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Liquid Crystals
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Heterocyclic benzothiazole-based liquid crystals:
synthesis and mesomorphic properties
Sie-Tiong Ha ab , Teck-Ming Koh a , Hong-Cheu Lin c , Guan-Yeow Yeap d , Yip-Foo Win b , Siew-Teng Ong ab , Yasodha Sivasothy e & Lay-Khoon Ong a
a
Faculty of Engineering & Science , Universiti Tunku Abdul Rahman, Jalan Genting Klang , Setapak, 53300, Kuala Lumpur, Malaysia
b
Faculty of Science, Engineering & Technology , Universiti Tunku Abdul Rahman, Jalan University , Bandar Barat, 31900, Kampar, Perak, Malaysia
c
Department of Materials Science & Engineering , National Chiao Tung Universiti , 1001 Ta-Hsueh Road, Hsinchu, 300, Taiwan, ROC
d
Liquid Crystal Research Laboratory, School of Chemical Sciences , University Sains Malaysia , 11800, Minden, Penang, Malaysia
e
Chemistry Department, Faculty of Science , Universiti Malaya , 50603, Kuala Lumpur, Malaysia
Published online: 22 Sep 2009.
To cite this article: Sie-Tiong Ha , Teck-Ming Koh , Hong-Cheu Lin , Guan-Yeow Yeap , Yip-Foo Win , Siew-Teng Ong , Yasodha Sivasothy & Lay-Khoon Ong (2009) Heterocyclic benzothiazole-based liquid crystals: synthesis and mesomorphic properties, Liquid Crystals, 36:9, 917-925, DOI: 10.1080/02678290903131278
To link to this article: http://dx.doi.org/10.1080/02678290903131278
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Heterocyclic benzothiazole-based liquid crystals: synthesis and mesomorphic properties
Sie-Tiong Haa,b*, Teck-Ming Koha, Hong-Cheu Linc, Guan-Yeow Yeapd, Yip-Foo Winb, Siew-Teng Onga,b, Yasodha Sivasothyeand Lay-Khoon Onga
a
Faculty of Engineering & Science, Universiti Tunku Abdul Rahman, Jalan Genting Klang, Setapak, 53300 Kuala Lumpur, Malaysia;bFaculty of Science, Engineering & Technology, Universiti Tunku Abdul Rahman, Jalan University, Bandar Barat, 31900 Kampar, Perak, Malaysia; cDepartment of Materials Science & Engineering, National Chiao Tung Universiti, 1001 Ta-Hsueh Road, Hsinchu 300, Taiwan, ROC; dLiquid Crystal Research Laboratory, School of Chemical Sciences, University Sains Malaysia, 11800 Minden, Penang, Malaysia;eChemistry Department, Faculty of Science, Universiti Malaya, 50603, Kuala
Lumpur, Malaysia
(Received 22 April 2009; final form 19 June 2009)
Two homologous series of 2-(4-alkanoyloxybenzylidenamino)benzothiazoles and 2-(2-hydroxy-4-alkanoyloxy-benzylidenamino)benzothiazoles were synthesised and characterised. Their molecular structures differed wherein the latter comprised a lateral hydroxyl group, unlike the former. Spectroscopic techniques such as FT-IR,1H &13C NMR and mass spectrometry together with elemental analysis were employed to elucidate the molecular structures. The transition temperatures and their mesophases were determined by differential scanning calorimetry, optical polarising microscopy and X-ray diffraction techniques. Members with decanoyloxy till hexadecanoloxy chain in the series without the lateral hydroxyl group each exhibited a smectic A phase, while those in the series with the lateral hydroxyl group were non-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-alkanoyloxybenzylidenamino)benzothiazole; 2-(2-hydroxy-4-alkanoyloxybenzylidenamino)benzo thiazole; Schiff base; smectic A; structure–property relationship
1. Introduction
The mesomorphic behaviour of an organic compound can be varied by modifying its molecular structure including linking, terminal and core groups. Schiff base (or azomethine), a linking group, is usually incor-porated into the molecular structure to increase the length and polarisability anisotropy of the molecular core in order to enhance liquid crystal phase stability (1). Schiff bases have been studied intensively (2–5) since the discovery of 4-methoxybenzylidene-4’-buty-laniline which exhibited a nematic phase at room tem-perature (6).
However, it is commonly believed that molecular order in liquid crystal phases depends largely on the mesogenic core structure: its geometry, polarisability, molecular conformation and length-to-breath ratio as well as the number and position of permanent dipole moments in the core. For this reason, altering the core structure has been regarded as one of the factors that bring significant changes to mesomorphic properties (1). Recently, several kinds of heterocyclic rings, such as pyridine (7), thiophene (8) and 1,3,4-thiadiazole (9), have been introduced into the compounds as mesogenic cores. Heterocycles are of great importance as core units in thermotropic liquid crystals owing to their ability to impart lateral and/or longitudinal dipoles combined with
changes in the molecular shape (10,11). Furthermore, the incorporation of heteroatoms results in considerable changes in the corresponding liquid crystalline phases and/or in the physical properties of the observed phases, as most of the heteroatoms (S, O, and N) commonly introduced are more polarisable than carbon (12).
In addition, heterocyclic rings fused with benzene rings are now becoming popular mesogenic cores to be incorporated into the molecular structure. The meso-morphic properties of heterocyclic fused-ring deriva-tives such as 2,1,3-benzothiadiazole (13), benzoxazole (14) and 2,1,3-benzoxadiazole (15) have been recently studied. The unsaturation and/or the more polarisable nature of the heterocyclic fused-ring systems have greatly affected the mesomorphic properties of cala-mitic molecules (16).
Benzothiazole, another kind of heterocyclic fused-ring system, exhibits good hole-transporting proper-ties with a low ionisation potential, making it of poten-tial interest as hole-transporting materials in organic light-emitting devices (OLEDs) (17). However, only scant information on the incorporation of benzothia-zole as a core in liquid crystalline compounds (18–20) is available. In addition, benzothiazole has also been proved to be a good core in mesogens in our recent study (21,22).
*Corresponding author. Emails: [email protected] or [email protected]
ISSN 0267-8292 print/ISSN 1366-5855 online #2009 Taylor & Francis
DOI: 10.1080/02678290903131278 http://www.informaworld.com
Previous study of a series of Schiff base esters has shown that the presence of a lateral hydroxyl group at the ortho position led to an increase in the molecular polarisability as well as in the clearing temperature (23). A lateral hydroxyl group, on the other hand, can enhance the stability of a molecule through intramole-cular hydrogen bonding (24). However, the lateral sub-stituents can also disturb the molecular order in liquid crystalline phases or even completely diminish the meso-phases (25). Therefore, the influence of a lateral hydroxyl group on the mesomorphic properties of benzothiazoles is also one of the interests of the current study.
Herein, we describe the preparation of two homo-logous series of Schiff base esters with benzothiazole and aromatic cores, 2-(4-alkanoyloxybenzylidena-mino) benzothiazoles and 2-(2-hydroxy-4-alkanoylox-ybenzylidenamino)benzothiazoles. Fourier transform infrared (FT-IR), 1H & 13C nuclear magnetic reso-nance (NMR), electron ionization mass spectroscopy (EI-MS) and elemental analysis were employed to elu-cidate the molecular structure of the title compounds, whereas the liquid crystal behaviours were determined by differential scanning calorimetry (DSC), polarising optical microscopy (POM) and X-ray diffraction (XRD) analysis. In addition, the relationship between the molecular structure and liquid crystal properties is also discussed in this paper.
2. Experimental 2.1 Characterisation
FT-IR analyses were performed on a Perkin-Elmer System 2000 FT-IR Spectrophotometer. Spectra were obtained by the KBr disc procedure and the range of measurement was from 4000 to 400 cm-1. 1
H NMR (400 MHz) and13C NMR (100 MHz) spec-tra were recorded in CDCl3 using a JEOL LA-400 MHz NMR spectrometer with TMS as the internal standard. EI-MS (70 eV) were measured with a Mass Spectrometer Finnigan MAT95XL-T at a source tem-perature of 200C. Microanalyses were carried out on
a Perkin Elmer 2400 LS Series CHNS/O analyser. TLC was carried out on aluminium-backed silica-gel plates (Merck 60 F254) and visualised under short-wave ultraviolet light.
Phase-transition temperatures and enthalpy changes were measured using a Differential Scanning Calorimeter Mettler Toledo DSC823eat heating and cooling rates of 10C/min and -10C/min, respectively. A polarising optical microscope (Carl Zeiss) equipped with a Linkam heating stage was used for tempera-ture-dependent studies of the liquid crystal textures. A video camera (Video Master coomo20P) installed on the polarising 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 polarised light with crossed polarisers. 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 (26,27).
Synchrotron powder XRD measurements were per-formed 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 area = 20 40 cm2
and 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 was programmed by a PC with a proportional–integral–derivative (PID) feed-back system. The scattering angle theta was calibrated by a mixture of silver behenate and silicon.
2.2 Synthesis
4-(N,N-dimethylamino)pyridine (DMAP) and fatty acids (Cn-1H2n-1COOH where n = 2, 4, 6, 12, 14, 16, 18) were obtained from Merck (Germany). 2-Aminobenzothiazole, 2,4-dihydroxybenzaldehyde, 4-hydroxybenzaldehyde, propanoic acid, pentanoic acid, octanoic acid, decanoic acid and N,N’-dicyclo-hexylcarbodiimide (DCC) were purchased from Acros Organics (USA). All solvents and reagents were pur-chased commercially and used without any further purification.
The synthetic routes for all the title compounds are depicted in Scheme 1. The synthesis of each compound in both the series was identical.
2.2.1 Synthesis of benzothiazole 1
2-Aminobenzothiazole (40 mmol, 6.01 g) and the appropriate benzaldehyde (40 mmol) were dissolved in 60 ml of ethanol. Two drops of acetic acid were added and the mixture was refluxed for 6 h upon stirring. The mixture was then filtered and the filtrate was left to evaporate to dryness. The yellow solid which was formed was recrystallised with ethanol for further reaction.
2.2.2 Synthesis of benzothiazole 2
Benzothiazole 1 (20 mmol), the appropriate fatty acid (20 mmol) and DMAP (4 mmol) were dissolved in a 918 S.T. Ha et al
50 ml mixture of dichloromethane (DCM) and dimethylformamide (DMF) with the ratio of 4:1 and stirred at 0C. DCC (20 mmol, 4.13 g) dissolved
in 10 ml of dichloromethane was added into the mixture dropwise and continuously stirred for 1 h at 0C. The mixture was then stirred at room tempera-ture for another 3 h following which it was filtered and the solvent removed by evaporation. The yellow solid which was obtained was recrystallised using ethanol.
The percentage yields and the analytical data of each compound in both the series are tabulated in Tables 1 and 2. The IR, NMR (1H and 13C) and mass spectral data of the representative compounds, 16BABTH and 16HBABTH, of both the series are summarised as follows.
16BABTH: IR (KBr) vmax cm-1 3067 (C-H aro-matic), 2943, 2891 (C-H aliphatic), 1762 (C=O ester), 1600 (C=N, thiazole).1H NMR (400 MHz, CDCl3): /ppm 0.9 (t, J = 6.6 Hz, 3H, CH3-), 1.2–1.4 (m, 24H, CH3-(CH2)12-CH2-CH2-COO-), 1.8 (quint., J = 7.4 Hz, 2H, CH2CH2COO), 2.6 (t, J = 7.4 Hz, 2H, -CH2-COO-), 7.2 (d, J = 8.5 Hz, 2H, Ar-H), 7.3 (t, J = 8.2 Hz, 1H, Ar-H), 7.5 (t, J = 7.2 Hz, 1H, Ar-H), 7.8 (d, J = 8.0 Hz, 1H, H), 8.0 (d, J = 8.2 Hz, 1H, Ar-H), 8.1 (d, J = 8.5 Hz, 2H, Ar-Ar-H), 9.0 (s, 1H, CH=N). 13 C NMR (100 MHz, CDCl3): /ppm 171.60 (-COO-), 164.71 (C=N), 154.57, 151.65, 134.63, 132.16, 131.44, 126.40, 125.03, 123.03, 122.27, 121.62 for aromatic carbons, 34.35, 31.87, 29.64, 29.62, 29.60, 29.59, 29.54, 29.39, 29.31, 29.19, 29.02, 24.78, 22.64 for methylene carbons [-COO-(CH2)14-CH3], 14.07
N S NH2
+
C2H5OH 1. DCM, DMF (reflux 6 hours) 2. DCC, DMAP R1 OH O H R1 OH N S N R1 O N S N O Cn-1H nBABTH nHBABTH where n = 2, 3, 4, 5, 6, 8, 10, 12, 14, 16, 18 where n = 8, 10, 12, 14, 16. R1 = H R1 = OH 1 2 3. Cn-1H2n-1COOHScheme 1. Synthetic route of nBABTH and nHBABTH.
[-COO-(CH2)14-CH3]. EI-MS m/z (rel. int. %): 492 (8) (M)+, 254 (100).
16HBABTH: IR (KBr) vmaxcm-13439 (O-H), 3053 (C-H aromatic), 2954, 2894 (C-H aliphatic), 1758 (C=O ester), 1618 (C=N, thiazole). 1H NMR (400 MHz, CDCl3): /ppm 0.9 (t, J = 6.0 Hz, 3H, CH3-), 1.2–1.4 (m, 24H, CH3-(CH2)12-CH2-CH2-COO-), 1.8 (quint., J = 7.4 Hz, 2H, -CH2-CH2-COO-), 2.6 (t, J = 7.4 Hz, 2H, -CH2-COO-), 6.7 (d, J = 8.5 Hz, 1H, Ar-H), 6.8 (s, 1H, Ar-Ar-H), 7.4 (d, J = 8.0 Hz, 1H, Ar-Ar-H), 7.5 (t, J = 8.5 Hz, 2H, Ar-H), 7.8 (d, J = 8.0 Hz, 1H, Ar-H), 8.0 (d, J = 8.2 Hz, 1H, Ar-H), 9.2 (s, 1H, CH=N), 12.5 (s, 1H, -OH). 13C NMR (100 MHz, CDCl3): /ppm 171.38 (-COO-), 168.85 (C=N), 166.35, 163.31, 156.32, 151.43, 135.02, 134.65, 126.71, 126.26, 123.00, 121.72, 116.20, 113.85, 110.70 for aromatic carbons, 34.42, 31.90, 29.65, 29.62, 29.57, 29.42, 29.33, 29.22, 29.04, 24.80, 22.67 for methylene carbons [-COO-(CH2)14-CH3], 14.10 [-COO-(CH2)14-CH3]. EI-MS m/z (rel. int. %): 508 (12) (M)+, 270 (100).
3. 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 nBABTH and nHBABTH. The pro-minent molecular ion peaks of 16BABTH and 16HBABTH at m/z 492 and 508, respectively, in the mass spectra established a molecular formula of C30H40N2O2S and C30H40N2O3S, supporting the pro-posed structures.
3.1 FT-IR,1H NMR and13C NMR spectral studies The FT-IR spectra of the title compounds in both the series showed similarities, thus, 16BABTH was discussed as a representative. From the FT-IR spectrum, the diagnostic absorption band resulting from the C=O stretch of the ester group was pre-sent at 1762 cm-1 while those of the alkyl groups were observed between 2894 and 2954 cm-1. The relative intensity of the absorption bands of the alkyl groups increased upon ascending the series due to the increasing number of carbons in the alkyl chain. The absorption peak of the azo-methine (C=N) group was overlapping with the absorption band arising from the C=N stretch of
Table 1. Percentage yields and analytical data of nBABTH.
% Found (% Calcd.)
Compound Yield (%) Formula C H N
2BABTH 26 C16H12N2O2S 64.94 (64.85) 3.97 (4.08) 9.40 (9.45) 3BABTH 30 C17H14N2O2S 65.72 (65.81) 4.63 (4.52) 9.03 (9.01) 4BABTH 32 C18H16N2O2S 66.70 (66.64) 4.91 (4.97) 8.53 (8.64) 5BABTH 31 C19H18N2O2S 67.54 (67.44) 5.33 (5.38) 8.22 (8.30) 6BABTH 29 C20H20N2O2S 68.12 (68.16) 5.79 (5.72) 7.90 (7.95) 8BABTH 37 C22H24N2O2S 69.38 (69.44) 6.41 (6.36) 7.33 (7.36) 10BABTH 42 C24H28N2O2S 70.66 (70.55) 6.83 (6.91) 6.80 (6.86) 12BABTH 44 C26H32N2O2S 71.59 (71.52) 7.36 (7.39) 6.39 (6.42) 14BABTH 51 C28H36N2O2S 72.47 (72.38) 7.77 (7.81) 5.94 (6.03) 16BABTH 66 C30H40N2O2S 73.19 (73.13) 8.19 (8.18) 5.60 (5.69) 18BABTH 73 C32H44N2O2S 73.71 (73.80) 8.55 (8.52) 5.47 (5.38)
Table 2. Percentage yields and analytical data of nHBABTH.
% Found (% Calcd.)
Compound Yield (%) Formula C H N
8HBABTH 36 C22H24N2O3S 66.71 (66.64) 6.02 (6.10) 7.05 (7.07) 10HBABTH 44 C24H28N2O3S 67.81 (67.90) 6.70 (6.65) 6.64 (6.60) 12HBABTH 39 C26H32N2O\3S 69.08 (69.00) 7.10 (7.13) 6.11 (6.19) 14HBABTH 52 C28H36N2O3S 69.89 (69.97) 7.63 (7.55) 5.87 (5.83) 16HBABTH 64 C30H40N2O3S 70.72 (70.83) 8.00 (7.93) 5.49 (5.51) 920 S.T. Ha et al
the benzothiazole ring, resulting in a sharp and strong absorption peak at 1600 cm-1.
There was a resemblance between the 1H NMR spectra of 16BABTH and 16HBABTH in the region between 0.9 and 2.6 ppm. The triplets at 0.9 ppm in both the spectra were assigned to the terminal methyl protons. The multiplets between 1.2 and 1.4 ppm, the quintet at 1.8 ppm and the triplet at 2.6 ppm were attributed to the remaining methylene protons [CH3-(CH2)14-] of the alkanoyloxy chain. The 1H NMR spectra of 16BABTH and 16HBABTH confirmed the presence of eight and seven aromatic protons, respec-tively. The singlet arising from the azomethine proton, was detected at 9.0 and 9.2 ppm for 16BABTH and 16HBABTH, respectively. However, in the case of 16HBABTH, an intense singlet at 12.5 ppm due to the presence of the lateral hydroxyl group was observed in its spectrum.
In the13C NMR spectra, the peaks at 171.60 and 171.38 ppm were respectively assigned to the carbonyl carbon (-COO-) of 16BABTH and 16HBABTH and this was in agreement with that reported in the litera-ture (22) while those of the azomethine carbon (CH=N) appeared at 164.71 and 168.85 ppm, respec-tively. The signals in the region between 110.70– 166.35 ppm supported the presence of the benzothia-zole and aromatic moieties. The carbon resonances between 14.07 and 34.35 ppm were indicative of the methylene and methyl carbons of the alkanoyloxy chain. The results as inferred from the IR and NMR spectral data of the title compounds were consistent with the proposed structure.
3.2 Mesomorphic properties of nBABTH and nHBABTH
The transition temperatures during the heating and cooling scans and associated enthalpy changes of nBABTH and nHBABTH were determined using DSC analysis and are summarised in Tables 3 and 4. The n-decanoyloxy up to the n-octadecanoyloxy deri-vatives of nBABTH exhibited smectic A phase while nHBABTH was a non-mesogenic series. The DSC thermograms of 8BABTH, 10BABTH and 12BABTH during the heating and cooling scans are depicted in Figure 1. Figure 1(a) revealed that 8BABTH is a non-mesogenic compound since only a crystal–isotropic transition temperature was observed. However, an additional phase transition was detected for 10BABTH (Figure 1(b)) and 18BABTH upon cooling from the isotropic liquid. Therefore it is a monotropic liquid crystal whereby the melting points are always equal to or higher than the clearing points, hence enabling it to exhibit super-cooling properties (1). Conversely, 12BABTH,
14BABTH and 16BABTH are enantiotropic liquid crystals as the liquid crystal phase was observed dur-ing the heatdur-ing and cooldur-ing scans.
Optical photomicrographs of 10BABTH and 16BABTH are shown in Figure 2 as the representative illustration. Upon cooling the isotropic liquid of 10BABTH, the SmA phase emerged as baˆtonnet (Figure 2(a)) coalescing to form a focal-conic fan-shaped texture. Upon cooling of 16BABTH, the co-existence of fan-shaped and homeotropic (dark
Table 3. Transition temperatures and associated enthalpy changes of nBABTH upon heating and cooling.
Compound Transition temperatures,C (H, kJmol-1)
2BABTH* Cr 125.5 (31.82) I 3BABTH Cr 141.1 (12.48) I Cr 114.7 (11.73) I 4BABTH Cr 99.7 (20.70) I Cr 63.0 (20.38) I 5BABTH Cr 83.6 (22.32) I Cr 54.2 (22.43) I 6BABTH Cr 92.0 (23.80) I Cr 45.4 (20.36) I 8BABTH Cr 94.2 (7.74) I Cr 82.3 (7.26) I 10BABTH Cr 88.2 (35.83) I Cr 51.5 (26.79) SmA 71.4 (6.34) I 12BABTH Cr 80.8 (45.07) SmA 85.6 (7.44) I Cr 52.7 (37.81) SmA 81.7 (8.17) I 14BABTH Cr155.7 (3.23) Cr282.3 (38.22) SmA 90.7 (10.32) I Cr149.5 (2.36) Cr261.9 (35.18) SmA 87.9 (10.23) I 16BABTH Cr164.9 (2.21) Cr279.5 (2.61) Cr387.3 (44.24) SmA 92.3 (8.73) I Cr157.3 (3.71) Cr267.8 (44.72) SmA 89.1 (9.79) I 18BABTH Cr169.6 (3.44) Cr291.5 (64.29) I Cr163.7 (3.84) Cr275.6 (48.69) SmA 89.4 (10.41) I
The values in italics were taken during the cooling cycle. Cr = Crystal; SmA = Smectic A; I = Isotropic liquid. *No cooling data due to decomposition.
Table 4. Transition temperatures and associated enthalpy changes of nHBABTH upon heating and cooling.
Compound Transition temperatures,C (H, kJmol-1)
8HBABTH Cr 124.5 (31.10) I Cr 102.0 (30.48) I 10HBABTH Cr 103.5 (31.02) I Cr 92.1 (29.59) I 12HBABTH Cr 99.9 (40.86) I Cr 88.5 (39.40) I 14HBABTH Cr 103.3 (42.07) I Cr 93.2 (39.90) I 16HBABTH Cr172.2 (9.37) Cr2106.9 (52.62) I Cr 79.2 (29.21) I
The values in italics were taken during the cooling cycle. Cr = Crystal; I = Isotropic liquid.
region) textures (Figure 2(b)) were observed. In the homeotropic region, the director of the phase is ortho-gonal to the layer planes. Consequently, the observed phase was assigned as a SmA phase. All observed liquid crystalline textures are typical according to the literatures (26,27).
Figure 3 shows the graph of phase transition temperature versus the number of carbons in the alkanoyloxy chain of nBABTH. It is obvious that the short chain derivatives (n = 2, 3, 4, 5, 6, 8) were non-mesogens. The liquid crystal phase starts to emerge as a monotropic (metastable) SmA phase beginning with the n-decanoyloxy derivative. As
the length of the carbon chain increases from the n-dodecanoyloxy to the n-hexadecanoyloxy deriva-tive, the enantiotropic (stable) SmA phase was observed. The SmA phase range increased from 12BABTH (4.8C) to 14BABTH (8.4C) and then
decreased for 16BABTH (5.0C). Upon further
increasing the length of the carbon chain, the SmA phase for 18BABTH turns to a monotropic phase. This phenomenon could have resulted from the flex-ibility provided by the carbon chain. Generally, high rigidity of a molecule can prevent the forma-tion of a mesophase (1). Once the length of the terminal chain is increased, the molecule becomes more flexible hence promoting a monotropic meso-phase in a particular compound. If the length of the carbon chain keeps increasing, it may generate an
Figure 1. Differential scaning colorimetry thermograms of 8BABTH, 10BABTH and 12BABTH during heating and cooling cycles.
(a)
(b)
Figure 2. (a) Optical photomicrograph of 10BABTH where smectic A phase emerged as baˆtonnet upon cooling from isotropic liquid; (b) optical photomicrograph of 16BABTH
exhibiting smectic A phase with fan-shaped and
homeotropic (dark area) textures.
922 S.T. Ha et al
enantiotropic mesophase. However, a continual increase in the length of the carbon chain will depress the stability of the mesophase or even com-pletely diminishing the mesophase formation.
3.3 X-ray diffraction studies
Generally, the types of liquid crystal phases can be preliminarily concluded based on the DSC and POM analysis. In order to further confirm the smectic phase, temperature-dependent XRD analysis was employed whereby the additional information regarding the layered structure was obtained. The XRD pattern of the representative compound 14BABTH is shown in Figure 4.
In Figure 4, the diffraction pattern showed one sharp peak at a lower region angle and one weak and
broad peak at a wider angle. This kind of diffraction pattern is characteristic of a layered structure observed for a smectic phase (23–25). The XRD patterns of 14BABTH at 80C revealed a sharp diffraction peak at 2.35, implying the formation of a layered structure.
In general, a sharp and strong peak at a low angle (1,2,6) in a small-angle X-ray scattering curve is
observed for smectic structures, unlike nematic and cholesteric structures (28–30).
The d-layer spacing upon cooling 14BABTH from the isotropic liquid was 35.52 A˚ whereas the molecular length obtained by MM2 molecular calculation was 31.93 A˚ . Since the d/l ratio was calculated to be 1.11 (d/ l , 1), the SmA phase for 14BABTH was suggested to have a monolayer arrangement (31).
3.4 Chemical structure–mesomorphic property relationship
The mesomorphic behaviour of an organic compound is dependant on its molecular architecture. Thus, a detailed study of a homologous series may help to derive some general rules on how chemical constitu-tion is able to affect nematogenic and smectogenic compounds, or even the mesophase ranges. In Table 5, the transition temperatures, types of meso-phases, mesophase range and molecular structures of nBABTH and nHBABTH are compared with structu-rally related compounds A (19), B (23) and C (32).
The difference between the molecular structures of 10BABTH and compound A is in the attachment of the alkoxyl chain (C10H21O-) to the benzothiazole core in the latter and the absence of it from the former. While 10BABTH exhibited a monotropic (metastable) SmA phase, compound A exhibited enantiotropic SmC and nematic phases. The presence of the terminal alkoxyl chains identical in length at both sides of the mesogenic core of compound A helped to generate the more stable (enantiotropic) smectic phase. Unlike 10BABTH which exhibited a non-tilted (SmA) phase, compound A exhibited a tilted (SmC) meso-phase. This could have resulted from the influence of the alkoxyl chain at the sixth position of the ben-zothiazole moiety. An ethoxyl or a longer alkoxyl chain tends to favour the intermolecular interactions giving rise to a tilted arrangement of the molecules in the smectic layers (21).
16HBABTH is a non-mesogenic compound whereas a SmA phase was observed for compound B. Both structures differ only in the mesogenic core whereby 16HBABTH comprised a benzothiazole ring while compound B a benzene ring. The fused-ring sys-tem of benzothiazole tends to increase the molecular breadth, resulting in the thickening effect which dis-rupts the liquid crystalline packing (33).
Figure 3. Plot of transition temperatures versus the number of carbons (n) in the alkanoyloxy chain of nBABTH during heating cycle.
Figure 4. X-ray diffraction patterns of 14BABTH at different temperatures upon cooling from the isotropic phase.
In order to reveal the effect of the lateral hydroxyl group on mesomorphic properties, the molecular structures and phase behaviours of both the title com-pounds were compared. nBABTH (where n = 10–18) without the lateral hydroxyl group, exhibited a SmA phase whereas nHBABTH with the lateral hydroxyl group is a non-mesogenic series. Generally, lateral substituents will disturb the molecular packing, hence reducing the mesophase stability (1). However, in this case, the absence of the mesophase in nHBABTH can be explained by the presence of the lateral hydroxyl
group that reduces the molecular length-to-breadth ratio, therefore diminishing the mesophase in nHBABTH. A similar structure–properties relationship can be inferred by comparing the molecular structure of 12HBABTH and compound C. The compound with-out a lateral hydroxyl group (compound C) is still able to exhibit mesophases despite having a broader naphthalene molecule. However, the presence of the lateral hydroxyl group in nHBABTH disturbed the molecular ordering required to generate a liquid crystal phase.
Table 5. Transition temperatures, mesophase range and molecular structures of nBABTH, nHBABTH, A, B and C.
N S N O O CnH2n+1 nBABTH N S N O O Cn-1H2n-1 O H nHBABTH N S N OC10H21 H21C10O Compound A N O O C16H33 O H Compound B N OC12H25 Compound C Mesophase range (C)
Compound Transition temperatures (C) Sm N
10BABTH Cr 88.2 (SmA 71.4)* I - -12HBABTH Cr 99.9 I - -16HBABTH Cr 106.9 I - -A Cr 81.3 SmC 121.6 N 122.7 I 40.3 1.1 B Cr 68 SmA 86 I 18.0 -C Cr 92 (SmA 76 N 79)* I -
-Note: ( )* Monotropic value
924 S.T. Ha et al
4. Conclusions
In this paper, we have described the synthesis and mesomorphic behaviour of two homologous series of 2-(4-alkanoyloxybenzylidenamino)benzothiazoles and 2-(2-hydroxy-4-alkanoyloxybenzylidenamino)-benzothiazoles. The n-decanoyloxy to the n-octadeca-noyloxy derivatives of nBABTH exhibited a SmA phase. nHBABTH is a non-mesogenic series due to the presence of the lateral hydroxyl group. The study also revealed that the mesophase range was greatly affected by the length of the terminal chain. In addi-tion, the terminal group at the lateral position was also found to influence the molecular tilting.
Acknowledgements
The author (S.T. Ha) would like to thank Universiti Tunku Abdul Rahman (UTAR) for the UTAR Research Fund 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 and teaching assistantships. The powder XRD measurements were supported by beamline BL17A (charged by Dr. Jey-Jau Lee) of the National Synchrotron Radiation Research Center, Taiwan.
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