Achiral bent-core salicylaldimine compounds exhibiting dark conglomerate and B₂ mesophases: effect of linkage groups and lateral substituents

Four new series of achiral bent-core compounds and two biphenyl derivatives of them have been synthesised and their mesomorphic properties are studied. These mesogens are salicylaldimine derivatives, designed with an aim to study the effect of salicylaldimine group and lateral substituents, viz., −F and −Cl at different locations of the molecular structure, on the mesomorphic properties. The newly synthesised compounds are characterised using organic spectroscopy and elemental analysis (EA). The liquid crystalline properties of these compounds are studied using polarising optical microscopy, differential scanning calorimetry, X-ray diffraction and electro-optical studies. They are found to exhibit dark conglomerate (DC) and B₂ mesophases. The DC phases are of sponge and HNC type, and the B₂ is SmC_aP_A in nature. The compounds, wherein the salicylaldimine group is present at the centre of the bent-core, mainly exhibit DC mesophases and when this group is present in one of the wings of the bent-core molecule, the compounds tend to show only a B₂ mesophase, irrespective of the nature of lateral substituents. Many of the DC liquid crystals synthesised here retained the chirality even in their crystalline states which are confirmed by the circular dichroism spectroscopic studies.