Pseudopolar smectic-C phases of azo-substituted achiral bent-core hockey-stick-shaped molecules

Leaning induced layer undulated tilted smectic phase of asymmetric bent-core liquid crystals

We report the synthesis and experimental studies on the liquid crystalline phase of a homologous series of compounds comprised of strongly asymmetric bent-core banana shaped molecules. Our x-ray diffraction studies clearly indicate that the compounds exhibit a frustrated tilted smectic phase with layer undulated structure. The low value of the dielectric constant as well as switching current measurements indicate the absence of polarization in this layer undulated phase. In spite of the absence of polarization, a planar aligned sample can be transformed irreversibly to a higher birefringent texture on the application of a high electric field. The zero field texture can only be retrieved by heating the sample to the isotropic phase and cooling it subsequently to the mesophase. We propose a double tilted smectic structure having layer undulation to account for the experimental observations with the layer undulation arising due to the leaning of the molecules in the layer.

Optical Microscopy Observations and Construction of Dupin Cyclides at the Isotropic/Smectic A Phase Transition

In this work, we are interested in the nucleation of bâtonnets at the Isotropic/Smectic A phase transition of 10CB liquid crystal. Very often, these bâtonnets are decorated with a large number of focal conics. We present here an example of a bâtonnet obtained by optical crossed polarized microscopy in a frequently observed particular area of the sample. This bâtonnet presents bulges and one of them consists of a tessellation of ellipses. These ellipses are two by two tangent, one to each other, and their confocal hyperbolas merge at the apex of the bâtonnet. We propose a numerical simulation with Python software to reproduce this tiling of ellipses as well as the shape of the smectic layers taking the well-known shape of Dupin cyclides within this particular bâtonnet area.

Stereochemical Rules Govern the Soft Self-Assembly of Achiral Compounds: Understanding the Heliconical Liquid-Crystalline Phases of Bent-Core Mesogens

A series of bent-shaped 4-cyanoresorcinol bisterephthalates is reported. Some of these achiral compounds spontaneously form a short-pitch heliconical lamellar liquid-crystalline phase with incommensurate 3-layer pitch and the helix axis parallel to the layer normal. It is observed at the paraelectric-(anti)ferroelectric transition, if it coincides with the transition from random to uniform tilt and with the transition from anticlinic to synclinic tilt correlation of the molecules in the layers of the developing tilted smectic phase. For compounds with long chains the heliconical phase is only field-induced, but once formed it is stable in a distinct temperature range, even after switching off the field. The presence of the helix changes the phase properties and the switching mechanism from the naturally preferred rotation around the molecular long axis, which reverses the chirality, to a precession on a cone, which retains the chirality. These observations are explained by diastereomeric relations between two coexisting modes of superstructural chirality. One is the layer chirality, resulting from the combination of tilt and polar order, and the other one is the helical twist evolving between the layers. At lower temperature the helical structure is replaced by a non-tilted and ferreoelectric switching lamellar phase, providing an alternative non-chiral way for the transition from anticlinic to synclinic tilt.