Mesophase study of pure and doped cyanobiphenyl liquid crystals with salen-type systems

Abstract

ABSTRACTTwo salen Schiff base ligands derived from the condensation of ethylendiamine on salicylaldehyde and 5-chlorosalicylaldehyde, namely N,N′-bis(salicylidene)ethylene-diamine (L1) and N,N′-bis(5-chlorosalicylidene)ethylene-diamine (L2) as well as two of their iron(III) and nickel(II) complexes, were prepared and then used as doping agents of two thermotropic liquid crystals (LCs) of cyanobiphenyl type, namely 4-cyano-4′-n-pentyl-biphenyl (5CB) and 4-cyano-4′-n-octyl-biphenyl (8CB). The study of the mesophase of pure and doped LCs was carried out by UV–visible spectroscopy equipped with a heating compartment for precise temperature control, differential scanning calorimetry and polarised optical microscopy. The characteristic nematic/isotropic and smectic-A/nematic transition temperatures of 5CB- and 8CB-based systems were measured and then compared to those of the literature concerning pure 5CB and 8CB. Optical microscopy results revealed the existence of Schlieren and focal conic textures of the nematic and smectic states, respectively, both of pure and doped LCs. The homogeneity of the obtained guest–host systems was proven by the linear evolution of their transition temperatures as function of the solute concentration, with correlation factors close to unity.