Solid-state photochemical and photomechanical properties of molecular crystal nanorods composed of anthracene ester derivatives
作者:Lingyan Zhu、Arun Agarwal、Jinfeng Lai、Rabih O. Al-Kaysi、Fook S. Tham、Tarek Ghaddar、Leonard Mueller、Christopher J. Bardeen
DOI:10.1039/c1jm10228a
日期:——
A series of 9-anthroate esters that can form photoresponsive molecular crystal nanorods is prepared and their properties are investigated. All crystal structures that can support a [4 + 4] photodimerization reaction lead to nanorods that undergo photomechanical deformations without fragmentation. In order to determine the molecular-level motions that give rise to the nanorod photomechanical response, the reaction of anthracene-9-carboxylic acid tert-butyl ester is studied in detail using X-ray diffraction and solid-state NMR techniques. The monomer crystal is well-aligned within the nanorod and reacts to form the photodimer crystal according to first-order kinetics. The solid-state reacted dimer crystal is a metastable intermediate that slowly converts into the low energy dimer crystal structure over the course of weeks. Based on single crystal X-ray diffraction studies and solid-state NMR data, this intermediate structure is likely composed of the [4 + 4] photodimer that has not yet undergone the ester group rotations and repacking is necessary to form the lower energy crystal polymorph that is produced directly by crystallization from solution. Our results show that the photomechanical response of these molecular crystal nanostructures is determined by nonequilibrium intermediate states and cannot be predicted based solely on knowledge of the equilibrium reactant and product crystal structures.
制备了一系列可形成光响应分子晶体纳米棒的9-蒽甲酸酯并研究了其性能。所有可以支持 [4 + 4] 光二聚反应的晶体结构都会导致纳米棒经历光机械变形而不会碎裂。为了确定引起纳米棒光机械响应的分子级运动,使用 X 射线衍射和固态 NMR 技术详细研究了蒽-9-甲酸叔丁酯的反应。单体晶体在纳米棒内排列良好,并根据一级动力学反应形成光二聚体晶体。固态反应二聚体晶体是一种亚稳态中间体,可在数周内缓慢转化为低能二聚体晶体结构。基于单晶 X 射线衍射研究和固态 NMR 数据,这种中间结构可能由尚未经历酯基旋转的 [4 + 4] 光二聚体组成,并且需要重新包装以形成较低能量的晶体多晶型物它是通过从溶液中结晶直接产生的。我们的结果表明,这些分子晶体纳米结构的光机械响应是由非平衡中间态决定的,不能仅根据平衡反应物和产物晶体结构的知识来预测。