Light-Driven Bending Crystals of Salicylidenephenylethylamines in Enantiomeric and Racemate Forms

The
photomechanical bending behavior of chiral crystals composed
of S- and R-enantiomers of photochromic N-3,5-di-tert-butylsalicylidene-1-phenylethylamine in enol form [enol-(S)-1 and enol-(R)-1] has been compared with that of achiral crystals of the racemic
compound [enol-(rac)-1] of S- and R-enantiomers.
Both platelike chiral and achiral crystals, a few millimeters in length,
exhibited similar reversible bending upon alternate irradiation with
ultraviolet (UV) and visible light. The reversible bending of the
achiral enol-(rac)-1 crystal could be
repeated over 100 times, while the tip displacement angles of the
chiral enol-(S)-1 and enol-(R)-1 crystals gradually decreased with repeated
cycles. In situ X-ray measurements revealed that the bending motion
of the chiral and achiral crystals was caused by a slight shrinkage
of the a and b axes of the unit
cell, respectively, corresponding to the long axis of the platelike
crystals; shrinkage was induced by the shape change of component salicylidenephenylethylamine
molecules upon photoinduced proton transfer. The Young’s modulus
of the enol-(S)-1 crystal was 0.8 GPa,
which is smaller than that of the enol-(rac)-1 crystal (2.6 GPa). The crystal cantilevers made of enol-(S)-1 and enol-(rac)-1 lifted metal rings that were up to 50 and 300 times heavier,
respectively, than the cantilevers upon UV irradiation. Overall, the
achiral enol-(rac)-1 crystal was found
to be superior to the chiral enol-(S)-1 crystal as a light-driven actuator.