Microwave-assisted Synthesis, Structure, and Tunable Liquid-Crystal Properties of 2,5-Diaryl-1,3,4-Thiadiazole Derivatives through Peripheral n-Alkoxy Chains

A series of substituted 2,5-diaryl-1,3,4-thiadiazole derivatives are prepared by microwave-assisted synthesis in the absence of an organic solvent. All derivatives are well characterized by (1)H and (13)C NMR, MS, and elemental analyses. The X-ray crystal structure of 2,5-di-(4-decyloxyphenyl)-1,3,4-thiadiazole reveals the till lamellar arrangement of rod-shaped molecules, which are stabilized by a variety of weak non-covalent interactions. The liquid crystalline properties are studied by polarized-light optical microscopy (POM), differential scanning calorimetry (DSC), and in situ variable temperature X-ray diffraction (VTXRD). By variations in the peripheral n-alkoxy chains. the calamitic mesogens exhibit enantiotropic smectic (SmC and/or SmA) mesophases with wide mesomorphic temperature ranges, whilst the disc-like mesogens form hexagonal columnar mesophase (Col(h)) at room temperature. The bulk electrical conductivity values of the smectic mesophases of 1-3 are in the range of 10(-3)-10(-4) Scm(-1), which are slightly higher than that of their solid films. in contrast. the solid film made front 2,5-di-(3,4,5-trioctyloxyphenyl)-1,3,4-thiadiazole shows poor conductivity (2 x 10(-7) Scm(-1)).