Abdelgawad A. Fahmi

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

Four pyridine-based derivatives (I 10, I 12, I 14, and I 18) with molecular formula CnH2n+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 (CmH2m+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.

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

Phase diagrams of the binary mixtures of the title compounds were constructed to investigate the effect of different polar substituents, on the pyridine-based derivatives, as well as the alkoxy-chain length, on the acids, on the extent and stability of the supramolecular liquid crystal phases induced by intermolecular hydrogen bonding. None of the newly prepared pyridine-based derivatives is mesomorphic, but the hydrogen-bonded complexes are. The mixtures prepared were characterized for their mesophase behavior by differential scanning calorimetry (DSC) and polarized light microscopy (PLM). Five pyridine-based derivatives (I a-e), with molecular formula X-C6H4COOC6H4-N=N-C5H4N, were prepared that differ from each other by the substituent X. The latter varies between CH3O, CH3, H, Br, and NO2. Four 4-alkoxybenzoic acids (CnH2n+1OC6H4COOH, II 10–II 18) were used; the number of carbons (n) of their alkoxy groups varies between 10, 12, 14, and 18. Smectic C mesophase is induced in most of the binary mixtures investigated.

Novel hydrogen-bonded angular supramolecular liquid crystals

Two types of new pyridine-based azo dyes, namely, 4-(3′-pyridylazo)-4″-alkoxybenzoates with various alkoxy chains and 4-(3′-pyridylazo)-4″-substituted benzoates bearing groups of different polarities, were prepared and investigated for their mesophase behaviour by differential scanning calorimetry and polarised light microscopy. All homologues of the former group were found to be mesomorphic, while all derivatives of the latter are non-mesomorphic. The structures of compounds prepared were confirmed via elemental analyses, IR, 1H-NMR and mass spectra. Four types of angular 1:1 hydrogen-bonded supramolecular complexes were prepared via hydrogen-bond formation between each of the pyridine-based derivatives and two types of benzoic acids, namely, 4-alkoxy and 4-substituted benzoic acids. The latter bear the polar groups (CH3O, CH3, Br and NO2) in addition to benzoic acid. Complexes were similarly investigated and it was found that three of them are mesomorphic while the fourth is not. The formation of 1:1 hydrogen-bonded supramolecular liquid crystals complexes was confirmed by FTIR spectroscopy.

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

Five homologous series of 4-substituted phenyl 4′-(4″-alkoxy phenylazo) benzoates (Ina–−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 CH3O, CH3, H, Br, and CN groups. Compounds prepared were characterised by infrared, mass, and H1-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, I8a-e, that showed a nematic phase, the nematic-to-isotropic transition temperatures (TN–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 (IIna–e), the ester groups are inverted. While in the second, 4-(4′-substituted phenylazo) phenyl 4″-alkoxy benzoates (IIIna–e), two modifications were made, inversion of the COO group, and exchange of the two wing substituents

Design and Synthesis of Some New Pyrazolo[1,5-a]pyrimidines, Pyrazolo[5,1-c]triazines, Pyrazolo[3,4-d]pyridazines, and Isoxazolo[3,4-d]pyridazines Containing the Pyrazole Moiety

Abstract GRAPHICAL ABSTRACT

The effect of lateral methyl substitution on the mesophase behaviour of aryl 4-alkoxyphenylazo benzoates

In order to investigate the effect of lateral methyl substitution on the mesophase behaviour of the previously investigated laterally neat 4-substituted phenyl 4ʹ-(4ʺ-alkoxyphenylazo) benzoates (Ina–e), five homologous series of 4-substituted phenyl 4ʹ-(3ʺ-methyl-4ʺ-alkoxyphenylazo) benzoates (IIna–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 between the electron-donating CH3O and CH3 groups, and the electron-withdrawing Br and CN groups, in addition to the unsubstituted analogue. Two other isomeric series (IIIna–e) were also prepared in which the methyl group is laterally attached, this time, to position 2 of the same ring, aiming to investigate the effect of the different orientation of the methyl groups on the mesophase behaviour of the produced isomers. The results were discussed in terms of steric, mesomeric, and polarisability effects.

Effect of exchange of terminal substituents on the mesophase behaviour of some azo/ester compounds

Six homologous series of 4-(4′-alkoxy phenylazo) phenyl 4″-substituted benzoates (Ina-f) were prepared in which, within each homologous series, the length of the terminal alkoxy chain varies between 6, 8, 10, and 12 carbons, while the other terminal substituent, X, is a polar group that alternatively changed from CH3O, CH3, H, Br, NO2 and CN. Compounds prepared were characterised by spectroscopic methods, and their mesophase behaviour investigated by differential scanning calorimetry (DSC) and polarised optical microscopy (POM). The results were discussed in terms of mesomeric and polarisability effects. In each group of compounds, bearing the same alkoxy substituent, the nematic-to-isotropic transition temperatures (TC) were successfully correlated with the polarisability anisotropy of bonds to the substituent X. A comparative study was made between the investigated compounds and their previously prepared isomers, namely, 4-(4′-substituted phenylazo) phenyl 4″-alkoxybenzoates (IIna-f) in which the two terminal (alkoxy- and X) groups are exchanged.

Effect of lateral substitution on supramolecular liquid crystal associates induced by hydrogen-bonding interactions between 4-(4′-pyridylazo-3-methylphenyl)-4′′-alkoxy benzoates and 4-substituted benzoic acids

Five laterally methyl-substituted pyridine-based derivatives of the title compounds (I 8–I 16), with molecular formula 4-CnH2n+1O-C6H4COOC6H3(3-CH3)-N=N-C5H4N were prepared and their molecular formulae elucidated via elemental analyses, infrared, nuclear magnetic resonance and mass spectra. The number of carbon atoms in the alkoxy chain (n) varies between 8, 10, 12, 14, and 16 carbons. The newly prepared pyridine-based derivatives were investigated for their mesophase behaviour by differential scanning calorimetry and polarised optical microscopy; most of them were found to possess monotropic smectic C (SmC) mesophase. Two groups (A and B) of the 1:1 hydrogen-bonded associates, formed between each of the derivatives I 8– I 16 and two types of 4-substituted benzoic acids (II), were prepared and similarly characterised to investigate the effect of lateral methyl substitution on the central phenylene ring, as well as terminal polar substituents and alkoxy-chain length on the stability of the mesophases induced by intermolecular hydrogen bonding. In Group A complexes, mesomorphic 4-alkoxy benzoic acids, that carry the terminal n-alkoxy group of varying chain length, were used. The other series of complexes (Group B) is composed from the same pyridine-based derivatives and each of the non-mesomorphic 4-substituted benzoic acids that carries small compact polar groups, varying between CH3O, CH3, H, Cl, Br, and CN. All complexes prepared were investigated for their mesophase behaviour by differential scanning calorimetry and polarised optical microscopy and found to be purely smectogenic, possessing SmC as the only mesophase observed. The formation of the hydrogen-bonded complexes was confirmed by constructing their binary phase diagrams, which cover the whole range of concentration of the two complements.

Effect of lateral substitution of different polarity on the mesophase behaviour in pure and mixed states of 4-(4′-substituted phenylazo)-2-substituted phenyl-4″-alkoxy benzoates

Two groups of 4-(4′-substituted phenylazo)-2-phenyl-4″-alkoxy benzoates were prepared and their mesophase formation and stability investigated. Each group (I n a-e and II n a-e) constituted five homologous series differing in their terminal substituent, CH3O, CH3, H, Cl or NO2. Within each series the number of carbons in the terminal alkoxy chain was 8, 10, 12, 14 or 16. The two main groups of compounds (I and II) differed in the polarity of the lateral substitutent attached to the central benzene ring. Group I had a chlorine atom adjacent to the ester group, replaced in Group II by the weakly electron-donating methyl group. Binary phase diagrams were constructed in which the two components were terminally-substituted analogues from within each group. The study was designed to investigate the effect of a lateral chloro or methyl group on the mesomorphic properties of the resulting derivatives, in pure or mixed states. The mesophase behaviour of the compounds themselves and their binary mixtures was investigated by differential scanning calorimetry and polarised light microscopy. The nematic phase was exhibited in most of the compounds, including their binary mixtures.

Supramolecular Liquid Crystals Induced by Hydrogen-Bonding Interactions Between Non-Mesomorphic Compounds. I. 4-(4′-Pyridylazophenyl)-4″-Substituted Benzoates and 4-Substituted Benzoic Acids

Equimolar binary mixtures of the title compounds were prepared to investigate the effect of different polar substituents, either on the pyridine-based derivatives or on the acid component, on the extent and stability of the supramolecular liquid crystal phases induced by intermolecular hydrogen bonding. None of the pyridine-based derivative or the acid complement is mesomorphic, but the hydrogen-bonded complexes are. The mixtures prepared were characterized for their mesophase behavior by differential scanning calorimetry, DSC, and polarized light microscopy, PLM. Five azo pyridine-based derivatives (I a–e), with molecular formula X-C6H4COOC6H4-N=N-C5H4N, were prepared that differ from each other by the substituent X. The latter varies between CH3O, CH3, H, Br, and NO2. Six 4-substitutedbenzoic acids (Y-C6H4COOH, II a-f) were used; the substituent Y varies between CH3O, CH3, H, Br, CN, and NO2. Nematic mesophase is induced in most of the binary mixtures investigated.