The Effect of Hydrogen Bonding and Azomethine Group Orientation on Liquid Crystal Properties in Benzylidene Aniline Compounds

Abstract

This study examines the effects of substituents and hydrogen bonding, orientations of imine linkage on the behavior of benzylidene aniline compounds as liquid crystals (LC). Compounds 4-carboxy benzylidene-4-X-aniline (X = H, F, Cl, Br, CH₃, OCH₃) 1a–6a were synthesized by the reaction of aniline and its substituted derivatives with 4-formylbenzoic acid. Compounds 4-X-benzylidene-4-carboxy aniline (X = H, F, Cl, Br, CH₃, OCH₃) 1b–6b were synthesized by the reaction of benzaldehyde and its substituted derivatives with 4-aminobenzoic acid using absolute ethanol as the solvent. Synthesized compounds were characterized by FT IR and ¹H NMR spectroscopy, liquid crystal properties were investigated using differential scanning calorimetry (DSC) and polarizing optical microscopy (POM) techniques. Based on the mesomorphic properties, it was proven that the compounds 2b–4b are dimorphic exhibiting a smectic and nematic phase, compounds 5b, 6b are monomorphic exhibiting a nematic phase, while compounds 1a–6a and 1b have not shown any mesophase. For compounds 1a–6a hydrogen bonding and reversing imine linkage (in comparison with compounds 1b–6b) caused the absence of their mesomorphic properties.