Mesogenic pyrazaboles: Synthesis, properties, and structural characterization

A series of disklike compounds containing the pyrazabole ring have been pre pared by reaction of 3,5-diarylpyrazoles and borane. The aryl groups are be nzene rings substituted with one, two, or three hexyloxy, decyloxy, or tetr adecyloxy groups. The study of the liquid crystalline properties shows that the optimal chain length to generate mesomorphism in this family of compou nds is 10 carbon atoms. Thus, three compounds with decyloxy substituents sh ow hexagonal columnar mesophases at relatively low temperatures. For two of them bearing 10 and 12 decyloxy chains the mesophase remains stable at roo m temperature for long periods of time, as confirmed by X-ray diffraction. The crystal structure of the nonmesogenic homologue 1,3,5,7-tetrakis(4'-met hoxyphenyl)pyrazabole has been determined by X-ray diffraction. The structu re analysis reveals that the pyrazabole cycle adopts a slightly twisted boa t conformation. In the crystal packing its molecules stack in columns, whic h is consistent with the fact that the homologous compounds with long chain s generate columnar mesophases.