Synthesis and characterization of thermotropic liquid crystals consisting heterocyclic benzothiazole core system

Article abstract of DOI:10.14233/ajchem.2014.17135

A new series of rod-shaped mesomorphic compounds, 6-methyl-2-(4-alkoxybenzylidenamino)benzothiazoles bearing a fused thiazole ring and imine central linkage, were prepared and their molecular structures were confirmed via mass spectrometric, IR and NMR spectroscopic techniques. The mesomorphic properties were identifed using differential scanning calorimetry, polarizing optical microscopy and X-ray diffraction analysis. All compounds showed enantiotropic mesomorphism. Whilst the lower members of the series, butyloxy, hexyloxy and octyloxy derivatives exhibited nematic phase, the medium members (decyloxy derivative) exhibited nematic and smectic A phases. The higher members (C12, C14, C16 and C18 derivatives) displayed smectic A phase only.

Full text of DOI:10.14233/ajchem.2014.17135

Asian Journal of Chemistry; Vol. 26, No. 22 (2014), 7627-7631
ASIAN JOURNAL OF CHEMISTRY
http://dx.doi.org/10.14233/ajchem.2014.17135
Synthesis and Characterization of Thermotropic Liquid
Crystals Consisting Heterocyclic Benzothiazole Core System
1
1,*
2
KOK-LEEI FOO , SIE-TIONG HA and SIEW-LING LEE
1
Department of Chemical Science, Faculty of Science, Universiti Tunku Abdul Rahman, Jln Universiti, Bandar Barat, 31900 Kampar, Perak, Malaysia
2
Ibnu Sina Institute for Fundamental Science Studies, Universiti Teknologi Malaysia, 81310 UTM Skudai, Johor, Malaysia
Corresponding author: Email: hast_utar@yahoo.com
Received: 28 January 2014; Accepted: 21 March 2014;
*
Published online: 6 November 2014;
AJC-16198
A new series of rod-shaped mesomorphic compounds, 6-methyl-2-(4-alkoxybenzylidenamino)benzothiazoles bearing a fused thiazole
ring and imine central linkage, were prepared and their molecular structures were confirmed via mass spectrometric, IR and NMR
spectroscopic techniques. The mesomorphic properties were identifed using differential scanning calorimetry, polarizing optical microscopy
and X-ray diffraction analysis. All compounds showed enantiotropic mesomorphism. Whilst the lower members of the series, butyloxy,
hexyloxy and octyloxy derivatives exhibited nematic phase, the medium members (decyloxy derivative) exhibited nematic and smectic A
phases. The higher members (C12, C14, C16 and C18 derivatives) displayed smectic A phase only.
Keywords: Imine, Benzothiazole, Mesomorphic, Nematic, Smectic.
elongation by addition of phenyl ring and a central ester linkage
INTRODUCTION
generated higher mesophase thermal stability. The nitro
At the present day, liquid crystals have turn out to be the
substituent is also more conductive to the generation of smectic
basic molecular electronic materials that used in our daily life.
mesophase as compared to the chloro and the methoxy substi-
These materials have useful applications in the field of
surfactant and detergents, high yield strength polymers,
5
tuent . The substitution at the sixth position was thermally more
6
stable compared to fifth position of benzothiazole ring . In
1
membrane, photonic, thin films, semi-conductors etc. . Interest
this paper, we reported the mesomorphic properties of hetero-
cyclic benzothiazole liquid crystal with a homologous series
of Schiff base ether, 6-methyl-2-(4-alkoxybenzylidenamino)-
in mesomorphic heterocyclic compounds has dramatically
increased in recent years due to their larger range of structural
templates, as well as their optical and photochemical pro-
7
benzothiazoles . The present study represents an expansion
2
perties .
4,8,9
of our existing works in this area
.
Ability to generate lateral and/or longitudinal dipoles is
among the great significance of heterocycles as core units in
thermotropic liquid crystal. The introduction of heterocyclic
fragment is always accompanied with changes in the molecular
EXPERIMENTAL
All solvents and reagents were obtained commercially and
used without any further purification. 4-Hydroxybenzaldehyde,
potassium carbonate and 1-bromoalkanes were obtained from
Merck (Darmstadt, Germany). 2-Amino-6-methylbenzothiazole
was purchased from Acros Organics (USA).
3
shape . These structures basically incorporated unsaturated
atoms, such as, oxygen, nitrogen, sulfur and/or others and the
presence of such electronegativity atoms often yielded in a
reduced symmetry for the overall molecules and a stronger
Infrared spectroscopy was performed on a Perkin-Elmer
2
1
polar induction . The heterocyclic compounds involved are
System 2000 FTIR spectrometer. H NMR (400 MHz) and
13
usually five- or six- membered and they form part of the central
core in a typical rod-shaped molecule. In previous studies,
benzothiazole ring was incorporated as a mesogenic core in
thermotropic liquid crystals which allowed them to display
C NMR (100 MHz) spectra were recorded in CDCl using
3
a Bruker Avance 400 MHz Spectrometer. EI-MS (70 eV)
spectrum was collected with a mass spectrometer Finnigan
MAT 95XL-T.
4
good mesomorphic properties . Prajapati and Bonde have
Phase transition temperatures and enthalpy changes were
obtained using Mettler Toledo DSC823e at heating and cooling
synthesized two series of mesogenic benzothiazole derivatives
with polar nitro substituent. It was found that the molecular
-1
rates of 10 °C min . Carl Zeiss polarizing optical microscope

7
628 Foo et al.
Asian J. Chem.
1
13
(
POM) equipped with Linkam hotstage was used for tempe-
The IR, NMR ( H and C) and mass spectral data for the
representative compound, 12MeBTH, are summarized as follow.
rature-dependent studies of the textures of liquid crystals.
Textures exhibited by the compounds were observed using
polarized light with crossed polarisers. Samples were prepared
as thin films sandwiched between a glass slide and cover slip.
Mesophase identification was done by comparing the observed
+
Yield 65 %; EI-MS m/z (rel. int. %): 283 (38.8) [M ], 452
(100); IR (KBr, νmax, cm ): 1286 (C-O aromatic ether), 1567
(C=N thiazole), 1597 (C=N Schiff base), 2919, 2849 (C-H
aliphatic), 3053 (C-H aromatic); H NMR (400 MHz, CDCl
-1
1
3
):
1
0,11
textures with those reported in the literature
.
δ 0.90 (t, 3H, J = 6.9 Hz, CH
(CH -), 1.80 (p, 2H, J = 6.9 Hz, -CH
CH -), 4.03 (t, 2H, J = 6.6 Hz, -CH -O-Ar-), 6.99 (d, 2H, J =
8.7 Hz, Ar-H), 7.27 (d, 1H, J = 8.1 Hz), 7.61 (s, 1H, Ar-H),
7.83 (d, 1H, J = 8.1 Hz, Ar-H), 7.96 (d, 2H, J = 8.7 Hz, Ar-H),
8.94 (s, 1H -N=CH-). C NMR (100 MHz, CDCl
(CH=N), 163.51, 149.84, 134.97, 134.49, 132.24, 127.84,
127.56, 122.40, 121.42, 114.99 for Ar-C, 68.39 (Ar-O-CH -),
31.92 (Ar-O-CH -CH -), 29.65, 29.63, 29.58, 29.55, 29.35,
29.11, 25.98, 22.68, 21.56 [CH (CH -O], 14.10 (-CH CH ).
3
-), 1.27-1.49 (m, 18H, CH
3
-
Synchrotron powder X-ray diffraction measurements were
carried out using beamline Bl17A at the National Synchrotron
Radiation Research Center in Taiwan, where the X-ray wave-
length used was 1,32633Å. The XRD data were obtained using
imaging plates (area = 20 cm × 40 cm with a pixel resolution
of 100) curved with a radius equivalent to a sample-to-image
plate distance of 280 mm and the diffraction signals were
accumulated for 3 min the powder samples were packed into
capillary tube and heated by a heat gun, where the temperature
controller was programmed by a PC with a PID feedback
system. The scattering angle θ was calibrated by a mixture of
sliver behenate and silicon.
Synthesis of benzothiazole 1: Equimolar of 2-amino-6-
methylbenzothiazole (40 mmol, 6.56 g) and 4-hydroxybenzal-
dehyde (40 mmol, 4.88 g) were refluxed for 3 h in an ethanol
solution (60 mL) with the addition of acetic acid (two drops)
as catalyst. The reaction mixture was then filtered and the
filtrate was left to evaporate in the fume hood. The yellow
solid thus obtained was recrystallized from ethanol before
being used for further reaction (Scheme-I).
2
)
9
2
-CH -O-), 2.48 (s, 3H,
2
3
2
13
3
): δ 164.93
2
2
2
3
2
)
9
2
3
RESULTS AND DISCUSSION
The synthetic route towards the targeted compounds is
shown in Scheme-I. Molecular structure of nMeBTH is confir-
med using mass spectrometric, infrared and nuclear magnetic
resonance spectroscopic techniques. The predominant mole-
cular ion peak at 452.3 m/z in the mass spectrum of 12MeBTH
establish its molecular formula as C27
proposed structure.
FT-IR spectra of all the members in the series showed the
similar characteristics, thus, 12MeBTH is discussed as a repre-
sentative case. The diagnostic absorption band resulting from
H
35
2
N OS, supporting the
Synthesis of benzothiazole 2: Benzothiazole 1 (20 mmol,
.8 g) dissolved in minimum amount of N,N'-dimethylformamide
5
-1
the alkyl groups were observed between 2849 and 2916 cm .
The relative intensity of the absorption bands of the alkyl
groups increased upon ascending the series due to increasing
number of carbons in the alkyl chain. The absorption band of
azomethine (C=N) group was overlapping with the absorption
band arising from the C=N stretch of the benzothiazole ring,
hence, resulting in a sharp and strong absorption bands at 1597
is etherified with the suitable 1-bromoalkanes. Anhydrous
potassium carbonate is added into the mixture. The mixture
was then heated at 80 °C for 5 h. Finally, the precipitate formed
upon cooling was filtered off and the solvent was removed by
slow evaporation. The yellow product was recrystallized several
times with ethanol where upon the pure compound was
obtained. The purity of all compounds was checked by thin
layer chromatography (Merck 60 F 254) and visualised under
shortwave UV light.
-1
-1
cm . Moreover, a strong band can be observed at 1286 cm
which attributed to the C-O ether bond.
1
According to H NMR spectrum of 12MeBTH, the triplet
at δ = 0.90 ppm in the spectra is assigned to the terminal methyl
protons. The multiplet signal at δ = 1.27-1.49 ppm, pentet at δ =
N
S
O
NH2
+
OH
1.80 ppm and a triplet at δ = 4.03 ppm were attributed to the
H
H C
3
remaining methylene protons of the alkoxy chain.A singlet signal
at δ = 2.48 ppm is attributed to the methyl protons that directly
attached to aromatic ring. The spectrum confirmed the presence
of seven aromatic protons. The singlet arising from the azome-
a
N
S
12,13
13
thine proton was detected at δ = 8.94 ppm . In the C NMR
spectra, the peak at δ = 164.93 ppm is assigned to the azomethine
N
C
OH
H C
3
12,13
1
carbon . The signals between δ = 114.99 to 163.51 ppm supp-
orted the presence of aromatic carbons whereas the carbons
resonances appeared between δ = 14.10 to 31.92 ppm were
indicative of the methylene and methyl carbons of the alkoxy
chain. The results from the IR and NMR spectral data of the
title compounds were consistent with the proposed structure.
The mesophases of all compounds were observed under
a POM during heating and cooling cycles. All the compounds
exhibited mesomorphism. The phase transition temperatures
and corresponding enthalpy changes of compounds nMeBTH
were determined using a DSC. The data obtained from the
b
N
S
N
C
2
OC H
H C
n
2n+1
3
Scheme-I:Reagents and reaction condition: (a) C
for 3 h; (b) (CH CO, DMF, K CO , C
, 10, 12, 14, 16, 18), reflux for 3 h
2
H
n
5
3
OH, CH COOH, reflux
3
)
2
2
3
H2n+1Br (where n = 4, 6,
8

Vol. 26, No. 22 (2014)
Synthesis and Characterization of Thermotropic Liquid Crystals 7629
DSC analysis are summarized in Table-1. The DSC thermo-
grams of 8MeBTH and 10MeBTH during heating and cooling
scans are depicted in Figs. 1 and 2, respectively. 8MeBTH
exhibited monotropic smectic A (SmA) phase whereby the
SmA phase can be observed during cooling cycle only. The
existence of SmA phase regardless during heating or cooling
processes and thus, it is confirmed that 10MeBTH possessed
enantiotropic SmA phase. Both thermograms also showed
enantiotropic nematic properties.
Cr
SmA
I
Heating scan
N
Cooling scan Cr
SmA
N
I
4
0
50
60
70
80
90
Temperature (°C)
100
110
120
130
140
Fig. 2. Differential scanning calorimetry thermogram of 10MeBTH during
heating and cooling scans
On cooling the isotropic liquid of whole series, C4, C6,
C8 and C10 members showed droplets which coalesce to a
classical schlieren texture of nematic phase. However, in
Fig. 3, upon cooling from isotropic liquid of 16MeBTH,
emergence of SmA phase with bâtonnets texture can be
observed. The presence of SmA phase was confirmed based
on the microscopic observation of the characteristic fan shaped
texture.All the observed liquid-crystalline textures were typical
Heating scan
Cr
N
I
Cooling scan
Cr
SmA
N
I
1
0,11
according to the literature
.
40
50
60
70
80
90
100
110
120
130
140
It is well known that thermotropic liquid crystals are highly
sensitive to their molecular constitution. The thermal stability
and mesophase length as a measure of mesomorphism can be
Temperature (°C)
Fig. 1. Differential scanning calorimetry thermogram of 8MeBTH during
heating and cooling scans
14
correlated with the molecular constitution of the compounds .
TABLE-1
PHASE TRANSITION TEMPERATURES AND ASSOCIATED ENTHALPY CHANGES OF SERIES nMeBTH
-1
Compound
Cycle
Temperature (ºC)
9.6
Phase transition
Associated enthalphy change (∆H, kJ mol )
9
Cr-to-N
N-to-I
I-to-N
N-to-Cr
Cr-to-N
N-to-I
I-to-N
N-to-Cr
Cr-to-N
31.02
1.13
1.15
26.38
39.83
1.03
Heating
1
1
25.3
25.1
4
MeBTH
MeBTH
Cooling
Heating
Cooling
Heating
6
3.8
105.5
119.5
119.4
6
1.47
8
3.3
35.60
49.10
1.33
1
1
08.2
18.3
N-to-I
8
MeBTH
118.2
96.2
I-to-N
1.84
1.43
33.94
49.84
3.41
1.79
1.94
3.13
Cooling
Heating
Cooling
N-to-SmA
SmA-to-Cr
Cr-to-SmA
SmA-to-N
N-to-I
7
9
0.4
9.0
110.7
1
1
18.4
18.1
10MeBTH
I-to-N
110.8
N-to-SmA
SmA-to-Cr
Cr-to-SmA
SmA-to-I
I-to-SmA
SmA-to-Cr
Cr-to-SmA
SmA-to-I
I-to-SmA
SmA-to-Cr
Cr-to-SmA
SmA-to-I
I-to-SmA
SmA-to-Cr
Cr -to-Cr
6
8
1.1
1.6
37.42
44.98
8.25
Heating
Cooling
Heating
Cooling
Heating
Cooling
1
1
16.1
16.7
12MeBTH
14MeBTH
16MeBTH
8.90
6
8
8.9
3.2
41.13
48.79
8.60
1
1
13.6
15.1
8.97
6
8
9.7
8.4
47.86
61.10
10.45
10.94
58.64
1.21
1
1
15.8
15.5
8
5
1.3
5.6
1
2
Heating
91.2
Cr -to-SmA
67.18
10.83
8.70
57.81
1.28
2
109.6
SmA-to-I
I-to-SmA
SmA-to-Cr2
Cr -to-Cr
18MeBTH
112.1
Cooling
86.6
4.7
Note: Cr = crystal; I = isotropic; N = nematic; SmA = smectic A
5
2
1

7
630 Foo et al.
Asian J. Chem.
mesophase formation (nematic phase). Generally, compounds
with short carbon chains exhibit wider nematic temperature
15
ranges than compounds with longer alkoxy chains (Table-2) .
Smectogenic properties were observed as the chain length
increased. In general, a metastable smectic phase commenced
from C member as monotropic phase and become a stable
8
enantiotropic smectic phase from C10 member and persistent
till the end of the series. It can be clearly deduced that the length
of the terminal chains can significantly affect the mesomorphic
16
properties of the compounds . The azomethine (C=N) linkage
which conferred a step like structure resulted in the thickening
effect which in turn enhanced the nematic phase for short to
7
medium chain compounds .
TABLE-2
MESOPHASE RANGE OF SERIES nMeBTH
(
a)
o
Phase range, C
Compound
∆SmA
∆N
25.7
14.0
10.1
7.7
-
-
-
-
4MeBTH
6MeBTH
8MeBTH
-
-
-
10MeBTH
12MeBTH
14MeBTH
16MeBTH
18MeBTH
11.7
34.5 (widest)
30.4
27.4
18.4
From the plot (Fig. 4), the clearing temperature showed
descending trends as the length of the carbon chain increased.
The flexible terminal alkoxy chain acts as a diluent to the meso-
genic core rings system, hence, depressed the clearing tempe-
rature. The descending trend was in agreement with those series
of 6-methoxy-2-(2-alkanoyloxybenzylidenamino)-benzo-
thiazoles in which the homologous with the longest chain
(
b)
Fig. 3. Optical photomicrographs of 16MeBTH on cooling. The battonets
a) coalesced to form the fan-shaped texture of SmA phase in (b)
7
possessed the lowest thermal stability . The increasing van der
Waals forces resulted from the lengthening of alkyl chain that
plays an important role in stabilizing the smectic phase by
favouring the lamellar packing. On the other hand, it suppre-
(
A plot of phase transition temperatures against the number of
carbons in the alkoxy chains for series nMeBTH, enable the
effects of the terminal chains on the mesomorphic properties
to be studied (Fig. 4).
17
ssed the nematic phase range . In addition, the long carbon
chain being attracted and intertwined which in turn facilitates
the lamellar packing causing a reduction in the nematic phase
18
range . However, as the length of the carbon chain increase
from C10 to C18 members, the smectic phase range (∆SmA) did not
show a specific trend. Among all the smectogens, C12 member
was found to possess the widest ∆mA phase range (34.5 °C).
In order to confirm the presence of the smectic A phase
in the series, XRD analysis was performed. The representative
compound 16MeBTH is shown in Fig. 5. From XRD pattern
of 16MeBTH, it has been revealed that the single sharp
diffraction peak appeared at 1.605° indicating the presence of
layered structure of smectic phase. It is well known that when
the reflection within the SmA layer corresponds to d appro-
1
1
1
30
25
20
115
110
N
1
05
00
SmA
Cr
1
9
9
8
8
5
0
5
0
19
ximate to L, the SmA arrangement is in the monolayer form .
However, when the d-layer spacing is intermediate between L
4
6
8
10
12
14
16
18
14
and 2L then it is called a partial bilayer phase . According to
Number of carbons (n) in the alkoxy chain
XRD data, the d-layer spacing was found to be 37.9 Å. The
layer spacing is much smaller than the molecular length and
the d/L ratio was found to be 1.12. By combining the data
from POM and XRD, SmA phase was identified to be present
Fig. 4. Plot of transition temperature versus the number of carbon (n) in
the alkoxy chain of nMeBTH
The present compounds exhibited liquid crystal properties
where the terminal chain lengths are long enough for promoting
20
in compound 16MeBTH . It can be seen that the layer spacing

Vol. 26, No. 22 (2014)
Synthesis and Characterization of Thermotropic Liquid Crystals 7631
d, is significantly larger than the calculated molecular length
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21
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1
Phase observed
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95 °C
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