The development of actuators based on materials that reversibly change shape and/or size in response to external stimuli has attracted interest for some time. A particularly intriguing possibility is offered by light-responsive materials, which allow remote operation without the need for direct contact to the actuator. The photo-response of these materials is based on the photoisomerization of constituent molecules (typically trans-cis isomerization of azobenzene chromophores), which gives rise to molecular motions and thereby deforms the bulk material. This effect has been used to create light-deformable polymer films and gels, but the response of these systems is relatively slow. Here we report that molecular crystals based on diarylethene chromophores and with sizes ranging from 10 to 100 micrometres exhibit rapid and reversible macroscopic changes in shape and size induced by ultraviolet and visible light. We find that on exposure to ultraviolet light, a single crystal of 1,2-bis(2-ethyl-5-phenyl-3-thienyl)perfluorocyclopentene changes from a square shape to a lozenge shape, whereas a rectangular single crystal of 1,2-bis(5-methyl-2-phenyl-4-thiazolyl)perfluorocyclopentene contracts by about 5-7 per cent. The deformed crystals are thermally stable, and switch back to their original state on irradiation with visible light. We find that our crystals respond in about 25 microseconds (that is, about five orders of magnitude faster than the response time of the azobenzene-based polymer systems) and that they can move microscopic objects, making them promising materials for possible light-driven actuator applications.

Correlation between the photoinduced shape changes of diarylethene single crystals and their molecular packing in the crystals was studied. Crystals of 1,2-bis(5-ethyl-2-phenyl-4-thiazolyl)perfluorocyclopentene (3a) and 1,2-bis(2-isopropyl-5-phenyl-3-thienyl)perfluorocyclopentene (4a) showed similar photoinduced deformation from square to lozenge as that of 1,2-bis- (2-ethyl-5-phenyl-3-thienyl)perfluorocyclopentene (1a). Although these three diarylethenes have different electronic structures and exhibit different colours upon UV irradiation, the crystallographic structures and molecular packing of the crystals are very similar to each other. The result indicates that the deformation mode is determined by the packing mode of component molecules in the crystal. X-Ray crystallographic analysis of a micrometre-sized crystal 1a (20 x 15 x 8 microm) prepared by sublimation revealed that the small-size crystal, which shows photoinduced deformation, has the same crystal structure as that of the large bulk crystal.

The closed-ring isomer of 1,2-bis(5-methyl-2-phenylthiazol-4-yl)perfluorocyclopentene 1b was found to be extraordinarily thermally stable (t1/2 = 4.7 × 105 years at 30 °C), while the photochemical cycloreversion efficiency is similar to the dithienylethene closed-ring isomer of 1,2-bis(2-methyl-5-phenylthiophen-3-yl)perfluorocyclopentene 2b.

Single crystals composed of two or three different kinds of diarylethenes, having similar geometrical structures but different colors in the closed-ring isomers, 1,2-bis(2-methyl-5-phenyl-3-thienyl)perfluorocyclopentene (1a), 1,2-bis(5-methyl-2-phenyl- 4-thiazolyl)perfluorocyclopentene (2a), and 1,2-bis(5-methyl-2-phenyl-4-oxazolyl)perfluorocyclopentene (3a) have been prepared in an attempt to form single crystals which exhibit different colors, depending on illumination wavelengths. When 1a and 2a are mixed in ethanol and the content of 2a in the feed is higher than that of 1a, only a needle-shaped crystal is obtained. The crystal shape is similar to that of 2a. On the other hand, high content of 1a in the feed leads to formation of a block-shaped crystal, of which the shape is similar to that of 1a. At a feed ratio of 1:1 for 1a and 2a, two types of crystals such as needle- and block-shapes are obtained from the same batch. Mixed crystals composed of three kinds of derivatives 1a/2a/3a have also been prepared. A crystal composed of the three derivatives in the molar ratio of 32 (1a):53 (2a):15 (3a) was obtained. X-ray analysis proved that 1a and 3a are packed in the crystal lattice of 2a. The mixed crystal 1a/2a/3a changed color from colorless to yellow, red, and blue upon irradiation with light of appropriate wavelengths.