Polymorphic Crystallization and Thermodynamic Phase Transition between the Polymorphs of a Photochromic Diarylethene

We synthesized a photochromic diarylethene, 1,2-bis­(2-methyl-5-(4-hexyloxyphenyl)-3-thienyl)­perfluorocyclopentene (1a), and found that it has two types of crystals, a needle-like α-crystal and a plate-like β-crystal. From X-ray crystallographic analysis, the space groups of the α- and β-crystals were determined to be P1̅ and C2/c, respectively. The molecular conformation and packing of two crystal forms are quite different. The α- and β-crystals can be obtained individually by recrystallization from acetone at different temperatures. The solvent-mediated phase transition from the α-crystal to the β-crystal was found to occur in the acetone solution at room temperature. Moreover, the thermodynamic phase transition from α-form to β-form was found to take place above 88 °C, as confirmed by differential scanning calorimetry measurement, optical microscopic observation under crossed Nicols, and powder X-ray diffraction measurement. The phase transition from α-form to β-form was also observed at 78 °C by photochromic reaction of 1a, and the phase transition proceeded from the UV irradiated part to the nonirradiated part.