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

ABSTRACT Two homologous series of 4-alkoxyphenylazo 4ʹ-(2ʺ- (and 3ʺ-) methyl-) 4ʹ-substituted benzoates (IIna–f and IIIna–f, six series each) were prepared and investigated. Within each series, the length of the terminal alkoxy group varies among 6, 8, 10 and 12 carbons, while the other terminal substituent, X, is a polar group that alternatively changes between the electron-donating CH3O, CH3, and the electron-withdrawing Br, NO2 and CN groups, in addition to the un-substituted analogue, X = H, aiming to investigate the effect of the different orientations of the methyl groups substituted on the central benzene ring, on the mesophase behaviour. The mesomorphic properties were discussed in terms of steric and polarisability effects. The mesophase stability was correlated with the polarisability anisotropy of bonds to the terminal substituent X. Comparative studies were made between the prepared isomers with each other and with the previously investigated laterally neat analogues 4-(4ʹ-alkoxyphenylazo) phenyl 4ʺ-substituted benzoates (Ina–f). GRAPHICAL ABSTRACT


Introduction
From the liquid crystalline point of view, the nature of the central linkage in a mesogen is of great importance. Linking units containing multiple bonds that maintain the rigidity and linearity of a mesomorphic structure are most satisfactory in promoting mesophase stability. The phenylazo group represents suitable fragments to design and synthesise new structures, giving stable mesophases often with very interesting polymorphism; in addition, azo compounds are thermally stable and are attractive from the point of view of studying photoinduced effects. [1] The ester linkage, on the other hand, contains no multiple bonds in the chain linking the two benzene rings; however, conjugative interactions within the ester moiety and the rings do lead to some double bond character and stiffer structure than might be expected. Aromatic esters are again known to be thermally stable and relatively resistant to CONTACT Magdi M. Naoum magdinaoum@yahoo.co.uk Supplemental data for this article can be accessed here.
hydrolysis. In addition, aromatic esters are, in fact, fairly planar systems and quite mesomorphic. [2][3][4][5][6] Based on their outstanding characteristics, the azo/ester groups in the mesogenic core are often used from the point of view of their interesting optical properties such as optical switching, optical image storage, optical display and optical computing. [7][8][9][10][11][12] Numerous studies have been carried out to investigate the effect of changes in molecular frameworks on the incidence and stability of liquid crystal phases in azo/ester containing compounds. [13][14][15][16][17][18][19][20][21][22][23][24][25][26] It has been realised that a terminal substituent that increases the length-to-breadth ratio of the molecule is superior to an H atom, and that groups such as cyano or alkoxy are more favourable than others in promoting high mesophase-isotropic transition temperatures. The stability of a mesophase should be greater, the greater the lateral adhesion of linear molecules, which in turn would be augmented by an increase of the polarity and/or polarisability of the mesogenic portion of the molecule. In this respect, terminal substituents would affect the polarisability of the aromatic ring to which they are attached. This is attributed to increased intermolecular attractions with increasing polarity and polarisability of the mesogenic group. [27,28] Generally, the liquid crystalline behaviour of a mesogen is strongly affected when a lateral substituent is appended to the mesogenic core. It widens the core effectively thus increasing intermolecular separation leading to a reduction in the lateral association and consequently the mesophase stability is reduced. [29,30] In a previous work in our laboratory, compounds In a-f were prepared and characterised for their mesophase behaviour. [31] All the compounds investigated were found to possess high transition temperature indicating strong intermolecular association either in the solid or in mesophase. Varying the electronic nature of the polar substituent X resulted in extremes in the electronic interactions between the substituent and the mesogenic part via the ester carbonyl group. Such differences have led to a significant variation in the mesophase behaviour of the compound. On the other hand, extension of the terminal n-alkoxy chain gives rise to a decrease in the mesophase-isotropic transition temperatures (T C ), suggesting that its effect becomes dominant over polarity variation.
The present work aims to prepare some laterally methyl substituted three-ring azo/ester compounds, IIn a-f and IIIn a-f , possessing two unsymmetrical terminal groups. The first terminal, attached to the phenyl azo moiety, is an alkoxy group with chain length that varies between 6 and 12 carbons. The second terminal group, attached to the benzoic acid moiety, is a polar substituent that changes between the electron-donating (CH 3 O and CH 3 ) and electronwithdrawing (Br, CN and NO 2 ) substituents, including the terminally un-substituted analogues. Methyl group is attached to positions 2-or 3-of the central ring. Lateral substituent is chosen to be of different orientation aiming to investigate their steric and polarisability effects that influence the lateral association and, consequently, the stability of the solid and mesophases. The effect of orientation of lateral methyl substituent, as well as the length of the alkoxy chain and the polarity of the other terminal group, on the mesophase behaviour of the prepared compounds (IIn a-f and IIIn a-f ) will be investigated and compared with their previously investigated [31] laterally neat isomers, In a-f .
In a , X = CH 3 O, In b , X = CH 3 , In c , X = H, In d , X = Br In e , X = NO 2 , In f X = CN.

Materials and technique
Chemicals used are of pure grades and purchased from the following companies: BDH, Poole, England; MP Biomedicals Inc., Illkirch, France, Aldrich, Wisconsin, USA; E. Merck, Darmstadt, Germany; and Fluka, Buchs, Switzerland. 1 H-MNR spectra of the representative compounds were measured with a Varian EM 350L, FTIR spectra with a Perkin-Elmer B25 spectrophotometer, mass spectra with GCMS-QP1000EX Mass spectrometer (Shimadtzu, Japan) and microanalyses with a Perkin-Elmer Series II 2400-CHN analyser (Shelton, CT USA).
Calorimetric measurements were carried out using a TA Instruments Co. Q200 Differential Scanning Calorimeter (DSC; USA). The DSC was calibrated using the melting temperature and enthalpy of indium and lead. DSC investigation was carried out for small samples (2-3 mg) placed in sealed aluminium pans. All measurements were achieved at a heating rate of 10°C/min in inert atmosphere of nitrogen gas (10 ml/ min) and all transition recorded from the second heating scan.
Transition temperatures were checked and type of mesophase was identified for all compounds prepared with a standard polarised light microscope PLM (Wild, Germany) attached to a home-made hot-stage.
The purity of the prepared samples was checked with thin-layer chromatography (TLC) using TLC sheets coated with silica gel (E Merck), and CH 2 Cl 2 / CH 3 OH (9:1) as eluent, whereby only one spot was detected by a UV-lamp.

Synthesis
All newly obtained compounds (IIn a-f and IIIn a-f ) were synthesised according to the following scheme: 2.2.1. Synthesis of 4-n-alkoxyphenylazo 2ʹ-(or 3ʹ-) methyl phenol (An) One molar equivalent of the 4-alkoxy aniline (n = 6, 8, 10 and 12 carbons) in ice-cold dilute hydrochloric acid is diazotised with cold sodium nitrite solution and then added slowly to a cold solution of cresol/sodium hydroxide (1:1). The solid product obtained was filtered and re-crystallised twice from glacial acetic acid to give TLC-pure compounds.
2.2.2. Synthesis of 4-(4ʹ-alkoxy phenylazo)-2ʹ-(or 3ʹ-) methyl phenyl 4,-substituted benzoates, (IIn a-f and IIIn a-f ) Molar equivalents of 4-substituted benzoic acid (X = CH 3 O, CH 3 , H, Br, NO 2 and CN) and the phenol An were dissolved in dry methylene chloride. To the resulting solution, 1.2 molar equivalents of dicyclohexylcarbodiimide (DCC) and few crystals of 4-(dimethylamino)-pyridine (DMAP), as catalyst, were added and the solution left to stand for 72 hours at room temperature with stirring. The solid separated was then filtered off and the solution evaporated. The solid residue obtained was re-crystallised twice from ethanol. The purity of the products obtained was checked by TLC using CH 2 CL 2 / CH 3 OH (9:1) as eluent; one spot was observed.

Confirmation of molecular structure
The molecular structures of the compounds were confirmed by FT-IR, 1  analyses. The main characteristic peaks of the FT-IR, 1 H-NMR spectra, mass spectra and elemental analyses of representative compounds of series IIn a-f and IIIn a-f are given as supplemental information Tables S1-S5. As observed before, [31] the nearly identical infrared absorption spectra for all individual series investigated revealed that neither the length of the alkoxy chain, C6-C12, in any homologous series, nor the substituent X in any group of derivatives bearing the same alkoxy chain, has a significant effect of the position of the infrared absorption bands. 1 H-NMR data showed the expected integrated aliphatic-toaromatic proton ratios in all compounds investigated. Mass spectra indicated exact molecular masses for the whole molecular structures as well as its expected fragmentations. These results confirm that the prepared compounds are consistent with the structures assigned.

Mesophase behaviour of compounds investigated
Phase transition temperatures and associated enthalpies (ΔH) of series IIn a-f and IIIn a-f are collected in Tables 1 and 2. DSC thermograms of representative compounds obtained on the second heating and the second cooling, under N 2 atmosphere are shown in Figure 1. The DSC observation reveals that all the investigated compounds are enantiotropically nematogenic with a schlieren or marble textures ( Figure 2).   Plots of the thermal transitions against the alkoxy chain length (n) during second heating scans are shown in Figures 3 and 4, respectively. As can be seen from these figures, there is, as usual, no systematic change of melting points with increase of n. As for their mesophase behaviour, independent of the polarity of the terminal substituent X, or the orientation of the lateral methyl substituent, or the length of the alkoxy chain, all derivatives possess relatively wide nematic temperature range. On the other hand, comparison of nematic-transition enthalpies of the laterally methyl substituted derivatives, IIn a-f and IIIn a-f as given in Table 1, with their previously reported unsubstituted analogues (In a-f ), [31] revealed that, irrespective of the alkoxy chain length (n) or the position and orientation of the lateral methyl group, the former groups of derivatives exhibit relatively lower values than that those of the latter group compounds. This may be rationalised in terms of the reduction of length-to-breadth ratio and/or the increased molecular biaxiality, due to lateral methyl substitution, which accordingly reduces the transition temperatures and enthalpies. Similar conclusion could be arrived at for liquid crystal dimers containing either branched terminal chains. [32][33][34] In order to investigate the effect of introducing a lateral methyl group in positions 2-and 3-of the central benzene ring, on the mesophase behaviour of series In a-f , correlations were individually made between the laterally substituted compounds (IIn a-f and IIIn a-f ) and their corresponding laterally neat analogues (In a-f ) at constant chain length (n), Figures 5   and 6, respectively. As can be seen from Figure 5, the introduction of a methyl group into position 2-is accompanied with a large decrease in T C for lower n, the slopes are 0.087 and 0.176 with poor correlation coefficients (r 2 ) = 0.094 and 0.263 for n = 6 and 8, respectively. Increasing of n diminishes the steric effect of the methyl group to give slopes equal to 0.351 and 0.409 with enhanced correlations (r 2 = 0.949 and 0.910) for n = 10 and 12, respectively. With respect to the 3methyl isomers (Figure 6), the slopes increase to 0.422 and 0.544 for n = 6 and 8, and to 0.672 and 0.636 for n = 10 and 12, respectively. These results indicate that the steric effect of the methyl group is much pronounced in position 2-than in position 3-.
In order to investigate similar comparison at constant X, T C values for IIn a-f and IIIn a-f were drawn versus those of their laterally neat analogues in Figures  7 and 8, respectively. Except for the bromo substituted analogues, where r 2 = 0.765, other derivatives, as shown in Figure 7, are well correlated where r 2 values vary between 0.864 (for X = CH 3 O) and 0.994 (for X = H). The slopes of the correlations decrease from 3.053 (for X = H) to 0.455 for X = NO 2 . This indicates that the steric effect of the methyl group in position 2is counteracted by the polar effect of the NO 2 group and the increase of the lateral association with increase of n. So, the resultant effect is not regular with either the increase of polarity of X or the lengthening of the alkoxy chain length n; similar aspects could be deduced from Figure 8 for the 3-methyl substituted isomers. The correlation parameters r 2 are somewhat better in this case which varies between 0.666 for X = NO 2 and In conclusion, substitution with a methyl group in position 2-more sterically affects the nematic stability than in position 3-. This can be further checked if correlations are made by correlating the nematic stability (T C ) of group III members against their isomers in group II, again once at constant n and another at constant X and represented graphically in Figures 9  and 10, respectively. It should be noted here that a slope greater the one indicates that the nematic stability of group III members is greater than that of II and vice versa. On the other hand, the correlation parameter r 2 is a measure of parallelism of the lateral effect in both cases. Thus Figure 9 shows that for short alkoxy chain (n = 6 and 8) the correlations are poor, indicating strong steric effect. For longer chain length n = 10 and 12, r 2 increases to 0.958 and 0.828, respectively. This means that the alkoxy chain length is determining factor, nevertheless, the polarity of the substituent X is also effective. Figure 10 shows that, independent of X, the correlations are nearly linear with r 2 > 0.9. On the other hand the slope decreases in the order: This indicates that the effect of orientation of the lateral methyl group is dependent on the polarity of the terminal substituent X. Thus, in the case of the electron withdrawing substituent Br, NO 2 and CN, introducing the lateral methyl group into position 3-of the central benzene ring enhances the mesophase stability (slope greater than one) compared to those substituted in When substitution is made in the lateral 2-or 3positions, that make, respectively an angle of 60°and 120°with the major longitudinal axis of the molecule, the dipole moment will be differently but slightly affected according to the position of the CH 3 group. In case of the electron-donating (CH 3 ) group, the reverse holds good but with very small differences in the dipole moments. Such variation in the polarity is expected to be accompanied by a parallel behaviour in the stability of the mesophase.
Generally, the mesophase behaviour of a calamitic mesogen is a direct effect of molecular-molecular interactions that depend mainly on molecular shape, polarisability anisotropy of the polar substituent X, as well as the stereo electronic properties of the whole molecule. Thus, in the present investigated two groups of compounds (IIn a-f and IIIn a-f ), molecular association of the rod-like molecules, and consequently their mesophases stability (T C ) depends on several interfering factors, namely: (1) Lateral adhesion of linear molecules that increases with the increase of the alkoxy-chain length (n). (2) End-to-end interaction that differs according to the polarity of the polar substituent X. (3) The position and orientation of the lateral CH 3 substituent leading to variable steric effects. The steric effects of the lateral methyl substituent may lead to variable conformations as a result of the enforced rotation around the X-phenyl-N-bond; the methyl-substituted derivatives will not be strictly planar but slightly twisted to an extent not to disturb the molecular arrangement within the mesophase. In addition, the methyl   group in position 2 (compared to that substituted in position 3) is likely to enhance this twisting; this is an additional factor in affecting the mesophase stability. [35,36] In order to investigate the resultant effect of all these three factors on the mesophases stability (T C ) of the differently substituted derivatives (In a-f -IIIn a-f ), T C values were found to decrease according to X in the following orders: Alternatively, the mesophase stabilities (T C ) were found to decrease according to the chain length n as: For n = 8

(v) X = NO 2 H > 3-CH 3 > 2-CH 3 (vi) X = CN H > 3-CH 3 > 2-CH 3
For n = 10 For n = 12 From these two types of comparisons, it can be concluded that no one order of decrease in the mesophase stability is applicable in all values of X or n. This behaviour indicates that the length of the alkoxy chain is a major factor that directs the resultant steric effect.

Clearing temperature and polarity anisotropy of the C ar -X of series II and III
The relationship between the stability of the mesophase, expressed as the clearing temperature, T C , and the anisotropy of polarisability (Δα X ) of bonds to small compact terminal substituent (C ar-X) was studied by van der Veen. [37] The relation has the form where T C is measured in kelvin. The term Δα M is the anisotropy of the polarisability for the whole molecular structure except the terminal substituent, X. Equation (1) can be put in the following form [38]: where 'a' is the proportionality constant. The values of Δα X are given elsewhere. [38] Thus, if p T C is plotted against Δα X for any series of liquid crystalline compounds, a straight line is expected, the slope of which equals 'a' and intercept equals 'a .Δα M '. Consequently, Δα M will be given by Δα M = intercept/slope. Applying Equation (2) on series II and III, the p T C values are plotted as a function of Δα X , for series II and III in Figure 11. For the sake of comparison, Δα M values of corresponding laterally neat analogues, groups I are included in Table 3. For series II, as can be seen from  Figure 11, fairly linear dependencies were observed for n = 10 and 12 (r 2 = 0.958 and 0.953, respectively), while at the lower n (6 and 8) r 2 = 0.156 and 0.267, respectively, indicating again that the chain length is a determining factor. For the 3-CH 3 analogues, the correlations are quite less, r 2 decreases from 0.945 for n = 12 to 0.762 for n = 6. Comparison of Δα M values for the two laterally CH 3 substituted derivatives II a-f and III a-f with those of the laterally neat analogues I a-f [31] (Table 3) revealed that the introduction of the lateral methyl group into position 3-of the central benzene ring (compounds III) resulted in a mild dipole-dipole repulsion between molecules that led to relatively smaller increase in the Δα M values compared to those of compounds I. With respect to the isomeric compounds II, Δα M possess still higher values with a tremendous decrease with the increase of n, indicating again that the polarity of X counteracted the effect of the alkoxy-chain length. These conclusions can be confirmed by examining the van der Veen plots in Figure 11. As can be seen from this figure, the absolute values of p T C for isomers II are greater than that of III indicating that the steric effect of the lateral methyl group in position 2-is comparatively lower than that in position 3. Conversely, the higher slopes observed with series III indicate that the effect of the polarisability of the lateral substituent X is more predominant.

Conclusions
Two newly mesogenic of 4-alkoxyphenylazo 4ʹ-(2ʺ-(and 3ʺ-) methyl-) 4ʺ-substituted benzoates (IIn a-f and IIIn a-f ) were synthesised and characterised; this is to investigate the effect of the different orientations of the methyl groups substituted on the central benzene ring, on the mesophase behaviour of the produced isomers. The mesomorphic properties of both series (IIn a-f and IIIn a-f ) were compared with each other and with the previously investigated laterally neat 4-(4ʹ-alkoxyphenylazo) phenyl 4ʺ-substituted benzoates (In a-f ). All homologous of both series are found to be purely nematogenic.
Comparison of the mesophase stability (T C ) of compounds IIn a-f and IIIn a-f individually with their laterally neat analogues (In a-e ) revealed that introduction of the methyl group, independent of its position, is associated with a decrease in T C, resulting from the less effective packing in the crystal lattice. Comparing T C values for compounds IIn a-f with IIIn a-f indicated that substitution with a methyl group in position 2-more sterically affects the nematic stability than in position 3. On the other hand, the length of the alkoxy chain was found to be a major factor in the mesophase stability than the resultant steric effect. The mesophase stability (√T C ) was found to be linearly correlated with the polarisability anisotropy of bonds to the substituent X. The estimated values of the anisotropy of polarisability (Δα M ) of series II are found to be nearly comparable with series I and almost independent on the alkoxy chain length, while higher in series III and declined with the increase of alkoxy chain length.

Disclosure statement
No potential conflict of interest was reported by the authors. Figure 11. Effect of polarisability anisotropy (Δα X ) of the C ar -X bonds on the clearing temperatures of both series IIn a-f and IIIn a-f . Table 3. Regression analyses data for the van der Veen correlations for two Series II6 a-f -II12 a-f and III6 a-f -III12 a-f , Compared to those of compound I6 a-f -I12 a-f [26].  [26]