Photochromism of 1,2-bis(2,5-dimethyl-3-thienyl)perfluorocyclopentene in a single crystalline phase

1,3-Bis(2,5-dimethyl-3-thienyl)perfluorocyclopentene (la) was found to undergo photochromism in a single crystalline phase. Upon irradiation with ultraviolet light the colorless crystal turned red with keeping the crystal shape. The red color is due to the closed-ring form Ib, The red color was bleached by irradiation with visible light (lambda > 450 nm). The intensity of the red color observed under polarized light dramatically changed by rotating the crystal sample, and the order parameter was as high as 0.84. The high order parameter confirmed that the molecules undergo photochromism in the crystalline lattice. The direction of the electronic transition moment of Ib estimated from the absorption anisotropy at 535 nln agreed with the long axis of la packed in the crystal, which was determined by X-ray crystallographic analysis. The movement of each atom of the molecule in the crystal during the cyclization reaction was estimated by comparing the structures of la and Ib, both of which were determined by X-ray analysis.