Isoxazoline- and isoxazole-liquid crystalline schiff bases: A puzzling game dictated by entropy and enthalpy effects

Two series of Schiff base (SB) liquid crystals (LC) containing the 5-membered rings isoxazoline or isoxazole were synthesized and characterized; 27 isoxazoline and 20 isoxazole compounds were obtained. Nematic, smectic A, and smectic C mesophases were found and characterized by Polarized optical microscopy (POM), Differential scanning calorimetry (DSC), and X-ray diffraction. The scientific problem addressed was how the isoxazoline and isoxazole rings affect the mesophase structure and stability. Molecular packing and anisotropic interactions based on enthalpy and entropy properties were used to explain the thermal and structural behaviour observed for both series. The proposed mechanism was assisted by Density functional theory (DFT) calculations, providing new insights into the molecular organization of this kind of system. [Display omitted] •Synthesis of Schiff bases (SBs) containing isoxazoline and isoxazole rings.•Entropy and enthalpy effects dictated the liquid crystals properties.•Molecular geometry of heterocyclic is critical on mesomorphic behaviour.•Isoxazoline SBs were predominantly smectics while isoxazoles SBs were nematics.•Lateral and longitudinal diffusion were express in terms of molecular geometry.