Effect of orientation of extra fused benzene ring and lateral methyl substituent on the mesophase behaviour of three-ring azo/ester molecules

ABSTRACT

Two homologous series of the three-ring azo/ester compounds 2-(or1-) naphthyl 4ʹ-(4ʹ’-alkoxy phenylazo) benzoates (In_a and IIn_a) were synthesised. A lateral methyl group was introduced in different positions of the alkoxy phenyl moiety, in position-2 to give series In_b and IIn_b and in position-3 to give series In_c and IIn_c. 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 (IIIn_a). 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 IIIn_a,b,c, indicated that the 2-naphthyl analogues, In_a,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.

Graphical abstract

KEYWORDS:

• Naphthyl azo/ester liquid crystals
• lateral methyl group
• orientation
• electronic transition

Disclosure statement

No potential conflict of interest was reported by the authors.