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

Abstract

A class of extended 2,5-disubstituted-1,3,4-oxadiazoles R 1-C 6H 4-{OC 2N 2}-C 6H 4-R 2 (R 1 = R 2 = C 10H 21O 1a, P-C 10H 21O-C 6H 4-C≡C 3a, p-CH 3O-C 6H 4-C≡C 3b; R 1=C 10H 21O, R 2 = CH 3O 1b, (CH 3) 2N 1c; F 1d; R 1 = C 10H 21O-C 6H 4- C≡C, R 2 = C 10H 21O 2a, CH 3O 2b, (CH 3) 2N 2c, F 2d) were prepared, and their liquid-crystalline properties were examined. In CH 2Cl 2 solution, these compounds displayed a room-temperature emission with λ max at 340-471 nm and quantum yields of 0.73-0.97. Compounds 1d, 2a-2d, 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 2b and 3 a, leading to different nematic mesophase behavior, with 2b being monotropic and 3 a enantiotropic in the narrow temperature range of 200-210°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 diffracto-grams of 1b, 1d, 2b, 2d, and 3a measured at 25-200°C. Powder X-ray diffraction and small-angle X-ray scattering data revealed that the structure of the annealed solid residue 2b 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. © 2006 Wiley-VCH Verlag GmbH & Co. KGaA.