H.A. Ahmed

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

Eight homologous series of 2- (or 3-) substituted phenyl 4ʹ-(4ʺ-alkoxy phenylazo) benzoates (Ina–h) were prepared in which, within each homologous series, the length of the terminal alkoxy group varies between 6, 8, 10 and 12 carbons, while the other substituent, X, is a laterally attached polar group that alternatively changed from CH3, H, F, Br and CN. Compounds prepared were characterised by infrared and 1H-NMR spectroscopy, and their mesophase behaviour investigated by differential scanning calorimetry and identified by polarised light microscopy. The results were discussed in terms of polarity and steric effects. The stability of the mesophase was correlated once with the dipolar anisotropy of the whole molecule and another with the dipolar anisotropy of the substituent, X. A comparative study was made between the investigated compounds and their previously prepared linear 4-substituted isomers, namely 4-substituted phenyl 4ʹ-(4ʺ-alkoxy phenylazo) benzoates (Ini–k).

Lateral protrusion and mesophase behaviour in pure and mixed states of model compounds of the type 4-(4′-substituted phenylazo)-2-(or 3-)methyl phenyl-4'-alkoxy benzoates

Two groups of the title compounds were prepared and investigated for their mesophase formation and stability. Each group constitutes five homologous series that differ from each other by the polar substituent X (CH3O, CH3, H, Cl, and NO2). Within each homologous series, the number (n) of carbons in the alkoxy chain varies between 8, 10, 12, 14 and 16. The difference between the two groups of compounds lies in the orientation protrusion of the lateral methyl group attached to the central benzene ring. In the first group (Group I) the methyl substituent, introduced into the o-position with respect to the ester group, makes an angle of 60° with the long axis of the molecule. In the other series of compounds (Group II), the orientation angle is 120° as it is introduced into the position-3. All possible binary phase diagrams could be constructed in which the two components are corresponding positional isomers from either group. The study aimed to investigate the effect of inclusion of the lateral methyl group, as well as its spatial orientation, on the mesomorphic properties of the produced derivatives in their pure and mixed states. The compounds prepared in both groups were characterised for their mesophase behaviour by differential scanning calorimetry and polarised light microscopy. The nematic phase is the mesophase observed in most of the compounds prepared and their binary mixtures.

Mesophase stability in binary mixtures of monotropic nematogens

Binary phase diagrams were constructed from laterally substituted methyl azo/ester derivatives, namely 4-(4″-substituted phenylazo)-3-methyl phenyl-4″-alkoxy benzoates (Ina–d). In this group of compound the unsubstituted and chloro-substituted derivatives possess the nematic phase monotropically, while the nitro and methyl analogues are enantiotropically nematogenic. The binary phase diagrams constructed were made once from the monotropic nematogens with each other, and another with the enantiotropic nematogens. In both the cases enantiotropic nematic phase was observed that covers a wide range of composition. The mesophase behaviour of all binary mixtures was investigated by differential scanning calorimetry (DSC) and polarised light microscopy (PLM). The nematic phase was exhibited in all binary mixtures. Independent of the alkoxy chain length, the entropy change, ΔSN–I of the N–I transition of pure components was found to vary irregularly with the anisotropy of polarisability (∆αX) of the polar substituent, X.

Mesophase behaviour of azobenzene-based angular supramolecular hydrogen-bonded liquid crystals

ABSTRACT New types of angular 1:1 hydrogen-bonded supramolecular complexes via hydrogen-bond formation between 4-alkoxyphenylazo benzoic acids (In) and 4-(3ʹ-pyridylazo)-4ʹʹ-alkoxybenzoates (IIm) with various alkoxy chains (from 6 to 16 carbons) were prepared and investigated for their mesophase behaviour by differential scanning calorimetry (DSC) and polarised-light microscopy (PLM). All prepared homologues were found to be dimorphic, possessing smectic C and nematic mesophases. The formation of 1:1 hydrogen-bonded supramolecular liquid crystals (LCs) complexes was confirmed by FTIR and UVʹvisible (UVʹvis) absorption spectroscopy. The study revealed that nematic transition enhancement (ΔT) decreases with the increase of the alkoxy chain length on the base complement, while it increases with the increase of the chain attached to the acid complement of the complex, that is the stability of the nematic phase is more dependent on the length of the acid component. Graphical Abstract

Liquid crystalline behaviour of model compounds di-laterally substituted with different polar groups

Five groups of laterally di-substituted three-ring compounds, 4-(3′-fluoro phenylazo)-2-(or 3-) substituted phenyl-4′-alkoxybenzoates, were prepared and investigated for their mesophase formation and stability. Each group of compounds, which have in common a lateral fluorine atom on the first terminal ring, differs from each other by a second lateral group substituted on the central ring. The central group varies between the electron-donating (CH3) and electron-withdrawing (Cl and CN) groups, including the hydrogen (H). In each group of homologues, the number of carbons in the other terminal, alkoxy group varies between eight and 16 carbons. The study investigated the effect of the di-lateral substitution, as well as the spatial orientation of the central substituent, on the mesomorphic properties of the produced derivatives either in their pure or mixed states. Molecular formulae of the compounds prepared were confirmed via elemental analyses, IR, NMR and mass spectroscopy. The compounds prepared were characterised for their mesophase behaviour by differential scanning calorimetry (DSC), and the mesophases were identified by polarised light microscopy (PLM) and confirmed by miscibility with a standard smectic C (SmC) compound.

Mesophase behaviour of 1:1 mixtures of 4-n-alkoxyphenylazo benzoic acids bearing terminal alkoxy groups of different chain lengths

ABSTRACT The thermal and mesophase behaviour of all possible 1:1 mixtures (In/Im) formed from any two homologues of 4-n-alkoxyphenylazo benzoic acids bearing different alkoxy chain lengths (n and m vary between 6 and 16 carbons), were investigated by differential scanning calorimetry (DSC) and phases identified by polarised light microscopy (PLM). Phase transitions were investigated as a function of the average alkoxy chain length (=(n + m)/2) of the mixed acids. A comparison, made between the present series and previously investigated simpler analogues, 4-n-alkoxy benzoic acids, revealed that increasing the length of the mesogenic core increased the stabilities of both the smectic C and nematic phases. GRAPHICAL ABSTRACT

Effect of alkoxy-chain length proportionation on the mesophase behaviour of terminally di-substituted phenylazo phenyl benzoates

A series of 4-alkoxyphenylazo-4′-phenyl-4′′-alkoxybenzoates bearing two terminal alkoxy groups (I m + n), where m and n denote the number of carbon atoms in the alkoxy groups attached to the phenylazo and benzoate moieties, respectively, was synthesised, and the molecular structures were established via elemental analyses, IR, NMR and mass spectroscopy. First, the thermal transitions and mesophase characteristics of any two isomeric molecules, having the same molecular length but with differently proportionated alkoxy groups, were investigated by differential scanning calorimetry and polarised-light microscopy. All compounds investigated proved to have a wide smectic C and nematic liquid crystalline ranges, except for the lower homologous (I 6 + 8 and I 8 + 6) where they are purely nematogenic. Mesophases were confirmed by the miscibility method, using dodecyloxybenzoic acid as the mesophase reference. Second, the mesophase behaviours of binary mixtures of isomeric compounds, bearing the same total alkoxy-chain length (m + n) but differ in the individual values of m and n, were studied and discussed. It was found that the smectic C and nematic phases behaved ideally in their binary mixture with the 4-alkoxy benzoic acid, independent of either n or m.

Mesophase behaviour of laterally di-fluoro-substituted four-ring compounds

Four new groups of the di-fluoro-substituted 4-(2′-(or 3′)-fluoro phenylazo)-2-(or 3-) fluoro phenyl-4″-alkoxyphenylazo benzoates (In–IVn) were prepared and investigated for their mesophase behaviour. An alkoxy group of variable chain length (n = 6, 10 and 14 carbons) is attached to the terminal phenylazo benzoate moiety, and two lateral fluoro substituents are attached individually with different orientations to the other two adjacent rings. The molecular structures of the prepared compounds were confirmed by Fourier transform infrared spectroscopy and 1H NMR spectroscopy. The study aims to investigate the steric effect of the spatial orientation and relative positions of the two lateral fluorine atoms on the mesomorphic properties in their pure states. The mesophase behaviour was investigated via differential scanning calorimetry and mesophases were identified by polarised light microscopy. The investigation shows that these compounds exhibit high enantiotropic mesophases (SmC and N) and broad mesophase temperature range. The type and stability of the mesophase depends on the length of the terminal alkoxy chain and the position the two fluoro substituents. A comparison between these investigated compounds with their corresponding three-ring analogues was discussed.

Effect of the Relative Orientation of the Two Fluoro-Substituents on the Mesophase Behavior of Phenylazophenyl Benzoates

Four novel types of laterally di-fluoro substituted 4-(2′-(or 3′-) fluoro phenylazo)-2- (or 3-) fluoro phenyl-4″-alkoxybenzoates were prepared and investigated for their mesophase behavior. In order to deduce the most probable conformation for each of the positional isomers investigated, the dipole moments for each of the four novel types have been determined experimentally in benzene at 30°C and compared with those theoretically calculated using molecular modeling program. Probable conformations deduced were found to vary according to the relative orientations of the two fluorine atoms. The results were used to correlate the mesophase behavior, in pure and mixed derivatives, with the conformation deduced for each series. The study aims to investigate the effect of the spatial orientation of the two lateral fluoro-substituents on the mesomorphic properties in their pure and mixed states.

Wide nematic phases induced by hydrogen-bonding

ABSTRACT New series of hydrogen-bonded supramolecular complexes, In/IIm, were prepared and investigated for their mesophase behaviour. The complexes were prepared through hydrogen-bonding between equimolar amounts of 4-alkoxyphenylazo-benzoic acids as the proton donors and non-mesomorphic 4-alkoxyphenylazo pyridines as the proton acceptors. The length of the terminal alkoxy chains was varied systematically on both terminals of the supramolecular complexes. The formation of 1:1 hydrogen-bonded complexes was confirmed by differential scanning calorimetry (DSC) and FTIR spectroscopy. All of new complexes were characterised by DSC and polarised light microscopy (PLM). It was found that the prepared complexes are dimorphic, possessing smectic C (SmC) and nematic (N) phases with relatively wide ranges. A comparison was made between the present series of complexes and the previously investigated isomers of the simple 4-alkoxybenzoic acids with the 4-(4ʹ-pyridylazophenyl)-4ʹʹ-alkoxybenzoates, revealed the stability of the nematic phase is dependent on the length of the acid component. GRAPHICAL ABSTRACT