Synthesis and mesomorphic properties of 2-(4-Alkyloxyphenyl)benzothiazoles

Article abstract of DOI:10.1080/15421400902987347

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. Taylor & Francis Group, LLC.

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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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Mol. Cryst. Liq. Cryst., Vol. 506, pp. 56–70, 2009
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421400902987347
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
2Liquid Crystal Research Laboratory, School of Chemical Sciences,
Universiti Sains Malaysia, Minden, Penang, Malaysia
3Department 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: hast@utar.edu.my
56

Properties of 2-(4-Alkyloxyphenyl)benzothiazoles
57
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

58
S.-T. Ha et al.
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 n ¼ 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 cmꢀ1 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 200ꢁC. 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 10ꢁC=min and ꢀ10ꢁC=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

Properties of 2-(4-Alkyloxyphenyl)benzothiazoles
59
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 ¼ 20 ꢂ 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 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.

60
S.-T. Ha et al.
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 20ꢁC, and the solid formed was filtered and
washed with cold ethanol:water (1:1.5) and dichloromethane. Yield
75%. IR (KBr) nmax cmꢀ1 3417 (O-H), 3053, 2996 (C-H aromatic), 1606
1
(C¼N, thiazole), 1483, 1455 (C¼C aromatic), 1074 (C-S-C). H 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, n ¼ 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
13C), and mass spectral data for the representative compound,
16O-PBTH, are summarized as follows.
16O-PBTH: Yield 63%. IR (KBr) vmax cmꢀ1 3057, 3030 (C-H
aromatic), 2955, 2849 (C-H aliphatic), 1608 (C¼N, thiazole), 1487, 1465
1
(C¼C aromatic), 1261 (Ar-O-R ether), 1073 (C-S-C, thiazole). H NMR
(400 MHz, CDCl3): d=ppm 0.9 (t, 3 H, CH3-), 1.3–1.5 (m, 26 H, CH3-
(CH2)13-CH2-CH2-O-), 1.8–1.9 (m, 2 H, -CH2-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).

Properties of 2-(4-Alkyloxyphenyl)benzothiazoles
61
TABLE 1 Percentage Yields and Analytical Data of nO-PBTH
% Found (% Calcd.)
Compound
Yield (%)
Formula
C
H
N
2O-PBTH
4O-PBTH
6O-PBTH
8O-PBTH
10O-PBTH
12O-PBTH
14O-PBTH
16O-PBTH
18O-PBTH
37
49
43
43
47
44
52
63
71
C15H31NOS
C17H17NOS
C19H21NOS
70.50 (70.56) 5.19 (5.13)
72.09 (72.05) 6.11 (6.05)
73.29 (73.31) 6.80 (6.75)
74.36 (74.34) 7.51 (7.37)
75.22 (75.20) 8.08 (7.90)
75.90 (75.95) 8.43 (8.35)
76.58 (76.60) 8.87 (8.75)
77.13 (77.16) 9.18 (9.09)
77.53 (77.66) 9.52 (9.39)
5.43 (5.49)
4.89 (4.94)
4.45 (4.50)
3.95 (4.13)
3.70 (3.81)
3.63 (3.54)
3.17 (3.31)
3.18 (3.10)
2.83 (2.92)
C
C
21H25NOS
23H29NOS
C25H33NOS
27H37NOS
C
C29H41NOS
C31H45NOS
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 n ¼ 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.
1
FTIR, H NMR, and 13C NMR Spectral Studies
The diagnostic absorption bands at 2849 and 2955 cmꢀ1 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 cmꢀ1. The absorption bands assignable to
the stretching of the C¼N and C-S-C bonds of benzothiazole were
observed at 1608 and 1073 cmꢀ1, respectively. These values conform
with those reported in the IR spectra for various substituted benzothia-
zoles [20,25]. The absorption band at 1261 cmꢀ1 was indicative of
the C-O stretching of the aromatic ether (Ar-O-R). Compounds
nO-PBTH (n ¼ 2, 4, 6, 8, 10, 12, 14, 18) exhibited similar characteristic
absorptions as discussed for 16O-PBTH.
The triplets at d ¼ 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 d ¼ 1.3–1.9 ppm were attributed to
the remaining methylene protons [CH3-(CH2)14-] of the alkoxyl chain.

62
S.-T. Ha et al.
1
The H 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 d ¼ 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 d ¼ 115.24–
161.96 ppm were assigned to the carbons of the benzothiazole and aro-
matic moieties. The carbon resonances between d ¼ 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 cooling 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 molꢀ1
)
2O-PBTH
Cr 125.7 (18.18) I
Cr 95.7 (16.16) I
4O-PBTH
6O-PBTH
8O-PBTH
10O-PBTH
12O-PBTH
14O-PBTH
16O-PBTH
18O-PBTH
Cr 89.5 (36.16) I
Cr 78.0 (34.71) I
Cr 85.1 (20.87) I
Cr 73.0 (20.12) I
Cr 75.5 (27.86) I
Cr 48.8 (17.12) SmA 74.8 (6.97) I
Cr 81.7 (44.42) I
Cr 63.1 (32.03) SmA 79.3 (8.80) I
Cr 86.9 (55.61) I
Cr 63.8 (42.11) SmA81.2 (10.41) I
Cr 91.7 (59.52) I
Cr 70.7 (47.00) SmA81.2 (9.79) I
Cr 95.7 (70.41) I
Cr 77.3 (57.37) SmA79.8 (11.20) I
Cr 96.8 (58.49) I
Cr 85.2 (56.27) I
The values in italics were taken during cooling cycle.
Cr ¼ Crystal; SmA ¼ Smectic A; I ¼ Isotropic liquid.

Properties of 2-(4-Alkyloxyphenyl)benzothiazoles
63
FIGURE 1 DSC thermograms of nO-PBTH during heating cycle.
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.

64
S.-T. Ha et al.
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.
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 n-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

Properties of 2-(4-Alkyloxyphenyl)benzothiazoles
65
FIGURE 4 Plot of transition temperatures versus the number of carbons (n)
in the alkoxyl chain of nO-PBTH during heating cycle.
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 (n ¼ 2, 4, 6, and 18) did not exhibit
liquid crystal phases. 2O-PBTH, 4O-PBTH, and 6O-PBTH were
FIGURE 5 Plot of transition temperatures versus the number of carbons (n)
in the alkoxyl chain of nO-PBTH during cooling cycle.

66
S.-T. Ha et al.
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 ꢃ 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 (d ¼ 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.

Properties of 2-(4-Alkyloxyphenyl)benzothiazoles
67
FIGURE 6 XRD patterns of 10O-PBTH and 12O-PBTH at different
temperatures upon cooling from the isotropic phase.
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,

68
S.-T. Ha et al.
TABLE 3 Transition Temperatures, Mesophase Range, and Molecular
Structures of 8O-PBTH, A, B, and C
8O-PBTH
Compound A
Compound B
Compound C
Mesophase range (ꢁC)
Compound
Transition temperatures (ꢁC)
Sm
N
8O-PBTH
Cr (SmA 74.8)ꢄ 75.5 I
Cr 94 N 136 I
Cr 106 SmC 134 I
Cr 70.7 SmC 95.0 N 123.8 I
–
–
28
24.3
–
42
–
A
B
C
28.8
()ꢄMonotropic value.
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.

Properties of 2-(4-Alkyloxyphenyl)benzothiazoles
69
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.
REFERENCES
[1] Collings, P. J., & Hird, M. (1998). Introduction to Liquid Crystals, Taylor & Francis
Ltd.: London, UK.
[2] Seed, A. (2007). Chem. Soc. Rev., 36, 2046.
[3] Parra, M., Alderete, J., Zuniga, C., Gallardo, H., Hindalgo, P., Veragara, J., &
Hernandez, S. (2001). Liq. Cryst., 28, 1659.
[4] Parra, M. L., Saavedra, C. G., Hildago, P. I., & Elgueta, E. Y. (2008). Liq. Cryst.,
35(1), 55.
[5] Lai, L. L., Wang, C. H., Hsien, W. P., & Lin, H. C. (1996). Mol. Cryst. Liq. Cryst.,
287, 177.
[6] Bartulin, J., Zuniga, C., Ramirez, A., Muller, H., & Taylor, T. R. (1990). Mol. Cryst.
Liq. Cryst., 185, 131.
[7] 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, 254.
[8] Karda, D., Mieczkowski, J., Pociecha, D., Szydloska, J., & Gorcecka, E. (2001).
J. Mater. Chem., 11, 741.
[9] Champa, R. A. (1973). Mol. Cryst. Liq. Cryst., 19, 23.
[10] Dave, J. S., & Menon, M. (2000). Bull. Mater. Sci., 23, 237.
[11] Milkhaleva, M. A. (2003). Chem. Heterocycl. Comp., 39, 1032.
[12] Thaker, B. T., Patel, P., Vansadia, A. D., & Patel, H. G. (2007). Mol. Cryst. Liq.
Cryst., 466, 13.

70
S.-T. Ha et al.
[13] Parra, M., Vergara, J., Zuniga, C., Soto, E., Sierra, T., & Serrano, J. L. (2005). Liq.
Cryst., 32(4), 457.
[14] Parra, M., Alderete, J., Zuniga, C., Gallardo, H., Hildago, P., Veragara, J., &
Hernandez, S. (2000). Liq. Cryst., 27, 995.
[15] Funahashi, M., & Hanna, J. I. (1996). Jpn J. Appl. Phys., 35, L703.
[16] Funahashi, M., & Hanna, J. I. (1997). Phys. Rev. Lett., 78, 2184.
[17] Funahashi, M., & Hanna, J. I. (1997). Mol. Cryst. Liq. Cryst., 304, 429.
[18] Pavluchenko, A. I., Smirnova, N. I., Titov, V. V., Kovahev, E. I., & Djumaev, K. M.
(1976). Mol. Cryst. Liq. Cryst., 37, 35.
[19] Belmar, J., Parra, M., Zuniga, C., Perez, C., & Munoz, C. (1999). Liq. Cryst., 26, 389.
[20] Prajapati, A. K., & Bonde, N. L. (2006). J. Chem. Sci., 118(2), 203.
[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] Toba, M., Takeoka, Y., Rikukawa, W., & Sanui, K. (2005). Synth. Met., 152,
197–200.
[24] Prajapati, A. K., Vora, R. A., & Pandya, H. M. (2001). Mol. Crys. Liq. Cryst., 369,
37–46.
[25] Vicini, P., Geronikaki, A., Incerti, M., Busonera, M., Poni, G., Cabras, C. A., &
Colla, P. L. (2003). Bioorg. & Med. Chem., 11, 4785.
[26] Gray, G. W. (1962). Molecular Structure and Properties of Liquid Crystals,
Academic Press: London.
[27] Liu, C.-T. (1981). Mol. Cryst. Liq. Cryst., 74, 25.
[28] Kumar, S. (2001). Liquid Crystals: Experimental Study of Physical Properties and
Phase Transitions, Cambridge University Press: U.K.
[29] Wang, Y., Zhang, B. Y., He, X. Z., Wang, J. W. (2007). Colloid Polym. Sci., 285,
1077.
[30] Meng, F. B., Gao, Y. M., Lian, J., Zhang, B. Y., & Zhang, F. Z. (2008). Colloid Polym.
Sci., 286, 873.
[31] Xiao, W., Zhang, B., & Cong, Y. (2008). Colloid Polym. Sci., 286, 267.
[32] Aldred, M. P., Vlachos, P., Dong, D., Kitney, S. P., Tsoi, W. C., O’Neill, M., &
Kelly, S. M. (2005). Liq. Cryst., 32(8), 951.
[33] Vieira, A. A., Cristiano, R., Bortoluzzi, A. J., & Gallardo, H. (2007). J. Mol. Struct.,
875, 364.