Polymorph-Derived Diversification of Crystal Actuation by Photoisomerization and the Photothermal Effect

Mechanically responsive materials
have been investigated extensively
over the past two decades. Diversification of actuation modes is essential
for the practical application of mechanical materials. Polymorphic
crystals with different crystal structures composed of the same compound
can exhibit distinct mechanical motions. Here, we focused on two polymorphs
of a salicylideneaniline derivative with a 4-fluoro substituent in
enol form, 1α and 1β, and investigated
their different photomechanical behaviors. Under ultraviolet (UV)
light irradiation, the thin plate-like 1α crystal
bent away gradually and strongly from the light source, with some
twist caused by enol–keto photoisomerization. In contrast,
the thin, needle-like 1β crystal did not bend by
photoisomerization; however, the thick 1β crystal
bent away quickly from the light source because of the photothermal
effect, ultimately achieving 500 Hz high-speed bending under pulsed
UV laser irradiation. Moreover, the thick plate-like 1α crystal exhibited two-step motion: fast bending forward by the photothermal
effect and then slow bending away by photoisomerization. We succeeded
in creating four motions using two polymorphic crystals and two distinct
mechanisms, thereby providing a novel approach to diversify the mechanical
motions of molecular crystals and expanding the potential and versatility
of molecular crystals as actuation materials.