Synthesis and terminal chain effect on the phase transition behavior of azo-bridged benzothiazole-phenyl ethers

Article abstract of DOI:10.1080/15421406.2013.766919

A series of azo-bridged benzothiazole-phenyl ethers 4-((6- ethoxybenzothiazol-2-yl)diazenyl)bromoalkoxybenzene in which an exocyclic N=N group is situated between an ethoxy-substituted benzothiazole and a bromoalkoxyphenyl have been synthesized and characterized. The phase transition behavior of these compounds have been correlated against the variation of the flexible alkyl chain with n methylene units (CH₂), ranging from 5 to 12. The thermal stability as reflected by the transition temperature range of the present homologues is found to be strongly dependent on the length of the CH₂ units. All the members of this series exhibit enantiotropic nematic phases. However, upon further cooling, the compounds with n = 9-12 also behave as smectogens.

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Synthesis and Terminal Chain Effect on
the Phase Transition Behavior of Azo-
Bridged Benzothiazole-Phenyl Ethers
Arwa Alshargabi a , Guan-Yeow Yeap a , Daisuke Takeuchi b & Masato
M. Ito c
a Liquid Crystal Research Laboratory, School of Chemical Sciences ,
Universiti Sains Malaysia , Penang , Malaysia
b Chemical Resources Laboratory , Tokyo Institute of Technology ,
Yokohama , Japan
c Department of Environmental Engineering for Symbiosis, Faculty of
Engineering , Soka University , Tokyo , Japan
Published online: 22 Apr 2013.
To cite this article: Arwa Alshargabi , Guan-Yeow Yeap , Daisuke Takeuchi & Masato M. Ito
(2013) Synthesis and Terminal Chain Effect on the Phase Transition Behavior of Azo-Bridged
Benzothiazole-Phenyl Ethers, Molecular Crystals and Liquid Crystals, 575:1, 128-139, DOI:
10.1080/15421406.2013.766919
To link to this article: http://dx.doi.org/10.1080/15421406.2013.766919
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Mol. Cryst. Liq. Cryst., Vol. 575: pp. 128–139, 2013
Copyright © Taylor & Francis Group, LLC
ISSN: 1542-1406 print/1563-5287 online
DOI: 10.1080/15421406.2013.766919
Synthesis and Terminal Chain Effect on the Phase
Transition Behavior of Azo-Bridged
Benzothiazole-Phenyl Ethers
ARWA ALSHARGABI,1 GUAN-YEOW YEAP,1,∗ DAISUKE
TAKEUCHI,2 AND MASATO M. ITO3
1Liquid Crystal Research Laboratory, School of Chemical Sciences, Universiti
Sains Malaysia, Penang, Malaysia
2Chemical Resources Laboratory, Tokyo Institute of Technology, Yokohama,
Japan
3Department of Environmental Engineering for Symbiosis, Faculty of
Engineering, Soka University, Tokyo, Japan
A series of azo-bridged benzothiazole-phenyl ethers 4-((6-ethoxybenzothiazol-2-
yl)diazenyl)bromoalkoxybenzene in which an exocyclic N N group is situated between
an ethoxy-substituted benzothiazole and a bromoalkoxyphenyl have been synthesized
and characterized. The phase transition behavior of these compounds have been cor-
related against the variation of the flexible alkyl chain with n methylene units (CH2),
ranging from 5 to 12. The thermal stability as reflected by the transition temperature
range of the present homologues is found to be strongly dependent on the length of the
CH2 units. All the members of this series exhibit enantiotropic nematic phases. However,
upon further cooling, the compounds with n = 9–12 also behave as smectogens.
Keywords Azo-bridged; benzothiazole; ethers; mesogenic; nematic; smectic
1. Introduction
Liquid crystals have been accepted as quintessential materials in our daily life for more
than a century. The workability of these materials can be exemplified by their application
in technological devices, as prototypical self-organizing molecular materials, as precursor
in biological as well as pharmacological fields [1].
In general, these materials possess a mesogenic core, terminal groups and a flexible
terminal chain, which are favorable in inducing mesomorphic behavior due to their high
shape anisotropy [2–5]. One of these materials contains an azo-based mesogenic core,
which has widely been used in optical switching, holography, and optical storage devices
[6–11]. The azo-based dyes have also been applied in liquid crystal display (LCD) governed
by the guest–host interaction [12]. In addition, the aromatic azo compounds undergo the
cis–trans isomerization in response to light or heat and thus offer many opportunities in
photonic liquid crystals applications [13–15]. Among these compounds, the analogues with
∗Address correspondence to Guan-Yeow Yeap, Liquid Crystal Research Laboratory,
School of Chemical Sciences, Universiti Sains Malaysia, 11800 Minden, Penang, Malaysia.
Tel.: 60-4-6533568; Fax: 60-4-6574854. E-mail: gyyeap@usm.my
128

Phase Transition Behavior of Azo-Bridged Benzothiazole-Phenyl Ethers
129
–COO– and –N N– central bridges have been synthesized and claimed as suitable candi-
dates to exhibit liquid crystal properties [16].
Earlier reports have shown that the introduction of heterocycles greatly influences
the mesomorphic properties of the calamitic molecules owing to their unsaturation and
more polarizable nature [17]. The effect of heteroatoms (S, O, and N) can change con-
siderably the polarity, polarizability and, sometimes, the geometric shape of the molecule,
thereby influencing the type of mesophase, the phase transition temperatures, and the di-
electric and other properties of the mesogens [17]. A typical heterocyclic compound is
benzothiazole, which could serve as a core unit in calamitic liquid crystals. Prajapati et al.
had reported a new mesogenic homologous series containing 6-nitrobenzothiazole and
6-methoxybenzothiazole in which the 6-position in benzothiazole group was substituted
by respective nitro and methoxy groups. These compounds with ether and ester linkages
on the other side of the azo seem to show enantiotropic nematic (N) and smectic A (SmA)
phases [18,19].
A recent finding by our group has shown that the azo-bridged benzothiazole-phenyl
esters favor the formation of the N phase [20]. The 2,6-di-substituted benzothiazole was
preferred over 2,5-di-substituted analogues due to its linearity [21] and thermal stability
[22].
The present study focuses on 2,6-di-substituted benzothiazole in which the bro-
moalkoxyphenyl will be linked to the 2-position of the 6-ethoxy-benzothiazole via the
N N azo bond. The synthetic route used for the preparation of 1 and 2a–2g is summarized
in Scheme 1.
N
N
NaNO2 / H2SO4
NH2
N2 HSO4
2 °C
S
H3C
O
S
H3C
O
N
N
OH
S
N
OH
H3C
O
1
NaOH
K2CO3
Aceton
KI
Br-(CH2)n-Br
n = 5; 2 a
n = 6; 2 b
n = 8; 2 c
n = 9; 2 d
n = 10; 2 e
n = 11; 2 f
n = 12; 2 g
N
N
S
N
O
(CH2)n Br
H3C
O
Scheme 1. Synthetic route to formation of the compounds 2a–2g.

130
A. Alshargabi et al.
2. Experimental
2.1. Materials
2-amino-6-ethoxybenzothiazole and dibromoalkane (CnH2nBr2, where n = 5, 6, 8, 9, 10,
11, and 12) were purchased from Acros Organics and TCI. Sodium nitrite and phenol were
obtained from M&R and Sigma, respectively. All of the reagents and solvents were used
without further purification.
2.2. Measurements
CHN microanalysis was carried out on the Perkin Elmer 2400 LS Series CHNS/O analyzer.
The infrared (IR) spectra for all intermediates and final compounds were recorded using
the Perkin Elmer 2000 FT-IR spectrophotometer, in which the samples were prepared
1
as KBr disks. H-NMR (nuclear magnetic resonance) as well as 13C-NMR spectra were
recorded in CDCl3 and deuterated DMSO using a Bruker 500 MHz UltrashieldTM FT-NMR
spectrometer. Tetramethylsilane (TMS) was used as an internal standard.
The phase transition temperature and associated enthalpy values were determined
using a differential scanning calorimeter (DSC) (Seiko DSC6200R), in which the samples
were heated and cooled at the rates of 5◦C min−1. Texture observation of the mesophase
was carried out by using polarizing optical microscopy (POM) (Carl Zeiss Axioskop 40
polarizing microscope) equipped with a hot stage (Linkam LTS350) and a temperature
controller (TMS94).
2.3. Synthesis
2.3.1. Synthesis of 4-((5-ethoxybenzothiazol-2-yl)diazenyl)phenol (1). A solution contain-
ing 20 ml diluted sulphuric acid (60%) and 6-ethoxy-2-aminobenzothiazole (1 g, 5.1 mmol),
which was earlier dissolved in 15 ml glacial acetic acid, was cooled down to 5◦C. Sodium
nitrite (0.35 g, 5.1 mmol) in 12.5 ml water was added dropwise to the cooled mixture and
stirred for 1 h in an ice bath. Phenol (0.47 g, 5.1 mmol) dissolved in 15 ml ethanol was
cooled to 5◦C and the diazonium salt solution was added dropwise at a temperature below
5◦C. The mixture was stirred for 1 h. The pH was subsequently increased to 6–7 by the
addition of 1 N NaOH and the mixture was stirred for 1 h in an ice bath. Finally, water was
added to the resulting mixture and the precipitate was collected by filtration. The product
was recrystallized from ethanol. Yield (70.4%) as red crystals and m.p. 286–287◦C. IR
(KBr) νmax/cm−1: 3300–2600 (OH broad), 3042 (C–H aromatic), 1605, 1579 (C C aro-
matic, C N thiazole), 2943 (C–H aliphatic), 1057 (bezothiazole), 1135 (C–O). 1H-NMR
(DMSO-d6) δ/ppm: 1.39 (t, 3H, –CH3), 4.1 (q, 2H, –OCH2), 7.01 (d, 2H, ArH), 7.1 (d, 1H,
ArH), 7.6 (d, 1H, ArH), 7.8 (d, 2H, Ar), 8.0 (s, 1H, ArH).
2.3.2. Synthesis of 4-((6-ethoxybenzothiazol-2-yl)diazenyl) bromo-alkoxyphenyl (2a–2g).
A mixture consisting of compound 1 (8.33 mmol) in 150 ml dry acetone, potassium
carbonate (K2CO3) (62 mmol), a catalytic amount of potassium iodide (100 mg), and a
five-fold excess of dibromoalkane (41.6 mmol) was refluxed for 24 h. K2CO3 was removed
by filtration while it was still hot and the acetone was removed under reduced pressure. The
product was recrystallized three times from ethanol whereupon the pure compound was
isolated.

Phase Transition Behavior of Azo-Bridged Benzothiazole-Phenyl Ethers
131
The analytical data, IR, 1H-NMR, and 13C-NMR spectra data for 2a–2g are summarized
as follows:
2a: Yield 53.5%. Orange. Elemental analysis (%): Found C 53.51, H 4.93, N 9.36;
calculated (C20H22BrN3O2S) C 53.57, H 4.95, N 9.37. IR (KBr) νmax/cm−1: 3071 (C–H
aromatic), 2975, 2865 (C–H aliphatic), 1600 (C C aromatic), 1578 (C N, thiazole),
1
1263 (Ar–O–R ether), 1053 (C–S–C, thiazole). H-NMR (500 MHz, CDCl3) δ/ppm:
1.47 (t, 3H, CH3–), 1.53–1.55 (m, 2H, –(CH2)2–), 1.8–1.9 (m, 4H, – (CH2)2–), 3.46
(t, 2H, –CH2Br), 4.09–4.14 (m, 4H, (CH2)O–), 7.01 (d, 2H, Ar–H), 7.08 (d-d, 1H, Ar–H),
7.26 (d, 1H, Ar–H), 7.9 (s, 1H, Ar–H), 8.0 (d, 2H, Ar–H). 13C-NMR (CDCl3) δ/ppm:
173.88 (C N), 163.39–105.02 (Ar–C), 68.15 (O–CH2–CH3), 64.15 (O–CH2–CH2), 33.49
(–CH2Br), 32.4–24.76 (–(CH2)3–), 14.79 (–CH3).
2b: Yield 64.7%. Orange. Elemental analysis (%): Found C 54.3, H 5.19, N 9.0;
calculated (C21H24BrN3O2S) C 54.5, H 5.2, N 9.0. IR (KBr) νmax/cm−1: 3071 (C–H aro-
matic), 2981, 2868 (C–H aliphatic), 1599 (C C aromatic), 1577 (C N, thiazole), 1261
1
(Ar–O–R ether), 1056 (C–S–C, thiazole). H-NMR (500 MHz, CDCl3) δ/ppm: 1.47 (t,
3H, CH3–), 1.53–1.55 (m, 4H, –(CH2)2–), 1.8–1.9 (m, 4 H, – (CH2)2–), 3.45 (t, 2H,
–CH2Br), 4.09–4.13 (m, 4H, (CH2)2O–), 7.01 (d, 2H, Ar–H), 7.07 (d-d, 1H, Ar–H), 7.29
(d, 1H, Ar–H), 8.0 (s, 1H, Ar–H), 8.0 (d, 2H, Ar–H). 13C-NMR (CDCl3)/δ: 173.89 (C N),
163.49–105.01 (Ar–C), 68.29 (O–CH2–CH3), 64.14 (O–CH2–CH2), 33.75 (–CH2Br),
32.62–25.0 (–(CH2)4–), 14.78 (–CH3).
2c: Yield 62.5%. Orange. Elemental analysis (%): Found C 55.86, H 4.73, N 8.51;
calculated (C23H28BrN3O2S) C 56.32, H 4.75, N 8.57. IR (KBr) νmax/cm−1: 3071 (C–H
aromatic), 2925, 2860 (C–H aliphatic), 1601 (C C aromatic), 1578 (C N, thiazole), 1263
1
(Ar–O–R ether), 1050 (C–S–C, thiazole). H-NMR (500 MHz, CDCl3) δ/ppm: 1.39 (t,
3H, CH3–), 1.47–1.45 (m, 8H, –(CH2)4–), 1.83–1.87 (m, 4H, –(CH2)2–), 3.42 (t, 2H,
–CH2Br), 4.07–4.14 (m, 4H, (CH2)2O–), 7.01 (d, 2H, Ar–H), 7.08 (d-d, 1H, Ar–H),
7.29 (d, 1H, Ar–H), 8.0 (s, 1H, Ar–H), 8.1 (d, 2H, Ar–H). 13C-NMR (CDCl3) δ/ppm:
173.92 (C N), 163.59–105.03 (Ar–C), 68.51 (O–CH2–CH3), 64.15 (O–CH2–CH2), 33.89
(–CH2Br), 33.47–25.89 (–(CH2)6–), 14.79 (–CH3).
2d: Yield 62.3%. Orange. Elemental analysis (%): Found C 56.97, H 5.88, N 8.42;
calculated (C24H30BrN3O2S) C 57.14, H 5.99, N 8.33. IR (KBr) νmax/cm−1: 3071 (C–H
aromatic), 2926, 2855 (C–H aliphatic), 1596 (C C aromatic), 1578 (C N, thiazole),
1
1261 (Ar–O–R ether), 1052 (C–S–C, thiazole). H-NMR (500 MHz, CDCl3) δ/ppm:
1.5 (t, 3H, CH3–), 1.39–1.47 (q, 10H, –(CH2)5–), 1.83–1.87 (m, 4H, –(CH2)2–), 3.41
(t, 2H, –CH2Br), 4.07–4.14 (m, 4H, (CH2)2O–), 7.0 (d, 2H, Ar–H), 7.07 (d-d, 1H, Ar–H),
7.29 (d, 1H, Ar–H), 7.9 (s, 1H, Ar–H), 8.0 (d, 2H, Ar–H). 13C-NMR (CDCl3) δ/ppm:
173.92 (C N), 163.63–105.03 (Ar–C), 68.55 (O–CH2–CH3), 64.15 (O–CH2–CH2), 34.03
(–CH2Br), 29.50–25.94 (–(CH2)7–), 14.79 (–CH3).
2e: Yield 65.8%. Orange. Elemental analysis (%): Found C 57.61, H 6.19, N 8.04;
calculated (C25H32BrN3O2S) C 57.91, H 6.22, N 8.10. IR (KBr) νmax/cm−1: 3077 (C–H
aromatic), 2980, 2851 (C–H aliphatic), 1598 (C C aromatic), 1579 (C N, thiazole),
1
1262 (Ar–O–R ether), 1058 (C–S–C, thiazole). H-NMR (500 MHz, CDCl3) δ/ppm:
1.49 (t, 3H, CH3–), 1.32–1.47 (q, 12H, –(CH2)6–), 1.83–1.89 (m, 4H, –(CH2)2–), 3.41
(t, 2H, –CH2Br), 4.0–4.1 (m, 4H, (CH2)2O–), 7.0 (d, 2H, Ar–H), 7.1 (d-d, 1H, Ar–H),
7.29 (d, 1H, Ar–H), 7.9 (s, 1H, Ar–H), 8.1 (d, 2H, Ar–H). 13C-NMR (CDCl3) δ/ppm:
173.92 (C N), 163.61–105.02 (Ar–C), 68.56 (O–CH2–CH3), 64.14 (O–CH2–CH2), 34.06
(–CH2Br), 33.47–25.89 (–(CH2)8–), 14.79 (–CH3).
2f: Yield 64.7%. Orange. Elemental analysis (%): Found C 58.30, H 6.46, N 7.93;
calculated (C26H34BrN3O2S) C 58.64, H 6.44, N 7.89. IR (KBr) νmax/cm−1: 3071

132
A. Alshargabi et al.
(C–H aromatic), 2938, 2851 (C–H aliphatic), 1597 (C C aromatic), 1578 (C N, thia-
zole), 1261 (Ar–O–R ether), 1058 (C–S–C, thiazole). 1H-NMR (500 MHz, CDCl3) δ/ppm:
1.48 (t, 3H, CH3–), 1.38–1.43 (q, 14H, –(CH2)7–), 1.81–1.84 (m, 4H, –(CH2)2–), 3.39 (t,
2H, –CH2Br), 4.05–4.12 (m, 4H, (CH2)2O–), 7.01 (d, 2H, Ar–H), 7.07 (d-d, 1H, Ar–H),
7.29 (d, 1H, Ar–H), 7.98 (s, 1H, Ar–H), 8.0 (d, 2H, Ar–H). 13C-NMR (CDCl3) δ/ppm:
173.92 (C N), 163.62–105.02 (Ar–C), 68.58 (O–CH2–CH3), 64.14 (O–CH2–CH2), 34.07
(–CH2Br), 33.54–25.95 (–(CH2)9–), 14.79 (–CH3).
2g: Yield 69.9%. Orange. Elemental analysis (%): Found C 59.31, H 6.65, N 7.66;
calculated (C27H36BrN3O2S) C 59.33, H 6.64, N 7.69. IR (KBr) νmax/cm−1: 3071 (C–H
aromatic), 2938, 2851 (C–H aliphatic), 1597 (C C aromatic), 1578 (C N, thiazole), 1261
1
(Ar–O–R ether), 1058 (C–S–C, thiazole). H-NMR (500 MHz, CDCl3) δ/ppm: 1.48 (t,
3H, CH3–), 1.38–1.48 (m, 16H, –(CH2)8–), 1.8–1.9 (m, 4H, – (CH2)2–), 3.45 (t, 2H,
–CH2Br), 4.09–4.13 (m, 4H, (CH2)2O–), 7.01 (d, 2H, Ar–H), 7.07 (d-d, 1H, Ar–H),
7.29 (d, 1H, Ar–H), 8.0 (s, 1H, Ar–H), 8.1 (d, 2H, Ar–H). 13C-NMR (CDCl3) δ/ppm:
173.93 (C N), 163.64–105.03 (Ar–C), 68.60 (O–CH2–CH3), 64.14 (O–CH2–CH2), 33.89
(–CH2Br), 33.55–25.96 (–(CH2)10–), 14.79 (–CH3).
3. Results and Discussion
3.1. Mesomorphic Behavior
The phase transition temperatures and corresponding enthalpy changes of compounds
2a–2g determined by DSC are summarized in Table 1. All compounds in this series show
enantiotropic N phase. However, compounds 2d–2g also exhibit monotropic SmA phase.
Table 1. Phase transition temperatures (◦C) and enthalpies (in kJ mol−1) for 2a–2g.
(Cr: crystal; SmA: smectic A; N: nematic; I: isotropic)
Compound
Transition temperatures (◦C) (ꢀH, kJ mol−1): Heating cooling
2a
Cr 132.6 (42.5) N 145.4 (0.7) I
I 143a N 84.7 (14.0) Cr
2b
2c
2d
2e
2f
Cr 109.9 (39.9) N 143.9 (1.0) I
I 135a N 77 Cr1 46.5 (5.2) Cr2
Cr 107.0 (43.8) N 138.8 (0.9) I
I 138a N 80 (24.7) Cr
Cr 120.6 (48.3) N 135.4 (0.8) I
I 122.6 (0.5) N 87.8 (0.4) SmA 74.0
(22.5) Cr
Cr 106.7 (47.4) N 130.8 (0.9) I
I 119.1 (0.7) N 89.5 (0.6) SmA 73.6
(30.5) Cr
2g
Cr 102.1 (37.6) N 128.3 (1.0) I
I 121.6 (0.8) N 93.5 (1.2) SmA 62.2 (25.2) Cr
Cr 97.4 (29.4) N 119.3 (0.7) I
I 113.6 (0.7) N 89.6 (1.1) SmA 73.6 (24.2) Cr
aDenotes transition temperatures determined via polarizing optical microscopy, but undetected via
DSC.

Phase Transition Behavior of Azo-Bridged Benzothiazole-Phenyl Ethers
133
Figure 1. Differential scanning calorimetry (DSC) thermogram of 2c during heating and cooling
scans.
The DSC thermograms of 2c and 2e during heating and cooling scans are shown in Figs
1 and 2, respectively. From Fig. 1, on heating, the DSC curve for compound 2c shows two
endothermic peaks at 107◦C and 138.8◦C, which represent the Cr–N and N–I transitions,
respectively.
However, on cooling, the DSC curve for compound 2e shows three exothermic peaks
at 119.1◦C, 89.5◦C, and 73.6◦C, which can be assigned to the I–N, N–SmA and SmA–Cr
transitions, respectively (Fig. 2).
Figure 2. DSC thermograms of 2e upon heating and cooling.

134
A. Alshargabi et al.
The mesophases for all the compounds are observed under a POM during heating and
cooling cycles. One of the notable features is that the compounds 2a–2c exhibit predominant
enantiotropic N phase, while 2d–2g exhibit monotropic SmA as well as enantiotropic
N phases.
Observation on cooling 2e from its isotropic (I) liquid phase shows the presence of two
mesophases. Initial cooling from the I liquid phase led to nematic droplets (Fig. 3(a)), which
appear and coalesce to form the typical schlieren texture with disclination lines (Fig. 3(b)).
Upon further cooling of the N phase, the emergence of the SmA phase is observed at
89.5◦C (Fig. 3(c)). All the observed liquid-crystalline textures resemble those reported in
the literature [23,24].
3.2. Structure–Mesomorphic Property Relationships
While Fig. 4 represents the correlation of transition temperatures with the number of carbons
in the terminal bromoalkoxyl chain of 2a–2g during the heating cycle, Fig. 5 illustrates the
thermal stability of mesophase upon the cooling cycle for the same series.
The thermogram indicates that the length of the bromoalkoxy chain also influences the
types of mesophase. The odd-even effect on the mesomorphic properties is not obvious,
but it can still be noted when moving from 2c to 2e (Fig. 4). According to the plot, the
clearing (N–I) point shows a descending trend from 2a (145.4◦C) to 2g (119.3◦C), while
the melting point (Cr–N) shows a slight increment for 2d (120.6◦C) and again descending
from 2e (106.7◦C) to 2g (79.4◦C). This behavior is due to the flexible terminal chain, which
acts as a diluent to the mesogenic core ring system, and hence depressed both the melting
and the clearing temperature of this series [25,26].
It is also important to mention that upon increasing the number of carbon in the CnH2n
unit, a significant decrease in the N phase temperature range (ꢀN) is observed for all the
members and an increase in the phase stability of the SmA phase in 2d–2g.
By referring to the graph in Fig. 5, ꢀN has apparently decreased by the increase in the
length of the terminal chain from 58.3◦C for 2a to 24◦C for 2g. This phenomenon results
from the long terminal chain being attracted and intertwined, which in turn facilitates
lamellar packing, causing a decrease in ꢀN [27,28]. Generally, members with shorter
terminal chain favor the formation of the N phase, whereas the smectic phase is more
favorable by members having longer chain [25]. However, Fig. 5 also indicates that the
SmA phase temperature range (ꢀSmA) was found to be 13.8◦C for 2d, and with an
additional methylene unit, the ꢀSmA increases from 15.9◦C to 31.3◦C for 2e and 2f,
respectively, and then decreases to 16◦C for 2g.
3.3. Spectroscopic Studies
The IR and NMR spectra obtained for all compounds in this series have shown similar
characteristics as those recorded for compound 2b. Hence, the characterization based on
compound 2b will be described.
From the FTIR (Fourier transform infrared spectroscopy) spectrum, the diagnostic
absorption bands resulting from the alkyl groups are observed at 2868 and 2981 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 alkoxyl chain. The absorption bands
assignable to the stretching of the C N and C C bonds in benzothiazole are observed at
1577 and 1599 cm−1, respectively. The absorption band observed at 1261 cm−1 is indicative
of the C–O stretching of the aromatic ether Ar–O–R.

Phase Transition Behavior of Azo-Bridged Benzothiazole-Phenyl Ethers
135
Figure 3. Optical photomicrographs of 2e. Upon cooling from the isotropic liquid, nematic droplets
(a) appeared and coalesced to form the typical schlieren texture (b). On further cooling, SmA phase
shown in (c) was observed.

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Figure 4. Dependence of the transition temperatures on the number of methylene units in the terminal
bromoalkoxyl chains for 2a–2g.
The 1H-NMR data for compound 2b show a triplet at δ 1.47 ppm, indicating that the
methyl protons from the ethoxy group attached to the benzothiazole. All the methylene
protons from the bromo-substituted terminal chain, except those from the carbons attached
directly to O and Br, gave multiplets at δ 1.53–1.55 ppm. The presence of a quintet at δ
1.7 ppm can be assigned to the methylene protons in –CH2–CH2–O– and –CH2–CH2–Br.
Both triplet (δ 3.45 ppm) and quadruplet (δ 4.10 ppm) can be assigned to the protons of
–CH2Br and –OCH2– originating from the same terminal chain.
Figure 5. Graphical representations of the phase transition temperature ranges for 2a–2g.

Phase Transition Behavior of Azo-Bridged Benzothiazole-Phenyl Ethers
137
The diagnostic peaks as inferred from the 13C-NMR spectrum for 2b has supported
the presence of 21 carbons within this molecule. The peak at δ 173.89 is due to the C N,
which formed part of the heterocyclic ring of benzothiazole. The resonances observed at
δ 105.01–163.49 ppm support the presence of the remaining sp2 carbons in benzothiazole
and aromatic units. While the resonance observed at δ 14.78 ppm supports the presence of
a methyl carbon from the ethoxy group, the peaks within the range of δ 25.0–31.62 ppm
are indicative of the methylene carbons that are not bonded directly to O and Br within the
terminal chain. However, the more downfield peaks at δ 33.75 ppm and 68.29 ppm can be
ascribed to the carbon attached to Br and O, respectively. On the other hand, the downfield
resonance recorded at δ 64.14 ppm can be assigned to the carbon attached to the oxygen in
the ethoxy group.
3.4. Comparison Studies Between Title Compounds with 4-((6-Ethoxybenzothiazol-
2-yl)Azenyl)Phenylalkanoate nEBTHDZ
A comparison of the title compound with the recently reported benzothiazole nEBTHDZ
[20] has been carried out. The purpose of making this comparison is based on the fact that
the presence of an ether linkage, along with the existence of a terminal Br atom, gives
rise to appreciable change in mesomorphic properties. The illustrations of the molecular
structure for the title compound and nEBTHDZ show that these compounds possess a
benzothiazole moiety attached to the N atom of the azo group, while the other N of the azo
group is attached by a phenyl ring in which the para position consists of a Br-substituted
alkyloxy chain linked to the phenyl ring via ether linkage.
nEBTHDZ
where n = 9 and 10
9EBTHDZ Cr 106.8 (38.8) N 137.7 (1.2) I 139.2 (0.9) N 76.1 (38.8) Cr
11EBTHDZ Cr 106.5 (40) N 132.5 (0.9) I 131.3 (0.9) N 77.9 (40.7) Cr
Compounds with alkyl chain of 9EBTHDZ and 11EBTHDZ are purely nematogenic,
while the compounds 2d and 2f exhibit enantiotropic N phase along with monotropic SmA
phase. The presence of SmA in 2d and 2f is due to (i) the presence of an ether-linking
group that provides greater linearity to molecules rather than the presence of an ester group,
and (ii) in terms of van der Waals radii, the Br atom is larger than the H atom. Besides,
the polar Br atom increases the molecular polarizability because the valence electrons of
this atom are positioned far apart from the nucleus [29]. However, the thermal stability of
the N phase for compound 9EBTHDZ is higher by 16.1◦C than that for compound 2d.
In addition, 11EBTHDZ possesses higher clearing temperature of 132.5◦C in comparison
with 2f, owing to the π-electrons associated with the carbonyl group, which provides
greater intermolecular interactions among molecules [30].

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4. Conclusion
A series of 4-((6-ethoxybenzothiazol-2-yl)diazenyl)bromoalkoxyphenyl has been synthe-
sized and characterized. All the target compounds exhibit enantiotropic N phase. However,
on further cooling, 2d–2g also show monotropic SmA phase.
The varying length of the terminal bromoalkoxyl chain is one of the determining fac-
tors that affects the melting and clearing temperatures. The melting temperatures decrease
when the length increases from 2a to 2g, except for compound 2d. Similarly, the clearing
temperatures also decrease from 2a to 2g with the elongation of the molecule. It is notewor-
thy to mention that the smectogenic properties will be observed only when the compounds
possess longer terminal chain (2d–2g).
Acknowledgment
The main author (G-Y. Yeap) would like to thank Universiti Sains Malaysia for funding
the project through an FRGS (Fundamental Research Grant Scheme) grant (account no.
203/PKIMIA/6711192).
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