Heterocyclic benzoxazole-based liquid crystals: Synthesis and mesomorphic properties

Article abstract of DOI:10.1016/j.cclet.2011.05.017

New Schiff base liquid crystals containing benzoxazole core and alkanoyloxy chain at the end group of the molecules (C_n-1H_2n-1COO-, n = 14, 16, 18) was synthesized. The present compounds are enantiotropic smectic A liquid crystals. I

Full text of DOI:10.1016/j.cclet.2011.05.017

Available online at www.sciencedirect.com
Chinese Chemical Letters 22 (2011) 1191–1194
www.elsevier.com/locate/cclet
Heterocyclic benzoxazole-based liquid crystals: Synthesis
and mesomorphic properties
a
b
c
Sie Tiong Ha a, , Kok Leei Foo , Ramesh T. Subramaniam , Masato M. Ito ,
*
S. Sreehari Sastry d, Siew Teng Ong a
a Department of Chemical Science, Faculty of Science, Universiti Tunku Abdul Rahman, Jln Universiti, Bandar Barat, 31900 Kampar, Perak,
Malaysia
b Center for Ionics University Malaya, Department of Physics, Faculty of Science, University of Malaya, 50603 Kuala Lumpur, Malaysia
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 18 February 2011
Available online 20 July 2011
Abstract
New Schiff base liquid crystals containing benzoxazole core and alkanoyloxy chain at the end group of the molecules
(Cnꢀ1H2nꢀ1COO–, n = 14, 16, 18) was synthesized. The present compounds are enantiotropic smectic A liquid crystals. It was also
found that the end groups of the molecules and polar chloro substituent at the benzoxazole fragment had effect on the mesomorphic
properties.
# 2011 Sie Tiong Ha. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
Keywords: Schiff bases; Benzoxazoles; Mesomorphic; Smectic A; Structure–property relationship
Interest in the study of mesomorphic heterocycles has dramatically increased in the recent years due to their wider
range of structural templates, as well as their optical and photochemical properties [1–3]. These heterocyclic structures
generally incorporated unsaturated atoms, such as, O, N, S and/or others, and the presence of such electronegative
atoms often resulted in a reduced symmetry for the overall molecules; and a stronger polar induction [1]. In addition,
heterocyclic ring fused with benzene ring has also become popular mesogenic core in liquid crystal research.
Examples of heterocyclic fused-ring derivatives are benzothiazole [4], benzopyran-4-one [5] and benzothiadiazole [6].
Materials with lower melting temperatures, potential candidates for further applications, are often generated by such
compounds.
Benzoxazole, another kind of heterocyclic fused-ring system, have been studied in a variety of research areas,
including non-linear optics, organic light-emitting diodes and polymeric materials. However, examples of
benzoxazole-based liquid crystals are relatively rare [7] and some of them are polymer liquid crystals [8–12].
Therefore, in the search of new benzoxazole liquid crystals, we describe here mesomorphic properties of 5-chloro-2-
(4-alkanoyloxybenzylidenamino)benzoxazoles. The synthetic route is illustrated in Scheme 1. 2-Amino-5-
* Corresponding author.
E-mail addresses: hast_utar@yahoo.com, hast@utar.edu.my (S.T. Ha).
1001-8417/$ – see front matter # 2011 Sie Tiong Ha. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
doi:10.1016/j.cclet.2011.05.017

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S.T. Ha et al. / Chinese Chemical Letters 22 (2011) 1191–1194
Cl
Cl
O
H
N
O
N
O
(i)
NH2
OH
N
+
C
OH
O
O
Cl
N
O
(ii)
C
n-1H2n-1
N
C
Cl-n-BZX
n = 14, 16, 18
Scheme 1. Synthetic route for the target compounds. (i) CH2Cl2 (ii) Cnꢀ1H2nꢀ1COOH, DCC, DMAP, CH2Cl2, DMF.
chlorobenzoxazole and 4-hydroxybenzaldehyde were coupled by reflux in dichloromethane for 24 h, following which
the Schiff base intermediate was subjected to Steglich esterification with the appropriate fatty acids in the presence of
DCC and DMAP [12]. The crude products were purified upon repeated recrystallization using ethanol and their
structures were elucidated via FT-IR, NMR and EI-MS spectroscopic techniques [13].
The liquid crystalline textures of the products were observed under a polarizing optical microscope equipped with a
Linkam hotstage and temperature regulator. Phase identification was made by comparing the observed textures with
those reported in the literature [14,15]. Transition temperatures and enthalpy changes were determined using a
differential scanning calorimeter. The results are summarized in Table 1.
All synthesized compounds showed two endotherms in the DSC thermograms (Fig. 1) which can be attributed to
the isotropic-mesophase and mesophase-crystal transitions. From Table 1, it is clearly noticed that the target
Table 1
Phase transition and transition enthalpy changes for Cl-n-BZX upon heating and cooling.
Compound
Cl-14-BZX
Phase transition, 8C (corresponding enthalpy changes, kJ molꢀ1
)
Heating: Cr 105.9 (47.4) SmA 143.0 (8.9) I
Cooling: Cr 67.7 (51.1) SmA 134.9 (7.0) I
Heating: Cr 108.5 (47.1) SmA 136.7 (7.7) I
Cooling: Cr 74.0 (46.7) SmA 125.4 (2.8) I
Heating: Cr 109.1 (44.9) SmA 130.3 (5.7) I
Cooling: Cr 75.2 (43.4) SmA 118.9 (1.7) I
Cl-16-BZX
Cl-18-BZX
Cr, crystal; SmA, smectic A; I, isotropic. Value in the bracket is enthalpy change.
Fig. 1. DSC thermogram of Cl-16-BZX during heating and cooling cycles.

S.T. Ha et al. / Chinese Chemical Letters 22 (2011) 1191–1194
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Fig. 2. Liquid crystal textures of Cl-14-BZX upon cooling. The filament textures (at 132 8C) (a) coalesced to form focal-conic fan texture of smectic
A phase in (b) (at 97 8C).
compounds showing enantiotropic properties as the mesophase were observed during heating and cooling cycles [16].
Upon cooling of the isotropic liquid, compound Cl-14-BZX exhibited filament textures which coalesce to form a
focal-conic fan texture on further cooling. Fig. 2a shows the filament growth pattern appearing below the isotropic.
During this growth process, the filaments buckle quasi-continuously and eventually coalesce to focal-conic domains of
smectic A phase (Fig. 2b). The characteristics of filament texture is similar to that reported for N,N’-bis[(2-hydroxy-4-
alkoxyphenyl)methylene]benzene-1,4-diamine and N-[4-(4-n-alkanoyloxybenzoyloxy)benzylidene]-4-cyanoaniline
[17].
Linking group is one of the factors influencing the target compounds in exhibiting the smectic phase instead of a
nematic phase. The ester linking group favour the lamellar packing due to the dipole–dipole interaction which
ultimately generated the smectic phase. Then, a conjugative interaction over the entire molecule is better achieved
between the polar (chloro) substituent and benzoxazole, and led to smectic property in Cl-n-BZX. This suggests that
polar terminal substituent has substantial effect on influencing certain phase.
Smectic-isotropic transition (clearing) temperatures exhibited ascending trend as length of alkanoyloxy chain
increased from n-tetradecanoyloxy to n-octadecanoyloxy. It was attributed to the increasing van der Waals forces
between the alkyl chains [18]. However, the clearing temperature descended with the increase in the length of the
carbon chain due to the dilution of the mesogenic core resulting from the flexibility provided by the terminal
alkanoyloxy chain [19]. Decrement of phase range {DSmA: 37 8C (C14) ! 28 8C (C16) ! 21 8C (C18)} was noticed
as the chain length keeps increasing. Certain extent of flexibility is essential for promoting liquid crystal phase [20,21].
However, further increasing of the carbon chain of compound (from C14 to C16) will cause the molecule to be too
flexible, hence, reducing the stability of mesophase (in term of phase range) [22,23].
In conclusion, all the target compounds exhibited enantiotropic smectic A phase. The length of the terminal
alkanoyloxy chain (end molecule) affects the melting, clearing temperatures and phase ranges. The ester linking group
and the polar (chloro) substituent at the benzothiazole moiety were among the factors influencing the formation of the
smectic phase in the target compounds [24].
Acknowledgments
The authors would like to thank Universiti Tunku Abdul Rahman for the research facilities and financial support
(UTAR Research Fund), and Ministry of Higher Education (MOHE) for funding this project. K.L. Foo would like to
acknowledge MOHE for the scholarship.
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