Bending Crystals By Light

Changes in azobenzene's molecular geometry can be translated into mechanical motion in a crystal, according to chemists in Japan. Hideko Koshima, Naoko Ojima, and Hidetaka Uchimoto of Ehime University show that platelike microcrystals of trans-4-(dimethylamino)azobenzene move in response to ultraviolet light (J. Am. Chem. Soc., DOI: 10.1021/ja8098596). When irradiated at 365 nm, the microcrystals bend away from the light, reaching a maximum deflection angle of 180° in half a second. The crystals return to their original shape 30 seconds after the light is shut off. The researchers attribute the motion to the trans-cis isomerization of the azobenzene as shown: UV light prompts some of the molecules to isomerize to the cis form, which bends the microcrystal. Thermal isomerization back to the trans form restores the crystal's original shape. "To our knowledge, trans-cis photoisomerization of azobenzene chromophores has not been observed in the crystalline state because of the large geometric changes that would be required in such densely packed crystal lattices," the researchers write. Nevertheless, they confirmed this photoisomerization near the surface of the microcrystals by atomic force microscopy. The light-induced motion could be harnessed for molecular machines, the Japanese team suggests.

Rapid bending of the platelike microcrystals (about 525 µm long) of trans-4-(dimethylamino)azobenzene takes place when the sample is irradiated with 365 nm light. The microcrystals whip forward in the direction opposite to the light source. After 30 seconds, the crystals return to the initial flat shape.
J. Am. Chem. Soc.