The nematic phase occurring in bent-core (or banana-shaped) molecule systems is conducive to the occurrence of biaxial (Nb) phase due to the inherent biaxial molecular shape. The tendency of bent-core molecules to stabilise a layered (smectic) structure poses one of the difficulties in obtaining the thermotropic Nb phase. To investigate the factors favouring the formation of the nematic phase, a number of bent-core nematic liquid crystals derived from 1, 3-phenylene central bent motifs were synthesised. Results of an investigation into the influence of the nature and position of substitutions on the central core and two rigid arms as well as the type of linkage units on the occurrence and stability of the nematic phase reveal important structure–property relationships. Textural and X-ray diffraction measurements have helped us identify molecules with the potential of exhibiting the Nb phase.
Surface Alignment, Anchoring Transitions, Optical Properties, and Topological Defects in the Nematic Phase of Thermotropic Bent-Core Liquid Crystal A13110/26/2010
We study optical, structural, and surface anchoring properties of thermotropic nematic bent-core material A131. The focus is on the features associated with orientational order as the material has been reported to exhibit not only the usual uniaxial nematic but also the biaxial nematic phase. We demonstrate that A131 experiences a surface anchoring transition from a perpendicular to tilted alignment when the temperature decreases. The features of the tilted state are consistent with surface-induced birefringence associated with smectic layering near the surface and a molecular tilt that changes along the normal to the substrates. The surface-induced birefringence is reduced to zero by a modest electric field that establishes a uniform uniaxial nematic state. Both refractive and absorptive optical properties of A131 are consistent with the uniaxial order. We found no evidence of the “polycrystalline” biaxial behavior in the cells placed in crossed electric and magnetic fields. We observe stable topological point defects (boojums and hedgehogs) and nonsingular “escaped” disclinations pertinent only to the uniaxial order. Finally, freely suspended films of A131 show uniaxial nematic and smectic textures; a decrease in the film thickness expands the temperature range of stability of smectic textures, supporting the idea of surface-induced smectic layering. Our conclusion is that A131 features only a uniaxial nematic phase and that the apparent biaxiality is caused by subtle surface effects rather than by the bulk biaxial phase.