Synthesis, mesomorphic behaviour, and DFT studies of biphenyl bis-ester Schiff base liquid crystals

Abstract

A novel series of calamitic liquid crystals with rod-like structures, derived from biphenyl bis-ester, has been successfully synthesized. This was achieved through the condensation reaction of [1,1′-biphenyl]-4-yl 4-formylbenzoate with n-alkyl 4-aminobenzoate, where n ranges from 2 to 8, 10, 12, 14, 16, and 18. The synthesized compounds were characterized using FT-IR, 1H NMR, 13C NMR, and mass spectroscopy techniques. The liquid crystal behaviour of all synthesized compounds was investigated using Polarizing Optical Microscopy (POM) and Differential Scanning Calorimetry (DSC). It was found that all compounds within the homologous series exhibited mesogenic properties. The effect of molecular flexibility on the mesophase type and transition temperature range was systematically explored. Some compounds displayed a nematic phase, while others showed characteristics of the smectic A phase; additionally, certain compounds exhibited properties of both the nematic and smectic A phases. Furthermore, utilizing B3LYP/6–31++G (d,p) basis sets DFT level calculations and complete geometric optimization of the synthesized compounds was conducted. This optimization facilitated the examination of optimized geometrical structures, aspect ratios, dipole moments, polarizabilities, Frontier molecular orbitals, and molecular electrostatic potentials through DFT calculations.