Synthesis and mesomorphic behaviour of heterocycle-based liquid crystals containing 1,3,4-oxadiazole/thiadiazole and thiophene units

A new class of thiophene-based 1,3,4-oxadiazole derivatives ( n -OXD-R) and the corresponding 1,3,4-thiadiazole derivatives ( n -THD-R) was synthesised and characterised by means of 1H NMR, 13C NMR, MS and HRMS techniques. Liquid crystal properties were investigated by differential scanning calorimetry, polarising optical microscopy and variable temperature X-ray diffraction measurements. All the 1,3,4-thiadiazole compounds ( n -THD-R) exhibited enantiotropic mesophases (smectic A, smectic C and nematic phases) with wide mesomorphic temperature ranges (68.5-109.5C). In contrast, only the oxadiazole compounds 8-OXD-Cl, 9-OXD-Cl and 10-OXD-Cl, bearing electron-withdrawing terminal group and longer alkoxy chain, displayed an enantiotropic smectic A phase with narrow mesomorphic temperature ranges (4.1-10.9C). The effects of central heterocyclic rings, the terminal groups and the length of the terminal alkoxy on the mesomorphic behaviour are discussed.