Mesomorphic behaviour of new azomethine liquid crystals having terminal bromo substituent

Article abstract of DOI:10.1016/j.cclet.2011.10.022

A homologous series of azomethine esters, 4-n-alkanoyloxybenzylidene- 4′-bromoanilines possessing even number of carbon atoms at the terminal alkanoyloxy chain (C_n-1H_2n-1COO-, n = 8, 10, 12, 14, 16, 18) was synthesized and characteri

Full text of DOI:10.1016/j.cclet.2011.10.022

Available online at www.sciencedirect.com
Chinese Chemical Letters 23 (2012) 177–180
www.elsevier.com/locate/cclet
Mesomorphic behaviour of new azomethine liquid crystals having
terminal bromo substituent
a
b
c
Sie Tiong Ha a, , Teck Leong Lee , Siew Ling Lee , Guan Yeow Yeap ,
*
Yip Foo Win a, Siew Teng Ong a
a Department of Chemical Science, Faculty of Science, Universiti Tunku Abdul Rahman, Jln Universiti, Bandar Barat, 31900 Kampar, Malaysia
b Ibnu Sina Institute for Fundamental Science Studies, Universiti Teknologi Malaysia, 81310 UTM Skudai, Malaysia
c Liquid Crystal Research Laboratory, School of Chemical Sciences, Universiti Sains Malaysia, 11800 Minden, Malaysia
Received 25 August 2011
Available online 21 December 2011
Abstract
A homologous series of azomethine esters, 4-n-alkanoyloxybenzylidene-40-bromoanilines possessing even number of carbon
atoms at the terminal alkanoyloxy chain (Cnꢀ1H2nꢀ1COO–, n = 8, 10, 12, 14, 16, 18) was synthesized and characterized. Whilst n-
octanoyloxy to n-dodecanoyloxy derivatives exhibited enantiotropic smectic A and smectic B phases, n-tetradecanoyloxy to n-
octadecanoyloxy derivatives possessed enantiotropic smectic A and monotropic smectic B properties. n-Decanoyloxy derivatives
demonstrated the optimum exhibition for both smectic A and smectic B phases. It was found that the length of terminal alkanoyloxy
chain has an influence on mesomorphic properties.
# 2011 Sie Tiong Ha. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
Keywords: Liquid crystals; Schiff base ester; Enantiotropic; Smectic A; Smectic B
Mesomorphic materials, either low molar mass or polymeric in nature, having an azo group in the mesogenic core,
are often studied owing to their interesting oxygen-enriching ability, enhanced air-separation performance and optical
properties, which enable applications in, for example, optical switching, holography and optical storage devices [1].
Dyes containing azo group are also being used in liquid crystal display (LCD) devices for the guest–host interaction
[2].
Low-mass molecules compounds containing as simple as two unsaturated (aromatic) rings with one or multiple
terminal substituents are capable of exhibiting mesomorphic properties [3]. Some studies showed that the influence of
a terminal alkyl chain upon the liquid crystalline properties [4] and the possibility of enhancing the rigidity of core
system of azo compounds through metal complexes formation [5,6] have been claimed as two of the favourable
pathways to improve mesogenic properties.
In our previous studies, the results revealed that azomethine and ester are useful linking units for generating
mesomorphism in two and three aromatic rings compounds. The presence of different polarity of terminal substituents
has been well reported that will either promote or suppress the mesomorphic properties [7–11].
* 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.2011.10.022

178
S.T. Ha et al. / Chinese Chemical Letters 23 (2012) 177–180
O
N
Br
(i)
HO
H2N
Br
HO
+
N
Br
(ii)
C
n-1H2n-1COO
nABBA
where n= 8, 10, 12, 14, 16, 18
Scheme 1. Synthetic route of nABBA. (i) CH3OH, reflux, 3 h (ii) Cnꢀ1H2nꢀ1COOH, DCC, DMAP, THF, r.t., 6 h.
In order to accomplish the research on mesomorphic properties of two aromatic rings Schiff base ester, we reported
another homologous series of liquid crystal, 4-n-alkanoyloxybenzylidene-40-bromoanilines (nABBA) and its
synthetic route is shown in Scheme 1. 4-Bromoaniline was condensed with 4-hydroxybenzaldehyde upon refluxing in
methanol for an hour [12]. Then, the Schiff base intermediate was subjected to Steglich esterification with fatty acids
in the presence of DCC and DMAP [13]. All the crude products were purified by repeated recrystallization using
ethanol and hexane until constant melting points were obtained. Molecular structure of nABBA was confirmed by
1
elemental analysis and spectroscopic techniques including FT-IR, H and 13C NMR and EI-MS [14].
The mesophase textures of the products were examined 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 [15,16]. Transition temperatures and enthalpy changes were determined using a
differential scanning calorimeter. The results are summarized in Table 1.
All synthesized compounds showed liquid crystal characteristics based on the endotherms appeared in the DSC
thermograms (Fig. 1) which can be attributed to the crystal-mesophase and mesophase-isotropic transitions.
Based on DSC data, n-octanoyloxy to n-dodecanoyloxy derivatives exhibited enantiotropic smectic A and smectic
B phases whilst n-tetradecanoyloxy to n-octadecanoyloxy derivatives displayed enantiotropic smectic A and
monotropic B phases. For enantiotropic compounds, similar mesophases were observed during heating and cooling
cycles. Upon cooling of 8ABBA from isotropic liquid, homogenous focal-conic texture of smectic A phase (Fig. 2a)
was first observed followed by temporary transition bars which indicate transition phase from smectic A to smectic B
phase (Fig. 2b).
A plot of phase transition temperatures against number of carbon atoms (n) in alkanoyloxy chain during cooling
scan is shown in Fig. 2c. Based on the plot, it can be deduced that length of terminal alkanoyloxy chain influenced the
Table 1
Phase transition temperatures and enthalpy changes of nABBA upon heating and cooling.
Compound
8ABBA
Phase transition temperature, 8C (enthalpy change, kJ molꢀ1
)
Cr 89.62 (25.09) SmB 94.40 (2.27) SmA 110.55 (6.39) I (heating)
I 106.78 (6.53) SmA 91.03 (3.02) SmB 46.16 (10.64) Cr (cooling)
10ABBA
Cr 86.70 (26.28) SmB 96.17 (2.78) SmA 113.55 (8.16) I (heating)
I 108.44 (8.65) SmA 92.33 (3.27) SmB 40.48 (28.73) Cr (cooling)
12ABBA
Cr 90.95 (31.54) SmB 96.50 (3.09) SmA 115.08 (8.16) I (heating)
I 108.82 (7.46) SmA 90.83 (3.20) SmB 53.47 (31.75) Cr (cooling)
14ABBA
Cr 91.26 (41.08) SmA 110.55 (8.08) I (heating)
I 105.72 (8.00) SmA 84.83 (3.25) SmB 54.15 (36.98) Cr (cooling)
16ABBA
Cr 94.35 (53.70) SmA 107.13 (9.27) I (heating)
I 102.16 (10.45) SmA 84.13 (3.79) SmB 70.74 (50.79) Cr (cooling)
18ABBA
Cr 97.11 (73.88) SmA 105.50 (11.44) I (heating)
I 102.39 (11.93) SmA 83.92 (4.52) SmB 74.26 (69.16) Cr (cooling)
Note: Cr = crystal; SmB = smectic B; SmA = smectic A; I = isotropic.

S.T. Ha et al. / Chinese Chemical Letters 23 (2012) 177–180
179
Fig. 1. DSC thermogram of 12ABBA.
mesomorphic properties. With the increasing length of terminal chain, the phase changed from enantiotropic to
monotropic smectic B phase. From the graph, it is apparent that mesophase range increased from n-octanoyloxy to n-
decanoyloxy members. This is because the increase of length of terminal alkanoyloxy chain led to the enhancement of
the smectic properties. However, the mesophase range of n-dodecanoyloxy till n-octadecanoyloxy decreased due to
the dilution of mesogenic core [17,18]. Amongst all, n-decanoyloxy derivatives demonstrated the optimum exhibition
(largest mesophase range) for both smectic A and smectic B phase.
Fig. 2. Liquid crystals textures of 8ABBA upon cooling. Homogenous focal-conic textures of smectic A phase (a) and temporary transition bars (b)
when changing from smectic A to smectic B phase. (c) Plot of phase transition temperatures against number of carbon atoms (n) in alkanoyloxy chain
during cooling scan.

180
S.T. Ha et al. / Chinese Chemical Letters 23 (2012) 177–180
Acknowledgments
The author (S.T. Ha) would like to thank Universiti Tunku Abdul Rahman and Malaysia Ministry of Higher
Education for the financial supports via LRGS and research facilities.
References
[1] (a) C. Ruslim, K. Ichimura, J. Mater. Chem. 9 (1999) 673;
(b) J. Belmar, M. Parra, C. Zuniga, et al. Liq. Cryst. 26 (1999) 389;
(c) L.L. Lai, E. Wang, Y.H. Liu, Y. Wang, Liq. Cryst. 29 (2002) 871;
(d) A.K. Prajapati, Mol. Cryst. Liq. Cryst. 364 (2001) 769;
(e) L. Komitov, K. Ichimura, A. Strigazzi, Liq. Cryst. 27 (2000) 51;
(f) L. Komitov, C. Ruslim, Y. Matsuzawa, K. Ichimura, Liq. Cryst. 27 (2000) 1011;
(g) X.G. Li, M.R. Huang, G. Lin, J. Membr. Sci. 116 (1996) 143;
(h) M.R. Huang, X.G. Li, G. Lin, Sep. Sci. Technol. 30 (1995) 449;
(i) X.G. Li, M.R. Huang, G. Lin, P.C. Yang, Colloid Polym. Sci. 273 (1995) 772;
(j) X.G. Li, M.R. Huang, Angew. Macromol. Chem. 220 (1994) 151.
[2] M. Matsui, Y.H. Nakagaway, B. Joglekary, et al. Liq. Cryst. 21 (1996) 669.
[3] A. Jacobi, W. Weissflog, Liq. Cryst. 22 (1997) 107.
[4] (a) J.S. Dave, G. Kurian, Mol. Cryst. Liq. Cryst. 42 (1977) 175;
(b) Z.Y. Cheng, B.Y. Ren, S.Y. He, et al. Chin. Chem. Lett. 22 (2011) 1375;
(c) X.D. Li, M.S. Zhan, K. Wang, Chin. Chem. Lett. 22 (2011) 1111.
[5] N. Hoshino, K. Takashi, T. Sekuchi, et al. Inorg. Chem. 37 (1998) 882.
[6] Y. Tian, F. Su, P. Xing, et al. Liq. Cryst. 20 (1996) 139.
[7] G.Y. Yeap, S.T. Ha, P.L. Lim, et al. Mol. Cryst. Liq. Cryst. 423 (2004) 73.
[8] G.Y. Yeap, S.T. Ha, P.L. Lim, et al. Mol. Cryst. Liq. Cryst. 452 (2006) 63.
[9] G.Y. Yeap, S.T. Ha, P.L. Boey, et al. Mol. Cryst. Liq. Cryst. 452 (2006) 73.
[10] G.Y. Yeap, S.T. Ha, P.L. Lim, et al. Liq. Cryst. 33 (2006) 205.
[11] S.T. Ha, T.L. Lee, G.Y. Yeap, et al. Chin. Chem. Lett. 21 (2010) 637.
[12] A.A. Jarrahpour, M. Zarei, Molbank (2004) M352.
[13] S.T. Ha, L.K. Ong, Y.F. Win, et al. Molbank 3 (2008) M582.
[14] Analytical and spectroscopic data for the representative compound 18ABBA: Yield 60.86%, EI-MS m/z (rel. int. %): 541.3 (5.34) [M+], 275.0
(100.00), IR nmax (KBr, cmꢀ1): 2951, 2919, 2850 (C–H aliphatic); 1749 (C O ester); 1619 (C N); 1211, 1100 (C–O ester), 1H NMR
(300 MHz, CDCl3): d 0.899 (t, 3H, J = 6.6 Hz, CH3–), 1.279 (m, 28H, CH3–(CH2)14–), 1.785 (qt, 2H, J = 7.5 Hz, –CH2–CH2–COO–), 2.597 (t,
2H, J = 7.5 Hz, –CH2–COO–), 7.098 (d, 2H, J = 8.7 Hz, Ar–H), 7.223 (d, J = 8.4 Hz, 2H, Ar–H), 7.522 (d, J = 8.7 Hz, 2H, Ar–H), 7.930 (d,
J = 8.4 Hz, 2H, Ar–H), 8.423 (s, 1H, –CH N–), 13C NMR (75 MHz, CDCl3): d 14.091 (CH3–), 22.675 (CH3CH2–), 24.881 (CH3CH2CH2–),
29.096, 29.238, 29.346, 29.443, 29.584, 29.648, 29.685 for methylene carbons (CH3CH2CH2–(CH2)14–), 31.917 (–CH2CH2COO–), 34.429 (–
CH2COO–), 119.369, 122.098, 122.559, 130.048, 132.203, 133.529, 150.880, 153.363 for aromatic carbons, 159.445 (–CH N–), 171.874 (–
COO–), anal. calcd. for C31H44BrNO2: C, 68.62%, H, 8.17%, N, 2.58%; found: C, 68.93%, H, 8.41%, N, 2.48%.
[15] D. Demus, L. Richter, Textures of Liquid Crystals, Verlag Chemie, New York, 1978.
[16] I. Dierking, Textures of Liquid Crystals, Wiley-VCH, Weinheim, 2003.
[17] G.W. Gray, Molecular Structure and Properties of Liquid Crystals, Academic Press, London, 1962.
[18] P. Berdague, J.P. Bayle, M.S. Ho, B.M. Fung, Liq. Cryst. 14 (1993) 667.