A polar biaxial smectic A phase in new unsymmetrical compounds composed of bent-core molecules

Abstract

The synthesis and liquid crystalline properties of several new compounds unsymmetrically substituted about two different central phenyl rings are reported. All the compounds contain a strongly polar terminal cyano/nitro substituent. Some of the nitro-substituted compounds derived from 3-hydroxybenzoic acid show a transition from the partial bilayer uniaxial smectic A (SmA_d) phase to a polar partial bilayer biaxial smectic A (SmA_dP_A) phase. The analogous cyano-substituted derivatives exhibit only a SmA_d phase. However, compounds containing a 4-cyanobiphenyl moiety in the terminal position and derived from 1,3-dihydroxybenzene show the SmA_d – SmA_dP_A transition. The mesophases were characterized using polarizing optical microscopy, differential scanning calorimetry, X-ray diffraction studies, conoscopic examination and miscibility studies.