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

Abstract

We synthesized a photochromic diarylethene, 1,2-bis­(2-methyl-5-(4-hexyloxyphenyl)-3-thienyl)­perfluorocyclopentene (<b>1a</b>), 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 <i>P</i>1̅ and <i>C</i>2/<i>c</i>, 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 <b>1a</b>, and the phase transition proceeded from the UV irradiated part to the nonirradiated part