Influence of Bromine Substitution on Mesophase Stability and Electro-Optical Performance of Schiff Base Liquid Crystals

This work reports the synthesis and characterization of two thermotropic liquid crystals (LCs) derived from Schiff bases of 4-bromophenylpyridyl (IeC8) and 2,4-dibromophenylpyridyl (ILC8), in combination with 4-octyloxybenzoic acid, with a focus on the influence of dibromo substitution on mesophase behavior and electro-optical properties. The compounds were synthesized and purified using established protocols. Phase transition temperatures were determined by differential scanning calorimetry (DSC), while mesophase textures were confirmed using polarized optical microscopy (POM), revealing nematic and smectic phases. Optical properties, including refractive indices, birefringence (Δn), and optical band gap, were measured using an Abbe refractometer and a modified spectrophotometer. ILC8 consistently exhibited higher birefringence, indicating enhanced molecular ordering and rigidity due to dibromo substitution. Impedance spectroscopy revealed thermally activated charge dynamics, with abrupt changes in relaxation time (τ) and conductance (G) near phase transitions. ILC8 demonstrated moderate dielectric anisotropy (Δε ≈ 0.40), lower optical conductivity anisotropy, and greater activation energy (Ea ≈ 0.083 eV), reflecting improved dipolar alignment and reduced optical losses. These results highlight the role of dibromo substitution in enhancing thermal stability, dielectric anisotropy, and birefringence, positioning ILC8 as a strong candidate for advanced electro-optic and display applications.