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Keywords: Natural Sciences
Chemistry & allied sciences
Issue Date: 2017
Abstract: One of the important properties of heterocyclic compounds is their thermal and chemical stability, which makes them significant in synthesis of thermally and chemically stable liquid crystalline compounds. Incorporation of heterocyclic ring in the molecules changes the polarizability, phase transition temperatures and type of mesophase. These heterocyclic liquid crystalline materials have great potential applications like spatial light modulation, optical signal processing, optical information storage, organic thin-film transistors, fast-switching ferroelectric materials and fluorescent probes. In the present work, azole (tetrazole, thiazole, oxadiazole) based liquid crystalline molecules were prepared. The tetrazole derivatives having two benzene rigid cores with various linking units, i.e., single bond, ester, an ester and a double bond, ether and triple bond, were synthesized. The tetrazole moiety was also incorporated in side chain tetrazole liquid crystalline polymers. Furthermore, molecular architecture was designed by incorporating thiazole, oxadiazole moiety and two benzene rings using a single bond as linking unit. In addition, molecular structures with pyridine and oxadiazole rigid cores and ester functionality as linking unit were also synthesized. The long alkyl and alkoxy groups were introduced at the peripheral rigid core units. The structure characterization of all the synthesized compounds was performed employing IR, NMR and CHNS analysis. The liquid crystalline properties of the compounds were evaluated through DSC, POM and XRD analysis. The tetrazole derivatives with ester linking group showed enantiotropic nematic (N), chiral nematic (N*), chiral smectic A (SmA*) and smectic C (SmC) phases. The smectic phase appeared with increased alkoxy chain length. The liquid crystalline range also increased with increasing length of alkyl chain. The tetrazole derivatives with ester and double bond linker also exhibited enantiotropic nematic phase. Moreover, the compounds with triple bond as linking unit manifested monotropic nematic phase with very narrow mesophase range. Furthermore, tetrazole derivatives with ether linking unit and oxadiazole and thiazole derivatives proved non-mesogenic. The tetrazole polymers demonstrated SmA phase. A single nematic mesophase with large transition temperature ranges was observed for oxadiazole and pyridine derivatives. The synthesized tertazole derivatives with perfluorobenzene ring, ether and single bond linking units showed sharp melting temperatures and were non-mesopmorphic
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