Synthesis and mesomorphic properties of 4-(4-bromopropyloxy)-4′-(4- alkyloxybenzylidene)anilines

Article abstract of DOI:10.1016/j.cclet.2012.05.008

This article describes the preparation and liquid crystalline properties of a new homologous series of 4-(4-bromopropyloxy)-4′-(4- alkyloxybenzylidene)anilines in which the phenyl ring is armed by a bromopropyloxy chain. The thermal behavior and mesomorph

Full text of DOI:10.1016/j.cclet.2012.05.008

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Chinese Chemical Letters 23 (2012) 769–772
www.elsevier.com/locate/cclet
Synthesis and mesomorphic properties of 4-(4-bromopropyloxy)-40-
(4-alkyloxybenzylidene)anilines
a
b
b
Guan-Yeow Yeap a, , Yew-Hong Ooi , Kenji Kubo , Masato M. Ito
*
a Liquid Crystal Research Laboratory, School of Chemical Sciences, Universiti Sains Malaysia, 11800 Minden, Penang, Malaysia
b Department of Environmental Engineering for Symbiosis, Faculty of Engineering, Soka University, Tokyo 192-8577, Japan
Received 27 February 2012
Available online 9 June 2012
Abstract
This article describes the preparation and liquid crystalline properties of a new homologous series of 4-(4-bromopropyloxy)-40-
(4-alkyloxybenzylidene)anilines in which the phenyl ring is armed by a bromopropyloxy chain. The thermal behavior and
mesomorphic properties of the synthesized compounds were studied with particular attention given to the correlation between their
phase transition temperatures and anisotropic change influenced by molecular structure. All the members displayed enantiotropic
smectic phase except for the homologue with the longest alkyloxy chain (R = C18H37), exhibiting only monotropic characteristic.
The presence of bromine atom from the propyloxy side chain is found to be capable of altering and influencing the mesomorphic
properties.
# 2012 Guan-Yeow Yeap. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
Keywords: 4-(4-Bromopropyloxy)-40-(4-alkyloxybenzylidene)anilines; Mesomorphic properties; Enantiotropic smectic; DSC
Apart from the utilization in display and imaging, liquid crystal also plays an important role in the other fields of
science. For instance, liquid crystal incorporating cellulose fragment have recently been used to study the air-separation
performance owing to its potential application in membrane separation [1,2]. In addition, liquid crystalline polymers can
also serve as thermotropic liquid crystals particularly in the preparation of high strength, good mechanical and heat-
resistance materials [3,4]. One of the essential parts in these liquid crystals is made up by Schiff base.
Schiff base is a very interesting organic precursor for material science particularly in the preparation of liquid
crystalline substance, thermochromic as well as photochromic material which may possess potential application in the
measurement of radiation intensity and in optical display system [5]. Thus, their properties can be varied by appropriate
molecular design. The aromatic imine compounds with rod-like structure arewell known for their ability to exhibit liquid
crystalline behaviours resulted from their high shape anisotropy and the presence of terminal halogen atom which help to
increase the molecular polarizability [6–15]. The synthesis of 4-methoxybenzylidene-40-butylaniline by Kelker and
Scheurle [16] yielded the first thermotropic liquid crystal which showed nematic phase at room temperature. In view of
the interesting mesomorphism of these imines, the exhaustic liquid crystalline series of 4-alkyloxybenzylidene-40-
alkyloxyanilinewith varying alkyl chain length have subsequently been studied [17–22]. Early findings revealed that this
type of compound was nematognic and also exhibited a rich smectic polymorphism such as SmA, SmB, SmC as well as
* Corresponding author.
E-mail address: gyyeap@usm.my (G.-Y. Yeap).
1001-8417/$ – see front matter # 2012 Guan-Yeow Yeap. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
http://dx.doi.org/10.1016/j.cclet.2012.05.008

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G.-Y. Yeap et al. / Chinese Chemical Letters 23 (2012) 769–772
Scheme 1. Synthetic pathway towards the title compounds. Reagent and reaction condition: (i) K2CO3 (3 equiv.), 1-bromoalkane (1.2 equiv.), heat
in DMF, 80 8C, 4 h; (ii) 4-aminophenol (1.0 equiv.), 3 drops glacial acetic acid, reflux in ethanol, 3 h; (iii) K2CO3 (3 equiv.), 1,3-dibromopropane
(4 equiv.), reflux in acetone, 8 h.
the rare SmI phases. Hence, in order to further investigate the factors which govern the thermal behavior of the aromatic
imines and their relationship with its molecular structures, we report herein the effect of terminal bromine atom from the
propyloxy side chain upon the mesomorphic properties of newly derived imines containing v-brominated alkyloxy side
chain, 4-(4-bromopropyloxy)-40-(4-alkyloxybenzylidene)-anilines 1a–1f.
Scheme 1 outlined the synthetic pathway towards the formation of all intermediates and title compounds. 4-
Alkyloxybenzaldehyde was prepared following the method described previously by Catanescu et al. [23] where 4-
hydroxybenzaldehyde was reacted through Williamson etherification with various bromoalkane, RBr (where
R = C8H17, C10H21, C12H25, C14H29, C16H33 and C18H37) of even parity. The intermediate underwent condensation
with 4-aminophenol under reflux for 3 h in ethanol with a catalytic amount of glacial acetic acid. Finally, the imine
thus obtained was reacted with an excess of 1,3-dibromopropane to yield compounds 1a–1f. Their molecular
structures were elucidated by elemental analysis, FT-IR and FT-NMR spectroscopic techniques [24]. All phase
transition temperatures and associated enthalpy changes of compounds 1a–1f were determined by DSC with the
heating and cooling rates of ꢀ2 8C. The results were summarized in Table 1. Polarizing optical microscope equipped
with a hot stage and temperature controller was used for the texture observation,.
Optical microscopy studies reveal that all the title compounds 1a–1f exhibit liquid crystalline properties. All of them
were enantiotropic except for compound 1f (R = C18H37). Upon heating, 1a–1d show three endothermic peaks. The first
peak at lower temperature correspondsto the Cr1-Cr2 transition. The second peak which occurs at higher temperature and
possesses small enthalpy change in comparison to the first peak can be ascribed to the transition from crystal to
mesophase. Upon heating, the focal conic fan-shaped texture characteristics of SmA phase can be observed prior to
isotropization. Compound 1e (R = C16H33) exhibits two endothermic peaks wherein the first and second peaks can be
ascribed to the phase transitions of Cr–SmA and SmA–I, respectively. As for compound 1f which possesses the longest
alkyl chain (R = C18H37), only one endotherm corresponding to direct melting of the crystal to isotropic transition was
Table 1
Phase transition temperatures (8C) and transition enthalpy changes for compounds 1a–1f upon heating and cooling.
Compound
Phase transition, 8C (enthalpy change, kJ molꢁ1
)
1a (R = C8H17
)
Heating Cr 69.9 (35.4) Cr2 98.3 (3.9) SmA 109.0 (7.2) I
Cooling I 105.3 (4.8) SmA 88.5 (5.2) SmC 47.8 (21.2) Cr
Heating Cr 78.8 (41.2) Cr2 99.4 (4) 99.4 (4.0) SmA 111.6 (8.6) I
Cooling I 107.3 (6.4) SmA 91.1 (3.0) SmC 63.6 (31.6) Cr
Heating Cr 82.1 (51.8) Cr2 98.5 (4.4) SmA 112.8 (9.4) I
Cooling I 109.0 (9.0) SmA 91.4 (3.5) SmC 73.6 (43.2) Cr
Heating Cr 88.9 (55.2) Cr2 96.2 (4.0) SmA 111.6 (10.0) I
Cooling I 108.5 (9.0) SmA 90.2 (3.5) SmC 78.5 (51.8) Cr
Heating Cr 93.3 (67.0) SmA 110.0 (11.6) I
1b (R = C10H21
)
1c (R = C12H25
)
1d (R = C14H29
)
1e (R = C16H33
)
Cooling I 107.2 (9.9) SmA 89.8 (3.4) SmC 83.8 (58.7) Cr
Heating Cr 95.5 (69.8) I
1f (R = C18H37
)
Cooling I 94.7 (4.9) SmA 87.5 (42.9) Cr
Note: Cr, crystal; SmA, smectic A; SmC, smectic C; I, isotropic.

G.-Y. Yeap et al. / Chinese Chemical Letters 23 (2012) 769–772
771
Fig. 1. The photomicrographs of compound 1c upon cooling. (a) The formation of batonets from isotropic. (b) The focal conic fan-shaped texture of
SmA phase. (c) The appearance of broken fan-shaped texture characteristics of the SmC phase.
observed as it has a large enthalpy value. Upon cooling from isotropic phase, all the compounds show SmA phase which
can be characterized by the formation of batonets that coalesce to form a focal conic fan-shaped texture as displayed in
Fig. 1a and b. On further cooling, the broken fan-shaped texture typical of a SmC phase can be observed (Fig. 1c). In
contrary to compound 1f, the SmC phase is absent on cooling from the SmA phase as it undergoes direct crystallization at
87.5 8C. Overall, these data indicate that compounds 1a–1d are enantiotropic smectogens while compound 1f is
monotropic smectogen of which the SmA phase is observed only in cooling cycle (Figs. 2 and 3).
A comparison between the present v-brominated alkoxy imines 1a–1f with the homologous series I [21] and II [19]
prepared by Godzwon et al. shows that the difference lies on the presence of brominated propyloxy at one of the
terminals lying along the long molecular axis. Inspection from the result shows that the present compounds display
SmA and SmC phases only while the earlier reported analogues exhibited a nematic phase as well as a rich smectic
mesomorphic behaviour such as SmA, SmB, SmC and SmI. Hence, it can be presumed that the introduction of
polarizable Br atom in the alkoxy chain for these aromatic imines compounds will suppress the formation of nematic
phase wherein the molecule will tend to self-assembly into smectic and tilted smectic arrangement. Thus, the presence
of polarizable Br atom from the alkoxy side chain at one end is found to be capable of altering the mesomorphic
properties as it increases the overall intermolecular attraction which can facilitate lamellar packing of molecules
leading to the formation of smectic A and tilted arrangement of smectic C [25,26]. In addition, the thermal stability of
the present series is quite similar to that in series I and II. The clearing temperature for compounds in both series is less
than 120 8C regardless of the presence of Br atom in the molecular structure.
In conclusion, all the title compounds exhibit liquid crystal properties. The all the members in this series are
enantiotropic while the higher member (R = C18H37) exhibits monotropic behaviour only. The tendency of forming
only smectic phase can be attributed to the presence of polarizable Br atom.
Fig. 2. DSC thermograms of compound 1c (R = C12H25).

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G.-Y. Yeap et al. / Chinese Chemical Letters 23 (2012) 769–772
Fig. 3. Comparison between the molecular structures of title compounds 1a–1f and their reported analogues (I and II).
Acknowledgments
The author (G.Y. Yeap) would like to thank Universiti Sains Malaysia for USM RU (No. 1001/PKIMIA/811159)
and the research facilities. Y.H. Ooi would like to acknowledge Ministry of Higher Education Malaysia (MOHE) for
scholarship of the research assistantship.
References
[1] M.R. Huang, et al. J. Appl. Polym. Sci. 54 (1994) 463.
[2] X.G. Li, et al. Sep. Sci. Technol. 29 (1994) 671.
[3] X.G. Li, M.R. Huang, J. Macromol. Sci. Pure Appl. Chem. A 36 (1999) 859.
[4] X.G. Li, Polym. Test. 19 (2000) 43.
[5] E. Hadjoudis, M. Vittorakis, I. Moustakali-Mavridis, Tetrahedron 43 (1987) 7.
[6] S.T. Ha, L.K. Ong, Y. Sivasothy, et al. World Appl. Sci. J. 8 (2010) 641.
[7] M.Y. Zheng, Z.W. An, Chin. J. Chem. 24 (2006) 1754.
[8] S.T. Ha, T.L. Lee, Y.F. Win, et al. World Acad. Sci. Eng. Technol. 81 (2011) 575.
[9] S.R. Keum, S.H. Lee, S.O. Kang, Liq. Cryst. 31 (2004) 1569.
[10] S.T. Ha, G.Y. Yeap, P.L. Boey, Aust. J. Basic Appl. Sci. 3 (2009) 3417.
[11] S.T. Ha, Y.M. Ng, R.T. Subramaniam, et al. Int. J. Phys. Sci. 5 (2010) 1256.
[12] G.Y. Yeap, S.T. Ha, P.L. Lim, et al. Mol. Cryst. Liq. Cryst. 33 (2006) 205.
[13] G.Y. Yeap, S.T. Ha, P.L. Lim, P.L. Boey, Mol. Cryst. Liq. Cryst. 452 (2006) 63.
[14] G.Y. Yeap, S.T. Ha, P.L. Boey, et al. Mol. Cryst. Liq. Cryst. 452 (2006) 73.
[15] G.Y. Yeap, S.T. Ha, P.L. Lim, P.L. Boey, W.A.K. Mahmood, Mol. Cryst. Liq. Cryst. 423 (2004) 73.
[16] H. Kelker, B. Scheurle, Angew. Chem. Int. Ed. 81 (1969) 903.
[17] N. Ajeetha, V.G.K.M. Pisipati, M.R.N. Rao, P.V.D. Prasad, Mol. Cryst. Liq. Cryst. 457 (2006) 3.
[18] N. Ajeetha, D.P. Ojha, Mol. Cryst. Liq. Cryst. 490 (2008) 16.
[19] J. Godzwon, M.J. Sienkowska, Z. Galewski, Phase Transit. 80 (2007) 217.
[20] J. Godzwon, M.J. Sienkowska, Z. Galewski, Liq. Cryst. 34 (2007) 911.
[21] J. Godzwon, M.J. Sienkowska, Z. Galewski, Thermochim. Acta 491 (2009) 71.
[22] J. Godzwon, M.J. Sienkowska, Z. Galewski, J. Mol. Struct. 844–845 (2007) 259.
[23] O. Catanescu, M. Grigoras, G. Colotin, et al. Eur. Polym. J. 37 (2001) 2213.
[24] Analytical and spectroscopic data for the representative compound 1e (R = C16H33): Yield 80%, Appearance white precipitate, IR vmax (KBr,
cm(1)): 2918, 2850 (C–H apliphatic), 1608–1622 (C N azomethine), 1251 (C–O ether), 1H NMR (300 MHz, CDCl3): d 0.90 (t, 3H,
J = 6.6 Hz, CH3), 1.28–1.51 (m, 26H, CH2), 1.78–1.87 (m, 2H, CH2), 2.30–2.39 (m, 2H, CH2), 3.64 (t, 2H, J = 6.5 Hz, BrCH2), 4.03 (t, 2H,
J = 6.6 Hz, OCH2), 4.14 (t, 2H, J = 5.7 Hz, OCH2), 6.94 (d, 2H, J = 9.0 Hz, Ar), 6.98 (d, 2H, J = 8.7 Hz, Ar), 7.21 (d, 2H, J = 8.4 Hz, Ar), 7.83
(d, 2H, J = 8.7 Hz, Ar), 8.41 (s, 1H, CH N), 13C NMR (75 MHz, CDCl3): d 158.10 (C N), 161.69, 157.02, 145.67, 130.25, 129.27, 122.08,
115.07, 114.70 (Caromatic), 68.22, 65.68 (OCH2), 32.47 (BrCH2), 22.69–31.92 (CH2), 14.10 (CH3). Anal. calcd. for C32H48NO2Br: C 68.80, H
8.66, N 2.51, found, C 68.94, H 8.70, N 2.55.
[25] G.Y. Yeap, T.N. Chan, W.S. Yam, D. Takeuchi, M.M. Ito, Chin. Chem. Lett. 22 (2011) 947.
[26] P.J. Collings, M. Hird, Introduction to Liquid Crystals: Chemistry and Physics, Taylor & Francis, Ltd., UK, 1998.