Mechanical Behavior of Molecular Crystals Induced by Combination of Photochromic Reaction and Reversible Single-Crystal-to-Single-Crystal Phase Transition

We herein report
a unique mechanical behavior of a molecular crystal
induced by combination of a photochromic reaction and a reversible
single-crystal-to-single-crystal (SCSC) phase transition. A crystal
of a diarylethene having octyl group at both sides (1a) was found to undergo a reversible thermodynamic SCSC phase transition
accompanying a change in crystal length, which was clarified by differential
scanning calorimetry measurement, X-ray crystallographic analysis,
and direct microscopic observation of the crystal length. Furthermore,
upon irradiation with ultraviolet light, the diarylethene crystal
exhibited an unusual photomechanical behavior. The mechanism of the
behavior was proposed based on photoisomerization of the diarylethene
from the open-ring isomer to the closed-ring isomer and a reversible
thermodynamic SCSC phase transition, which was well-supported by thermal
bending behavior of a photoirradiated crystal.