Effect of orientation of extra fused benzene ring and lateral methyl substituent on the mesophase behaviour of three-ring azo/ester molecules
Two homologous series of the three-ring azo/ester compounds 2-(or1-) naphthyl 4ʹ-(4ʹ’-alkoxy phenylazo) benzoates (Ina and IIna) were synthesised. A lateral methyl group was introduced in different positions of the alkoxy phenyl moiety, in position-2 to give series Inb and IInb and in position-3 to give series Inc and IInc. Molecular structures were characterised via elemental analyses, infrared and 1H-NMR. Their mesophase characteristics were investigated by differential scanning calorimetry (DSC) and their phases identified via polarised light microscopy (PLM). Transition temperatures were correlated with the alkoxy-chain length (n) that varies between 6, 8, 10, 12, 14, and 16 carbons. Comparative studies were first made to investigate the effect of including the extra fused benzene ring, and its orientation, into the previously investigated three-ring compounds, 4-phenyl 4ʹ-(4′′-alkoxyphenylazo) benzoates (IIIna). Investigation of the mesophase behaviour was extended to cover the effect of introducing the lateral methyl group and its position. The comparison between the present six series and their corresponding phenyl analogues IIIna,b,c, indicated that the 2-naphthyl analogues, Ina,b,c, exhibit the highest mesophase stability. Whereas, the steric effect of the protruded naphthalene group destabilises all mesophases and appear only monotropically. The results were discussed in terms of polarisability effect.
