Synthesis and mesomorphic evaluation of new calamitic liquid crystals containing benzothiazole core

Article abstract of DOI:10.1016/j.cclet.2010.10.042

The design and synthesis of new calamitic benzothiazole-based liquid crystals, 2-[4-(4-alkyloxybenzoyloxy)-phenyl]benzothiazoles are presented. The target compound was characterized using spectroscopic techniques, such as FT-IR, NMR (¹H and ¹³C), microanalysis and EI-MS. The liquid crystalline behaviours of these compounds were thoroughly examined by differential scanning calorimetry and polarizing optical microscope techniques. These materials exhibited enantiotropic nematic phase with high thermal stability (>168 °C). Smectic A phase starts to emerge as monotropic (metastable) phase from C10 member and changes into enantiotropic (stable) phase from C12 and persists up to C16 members.

Full text of DOI:10.1016/j.cclet.2010.10.042

Available online at www.sciencedirect.com
Chinese Chemical Letters 22 (2011) 619–622
www.elsevier.com/locate/cclet
Synthesis and mesomorphic evaluation of new calamitic liquid
crystals containing benzothiazole core
c
d
Teck Ming Koh a,b, Sie Tiong Ha c, , Teck Leong Lee , Siew Ling Lee ,
*
Guan Yeow Yeap e, Hong Cheu Lin f, Ramesh T. Subramaniam g
a Department of Science, Faculty of Engineering & Science, Universiti Tunku Abdul Rahman, Jln Genting Klang, Setapak,
53300 Kuala Lumpur, Malaysia
b Department of Chemistry, National University of Singapore, Singapore 117543, Singapore
c Department of Chemical Science, Faculty of Science, Universiti Tunku Abdul Rahman, Jln Universiti, Bandar Barat,
31900 Kampar, Perak, Malaysia
d Ibnu Sina Institute for Fundamental Science Studies, Universiti Teknologi Malaysia, 81310 UTM Skudai, Johor, Malaysia
e Liquid Crystal Research Laboratory, School of Chemical Sciences, Universiti Sains Malaysia, 11800 Minden, Penang, Malaysia
f Department of Materials Science & Engineering, National Chiao Tung University, Hsinchu 300, Taiwan, ROC
g Center for Ionics University Malaya, Department of Physics, Faculty of Science, University of Malaya, 50603 Kuala Lumpur, Malaysia
Received 10 September 2010
Available online 1 March 2011
Abstract
The design and synthesis of new calamitic benzothiazole-based liquid crystals, 2-[4-(4-alkyloxybenzoyloxy)-phenyl]ben-
zothiazoles are presented. The target compound was characterized using spectroscopic techniques, such as FT-IR, NMR (1H and
13C), microanalysis and EI-MS. The liquid crystalline behaviours of these compounds were thoroughly examined by differential
scanning calorimetry and polarizing optical microscope techniques. These materials exhibited enantiotropic nematic phase with
high thermal stability (>168 8C). Smectic A phase starts to emerge as monotropic (metastable) phase from C10 member and
changes into enantiotropic (stable) phase from C12 and persists up to C16 members.
# 2010 Sie Tiong Ha. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
Keywords: Benzothiazole; Alkoxy chain; Nematic; Smectic A; Liquid crystal
Liquid crystals (LCs) have gained additional attention as new type of organic semiconductor exhibiting self-
organization, liquid crystal displayer and separation membrane materials [1]. In the 1970s, much attention has
been focused on the electrical properties of LCs, as a result, the electrical properties of different calamitic LCs
have been examined [2,3]. Research focus shifted from calamitic to discotic LCs since the discovery of discotic
LCs in 1977 [4]. High hole mobility found in the hexagonal columnar phase of hexapenthyloxy-triphenylene
becomes an important milestone in the electronic studies of LCs [1]. An important interest, however, still remains
to be explored in calamitic mesophase due to their different degrees of molecular order and arrangement. As for
calamitic LCs, the electronic conduction was first established in the SmA phase of 2-phenylbenzothiazole
* Corresponding author.
E-mail addresses: hast_utar@yahoo.com, hast@utar.edu.my (S.T. Ha).
1001-8417/$ – see front matter # 2010 Sie Tiong Ha. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
doi:10.1016/j.cclet.2010.10.042

[()TD$FIG]
620
T.M. Koh et al. / Chinese Chemical Letters 22 (2011) 619–622
NH2
N
S
O
H
(i)
OH
OH
+
SH
[A]
O
O
O
(iii)
(ii)
OCnH2n+1
OH
OCnH2n+1
H5C2O
H5C2O
HO
[B]
O
O
OCnH2n+1
N
S
(iv)
[A]
[B]
+
nBPEP
Scheme 1. Synthetic route for the target compounds. (i) EtOH, reflux 6 h, (ii) CnH2n+1Br, K2CO3, CH3COCH3, reflux 5 h, (iii) H2O:EtOH (1:1)
reflux 5 h, KOH, conc. HCl, (iv) DCC, DMAP, DCM, DMF, reflux 5 h. Yield of 8BPEP (48%), 10BPEP (51%), 12BPEP (56%), 14BPEP (66%), and
16BPEP (74%).
derivatives [5,6]. In fact, smectic phases exhibit stronger intermolecular interaction than that of the nematic phase,
resulting in a higher order in terms of the molecular alignment and in a higher viscosity [5]. Furthermore, the
layer structure present in all smectic LCs induces p–p stacking interactions which facilitate hopping of charge
carrier mobility.
In our previous studies on LCs [7–10], we found that rod-like molecules incorporated with benzothiazole core ring
enabled them to exhibit mesophase easily. Here, we designed and synthesized a homologues series of compounds
according to Scheme 1. The heterocyclic benzothiazole ring containing electron-rich sulfur atom was used to induce a
smectic phase. Three core unit (one benzothiazole and two phenyl rings) was coupled together in order to enhance the
p stacking within the smectic layer structures. An ester linkage that connected two phenyl rings increases the
anisotropy polarizability which in turn strengthen the mesomorphic properties.
The synthetic route toward the title compounds, 2-[4-(4-alkyloxybenzoyloxy)phenyl]benzothiazoles (nBPEP,
where n = 8, 10, 12, 14, 16) is illustrated in Scheme 1. 2-Aminothiophenol was condensed with 4-hydroxy-
benzaldehyde upon refluxing in ethanol for 5 h and yield intermediate A [7]. Ethyl-4-hydroxybenzoate undergone
Williamson ether synthesis in acetone with appropriate 1-bromoalkane and followed by acidification by concentrated
hydrochloric acid to produce intermediate B [11]. Intermediate A and B were then subjected to Steglich esterification
with the appropriate fatty acid in the presence of DCC and DMAP according to previously reported procedure [12]. All
the crude products were purified upon repeated recrystallization using ethanol until constant melting points were
obtained. Structural elucidation of the title compound was carried out via elemental analysis, FT-IR, NMR and EI-MS
spectroscopic techniques [13].
The liquid crystalline textures of the title compounds were observed under polarizing optical microscope (POM)
and phase identification was made by comparing the observed textures with those reported in the literature [14,15].
Transition temperatures and corresponding enthalpy changes were determined using a differential scanning
calorimeter (DSC) and the data was tabulated in Table 1.
From Table 1, it can be noted that all synthesized compounds are mesogenic. Nematic phase was observed in all the
compounds, however, only 8BPEP is pure nematogenic compound. SmA phase emerged as monotropic phase
(metastable) in 10BPEP. From C12 derivative onwards, SmA phase exists as enantiotropic phase (stable) and persists
to C16 derivative.
The occurrence of nematic phases in 8BPEP was evidenced by observation of Schlieren and marble textures of
nematic phase. As a representative case of nematic phase, the optical photomicrograph of 8BPEP was illustrated in
Fig. 1(a). Upon cooling the isotropic liquid, the appearance of colorful birefringence domains was noted.
Representative optical photomicrographs of 16BPEP were depicted in Fig. 1. By cooling the isotropic liquid phase,
Schlieren texture showing a network of black brushed connecting centers of point and line defects, Fig. 1(b), was
observed. On further cooling the nematic phase, more ordered SmA phase was observed at lower temperature,
Fig. 1(c). The co-existence of the homogeneous (fan-shaped texture) and homeotropic texture (dark area) was
observed for the presence of SmA. The appearance of the homeotropic area is a diagnostic feature of SmA phase [16].

T.M. Koh et al. / Chinese Chemical Letters 22 (2011) 619–622
621
Table 1
Phase transition and transition enthalpy changes for nBPEP upon heating and cooling.
Compound
Phase transition, 8C
Heating
(corresponding enthalpy changes, kJ molÀ1
)
Cooling
8BPEP
Cr 137.3 (30.6) N 187.9 (0.6) I
Cr 109.2 (33.6) N 184.4 (1.2) I
10BPEP
12BPEP
14BPEP
16BPEP
Cr 134.3 (29.0) N 186.2 (0.9) I
Cr 100.0 (23.8) SmA 132.3 (0.8) N 183.8 (1.0) I
Cr 127.7 (33.3) SmA 142.5 (1.0) N 176.1 (0.9) I
Cr 92.3 (27.7) SmA 140.4 (1.2) N 173.7 (1.0) I
Cr1 103.6 (16.8) Cr2 121.4 (31.2) SmA 156.0 (1.3) N 174.5 (1.2) I
Cr1 87.0 (29.9) Cr2 95.6 (4.23) SmA 154.5 (1.4) N 172.8 (1.5) I
Cr 110.3 (49.5.6) SmA 158.8 (1.4) N 168.8 (1.5) I
Cr1 81.7 (37.6) Cr2 95.5 (6.4) SmA 157.4 (1.6) N 167.4 (1.6) I
Cr = crystal; N = nematic; SmA = smectic A; I = isotropic.
[()TD$FIG]
Fig. 1. Polarizing optical photomicrographs (100Â) of (a) nematic phase in 8BPEP, (b) nematic phase in 16BPEP and (c) SmA phase in 16BPEP.
A plot of transition temperatures against the number of carbons in the alkoxy chain during the heating cycle is
shown in Fig. 2. Based on the plot, both melting (Cr-SmA/N) and clearing (N-I) points showed a descending trend as
the length of the carbon chain increased. The flexible terminal alkoxy chain acts as a diluent to the mesogenic core ring
system, hence, depressed both melting and clearing temperatures of compounds nBPEP [17]. As can be seen from the
graph, the length of alkoxy chain also influenced the types of mesophase formed. All the compounds exhibited
enantiotropic nematic phase and smectogenic properties commenced only as the chain length increased. Furthermore,
the nematic phase range (DN) is reduced and the SmA phase range (DSmA) is increased as the alkyl chain length
ascended. The increasing van der Waals forces tend to stabilize the SmA phase by favouring the lamellar packing, on
the other hand, suppressed the nematic phase range.
In conclusion, all the synthesized compounds exhibited mesomorphic properties whereby nematic phase with high
thermal stability exists throughout the whole series and SmA phase emerged from the C10 derivatives onwards. The
presence of the ordered smectic structure in the title compounds becomes potential interest in electrical studies for
device application.
[()TD$FIG]
Fig. 2. Liquid crystal phase behaviour of nBPEP.

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T.M. Koh et al. / Chinese Chemical Letters 22 (2011) 619–622
Acknowledgments
The author (S.T. Ha) would like to thank Universiti Tunku Abdul Rahman (UTAR) for the UTAR Research Fund
(No. 6200/H02) and the Malaysia Toray Science Foundation (No. 4359/000) for funding this project. T.M. Koh would
like to acknowledge UTAR for the award of the research and teaching assistantships.
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