الفهرس 
Title : Effect of lateral methyl substitution on the mesophase behaviour of some new four-ring azo-ester-azo
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Abstract
The effect of introducing a lateral methyl substitution into the previously investigated laterally neat four-ring analogues, 4-substituted phenylazo phenyl 4ʹ-(4ʺ-alkoxyphenylazo) benzoates (Ina{u2013}e), on their mesophase behaviour was investigated for the newly prepared five homologous series of 4-substituted phenylazo phenyl 4ʹ-(3ʺ-methyl-4ʺ-alkoxyphenylazo) benzoates (IIna-e). Within each homologous series, the alkoxy group varies between 6, 8, 10, 12, 14, and 16 carbons, while the substituent, X, is a polar group that alternatively changes between the electron-donating (CH³O and CH³) and the electron-withdrawing (Br and NO²) groups, including the un-substituted homologues (Hnc). Their mesophase stabilities were determined by differential scanning calorimetry (DSC) and phases identified by polarized light microscopy (PLM). The results showed that, independent of either of the alkoxy-chain length (n) or the polarity of the substituent X, the nematic phase is predominant with relatively high stability and wide temperature ranges. All compounds show a good thermal stability in the mesophases domain, except the nitro and Br substituted derivatives bearing short alkoxy chain length. Comparison of the mesophase behavior was also made between the present series and corresponding three-ring laterally CH³-substituted azo/ester analogues. UV-vis absorption spectra revealed that both derivatives with the electron donating or the electron withdrawing groups exhibited red shifts of the s{u2192}s* transition compared with that of the un-substituted ones