Thermotropic Liquid Crystals Based on Extended 2,5‐Disubstituted‐1,3,4‐Oxadiazoles: Structure–Property Relationships, Variable‐Temperature Powder X‐ray Diffraction, and Small‐Angle X‐ray Scattering Studies

A class of extended 2,5-disubstituted-1,3,4-oxadiazoles R1-C6H4-{OC2N2}-C6H4-R2 (R1 = R2 = C10H21O 1 a, p-C10H21O-C6H4-C[triple bond]C 3 a, p-CH3O-C6H4-C[triple bond]C 3 b; R1 = C10H21O, R2 = CH3O 1 b, (CH3)2N 1 c; F 1 d; R1 = C10H21O-C6H4-C[triple bond]C, R2 = C10H21O 2 a, CH3O 2 b, (CH3)2N 2 c, F 2 d) were prepared, and their liquid-crystalline properties were examined. In CH2Cl2 solution, these compounds displayed a room-temperature emission with lambda(max) at 340-471 nm and quantum yields of 0.73-0.97. Compounds 1 d, 2 a-2 d, and 3 a exhibited various thermotropic mesophases (monotropic, enantiotropic nematic/smectic), which were examined by polarized-light optical microscopy and differential scanning calorimetry. Structure determination by a direct-space approach using simulated annealing or parallel tempering of the powder X-ray diffraction data revealed distinctive crystal-packing arrangements for mesogenic molecules 2 b and 3 a, leading to different nematic mesophase behavior, with 2 b being monotropic and 3 a enantiotropic in the narrow temperature range of 200-210 degrees C. The structural transitions associated with these crystalline solids and their mesophases were studied by variable-temperature X-ray diffractometry. Nondestructive phase transitions (crystal-to-crystal, crystal-to-mesophase, mesophase-to-liquid) were observed in the diffractograms of 1 b, 1 d, 2 b, 2 d, and 3 a measured at 25-200 degrees C. Powder X-ray diffraction and small-angle X-ray scattering data revealed that the structure of the annealed solid residue 2 b reverted to its original crystal/molecular packing when the isotropic liquid was cooled to room temperature. Structure-property relationships within these mesomorphic solids are discussed in the context of their molecular structures and intermolecular interactions.