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