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

ABSTRACT

A 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.

KEYWORDS:

• Chalconyl ester
• Liquid Crystal
• Mesomorphism
• Smectic
• Nematic
• Smectic: -CO-CH = CH- > -N = N- > -CH = CH-CO-
• Nematic: -N = N > -CH = CH-CO > -CO-CH = CH-

Acknowledgments

Authors acknowledge thanks to the Dr. N.N. Vyas, the head of chemistry department and the principal Dr. M.B. Patel and Prof. M.L. Chauhan of the college for their supportive view and providing research facility of present investigation. Authors are also thankful to Dr. A.V. Doshi, Ex. Principal, M.V.M. Science and Home. Sc. College, Rajkot for his valuable suggestions, comments, co operation and support as and when needed throughout this present investigation. Authors acknowledge thanks to NFDD center, dept. of chemistry, Rajkot, for analytical and spectral services.