Synthesis and mesomorphic properties of 6-methoxy- and 6-ethoxy-2-(2- hydroxy-4-alkanoyloxybenzylidenamino)benzothiazoles

Article abstract of DOI:10.1080/15421406.2010.504510

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 etho

Full text of DOI:10.1080/15421406.2010.504510

This article was downloaded by: [McMaster University]
On: 29 December 2014, At: 15:38
Publisher: Taylor & Francis
Informa Ltd Registered in England and Wales Registered Number: 1072954 Registered
office: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK
Molecular Crystals and Liquid Crystals
Publication details, including instructions for authors and
subscription information:
http://www.tandfonline.com/loi/gmcl20
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 6-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
PLEASE SCROLL DOWN FOR ARTICLE
Taylor & Francis makes every effort to ensure the accuracy of all the information (the
“Content”) contained in the publications on our platform. However, Taylor & Francis,
our agents, and our licensors make no representations or warranties whatsoever as to
the accuracy, completeness, or suitability for any purpose of the Content. Any opinions
and views expressed in this publication are the opinions and views of the authors,
and are not the views of or endorsed by Taylor & Francis. The accuracy of the Content
should not be relied upon and should be independently verified with primary sources
of information. Taylor and Francis shall not be liable for any losses, actions, claims,

proceedings, demands, costs, expenses, damages, and other liabilities whatsoever or
howsoever caused arising directly or indirectly in connection with, in relation to or arising
out of the use of the Content.
This article may be used for research, teaching, and private study purposes. Any
substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing,
systematic supply, or distribution in any form to anyone is expressly forbidden. Terms &
Conditions of access and use can be found at http://www.tandfonline.com/page/terms-
and-conditions

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
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 ONG1
1Department of Chemical Science, Faculty of Science, University Tunku
Abdul Rahman, Jln University, Bandar Barat, Kampar, Perak, Malaysia
2Department of Science, Faculty of Engineering & Science, University
Tunku Abdul Rahman, Jln Genting Kelang, Setapak, Kuala Lumpur,
Malaysia
3Liquid Crystal Research Laboratory, School of Chemical Sciences,
University Sains Malaysia, Minden, Penang, Malaysia
4Department of Materials Science & Engineering, National Chiao Tung
University, Hsinchu, Taiwan, Republic of China
5Ibnu 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), 1H and 13C 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 (n ¼ 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: hast@utar.edu.my
10

New Series of Schiff Base Thermotropic Liquid Crystals
11
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 (OCH3 and OC2H5).
Experimental
4-Dimethylaminopyridine (DMAP) and fatty acids (Cn-1H2n-1COOH where n ¼ 2,
4, 6, 7, 12, 14, 16, 18) were obtained from Merck (Darmstadt, Germany).

12
S.-T. Ha et al.
2-Amino-6-ethoxybenzothiazole, 2-amino-6-methoxybenzothiazole, 2,4-dihydroxy-
benzaldehyde, fatty acids (Cn-1H2n-1COOH where n ¼ 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 cmꢀ1. The 1H nuclear magnetic resonance (NMR) (400MHz)
and 13C NMR (100MHz) spectra were recorded in CDCl3 using 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 200ꢁC. 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 DSC823e differential scanning calorimeter at heating and cooling rates of 10ꢁC=
min and ꢀ10ꢁC=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 20ꢁC=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 0ꢁC. 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 0ꢁC. 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) vmax cmꢀ1 3431 (OꢀH), 3069 (CꢀH aromatic), 2921,
2851 (CꢀH aliphatic), 1758 (C O ester), 1611 (C N thiazole). 1H NMR
(400 MHz, CDCl3): d=ppm 0.9 (t, J ¼ 6.6 Hz, 3H, CH3ꢀ), 1.2–1.4 (m, 16H,
CH3(CH2)8ꢀ), 1.8 (q, J ¼ 7.3 Hz, 2H, ꢀCH2CH2COOꢀ), 2.6 (t, J ¼ 7.3 Hz, 2H,
ꢀCH2COOꢀ), 3.9 (s, 3H, CH3Oꢀ), 6.7 (d, J ¼ 8.5 Hz, 1H, Ar-H), 6.8 (s, 1H,
=
=

New Series of Schiff Base Thermotropic Liquid Crystals
13
Scheme 1. Synthetic route of nMHBABTH and nEHBABTH.
Ar-H), 7.1 (d, J ¼ 9.0 Hz, 1H, Ar-H), 7.2 (s, 1H, Ar-H), 7.5 (d, J ¼ 8.5 Hz, 1H,
13
=
Ar-H), 7.8 (d, J ¼ 8.8 Hz, 1H, Ar-H), 9.2 (s, 1H, ꢀN CHꢀ), 12.5 (s, 1H, OH).
=
C NMR (400 MHz, CDCl3): d=ppm 171.54 (ꢀCOOꢀ), 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 (CH3Oꢀ), 34.53 (ꢀCH2COOꢀ), 32.01
(ꢀCH2CH2COOꢀ), 29.70, 29.55, 29.43, 29.35, 29.15, 24.93, 22.78 for methylene
carbons [CH3(CH2)12CH2CH2COOꢀ], 14.23 [CH3(CH2)14COOꢀ]. EI-MS m=z
(rel. int. %): 482 (9) [Mþ], 300 (100).
16EHBABTH. The synthetic method is identical to that of 12MHBABTH. IR
(KBr) vmax cmꢀ1 3420 (OꢀH), 3075 (CꢀH aromatic), 2922, 2850 (CꢀH aliphatic),
1
=
=
1758 (C O ester), 1609 (C N, thiazole). H NMR (400 MHz, CDCl3): d=ppm 0.9
(t, J ¼ 6.6 Hz, 3H, CH3ꢀ), 1.2 (m, 24H, CH3(CH2)12CH2CH2COOꢀ), 1.5 (t,
J ¼ 7.2 Hz, 3H, CH3CH2Oꢀ), 1.8 (q, J ¼ 7.1 Hz, 2H, ꢀCH2CH2COOꢀ), 2.6 (t,
J ¼ 7.4 Hz, 2H, ꢀCH2COOꢀ), 4.1 (q, J ¼ 6.9 Hz, 2H, CH3CH2Oꢀ), 6.7 (d,
J ¼ 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,

14
S.-T. Ha et al.
Ar-H), 7.5 (d, J ¼ 8.5 Hz, 1H, Ar-H), 7.8 (d, J ¼ 9.1 Hz, 1H, Ar-H), 9.2 (s, 1H,
ꢀN CHꢀ), 12.5 (s, 1H, OH). 13C NMR (400 MHz, CDCl3): d=ppm 171.40
=
=
(ꢀCOOꢀ), 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
(CH3CH2Oꢀ), 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)14COOꢀ], 14.78 (CH3CH2Oꢀ), 14.10
[CH3(CH2)14COOꢀ]. 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 (n ¼ 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
3MHBABTH
4MHBABTH
5MHBABTH
6MHBABTH
7MHBABTH
8MHBABTH
10MHBABTH
12MHBABTH
14MHBABTH
16MHBABTH
18MHBABTH
26
21
32
36
40
48
41
55
46
59
63
67
C17H14N2O4S 59.72 (59.64) 4.01 (4.12) 8.05 (8.18)
C18H16N2O4S 60.78 (60.66) 4.48 (4.53) 7.79 (7.86)
C19H18N2O4S 61.69 (61.61) 4.83 (4.90) 7.50 (7.56)
C20H20N2O4S 62.43 (62.48) 5.27 (5.24) 7.37 (7.29)
C21H22N2O4S 63.41 (63.30) 5.50 (5.56) 6.94 (7.03)
C22H24N2O4S 64.16 (64.06) 5.80 (5.86) 6.71 (6.79)
C23H26N2O4S 64.83 (64.77) 6.10 (6.14) 6.47 (6.57)
C25H30N2O4S 66.11 (66.05) 6.57 (6.65) 6.12 (6.16)
C27H34N2O4S 67.07 (67.19) 7.16 (7.10) 5.75 (5.80)
C29H38N2O4S 68.27 (68.20) 7.48 (7.50) 5.47 (5.49)
C31H42N2O4S 69.19 (69.11) 7.79 (7.86) 5.22 (5.20)
C33H46N2O4S 69.99 (69.93) 8.10 (8.18) 4.91 (4.94)

New Series of Schiff Base Thermotropic Liquid Crystals
15
Table 2. Percentage yields and analytical data of nEHBABTH
% Found (% Calcd.)
Compound
Yield (%)
Formula
C
H
N
2EHBABTH
3EHBABTH
4EHBABTH
5EHBABTH
6EHBABTH
7EHBABTH
8EHBABTH
10EHBABTH
12EHBABTH
14EHBABTH
16EHBABTH
18EHBABTH
23
28
31
37
35
36
42
40
55
58
61
65
C18H16N2O4S 60.78 (60.66) 4.50 (4.53) 7.75 (7.86)
C19H18N2O4S 61.66 (61.61) 4.85 (4.90) 7.50 (7.56)
C20H20N2O4S 62.40 (62.48) 5.13 (5.24) 7.31 (7.29)
C21H22N2O4S 63.25 (63.30) 5.63 (5.56) 7.09 (7.03)
C22H24N2O4S 64.00 (64.06) 5.90 (5.86) 6.82 (6.79)
C23H26N2O4S 64.69 (64.77) 6.21 (6.14) 6.59 (6.57)
C24H28N2O4S 65.47 (65.43) 6.30 (6.41) 6.66 (6.63)
C26H32N2O4S 66.72 (66.64) 6.79 (6.88) 5.90 (5.98)
C28H36N2O4S 67.76 (67.71) 7.24 (7.31) 5.67 (5.64)
C30H40N2O4S 68.61 (68.67) 7.66 (7.68) 5.31 (5.34)
C32H44N2O4S 69.62 (69.53) 7.98 (8.02) 5.02 (5.07)
C34H48N2O4S 70.39 (70.31) 8.24 (8.33) 4.85 (4.82)
Table 3. Transition temperatures and associated enthalpy changes of nMHBABTH
upon heating
Transition temperature (ꢁC)
Compounds
(associated enthalpy changes, kJ molꢀ1
)
2MHBABTHa
3MHBABTHa
4MHBABTHa
5MHBABTH
Cr 186.8 (45.57) I
Cr 143.0 (35.55) I
Cr 154.1 (36.58) N 162.4 (0.60) I
Cr 137.8 (33.89) N 147.6 (0.32) I
Cr 92.8 (26.86) N 138.2 (0.33) I
Cr 131.1 (38.74) N 155.6 (0.63) I
Cr 91.8 (32.89) N 141.8 (0.81) I
6MHBABTH
7MHBABTH
8MHBABTH
10MHBABTH
12MHBABTH
14MHBABTH
16MHBABTH
18MHBABTH
Cr1 121.6 (1.13) Cr2 133.6 (38.63) N 145.8 (0.49) I
Cr 114.3 (40.13) N 143.6 (0.39) I
Cr 133.2 (37.66) N 145.8 (0.60) I
Cr 100.7 (33.72) N 132.5 (0.84) I
Cr 127.5 (90.04) N 139.8 (1.81) I
Cr 98.9 (89.07) N 135.9 (2.55) I
Cr 124.9 (43.25) N 136.8 (0.98) I
Cr 107.7 (43.28) N 133.8 (0.53) I
Cr 123.6 (54.67) N 133.6 (0.84) I
Cr 103.7 (53.75) N 130.7 (0.88) I
Cr 123.2 (61.86) N 129.7 (1.51) I
Cr 95.7 (61.22) N 126.5 (1.51) I
Cr 122.3 (61.27) N 125.2 (1.23) I
Cr 110.8 (58.31) N 117.0 (0.89) I
Cr ¼ Crystal; N ¼ nematic; I ¼ isotropic liquid.
aOnly heating data are provided. No distinct peak was detected during the cooling scan due
to partial decomposition.
Cooling data was shown in italics.

16
S.-T. Ha et al.
Table 4. Transition temperatures and associated enthalpy changes of
nEHBABTH upon heating
Transition temperature (ꢁC)
Compounds
(associated enthalpy changes, kJ molꢀ1
)
2EHBABTHa
3EHBABTHa
4EHBABTHa
5EHBABTHa
6EHBABTHa
7EHBABTHa
8EHBABTHa
10EHBABTHa
12EHBABTHa
14EHBABTHa
16EHBABTHa
18EHBABTHa
Cr 173.8 (50.4) I
Cr 154.2 (54.9) I
Cr 136.0 (33.9) N 175.9 (0.7) I
Cr1 80.6 (10.7) Cr2 128.7 (34.0) N 162.1 (0.7) I
Cr 128.6 (40.8) N 147.9 (0.7) I
Cr 118.5 (38.5) N 155.1 (1.0) I
Cr 128.6 (38.2) N 152.0 (0.8) I
Cr 126.8 (47.5) N 149.0 (0.9) I
Cr 126.5 (52.0) N 137.6 (1.1) I
Cr 124.4 (49.7) N 129.5b I
Cr 124.0 (59.7) N 130.0b I
Cr 124.4 (73.0) N 127.9b I
Cr ¼ Crystal; N ¼ nematic; I ¼ isotropic liquid.
aOnly heating data are provided. No distinct peak was detected during the cool-
ing scan due to its partial decomposition.
bPOM data were used. The N-I transition was untraced by DSC analysis
although this transition was observed through microscope studies.
Fig. 1). For instance, the decomposition temperature of 4MHBABTH was found to
be 168.38ꢁC and the melting temperature from the DSC analysis was 162.4ꢁC. 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 (n ¼ 4, 5, 6, 7, and 8). In addition, the clear-
ing temperatures of nMHBABTH exhibited a typical descending 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].

New Series of Schiff Base Thermotropic Liquid Crystals
17
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 141ꢁC and (b) coalesced to form the classical marble-like tex-
ture of the nematic phase.

18
S.-T. Ha et al.
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).
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.

New Series of Schiff Base Thermotropic Liquid Crystals
19
Figure 5. Plot of transition temperatures versus the number of carbons (n) in the alkanoyloxy
chain of nEHBABTH during the heating cycle.
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

20
S.-T. Ha et al.
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
Mesophase
range (ꢁC)
Transition
temperatures (ꢁC)
Commencement
of smectic phase
Compound
Sm
N
12MHBABTH Cr 124.9 N 136.8 I
12EHBABTH
—
—
—
—
38.0
90.0
11.9
11.1
—
22.0
—
—
—
—
C12
C4
C5
Cr 126.5 N 137.6 I
Cr 99.9 I
A
B
C
D
Cr 113 (SmA 108)ꢃ N 135 I
Cr 156.0 SmA 194.0 I
Cr 80.0 SmA 172.0 I
—
ꢃMeans monotropic data.
absence of the lateral group. The melting temperature of 12MHBABTH (124.9ꢁC)
was found to be higher than that of compound B (113ꢁC), 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
(11.9ꢁC) was smaller than that of compound B (22.0ꢁC) resulting from the lateral
hydroxyl group involved in a strong intermolecular hydrogen bonding (OH ꢂꢂꢂꢂ 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

New Series of Schiff Base Thermotropic Liquid Crystals
21
have increased dramatically to 38.0ꢁC and 90ꢁC. 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 (n ¼ 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.
References
[1] Imrie, C. T., & Henderson, P. A. (2002). Cur. Op. Colloid. Inerface. Sci., 7, 298–311.
[2] Yoshizawa, A., Kinbara, H., Narumi, T., Yamaguchi, A., & Dewa, H. (2005). Liq.
Cryst., 32, 1175–1181.
[3] Imrie, C. T., & Luckhurst, G. R. (1998). J. Mater. Chem., 8, 1339–1343.
[4] Gray, G. W. (1987). Thermotropic Liquid Crystals, John Wiley, Chichester, England.
[5] Collings, P. J., & Hird, M. (1998). Introduction to Liquid Crystals: Chemistry and Physics.
Taylor & Francis Ltd.: London, UK.
[6] Petrov, V. F., Pavluchenko, A. I., & Smirnova, N I. (1995). Mol. Cryst. Liq. Cryst., 265,
47–53.
[7] Parra, M., Belmar, J., Zunza, H., Zuniga, C., Villouta, S., & Martinez, R. (1993). Bol.
Soc. Chil. Quim., 38, 325–330.
[8] Campbell, N. L., Duffy, W. L., Thomas, G. L., Wild, J. H., Kelly, S. M., Bartle, K.,
O’Neill, M., Minter, V., & Tuffin, R. P. (2002). J. Mater. Chem., 12, 2706–2721.
[9] Bartulin, J., Zuniga, C., Ramirez, A., Muller, H., Taylor, T. R. (1990). Mol. Cryst. Liq.
Cryst., 185, 131–140.
[10] Karamysheva, L. A., Kovshev, E. L., Pavluchenko, A. I., Roitman, K. V., Titov, V. V.,
Torgova, S. I., & Grebenkin, M. F. (1981). Mol. Cryst. Liq. Cryst., 67, 241–251.
[11] (a) Racane, L., Stojkovic, R., Tralic-Kulenovic, V., & Karminski-Zamola, G. (2006).
Molecules, 11, 325–333; (b) Kini, S., Swain, S. P., & Gandhi, A. M. (2007). Indian J.
Pharmacent. Sci., 69, 46–50; (c) Wells, G., Lowe, P. R., & Stevens, M. F. G. (2000).
Arkivoc., 1, 779–797; (d) Chavan, A. A., & Pai, N. R. (2007). Molecules, 12, 2467–2477.

22
S.-T. Ha et al.
[12] (a) Funahashi, M., & Hanna, J. I. (1996). Jpn. J. Appl. Phys., 35, L703–L705; (b)
Funahashi, M., & Hanna, J. I. (1997). Phys. Rev. Lett., 78, 2184–2187; (c) Funahashi,
M., & Hanna, J. I. (1997). Mol. Cryst. Liq. Cryst., 304, 429–434.
[13] Pavluchenko, A. I., Smirnova, N. I., Titov, V. V., Kovahev, E. I., & Djumaev, K. M.
(1976). Mol. Cryst. Liq. Cryst., 37, 35–46.
[14] Prajapati, A. K., & Bonde, N. L. (2009). Mol. Cryst. Liq. Cryst., 501, 72–85.
[15] Prajapati, A. K., & Bonde, N. L. (2005). Mesogenic benzothiazole derivative,
Proceedings of the 11th National Conference on Liquid Crystals, 1–3 November
Allahabad, India.
[16] Prajapati, A. K., & Bonde, N. L. (2006). J. Chem. Sci., 118, 203–210.
[17] Gray, G. W. (1962). Molecular Structure and Properties of Liquid Crystals, Academic
Press: London.
[18] Yeap, G. Y., Ha, S. T., Lim, P. L., Boey, P. L., & Mahmood, W. A. K. (2004). Mol.
Cryst. Liq. Cryst., 423, 73–84.
[19] Hallsby, A., Nilsson, M., & Otterholm, B. (1982). Mol. Cryst. Liq. Cryst., 82, 61–68.
[20] Wei, Q., Shi, L., Cao, H., Wang, L. P., Yang, H., Wang, Y. B. (2008). Liq. Cryst., 35,
581–585.
[21] Demus, D., & Richter, L. (1978). Textures of Liquid Crystals, Verlag Chemie: New York.
[22] Dierking, I. (2003). Textures of Liquid Crystals, Wiley-VCH: Weinheim.
[23] Berdague, P., Bayle, J. P., Ho, M. S., & Fung, B. M. (1993). Liq. Cryst., 14, 667–674.
[24] Ha, S. T., Koh, T. M., Lin, H. C., Yeap, G. Y., Win, F. W., Ong, S. T., Sivasothy, Y., &
Ong, L. K. (2009). Liq. Cryst., 36, 917–925.
[25] Ha, S. T., Koh, T. M., Yeap, G. Y., Lin, H. C., Boey, P. L., Win, Y. F., Ong, S. T., &
Ong, L. K. (2009). Mol. Cryst. Liq. Cryst., 506, 56–70.
[26] Dixit, S., & Vora, R. A. (2009). Mol. Cryst. Liq. Cryst., 501, 43–52.