Department of Physics & Astronomy, Swarthmore College, Swarthmore, PA, USA.
Department of Chemistry and Biochemistry, Swarthmore College, Swarthmore, PA, USA.
Soft Matter. 2018 Dec 19;15(1):109-115. doi: 10.1039/c8sm02091d.
One of the most simple cases in which chirality at the microscopic level produces a chiral macroscopic structure is the chiral nematic liquid crystal phase. In such a phase, the preferred direction of molecular orientation rotates in helical fashion, with the pitch of the helix in different systems ranging from around 100 nm to as large as can be measured (∼10 mm). For almost all thermotropic and lyotropic liquid crystals, the ordered entities are formed from strong bonds, so the pitch varies in accordance with how the interactions between these largely immutable entities are affected by changing conditions. A unique exception are lyotropic chromonic liquid crystals (LCLCs) that spontaneously form weakly bound assemblies in solution, the size of which depends strongly on experimental parameters. While the temperature dependence of the pitch has been measured for chiral LCLCs formed by short strands of DNA (DNA-LCLCs), such is not the case for chiral LCLCs formed by small molecules. Polarized optical microscopy experiments on small molecule chiral LCLCs reveal the changing assembly size through a temperature dependence of the pitch not typical for many other systems, including the most recent measurements on DNA-LCLCs. In fact, the pitch measurements in small molecule chiral LCLCs strongly increase in value as the temperature is increased and the assemblies shrink in size. Theoretical considerations provide some help in understanding this phenomena, but leave much to be explained.
在微观水平的手性产生手性宏观结构的最简单情况之一是手性向列液晶相。在这样的相中，分子取向的优选方向以螺旋方式旋转，不同系统中的螺旋节距范围从大约 100nm 到可以测量的最大范围（∼10mm）。对于几乎所有热致性和溶致性液晶，有序实体是由强键形成的，因此节距的变化取决于这些基本不变的实体之间的相互作用如何受到变化条件的影响。溶致性变色液晶（LCLC）是一个独特的例外，它在溶液中自发形成弱结合的组装体，其大小强烈依赖于实验参数。虽然已经测量了由短链 DNA（DNA-LCLC）形成的手性 LCLC 的节距对温度的依赖性，但由小分子形成的手性 LCLC 并非如此。对小分子手性 LCLC 的偏光显微镜实验通过对许多其他系统（包括最近对 DNA-LCLC 的测量）不典型的节距对温度的依赖性来揭示组装体大小的变化。事实上，小分子手性 LCLC 的节距测量值随着温度升高和组装体缩小而强烈增加。理论考虑提供了一些理解这一现象的帮助，但仍有很多需要解释。
