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.