Origin of the Monolayer and Bilayer Arrangement in Smectic A and C Mesophases and Nematic Cybotactic Domains: The Case Study of Benzyloxy-Terminated Isoxazole Liquid Crystals

The synthesis and characterization of two benzyloxy-terminated isoxazole liquid crystals, (ILCs) 7a (n-octyl) and 7b (n-decyl), are described. The synthesis of the five-membered isoxazole ring was based on the (3 + 2) 1,3-dipolar cycloaddition to furnish the isoxazoline cycloadduct and the subsequent MnO₂-oxidation. The liquid crystal (LC) behavior was accomplished by polarized optical microscopy (POM), differential scanning calorimetry, single-crystal X-ray diffraction, X-ray diffraction (XRD), 7-lithium nuclear magnetic resonance (⁷Li-NMR), and density functional theory (DFT) calculations. For both ILCs, the nematic cybotactic (Ncyb) and smectic A and C mesophases were characterized. Two organizational monolayer and bilayer regimes for the smectic A and C mesophases were characterized by the XRD pattern, and the Ncyb mesophase was assigned by the analysis of the low intensity of the peak at 2θ = 2.70°, suggesting some smectic character for the nematic mesophase. A correlation length (ξ) of around 20 layers and 7 layers was established by XRD for the monolayer and bilayer organization in the SmC mesophase. Single-crystal XRD, ⁷Li-NMR experiments, and DFT calculations were performed and are discussed in the context of the LC behavior influenced by the benzyloxy-terminated group. A model for the layering organization is presented as well as the organization in the Ncyb, SmA, and SmC mesophases.