Mesomorphic behavior of new azomethine liquid crystals having terminal iodo group

Article abstract of DOI:10.1016/j.cclet.2010.09.017

A series of azomethine esters, 3-hydroxy-4-{[(4-iodophenyl)imino]methyl} phenyl alkanoates possessing even number of carbon atoms at the terminal alkanoyloxy chain (C_n-1H_2n-1COO-, n = 4,6,8,10,12,14) was synthesized. n-Butanoyloxy wa

Full text of DOI:10.1016/j.cclet.2010.09.017

Available online at www.sciencedirect.com
Chinese Chemical Letters 22 (2011) 260–263
www.elsevier.com/locate/cclet
Mesomorphic behavior of new azomethine liquid crystals
having terminal iodo group
a
b
c
Sie-Tiong Ha a, , Teck-Leong Lee , Guan-Yeow Yeap , Masato M. Ito ,
*
Ayumu Saito c, Masaaki Watanabe c
a Department of Chemical Science, Faculty of Science, Universiti Tunku Abdul Rahman, Jln Universiti, Bandar Barat,
31900 Kampar, Perak, Malaysia
b Liquid Crystal Research Laboratory, School of Chemical Sciences, Universiti Sains Malaysia, 11800 Minden, Penang, Malaysia
c Faculty of Engineering, Soka University, 1-236 Tangi-cho, Hachioji, Tokyo 192-8577, Japan
Received 10 June 2010
Available online 19 December 2010
Abstract
A series of azomethine esters, 3-hydroxy-4-{[(4-iodophenyl)imino]methyl}phenyl alkanoates possessing even number of
carbon atoms at the terminal alkanoyloxy chain (Cn-1H2n-1COO-, n = 4,6,8,10,12,14) was synthesized. n-Butanoyloxy was found
non-mesogenic, whilst n-hexanoyloxy to n-tetradecanoyloxy exhibited enantiotropic smectic A phase with fan-shaped texture. It
was found that the length of terminal alkanoyloxy chain influenced the mesomorphic properties.
# 2010 Sie-Tiong Ha. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
Keywords: Liquid crystals; Azomethine ester; Enantiotropic; Smectic A
In August 1889, Otto Lehman, a physicist first reported the fourth phase of matter called liquid crystals phase after
investigating the double-melting phenomenon of molten cholesteryl benzoate observed by Friedrich Reinitzer [1,2].
Understanding of structure–property relationship is elemental on molecular modifications for synthesis of new
mesogens with desirable properties and future applications [3]. An earlier report has shown that low-mass molecules
containing two unsaturated rings with one or multiple terminal substituents are capable of exhibiting mesomorphic
properties [4]. Discovery of 4-methoxybenzylidene-40-butylaniline (MBBA) and its application of nematic phase at
room temperature in displays sparked a renewed interest of researcher on establishing structure–property relationships
of Schiff base esters [5,6].
In our previous studies, the results revealed that azomethine and ester are useful connecting units for generating
mesomorphism in thermotropic liquid crystals with two and three aromatic rings. Aromatic azomethine ester comprising
of different polarity of substituents has been known to either promote or suppress the mesomorphic properties [7–10].
In order to accomplish the research on mesomorphic properties of two aromatic rings Schiff base ester, we reported
another homologous series of Schiff base esters, 3-hydroxy-4-{[(4-iodophenyl)imino]methyl}phenyl alkanoates and its
synthetic route is shown in Scheme 1. 4-Iodoaniline was condensed with 4-hydroxybenzaldehyde upon refluxing in
* 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.2010.09.017

[()TD$FIG]
S.-T. Ha et al. / Chinese Chemical Letters 22 (2011) 260–263
261
O
N
I
(i)
HO
H2N
I
HO
+
N
I
(ii)
Cn-1H2n-1COO
nABIA
where n= 4, 6, 8, 10, 12, 14
Scheme 1. Synthetic route of the target compounds. (i) CH3OH (ii) Cn-1H2n-1COOH, DCC, DMAP, THF.
Table 1
Phase transition temperatures and associated enthalpy changes of nABIA upon heating and cooling.
Compound
4ABIA
Phase transition temperature, 8C (enthalpy change, kJ molꢀ1
)
Cr 116.1 (29.9) I
I 112.1 (29.6) Cr
6ABIA
Cr 101.3 (52.2) SmA 111.1 (14.6) I
I 104.3 (23.1) SmA 31.4 (36.6) Cr
Cr 86.6 (57.9) SmA 115.2 (10.4) I
I 107.8 (7.0) SmA 36.4 (43.6) Cr
Cr 86.6 (61.6) SmA 115.7 (6.4) I
I 108.3 (5.8) SmA 27.5 (38.7) Cr
Cr 101.6 (78.2) SmA 109.6 (11.7) I
I 102.0 (23.9) SmA 84.3 (70.2) Cr
Cr 94.3 (70.5) SmA 116.5 (16.2) I
I 110.1 (16.3) SmA 61.6 (74.3) Cr
8ABIA
10ABIA
12ABIA
14ABIA
Note: Cr = crystal; SmA = smectic A; I = isotropic.
methanol for an hour [11]. Then, the Schiff base intermediate was esterified with fatty acids in the presence of DCC/
DMAP [12]. All the crude products were purified by repeated recrystallization using ethanol and hexane until constant
melting points were obtained. Structure of the products was characterized by elemental analysis and spectroscopic
techniques including FT-IR, 1H and 13C NMR and EI-MS [13].
Transition temperatures and enthalpy changes were determined using a differential scanning calorimeter. The
results obtained were listed in Table 1. Liquid crystalline textures were observed under a polarizing optical microscope
equipped with hotstage and temperature regulator. Phase identification was made by comparing the observed textures
with those reported in the literature [14,15].
All the compounds exhibited enantiotropic smectic A phase with fan-shaped texture except n-butanoyloxy
derivative. Upon cooling of 12ABIA from isotropic liquid, fan-shaped texture (Fig. 1a) of smectic A phase was first
observed followed by coexisting planar and homeotropic regions of soft crystal E phase (Fig. 1b).
[()TD$FIG]
Fig. 1. Liquid crystals textures (100ꢁ) of 12ABIA upon cooling exhibiting (a) fan-shaped texture of smectic A phase and (b) coexisting planar and
homeotropic regions of soft crystal E phase (b).

[()TD$FIG]
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S.-T. Ha et al. / Chinese Chemical Letters 22 (2011) 260–263
Fig. 2. Plot of phase transition temperatures against number of carbon atoms (n) in alkanoyloxy chain during heating scan.
A plot of phase transition temperatures against number of carbon atoms (n) in alkanoyloxy chain during heating
scan is shown in Fig. 2. Based on the plot, it can be deduced that length of terminal alkanoyloxy chain influenced the
mesomorphic properties. With the increasing length of terminal chain, the phase changed from non-mesogenic to
enantiotropic smectic A phase. The molecule of n-butanoyloxy derivative is too rigid for it to be mesogenic [14]. As
from n-hexanoyloxy to n-tetradecanoyloxy derivatives, the homologous members showed enantiotropic smectic A
phase. From the graph, it is apparent that range of smectic A phase increased from n-hexanoyloxy to n-decanoyloxy
members. This is because the increase of length of terminal alkanoylxy chain led to the enhancement of the smectic
properties. However, the range of smectic A phase of n-dodecanoyloxy and n-tetradecanoyloxy decreased due to the
dilution of mesogenic core [16,17].
Acknowledgment
The author (S.T. Ha) would like to thank Universiti Tunku Abdul Rahman for the research facilities and
financial support. T.L. Lee would like to acknowledge Ministry of Higher Education for the award of MOHE
Scholarship.
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[13] Analytical and spectroscopic data for the representative compound 12ABIA: Yield 63%, EI-MS m/z (rel. int. %): 505 (11) [M+], 323 (100), IR
(max (KBr, cm(1): 2920, 2850 (C-H aliphatic); 1751 (C=O ester); 1600 (C=N), 1H NMR (300 MHz, CDCl3): d 0.88 (t, 3H, J=6.6 Hz, CH3-),
1.30 (m, 16H, CH3-(CH2)8-), 1.76 (qt, 2H, J=7.4 Hz, -CH2-CH2-COO-), 2.57 (t, 2H, J=7.5 Hz, -CH2-COO-), 6.95 (d, 2H, J = 8.7 Hz, Ar-H),
7.20 (d, J = 8.4 Hz, 2H, Ar-H), 7.69 (d, J=8.7 Hz, 2H, Ar-H), 7.90 (d, J = 8.4 Hz, 2H, Ar-H), 8.39 (s, 1H, -CH = N-), 13C NMR (100 MHz,
CDCl3): d 14.11 (CH3-), 22.68 (CH3CH2-), 24.89 (CH3CH2CH2-), 29.10, 29.25, 29.33, 29.45, 29.60 for methylene carbons (CH3CH2CH2-

S.-T. Ha et al. / Chinese Chemical Letters 22 (2011) 260–263
263
(CH2)8-), 31.91 (-CH2CH2COO-), 34.43 (-CH2COO-), 90.34, 122.11, 122.94, 130.08, 133.51, 138.20, 151.57, 153.37 for aromatic carbons,
159.52 (-CH = N-), 171.89 (-COO-), Anal. calcd. for C25H32INO2: C, 59.41%, H, 6.38%, N, 2.77%; found: C, 59.55%, H, 6.40%, N, 2.57%.
[14] D. Demus, L. Richter, Textures of Liquid Crystals, Verlag Chemie, New York, 1978.
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