Liquid crystalline paracyclophanes and ansa compounds—series of polyether macrocycles incorporating diacetylene, phenyl, biphenyl, p-terphenyl and 2,5-diphenyl-1,3,4-thiadiazole rigid cores
作者:Torsten Hegmann、Bernhard Neumann、Raik Wolf、Carsten Tschierske
DOI:10.1039/b416091f
日期:——
Series of different macrocyclic liquid crystals consisting of two rod-like aromatic cores connected at both terminal ends by polyether chains have been synthesised by various macrocyclisation reactions (template assisted etherification, Yamaguchi macrolactonisation, Glaser-coupling reaction). The nature and length of the rigid core have a dominant impact on the liquid crystalline phases formed. A stepwise increase of the length of one of the rigid cores leads to significant mesophase stabilisation, to the induction of lamellar phases and, in general, to wider mesophase ranges. The same types of mesophases but at much reduced transition temperatures are observed when flexible methylene linkages are incorporated into the macrocycles. Parallel pre-organisation of the calamitic cores and micro-segregation of chemically distinct molecular parts are discussed as the main driving forces for the mesophase behaviour of these macrocyclic compounds. Additionally, chiral macrocyclic liquid crystals exhibiting SmC* phases, cholesteric phases and blue phases have been synthesised. In contrast to the cyclophanes, ansa-compounds incorporating only one aromatic core within the macrocycle show either a monotropic nematic phase only or no liquid crystalline behaviour at all. An architecture combining the cyclophane structure with an ansa-unit and a molecule with a non-parallel pre-organisation of rod-like mesogenic cores as well does not show LC phases. The lack of LC behaviour for these compounds might be due to the molecular architecture resulting in a steric disturbance of packing that, in this way, does not allow for segregation of rigid and flexible units into distinct sub-domains.
由两个棒状芳香核通过聚醚链在两端连接而成的一系列不同的宏环液晶已通过各种宏环化反应合成(模板辅助醚化、山口巨环化反应、格拉泽偶联反应)。刚性核心的性质和长度对形成的液晶相具有显著影响。逐步增加其中一个刚性核心的长度会显著稳定中间相,诱导出层状相,并且总体上扩大了中间相的范围。当将柔性亚甲基链接体引入宏环时,观察到相同类型的中间相,但其相变温度明显降低。讨论了直链核心的平行预先组织和化学不同分子部分的微相分离作为这些宏环化合物中间相行为的主要驱动力。此外,还合成了表现出SmC*相、胆甾相和蓝相的手性宏环液晶。不同于环芳烃,仅在宏环内部包含一个芳香核心的安萨化合物仅显示出单调的向列相或根本没有液晶行为。结合环芳烃结构与安萨单位的架构以及不平行预先组织的棒状介导核心的分子也未显示出液晶相。这些化合物缺乏液晶行为可能是由于分子架构导致的堆积立体障碍,从而不允许刚性和柔性单位分离成不同的子域。