Synthesis and Mesomorphic Properties of 6-Methoxy- and 6-Ethoxy-2-(2-Hydroxy-4-Alkanoyloxybenzylidenamino)Benzothiazoles

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Synthesis and Mesomorphic

Properties of 6-Methoxy- and

6-Ethoxy-2-(2-Hydroxy-4-Alkanoyloxybenzylidenamino)Benzothiazoles

Sie-Tiong Ha a , Teck-Ming Koh b , Guan-Yeow Yeap c , Hong-Cheu Lin d

, Siew-Ling Lee e , Yip-Foo Win a & Siew-Teng Ong a a

Department of Chemical Science, Faculty of Science , University Tunku Abdul Rahman, Jln University, Bandar Barat , Kampar, Perak, Malaysia

b

Department of Science, Faculty of Engineering & Science ,

University Tunku Abdul Rahman, Jln Genting Kelang , Setapak, Kuala Lumpur, Malaysia

c

Liquid Crystal Research Laboratory, School of Chemical Sciences, University Sains Malaysia , Minden, Penang, Malaysia

d

Department of Materials Science & Engineering , National Chiao Tung University , Hsinchu, Taiwan, Republic of China

e

Ibnu Sina Institute for Fundamental Science Studies, University Teknologi Malaysia , Skudai, Johor, Malaysia

Published online: 20 Oct 2010.

To cite this article: Sie-Tiong Ha , Teck-Ming Koh , Guan-Yeow Yeap , Hong-Cheu Lin , Siew-Ling Lee ,

Yip-Foo Win & Siew-Teng Ong (2010) Synthesis and Mesomorphic Properties of Methoxy- and 6-Ethoxy-2-(2-Hydroxy-4-Alkanoyloxybenzylidenamino)Benzothiazoles, Molecular Crystals and Liquid Crystals, 528:1, 10-22, DOI: 10.1080/15421406.2010.504510

To link to this article: http://dx.doi.org/10.1080/15421406.2010.504510

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Synthesis and Mesomorphic Properties of

6-Methoxy- and

6-Ethoxy-2-(2-Hydroxy-4-Alkanoyloxybenzylidenamino)Benzothiazoles

SIE-TIONG HA,

1

TECK-MING KOH,

2

GUAN-YEOW YEAP,

3

HONG-CHEU LIN,

4

SIEW-LING LEE,

5

YIP-FOO WIN,

1

AND

SIEW-TENG ONG

1

1

Department of Chemical Science, Faculty of Science, University Tunku Abdul Rahman, Jln University, Bandar Barat, Kampar, Perak, Malaysia

2

Department of Science, Faculty of Engineering & Science, University Tunku Abdul Rahman, Jln Genting Kelang, Setapak, Kuala Lumpur, Malaysia

3

Liquid Crystal Research Laboratory, School of Chemical Sciences, University Sains Malaysia, Minden, Penang, Malaysia

4

Department of Materials Science & Engineering, National Chiao Tung University, Hsinchu, Taiwan, Republic of China

5

Ibnu Sina Institute for Fundamental Science Studies, University Teknologi Malaysia, Skudai, Johor, Malaysia

Two new series of Schiff base thermotropic liquid crystals, 6-methoxy-2-(2-hydroxy-4-alkanoyloxybenzylidenamino)benzothiazoles and 6-ethoxy-2-(2-hydroxy-4-alkanoyloxybenzylidenamino)benzothiazoles, comprising different terminal groups, a methoxyl and ethoxyl group, respectively, were synthesized. Structural elucidation was carried out using elemental analysis and spectroscopic techniques such as Fourier transform infrared (FTIR),1_{H and}13_{C nuclear magnetic}

resources (NMR), and mass spectrometry. The mesomorphic properties and ther-mal stabilities of the title compounds were studied by using differential scanning calorimetry, optical polarizing microscopy, and thermogravimetric analysis. No liquid-crystal phases were observed for the short-chain members (n¼ 2 and 3) in both series, and the remaining members all exhibited nematic phase with Schlieren or marble-like textures. Effects of the lateral hydroxyl group, terminal group, and the length of the terminal alkanoyloxy chain on the mesomorphic properties are dis-cussed. Structure-property relationships were established upon comparison with other structurally related compounds.

Keywords Benzothiazole liquid crystals; lateral hydroxyl group; nematic; structure-property relationship; terminal group

Address correspondence to Sie-Tiong Ha, Department of Chemical Science, Faculty of Science, University Tunku Abdul Rahman, Jln University, Bandar Barat, 31900 Kampar, Perak, Kuala Lumpur 53300, Malaysia. E-mail: [email protected]

Mol. Cryst. Liq. Cryst., Vol. 528: pp. 10–22, 2010 Copyright # Taylor & Francis Group, LLC ISSN: 1542-1406 print=1563-5287 online DOI: 10.1080/15421406.2010.504510

10

Introduction

Since the 1960s, the unique properties of liquid crystals have been extensively applied in modern technology such as liquid-crystal displays (LCDs), thermocromic materials, and many other areas. In general, rod-like (calamitic) molecules are highly favorable in inducing mesomorphic behavior due to their high shape anisotropy [1–3]. A mesogenic core, terminal groups, and a flexible terminal chain are the fundamental prerequisites in designing new thermotropic liquid crystals [4].

It is also commonly believed that molecular order in liquid-crystal phases depends largely on the structure of the mesogenic core: its geometry, polarizability, molecular conformation, length-to-breath ratio, as well as the number and position of permanent dipole moments [5]. Mesogenic compounds having single-ring hetero-cyclic moieties, such as pyridine [6], thiophene [7] and 1,3,4-thiadiazole [8], have been reported in the literature. These heterocyclic mesogens are usually incorporated with heteroatoms, such as N, O, and S, resulting in a reduced symmetry in the overall molecule together with a stronger polar induction. The inclusion of the heteroatoms can significantly change the polarity, polarizability, and, to a certain extent, geometry of a molecule, thereby influencing its type of mesophase, the phase tran-sition temperatures, dielectric constants, and other properties [9,10].

Benzothiazole derivatives, an important class of heterocyclic compounds, have been extensively studied due to their potent antitumor and antibacterial activities [11]. Benzothiazole-type liquid crystals have been proven to exhibit good hole-transporting properties with a low ionization potential, making them potential hole-transporting materials in organic light-emitting devices (OLEDs) [12]. However, examples of benzothiazole-based liquid crystals are relatively scant. Pavluchenko et al. [13] reported on mesogens comprising benzothiazole and benzoxazole moieties with different central linkages and lateral substituents at different positions to evaluate the effect of structural changes on mesomorphic properties. Additionally, benzothiazole-type mesogens having an azo central linkage with different terminal groups (OCH3, Cl, and NO2) at the sixth

position of the benzothiazole moiety have been reported recently [14–16] in which the nitro substituent was found to be more conducive to the generation of the smectic mesophase compared to the chloro and methoxyl substituents.

Furthermore, it was believed that the lateral hydroxyl group could have dis-rupted or even destroyed mesomorphism due to strong intermolecular hydrogen bonding [17]. However, previous study has shown that the presence of the lateral hydroxyl group at the ortho-position may have led to an increase in the molecular polarizability as well as in the clearing temperature [18]. The lateral hydroxyl group, on the other hand, can also enhance the stability of a molecule through intramolu-cular hydrogen bonding [19].

As a continuation of our previous work on the synthesis and study of mesomorphic properties of compounds bearing a benzothiazole core unit, here we have synthesized two new mesogenic homologous series of benzothiazole derivatives, each having a Schiff base central linkage, a lateral hydroxyl group, and different terminal groups (OCH3and OC2H5).

Experimental

4-Dimethylaminopyridine (DMAP) and fatty acids (Cn-1H2n-1COOH where n¼ 2,

4, 6, 7, 12, 14, 16, 18) were obtained from Merck (Darmstadt, Germany).

2-Amino-6-ethoxybenzothiazole, 2-amino-6-methoxybenzothiazole, 2,4-dihydroxy-benzaldehyde, fatty acids (Cn-1H2n-1COOH where n¼ 3, 5, 8, 10), and N,N0

-dicyclohexylcarbodiimide (DCC) were purchased from Acros Organics (New Jersey, USA). All solvents and reagents were purchased commercially and used without any further purification.

Fourier transform infrared (FTIR) analyses were performed on a Perkin-Elmer System 2000 FTIR Spectrometer (Universiti Tunku Abdul Rahman and Universiti Sains Malaysia). All compounds were analyzed using KBr discs with a measurement range from 4000 to 400 cm
1. The1H nuclear magnetic resonance (NMR) (400 MHz) and13C NMR (100 MHz) spectra were recorded in CDCl3using a JEOL LA-400 MHz

NMR spectrometer with tetramethylsilane (TMS) as the internal standard. Electron ionization mass spectrometry (EI-MS) (70 eV) were measured with a Finnigan MAT95XL-T mass spectrometer at a source temperature of 200C. Microanalyses were carried out on Perkin Elmer 2400 LS Series CHNS=O analyzer. Thin-layer Chromato-graphy (TLC) was carried out on aluminum-backed silica-gel plates (Merck 60 F254

(Darmstadt, Germany)) and visualized under short-wave ultraviolet (UV) light. Phase-transition temperatures and enthalpy changes were measured using a Mettler Toledo DSC823edifferential scanning calorimeter at heating and cooling rates of 10C= min and
10_{C=min, respectively. Thermogravimetric analysis was performed on a} Mettler Toledo thermal gravimetric Analyzer, which consisted of a TGA=SDTA851e main unit and a STARe software at a heating rate of 20_{C=min in nitrogen atmosphere.} A polarizing optical microscope (Carl Zeiss, Universiti Sains Malaysia) 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 micro-scope was coupled to a video capture card (Video Master coomo600), allowing real-time video capture and image saving. The 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 coverslip.

Synthesis

12MHBABTH. The synthetic route for the title compounds is depicted in Scheme 1. 2-Amino-6-methoxybenzothiazole (40 mmol, 7.21 g) and 2,4-dihydroxybenzaldehyde (40 mmol, 5.52 g) were dissolved in 60 mL ethanol. Two drops of acetic acid were added and the mixture was refluxed for 3 h upon stirring. The mixture was then filtered and the filtrate was left to evaporate to dryness. The yellow solid that was formed was recrystallized with ethanol for further reaction. The benzothiazole intermediate 1 (20 mmol), along with the appropriate fatty acid (20 mmol) and DMAP (4 mmol, 0.49 g) were dissolved in a 50-mL mixture of dichloromethane (DCM) and dimethylformamide (DMF) and stirred at 0_{C. DCC (20 mmol,} 4.13 g) dissolved in 10 ml of DCM was added into the mixture dropwise and continuously stirred for an hour at 0_{C. The mixture was then stirred at room} temperature for another 3 h. Finally, the mixture was filtered and the solvent was removed by evaporation. The yellow solid obtained was recrystallized using ethanol. Infrared (IR) (KBr) vmaxcm
1 3431 (O
H), 3069 (C
H aromatic), 2921,

2851 (C
H aliphatic), 1758 (C=_{O ester), 1611 (C}=_{N thiazole).} 1

H NMR (400 MHz, CDCl3): d=ppm 0.9 (t, J¼ 6.6 Hz, 3H, CH3
), 1.2–1.4 (m, 16H,

CH3(CH2)8
), 1.8 (q, J ¼ 7.3 Hz, 2H,
CH2CH2COO
), 2.6 (t, J ¼ 7.3 Hz, 2H,

CH2COO
), 3.9 (s, 3H, CH3O
), 6.7 (d, J ¼ 8.5 Hz, 1H, Ar-H), 6.8 (s, 1H,

12 S.-T. Ha et al.

Ar-H), 7.1 (d, J¼ 9.0 Hz, 1H, Ar-H), 7.2 (s, 1H, Ar-H), 7.5 (d, J ¼ 8.5 Hz, 1H, Ar-H), 7.8 (d, J¼ 8.8 Hz, 1H, Ar-H), 9.2 (s, 1H,
N=_{CH
), 12.5 (s, 1H, OH).}

13

C NMR (400 MHz, CDCl3): d=ppm 171.54 (
COO
), 166.45 (C=N), 165.36,

163.20, 157.88, 156.15, 145.90, 136.02, 134.88, 123.76, 116.41, 116.12, 113.87, 110.76, 104.42 for aromatic carbons, 55.86 (CH3O
), 34.53 (
CH2COO
), 32.01

(
CH2CH2COO
), 29.70, 29.55, 29.43, 29.35, 29.15, 24.93, 22.78 for methylene

carbons [CH3(CH2)12CH2CH2COO
], 14.23 [CH3(CH2)14COO
]. EI-MS m=z

(rel. int. %): 482 (9) [Mþ], 300 (100).

16EHBABTH. The synthetic method is identical to that of 12MHBABTH. IR (KBr) vmaxcm
13420 (O
H), 3075 (C
H aromatic), 2922, 2850 (C
H aliphatic),

1758 (C=O ester), 1609 (C=N, thiazole). 1H NMR (400 MHz, CDCl3): d=ppm 0.9

(t, J¼ 6.6 Hz, 3H, CH3
), 1.2 (m, 24H, CH3(CH2)12CH2CH2COO
), 1.5 (t,

J¼ 7.2 Hz, 3H, CH3CH2O
), 1.8 (q, J ¼ 7.1 Hz, 2H,
CH2CH2COO
), 2.6 (t,

J¼ 7.4 Hz, 2H,
CH2COO
), 4.1 (q, J ¼ 6.9 Hz, 2H, CH3CH2O
), 6.7 (d,

J¼ 8.5 Hz, 1H, Ar-H), 6.8 (s, 1H, Ar-H), 7.1 (d, J ¼ 8.8 Hz, 1H, Ar-H), 7.3 (s, 1H, Scheme 1. Synthetic route of nMHBABTH and nEHBABTH.

Ar-H), 7.5 (d, J¼ 8.5 Hz, 1H, Ar-H), 7.8 (d, J ¼ 9.1 Hz, 1H, Ar-H), 9.2 (s, 1H,
N=_{CH
), 12.5 (s, 1H, OH).} 13

C NMR (400 MHz, CDCl3): d=ppm 171.40

(
COO
), 166.24 (C=_{N), 165.18, 163.07, 157.15, 156.02, 145.70, 135.89, 134.74,} 123.63, 116.42, 116.31, 113.74, 110.63, 104.96 for aromatic carbons, 64.08 (CH3CH2O
), 34.41, 31.90, 29.68, 29.64, 29.57, 29.42, 29.34, 29.22, 29.04, 24.82,

22.67 for methylene carbons [CH3(CH2)14COO
], 14.78 (CH3CH2O
), 14.10

[CH3(CH2)14COO
]. EI-MS m=z (rel. int. %): 552(15) (M)þ, 312 (100).

Results and Discussion

Structural identification of the title compounds was carried out by employing a combination of elemental analysis and spectroscopic techniques (FTIR, NMR, and EI-MS). The percentages of C, H, and N from the elemental analysis (Tables 1 and 2) conform with the calculated values for compounds nMHBABTH and nEHBABTH. The prominent molecular ion peaks in the mass spectra of 12MHBABTH and 16EHBABTH at m=z 482 and 552, respectively, established a molecular formula of C27H34N2O4S and C32H44N2O4S, thus supporting the proposed structures.

Mesomorphic Behavior and Thermogravimetric Analysis

All the title compounds were investigated by polarizing optical microscopy (POM) and differential scanning calorimetry (DSC) to determine their mesomorphic proper-ties. Phase transition temperatures and corresponding enthalpy changes of com-pound nMHBABTH and nEHBABTH were determined using DSC. The data for nMHBABTH and nEHBABTH obtained from the DSC analysis are tabulated in Tables 3 and 4, respectively. The short-chain members (n¼ 2, 3, and 4) of nMHBABTH and all members of nEHBABTH exhibited no distinct exothermic peaks during the cooling cycle due to the partial decomposition of the compounds. This phenomenon was also observed in the work of Wei et al. [20]. The decompo-sition temperatures were further confirmed by thermogravimetric analysis (TGA; Table 1. Percentage yields and analytical data of nMHBABTH

% Found (% Calcd.)

Compound Yield (%) Formula C H N 2MHBABTH 26 C17H14N2O4S 59.72 (59.64) 4.01 (4.12) 8.05 (8.18) 3MHBABTH 21 C18H16N2O4S 60.78 (60.66) 4.48 (4.53) 7.79 (7.86) 4MHBABTH 32 C19H18N2O4S 61.69 (61.61) 4.83 (4.90) 7.50 (7.56) 5MHBABTH 36 C20H20N2O4S 62.43 (62.48) 5.27 (5.24) 7.37 (7.29) 6MHBABTH 40 C21H22N2O4S 63.41 (63.30) 5.50 (5.56) 6.94 (7.03) 7MHBABTH 48 C22H24N2O4S 64.16 (64.06) 5.80 (5.86) 6.71 (6.79) 8MHBABTH 41 C23H26N2O4S 64.83 (64.77) 6.10 (6.14) 6.47 (6.57) 10MHBABTH 55 C25H30N2O4S 66.11 (66.05) 6.57 (6.65) 6.12 (6.16) 12MHBABTH 46 C27H34N2O4S 67.07 (67.19) 7.16 (7.10) 5.75 (5.80) 14MHBABTH 59 C29H38N2O4S 68.27 (68.20) 7.48 (7.50) 5.47 (5.49) 16MHBABTH 63 C31H42N2O4S 69.19 (69.11) 7.79 (7.86) 5.22 (5.20) 18MHBABTH 67 C33H46N2O4S 69.99 (69.93) 8.10 (8.18) 4.91 (4.94) 14 S.-T. Ha et al.

Table 3. Transition temperatures and associated enthalpy changes of nMHBABTH upon heating

Compounds

Transition temperature (C) (associated enthalpy changes, kJ mol
1) 2MHBABTHa Cr 186.8 (45.57) I 3MHBABTHa Cr 143.0 (35.55) I 4MHBABTHa Cr 154.1 (36.58) N 162.4 (0.60) I 5MHBABTH Cr 137.8 (33.89) N 147.6 (0.32) I Cr 92.8 (26.86) N 138.2 (0.33) I 6MHBABTH Cr 131.1 (38.74) N 155.6 (0.63) I Cr 91.8 (32.89) N 141.8 (0.81) I 7MHBABTH Cr1121.6 (1.13) Cr2133.6 (38.63) N 145.8 (0.49) I Cr 114.3 (40.13) N 143.6 (0.39) I 8MHBABTH Cr 133.2 (37.66) N 145.8 (0.60) I Cr 100.7 (33.72) N 132.5 (0.84) I 10MHBABTH Cr 127.5 (90.04) N 139.8 (1.81) I Cr 98.9 (89.07) N 135.9 (2.55) I 12MHBABTH Cr 124.9 (43.25) N 136.8 (0.98) I Cr 107.7 (43.28) N 133.8 (0.53) I 14MHBABTH Cr 123.6 (54.67) N 133.6 (0.84) I Cr 103.7 (53.75) N 130.7 (0.88) I 16MHBABTH Cr 123.2 (61.86) N 129.7 (1.51) I Cr 95.7 (61.22) N 126.5 (1.51) I 18MHBABTH Cr 122.3 (61.27) N 125.2 (1.23) I Cr 110.8 (58.31) N 117.0 (0.89) I Cr¼ Crystal; N ¼ nematic; I ¼ isotropic liquid.

a

Only heating data are provided. No distinct peak was detected during the cooling scan due to partial decomposition.

Cooling data was shown in italics.

Table 2. Percentage yields and analytical data of nEHBABTH % Found (% Calcd.)

Compound Yield (%) Formula C H N 2EHBABTH 23 C18H16N2O4S 60.78 (60.66) 4.50 (4.53) 7.75 (7.86) 3EHBABTH 28 C19H18N2O4S 61.66 (61.61) 4.85 (4.90) 7.50 (7.56) 4EHBABTH 31 C20H20N2O4S 62.40 (62.48) 5.13 (5.24) 7.31 (7.29) 5EHBABTH 37 C21H22N2O4S 63.25 (63.30) 5.63 (5.56) 7.09 (7.03) 6EHBABTH 35 C22H24N2O4S 64.00 (64.06) 5.90 (5.86) 6.82 (6.79) 7EHBABTH 36 C23H26N2O4S 64.69 (64.77) 6.21 (6.14) 6.59 (6.57) 8EHBABTH 42 C24H28N2O4S 65.47 (65.43) 6.30 (6.41) 6.66 (6.63) 10EHBABTH 40 C26H32N2O4S 66.72 (66.64) 6.79 (6.88) 5.90 (5.98) 12EHBABTH 55 C28H36N2O4S 67.76 (67.71) 7.24 (7.31) 5.67 (5.64) 14EHBABTH 58 C30H40N2O4S 68.61 (68.67) 7.66 (7.68) 5.31 (5.34) 16EHBABTH 61 C32H44N2O4S 69.62 (69.53) 7.98 (8.02) 5.02 (5.07) 18EHBABTH 65 C34H48N2O4S 70.39 (70.31) 8.24 (8.33) 4.85 (4.82)

Fig. 1). For instance, the decomposition temperature of 4MHBABTH was found to be 168.38_{C and the melting temperature from the DSC analysis was 162.4}_{C. A} similar situation was observed for 6EHBABTH.

From Tables 3 and 4 it is obvious that the ethanoyloxy and propanoyloxy deri-vatives are nonmesogenic compounds and the liquid-crystal phase only started to emerge from the butanoyloxy derivative onwards in both series. Observation under polarizing optical microscopy revealed that all the compounds are pure nematogens. Representative optical photomicrographs of 6MHBABTH are depicted in Fig. 2. Upon cooling of 6MHBABTH from its isotropic liquid phase, nematic droplets (Fig. 2a) appeared and coalesced with each other to form the classical nematic phase with a marbled-like texture (Fig. 2b). For series nEHBABTH, the representative optical photomicrographs of 10EHBABTH are given in Fig. 3. Similarly, upon cool-ing of its isotropic liquid, the nematic phase was the first to emerge as small droplets (Fig. 3a) and, upon further cooling, nematic phase with disclination lines was formed (Fig. 3b). All observed liquid-crystalline textures are typical according to the litera-ture [21,22].

The effects of the terminal alkanoyloxy chain on mesomorphic properties can be established by plotting a graph of phase transition temperature against the number of carbon atoms in the alkanoyloxy chain. According to Fig. 4, the odd–even effect was noticed in the short-chain members (n¼ 4, 5, 6, 7, and 8). In addition, the clear-ing temperatures of nMHBABTH exhibited a typical descendclear-ing trend as the length of the carbon chain increased. This was attributed to the dilution of the mesogenic core, affected by the increase in the flexibility of the terminal alkanoyloxy chain [23]. This trend was in agreement with those homologous series of 2-(4-n-alkoxyphenylazo)-6-methoxybenzothiazoles reported by Prajapati and Bonde [16].

Table 4. Transition temperatures and associated enthalpy changes of nEHBABTH upon heating

Compounds

Transition temperature (_C) (associated enthalpy changes, kJ mol
1) 2EHBABTHa Cr 173.8 (50.4) I 3EHBABTHa Cr 154.2 (54.9) I 4EHBABTHa Cr 136.0 (33.9) N 175.9 (0.7) I 5EHBABTHa Cr180.6 (10.7) Cr2128.7 (34.0) N 162.1 (0.7) I 6EHBABTHa Cr 128.6 (40.8) N 147.9 (0.7) I 7EHBABTHa Cr 118.5 (38.5) N 155.1 (1.0) I 8EHBABTHa Cr 128.6 (38.2) N 152.0 (0.8) I 10EHBABTHa Cr 126.8 (47.5) N 149.0 (0.9) I 12EHBABTHa Cr 126.5 (52.0) N 137.6 (1.1) I 14EHBABTHa Cr 124.4 (49.7) N 129.5bI 16EHBABTHa Cr 124.0 (59.7) N 130.0bI 18EHBABTHa Cr 124.4 (73.0) N 127.9bI

Cr¼ Crystal; N ¼ nematic; I ¼ isotropic liquid.

a

Only heating data are provided. No distinct peak was detected during the cool-ing scan due to its partial decomposition.

b

POM data were used. The N-I transition was untraced by DSC analysis although this transition was observed through microscope studies.

16 S.-T. Ha et al.

Figure 1. Thermogravimetric analysis curves of (a) 4MHBABTH and (b) 6EHBABTH.

Figure 2. Optical photomicrographs of 6MHBABTH. Upon cooling from isotropic liquid, nematic droplets (a) appeared at 141_{C and (b) coalesced to form the classical marble-like}

tex-ture of the nematic phase.

Generally, short-chain members favor nematic formation, whereas the smectic phase is more favorable in long-chain members [5]. This general trend was obeyed by the nMHBABTH series depicted in Fig. 4 in which the nematic phase range reduced as the length of the terminal chain increased.

The plot of phase transition temperature against the number of carbon atoms in the terminal chain of nEHBABTH is depicted in Fig. 5. The odd–even effects on the mesomorphic properties are not that obvious but still observed for the C5 to C8 deri-vatives. The clearing temperatures showed a descending trend with the increase in the length of the carbon chain. Due to the same reason as discussed for series nMHBABTH, the nematic phase range was found to decrease as the length of the terminal alkyl chain is increased.

Figure 4. Plot of transition temperatures versus the number of carbons (n) in the alkanoyloxy chain of nMHBABTH during the heating cycle.

Figure 3. Optical photomicrographs of 10EHBABTH. During the first cooling under POM, the nematic phase emerged as small droplets (a) and coalesced with each other to form a Schlieren nematic phase with disclination lines (b).

18 S.-T. Ha et al.

Chemical Structure-Mesomorphic Property Relationship

By comparing the present compounds to other structurally related compounds having different types of terminal groups, general rules for the effect of chemical constitution in nematogenic and smectogenic compounds can be deduced. In Table 5, the transition temperatures, types of mesophases, mesophase range, and molecular structures of 12MHBABTH and 12EHBABTH are compared with other structurally related compounds A [24], B [16], C [14], and D [15].

In order to clarify the importance of the terminal group on mesomorphic properties, we first compared 12MHBABTH to compound A. Both compounds have a similar molecular structure, the only difference being in the terminal group, in which 12MHBABTH possessed a terminal methoxyl group attached to the ben-zothiazole fragment unlike compound A. It can be noticed that 12MHBABTH exhibited a nematic phase, whereas compound A is a non mesogenic compound. Therefore, the introduction of a polar group (
OCH3) to 12MHBABTH increased

its polarizability, resulting in the emergence of a mesophase.

Comparison of the transition temperatures and the mesophase ranges between 12MHBABTH and 12EHBABTH revealed similarities. The similar polarities of the terminal methoxyl and ethoxyl groups resulted in both compounds exhibiting similar melting and clearing temperatures. An earlier report claimed that the ter-minal methoxyl group stabilizes the nematic phase, whereas the ethoxyl group or a longer alkoxyl chain at the sixth position of the benzothiazole ring will stabilize the smectic phase [25]. However, in the case of 12EHBABTH, it exhibited a nematic phase instead. This is explained by the presence of the lateral hydroxyl group, which increased the broadness of the molecule, in turn facilitating the nematic phase instead of the smectic phase [5].

The main differences between 12MHBABTH and compound B are in the linking group and the lateral substituent. 12MHBABTH possesses a Schiff base linkage and a lateral hydroxyl group, and compound B consists of an azo linkage with an Figure 5. Plot of transition temperatures versus the number of carbons (n) in the alkanoyloxy chain of nEHBABTH during the heating cycle.

absence of the lateral group. The melting temperature of 12MHBABTH (124.9C) was found to be higher than that of compound B (113C), which could have been due to the intramolecular hydrogen bonding between the lateral hydroxyl (OH) and imine (CH=_{N) groups and the enhanced interactions arising from the overall} geometry of the molecule [26]. However, the mesophase range of 12MHBABTH (11.9C) was smaller than that of compound B (22.0C) resulting from the lateral hydroxyl group involved in a strong intermolecular hydrogen bonding (OH N), leading to the disruption in the mesomorphism of the compound, hence depressing the mesophase range [17].

On the other hand, by incorporating more polar terminal groups (such as
NO2

and
Cl) to compounds C and D, respectively, their mesophase ranges were found to Table 5. Transition temperatures, mesomorphic properties, and molecular structures of 12MHBABTH, 12EHBABTH, A, B, and C

12MHBABTH 12EHBABTH Compound A Compound B Compound C Compound D Compound Transition temperatures (_C) Mesophase range (C) Commencement of smectic phase Sm N 12MHBABTH Cr 124.9 N 136.8 I — 11.9 — 12EHBABTH Cr 126.5 N 137.6 I — 11.1 — A Cr 99.9 I — — — B Cr 113 (SmA 108) N 135 I — 22.0 C12 C Cr 156.0 SmA 194.0 I 38.0 — C4 D Cr 80.0 SmA 172.0 I 90.0 — C5  20 S.-T. Ha et al.

Means monotropic data.

have increased dramatically to 38.0C and 90C. Moreover, another interesting behavior that was also observed for compounds C and D was that both exhibited an SmA phase instead of the nematic phase like in 12MHBABTH. It might have been due to the presence of the polar group that favoured the lamellar packing, resulting in generation of the smectic phase.

Conclusions

Two nematic homologues series of 6-methoxy-2-(2-hydroxy-4-alkanoyloxybenzyli-denamino)benzothiazoles and 6-ethoxy-2-(2-hydroxy-4-alkanoyloxybenzylidenami-no)benzothiazoles were successfully synthesized and characterized. The short-chain derivatives (n¼ 2 and 3) were non-mesogenic compounds and the nematic phase was observed for the remaining members in both series. A polar terminal group is crucial in inducing mesophase formation. Although the lateral hydroxyl group increases the melting temperature due to intramolecular hydrogen bonding, it also decreases the mesophase range. More polar terminal groups such as nitro or chloro can be introduced to the compound in order to generate a smectic phase with a large mesophase range.

Acknowledgment

S. T. Ha is grateful to Universiti Tunku Abdul Rahman (UTAR) for the research facilities and financial support through the UTAR Research Fund (Vote No. 6200=H02). T. M. Koh acknowledges UTAR for the award of research and teaching assistantships.

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