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 photo chromic reaction and a reversible single-crystal-to-single-crystal (SCSC) phase transition. A crystal of a diarylethene having octyl group at both sides (la) 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.