Synthesis of liquid crystals with lateral substituents in central and terminal benzene cores and their mesomorphic characterization

Article abstract of DOI:10.1080/15421406.2013.768497

A new homologous series consisting of 12 homologues central azo, and ester linkage with chloro lateral group at central and methyl group at the terminal benzene ring is synthesized and their mesomorphic properties are studied. All the homologues are mesogenic in nature. All the compounds are exhibiting enantiotropic nematic phase; no homologues shows smectic phase even in monotropic form. To evaluate the effect of lateral chloro group at central benzene ring and methyl group at terminal benzene ring, the present series is compared with structurally related homologous series. The structure of the synthesized compounds is characterized by standard spectroscopic method, viz., IR, UV, NMR, and MASS.

Full text of DOI:10.1080/15421406.2013.768497

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Synthesis of Liquid Crystals with Lateral
Substituents in Central and Terminal
Benzene Cores and Their Mesomorphic
Characterization
Himanshu S. Bhatt a , Purvang D. Patel a & Jayrang S. Dave a
a Applied Chemistry Department, Faculty of Technology and
Engineering , The M. S. University of Baroda , Vadodara , India
Published online: 22 Apr 2013.
To cite this article: Himanshu S. Bhatt , Purvang D. Patel & Jayrang S. Dave (2013) Synthesis
of Liquid Crystals with Lateral Substituents in Central and Terminal Benzene Cores and Their
Mesomorphic Characterization, Molecular Crystals and Liquid Crystals, 575:1, 104-111, DOI:
10.1080/15421406.2013.768497
To link to this article: http://dx.doi.org/10.1080/15421406.2013.768497
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Mol. Cryst. Liq. Cryst., Vol. 575: pp. 104–111, 2013
Copyright © Taylor & Francis Group, LLC
ISSN: 1542-1406 print/1563-5287 online
DOI: 10.1080/15421406.2013.768497
Synthesis of Liquid Crystals with Lateral
Substituents in Central and Terminal Benzene
Cores and Their Mesomorphic Characterization
HIMANSHU S. BHATT, PURVANG D. PATEL,
AND JAYRANG S. DAVE∗
Applied Chemistry Department, Faculty of Technology and Engineering,
The M. S. University of Baroda, Vadodara, India
A new homologous series consisting of 12 homologues central azo, and ester linkage
with chloro lateral group at central and methyl group at the terminal benzene ring
is synthesized and their mesomorphic properties are studied. All the homologues are
mesogenic in nature. All the compounds are exhibiting enantiotropic nematic phase;
no homologues shows smectic phase even in monotropic form. To evaluate the effect of
lateral chloro group at central benzene ring and methyl group at terminal benzene ring,
the present series is compared with structurally related homologous series. The structure
of the synthesized compounds is characterized by standard spectroscopic method, viz.,
IR, UV, NMR, and MASS.
Keywords Azo-ester mesogen; lateral chloro group; lateral methyl group; mesomor-
phism; nematic
Introduction
Lateral substituents play an effective role in mesogenic properties of a mesogenic compound
as it makes molecules broad. Studies on the effect of lateral substitution have been carried
out by several researchers. A literature survey indicates that generally the mesophase
range of mesogens with lateral substituent is less than that for laterally unsubstituted
mesogens [1–11]. Mesomorphic behaviors of mesogens depends on the molecular structures
of molecules and molecular forces arising; therefore, a slight change in the molecular
geometry brings about considerable change in its mesomorphic properties [1–9,12–17].
Literature survey indicates that the study of lateral substitution at central and terminal
benzene ring is rare, however [6–8,14,17]. Our group has reported the effect of lateral
substitution on mesomorphic properties of fluoro aniline derivatives [7] and also lateral
methyl substitution on azo-ester mesogens [18]. Keeping this in view, we have synthesized
a new homologous series consisting of azo and ester central linkages and lateral chloro
group at central and methyl group at terminal benzene core in order to study the effect
of lateral substitution on mesomorphism at different lateral position on different benzene
cores.
∗Address correspondence to Jayrang S. Dave, Applied Chemistry Department, Faculty of Tech-
nology and Engineering, The M. S. University of Baroda, Vadodara 390 001, India. Tel.: +91-265-
2434188 Ext. 212; Fax: +91-265-2423898. E-mail: jayrangdave@yahoo.com
104

Synthesis of Liquid Crystals with Lateral Substituents
105
Experimental
4-Hydroxy benzoic acid, the appropriate n-alkyl halides, 2, 4-dimethylaniline, 2-Chloro
phenol were of Loba grade, used as received. Solvents were dried and distilled prior to use.
Structure of the synthesized compounds were confirmed by the spectroscopic techniques,
viz., mass spectra were recorded on Applied biosystems API 2000 mass spectrometer;
infrared (IR) spectra were recorded on a Shimadzu IR-408, and nuclear magnetic resonance
(NMR) spectra were measured on a Varian mercury-300 spectrometer. Liquid crystalline
properties were investigated on a Leitz Laborlux 12 POL optical polarizing microscope
provided with a Kofler heating stage.
1. 4-n-Alkoxy benzoic acids and 4-n-alkoxy benzoyl chlorides were synthesized by
the reported method [9].
2. 4-Hydroxyphenylazo-2ꢀ, 4ꢀ-dimethylbenzene, and 3-methyl-4-hydroxyphenylazo-
2ꢀ, 4ꢀ-dimethylbenzene were prepared by a known method [10].
3. The Series I, namely 4-(4ꢀ-n-alkoxybenzoyloxy)-3-chlorophenylazo-2ꢀꢀ, 4ꢀꢀ-dime-
thylbenzenes were synthesized by adding drop wise a cold solution of 3-chloro-4
hydroxyphenylazo-2ꢀꢀ, 4ꢀꢀ-dimethylbenzene, in dry pyridine to a cold solution of
4-n-alkoxybenzoyl chlorides. The mixture was allowed to stand overnight at room
temperature. It was acidified with 1:1 cold HCl and the separated solid was filtered
and column chromatographed on silica gel (60–120 mesh) with 2% ethyl acetate -
petroleum ether (60◦C–80◦C) (2:98) mixture as eluent. Removal of solvent from the
eluate afforded a solid material, which was crystallized repeatedly from methanol
until constant transition temperatures were obtained. These are recorded in Table 2.
The mass analyses of C7 member (Series I) were found to be satisfactory. The
synthetic route of the series is shown in Scheme 1.
Scheme 1. Synthetic route.

106
H. S. Bhatt et al.
IR Data
FTIR (Nujol, KBr pellets, cm−1) were recorded on a Shimadzu IR-408.
Compounds 7: -4-(4ꢀ-n-heptyloxybenzoyloxy)-3-chlorophenylazo-2ꢀꢀ, 4ꢀꢀ-dimethyl
benzene
2952–2858 (C H St, Alkyl CH3, SP3 hybridization), 1730 ( C O St. of ester),
1602 ( N N St.), 1570–1500 ( C C St. of Aromatic ring), 1477( C H bending
of CH2 ), 1250(Aromatic ether St.), 1061( C O St. of ester ), 893(strong C H
bending for 1:2:4 tri substituted benzene ring), 725 (weak C H bending for (CH2)6
cm−1
Compounds 8: -4-(4ꢀ-n-octyloxybenzoyloxy)-3-chlorophenylazo-2ꢀꢀ, 4ꢀꢀ-dimethyl
)
.
benzene
2954–2857 (C H St, Alkyl CH3, SP3 hybridization), 1735 ( C O St. of ester),
1604 ( N N St.), 1573–1502 ( C C St. of Aromatic ring), 1478( C H bending
of CH2 ), 1256(Aromatic ether St.), 1065( C O St. of ester), 890(strong C H
bending for 1:2:4 tri substituted benzene ring), 728 (weak C H- bending for (CH2)7
cm−1
Compounds 10: -4-(4ꢀ-n-decyloxybenzoyloxy)-3-chlorophenylazo-2ꢀꢀ, 4ꢀꢀ-dimethyl
)
.
benzene
2952–2856 (C H St, Alkyl CH3, SP3 hybridization), 1733 ( C O St. of ester),
1602 ( N N St.), 1577–1500 ( C C St. of Aromatic ring), 1477( C H bending
of CH2 ), 1253(Aromatic ether St.), 1063( C O St. of ester), 899(strong C H
bending for 1:2:4 tri substituted benzene ring), 722 (weak C H bending for (CH2)9
cm−1
Compounds 12: -4-(4ꢀ-n-dodecyloxybenzoyloxy)-3-chlorophenylazo-2ꢀꢀ, 4ꢀꢀ-dimethyl
)
.
benzene
2954–2858 (C H St, Alkyl CH3, SP3 hybridization), 1733 ( C O St. of es-
ter), 1604 ( N N St.), 1574–1505 ( C C St. of Aromatic ring), 1482( C H
bending of CH2 ), 1248(Aromatic ether St.), 1067( C O St. of ester), 895(strong
C H bending for 1:2:4 tri substituted benzene ring), 725 (weak C H bending for
(CH2)11 ) cm−1
.
UV Data
The UV absorption data for the solution of Series I compounds (7, 8, 10, 12) in ethyl acetate
at 25◦C are recorded in Table 1. For compounds (7, 8, 10, 12) of Series I, Wavelength ranging
from 450 and 341 corresponds to π→π∗ and n→π∗, respectively.
Table 1. UV data of compounds (7, 8, 10, 12)
(UV) Wavelength
N
π→π∗
n→π∗
7
8
10
12
444.50
445.50
450.50
442.00
341.00
341.50
341.50
341.50

Synthesis of Liquid Crystals with Lateral Substituents
107
◦
Table 2. Transition temperature C of Series I. Series I: -4-(4ꢀ-
n-alkyloxybenzoyloxy)-3-chlorophenylazo-2ꢀꢀ, 4ꢀꢀ-dimethylben
zenes
Transition temperature ◦C
R = n-Alkyl group
Nematic
Isotropic
Methyl
Ethyl
Propyl
Butyl
Pentyl
Hexyl
Heptyl
Octyl
Decyl
Dodecyl
Tetradecyl
Hexadecyl
121
118
109
98
92
80
79
84
72
75
163
160
153
141
139
133
126
129
124
102
93
72
63
85
NMR Data
1H NMR (CDCl3, 300 MHz, δ ppm, standard TMS) were recorded on Varian mercury 300
(CDCl3, 300 MHz, δ, ppm, standard TMS)
Compound 10: -4-(4ꢀ-n-decyloxybenzoyloxy)-3-chlorophenylazo-2ꢀꢀ, 4ꢀꢀ-dimethyl
benzene.
δ = 0.88 (t, 3H, aliphatic CH3), 1.25-1.83 (m, 18H, 9( CH2-)), 1.85 (Qunt, 2H,
COCH2), 2.39 (s, 3H, Ar-CH3) 2.70 (s, 3H, Ar-CH3), 4.04 (t, 3H, OCH2), 6.98–8.21
(m, 11H, Ar-H)
Mass Analysis
MS (FAB+): m/z for C28H31ClN2O3: calculated (M+1), 278.2; found (M+1), 279.0
Results and Discussion
In the present investigation, a new homologous series of 12 compounds, viz., 4-(4ꢀ-n-
alkyloxybenzoyloxy) 3-chlorolphenyl azo-2ꢀꢀ, 4ꢀꢀ-dimethylbenzenes (Series I), was synthe-
sized and their mesomorphic properties were studied.
All the 12 homologs of the series are mesogens; the nematic phase commences
from the very first derivative and remains up to the last hexadecyl derivative synthesized.
Figure 1 shows the plot of transition temperatures against number of carbon atoms in the
n-alkoxy chain; it indicates that Cr-M transitions show overall falling tendency with a
jump at the C8 derivative; the N-I curve shows overall falling tendency with a clear rising
tendency at C8 and C10 homologs as the series is ascended. The nematic phase of the
series shows marble texture.
The present series is compared with structurally related homologous series and Figure 3
shows 3D molecular structure of those series in comparison from ChemDraw Ultra 8.0

108
H. S. Bhatt et al.
170
160
150
140
130
120
110
100
90
Cr - M
N - I
80
70
60
0
2
4
6
8
10
12
14
16
18
No. of carbon atoms in n-Alkyl chain
Figure 1. Series I: 4-(4ꢀ-n-alkyloxybenzoyloxy)-3-chlorophenylazo-2ꢀꢀ, 4ꢀꢀ-dimethylbenzenes.
Software; table 3 indicates thermal stabilities of that series in comparison. Figure 4 shows
optical polarising microscopic images of C7 and C10 homolog of the series.
Series I is purely nematogenic in nature as it does not exhibit smectic mesophase till
C16 derivative studied, whereas Series A [2] and B [18] exhibit smectogenic behavior at
C16 and C10 derivative respectively. Series C [19] is again purely nematogenic in behavior.
The purely nematogenic behavior of the entire Series I can be due to the presence of two
lateral substituents namely Cl in central benzene ring and CH3 in terminal benzene ring
O
RO
H3C
O
N
N
CH3
[I]
Cl
O
O
O
O
RO
RO
N
N
N
CH3
CH3
[A]
[B]
H3C
N
O
O
RO
N
N
CH3
[C]
Cl
Figure 2. Show molecular structure of the series in comparison.

Synthesis of Liquid Crystals with Lateral Substituents
109
Table 3. Average thermal stabilities ◦C
Smectic-isotropic ◦C
or smectic-nematic ◦C
Nematic-
Commencement of
smectic mesophase
Series
isotropic ◦C
1
—
106.5 (C10-C16)
62 (C16)
—
129 (C1-C16)
242 (C1-C8)
155.6 (C1-C16)
164.2 (C3-C16)
—
C10
C16
—
A
B
C
which broadens the molecules and makes molecule not to remain along major molecular
axis. Series A has higher N-I thermal stability as compared with other structurally related
homologous Series I, B, C; this can be due to no lateral substituent in Series A; similarly
Series A has higher S-I thermal stabilities than Series B due to the same reason.
Theintroductionofthelateral chloroandmethylgroupsinSeries Ichangethemolecular
conformation and broaden the molecules, which decreases the transition temperatures and
thermal stabilities of smectic and nematic mesophase [13,8,9]. This factor is a direct result of
an increase in the thickness of the molecule [11]. The increased dissymmetry resulting from
the lateral chloro and methyl group leads to less effective molecular packing in the crystal
lattice and, therefore, lowers the crystal to mesophase transition temperature as shown in
Table 2[9]. Thus, the effect of substitution, which leads to an increase in molecular breadth,
is that the long narrow molecules will be forced further apart so reducing the strength
Figure 3. 3D molecular structure of the series in comparison from ChemDraw Ultra 8.0 Software.

110
H. S. Bhatt et al.
Figure 4. (a) Marble texture of nematic phase of compound C7 Series I at 92◦C on cooling. (b)
Marble texture of the nematic phase of Compound C10 Series I at 98◦C on cooling.
of the intermolecular lateral attractions. Both smectic and nematic mesophases depend
to a greater or lesser extent on these cohesive forces to maintain the parallel orientation
of the molecules. Therefore, substitution leading to an increase in molecular breadth will
decrease the smectic and nematic thermal stabilities, which is actually found in the present
Series I (2)
.
Conclusion
A new homologous series consisting of 12 homologs with central azo and ester linkage and
with chloro and methyl lateral groups at central and terminal benzene rings, respectively,
is synthesized and their mesomorphic properties are studied. All the compounds exhibit
only enantiotropic nematic phase with marble texture; the present series does not exhibit
smectic mesophase which may be due to the presence of two lateral substitutions, namely
Cl and CH3 in central and terminal benzene rings, respectively. Present homologous
series is compared with other structurally related homologous series to evaluate the effect
of lateral substitutions on mesomorphism.
Acknowledgment
The authors are thankful to the Head of the Applied Chemistry Department, Faculty of
Technology and Engineering, The M. S. University of Baroda, Vadodara, for providing the
laboratory facilities as well as analytical instruments facilities.
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Synthesis of Liquid Crystals with Lateral Substituents
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