Solid Crystal Network of Self-Assembled Cyclodextrin and Nonionic Surfactant Pseudorotaxanes

The title system allows the straightforward formation of three-dimensional crystals of self-assembled pseudorotaxanes formed by the nonionic surfactant Igepal CO-520 and β-cyclodextrin (β-CD) in aqueous solution. The work involves a combination of X-ray powder diffraction, high resolution electron transmission microscopy, and 13C CP/MAS NMR studies of the solid crystal, supported by single crystal structural analysis. The results indicate a lamellar self-assembly of pseudorotaxanes with preferential orientation and disorder in the structure. For the single crystal, the unit cell was found to be triclinic (P1) and contains a β-CD dimer. The surfactant molecules are located in the channel formed by these dimers along the c axis of the crystal network. The individual pseudorotaxane structure is formed by a dimer of β-CDs threaded by the oxyethylene hydrophilic segment of Igepal CO-520, and a β-CD dimer that binds the hydrophobic region of the surfactant. Thus, as in a CD polyrotaxane structure, this system results in an ordered self-assembly of pseudorotaxanes through the formation of a network of hydrogen bonds between head-to-head β-CD dimers. Moreover, the analysis of the 1H NMR spectra in solutions of pseudorotaxanes formed by β-CD and Igepals with different lengths of the hydrophilic tails indicates equal stoichiometry patterns of both oxyethyelene and hydrophobic regions for the different supramolecules. Whereas the common hydrophobic moiety threads two macrocycles, the ratio between complexed oxyehtlyene segments and β-CD is 2.5 for the hydrophilic tails. All these results show that nonionic surfactants can be used as alternative and effective linear threads to polymers and copolymers in the synthesis of supramolecular polyrotaxane solid crystals with CDs.

本研究所涉体系可在水溶液中，由非离子表面活性剂伊格帕尔CO-520（Igepal CO-520）与β-环糊精(β-cyclodextrin, β-CD)自组装形成准轮烷(pseudorotaxane)三维晶体，且制备过程简便直接。本研究结合X射线粉末衍射(X-ray powder diffraction)、高分辨透射电子显微镜(high resolution electron transmission microscopy)与固体晶体的13C交叉极化/魔角旋转核磁共振(13C CP/MAS NMR)表征，并辅以单晶结构分析。结果表明，准轮烷呈层状自组装结构，在晶体中表现出优先取向与无序性。针对该单晶，其晶胞为三斜晶系(P1空间群)，包含一对β-环糊精二聚体。表面活性剂分子沿晶体网络的c轴嵌入由该二聚体形成的通道中。单个准轮烷结构由两部分构成：一是被伊格帕尔CO-520的氧乙烯亲水链段穿入的β-环糊精二聚体，二是结合表面活性剂疏水区域的β-环糊精二聚体。因此，与环糊精基聚轮烷(polyrotaxane)结构类似，本体系通过头对头排布的β-环糊精二聚体间形成氢键网络，实现了准轮烷的有序自组装。此外，对由β-环糊精与不同亲水尾链长度的伊格帕尔表面活性剂形成的准轮烷溶液的1H核磁共振谱分析显示，不同超分子(supramolecular)结构的氧乙烯链段与疏水区域均遵循相同的化学计量模式。尽管常规疏水基团可穿入两个大环主体，但络合的氧乙烯链段与β-环糊精的比例为2.5，针对亲水尾链而言。综上，本研究结果表明，非离子表面活性剂可作为聚合物与共聚物的替代有效线性穿链剂，用于合成含环糊精的超分子聚轮烷固体晶体。