# Influence of Linking Group Orientation on Mesomorphism of Two Aromatic Ring Mesogens

Authors: L. K. Ong; S. T. Ha
Journal: Journal of Chemistry
 (2013)

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## Abstract

A new homologous series of alkyl 4-{[(4-chlorophenyl)imino]methyl}benzoates were prepared, and all the members are differentiate by the alkoxy chain length,CnH2n+1O, where n=2–7, 9, 11, 13, 15. Their phase transition behaviors and mesophase characteristics were studied by differential scanning calorimetry (DSC) and optical polarizing microscopy techniques. DSC thermograms show direct isotropization and recrystallization during heating and cooling processes, respectively. The crystal phase changed directly to dark area textures (isotropic phase) without displaying any mesophase. The mesomorphic properties of compounds studied are strongly dependent on the orientation of the ester linkage. Reversed ester linkage has caused depression of mesomorphic property in the compounds studied.


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## Body

## 1.  Introduction

Development of liquid crystal science and technology has led to the study of numerous newly synthesized mesogens, in particular, thermotropic liquid crystals [1, 2]. Most thermotropic liquid crystals are rod-like molecules having core system composed of two or more aromatic rings and one or more flexible terminal chains. Molecules containing C=N double are known as Schiff base, a well-known linking group used in connecting two core groups. It is formed by a reaction of  primary amine and aldehyde. Though it provides a stepped core structure, it retains molecular linearity, hence providing higher stability and enabling mesophase formation [3, 4]. Schiff base system has received overwhelming responses ever since the discovery of MBBA which exhibited nematic phase at room temperature [5]. Several studies have been conducted on Schiff base esters owing to their interesting properties and considerable temperature range [6–15].In  recent studies [16, 17], modification on orientation of ester linkage between aromatic units led to interesting variations of mesogenic properties in bent core molecules. Schiff base 4-chlorobenzylidene-4′alkanoyloxyanilines with ester linkage exhibited smectic A and B phases [18]. In this continuation study, we aim to change the orientation of ester linkage on the existing system and to study the influence of reversed ester linkage on mesomorphism of Schiff bases. In spite of the structural similarity, the smectic nature of the series is quite different.


## 2. Materials and Methods

4-Carboxybenzaldehyde, 4-chloroaniline, 4-dimethylaminopyridine (DMAP), N,N-dicyclohexylcarbodiimide (DCC), and straight chain alcohols (CnH2n+1OH, where n=2–7, 9, 11, 13, 15) were obtained commercially. The synthetic route for the title compounds is illustrated in Scheme 1.Scheme 1
Synthetic route of alkyl 4-{[(4-chlorophenyl)imino]methyl}benzoates.Electron ionization mass spectrum (EI-MS) was obtained using a Finnigan MAT95XL-T mass spectrometer operating at 70 eV ionizing energy. FT-IR data were acquired on Perkin Elmer 2000-FTIR spectrophotometer in the frequency range of 4000–400 cm−1 with samples embedded in KBr discs. NMR spectra were recorded in deuterated chloroform by utilizing Bruker Avance 400 MHz NMR spectrometer with TMS as internal standard. The phase transition temperatures were measured by Mettler Toledo DSC823 Differential Scanning Calorimeter (DSC) at a scanning rate of 10°C/min. Liquid crystalline properties were investigated using a Carl Zeiss Polarizing Optical Microscope (POM) attached to a Linkam Hotstage.


### 2.1. Synthesis of Alkyl 4-Formylbenzoates, nCB

A mixture of 4-carboxybenzaldehyde (1 mmol), straight chain alcohol (1 mmol), and DMAP (0.02 mmol) was dissolved in tetrahydrofuran (20 mL) in a round bottom flask. DCC (1 mmol) in 5 mL THF was added dropwise to mixture and stirred at 0°C for an hour and then stirring was continued at room temperature for three hours [12]. The reaction mixture was filtered and excess solvent was removed by evaporation. The white intermediate was recrystallized with hexane until transition temperature remained constant.


### 2.2. Synthesis of Alkyl 4-{[(4-Chlorophenyl)imino]methyl}benzoates, nClCBA

A solution of alkyl 4-formylbenzoate (1 mmol) and 4-chloroaniline (1 mmol) in ethanol (50 mL) was heated under reflux for three hours. The product obtained was repeatedly recrystallized with absolute ethanol whereupon the pure compound was isolated as a yellow solid.The IR, NMR (1H and 13C), and mass spectral data of the representative compound, 11ClCBA, are summarized as follows.11ClCBA: EI-MSm/z (rel. int. %): 413.2(53) [M]+, 260.0(100), 242.0(37), 215.0(38); IR (KBr) υmaxcm−1: 2953, 2919, 2848 (C–H aliphatic), 1714 (C=O), 1625 (C=N), 1284 (C–O); 1H NMR (400 MHz, CDCl3, δppm): 0.90 (t, 3H, CH3), 1.28–1.47 {m, 16H, CH3(CH2)8–}, 1.80 (quint, 2H, –CH2CH2O–), 4.36 (t, 2H, –CH2O–), 7.19 (d, 2H, Ar-H), 7.39 (d, 2H, Ar-H), 7.98 (d, 2H, Ar-H), 8.16 (d, 2H, Ar-H), 8.33 (s, 1H, CH=N); 13C NMR (100 MHz, CDCl3δppm): 14.08 (CH3), 22.66, 26.02, 28.70, 29.27, 29.31, 29.51, 29.58, 31.89 for methylene carbons [CH3(CH2)9–], 65.50 (–CH2O–), 122.37, 128.66, 129.33, 129.96, 132.07, 132.98, 139.62, 149.99 for aromatic carbons, 159.41 (CH=N), 166.08 (C=O).


## 2.1. Synthesis of Alkyl 4-Formylbenzoates, nCB

A mixture of 4-carboxybenzaldehyde (1 mmol), straight chain alcohol (1 mmol), and DMAP (0.02 mmol) was dissolved in tetrahydrofuran (20 mL) in a round bottom flask. DCC (1 mmol) in 5 mL THF was added dropwise to mixture and stirred at 0°C for an hour and then stirring was continued at room temperature for three hours [12]. The reaction mixture was filtered and excess solvent was removed by evaporation. The white intermediate was recrystallized with hexane until transition temperature remained constant.


## 2.2. Synthesis of Alkyl 4-{[(4-Chlorophenyl)imino]methyl}benzoates, nClCBA

A solution of alkyl 4-formylbenzoate (1 mmol) and 4-chloroaniline (1 mmol) in ethanol (50 mL) was heated under reflux for three hours. The product obtained was repeatedly recrystallized with absolute ethanol whereupon the pure compound was isolated as a yellow solid.The IR, NMR (1H and 13C), and mass spectral data of the representative compound, 11ClCBA, are summarized as follows.11ClCBA: EI-MSm/z (rel. int. %): 413.2(53) [M]+, 260.0(100), 242.0(37), 215.0(38); IR (KBr) υmaxcm−1: 2953, 2919, 2848 (C–H aliphatic), 1714 (C=O), 1625 (C=N), 1284 (C–O); 1H NMR (400 MHz, CDCl3, δppm): 0.90 (t, 3H, CH3), 1.28–1.47 {m, 16H, CH3(CH2)8–}, 1.80 (quint, 2H, –CH2CH2O–), 4.36 (t, 2H, –CH2O–), 7.19 (d, 2H, Ar-H), 7.39 (d, 2H, Ar-H), 7.98 (d, 2H, Ar-H), 8.16 (d, 2H, Ar-H), 8.33 (s, 1H, CH=N); 13C NMR (100 MHz, CDCl3δppm): 14.08 (CH3), 22.66, 26.02, 28.70, 29.27, 29.31, 29.51, 29.58, 31.89 for methylene carbons [CH3(CH2)9–], 65.50 (–CH2O–), 122.37, 128.66, 129.33, 129.96, 132.07, 132.98, 139.62, 149.99 for aromatic carbons, 159.41 (CH=N), 166.08 (C=O).


## 3. Results

Spectral characteristics ofnClCBA were studied by using mass spectrometric and spectroscopic methods. The percentage yields and the infrared data are tabulated in Table 1. The purity of the products was analyzed by thin layer chromatography (TLC) on silica gel plates and was chromatographically pure, as indicated by a single spot. The phase transition temperatures and their associated enthalpy changes during heating and cooling cycles are tabulated in Table 2. Representative EI mass spectrum is depicted in Figure 1. DSC thermogram of 11ClCBA upon heating and cooling cycles is shown in Figure 2.Table 1
Percentage yields and IR frequencies of alkyl 4-{[(4-chlorophenyl)imino]methyl}benzoates.
Compound
Yield  (%)
IR (cm−1)
v (C–H aliphatic)
v (C=O)
v (C=N)
v (C–O)
2ClCBA
42
2927, 2852
1719
1625
1282
3ClCBA
39
2963, 2845
1712
1623
1273
4ClCBA
43
2965, 2873
1716
1623
1281
5ClCBA
40
2953, 2954, 2873
1713
1625
1285
6ClCBA
35
2953, 2959, 2856
1711
1623
1281
7ClCBA
44
2953, 2923, 2852
1717
1623
1289
8ClCBA
42
2953, 2917, 2852
1717
1623
1298
9ClCBA
54
2953, 2917, 2848
1715
1623
1294
10ClCBA
62
2953, 2917, 2853
1709
1624
1289
11ClCBA
52
2953, 2917, 2848
1714
1625
1284
12ClCBA
67
2953, 2916, 2852
1708
1625
1281
13ClCBA
43
2953, 2917, 2848
1717
1623
1286
14ClCBA
52
2953, 2917, 2850
1717
1620
1284
15ClCBA
57
2953, 2917, 2848
1718
1623
1289
16ClCBA
64
2953, 2916, 2850
1719
1625
1287Table 2
Transition temperature and associated enthalpy changes of alkyl 4-{[(4-chlorophenyl)imino]methyl}benzoates upon heating and cooling cycles.
Compound
Transition temperature, °C (ΔH, kJ mol−1)Heating *Cooling
2ClCBA
Cr 117.3 (25.5) ICr 82.6 (25.9) I
3ClCBA
Cr 62.5 (22.0) ICr 37.6 (21.8) I
4ClCBA
Cr 57.7 (18.7) ICr 31.8 (17.2) I
5ClCBA
Cr 55.8 (23.6) ICr 20.1 (22.3) I
6ClCBA
Cr 41.4 (22.8) ICr 18.2 (23.2) I
7ClCBA
Cr 46.6 (24.4) ICr 21.3 (25.3) I
8ClCBA
Cr 44.5 (32.5) ICr 14.8 (31.8) I
9ClCBA
Cr 53.3 (41.3) ICr 25.6 (34.1) I
10ClCBA
Cr 53.1 (42.9) ICr 39.7 (45.5) I
11ClCBA
Cr 63.1 (45.8) ICr 42.1 (49.0) I
12ClCBA
Cr 62.5 (48.7) ICr 42.0 (47.9) I
13ClCBA
Cr 66.6 (51.9) ICr 44.0 (55.3) I
14ClCBA
Cr 70.3 (49.1) ICr 47.7 (51.3) I
15ClCBA
Cr 71.4 (45.3) ICr 50.1 (50.3) I
16ClCBA
Cr 75.6 (61.8) ICr 53.6 (72.0) I
Cr: crystal; I: isotropic. *Cooling data is presented initalicsfont.Figure 1
EI mass spectrum of undecyl 4-{[(4-chlorophenyl)imino]methyl}benzoate showing molecular ion peak (M+).Figure 2
DSC thermogram of undecyl 4-{[(4-chlorophenyl)imino]methyl}benzoate during heating and cooling cycles.


## 4. Discussion

### 4.1. Physical Characterization

EI mass spectrum (Figure1) showed the molecular ion peak at m/z=413 which was corresponding to molecular mass of C25H32N1O2Cl suggesting that 11ClCBA was successfully synthesized.Strong absorption bands appeared at 2917 and 2848 cm−1 in FTIR spectrum of 11ClCBA indicating the presence of aliphatic C–H in alkyl chain. A sharp absorption band centered at 1284 cm−1 is attributed to C–O bond. Absorption band emerged at 1625 cm−1 was designated for C=N linking group. This value falls within the frequency range reported for Schiff base linkage [19, 20].In1H NMR spectrum of 11ClCBA, two triplets appeared at δ=0.90 ppm and δ=4.36 ppm, which can be ascribed to the methyl and methylene protons (–OCH2–), respectively. The chemical shifts at δ=1.28–1.47 ppm can be assigned to methylene protons of long alkyl chain {CH3– (CH2)8–}. Four distinct doublets detected at δ=7.19,7.39,7.98, and 8.16 ppm can be assigned to eight aromatic protons. A singlet was observed at the most downfield region; δ=8.33 ppm is due to azomethine proton [13–15]. The molecular structure of 11ClCBA was further ascertained by using 13C NMR spectroscopy. A signal at δ=14.08 ppm is attributed to the methyl carbon, and peaks at δ=22.66–31.89 ppm are contributed to the methylene carbons of long alkyl chain. The signals appeared at δ=122.37–149.99 ppm is belonged to twelve aromatic carbons. Azomethine carbon exhibited its signal at δ=159.41 ppm. The most downfield peak at δ=166.08 ppm is due to the existence of carbonyl carbon of ester group.


### 4.2. Phase Transition Behaviors and Optical Texture Studies

All the members in thenClCBA series were nonmesogenic compounds. In the representative DSC thermogram of 11ClCBA (Figure 2), it showed an endotherm and exotherm, respectively, during both heating and cooling cycles. This observation indicates direct melting of crystal to isotropic liquid phase and vice versa. Under polarizing optical microscopy (POM) observation, The crystal changed to dark region isotropic during heating run. No liquid crystal texture was observed during cooling process. The rest of members showed similar characteristics as those found for 11ClCBA.


### 4.3. Effect of Alkyl Chain Length on Transition Temperatures

A further examination of DSC data also illustrates the evolution of the Cr-I transition with the lengthening of terminal chains. Based on phase transition data in Table2, it can be deduced that the thermal properties were greatly influenced by the length of the terminal chains. Melting temperatures exhibited a typical descending trend as the length of the carbon chain increased from the ethyl to the hexyl. This was attributed to the dilution of the mesogenic core, affected by the increase in the flexibility of the alkyl chain [21]. However, the lengthening of carbon chain from the hexyl to the pentadecyl derivatives led to ascending trend of melting temperatures. This phenomenon can be attributed to the increased intermolecular Van der Waals attraction [22]. A similar phenomenon was also reported for two analogous series of Schiff bases, 2-hydroxy-4-methoxy-4′-alkanoyloxyanilines and 3-methoxy-4-alkanoyloxybenzylidene-4′-alkanoyloxyanilines [15]. The recrystallization points also show similar trend to that observed for melting temperatures.


### 4.4. Influence of Direction of Ester Linkage on Mesomorphism

Mesomorphic properties of two aromatic ring compounds, 4-chlorobenzylidene-4′alkanoyloxyanilines (Series 1) [18] and alkyl 4-{[(4-chlorophenyl)imino]methyl}benzoates (Series 2), are strongly dependent on the direction of the ester linkage. Series 1 having molecular arrangement of alkyl-COO-phenyl exhibited smectic A and B phases. However, Series 2 (present compounds) with the reversed ester linkage were nonmesomorphic compounds. The position of the ester linking group defines the direction of the carbonyl group. The different directions of the carboxyl groups between the phenyl and alkyl units cause significant changes on the dipole moment, which in turn result in a reduction of the polarizability anisotropy and geometric anisotropic of the molecule [23]. A similar phenomenon has been reported for some banana-type mesogens [16]. The smectic properties of the molecules are strongly affected not only by the electrostatic interactions but also by the geometrical circumstances [24]. In addition, Series 1 possessed higher melting point compared to Series 2 having the same number of carbons (n) at the alkyl chain. This suggests that the direction of ester linkage can also change the thermal stability of a compound. The interaction between ester and phenyl groups in Series 1 seems to afford molecule with a higher polarity which would be expected to enhance lateral interactions of a polar nature and hence might lead to a higher clearing point (phase stability) [23].


## 4.1. Physical Characterization

EI mass spectrum (Figure1) showed the molecular ion peak at m/z=413 which was corresponding to molecular mass of C25H32N1O2Cl suggesting that 11ClCBA was successfully synthesized.Strong absorption bands appeared at 2917 and 2848 cm−1 in FTIR spectrum of 11ClCBA indicating the presence of aliphatic C–H in alkyl chain. A sharp absorption band centered at 1284 cm−1 is attributed to C–O bond. Absorption band emerged at 1625 cm−1 was designated for C=N linking group. This value falls within the frequency range reported for Schiff base linkage [19, 20].In1H NMR spectrum of 11ClCBA, two triplets appeared at δ=0.90 ppm and δ=4.36 ppm, which can be ascribed to the methyl and methylene protons (–OCH2–), respectively. The chemical shifts at δ=1.28–1.47 ppm can be assigned to methylene protons of long alkyl chain {CH3– (CH2)8–}. Four distinct doublets detected at δ=7.19,7.39,7.98, and 8.16 ppm can be assigned to eight aromatic protons. A singlet was observed at the most downfield region; δ=8.33 ppm is due to azomethine proton [13–15]. The molecular structure of 11ClCBA was further ascertained by using 13C NMR spectroscopy. A signal at δ=14.08 ppm is attributed to the methyl carbon, and peaks at δ=22.66–31.89 ppm are contributed to the methylene carbons of long alkyl chain. The signals appeared at δ=122.37–149.99 ppm is belonged to twelve aromatic carbons. Azomethine carbon exhibited its signal at δ=159.41 ppm. The most downfield peak at δ=166.08 ppm is due to the existence of carbonyl carbon of ester group.


## 4.2. Phase Transition Behaviors and Optical Texture Studies

All the members in thenClCBA series were nonmesogenic compounds. In the representative DSC thermogram of 11ClCBA (Figure 2), it showed an endotherm and exotherm, respectively, during both heating and cooling cycles. This observation indicates direct melting of crystal to isotropic liquid phase and vice versa. Under polarizing optical microscopy (POM) observation, The crystal changed to dark region isotropic during heating run. No liquid crystal texture was observed during cooling process. The rest of members showed similar characteristics as those found for 11ClCBA.


## 4.3. Effect of Alkyl Chain Length on Transition Temperatures

A further examination of DSC data also illustrates the evolution of the Cr-I transition with the lengthening of terminal chains. Based on phase transition data in Table2, it can be deduced that the thermal properties were greatly influenced by the length of the terminal chains. Melting temperatures exhibited a typical descending trend as the length of the carbon chain increased from the ethyl to the hexyl. This was attributed to the dilution of the mesogenic core, affected by the increase in the flexibility of the alkyl chain [21]. However, the lengthening of carbon chain from the hexyl to the pentadecyl derivatives led to ascending trend of melting temperatures. This phenomenon can be attributed to the increased intermolecular Van der Waals attraction [22]. A similar phenomenon was also reported for two analogous series of Schiff bases, 2-hydroxy-4-methoxy-4′-alkanoyloxyanilines and 3-methoxy-4-alkanoyloxybenzylidene-4′-alkanoyloxyanilines [15]. The recrystallization points also show similar trend to that observed for melting temperatures.


## 4.4. Influence of Direction of Ester Linkage on Mesomorphism

Mesomorphic properties of two aromatic ring compounds, 4-chlorobenzylidene-4′alkanoyloxyanilines (Series 1) [18] and alkyl 4-{[(4-chlorophenyl)imino]methyl}benzoates (Series 2), are strongly dependent on the direction of the ester linkage. Series 1 having molecular arrangement of alkyl-COO-phenyl exhibited smectic A and B phases. However, Series 2 (present compounds) with the reversed ester linkage were nonmesomorphic compounds. The position of the ester linking group defines the direction of the carbonyl group. The different directions of the carboxyl groups between the phenyl and alkyl units cause significant changes on the dipole moment, which in turn result in a reduction of the polarizability anisotropy and geometric anisotropic of the molecule [23]. A similar phenomenon has been reported for some banana-type mesogens [16]. The smectic properties of the molecules are strongly affected not only by the electrostatic interactions but also by the geometrical circumstances [24]. In addition, Series 1 possessed higher melting point compared to Series 2 having the same number of carbons (n) at the alkyl chain. This suggests that the direction of ester linkage can also change the thermal stability of a compound. The interaction between ester and phenyl groups in Series 1 seems to afford molecule with a higher polarity which would be expected to enhance lateral interactions of a polar nature and hence might lead to a higher clearing point (phase stability) [23].


## 5. Conclusion

Spectral, optical, and thermal behaviors of a homologous series of two aromatic ring compounds possessing chloroend group were studied. Melting and recrystallization points were greatly influenced by the changes of alkyl chain length. All compounds innClCBA series were nonmesomorphic derivatives. The orientation of ester linkage in nClCBA has significantly suppressed the formation of liquid crystal phase and thermal stability.



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