Polar nematic phase in short-chained fluorinated hydrogen-bonded liquid crystals

Abstract

So as to investigate structure–activity relationships in hydrogen bonded liquid crystals, a new short-chained (7 carbon atoms) fluorinated hydrogen-bonded liquid crystal 4-heptyloxy-2-fluorobenzoic acid, 7OBAF, is prepared. Differential scanning calorimetry, polarized optical microscopy and X-ray diffraction disclose a phase sequence as two crystalline, a nematic and the isotropic phases. Dielectric spectroscopy of the nematic phase in the frequency range of 1 Hz–6 MHz reveals a low frequency mode ascribed to ferroelectric clusters. A high polarization is induced by an external electric field, in agreement with results of dielectric measurements. Such polarization increases as the alkyl chain length is shorter. Results of DFT calculations using the Maier–Meier relationship are consistent with the dielectric measurements; they reveal the parallel molecular distribution and provide the dipole moment, polarization anisotropy, birefringence and dielectric anisotropy. Intermolecular hydrogen bonds linking the fluorine substituent and the aromatic hydrogen atoms are also disclosed.