Laboratory of Macromolecular and Organic Chemistry and Institute for Complex Molecular Systems (ICMS), Eindhoven University of Technology , P.O. Box 513, 5600 MB Eindhoven, The Netherlands.
J Am Chem Soc. 2017 Oct 4;139(39):13867-13875. doi: 10.1021/jacs.7b07639. Epub 2017 Sep 26.
Control over the helical organization of synthetic supramolecular systems is intensively pursued to manifest chirality in a wide range of applications ranging from electron spin filters to artificial enzymes. Typically, switching the helicity of supramolecular assemblies involves external stimuli or kinetic traps. However, efforts to achieve helix reversal under thermodynamic control and to understand the phenomena at a molecular level are scarce. Here we present a unique example of helix reversal (stereomutation) under thermodynamic control in the self-assembly of a coronene bisimide that has a 3,5-dialkoxy substitution on the imide phenyl groups (CBI-35CH), leading to "molecular pockets" in the assembly. The stereomutation was observed only if the CBI monomer possesses molecular pockets. Detailed chiroptical studies performed in alkane solvents with different molecular structures reveal that solvent molecules intercalate or form clathrates within the molecular pockets of CBI-35CH at low temperature (263 K), thereby triggering the stereomutation. The interplay among the helical assembly, molecular pockets, and solvent molecules is further unraveled by explicit solvent molecular dynamics simulations. Our results demonstrate how the molecular design of self-assembling building blocks can orchestrate the organization of surrounding solvent molecules, which in turn dictates the helical organization of the resulting supramolecular assembly.
控制合成超分子体系的螺旋结构，对于在从电子自旋滤波器到人工酶等广泛的应用中表现出手性，是一项备受关注的研究课题。通常，改变超分子组装体的螺旋性需要外部刺激或动力学陷阱。然而，实现热力学控制下的螺旋反转并从分子水平上理解相关现象的努力却很少。在本研究中，我们提出了一个独特的例子，即通过对具有 3,5-二烷氧基取代的苯并咪唑的冠醚二聚体（CBI-35CH）的自组装来实现螺旋反转（立体异构转变），从而在组装体中形成“分子口袋”。只有当 CBI 单体具有分子口袋时，才会观察到立体异构转变。在具有不同分子结构的烷烃溶剂中进行的详细手性研究表明，溶剂分子在低温（263 K）下会插入或形成 CBI-35CH 的分子口袋内的包合物，从而引发立体异构转变。通过显式溶剂分子动力学模拟进一步揭示了螺旋组装体、分子口袋和溶剂分子之间的相互作用。我们的研究结果证明了自组装构筑块的分子设计如何协调周围溶剂分子的组织，进而决定了所得超分子组装体的螺旋结构。
