New mesogenic materials bearing 1,2,4-oxadiazole moiety: Synthesis, characterization and investigation their mesomorphic behaviors

In this work, two series containing a 1,2,4-oxadiazole ring as a central core were synthesized and characterized by common spectroscopic techniques, including FT-IR, H-1-NMR, and elemental analysis. The first series was 3-(4-alkoxyphenyl)-5-(p-methylphenyl)-1,2,4-oxadiazole (C-n), which consisted of alkoxy group in the terminal arm and a methyl group in the other, while, the oxidation reaction of methyl group in a series (C-n) to the carboxy group was the method used to synthesize the second series in this work, 4-(3-(4-alkoxyphenyl)-1,2,4-oxadiazol-5-yl)benzoic acid (D-n). The mesophase behaviors of these two series were studied by optical polarized microscopy (OPM) and differential scanning calorimetry (DSC). The liquid crystalline investigations of the compounds (C-n and D-n) show that the last six homologeus of the series (C-n), (C-6-C-11), have a monotropic nematic phase, while only the intermediate compounds in the series (D-n), (D-3-D-9), displayed monotropic nematic phase, also, the liquid crystalline properties in the first and last two compounds in this series (D-1, D-2, D-10, and D-11) had disappeared. The differences in liquid crystalline properties between the two series, (C-n and D-n), were discussed through the influences of the different terminal groups (-CH3 and -COOH) in addition to the effect of the 1,2,4-oxadiazole ring and the length of the terminal alkyl chain.