Dimethyl-2(3-anthracen-9-yl)allylidene)malonate (DMAAM) is a divinylanthracene derivative that photoisomerizes between its (E) and (Z) conformations. Crystalline nanowires composed of this molecule undergo a rapid coiling motion when exposed to visible light. In this paper, a variety of experimental techniques are used to investigate the mechanism of this transformation, including powder X-ray diffraction, polarized light microscopy, ¹H NMR, and absorption spectroscopy. The results show that the presence of a surfactant like cetyltrimethylammonium bromide (CTAB) accelerates the photochemical reaction rate by at least a factor of 10 within the nanowire and is required to observe the photoinduced coiling. The accelerated reaction facilitates the transition to an amorphous phase composed of reactant and photoproduct, which leads to the rapid, large-scale shape changes that the nanowires undergo. Disruption of the highly packed crystal structure by photoisomerization also enhances the dissolution rate by a factor of about 30. The fact that the nanowires have a nominal diameter of 200 nm suggests that the presence of surface species can influence the reaction dynamics deep inside the crystal. These results show that the reaction dynamics and photomechanical motions of nanoscale molecular crystals can be extremely sensitive to surface species.

中文翻译：

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分子晶体纳米线中表面活性剂增强的光异构化和光机械响应

二甲基-2（3-蒽-9-基）亚烷基）丙二酸酯（DMAAM）是一种二乙烯基蒽并在其（E）和（Z）构象之间发生光异构化的衍生物。由该分子组成的结晶纳米线在暴露于可见光时会经历快速的盘绕运动。本文采用了多种实验技术来研究这种转变的机理，包括粉末X射线衍射，偏光显微镜，¹1 H NMR和吸收光谱。结果表明，表面活性剂（如十六烷基三甲基溴化铵（CTAB））的存在将纳米线内的光化学反应速率提高了至少10倍，是观察光诱导卷曲所必需的。加速的反应有助于过渡到由反应物和光产物组成的非晶相，从而导致纳米线经历快速，大规模的形状变化。通过光异构化破坏高度堆积的晶体结构也将溶解速率提高了约30倍。纳米线的标称直径为200 nm，这一事实表明表面物质的存在会影响晶体内部深处的反应动力学。