Taniguchi Takuya, Kubota Ayumi, Moritoki Tatsuya, Asahi Toru, Koshima Hideko
Department of Advanced Science and Engineering, Graduate School of Advanced Science and Engineering, Waseda University 3-4-1 Okubo, Shinjuku-ku Tokyo 169-8555 Japan.
Department of Life Science and Medical Bioscience, School of Advanced Science and Engineering, Waseda University 3-4-1 Okubo, Shinjuku-ku Tokyo 169-8555 Japan.
RSC Adv. 2018 Oct 5;8(60):34314-34320. doi: 10.1039/c8ra06639f. eCollection 2018 Oct 4.
Photomechanical crystals are interesting from both basic and applied perspectives, and thus it is important to develop new examples. We investigated the photomechanical bending behaviour of a photochromic crystal of a dibenzobarrelene derivative. When a plate-like crystal was irradiated with ultraviolet (UV) light at 365 nm, two-step bending was observed. In the first step, the crystal quickly bent away from the light source, with an accompanying crystal colour change from colourless to purple. In the second step, under prolonged UV light, the bending returned slowly and then the crystal bent up towards the opposite direction, accompanied by an additional colour change to light yellow. Spectroscopic measurements and X-ray crystallographic analysis suggested that a long-lived biradical species is generated immediately upon UV light irradiation a Norrish type II intramolecular hydrogen abstraction, and then the final photoproducts are formed under continuous UV exposure. X-ray crystallographic analysis before and after UV light irradiation for a few seconds revealed that the longitudinal axis ( axis) of the crystal elongated slightly after irradiation, which is consistent with the direction of the first-step bending. Based on these results, we propose that first-step bending could be induced by a biradical species, generated a Norrish type II intramolecular hydrogen abstraction, and the second-step bending could originate from the formation of a mixture of final photoproducts under prolonged light irradiation.
光机械晶体无论从基础还是应用的角度来看都很有趣，因此开发新的实例很重要。我们研究了二苯并桶烯衍生物的光致变色晶体的光机械弯曲行为。当用365nm的紫外光照射板状晶体时，观察到两步弯曲。第一步，晶体迅速向远离光源的方向弯曲，同时晶体颜色从无色变为紫色。第二步，在长时间紫外光照射下，弯曲缓慢恢复，然后晶体向相反方向向上弯曲，同时伴随颜色额外变为浅黄色。光谱测量和X射线晶体学分析表明，在紫外光照射时立即产生一种长寿命的双自由基物种——Norrish II型分子内氢抽取，然后在连续紫外光照射下形成最终光产物。对紫外光照射几秒钟前后进行的X射线晶体学分析表明，照射后晶体的纵轴（轴）略有伸长，这与第一步弯曲的方向一致。基于这些结果，我们提出第一步弯曲可能由通过Norrish II型分子内氢抽取产生的双自由基物种诱导，第二步弯曲可能源于长时间光照下最终光产物混合物的形成。
