Substitution effects on the liquid crystalline properties of thermotropic liquid crystals containing schiff base chalcone linkages

Article abstract of DOI:10.1080/15421400903291533

Two homologous series of Schiff base Chalcones, Series-A: 1-{4-[(2,4-dihydroxy benzylidene) amino] phenyl}-3-(4-alkoxy phenyl) prop-2-en-1-one and series-B: 1-(4-{[1-(2,4-dihydroxyphenyl)ethylidene]amino} phenyl)-3-(4-methoxyphenyl)prop-2-en-1-one were sy

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Substitution Effects on the
Liquid Crystalline Properties
of Thermotropic Liquid
Crystals Containing Schiff Base
Chalcone Linkages
B. T. Thaker a , P. H. Patel a , A. D. Vansadiya a & J.
B. Kanojiya a
a Department of Chemistry, Veer Narmad South
Gujarat University, Surat, India
Version of record first published: 17 Dec 2009.
To cite this article: B. T. Thaker, P. H. Patel, A. D. Vansadiya & J. B. Kanojiya (2009):
Substitution Effects on the Liquid Crystalline Properties of Thermotropic Liquid
Crystals Containing Schiff Base Chalcone Linkages, Molecular Crystals and Liquid
Crystals, 515:1, 135-147
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Mol. Cryst. Liq. Cryst., Vol. 515, pp. 135–147, 2009
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421400903291533
Substitution Effects on the Liquid Crystalline
Properties of Thermotropic Liquid Crystals
Containing Schiff Base Chalcone Linkages
B. T. Thaker, P. H. Patel, A. D. Vansadiya, and
J. B. Kanojiya
Department of Chemistry, Veer Narmad South Gujarat University,
Surat, India
Two homologous series of Schiff base Chalcones, Series-A: 1-{4-[(2,4-dihydroxy
benzylidene) amino] phenyl}-3-(4-alkoxy phenyl) prop-2-en-1-one and series-B:
1-(4-{[1-(2,4-dihydroxyphenyl)ethylidene]amino}phenyl)-3-(4-methoxyphenyl)prop-
2-en-1-one were synthesized and the physical characterization was carried out
along with spectroscopic techniques (FT-IR, 1H NMR,13C NMR, and MS). The
thermotropic properties were investigated by optical polarized light microscopy
and differential scanning calorimetry and evaluated as a function of chain length
and linking group. In series-A compounds C3 to C8 exhibit the nematic mesophase
while compounds C10, C12, C14, and C16 exhibit a smectic phase as well as the
nematic mesophase. Compounds C1 and C2 do not exhibit any mesophase. In
series-B, all the compounds (C1 to C8, C10, C12, C14, and C16) exhibit only the
nematic mesophase.
Keywords: chalcone; mesomorphic; nematic mesophase; Schiff base; smectic meso-
phase
INTRODUCTION
Mesogens with a chalcone central linkage are relatively rare. It
=
has been observed that ꢀCOꢀCH CHꢀ linkage is less conducive to
=
mesomorphism compared to ꢀCH Nꢀ(azomethine), ꢀCOOꢀ (ester),
=
ꢀN Nꢀ(azo) linkages due to the non linearity and angle strain arising
=
from the keto group [1]. But when ꢀCOꢀCH CHꢀ linkage is present
This paper was presented at the 15th National conference on Liquid Crystals at
Indian Institute of Science (I.I.Sc.), Banglore, Dec. 13th–15th, 2008.
Address correspondence to B. T. Thaker, Department of Chemistry, Veer Narmad
South Gujarat University, Udhana-Magdalla Road, Surat, 395 007, India. Tel: þ91
261 2227141-46; Fax: þ91 261 2227312. E-mail: btthaker1@yahoo.co.in
135

136
B. T. Thaker et al.
with other central linkages it becomes condusive to mesomorphism. In
the literature there are several reports of mesogenic compounds
having chalcone linkage. Many years ago Vora et al. [2] reported a
homologous series of polymers containing a chalcone linkage. Soon
after words Chudghar et al. [3] reported a homologous series contain-
ing on ester-chalcone linkages. Recently Yeap et al. [4] have also
synthesized mesomorphic compounds containing on ester-chalcone
linkage. In our previous work we have reported the homologous series
containing a Schiff base-chalcone linkage [5].
Chalcone is an important class of chemical compound and is being
studied extensively because of its significant use or application in
various sectors. In the fields of biology and biochemistry, chalcone
has been claimed to be one of the compounds that plays a vital role in
anti-tumor [6,7], anti-inflammatory [8,9], and anti-malaria [10] activ-
ities. It has also been documented that the chalcone possesses a
remarkable nonlinear optical (NLO) property, which is an essential ele-
ment for optical communications devices. The other importance of this
compound is its high photosensitivity and thermal stability, which are
used in developing various crystalline electro-optical devices [11–13].
In view of the outstanding behavior of chalcone compounds, a
systematic study focusing on the synthesis and characterization of this
class of compound has been carried out in our present laboratory over
past few years [5,14]. In addition, terminal and lateral subtituents
also play a vital role in imparting liquid crystallinity to potentially
mesogenic compounds [15–19]. The correlation between chemical
structure and mesomorphic properties is one of the most important
problems in liquid crystals science [20]. An understanding of the
influence of different structural elements of the molecules on the
physico-chemical characteristics of mesomorphic organic compounds
allows chemists to synthesize liquid crystals with required properties.
Laterally substituted mesogens are of considerable interest because
these compounds deviate from the classical rod-like shape. However
lateral hydroxyl groups are little exploited as it was reported that
the phenolic ‘hydroxy’ group may destroy mesomorphism. Gray [21]
has proposed that this rarity is associated with intermolecular hydro-
gen bonding raising the melting point above the mesophase-isotropic
liquid transition temperature, and perhaps also encouraging a non-
linear molecular arrangement that is incompatible with mesophase
formation. After an initial investigation by Weissflog and Demus
[22], a number of homologues series with a trisubstituted benzene
nucleus have been reported [23–30]. Schroeder and Schroeder [31]
reported a few terminally hydroxy substituted mesogens. Vora and
Gupta [32] for the first time reported homologous series with terminal

Properties of Thermotropic Liquid Crystals
137
and lateral hydroxy groups. Subsequently a few more mesogenic series
with a lateral hydroxy group have been reported [33–35]. In general, a
rigid lateral substituent disrupts the ordering of the liquid crystalline
phase [36–38], causing a significant depression in the clearing point
and liquid crystal phase transition.
In our previous work [5] we have synthesized two homologous series
containing Schiff base-chalcone linkages having substituted pyrazo-
lone ring as a terminal group. An attempt has been made to synthesize
two new homologous series containing the same central linkages, with
three aromatic rings in the main core and substituted by a lateral
aromatic branch on the terminal ring to study the influence of the
terminal and lateral hydroxy group on mesomorphic and thermal
stability of these compounds.
EXPERIMENTAL
Reagents and Techniques
For the synthesis of compounds of the homologous series, following
materials were used. 4-Hydroxy benzaldehyde, alkyl bromides
(Lancaster, England), 2-4 dihydroxy acetophenone, p-amino acetophe-
none (Lancaster, England) were used without further purification. All
the solvents were used after purification using the standard methods
described in literature [39].
Elemental analyses (C, H, N) were performed at CDRI, Lucknow.
Infrared spectra were recorded with a Perkin-Elmer 2000 FT-IR spectro-
photometer in the frequency range 4000–400cmꢀ1 with samples
embedded in KBr discs. 1H-NMR spectra of the compound were recorded
with JEOL-GSX-400 using CDCl3 as a solvent and TMS as an internal
reference at SAIF, IIT Madras, Chennai. 13C NMR spectra of the comp-
ound were recorded with BRUKER AVANCE II 400 NMR Spectrometer,
SAIF, Chandigarh. Mass spectra (EI) of the compounds were recorded at
SAIF, IIT Madras, Chennai. Thin-layer chromatography analyses were
performed by using aluminium-backed silica-gel plates (Merck 60 F524)
and examined under short-wave UV light.
The phase-transition temperatures were measured using
a
Shimadzu DSC-50 at heating and cooling rates of 5ꢁC minꢀ1. The
DSC data are shown in Table-III. The optical microscopy studies
were carried out with a ‘‘Leitz Loborlux 12’’ POL (Wetzler, Germany)
polarizing microscope equipped with a Mettler FP52 hot stage and
temperature controller. The textures of the compounds were observed
using polarized light with crossed polarizers with sample in a thin film
between a glass slide and a cover slip.

138
B. T. Thaker et al.
SYNTHESIS
Synthesis of n-Alkoxy Benzaldehydes
n-Alkoxy benzaldehydes were prepared by a reported method [40–43].
Synthesis of 1-{4-[(20,40-Dihydroxybenzylidene) Amino]
Phenyl} Ethanone
A mixture of 2,4-dihydroxy benzaldehyde (1 mmol) and 4-aminoaceto-
phenone(1 mmol) and three drops of acetic acid in absolute ethanol
(10 ml) was heated at reflux for 4 hr. The reaction mixture was allowed
to cool and stirred at room temperature overnight. The solid was
collected and recrystallized from acetone.
Synthesis of 1-(4-{[(1-(2,4-Dihydroxyphenyl)ethylidene]amino}
phenyl)ethanone
A mixture of 2,4-dihydroxy acetophenone (1 mmol) and 4-aminoaceto-
phenone(1 mmol) and three drops of acetic acid in absolute ethanol
(10 ml) was heated at reflux for 4 hr. The reaction mixture was allowed
to cool and stirred at room temperature overnight. The solid was
collected and recrystallized from acetone.
Synthesis of 1-{4-[(2,4-Dihydroxybenzylidene) Amino] Phenyl}-
3-(4-alkoxyphenyl) Prop-2-en-1-One (Series-A)
A mixture of 1-{4-[(20,40-dihydroxybenzylidene) amino] phenyl} etha-
none (1 mmol) and n-alkoxy benzaldehyde (1 mmol) was dissolved in
the alcoholic sodium hydroxide solution (80 ml ethanol and 10% NaOH
solution). The reaction mixture was heated at 80ꢁC for 7 hr. The mix-
ture was left at room temperature overnight. An HCl aqueous solution
was added to the mixture, and then yellow precipitate was obtained.
The precipitate was washed with water until a neutral aqueous solu-
tion was obtained. Then, the solid was washed with methanol and dried
under vacuum at 60ꢁC. The obtained yellow solid was purified by
recrystallization from acetone. The product was obtained in 45% yield.
The physical data of the series A compounds are given in Table 1.
Data
A12, Molecular Formula: C34H41O4N
Elemental analysis, calculated for C 71.70%; H 7.20% and N 9.84%,
found: C 71.78%; H 7.12% and N 9.93%, FT-IR (in KBr), 3432 cmꢀ1

Properties of Thermotropic Liquid Crystals
139
TABLE 1 Transition Temperature Data of the Series (Series-A)
Transition temperature in ꢁC
Code No.
R ¼ n-alkyl
S
N
I
A2
A3
A4
A5
A6
A7
A8
A10
A12
A14
A16
A18
Ethyl
Propyl
Butyl
Pentyl
Hexyl
Heptyl
Octyl
Decyl
Dodecyl
Tetradecyl
Hexadecyl
Octadecyl
–
–
–
–
–
–
–
–
81
85
66
69
–
–
183
167
159
154
146
141
135
121
123
121
109
98
146
131
129
112
106
103
99
94
79
82
(OH-phenolic); 2879, 2968 cmꢀ1 (C-H aliophatic) 1ꢀ5173 cmꢀ1 (C-H
phenyl ring); 1681 cmꢀ1 (C O, chalcone group); 1641 cm (-C N of azo-
=
=
methine); 1597 cmꢀ1 (-C C-, vinyl group of chalcone); 1120–1029 cm
ꢀ1
=
(C-O-C); 1H-N MR (CDCl3, d, ppm): 0.96–1.33 (3 H, t, CH3, alkyl chain),
1.29–1.71 (m, -(CH2)n, alkyl chain), 3.94–4.03 (2 H, t, -CH2O, alkoxy
chain), 7.56 & 7.90 (2 H, d, olefinic H), 6.32–7.80 (m, phenyl protons),
11.09 (s, phenolic OH), Mass (FAB): Molecular ion peak: A12 {m=z
ꢀ528 (M þ 1)þ}.
1-(4-{[1-(2,4-Dihydroxyphenyl)ethylidene]amino}phenyl)
-3-(4- methoxyphenyl) Prop-2-en-1-one (Series-B)
A mixture of 1-(4-{[1-(2,4-dihydroxyphenyl)ethylidene]amino}phenyl)
ethanone (1 mmol) and n-alkoxy benzaldehyde (1 mmol) was dissolved
in the alcoholic sodium hydroxide solution (80 ml ethanol and 10%
NaOH solution). The reaction mixture was heated at 80ꢁC for 7 hr.
The mixture was left at room temperature overnight. An HCl aqueous
solution was added to the mixture, and then yellow precipitate was
obtained. The precipitate was washed with swater until a neutral
aqueous solution was obtained. Then, the solid was washed with
methanol and dried under vacuum at 60ꢁC. The obtained yellow solid
was purified by recrystallization from acetone. The product was
obtained in 45% yield. The physical data of the series-B compounds
are given in Table 2.

140
B. T. Thaker et al.
TABLE 2 Transition Temperature Data of the Series (Series-B)
Transition temperature in ꢁC
Code No.
R ¼ n-alkyl
S
N
I
B2
B3
B4
B5
B6
B7
B8
B10
B12
B14
Bl6
Bl8
Ethyl
Propyl
Butyl
Pentyl
Hexyl
Heptyl
Octyl
Decyl
Dodecyl
Tetradecyl
Hexadecyl
Octadecyl
–
–
–
–
–
–
–
–
–
–
–
–
124
111
116
110
113
97
102
95
98
132
120
126
118
124
115
121
108
112
110
120
119
99
112
78
Data
B12, Molecular Formula: C35H43O4N
Elemental analysis, calculated for C 77.63%; H 7.94% and
N
2.58%, found: C 77.58%; H 7.87% and N 2.54%, FT-IR
(in KBr), 3439 cmꢀ1 (OH-phenolic); 2000–1667 cmꢀ1 (C-H phenyl
ring, out of plane bending); 1684 cmꢀ1 (C O, chalcone group);
=
1632 cmꢀ1 (-C N); 1606 cm
ꢀ1
=
=
(-C C-, vinyl group of chalcone);
1120–1030 cmꢀ1 (C-O-C); 1H-NMR (CDCl3, d, ppm): 0.89–0.93
(3H, t, CH3, alkyl chain), 0.94 (3H, s, CH3 of acetophenone)
1.44–1.86 (m, -(CH2)n, alkyl chain), 4.03–4.06 (2H, t, -CH2O, alkoxy
chain), 6.68 and 7.73 (2 H, d, olefinic H), 7.0–7.97 (m, phenyl pro-
tons), 9.90 (s, phenolic OH), Mass (FAB): Molecular ion peak: B12
{m=z ꢀ 540 (Mꢀ1)þ}.
Scheme: Synthetic Route for Series-A & Series-B
Synthesis of -n-alkoxy benzaldehydes

Properties of Thermotropic Liquid Crystals
141
Synthesis of 1-{4-[(20,40-dihydroxybenzylidene) amino] phenyl}
ethanone
Synthesis of 1-(4-{[(1-(2,4-dihydroxyphenyl)ethylidene]amino}phenyl)
ethanone
Synthesis of 1-{4-[(2,4-dihydroxybenzylidene) amino] phenyl}-3-
(4-alkoxyphenyl) prop-2-en-1- one (Series-A)

142
B. T. Thaker et al.
Synthesis
of
1-(4-{[1-(2,4-dihydroxyphenyl)ethylidene]amino}-
phenyl)-3-(4-methoxyphenyl)prop-2-en-1-one (Series-B)
RESULT AND DISCUSSION
In the present study, 12 homologues from each of the two series,
1-{4-[(2,4-dihydroxybenzylidene) amino] phenyl}-3-(4-alkoxyphenyl)
prop-2-en-1-one (Series-A) and 1-(4-{[1-(2,4-dihydroxyphenyl)ethylidene]
amino}phenyl)-3-(4-methoxyphenyl)prop-2-en-1-one (Series-B) were
synthesized and their mesomorphic properties studied. The meso-
morphic properties of all the synthesized compounds have been charac-
terized by differential scanning calorimetry (DSC) and polarizing
optical microscope (POM) attached with Mettler hot stage. Phase
identification was based on the optical textures, and the magnitude of
the isotropization on enthalpies is consistent with the assignment of
each mesophase type, using the classification systems reported by Sack
Mann and Demus [44], and Gray and Goodby [45].
In series-A, out of twelve compounds, only ten compounds exhibit
the enantiotropic nematic mesophase along with the Smectic-C phase.
In series-A, the first two compounds C1 & C2 do not exhibiting
mesomorphism. The terminal benzene ring is disubstituted by –OH
group. One –OH group is ortho to the imine linkage and meta to
another –OH group which does not show linearity in the molecule.
The two ends of the molecule possess –OH and either –OCH3 or
–OC2H5 groups which do not increase polarity and polarizability of
the molecule, and hence the first two compounds do not show meso-
morphism. As the alkoxy chain length increases, the linearity and
polarizability of the molecules increase which increases the cohesive
forces, resulting in showing nematic mesomorphism.

Properties of Thermotropic Liquid Crystals
143
FIGURE 1 Mesomorphic behavior as a function of the number of carbon
atoms (n) in the terminal alkoxy chain for series-A.
The transition temperature ranges Dt (S ꢀ N) and Dt (Nꢀ I) are
between 9–18ꢁC and 13–30ꢁC respectively. No odd-even effect is observed
for N-I transition temperature in series-A. The smectic mesophase
appears for the n-decyloxy to n-hexadecyloxy derivatives. The transition
temperature are recorded in Table 1. The plot of transition temperature
against the number of carbon atoms in the alkoxy chain (Figure 1)
shows a smooth falling tendency for the nematic-isotropic transition
temperature throughout the series. Series-A also exhibits a falling
tendency of the SC-N transition temperature for higher homologous.
By incorporating on electron donating group (–CH3) on the imine
linkage in series-B, leading to a disruption of SC phase and it’s only
appearance of the nematic phase. The transition temperature are
recorded in Table 2. All phase transition temperatures for the methyl
substituted compounds are significantly lower compared with the
non-substituted series-A. The plot of transition temperature against
the no. of carbon atoms in the alkoxy chain shown in Figure 2. This
effect is well known and can be explained in terms of steric influence
of the methyl group on molecular packing [46].
Comparison of series-A and B indicates that the smectic phase was
disappeared in series-B, which was observed in series-A. The methyl
lateral group on the imine linkage decreases the SC phase stability
and this also affects the stability of nematic phase, which results in
a reduced N–I range compared to series-B [47].
The increase in the SC-N transition temperature with increasing
the alkoxy chain length in compounds of series-A (n ¼ 10, 12, . . . .
etc.), can be explained by the increasing polarisability of the molecule.

144
B. T. Thaker et al.
FIGURE 2 Mesomorphic behavior as a function of the number of carbon
atoms (n) in the terminal alkoxy chain for series-B.
This will increase the cohesive forces, acting between the sides and
plane of the molecules, increasing the tendency for forming the smec-
tic layers. An increase in the nematic and smectic mesophase range is
observed with increasing number of methylene units in the terminal
group [36]. The enthalpies of the n-tetradecyloxy derivatives of each
of the series were measured by DSC. The data is recorded in Table 3.
Both the series are structurally similar, consisting of the three
aromatic cores, imine and chalcone linkages, and n-alkoxy as one of
the terminal group. Molecules of series-A & B differ only in the lateral
group on imine linkage. Series-A has a simple imine linkage and
series-B has a lateral methyl group on imine central linkage. It should
be remembered that the one central group in both the series is chalconyl
group and its effect has been taken to be similar for both series [48,49].
TABLE 3 Transition Temperature and DSC Data of the Series-A & B
Compound Transition Peak Temp. (Microscopic temp.) ꢁC DH (Jgꢀ1
)
DS (jg-lkꢀ1
)
A12
Cr–S
S–N
N–I
Cr–S
S–N
N–I
Cr–N
N–I
Cr–N
N–I
81 (81)
99 (99)
123 (124)
85 (86)
16.07
18.01
20.67
11.09
6.63
13.06
7.90
10.78
9.89
0.0453
0.0484
0.0520
0.0308
0.0180
0.0331
0.0213
0.0280
0.0265
0.1324
A14
94 (95)
121 (121)
97.89 (98)
111.34 (112)
98.79 (99)
109.69 (110)
B12
B14
14.56

Properties of Thermotropic Liquid Crystals
145
There is a decrease in thermal stabilities of the liquid crystal phases
in the series-B compounds, this may be attributed to the broadening
effect caused by the lateral methyl group in an otherwise relatively
linear molecule [50]. An increase in the molecular breadth forces the
long axes of the molecules apart, as a result of which the interactions
are decreased and consequently the liquid crystal temperatures are
lowered. A change in the degree of conjugation between the alkene
and the carbonyl group in the chalcone linkage will alter the polarisa-
bility. This effect of the change in resultant moments is subtle; how-
ever, consequentially, the decrease in the polarisability will cause a
decrease in the thermal stability of liquid crystal phase in series-B.
Variation of –CH3 group in series-B made it possible to observe the
effects of structural changes on mesomorphic behavior in a system
which had not been studied previously.
CONCLUSION
Systematic studies on a homologous series of compounds have allowed
for better understanding of the relationship between the molecular
structures and mesomorphic properties. Mesomorphic data obtained
in this work suggests that the smectic mesophases might be thermody-
namically disrupted due to the presence of a lateral methyl group on
imine central linkage. Strong molecular interactions in the mesophase
could be partially overcome with surrounding by a more or a less
polarisable terminal group and mesophase stabilities of chalcone link-
age is less compare to ester linkage liquid crystals. The present study
completes our objective of analyzing and establishing the effect of
different structural modifications on mesomorphism.
ACKNOWLEDGMENT
We are thankful to I.I.T. Bombay, CDRI Lucknow and Garda chemicals
Ltd., Ankleshwar (Gujarat) to providing an elemental analysis, FT-IR,
1H-NMR, 13C NMR Mass, and Thermal studies and Department of
Applied Chemistry, Faculty of Technology and Engineering, M. S.
University of Baroda, Vadodara for providing us Optical polarizing
microscope for mesophase study.
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