Bent-Core Liquid Crystals from Roof-Shaped Boron Coordination Compounds: Synthesis, Characterization, Dielectric, and Electro-Optic Studies

Bent-core liquid crystals derived from boron coordination compounds have been obtained using a novel approach that incorporates a roof-shaped pyrazabole ring at the core, and lateral wings of different lengths in their aromatic and aliphatic parts. The materials were characterized in solution and in the pure state. Mesophase behavior was studied by polarized light microscopy, differential scanning calorimetry, X-ray scattering, and solid-state NMR. All of the compounds display a nematic mesophase that is not typical of standard nematic phases as a consequence of their bent shape and their transverse dipole moment. The nonstandard behavior was investigated by dielectric spectroscopy and electro-optic studies. Additionally, some compounds display a direct transition from the fluid nematic phase to a crystal-like phase with a very well-defined layer structure. Compounds with extended aromatic structures exhibit additional mesophases, and these were characterized as intercalated smectic mesophases (B6). A novel transition from a tilted B6 phase to a nontilted B6 phase is described for one of these extended derivatives.