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

Abstract

The photomechanical bending behavior of chiral crystals composed of S- and R-enantiomers of photochromic <i>N</i>-3,5-di-<i>tert</i>-butylsalicylidene-1-phenylethylamine in enol form [enol-(<i>S</i>)-<b>1</b> and enol-(<i>R</i>)-<b>1</b>] has been compared with that of achiral crystals of the racemic compound [enol-(<i>rac</i>)-<b>1</b>] 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-(<i>rac</i>)-<b>1</b> crystal could be repeated over 100 times, while the tip displacement angles of the chiral enol-(<i>S</i>)-<b>1</b> and enol-(<i>R</i>)-<b>1</b> 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 <i>a</i> and <i>b</i> 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-(<i>S</i>)-<b>1</b> crystal was 0.8 GPa, which is smaller than that of the enol-(<i>rac</i>)-<b>1</b> crystal (2.6 GPa). The crystal cantilevers made of enol-(<i>S</i>)-<b>1</b> and enol-(<i>rac</i>)-<b>1</b> lifted metal rings that were up to 50 and 300 times heavier, respectively, than the cantilevers upon UV irradiation. Overall, the achiral enol-(<i>rac</i>)-<b>1</b> crystal was found to be superior to the chiral enol-(<i>S</i>)-<b>1</b> crystal as a light-driven actuator