Dependence of thermotropic mesomorphism on varying rigidity of central bridge in liquid crystals

Abstract

ABSTRACTA novel liquid crystalline(LC) homologous series of chalconyl esters: RO-C6H4-COO-C6H4-CH = CH-CO-C6H4F has been synthesized and studied with a view to understanding and establishing the relationship between thermotropic LC properties and molecular structure with reference to varying molecular rigidity due to linking groups. The novel homologous series consists of eleven members (C1 to C16). C1 and C2 homologues are nonliquid crystals (NLC) and the rest of the homologues are enantiotropically nematic (C3 to C16). The smectogenic character, either in enantiotropic or monotropic manner is totally absent for a series. Transition temperatures and textures of nematic phase were determined by an optical polarizing microscope (POM) equipped with a heating stage. Textures of the nematic phase are threaded or Schlieren. The analytical, thermal and spectral data support the molecular structures. Cr-N/I and N-I transition curves behaved in normal manner except for the C14 homologue. N-I transition curve exhibits an odd- even effect with a negligible deviating effect from the C10 homologue. Thermal stability for the nematic is 105.2°C and mesophase lengths range from 06.0°C to 46.0°C at C8 and C14 homologues respectively. The group efficiency order derived for nematic from the comparative study of present novel series with structurally similar analogous series on the basis of thermal stability as under.