Synthesis and liquid crystal properties of a new class of calamitic mesogens based on substituted 2,5-diaryl-1,3,4-thiadiazole derivatives with wide mesomorphic temperature ranges

Abstract

Liquid crystals based on substituted 2,5-diaryl-1,3,4-thiadiazole derivatives (1a-1f, 3a and 3b) and 1,3,4-oxadiazole analogues (2a-2f, 4a and 4b) were synthesised and characterised by 1H, 13C nuclear magnetic resonance, Fourier transform infrared, mass spectrometry, high-resolution mass spectrometry techniques and elemental analyses. The X-ray crystal structure of 1e revealed that it contains tilted lamellar arrangement of molecules in the crystalline solid. The liquid crystal properties have been investigated by polarised-light optical microscopy, differential scanning calorimetry and in-situ variable-temperature X-ray diffraction. All compounds (except 2e and 2f) exhibited thermotropic liquid crystal behaviours with various mesophases (smectic A and C, nematic N or soft crystal E phases). Notably, the 1,3,4-thiadiazole derivatives consistently have wider mesomorphic temperature ranges than those of the respective 1,3,4-oxadiazole analogues. The solutions of all compounds in CH2Cl2 individually displayed one or two absorption bands with λ max values at 297-355 nm and emitted with λ max values at 363-545 nm and quantum yields of 0.12-0.73. Structure-property relationships of these compounds are discussed in the contexts of their molecular structures and weak intermolecular interactions. © 2008 Taylor & Francis.