Mesomorphism dependence on molecular rigidity with reference to ‒CH˭CH‒ unit of central bridge

ABSTRACT

A novel ester homologous series of rich mesomorphism and low temperatures with unexpected phase behaviors of eleven homologues was synthesized and studied with a view to understanding and establishing the relation between mesomorphic behaviour and the molecular structure of a series 4-(4′-n-alkoxybenzoyloxy)-4″-chlorobenzyl cinnamates. All the members of the novel series are enantiotropically smectogenic and the octyloxy (C₈) to hexadecyloxy (C₁₆) homologues are enantiotropically nematogenic, in addition to smectogenic. Odd–even effect is observed for Sm‒I/Sm‒N transition curve but it is absent for N‒I transition curve. Textures of nematic phase are threaded or Schlieren and that of the smectic phase are fan shaped or batonates of smectic-A type phase or Smectic-C type for C₁₆ homologues as judged directly from a heating stage of an optical polarizing microscopy. Analytical and spectral data confirmed the molecular structures of novel homologues. Mesomorphic properties of present series are compared with the structurally similar other known series. The average smectic and nematic thermal stabilities are 92.78°C and 100.8°C, respectively. Mesophase length minimum to maximum for smectic and nematic are 21.0°C to 31.1°C and 8.4°C to 42.6°C respectively. Thus, the present novel series is partly nematogenic and fully smectogenic with considerable degree of mesomorphism and low melting type.

KEYWORDS:

• Liquid crystals
• mesogen
• mesomorphism
• nematic
• smectic

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Acknowledgments

Authors acknowledge thanks to the Green Circle Inc. Laboratory for providing research facilities services as and when needed. Authors acknowledge thanks to Dr. Keyur Trivedi of Solaris Industries Ltd., Vadodara, for providing the free sample of 4-chlorobenzyl bromide. Authors are also thankful to Dr. N. N. Vyas, Dr. Vipul Patel, and Dr. M. L.Chauhan, P.T. Arts and Science College, Godhara, for their valuable helping hand and microscopic facility. Also thanks are due to the Sophisticated Analytical Instrumentation Facility, Punjab University, (Chandigarh) for extending their help for analytical services.