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Molecular Crystals and Liquid
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Synthesis and Mesomorphic
Properties of
2-(4-Alkyloxyphenyl)benzothiazoles
Sie-Tiong Ha a , Teck-Ming Koh a , Guan-Yeow Yeap b , Hong-Cheu Lin c , Peng-Lim Boey b , Yip-Foo Win a , Siew-Teng Ong a & Lay-Khoon Ong a
a
Faculty of Engineering and Science, Universiti Tunku Abdul Rahman, Jalan Genting Klang , Setapak, Kuala Lumpur, Malaysia
b
Liquid Crystal Research Laboratory, School of Chemical Sciences, Universiti Sains Malaysia , Minden, Penang, Malaysia
c
Department of Materials Science and Engineering , National Chiao Tung University , Hsinchu, Taiwan, China
Published online: 03 Aug 2009.
To cite this article: Sie-Tiong Ha , Teck-Ming Koh , Guan-Yeow Yeap , Hong-Cheu Lin , Peng-Lim Boey , Yip-Foo Win , Siew-Teng Ong & Lay-Khoon Ong (2009) Synthesis and Mesomorphic Properties of 2-(4-Alkyloxyphenyl)benzothiazoles, Molecular Crystals and Liquid Crystals, 506:1, 56-70, DOI: 10.1080/15421400902987347
To link to this article: http://dx.doi.org/10.1080/15421400902987347
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Synthesis and Mesomorphic Properties
of 2-(4-Alkyloxyphenyl)benzothiazoles
Sie-Tiong Ha1, Teck-Ming Koh1, Guan-Yeow Yeap2, Hong-Cheu Lin3, Peng-Lim Boey2, Yip-Foo Win1, Siew-Teng Ong1, and Lay-Khoon Ong1
1Faculty of Engineering and Science, Universiti Tunku Abdul Rahman,
Jalan Genting Klang, Setapak, Kuala Lumpur, Malaysia
2
Liquid Crystal Research Laboratory, School of Chemical Sciences, Universiti Sains Malaysia, Minden, Penang, Malaysia
3
Department of Materials Science and Engineering, National Chiao Tung University, Hsinchu, Taiwan, China
A homologous series of 2-(4-alkyloxyphenyl)benzothiazoles with an even number of carbons in the terminal alkoxyl chain was synthesized and characterized. The structure of the compounds was postulated using elemental analyses, FT-IR, NMR, and mass spectrometry techniques. Their liquid crystalline properties have been investigated by optical polarizing microscopy, DSC, and X-ray diffraction (XRD) techniques. Whilst the n-octyloxy to n-hexadecyloxy derivatives exhibited a monotropic smectic A phase, the n-ethoxy to n-hexyloxy and the n-octadecyloxy derivatives were not mesogenic. The mesomorphic properties of the present series were compared with other structurally related series to establish the chemical structure-mesomorphic properties relationship.
Keywords: 2-(4-alkyloxyphenyl)benzothiazole; heterocycles; Smectic A; structure-property relationship
INTRODUCTION
Conventional thermotropic liquid crystals consist of anisotropic molecules that are either rod-shaped (calamitic) or disc-shaped (discotic) [1]. The 1,4-disubstituted phenyl rings forms the linear core in majority of the low molar mass calamitic mesogen systems. Over the
Address correspondence to Sie-Tiong Ha, Faculty of Engineering and Science, University Tunku Abdul Rahman, Jalan Genting Klang, Setapak, Kuala Lumpur 53300, Malaysia. E-mail: [email protected]
Copyright # Taylor & Francis Group, LLC ISSN: 1542-1406 print=1563-5287 online DOI: 10.1080/15421400902987347
56
past few decades, liquid crystals comprising heterocycles as core units have received overwhelming attention due to their unique properties [2–4]. Earlier report has shown that the introduction of heterocycles as the central core has greatly influenced the mesomorphic properties of the calamatic molecules owing to their unsaturation and=or their more polarizable nature [5]. It has also been claimed that the inclusion of the heteroatom considerably changes the polarity, polarizability, and to a certain extend the geometry of the molecule, thereby influen-cing the type of mesophase, the phase transition temperatures, dielec-tric constants, and other properties of the mesogens [6,7].
The slightly bent structure of mesogen possessing a single hetero-cyclic ring such as thiophene has led to unique physical properties such as reduced packing ability, medium to strong lateral dipole, high anisotropy of polarizability and low viscosity. Materials consisting of thiophene, 1,3-thiazole, or 1,3,4-thiadiazole as the core unit have sig-nificant lateral dipole moments which would contribute to physical parameters such as increased dielectric anisotropy and dielectric biaxiality [2]. Other examples of single heterocyclic ring liquid crystals include those with furyl [8], pyridine [8–10], substituted pyrimidine carboxylic acid [11], thiazole [12], thiadiazole [13], and oxadiazole [14] moieties.
Benzothiazole, another type of heterocycle, has emerged in its usage as a core unit in calamitic liquid crystals. It has been reported that liquid crystalline compounds incorporating a benzothiazole-fused ring exhibit good hole-transporting properties with a low ionization poten-tial, making them of potential interest as hole-transporting materials in organic light-emitting devices (OLEDs) [15–17]. Pavluchenko et al. [18] reported on mesogens comprising benzothiazole and benzoxazole with central linkages and lateral substituents at different positions so as to evaluate the effect of structural changes on mesomorphic prop-erties. Substitution at the sixth position of the benzothiazole ring was found to be thermally more stable as compared to that of the fifth posi-tion. Belmar et al. [19] studied the influence of central linking groups on the mesomorphic properties of 6-n-decyloxy-2-(4-alkoxybenzylide-amino)benzothiazoles, n-[2-(6-n-decyloxybenzothiazolyl)]-4-n-alkoxy-benzamides, and 6-n-decyloxy-2-(4-n-alkoxyphenylazo)benzothiazoles. Results indicated that the imine and azo derivatives both exhibited smectic and nematic phases while only the smectic phase was obser-ved for the amide derivatives. This observation can be ascribed to the hydrogen bonding between the molecules of the amide derivatives, which prevent the formation of nematic phase [19].
Prajapati and Bonde reported two mesogenic homologous series comprising 6-substituted-benzothiazole ring systems with an azo
centre linkage [20]. The study revealed that the methoxyl substituent at the sixth position of the benzothiazole ring favored the formation of the nematic phase. As part of our studies on heterocyclic mesogens, we report in this article the synthesis and the mesomorphic properties of a homologous series of 2-(4-alkyloxyphenyl)benzothiazoles without central linking groups and lateral substituents. In addition, they were compared to structurally related compounds to establish the chemical structure-mesomorphic properties relationship.
EXPERIMENTAL
2-Aminothiophenol, 1-bromoalkane (CnH2nþ1Br where n¼ 2, 4, 6, 8, 10, 12, 14, 16, 18), and potassium iodide were obtained from Merck (Germany). 4-Hydroxybenzaldehyde was purchased from Acros Organics (USA). Potassium hydroxide was obtained from R&M Chemicals (U.K.). IR spectra were recorded using a Perkin-Elmer System 2000 FT-IR Spectrometer via KBr disc procedure. The range of measurement was from 4000–400 cm1. 1H NMR (400 MHz) and 13C NMR (100 MHz) spectra were recorded in CDCl3 using a Bruker Avance 400 MHz Spectrometer with TMS as the internal standard. EI-MS (70 eV) were measured with a Mass Spectrometer Finnigan MAT95XL-T at a source temperature of 200C. The sample was introduced using direct inlet probe. Microanalyses were carried out on Perkin Elmer 2400 LS Series CHNS=O analyzer. Melting points of the synthesized com-pounds were determined using a Barnstead Electrothermal Melting Point Apparatus. Thin Layer Chromatography (TLC) was carried out on aluminium-backed silica-gel plates (Merck 60 F254) and visua-lized under short-wave UV light.
Phase-transition temperatures and enthalpy changes were measured using a Differential Scanning Calorimeter Mettler Toledo DSC823e at heating and cooling rates of 10C=min and10C=min, respectively. A polarizing optical microscope (Carl Zeiss) equipped with a Linkam heating stage was used for temperature dependent studies of the liquid crystal textures. A video camera (Video Master coomo20P) installed on the polarizing microscope was coupled to a video capture card (Video Master coomo600), allowing real-time video capture and image saving. Textures exhibited by the compounds were observed using polarized light with crossed polarizers. Samples were prepared as thin films sandwiched between a glass slide and a cover slip. Phase identification was made by comparing the observed textures with those reported in the literature [21,22].
Synchrotron powder X-ray diffraction (XRD) measurements were performed at beamline BL17A of the National Synchrotron Radiation
Research Center (NSRRC) in Taiwan, where the X-ray wavelength used was 1.32633 A˚ . XRD data were collected using imaging plates (IP, of an area¼ 20 40 cm2and a pixel resolution of 100) curved with a radius equivalent to the sample-to-image plate distance of 280 mm, and the diffraction signals were accumulated for 3 min. The powder samples were packed into a capillary tube and heated by a heat gun, where the temperature controller is programmable by a PC with a PID feedback system. The scattering angle theta was calibrated by a mixture of silver behenate and silicon.
The synthetic route of 2-(4-alkyloxyphenyl)benzothiazoles, nO-PBTH is illustrated in Scheme 1.
SCHEME 1 Synthetic route of nO-PBTH.
Synthesis
Synthesis of 2-(4-Hydroxyphenyl)benzothiazole, HPBTH
The compound was synthesized according to the method described by Toba et al. [23]. Five mmol (0.6260 g) of 2-aminothiophenol and 5 mmol (0.6106 g) of 4-hydroxybenzaldehyde were refluxed upon stirring for 6 hours in 40 mL of ethanol. The reaction mixture was subsequently cooled to room temperature following which to it 60 mL of distilled water was added slowly until the mixture turned cloudy. It was refriger-ated overnight at about 20C, and the solid formed was filtered and washed with cold ethanol:water (1:1.5) and dichloromethane. Yield 75%. IR (KBr) nmaxcm13417 (O-H), 3053, 2996 (C-H aromatic), 1606 (C¼N, thiazole), 1483, 1455 (C¼C aromatic), 1074 (C-S-C). 1H NMR (400 MHz, CDCl3): d=ppm 5.3 (1 H, s, OH), 6.9 (2 H, d, Ar-H), 7.3 (1 H, t, Ar-H), 7.4 (1 H, t, Ar-H), 7.9 (1 H, d, Ar-H), 8.0 (3 H, m, Ar-H).
Synthesis of 2-(4-Alkyloxyphenyl)benzothiazoles, nO-PBTH
2-(4-Alkyloxyphenyl)benzothiazoles were synthesized via modifica-tion of the method described by Prajapati et al. [24]; 0.9 mmoles (0.2046 g) of HPBTH in 35 ml of acetone and 0.9 mmoles (0.0505 g) of KOH in 5 ml of distilled water were mixed in a round bottom flask to which a small amount of KI was added as a catalyst. The mixture was refluxed upon stirring for 1 hour following which 1.35 mmoles of the appropriate 1-bromoalkane (CnH2nþ1Br, n¼ 2, 4, 6, 8, 10, 12, 14, 16, 18) was added and was further refluxed for another 20 hours. The mixture was cooled to room temperature, and the white precipi-tate formed was separated by filtration. The solid obtained was recrys-tallized several times with absolute ethanol whereupon the pure compound was isolated. The percentage yields and analytical data of the title compounds are tabulated in Table 1. The IR, NMR (1H and 13
C), and mass spectral data for the representative compound, 16O-PBTH, are summarized as follows.
16O-PBTH: Yield 63%. IR (KBr) vmaxcm1 3057, 3030 (C-H aromatic), 2955, 2849 (C-H aliphatic), 1608 (C¼N, thiazole), 1487, 1465 (C¼C aromatic), 1261 (Ar-O-R ether), 1073 (C-S-C, thiazole).1H NMR (400 MHz, CDCl3): d=ppm 0.9 (t, 3 H, ), 1.3–1.5 (m, 26 H, CH3-(CH2)13CH2-O-), 1.8–1.9 (m, 2 H, CH2-O-), 4.1 (t, 2H, -CH2-O-), 7.0 (d, 2 H, Ar-H), 7.4 (t, 1 H, Ar-H), 7.5 (t, 1 H, Ar-H), 7.9 (d, 1 H, Ar-H), 8.0 (m, 3 H, Ar-H).13C NMR (100 MHz, CDCl3): d=ppm 168.44 (C¼N), 161.96, 154.59, 135.21, 129.49, 126.62, 126.50, 125.18, 123.17, 121.93, 115.24 for aromatic carbons, 68.64 (-O-CH2-), 32.38, 30.16, 30.14, 30.12, 30.05, 30.02, 29.83, 29.60, 26.44, 23.15 for methylene car-bons [-(CH2)14CH2O-], 14.61 (-CH3). EI-MS m=z (rel. int. %): 451(42) (M)þ, 227(100).
RESULTS AND DISCUSSION
Structural identification of the title compounds was carried out by employing a combination of elemental analysis and spectroscopic techniques (FT-IR, NMR, and EI-MS). The percentages of C, H, and N from the elemental analysis conform with the calculated values for compounds nO-PBTH (where n¼ 2, 4, 6, 8, 10, 12, 14, 16, 18) and are presented in Table 1. The prominent molecular ion peak at m=z 451 in the mass spectrum of 16O-PBTH, establishing a molecular formula of C29H41ONS, which supports the proposed structure.
FTIR,
1H NMR, and
13C NMR Spectral Studies
The diagnostic absorption bands at 2849 and 2955 cm1 in the FTIR spectrum of 16O-PBTH provides evidence for the alkoxyl chain. The presence of the aromatic rings was inferred from the absorption bands at 3030, 3057, 1487, and 1465 cm1. The absorption bands assignable to the stretching of the C¼N and C-S-C bonds of benzothiazole were observed at 1608 and 1073 cm1, respectively. These values conform with those reported in the IR spectra for various substituted benzothia-zoles [20,25]. The absorption band at 1261 cm1 was indicative of the C-O stretching of the aromatic ether (Ar-O-R). Compounds nO-PBTH (n¼ 2, 4, 6, 8, 10, 12, 14, 18) exhibited similar characteristic absorptions as discussed for 16O-PBTH.
The triplets at d¼ 0.9 and 4.1 ppm in the 1H NMR spectrum of 16O-PBTH were assigned to methyl protons and methylene protons (Ar-O-CH2-) that bonded directly to aromatic ether group, respec-tively. The multiplets between d¼ 1.3–1.9 ppm were attributed to the remaining methylene protons [CH3-(CH2)14-] of the alkoxyl chain.
TABLE 1 Percentage Yields and Analytical Data of nO-PBTH
Compound Yield (%) % Found (% Calcd.) Formula C H N 2O-PBTH 37 C15H31NOS 70.50 (70.56) 5.19 (5.13) 5.43 (5.49) 4O-PBTH 49 C17H17NOS 72.09 (72.05) 6.11 (6.05) 4.89 (4.94) 6O-PBTH 43 C19H21NOS 73.29 (73.31) 6.80 (6.75) 4.45 (4.50) 8O-PBTH 43 C21H25NOS 74.36 (74.34) 7.51 (7.37) 3.95 (4.13) 10O-PBTH 47 C23H29NOS 75.22 (75.20) 8.08 (7.90) 3.70 (3.81) 12O-PBTH 44 C25H33NOS 75.90 (75.95) 8.43 (8.35) 3.63 (3.54) 14O-PBTH 52 C27H37NOS 76.58 (76.60) 8.87 (8.75) 3.17 (3.31) 16O-PBTH 63 C29H41NOS 77.13 (77.16) 9.18 (9.09) 3.18 (3.10) 18O-PBTH 71 C31H45NOS 77.53 (77.66) 9.52 (9.39) 2.83 (2.92)
The 1H NMR spectrum of 16O-PBTH also suggests the presence of eight aromatic protons.
The 13C NMR spectrum of 16O-PBTH suggests the presence of 29 carbons. The peak at d¼ 168.44 ppm owing to the presence of the imine carbon (-C¼N-, thiazole), was in agreement with that reported in literature [19]. The signals within the range of d¼ 115.24– 161.96 ppm were assigned to the carbons of the benzothiazole and aro-matic moieties. The carbon resonances between d¼ 14.61–68.64 ppm were indicative of the methylene and methyl carbons of the alkoxyl chain. The results as inferred from the IR and NMR spectral data of the title compounds were consistent with the proposed structure.
Phase-Transition Behavior and Liquid Crystallinity
of nO-PBTH
The transition temperatures and associated enthalpy changes of nO-PBTH obtained from the DSC measurements are summarized in Table 2. The DSC thermograms of all the members during the heat-ing and coolheat-ing cycles are depicted in Figures 1 and 2, respectively. The representative optical photomicrographs are shown in Figure 3. A plot of the transition temperatures against the number of carbons
TABLE 2 Transition Temperatures and Associated Enthalpy Changes of nO-PBTH Upon Heating and Cooling
Compound Transition temperatures,C (DH, kJ mol1)
2O-PBTH Cr 125.7 (18.18) I Cr 95.7 (16.16) I 4O-PBTH Cr 89.5 (36.16) I Cr 78.0 (34.71) I 6O-PBTH Cr 85.1 (20.87) I Cr 73.0 (20.12) I 8O-PBTH Cr 75.5 (27.86) I Cr 48.8 (17.12) SmA 74.8 (6.97) I 10O-PBTH Cr 81.7 (44.42) I Cr 63.1 (32.03) SmA 79.3 (8.80) I 12O-PBTH Cr 86.9 (55.61) I Cr 63.8 (42.11) SmA81.2 (10.41) I 14O-PBTH Cr 91.7 (59.52) I Cr 70.7 (47.00) SmA81.2 (9.79) I 16O-PBTH Cr 95.7 (70.41) I Cr 77.3 (57.37) SmA79.8 (11.20) I 18O-PBTH Cr 96.8 (58.49) I Cr 85.2 (56.27) I The values in italics were taken during cooling cycle. Cr¼ Crystal; SmA ¼ Smectic A; I ¼ Isotropic liquid.
in the alkoxyl chain during the heating and cooling cycles are shown in Figures 4 and 5. Based on the plot, it can be deduced that the meso-phase behaviors were greatly influenced by the length of the terminal chains. The crystal-isotropic transition (melting) temperatures exhib-ited a descending trend as the length of the terminal alkoxyl chain of the derivatives increased from C2 to C8. However, the lengthening of carbon chain from the n-octyloxy to the n-octadecyloxy derivatives led to ascending trend of melting temperatures. This phenomenon can be
FIGURE 2 DSC thermograms of nO-PBTH during cooling cycle. FIGURE 1 DSC thermograms of nO-PBTH during heating cycle.
attributed to the the increased intermolecular Van der Waals attrac-tion as the length of the alkoxyl chain increased [26].
Under the polarizing microscope, the octyloxy to the n-hexadecyloxy derivatives exhibited monotropic smectic A phase. In the monotropic mesogens, the melting points were always equal to or higher than the clearing points, hence exhibiting supercooling prop-erties [27]. As a representative illustration, the optical photomicro-graphs of compounds 10O-PBTH and 12O-PBTH are illustrated in Fig. 3. 10O-PBTH exhibited homogenously aligned fan-shaped
FIGURE 3 (a) Optical photomicrograph of 10O-PBTH exhibiting homoge-nously aligned smectic A phase with fan-shaped textures; (b) optical photo-micrograph of 12O-PBTH exhibiting smectic A phase with fan-shaped and homeotropic (dark area) textures.
textures whereas the co-existence of homogenous and homeotropic (dark region) smectic A with fan-shaped textures were observed for 12O-PBTH. The assignments of the mesophases were made on the basis of the textures observed from POM as the mesophases displayed the typical fan-shaped textures for a smectic A phase [21,22].
The rest of the members (n¼ 2, 4, 6, and 18) did not exhibit liquid crystal phases. 2O-PBTH, 4O-PBTH, and 6O-PBTH were
FIGURE 4 Plot of transition temperatures versus the number of carbons (n) in the alkoxyl chain of nO-PBTH during heating cycle.
FIGURE 5 Plot of transition temperatures versus the number of carbons (n) in the alkoxyl chain of nO-PBTH during cooling cycle.
non-mesomorphic perhaps due to the shorter terminal alkoxyl chains, resulting in higher melting points, thus suppressing the liquid crystal phase. The high flexibility owing to the long terminal chain (C18H36O-) in 18O-PBTH also depressed the formation of liquid crystal phase [28].
XRD Studies of nO-PBTH
Although the liquid crystal phase type of the title compound can be concluded preliminary by DSC and POM, it is necessary to measure them by XRD analysis and give additional information about their structure parameters. XRD patterns of representative compounds 10O-PBTH and 12O-PBTH are shown in Fig. 6.
In general, a smectic, nematic, cholesteric structure has a broad peak associated with lateral packing at 2h 16–21o in a wide-angle XRD curve. A sharp and strong peak at a low angle (1o< 2h < 6o) in a small angle X-ray scattering curve can be observed for smectic struc-tures, but it cannot be seen for nematic and cholesteric structures [29–31]. An XRD pattern of 10O-PBTH and 12O-PBTH showed sharp reflection peaks at 2h of 2.99 and 2.65 (d¼ 25.56 and 28.84 A˚ ), which were corresponding to the smectic layer spacing. Combining the polarized microscopy with XRD measurements confirmed that these compounds were smectogenic A.
Chemical Structure–Mesomorphic Property Relationship
There is a close relationship between the mesomorphic properties and molecular structure of organic compounds. Thus, transition temperatures, mesophase range, and types of mesophases can be correlated with the molecular structure of the compounds. In Table 3, the transition temperatures, types of mesophases, mesophase range, and molecular structure of 8O-PBTH are compared with other structurally related compounds A [20], B [32], and C [19].
Whilst 8O-PBTH exhibited a smectic A phase, compound A was nematogenic. The difference could be resulted from the additional methoxyl group at the sixth position of the benzothiazole ring along with the additional azo linkage between the two aromatic rings in compound A. Hence, the nematic phase in compound A could be due to the methoxyl substituent which diminished the formation of the smectic A [9]. The azo linkage between the two aromatic core units in compound A conferred stepped core structure leading to the broad-ening effect, thus disrupting the lamellar packing, and therefore gen-erating the nematic phase [1]. Similar structure–property relationship was also inferred from compound C whereby this structurally related compound exhibited a nematic phase owing to the presence of an azo linkage.
Meanwhile, the additional azo linkage also tends to increase the transition temperatures and mesophase range of compound A when compared to 8O-PBTH. The increase in the mesophase range of compound A could have originated from (i) an increase in the length-to-breadth ratio of the molecule and (ii) an increase in the polarizability due to the extension of p-conjugation [33].
Structurally related compound B exhibited a smectic C phase instead of a nematic or smectic A phase. Upon comparison to 8O-PBTH,
FIGURE 6 XRD patterns of 10O-PBTH and 12O-PBTH at different temperatures upon cooling from the isotropic phase.
it can be rationalized that the presence of the ethoxyl substituent at the sixth position of the benzothiazole ring promoted the formation of the smectic C phase in compound B. The ethoxyl group in compound B favoured the intermolecular interaction that gave rise to a tilted arrangement of the molecules in the smectic layers. The similar pat-tern can also be inferred by comparing the molecular structures of compound A and C. It is clearly observed that as the length of the alkoxyl chain at the sixth position of the benzothiazole ring increases, the molecules favour the tilted SmC phase.
CONCLUSION
In this article, we have reported the synthesis and mesomorphic prop-erties of a homologues series of 2-(4-alkyloxyphenyl)benzothiazoles.
TABLE 3 Transition Temperatures, Mesophase Range, and Molecular Structures of 8O-PBTH, A, B, and C
8O-PBTH
Compound A
Compound B
Compound C
Compound Transition temperatures (C)
Mesophase range (C) Sm N 8O-PBTH Cr (SmA 74.8)75.5 I – – A Cr 94 N 136 I – 42 B Cr 106 SmC 134 I 28 – C Cr 70.7 SmC 95.0 N 123.8 I 24.3 28.8 ()Monotropic value.
The n-octyloxy to n-hexadecyloxy derivatives exhibited a monotropic smectic A phase. The remaining members were non-mesomorphic. Comparison of the present series with other structurally related compounds revealed that transition temperatures and mesophase stability were greatly affected by the linking (terminal and centre) groups. The presence of the azo linkage disrupts the lamellar packing hence generating the nematic phase. The study also revealed that the methoxyl group at the sixth position of the benzothiazole ring stabi-lized the nematic phase whereas the ethoxyl group or a longer alkoxyl chain at the sixth position of the benzothiazole ring will stabilize the smectic C phase.
ACKNOWLEDGMENTS
Author S. T. Ha would like to thank Universiti Tunku Abdul Rahman (UTAR) for the research facilities and the Malaysia Toray Science Foundation (UTAR Vote No. 4359=000) for funding this project. T. M. Koh would like to acknowledge UTAR for the award of the research assistantship. The partial financial support from MOSTI via the eSciencefund (Project No. 03-01-05-SF0366) is also essential to make this project complete. The powder XRD measurements are supported by beamline BL17A (charged by Dr. Jey-Jau Lee) of the National Synchrotron Radiation Research Center, Taiwan.
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