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 ¹H and ¹³C NMR, MS, and elemental analyses. The X-ray crystal structure of 2, 5-di-(4-decyloxyphenyl)-1, 3, 4-thiadiazole reveals the tilt 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 from 2, 5-di-(3, 4, 5-tri-ACHTUNGTRENUNGoctyloxyphenyl)-1, 3, 4-thiadiazole shows poor conductivity (2×10^-7 Scm^-1).