)perfluorocyclopentene crystal upon irradiation with ultraviolet light showed an absorption maximum at 535 nm, which is 30 nm longer than that in hexane, 505 nm. The spectral shift is ascribed to the strained structure of the photogenerated closed-ring isomer in the crystal.