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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.
Koshima et al. (Fri,) studied this question.