Supramolecular Hydrogen-Bonded Liquid Crystals Formed from 4-(4′-Pyridylazophenyl)-4″-alkoxy Benzoates and 4-Substituted Benzoic Acids

Abstract

Four pyridine-based derivatives (I 10, I 12, I 14, and I 18) with molecular formula C_nH_2n+1O-C₆H₄COOC₆H₄-N = N-C₅H₄N, that differ from each other in the number of carbon atoms, n, were prepared and characterized for their mesophase behavior. The number of carbons in the alkoxy chain varies between 10, 12, 14, to 18 atoms, respectively. Liquid crystalline properties were studied for two series of hydrogen-bonded liquid crystal complexes, consisting of the central 4-pyridylazophenyl benzoate and substituted benzoic acids. In the first series of the supramolecular complexes (group A), the lengths of the alkoxy chains on both of the azo pyridine and the benzoic acid components have varied between 10, 12, 14, and 18 carbons. In the second series (group B), the same proton-acceptors, i.e., the pyridyl azo derivatives, were used but with benzoic acids para-substituted with small compact polar groups, X. All of the newly prepared pyridine-based derivatives and their hydrogen-bonded complexes were found to be mesomorphic. The newly prepared pyridine-based derivatives and their supramolecular complexes in both groups were characterized for their mesophase behavior by differential scanning calorimetry, DSC, and polarized light microscopy, PLM. Four 4-alkoxybenzoic acids (C_mH_2m+1OC₆H₄COOH, II 10, II 12, II 14, and II 18) were used in group A series; the number of carbons (m) of their alkoxy groups also varies between 10, 12, 14, and 18, respectively. Substituents on the other group of acids (III _a−f) in group B series vary, respectively, between CH₃O, CH₃, H, Br, CN, and NO₂. Smectic C mesophase is induced in most of the binary mixtures investigated.

Keywords:

• 4-(4′-pyridylazophenyl)-4″-alkoxy benzoates
• 4-substituted benzoic acids
• supramolecular LCs