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Photomechanical bending of trans-4-(dimethylamino)azobenzene microcrystals was observed. Upon UV irradiation, the (001) face of the platelike microcrystals bent quickly in the direction opposite the light source, reaching the maximum deflection after 0.5 s. The microcrystal returned to its initial flat shape 30 s after the illumination was stopped. This mechanical motion was reversible over repeated cycles of UV irradiation. The bending effect was attributed to a gradient in the extent of UV-induced trans-cis photoisomerization as a function of light penetration, causing the expansion of the irradiated crystal surface along the b axis to result in a bent macrostructure.
Koshima et al. (Wed,) studied this question.