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Journal of Chemistry
Volume 2013 (2013), Article ID 864819, 5 pages
http://dx.doi.org/10.1155/2013/864819
Research Article

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

1Faculty of Engineering & Science, Universiti Tunku Abdul Rahman, Jalan Genting Klang, Setapak, Lumpur, 53300 Kuala, Malaysia
2Faculty of Science, Universiti Tunku Abdul Rahman, Jalan Universiti, Bandar Barat, Perak, 31900 Kampar, Malaysia
3Centre for Biodiversity Research, Universiti Tunku Abdul Rahman, Jalan Universiti, Bandar Barat, Perak, 31900 Kampar, Malaysia

Received 14 May 2013; Accepted 27 June 2013

Academic Editor: Iciar Astiasaran

Copyright © 2013 L. K. Ong and S. T. Ha. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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, , where –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.

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 [615].

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 ( , where –7, 9, 11, 13, 15) were obtained commercially. The synthetic route for the title compounds is illustrated in Scheme 1.

864819.sch.001
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-MS (rel. int. %): 413.2(53) , 260.0(100), 242.0(37), 215.0(38); IR (KBr) cm−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 of ClCBA 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.

tab1
Table 1: Percentage yields and IR frequencies of alkyl 4-{[(4-chlorophenyl)imino]methyl}benzoates.
tab2
Table 2: Transition temperature and associated enthalpy changes of alkyl 4-{[(4-chlorophenyl)imino]methyl}benzoates upon heating and cooling cycles.
864819.fig.001
Figure 1: EI mass spectrum of undecyl 4-{[(4-chlorophenyl)imino]methyl}benzoate showing molecular ion peak (M+).
864819.fig.002
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 (Figure 1) showed the molecular ion peak at 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].

In 1H NMR spectrum of 11ClCBA, two triplets appeared at  ppm and  ppm, which can be ascribed to the methyl and methylene protons (–OCH2–), respectively. The chemical shifts at –1.47 ppm can be assigned to methylene protons of long alkyl chain {CH3– (CH2)8–}. Four distinct doublets detected at , and  ppm can be assigned to eight aromatic protons. A singlet was observed at the most downfield region;  ppm is due to azomethine proton [1315]. The molecular structure of 11ClCBA was further ascertained by using 13C NMR spectroscopy. A signal at  ppm is attributed to the methyl carbon, and peaks at –31.89 ppm are contributed to the methylene carbons of long alkyl chain. The signals appeared at –149.99 ppm is belonged to twelve aromatic carbons. Azomethine carbon exhibited its signal at  ppm. The most downfield peak at  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 the ClCBA 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 Table 2, 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 ( ) 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 in ClCBA series were nonmesomorphic derivatives. The orientation of ester linkage in ClCBA has significantly suppressed the formation of liquid crystal phase and thermal stability.

Acknowledgment

The authors would like to thank Universiti Tunku Abdul Rahman for the research facilities and financial supports.

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