Mesomorphic behavior of new benzothiazole liquid crystals having Schiff base linker and terminal methyl group

Article abstract of DOI:10.1016/j.cclet.2012.04.025

A homologous series of heterocycles, 6-methyl-2-(4-alkoxybenzylidenamino) benzothiazoles, were synthesized and characterized using FT-IR, ¹H and ¹³C NMR and mass spectrometric analysis. Enantiotropic nematic phase was observed for sh

Full text of DOI:10.1016/j.cclet.2012.04.025

Available online at www.sciencedirect.com
Chinese Chemical Letters 23 (2012) 761–764
www.elsevier.com/locate/cclet
Mesomorphic behavior of new benzothiazole liquid crystals
having Schiff base linker and terminal methyl group
a
b
c
Sie Tiong Ha a, , Kok Leei Foo , Hong Cheu Lin , Masato M. Ito ,
*
Kazuma Abe c, Kenji Kunbo c, S. Sreehari Sastry d
a Department of Chemical Science, Faculty of Science, Centre for Biodiversity Research, Universiti Tunku Abdul Rahman,
Jln Universiti, Bandar Barat, 31900 Kampar, Perak, Malaysia
b Department of Material Science & Engineering, National Chiao Tung University, Hsinchu 300, Taiwan, China
c Faculty of Engineering, Soka University, 1-236 Tangi-cho, Hachioji, Tokyo 192-8577, Japan
d Department of Physics, Acharya Nagarjuna University, Nagarjuna Nagar 522 510, India
Received 15 February 2012
Available online 9 June 2012
Abstract
A homologous series of heterocycles, 6-methyl-2-(4-alkoxybenzylidenamino)benzothiazoles, were synthesized and character-
1
ized using FT-IR, H and 13C NMR and mass spectrometric analysis. Enantiotropic nematic phase was observed for shorter
members. Smectic A phase only emerged from octyloxy derivative onwards. The terminal methyl group at the benzothiazole
fragment and the Schiff base linkage influenced the mesomorphic behavior of the present series.
# 2012 Sie Tiong Ha. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
Keywords: Schiff bases; Benzothiazole; Liquid crystal; Smectic A; Nematic
Over the past thirty years, liquid crystals have become the quintessential molecular electronic materials of our
present day era. Their applications encompass research activities in the field of surfactant and detergents, membrane
for air separation, high strength polymers, heat resistant materials, photonic, thin films, semiconductors etc. [1].
Heterocycles are of great importance as core units in thermotropic liquid crystals due to their ability to impart lateral
and/or longitudinal dipoles combined with changes in the molecular shape [2]. A great deal of mesomorphic
compounds containing heterocyclic units have been synthesized, and interest in such structures is constantly growing
[2,3]. However, only scant information on the inclusion of benzothiazole core into liquid crystal system is available
[4–6]. Funahashi and Hanna have reported the fast hole transport property of the photoconductive calamitic liquid
crystal, 2-(4-heptyloxyphenyl)-6-dodecylthiobenzothiazole [7]. A benzothiazole ring containing the electron-rich
sulphur atom can contribute to a low ionization potential and also induce a smectic phase. The flat molecular shape
with little interannular twisting in fused heterocyclic rings may also facilitate a high degree of overlapping molecular
wave function for an efficient hopping mechanism of charge transport [8]. Benzothiazole unit was also found in
fluorescent compounds, which is useful in applications as a result of high fluorescence quantum yields in the presence
* Corresponding author.
E-mail addresses: hast_utar@yahoo.com, hast@utar.edu.my (S.T. Ha).
1001-8417/$ – see front matter # 2012 Sie Tiong Ha. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
http://dx.doi.org/10.1016/j.cclet.2012.04.025

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S.T. Ha et al. / Chinese Chemical Letters 23 (2012) 761–764
N
O
N
S
C2H5OH
N
NH2
OH
+
S
C
OH
H3C
CH3COOH
H
H3C
CH3COCH3
K2CO3
CnH2n+1Br
N
N
S
C
OCnH2n+1
H3C
nMeBTH
n = 6,8,10,12,14
Scheme 1. Synthetic route for the target compounds.
of the rigid core structure [9]. Recently, benzothiazole derivatives have been continuously investigated for application
in thin-film, organic field-effect transistors [10].
In order to further explore benzothiazole as a mesogenic core in Schiff base liquid crystals, here, we reported a new
homologous series of 6-methyl-2-(4-alkoxybenzylidenamino)benzothiazoles. The synthetic route for the
benzothiazole derivatives is illustrated in Scheme 1. 2-Amino-6-methylbenzothiazole and 2, 4-dihydroxybenz-
aldehyde were coupled by reflux in ethanol for three hours, following which the Schiff base intermediate was subjected
to Williamson etherification with the appropriate bromoalkanes in the presence of potassium carbonate anhydrous.
The crude products were purified upon repeated recrystallization using ethanol and their structures were elucidated via
elemental analysis, FT-IR, NMR and EI-MS spectroscopic techniques [11].
The liquid crystalline textures of the products were observed under a polarizing optical microscope equipped with a
hotstage and temperature regulator. Phase identification was made by comparing the observed textures with those
reported in the literature [12,13]. Transition temperatures and associated enthalpy changes were determined using a
differential scanning calorimeter. The results are summarized in Table 1.
Under the polarizing microscope, shortest member of the series, 6MeBTH exhibited nematic phase. The mesophase
was identified by the nematic droplets texture. Brownian flashes, a feature of the nematic phase were also observed
before recrystallization [14]. By referring to the chemical constitution of the present series, the nematic phase which
was observed could have resulted from the terminal methyl group at the sixth position which disrupted the lamellar
arrangement of the smectic phase. The similar phenomenon was also reported for a homologous series of azo liquid
crystals with a benzothiazole core [5]. Furthermore, the Schiff base (CH N) linker which conferred a step like
structure resulted in the thickening effect which in turn enhanced the nematic phase stability [15].
An interesting phenomenon was observed for 8MeBTH where an additional phase with fan-shaped texture assigned
as smectic A (SmA) phase (Fig. 1b) was observed after the formation of the nematic droplets (Fig. 1a) on the cooling
run. SmA phase emerged as monotropic phase (metastable) in 8MeBTH. From C10 derivative onwards, SmA phase
Table 1
Phase transition and transition enthalpy for nMeBTH upon heating and cooling cycle.
Compound
Phase transition, 8C (corresponding enthalpy changes, kJ molÀ1
)
Heating
Cooling
6MeBTH
8MeBTH
10MeBTH
12MeBTH
14MeBTH
Cr 105.5 (39.83) N 119.5 (1.03) I
Cr 83.3 (35.60) N 119.4 (1.47) I
Cr 108.2 (49.10) N 118.3 (1.33) I
Cr 70.4 (33.94) SmA 96.2 (1.43) N 118.2 (1.84) I
Cr 99.0 (49.84) SmA 110.7 (3.41) N 118.4 (1.79) I
Cr 61.1 (37.42) SmA 110.8 (3.13) N 118.1 (1.94) I
Cr 81.6 (44.98) SmA 116.1 (8.25) I
Cr 68.9 (41.13) SmA 116.7 (8.90) I
Cr 83.2 (48.79) SmA 113.6 (8.6) I
Cr 69.7 (47.86) SmA 115.1 (8.97) I
Cr = crystal; N = nematic; SmA = smectic A; I = isotropic.

S.T. Ha et al. / Chinese Chemical Letters 23 (2012) 761–764
763
Fig. 1. Optical photomicrographs of 8MeBTH upon cooling. Formation of nematic droplets (a) cooled from isotropic and upon further cooling,
Smectic A phase with focal conic texture (b) was observed.
130
I
120
N
110
100
90
Cr-SmA/N
SmA-N
SmA
SmA/N-I
Cr
80
70
6
8
10
12
14
Number of carbon atoms (n) in alkyloxy chain
Fig. 2. Plot of transition temperature versus the number of carbon (n) in the alkyloxy chain during heating cycle.
exists as enantiotropic phase (stable) and persists to C14 derivative. Higher members of the series (C12 and C14)
displayed only smectic phase. This suggested that with increasing the length of the terminal chain, the nematic
properties decreased and led to the emergence of the smectic phase. This is due to attraction between the long
alkyloxy chains leading to their intertwining, which facilitates the lamellar packing and is vital for the smectic
phase [14]. It can therefore be proposed that in order to generate the enantiotropic (stable) smectic phase in the
analogous substituted C7H5SNCH NC6H5 compounds, the number of carbons in the alkyl chain must be at least
10 (n ꢀ 10) [16].
A plot of the transition temperature against the number of carbons in the alkyloxy chain during the heating scan is
depicted in Fig. 2. The melting temperature was considerably reduced by the increase in the length of the chain owing
to the increase in its flexibility [15]. As for clearing temperature, it descended as the number of carbon atoms
increased, resulting from the dilution of the core induced by the increase in the length of the terminal chain [17].
Furthermore, the nematic phase range is reduced and the SmA phase range is increased as the alkyl chain length
ascended. The increasing van der Waals forces tend to stabilize the SmA phase by favoring the lamellar packing, on the
other hand, suppressed the nematic phase range.
In conclusion, all the synthesized compounds exhibited mesomorphic properties whereby nematic phase exists for
the shorter members and SmA phase emerged from the C8 derivatives onwards. The presence of the ordered smectic
structure in the title compounds becomes potential interest in electrical studies for device application.
Acknowledgment
The author (S.T. Ha) would like to thank Universiti Tunku Abdul Rahman and Ministry of Higher Education
(MOHE) for the financial supports via LRGS (No. LR003-2011A) and research facilities.

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S.T. Ha et al. / Chinese Chemical Letters 23 (2012) 761–764
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