Azomethine Phenyl Benzoate Based Molecules: Synthesis, Characterization and Mesomorphic Behaviour

Abstract

Liquid crystalline materials have been of great interest in recent past due to their wide applications in scientific and technological areas, such as non-linear optics, display devices, anisotropic networks, photo-conductor and semi-conductor materials. Azomethine based liquid crystals are used in many fields like catalysis, photochemistry, biochemistry and organometallic chemistry. Azomethinephenyl benzoate based mesogens have special importance in the field of electronics, electro-optical devices, electronic screens and are used as temperature sensors. Keeping in view the importance of azomethine phenylbenzoate liquid crystals in different fields and their growing demand in electro-optical devices, in the present project, azomethine phenyl benzoate was used as central core that is composed of three aromatic rings linked with each other through azomethine and ester linkages. Azomethine linkage increased the molecular length and polarizability of the molecule that enhance the liquid crystalline properties. Ester linkage generates a bend in the molecular structure that influence the anisotropic properties of liquid crystals and strengthens the mesomorphic properties. In the present work, azomethine linkage was formed by the condensation of 4-aminophenol and 4-formylphenyl 4-methylbenzoate (4). Flexibility in the target mesogens was induced by alkylation of free hydroxyl group. Ten mesogens were synthesized with terminal methyl group at one end and varying chain length of 5-14 carbon-atoms at other end of the mesogen. The target mesogens (8a-j) were synthesized by coupling alkyl halide or alkyl tosylate and (E)-4-((4-hydroxyphenylimino)methyl) phenyl 4-methylbenzoate (6), itself synthesized by condensing 4-formyl phenyl 4-methylbenzoate (4) with 4-aminophenol (5). All the synthesized compounds were characterized using IR, 1H NMR and 13C NMR spectroscopy. Mesomorphic properties were studied using differential scanning calorimetry (DSC) and polarizing optical microscopy (POM). All the synthesized compound showed liquid crystalline properties. Lower homologues of the synthesized series having alkyl chain length of five to ten carbon atoms exhibited nematic phase while higher homologues exhibited both nematic and smectic phases. Compound (8j) exhibited nematic phase in the temperature range from 181.6 to 175.1 °C during cooling scan, while smectic phase appeared in the range of 104.2-84.6 °C.