Layer frustration, polar order and chirality in liquid crystalline phases of silyl-terminated achiral bent-core molecules.
Layer frustration, polar order and chirality in liquid crystalline phases of silyl-terminated achiral bent-core molecules.
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甲硅烷基封端的非手性弯核分子液晶相中的层挫败、极性顺序和手性。
DOI：
10.1002/chem.200600876
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发表时间：
2007
期刊：
影响因子：
--
通讯作者：
C. Tschierske
中科院分区：
文献类型：
--
作者：
Christina Keith;R. Amaranatha Reddy;M. Prehm;U. Baumeister;H. Kresse;J. L. Chao;H. Hahn;H. Lang;C. Tschierske
合成了一类具有低聚硅氧烷或碳硅烷两端的弯核分子，并通过偏光显微镜、DSC、x射线散射、介电和电光等方法研究了这些分子的自组织。根据含硅段的大小，可以形成近晶和柱状液晶相。大多数近晶相是低双折射的，由相反手性的宏观畴（暗砾岩相）组成。在这些近晶相中的转换过程是表面稳定的铁电转换过程，并且根据不同的条件，观察到两个不同的缓慢弛豫过程到非极性结构。提出近晶相是由反斜界面分离的手性和极性SmCsPF层叠加形成的。如果这些层堆的尺寸足够大，就会发生与衬底表面的耦合，并观察到铁电开关。此外，在低双折射中间相出现的海绵状层畸变是由于这些SmCsPF层堆栈内部的局部极序逃逸所致。对于具有较大硅基化单元的化合物，会产生位阻，导致层调制（柱状带相），这与从铁电转换到反铁电转换有关。所有化合物都表现出分子在长轴上的转换，这与层的手性相反。
A novel class of bent-core molecules with oligo(siloxane) or carbosilane units at both ends was synthesized and the self-organization of these molecules was investigated by polarizing microscopy, DSC, X-ray scattering, dielectric and electrooptical methods. Depending on the size of the silicon-containing segments, smectic and columnar liquid crystalline phases are formed. Most smectic phases are low birefringent and composed of macroscopic domains of opposite handedness (dark conglomerate phases). The switching process in these smectic phases is surface stabilized ferroelectric and, depending on the conditions, two distinct slow relaxation processes to nonpolar structures were observed. It is proposed that the smectic phases are built up by chiral and polar SmCsPF layer stacks which are separated by anticlinic interfaces. If the size of these layer stacks is sufficiently large a coupling to the substrate surfaces takes place and ferroelectric switching is observed. It is also suggested that the sponge-like layer distortion, occurring in the low birefringent mesophases, is due to an escape from the local polar order within these SmCsPF layer stacks. For compounds with larger silylated units a steric frustration arises, which leads to layer modulation (columnar ribbon phases) and this is associated with a transition from ferroelectric to antiferroelectric switching. All compounds show a switching of the molecules around the long axis which reverses the layer chirality.