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

Chain change : Variations in the position, number, and chain length of the n ‐alkoxy chains readily tune the liquid‐crystalline properties of 2,5‐diaryl‐1,3,4‐thiadiazole derivatives from smectic to hexagonal columnar mesophases at room temperature. The calamitic S‐heterocyclic compounds show electrical conductivity in both solid and liquid‐crystal states. magnified image 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 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 S cm −1 , which are slightly higher than that of their solid films. In contrast, the solid film made from 2,5‐di‐(3,4,5‐trioctyloxyphenyl)‐1,3,4‐thiadiazole shows poor conductivity (2×10 −7 S cm −1 ).