Synthesis and mesomorphic properties of Schiff base esters possessing terminal chloro substituent

Article abstract of DOI:10.1016/j.cclet.2010.01.023

A homologous series of Schiff base esters, 4-chlorobenzylidene-4′-n-alkanoyloxyanilines, containing even number of carbons at the end groups of the molecules (C_n-1H_2n-1COO-, n = 4, 6, 8, 10, 12, 14, 16) were synthesized. The mesomorp

Full text of DOI:10.1016/j.cclet.2010.01.023

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Chinese Chemical Letters 21 (2010) 637–640
www.elsevier.com/locate/cclet
Synthesis and mesomorphic properties of Schiff base esters
possessing terminal chloro substituent
b
c
d
Sie Tiong Ha a, , Lay Khoon Ong , Siew Ling Lee , Hong Cheu Lin ,
*
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, Perak, Malaysia
b Department of Science, Faculty of Engineering & Science, Universiti Tunku Abdul Rahman, Jln Genting Klang, Setapak,
53300 Kuala Lumpur, Malaysia
c Ibnu Sina Institute for Fundamental Science Studies, Universiti Teknologi Malaysia, 81310 UTM Skudai, Johor, Malaysia
d Department of Materials Science & Engineering, National Chiao Tung University, Hsinchu 300, Taiwan, China
Received 25 November 2009
Abstract
A homologous series of Schiff base esters, 4-chlorobenzylidene-40-n-alkanoyloxyanilines, containing even number of carbons at
the end groups of the molecules (Cnꢀ1H2nꢀ1COO–, n = 4, 6, 8, 10, 12, 14, 16) were synthesized. The mesomorphic properties were
investigated by differential scanning calorimetry (DSC) and polarizing optical microscopy (POM). It was found that the end groups
of the molecules had an effect on the mesomorphic properties. n-Butanoyloxy was found non-mesogenic, whilst n-hexanoyloxy
exhibited monotropic smectic phase. The higher members in this homologous series were enantiotropic smectogens.
# 2010 Sie Tiong Ha. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
Keywords: Schiff base esters; Smectic A; Smectic B; Liquid crystal
The discovery of thermotropic liquid crystal 4-methoxybenzylidene-40-butylaniline and the application of its room
temperature nematic phase in displays sparked a renewed interest liquid crystals and in research to establish structure–
liquid crystal property relationships [1,2]. The understanding of structure–property relationships is elemental to
selecting molecular modifications for the synthesis of new mesogens with desirable properties and future applications
[3].
In the previous studies, it was revealed that ester and Schiff base linking units are the useful structural components
for generating mesomorphism in two and three aromatic rings thermotropic liquid crystals [4–7]. Therefore, a
continuing effort has been carried out which focused on the synthesis and characterization of a new series of Schiff
base esters possessing terminal halogen substituent, 4-chlorobenzylidene-40-n-alkanoyloxyanilines. The synthesis of
the target molecules is shown in Scheme 1, where 4-chlorobenzaldehyde was coupled with 4-aminophenol by heating
under reflux for about 3 h in ethanol solution. Then, the Schiff base intermediate was subjected to esterification with
* 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.2010.01.023

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S.T. Ha et al. / Chinese Chemical Letters 21 (2010) 637–640
Scheme 1. Synthetic route of 4-chlorobenzylidene-40-alkanoyloxyanilines, nClBA. (i) C2H5OH and (ii) Cnꢀ1H2nꢀ1COOH, DCC, DMAP, DMF,
CH2Cl2.
Table 1
Phase transitions and transition enthalpy for nClBA upon heating and cooling cycle.
Compound
Transition temperatures, 8C (DH, kJ molꢀ1
)
Heating
Cooling
4ClBA
Cr 108.3 (30.1) I
I 83.5 (32.3) Cr
6ClBA
Cr 100.2 (33.9) I
I 88.7 (5.4) SmA 81.5 (3.3) SmB 66.0 (21.8) Cr
I 96.6 (6.7) SmA 83.4 (3.8) SmB 70.8 (32.2) Cr
I 99.8 (5.6) SmA 82.2 (2.8) SmB 55.5 (29.6) Cr
I 100.3 (6.5) SmA 79.9 (2.8) SmB 66.9 (36.7) Cr
I 98.7 (6.7) SmA 76.7 (2.8) SmB 70.3 (42.1) Cr
I 99.3 (9.0) SmA 82.0 (4.5) SmB 51.0 (44.1) Cr
8ClBA
Cr 93.0 (38.2) SmA 98.1 (6.4) I
Cr1 93.4 (33.5) SmA 101.5 (5.6) I
Cr 94.6 (39.8) SmA 101.3 (6.3) I
Cr 97.0 (43.5) SmA 100.1 (5.5) I
Cr 92.9 (50.6) SmA 101.0 (8.3) I
10ClBA
12ClBA
14ClBA
16ClBA
Cr, crystal; SmA, smectic A; SmB, smectic B; I, isotropic.
appropriate fatty acids in the presence of DCC and DMAP [8]. The final products were recrystallised with ethanol until
constant transition temperature was obtained. The structure of the products was elucidated using elemental analysis,
IR, NMR and EI-MS spectroscopic techniques [9].
Mesophase texture studies and phase transition temperatures were determined by means of polarizing microscope
equipped with hotstage and temperature regulator. Phase identification was made by comparing the observed textures
with those reported in the literature [10,11]. Transition temperatures and associated enthalpy changes were measured
using differential scanning calorimeter (DSC). The results obtained were listed in Table 1.
n-Butanoyloxy having the shortest alkyl chain length was found to be non-mesogenic. This is evident from the DSC
thermogram whereby an endotherm and an exotherm were observed during heating and cooling cycles, respectively.
However, three exotherms were observed for n-hexanoyloxy during cooling cycle and this suggested that the presence
of two mesophases in this compound. Higher members of the series, n-octanoyloxy to n-hexadecanoyloxy showed two
endotherms in the DSC thermograms (Fig. 1) which can be attributed to the isotropic liquid to mesophase and
mesophase-crystal transitions.
With aids of polarized light, n-butanoyloxy exhibited direct changing of crystal to isotropic upon heating, and vice
versa. n-Hexanoyloxy derivatives exhibited monotropic smectic phases. In the monotropic mesogens, the melting
Fig. 1. DSC thermograms of 16ClBA.

S.T. Ha et al. / Chinese Chemical Letters 21 (2010) 637–640
639
Fig. 2. Liquid crystal texture (100ꢁ) of 8ClBA upon cooling. Fan-conic shaped textures (a) which indicates smectic A phase and temporary
transition bars (b) when changing from smectic A to smectic B.
Fig. 3. Plot of transition temperatures versus number of carbon in alkanoyloxy chain of nClBA during heating cycle.
points were always equal to or higher than the clearing points, hence exhibiting supercooling properties [12].
Homologous members C8 to C16 are enantiotropic mesogens. During cooling cycle, these compounds exhibited fan-
shaped textures which can be assigned as smectic A phase. Further cooling, the fan-shaped textures changes to a series
of temporary dark-lines, running parallel to the layers, which identified as smectic B phase. Smectic B phase exhibited
metastable state below the melting point, indicated monotropic properties [13] (Fig. 2).
A plot of transition temperatures versus number of carbon in alkanoyloxy chain of nClBA during heating cycle is
shown in Fig. 3. Based on the plot, it can be deduced that the mesophase was greatly influenced by the length of terminal
chain. As increasing length of terminal chain, the homologous series had shown from non-mesogenic change to
monotropic and change to enantiotropic properties. For n-butanoyloxy, the core system is too rigid for it to form
mesophase [10]. The packing of molecules in hexanoyloxy tends to be less rigid, hence showing monotropic properties.
As from n-octanoyloxy to n-hexadecanoyloxy, the homologous members showed enantiotropic properties. This is
because the increase number of carbons enhances the flexibility of the chain and subsequently promotes enantiotropic
phase [10]. In addition, the melting points are gradually decreased from C4 to C16 due to the dilution of core system.
Acknowledgments
The author (S.T. Ha) would like to thank Universiti Tunku Abdul Rahman (UTAR) for the financial support through
UTAR Vote No. 6202/O05 and the Malaysia Toray Science Foundation (UTAR Vote No. 4359/000) for funding this
project. L.K. Ong would like to acknowledge UTAR for the award of the research and teaching assistantships.
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[M]+, 240 (100), IR (KBr, cm(1): 2953, 2918, 2850 (C–H aliphatic); 1752 (C O ester); 1608 (C N); 1590, 1498 (C C aromatic), 1H NMR
(300 MHz, CDCl3): d 0.90 (t, 3H, J = 6.5Hz, CH3), 1.28–1.42 {m, 20H, CH3(CH2)10-}, 1.77 (qt, 2H, J = 7.3Hz, –CH2CH2COO–), 2.58 (t, 2H,
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methylene carbons (CH3(CH2)10-), 25.41 (–CH2CH2COO–), 34.85 (–CH2COO–), 40.63 {N(CH3)2}, 111.92, 122.18, 122.48, 124.53, 130.91,
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