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Photoresponsive Diarylethene Molecular Crystals for Photoactuators
Osaka City University1
Photochromic compounds undergo a photochemically reversible transformation reaction between two isomers. Such molecules in organic crystals, which are regularly oriented and fixed in the crystal lattice, may be potentially useful for optoelectronic devices. To apply photochromic diarylethene crystals to functional materials in photonics, electronics, mechanics, and medical fields, the materials are required to change large physical property by photoirradiation. We have reported photoinduced crystal shape change of diarylethene crystals.1 In the photoinduced crystal shape changes, there are contraction/expansion, bending, twisting, and so on,2 which depend on the crystal structure and the size of the crystal. In this work, we have focused on the photoinduced crystal shape changes of diarylethene crystals depending on the crystal size, irradiation power, irradiation wavelength, irradiation direction with polarized UV light, and the direction of the incident light.
The rod-like crystals composed of the diarylethene derivatives showed the crystal bending away from the incident UV light or toward the incident UV light, depending on the molecular structure or the crystal packing. The bending velocity depended on the crystal thickness, and the curvature change against the crystal thickness was well fitted to Timoshenko’s bimetal model.3 When the irradiation power was changed, the bending velocity was proportional to the power of the UV irradiation, indicating that local strain acts cumulatively due to the structural changes in the individual diarylethene molecule at the initial stage of the photochromic reaction.4 Moreover, the bending behavior depended on the wavelength of the incident light.5 When the crystal was irradiated under polarized UV light, the bending speed depended on the polarization angles.6 A unique mechanical behavior of a molecular crystal was induced by a combination of a photochromic reaction and a reversible single-crystal-to-single-crystal phase transition.7 The photoinduced reversible crystal twisting of a diarylethene crystal was observed upon alternating irradiation with UV and visible light. The crystal twisting takes place in both a left-handed helix and a right-handed helix. The direction of the twisting depends on the face irradiated with UV light. The control of the photomechanical twisting of a diarylethene crystal was studied from the viewpoint of illumination direction.8 Changing the UV illumination direction for the crystal resulted in different twisting modes, ranging from helicoid to cylindrical. The control of photomechanical crystal deformation by illumination direction provides a convenient and useful way to generate a variety of photomechanical motions from a single crystal.
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2. Irie, M.; Fukaminato, T.; Matsuda, K.; Kobatake, S. Chem. Rev. 2014, 114, 12174−12277.
3. Kitagawa, D.; Kobatake, S. J. Phys. Chem. C 2013, 117, 20887−20892.
4. Hirano, A.; Hashimoto, T.; Kitagawa, D.; Kono, K.; Kobatake, S. Cryst. Growth Des. 2017, 17, 4819−4825.
5. Kitagawa, D.; Tanaka, R.; Kobatake, S. Phys. Chem. Chem. Phys. 2015, 17, 27300−27305.
6. Hirano, A.; Kitagawa, D.; Kobatake, S. CrystEngComm 2019, 21, 2495−2501.
7. Kitagawa, D.; Kawasaki, K.; Tanaka, R.; Kobatake, S. Chem. Mater. 2017, 29, 7524−7532.
8. Kitagawa, D.; Tsujioka, H.; Tong, F.; Dong, X.; Bardeen, C. J.; Kobatake, S. J. Am. Chem. Soc. 2018, 140, 4208−4212.