Thermodynamic Phase Transition Through Crystal-to-Crystal Process of Photochromic 1,2-Bis(5-phenyl-2-propyl-3-thienyl)perfluorocyclopentene

A photochromic diarylethene, 1,2-bis(5-phenyl-2-propyl-3-thienyl)perfluorocyclopentene (1a), was found to have two polymorphic crystal forms, - and -crystals. From X-ray crystallographic analysis, the space groups of - and -crystals were determined to be P2(1)/c and C2/c, respectively. The difference between two crystal forms is ascribed to the orientation of two of four molecules in the unit cell. The thermodynamic phase transition from - to -forms occurred via a crystal-to-crystal process, as confirmed by differential scanning calorimetry measurements, optical microscopic observations in the reflection mode and under crossed Nicols, and powder X-ray diffraction measurements. The movement of the molecules in the crystal was evaluated by analyzing the change of face indices before and after the phase transition.