The effect of inversion of the ester group on the mesophase behaviour of some azo/ester compounds

Abstract

Five homologous series of 4-substituted phenyl 4′-(4″-alkoxy phenylazo) benzoates (In a-−e ) were prepared in which, within each homologous series, the length of the terminal alkoxy group varies between 8, 10, 12, 14 and 16 carbons, while the other terminal substituent, X, is a polar group that alternatively changed from CH 3 O, CH 3 , H, Br, and CN groups. Compounds prepared were characterised by infrared, mass, and H 1 -NMR spectroscopy and their mesophase behaviour investigated by differential scanning calorimetry (DSC) and polarised light microscopy (PLM). The results were discussed in terms of mesomeric and polarisability effects. Only for the lower group of compounds, I8 a-e , that showed a nematic phase, the nematic-to-isotropic transition temperatures (T N-I ) were successfully correlated to the polarisability anisotropy of bonds to the substituent X. A comparative study was made between the investigated compounds and two previously prepared isomeric groups. In the first group of isomers, 4-(4′alkoxy phenylazo) phenyl 4″-substituted benzoates (IIn a-e ), the ester groups are inverted. While in the second, 4-(4′-substituted phenylazo) phenyl 4″-alkoxy benzoates (IIIn a-e ), two modifications were made, inversion of the COO group, and exchange of the two wing substituents Keywords: phenylazo phenyl benzoates; mesophase behaviour; inversion of ester group; polarisability anisotropy

Polarity and steric effect of the lateral substituent on the mesophase behaviour of some newly prepared liquid crystals

“…It should be mentioned here that only planar conformations are possible since all the investigated derivatives (In a-k ) are mesomorphic, while a non-planar conformation definitely destroys any possible mesophases. In this respect, linear substitution, in position-4, will lead to one conformation and, consequently, the stability of the mesophases (T C ) was found [21] to correlate with the polarisability anisotropy, Δα X , of the 4-substituent, X. …”

“…Since almost identical infrared absorption spectra were observed for all the members of the eight homologous series (In a -In h ), the absorption bands of compounds in series I12 a-h , with their assignments, are given as representative examples in Table 2. Corresponding absorption bands [21] for the 4-substituted isomers, I8 i-k , are given before. [21] Investigation of the infrared absorption bands of the ester carbonyl group revealed that neither the Scheme 1.…”

“…Corresponding absorption bands [21] for the 4-substituted isomers, I8 i-k , are given before. [21] Investigation of the infrared absorption bands of the ester carbonyl group revealed that neither the Scheme 1. Steps for preparation of compounds In a-h .…”

“…[21] The effect of increasing alkoxy-chain length on the mesophase behaviour of compounds in series In a-h is depicted graphically in Figure 1 and those of their isomers In i-k , for comparison, are depicted with their corresponding 2-(and 3-) substituted isomers in Figure 2. It will be seen from Table 1 and Figures 1 and 2 that, when comparison is made between the three differently substituted positional isomers (i.e.…”

“…On the other hand, azo-based liquid crystals have attracted considerable attention because of their potential application in optical storage. [21] Therefore, the azo-ester mesogens were chosen because of their remarkable thermal stability; in addition, they are attractive with regard to their photo-induced effect. [21][22][23][24][25][26][27] The present work aims to prepare some three-ring azo/ester compounds, In a-h , possessing two unsymmetrical wing groups.…”