Photochromism of 1,2-Bis(2,5-dimethyl-3-thienyl)perfluoro- cyclopentene in a Single Crystalline Phase

1,2-Bis(2,5-dimethyl-3-thienyl)perfluorocyclopentene (1a) 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 1b. The red color was bleached by irradiation
with visible light (λ > 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 1b estimated from the absorption anisotropy at 535 nm agreed with the long axis of 1a
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 1a
and 1b, both of which were determined by X-ray analysis.