Molecular machines based on the conversion of changes in molecular geometry to mechanical motion in macroscale materials are interesting from the perspective of basic research and offer a wide range of potential applications. Such machines are amenable to remote operation with external stimuli such a photon impingement and do not require direct contact. Azobenzenes are typical chromophores that undergo
trans-cis photoisomerization. We have recently found photomechanical bending of
trans-4-(dimethylamino)azobenzene microcrystals. Upon UV irradiation, the platelike microcrystal bent quickly in the direction opposite to 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 100 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 to result in a bent macrostructure.
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