Movie from the Supplementary Information of the paper "A Diarylethene Cocrystal that Converts Light into Mechanical Work," by Masakazu Morimoto and Masahiro Irie [Department of Chemistry and Research Center for Smart Molecules, Rikkyo University, Tokyo Japan], J. Am. Chem. Soc., 2010, 132 (40), pp 14172--14178. http://dx.doi.org/10.1021/ja105356w [ http://pubs.acs.org/doi/suppl/10.1021/ja105356w ].
The photomechancial effect of a rectangular plate two-component cocrystal composed of a photochromic diarylethene derivative, 1,2-bis(2-methyl-5-(1-naphthyl)-3-thienyl)perfluorocyclopentene (1o), and perfluoronaphthalene (FN) has been examined. The crystal of 1o·FN with the size of 1−5 mm in length exhibits reversible bending motion upon alternate irradiation with ultraviolet (UV) and visible light. The reversible bending could be repeated over 250 times. In situ X-ray crystallographic analysis revealed that the deformation of the crystal is due to the elongation of the b-axis of the unit cell, which corresponds to the long axis of the plate crystal, induced by the shape change of component diarylethene molecules upon photocyclization. The bending motion was observed even at 4.7 K, and dynamic measurement of the bending proved that the anisotropic expansion of the crystal takes place in the microsecond time scale at the low temperature. Molecular crystal cantilevers made of 1o·FN can lift metal balls, the weight of which is 200−600 times heavier than the weight of the crystal, upon UV irradiation. The maximum stress generated by UV irradiation was estimated to be 44 MPa, which is 100 times larger than that of muscles (0.3 MPa) and comparable to that of piezoelectric crystals, such as lead zirconate titanate (PZT) (50 MPa).
The movie shows that the photoinduced bending of a crystal upon irradiation with UV light, the crystal turned blue and bent, moving away from the direction of the incident light. After switching off the illumination, the crystal ceases the bending and keeps the bent shape over one hour in the dark at the low temperature. This result indicates that the bending is not due to surface heating by the incident light but due to photochemical reaction of diarylethene molecules in the crystal. Upon irradiation with visible light, the blue color of the crystal slowly disappears and the bent crystal returns to the original straight shape.
The movie also shows the weight lifting of a ball by the crystal. The crystal and the ball weigh 0.17 mg and 46.77 mg, respectively. The weight of the lead ball is 275 times heavier than that of the crystal. Upon irradiation with UV light from below, the heavy ball was lifted as high as 0.95 mm. Also the weight lifting of a 3-mm steel ball (110.45 mg) was loaded onto a crystal plate (0.18 mg), and the crystal was irradiated with UV light. The weight of the ball is 614 times heavier than that of the crystal. Even such a heavy steel ball is lifted upon irradiation with UV light. The former example shows that the cantilever arm performs lifting work and the amount of the work is as large as 0.43 μJ. The latter example indicates that the force which is generated by UV irradiation is larger than 1.1 mN. The crystals, which weigh less than 0.2 mg, can generate a strong force and perform large mechanical work.
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