Celebre Giorgio, De Luca Giuseppina, Maiorino Michela, Iemma Francesca, Ferrarini Alberta, Pieraccini Silvia, Spada Gian Piero
Dipartimento di Chimica, Università della Calabria, via P. Bucci, 87036 Rende (CS), Italy.
J Am Chem Soc. 2005 Aug 24;127(33):11736-44. doi: 10.1021/ja051589a.
The induction of a cholesteric phase by doping an achiral nematic liquid crystal with an enantiopure solute is a phenomenon that, as in all general supramolecular phenomena of chiral amplification, depends in a subtle way on intermolecular interactions. The micrometric helical deformation of the phase director in the cholesteric phase is generated by the interplay of anisotropy and chirality of probe-medium interactions. In the case of a flexible chiral dopant, the solvent can influence the twisting power in two ways, difficult to disentangle: it is responsible for the solute orientational order, an essential ingredient for the emergence of phase chirality; but also it can affect the dopant conformational distribution and then the chirality of the structures present in the solution. In this work we have investigated methyl phenyl sulfoxide, a flexible, chiral molecule that, when dissolved in different nematics, can produce cholesteric phases of opposite handedness. This peculiar, intriguing sensitivity to the environment makes MPS a suitable probe for a thorough investigation of the effects of solute-solvent interactions on chiral induction in liquid crystals. NMR experiments in various nematic solvents have been performed in addition to twisting power measurements. From the analysis of partially averaged 1H-1H and 13C-1H dipolar couplings, the effects of solvent on solute conformation and orientational order are disentangled, and this information is combined with the modeling of the chirality of intermolecular interactions, within a molecular field theory. The integration of different techniques allows an unprecedented insight into the role of solvent in mediating the chirality transfer from molecule to phase.
通过用对映体纯溶质掺杂非手性向列型液晶来诱导胆甾相，这一现象如同所有手性放大的一般超分子现象一样，微妙地取决于分子间相互作用。胆甾相中相指向矢的微米级螺旋形变是由探针 - 介质相互作用的各向异性和手性的相互作用产生的。对于柔性手性掺杂剂，溶剂可以通过两种难以区分的方式影响扭曲能力：它决定了溶质的取向有序性，这是相手性出现的关键要素；但它也会影响掺杂剂的构象分布，进而影响溶液中存在的结构的手性。在这项工作中，我们研究了甲基苯基亚砜，一种柔性手性分子，当它溶解在不同的向列型液晶中时，可以产生相反手性的胆甾相。这种对环境独特而有趣的敏感性使得甲基苯基亚砜成为深入研究溶质 - 溶剂相互作用对液晶中手性诱导影响的合适探针。除了扭曲能力测量外，还在各种向列型溶剂中进行了核磁共振实验。通过对部分平均的1H - 1H和13C - 1H偶极耦合的分析，区分了溶剂对溶质构象和取向有序性的影响，并将这些信息与分子场理论中分子间相互作用手性的建模相结合。不同技术的整合使得我们能够前所未有的深入了解溶剂在介导从分子到手性相的手性转移中所起的作用。
