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.

Platelike microcrystals of N-3,5-di-tert-butylsalicylidene-3-nitroaniline repeatedly bend and straighten upon alternate irradiation with UV and visible light. The mechanism of bending was elucidated by X-ray crystallographic analyses before and after photoirradiation.

Plate-like microcrystals of a photochromic furylfulgide bend toward the light upon UV irradiation and then straighten upon visible light irradiation. The reversible bending was observed over 200 cycles of alternating irradiation with UV and visible light. The mechanism of bending could be explained by the X-ray crystallographic data.

The photomechanical bending of crystals of a stilbene-type compound substituted with anthracene and indanone groups, (E)-2-(9-anthrylmethylene)-1-indanone (trans-1), was investigated. When a narrow platelike microcrystal was irradiated with ultraviolet (UV) light at 365 nm, the crystal gradually bent away from the light source and finally reached a semicircular shape after more than 10 min. A larger rod-like crystal, approximately 10 mm in length, also exhibited a slight bending motion. The cessation of UV irradiation caused the bent crystals to return very slowly to their straight form. In the crystals, the anthracene planes of two neighbouring trans-1 molecules are arranged in a head-to-tail parallel manner, with a short planeto-plane distance of only 3.72 Å. The 1 H nuclear magnetic resonance spectra of trans-1 crystals before and after UV irradiation revealed the intermolecular [4 + 4] photodimerisation of the two anthracene planes, while the trans-to-cis photoisomerisation was not significant. The UV-vis absorption spectra of the trans-1 powder crystals, obtained using a diffuse reflectance spectrophotometer, showed a gradual increase in absorbance between 200 and 500 nm with increasing UV irradiation time, reaching a maximum after 1 h. Thermal back-monomerisation was very slow in the dark, not recovering the initial spectrum even after 25 days. The fluorescence spectra at 570 nm, derived from the anthracene excimer, decreased in intensity with increasing UV irradiation time due to a decrease in the amount of anthracene chromophore via photodimerisation. In situ X-ray measurements revealed that the bending of the crystals was caused by slight elongation of the b axis of the unit cell, corresponding to the long axis of the rod-like crystals. Calculations revealed that the observed crystal elongation could be explained by an optimised head-to-tail [4 + 4] orientation of the anthracene dimer.