Magdi M. Naoum

Effect of molecular structure on the phase behaviour of some liquid crystalline compounds and their binary mixtures II. 4-Hexadecyloxyphenyl arylates and aryl 4-hexadecyloxy benzoates

4-Hexadecyloxyphenyl-4-substituted benzoates, Ia-d, and 4-substituted phenyl-4- hexadecyloxy benzoates, IIa-d, with the substituents CH3O, Cl, CN, and NO2, respectively, were prepared and characterized by infrared spectroscopy and the apparent solution dipole moment measured using cyclohexane as a solvent. Smectic A mesophase stability was investigated by differential scanning calorimetry and polarized light microscopy. The effects of structural changes on phase transitions in these two series of compounds are discussed in terms of dipole and mesomeric effects.

Effect of molecular structure on the phase behaviour of some liquid crystalline compounds and their binary mixtures IV. Dependence of Tc on the anisotropy of the aryl-X of polarizability bond

Transition temperatures of binary mixtures of the two series, 4-hexadecyloxyphenyl-4- substituted benzoates (Ia-d) and 4-substituted phenyl-4-hexadecyloxybenzoates (IIa-d), have been determined on cooling and the phase diagrams compared with those previously obtained on heating. Tc values for the monotropes and the non-LC analogues have been evaluated by extrapolation. Examination of both series revealed that the square root of Tc is related to the polarizability anisotropy, Delta alphaX, of the CAr-X bond, except for the methoxy derivatives.

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.

Effect of molecular structure on the phase behaviour of some liquid crystalline compounds and their mixtures XIII. 4-(4-Substituted phenylazo)phenyl 4-alkoxybenzoates

Five homologous series of 4-(4-substituted phenylazo)phenyl 4-alkoxybenzoates (Ia–e) were prepared in which the substituent (X) was taken from CH3O, CH3, Cl, NO2, and CN, while, within each homologous series, the length of the terminal alkoxy group varied between 6 and 16 carbon atoms. Compounds prepared were characterized by infrared spectroscopy, and their mesophase behaviour investigated by differential scanning calorimetry and polarizing optical microscopy. The results are discussed in terms of mesomeric and polarizability effects. In each group of compounds bearing the same alkoxy group, the mesophase – isotropic transition temperatures (T c) were successfully correlated with the polarizability anisotropy of bonds to the substituent X.

Effect of molecular structure on the phase behaviour of some liquid crystalline compounds and their binary mixtures III. Binary mixtures of alkoxyphenyl arylates and arylalkoxybenzoates

4-Substituted phenyl-4-hexadecyloxybenzoates and 4-hexadecyloxyphenyl-4-substituted benzoates were prepared and thermally characterized by differential scanning calorimetry. Transition temperatures were identified by polarized-light microscopy. Substituents were chosen to cover a wide range of electronic and dipolar characteristics. Binary mixtures of different components were prepared and similarly characterized to construct their phase diagrams. In mixtures of the 4-CN and 4-NO2 analogues, enhanced SmA mesophases were found when either of these esters was mixed with one of the methoxy analogues. A weak molecular complex is proposed to account for this non-ideal solution behaviour. Linear or slightly enhanced mixture behaviours were found for the nitro or cyano derivatives when mixed with each other. The results are interpreted in terms of electronic, dipolar, and molecular interactions.

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

Effect of steric factors on mesomorphic stability I. 4‐(4‐Substituted phenylazo)‐1‐naphthyl 4‐alkoxybenzoates

Four homologous series belonging to the family of 4‐(4‐substituted phenylazo)‐1‐naphthyl 4‐alkoxybenzoates (I a–d) (C n H2 n +1O–C6H4–COO–C10H6–N = N–C6H4–X) were prepared in which the 4‐ substituent (X) was varied between CH3O, CH3, Cl and NO2; while, within each homologous series, the number of carbon atoms, n, was varied between 8 and 14. The compounds were characterized by infrared spectroscopy, 1H NMR spectroscopy and elemental analysis. Their mesophase behaviour was investigated by differential scanning calorimetry and polarizing‐optical microscopy. The results are discussed in terms of mesomeric, polarizability, and steric effects. In each group of compounds bearing the same alkoxy group, attempts were made to correlate the mesophase–isotropic transition temperatures (T c) with the polarizability anisotropy, Δα x , of bonds to the small compact substituent X.

Effect of molecular structure on the phase behaviour of some liquid crystalline compounds and their binary mixtures. I. 4-Hexadecyloxyphenyl-4-carboxybenzoate and 4-carboxyphenyl-4-hexadecyloxybenzoate

4-Hexadecyloxyphenyl-4-carboxybenzoate (A) and 4-carboxyphenyl-4-hexadecyloxy benz- oate (B) were prepared and thermally characterized by polarized light microscopy and differential scanning calorimetry. Both compounds possess high transition temperatures indicating the formation of linear dimer molecules in the solid and liquid phases. The infrared data support the conclusion that the alkoxyphenyl carbonyl unit in A is more polarizable and hence stabilizes the intermolecular packing, thus leading to enhanced mesogenicity. Mixtures of the two compounds were prepared and thermally characterized to construct the phase diagram that showed a simple eutectic behaviour in the solid-mesophase transitions, and a linear mesophase-isotropic transition temperatures. The latter behaviour was attributed to similar electronic factors in both molecules.