Synthesis and study of novel liquid crystalline homologous series: 4-(4′- n -alkoxy benzoyloxy)-3-methoxy phenyl azo-3″, 4″ dichlorobenzenes

Article abstract of DOI:10.1080/15421406.2012.720856

The synthesis and mesomorphic properties of a new homologous series of azomesogens, 4-(4′-n-alkoxy benzoyloxy)-3-methoxy phenyl azo-3″,4″ dichlorobenzene is detailed. The first four members of the series are nonmesomorphic, and the other eight members are smectogenic in character. The smectic mesophase, which is exhibited montropically, commences from the pentyl to the hexadecyl derivatives. The usual odd-even effect is observed in the isotropic-smectic transition curve. The isotropic-smectic transition curve behaves in a normal manner. The smectic mesophase shows a focal conic fan shaped texture of the smectic A type. An important feature of the series is the absence of the nematic property. The thermal stability and mesomorphic characteristic are compared with two structurally similar homologous series.

Full text of DOI:10.1080/15421406.2012.720856

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Synthesis and Study of Novel Liquid
Crystalline Homologous Series: 4-(4′-n-
Alkoxy Benzoyloxy)-3-Methoxy Phenyl
azo-3″, 4″ Dichlorobenzenes
B. C. Chauhan a , Ankita A. Doshi b & A. V. Doshi c
a Chemistry Department, Shri. P. H. G. Muni. Arts & Science College,
Kalol. (N.G.), Gandhinagar, Gujarat, India
b Department of Biochemistry, M. S. University of Baroda, Vadodara,
Gujarat, India
c Chemistry Department, Matushri Virbaima Mahila Science & Home
Science College, Rajkot, Gujarat, India
Published online: 22 Jan 2013.
To cite this article: B. C. Chauhan , Ankita A. Doshi & A. V. Doshi (2013) Synthesis and Study
of Novel Liquid Crystalline Homologous Series: 4-(4′-n-Alkoxy Benzoyloxy)-3-Methoxy Phenyl
azo-3″, 4″ Dichlorobenzenes, Molecular Crystals and Liquid Crystals, 570:1, 84-91, DOI:
10.1080/15421406.2012.720856
To link to this article: http://dx.doi.org/10.1080/15421406.2012.720856
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Mol. Cryst. Liq. Cryst., Vol. 570: pp. 84–91, 2013
Copyright © Taylor & Francis Group, LLC
ISSN: 1542-1406 print/1563-5287 online
DOI: 10.1080/15421406.2012.720856
Synthesis and Study of Novel Liquid Crystalline
Homologous Series: 4-(4ꢀ-n-Alkoxy
Benzoyloxy)-3-Methoxy Phenyl azo-3ꢀꢀ, 4ꢀꢀ
Dichlorobenzenes
B. C. CHAUHAN,1,∗ ANKITA A. DOSHI,2 AND A. V. DOSHI3
1Chemistry Department, Shri. P. H. G. Muni. Arts & Science College,
Kalol. (N.G.), Gandhinagar, Gujarat, India
2Department of Biochemistry, M. S. University of Baroda, Vadodara,
Gujarat, India
3Chemistry Department, Matushri Virbaima Mahila Science & Home Science
College, Rajkot, Gujarat, India
The synthesis and mesomorphic properties of a new homologous series of azomesogens,
4-(4ꢀ-n-alkoxy benzoyloxy)-3-methoxy phenyl azo-3ꢀꢀ,4ꢀꢀ dichlorobenzene is detailed. The
first four members of the series are nonmesomorphic, and the other eight members are
smectogenic in character. The smectic mesophase, which is exhibited montropically,
commences from the pentyl to the hexadecyl derivatives. The usual odd-even effect is
observed in the isotropic-smectic transition curve. The isotropic-smectic transition curve
behaves in a normal manner. The smectic mesophase shows a focal conic fan shaped
texture of the smectic A type. An important feature of the series is the absence of the
nematic property. The thermal stability and mesomorphic characteristic are compared
with two structurally similar homologous series.
Keywords Liquid crystal; mesogens; mesomorphism; nematic; smectic
Introduction
This research of novel liquid crystal materials is aimed at enhancing the understanding
of the effect of molecular structure on liquid crystal behavior of a substance depending
upon molecular rigidity and flexibility [1–3]. The molecular structure in which three phenyl
rings are linked through –COO– and –N N– central bridges, to which the middle and third
phenyl rings are laterally substituted by –OCH3 and –Cl moieties in addition to a terminal
n-alkoxy chain and a terminal –Cl unit. Thus, three phenyl rings bridged through two central
linking groups serve as a rigid core and the remaining molecular part acts as flexible units.
Therefore, liquid crystal behavior of a homologous series and its relation with molecular
rigidity and flexibility of molecular structure can be correlated and discussed with reference
to molecular length to breadth ratio, polarity, and polarisability.
∗Address correspondence to B. C. Chauhan, Associate Professor, Shri. P. H. G. Muni. Arts &
Science College, Nr. Rotary Hall, High Way Road, Kalol. (N.G.), North Gujarat, India. E-mail: bakul.
chauhan2010@gmail.com
84

Novel LC Phenylazo Dichlorobenzenes
85
Experimental
Characterization
Some representative homologues of the series were selected for characterization of the
1
structures by elemental analysis, IR, and H-NMR techniques. Microanalysis was per-
formed on Perkin Elmer PE 2400 CHN analyzer. IR spectra were recorded on Perkin Elmer
Spectrum GX spectrometer and 1H-NMR was recorded on a Bruker instrument using CDCl3
as the solvent. The liquid crystalline properties were investigated using a Leitz Labourlux
12 POL polarizing microscope with a heating stage. The textures of the mesophases were
determined by a miscibility method.
Synthesis
The 3,4-Dichloroaniline, sodium nitrite, 2-methoxyphenol, pyridine, thionyl chloride
(GSC), 4-hydroxybenzoic acid, and n-alkyl halides (SRL, Lancaster) were used as re-
ceived. The synthetic route to the novel homologous series is outlined in Scheme 1. The 4-n-
Alkoxybenzoic acids and corresponding acid chlorides [x] were synthesized by a modified
method of Dave and Vora [4]. The 4-Hydroxy-3-methoxy phenyl azo-3ꢀ, 4ꢀ-dichlorobenzene
[y] was prepared by a known method [5]. The 4-(4ꢀ-n-Alkoxy benzoyloxy)-3-methoxy
phenyl azo-3ꢀꢀ, 4ꢀꢀ-dichlorobenzenes were synthesized by condensing the acid chlorides [x]
with 4-hydroxy-3-methoxy phenyl azo-3ꢀ, 4ꢀ-dichlorobenzene [y] in pyridine [6,7]. The
azoesters were individually crystallized from alcohol until constant transition temperatures
were obtained. The elemental analysis of compounds was found to be satisfactory and have
been listed in Table 1.
Analytical Data
IR (cm−1) for Heptyloxy Derivative. 2922, 2856.4 (–C–H aliphatic), 1732, 1255, 1166
(ester group), 1407 (–N N–group), 1540, 1470 (–C C– aromatic), 1107, 1203 (ether
group), 842 (p-sub. benzene ring), 1066.6 (C–Cl aromatic).
R-Br,KOH
SOCl2
HO-
-COOH
RO-
-COOH
RO-
-COCl
MeOH
Excess
[X]
NaNO2, HCl
0-5ºC
Cl+ N2-
H2N-
-Cl
-Cl
[1] O-Methoxyphenolin NaoH
[2] HCl
-Cl
HO-
-N=N-
[Y]
Dry pyridine
Cold 1:1 HCl
[X]
[Y]
+
-N=N-
-Cl
RO-
-COO-
Scheme 1. Synthetic route to Series 1.

86
B. C. Chauhan et al.
Table 1. Elemental analysis for butoxy, pentyloxy, and octyloxy derivatives
Calculated%
H
Observed%
H
Sr. no.
Molecular formula
C
N
C
N
1
2
3
C24H22N2O4Cl2
C25H24N2O4Cl2
C26H26N2O4Cl2
60.88
61.60
62.27
4.65
4.92
5.18
5.91
5.74
5.58
60.68
61.78
62.31
4.87
5.06
5.73
5.95
5.48
5.56
IR (cm−1) for Dodecyloxy Derivative. 2920, 2850 (–C–H aliphatic), 1683, 1255, 1166 (ester
group), 1419.5 (–N N– group), 1583.4 (–C C– aromatic), 1201, 1105 (ether group), 842
(p-sub. benzene ring), 1022 (C–Cl aromatic).
1H-NMR (CDCl3, Standard TMS, 400 MHz) for Heptyloxy Derivative. δppm: 0.91
(t, 3H,–CH3) 1.33(m, 2H, –OCH2–CH2–), 4.050 (t, 2H, –OCH2–), 3.912 (s,–OCH3),
6.9–8.188 (m,11H, Ar-H).
1H-NMR (CDCl3, Standard TMS, 400 MHz) for Decyloxy Derivatives. δppm: 0.91(t,
3H,–CH3) 1.28(s, 2H, –OCH2–CH2–), 4.05(t, 2H, –OCH2–), 4.01 (s,-OCH3), 6.898-
8.186(m,11H, Ar-H).
Results and Discussion
The transition temperatures and the degree of mesomorphism are lowered by linking
dimerized 4-n-alkoxybenzoic acids through their corresponding acid chlorides with 4-
hydroxy-3-methoxy phenyl azo-3ꢀ,4ꢀ-dichlorobenzene by breaking of hydrogen bonding of
dimerised n-alkoxy acids. The reduction in mesomorphic properties is attributed to the odd
geometrical shape, which reduces the intermolecular attractions. The methoxy to butoxy
homologues are nonmesomorphic while the pentyloxy to hexadecyloxy homologues are
monotropic smectogenic without the exhibition of any nematogenic character. The tran-
sition temperatures of the homologues were determined by hot stage optical polarizing
microscopy as recorded in Table 2, which are plotted versus the number of carbon atoms
present in the n-alkoxy terminal chain. A phase diagram (Fig. 1) for the novel homolo-
gous series is obtained by joining the related points of a plot. the solid-isotropic transition
curve adopts a zig-zag path of rising and falling values as the series is ascended with an
overall descending tendency. The isotropic-smectic transition curve initially rises, passes
through a maxima and then smoothly descends as the series is ascended up to the decyloxy
homologue, but further it adopts rising tendency of about 17.0◦C from and beyond the
decyloxy to the hexadecyloxy derivative. The isotropic-smectic transition curve showed
an odd–even effect and both curves merge into each other at the octyloxy homologue.
Nematogenic character is totally absent. The mesomorphic properties varying from ho-
mologue to homologue in the same series due to the sequential addition of a methylene
unit in the n-alkoxy terminal chain. The nonmesomorphic behavior of the methoxy to the
butoxy homologues is due to their high crystallizing tendency because molecules are unable
to withstand exposed thermal vibrations and sharply melt directly to the isotropic liquid
state. The molecules of the homologues from the pentyloxy to the hexadecyloxy deriva-
tives also melt directly to the isotropic liquid, but on supercooling, the molecules adopt a

Novel LC Phenylazo Dichlorobenzenes
87
Table 2. Transition temperatures of the homologous series: 4-(4ꢀ-n-alkoxy benzoyloxy)-3-
methoxyphenyl azo-3ꢀꢀ,4ꢀꢀ-Dichlorobenzenes
Transition temperatures in ◦C
No.
R n-alkyl group
Smectic
Nematic
Isotropic
1
2
3
4
5
6
7
8
9
Methyl
Ethyl
Propyl
Butyl
Pentyl
Hexyl
Heptyl
Octyl
Decyl
Dodecyl
Tetradecyl
Hexadecyl
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
171.0
136.0
107.0
94.0
121.0
88.0
(47.0)∗
(57.0)∗
(61.0)∗
(68.0)∗
(62.0)∗
(66.0)∗
(72.0)∗
(79.0)∗
98.0
138.0
101.0
116.0
109.0
89.0
10
11
12
( )∗Monotropic.
sliding layered molecular arrangement in a floating condition monotropically and showed
smectogenic character. Thus, the magnitude of the anisotropic intermolecular forces of
attraction happens to be favorable to display smectogenic character, but not for nemato-
genic behavior. The mesomorphic properties of the novel homologous series-1 are com-
pared with structurally similar homologous series [A] [8] and series [B] [9] as shown in
Fig. 2.
Homologous series 1, A, and B are identical with respect to three phenyl rings, molec-
ular linearity, n-alkoxy terminal chains, the lateral –OCH3 group, and the –COO– and
–N N– linking groups. However, they differ in that the lateral –Cl unit is missing in series
A and B, and the terminal –Cl unit of series 1 is replaced with ethanoyl and methyl in
series A and B respectively. Therefore, variations in mesomorphic properties, that is, extent
of mesomorphism exhibited, mesophase range, commencement of mesophase formation,
thermal stability, extent of noncoplanarity, and end to end intermolecular attractions are
attributed to the varying end groups of fixed polarity for the homologous series 1, A, [8] or
B [9]. The geometrical shape of all the series under comparison and discussion (Fig. 2) is
rod-like and linear. The different structural units in series 1, A, and B will cause variation in
molecular rigidity and flexibility [1–3], the length to breadth ratio, the ratio of the polarity
to polarisability, the magnitude of anisotropic intermolecular forces of attraction, which
will affect mesophase formation and the commencement of the mesophase or mesophases.
The variation of mesomorphic properties can then be linked to the molecular structural
changes [10]. Table 3 represents the average thermal stabilities of the mesophase and the
commencement of the mesophase for series under comparison.
The average thermal stability of titled series 1 is the lowest and that of series A is
the highest amongst the three series under comparison. The –COCH3 end group more
highly polar than the dichloro and –CH3 units, this also affects the molecular rigidity and
flexibility, [1–3] which are directly related to magnitudes of aniosotropic intermolecular
forces of attraction. Thus, molecules of series A can resist the exposed thermal vibrations

88
B. C. Chauhan et al.
200
180
160
140
120
100
80
Isotropic-Smectic
Solid-Isotropic
Isotropic
60
40
20
1
2
3
4
5
6
7
8
9 10 11 12 13 14 15 16
No. of carbon atoms in n-alkyl chain
Figure 1. Phase behavior of the series.
relatively more efficiently causing end to end attractions and attractions due to lamellar
packing as compared to the series 1 and B. Homologous series 1 and B individually are
only smectogenic and only nematogenic respectively while series A exhibits smectogenic
mesophase before the nematogenic mesophase. The nematic thermal stability of series B
is highest amongst the series under comparison. However, the total mesophase range (i.e.,
smectic + nematic) of series A exceeds the mesophase range of series B. The commence-
ment of the smectic mesophase takes place in series 1 from the pentyloxy homologue while
it commences from the dodecyloxy homologue in series A and, it does not occur until
the final homologue of series B. The extent of molecular noncoplanarity caused by the
–COCH3 terminal end group and dichloro group differs appreciably due to penta atomic
–COCH3 group and monoatomic –Cl group. Thus, early or late commencement of smectic
phase depends upon the extent of noncoplanarity caused by a molecule. The molecular
noncoplanarity caused by the common presence of –COO–, and –N N–, central bridges
including the common –OCH3 and n-alkoxy terminal end group for the same homologue
which remains unaltered for all the three series under comparison. However, variation
arises due to right handed terminal end group –Cl, –COCH3, and –CH3 of fixed polari-
ties. In all other respects except the central bridges which are azo –N N– and carboxy

Novel LC Phenylazo Dichlorobenzenes
89
l
C
-
-
Cl
RO-
-COO-
-N=N-
Series 1
O
C
H
4-(4’-n-alkoxy benzoyloxy )-3- methoxy phenyl azo- 3”,4”- dichlorobenzene.
Series-A
RO-
-COCH3
-COO-
-N=N-
-
O
C
3
H
4-(4’-n-alkoxy benzoyloxy) -3-Methoxyphenylazo - 4”-Acetylbenzenes.
Series-B
RO-
-CH3
-COO-
-N=N-
4-(4’-n-alkoxy benzoyloxy) -3-Methoxyphenylazo - 4”-Methylbenzenes
Figure 2. Structurally similar homologous series.
–COO– for the series under comparison are closely similar to each other. The oxygen
atom of the central –COO– group bump into the nonbonded sides of the adjacent hydrogen
atom of the aromatic phenyl ring which will cause considerable strain on the molecule.
Consequently, a twist around the C–O bond will occur forcing the phenyl ring out of the
plane of the molecule. Thus, the coplanarity of the molecule is reduced to some extent
making them broader. The other central bridge –N N– present on the major axis of the
core certainly endows the molecules with co-planarity. Moreover, the extent of coplanarity
variation caused by the varying –Cl, –COCH3, and –CH3 as right terminal end groups of
fixed polarities. On account of these differences the smectic–isotropic or vice versa thermal
stabilities and the commencement of smectic phase differs. The isotropic–smectic tran-
sition temperatures range between 47◦C and 79◦C from the pentyloxy homologue to the
hexadecyloxy homologue. These irreversible transition temperatures gradually decreases
from the pentyloxy derivatives to the decyloxy derivatives and then smoothly increases from
the dodecyloxy to the hexadecyloxy homologue below the melting point on cooling. The
melting point of the mesophase lies between 121◦C and 89◦C. Thus, the novel series 1 is a
low melting type series whose mesomorphic character starts to appear between 47◦C and
79◦C.

90
B. C. Chauhan et al.
Table 3. Average thermal stabilities in ◦C
Series
I
A
B
–
Smectic—isotropic or isotropic—smectic
smectic—nematic
1.0 to 2.0
(few degrees)
(C5–C16)
(C5–C16)
C5
112.0
(C12–C16)
Commencement of smectic phase
Nematic–isotropic or isotropic–nematic
C12
124.8
(C6–C16)
C6
–
134.73
(C5–C14)
C5
–
–
Commencement of nematic phase
Conclusions
(1) The novel series exhibits only smectogenic character below isotropic temperature
irreversibly.
(2) Molecular rigidity and flexibility tend to create a favorable anisotropic environment
to induce smectogenic character for the molecules of odd fitting shaped geometry
like the dichloro end group.
(3) End to end attractions are not favored comfortably at any temperature below the
melting point.
(4) The novel series is of the low melting type. Widening or broadening of a molecule
through lateral substitution can reduce transition temperatures and the degree of
mesomorphism, if the molecular polarisability is poorly effective.
Acknowledgments
The authors are thankful to Dr. N. D. Jadav Ex-Head department of Applied Chemistry,
M. S. university Vadodara and present head for their valuable co-operation in the work.
The authors are also thankful to the director, Central Salt and Marine Chemical Research
Laboratory, Bhavnagar for analysis of sample.
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