The mesogenic behavior of isomeric and nonisomeric series of novel azoesters. 4-(4′-n-Alkoxy Benzoyloxy) Phenylazo-2″-bromobenzenes

Article abstract of DOI:10.1080/15421406.2014.881235

A novel homologous series: 4-(4′-n-alkoxy benzoyloxy) phenylazo-2″-bromobenzenes is synthesized and studied with a view to understanding and establishing the effects of molecular structure on mesogenic behavior in a series. The mesogenic property commences from fifth homologue to the last homologue. The transition temperatures of the series are relatively low ranging between 68.C and 143.C. The mesogenic range varies between 6°C and 22°C. The novel azoester series is nematogenic without exhibition of any smectogenic property and an average thermal stability of 89.4°C. The mesogenic behavior of the novel series is compared with structurally similar isomeric/nonisomeric other known series.

Full text of DOI:10.1080/15421406.2014.881235

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The Mesogenic Behavior of Isomeric
and Nonisomeric Series of Novel
Azoesters. 4-(4′-n-Alkoxy Benzoyloxy)
″
Phenylazo-2 -Bromobenzenes
a
a
a
V. C. Kotadiya , M. D. Khunt & U. C. Bhoya
a
Department of Chemistry, Saurashtra University, Rajkot, Gujrat,
India
Published online: 27 May 2014.
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To cite this article: V. C. Kotadiya, M. D. Khunt & U. C. Bhoya (2014) The Mesogenic
Behavior of Isomeric and Nonisomeric Series of Novel Azoesters. 4-(4′-n-Alkoxy Benzoyloxy)
″
Phenylazo-2 -Bromobenzenes, Molecular Crystals and Liquid Crystals, 593:1, 165-176, DOI:
10.1080/15421406.2014.881235
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Mol. Cryst. Liq. Cryst., Vol. 593: pp. 165–176, 2014
Copyright © Taylor & Francis Group, LLC
ISSN: 1542-1406 print/1563-5287 online
DOI: 10.1080/15421406.2014.881235
The Mesogenic Behavior of Isomeric
and Nonisomeric Series of Novel Azoesters.
ꢀ
4
-(4 -n-Alkoxy Benzoyloxy)
ꢀ
ꢀ
Phenylazo-2 -Bromobenzenes
∗
V. C. KOTADIYA, M. D. KHUNT, AND U. C. BHOYA
Department of Chemistry, Saurashtra University, Rajkot, Gujrat, India
ꢀ
ꢀꢀ
A novel homologous series: 4-(4 -n-alkoxy benzoyloxy) phenylazo-2 -bromobenzenes
is synthesized and studied with a view to understanding and establishing the effects of
molecular structure on mesogenic behavior in a series. The mesogenic property com-
mences from fifth homologue to the last homologue. The transition temperatures of the
◦
◦
series are relatively low ranging between 68 C and 143 C. The mesogenic range varies
between 6 C and 22 C. The novel azoester series is nematogenic without exhibition of
any smectogenic property and an average thermal stability of 89.4 C. The mesogenic
◦
◦
◦
behavior of the novel series is compared with structurally similar isomeric/nonisomeric
other known series.
Keywords Liquid crystal; mesogen; mesomorphism; nematic; smectic
Introduction
Molecular rigidity and flexibility [1–3] are important factors related to intermolecular forces
of attractions. Suitable magnitudes of anisotropic forces [4] of intermolecular attractions
can induce mesogenic characteristics in a molecule. Thus, introduction of proper rigidity
by two or more phenyl rings bridged through central groups as well as substitution of
suitable lateral or terminal functional groups positioned properly at phenyl ring or rings
as flexible molecular part can yield liquid crystal substances [5–7] of desired range of
temperature. The present investigation concerns a molecular geometry constructed with
three phenyl rings bridged through –COO– and –N N– central groups as the rigid core
and n-alkoxy (terminal) and –Br (lateral) group as flexible part of a molecule, with a view to
understanding and establishing the effect of molecular structure on liquid crystal behavior.
Experimental
Synthesis
4-Hydroxy benzoic acid was alkylated by suitable alkylating agents by the modified method
of Dave and Vora [8]. n-Alkoxy benzoic acids were converted to corresponding n-alkoxy
∗
Address correspondence to U. C. Bhoya, Department of Chemistry, Saurashtra Univer-
ds ir t uy c, b Rh oa yj ka o@t g 3m 6 a0 i 0l . 0c 5o ,m Gujrat, India, Tel.: +919824203183; Fax: +912812583984. E-mail:
165

166
V. C. Kotadiya et al.
Table 1. Elemental analysis for (1) propyloxy, (2) hexyloxy, and (3) dodecyloxy derivatives
Elements% calculated
Elements% found
Sr. no. Molecular formula
C
H
N
C
H
N
1
2
3
C22H19BrN2O3
C25H25BrN2O3
C31H37BrN2O3
60.01
62.25
65.65
4.25 6.31 60.15 4.36
5.11 5.76 62.38 5.23
6.46 4.89 65.84 6.59
6.38
5.82
4.95
benzoyl chlorides (A) using thionyl chloride by the usual established method [9]. Azo
ꢀ
◦
dye, 4-hydroxy phenyl azo-2 -bromobenzene (B) (M.P. = 93 C) was prepared by usual
method [10] of diazotization. Components (A) and (B) were condensed in dry cold
pyridine by known method [11–13]. The final novel azoester homologues were decom-
posed, filtered, washed, dried, and purified until they gave constant transition temperatures.
Transition temperatures were determined on an optical polarizing microscope, equipped
with a heating stage. The chemicals required for the synthesis, viz., 4-hydroxy ben-
zoic acid, alkylating agents R-X, ortho-bromo aniline, phenol, thionyl chloride, KOH,
methanol, ethanol, etc., were used as received except solvents that were purified prior
to use.
Synthetic route to the series is outlined in Scheme 1.
Characterization
Representative members of a novel series were characterized by elemental analysis
1
(
Table 1), Infrared spectroscopy, H NMR spectroscopy, and mass spectroscopy. Mi-
croanalysis was performed on Perkin-Elmer PE 2400 CHN analyzer (Waltham, Mas-
sachusetts, USA). IR spectra were recorded on Shimadzu FTIR-8400 (Nishinokyou
1
Kuwabara-cho, Nakagyo-Ku Kyoto, Japan), H NMR spectra were recorded on Bruker
spectrometer (Billerica, Massachusetts, USA) using DMSO-d6 as solvent, and mass spec-
tra were recorded on Shimadzu GC-MS Model No. QP-2010 (Nishinokyou Kuwabara-
cho, Nakagyo-Ku Kyoto, Japan). The mesogenic behavior of the homologue derivatives
was observed through polarizing microscopy. The textures of nematogenic mesophase
were determined by miscibility method. Entropy(ꢀS) and enthalpy(ꢀH) are qualitatively
discussed.
Analytical Data: Spectral Data
NMR in ppm for Decyloxy Derivative. 0.84–0.88 (t, 3H, -CH3 of –OC10H21 group),
1
.26–1.77 (m, 16H, -CH3-(CH2-)8-CH2-O-), 4.08–4.12 (t, 2H, -CH2-O-), 7.51–7.69 (m, 7H,
o-substituted phenyl ring), 7.90–7.92 (q, 1H, in phenyl ring ortho to –N N– group),
.03–8.12 (m, 4H, in phenyl ring ortho to –COO– and –N N– group). The NMR data are
reliable with the molecular structure.
8

Novel Azoester Liquid Crystals
167
Where, R= C H and n = 1 to 8 and 10,12,14,16
n
2n+1
Scheme 1. Synthetic route to the novel series.

168
V. C. Kotadiya et al.
NMR in ppm for Tetradecyloxy Derivative. 0.84–0.85 (t, 3H, -CH3 of –OC14H29 group),
1.24–1.77 (m, 24H, -CH3-(CH2-)12-CH2-O-), 4.09–4.14 (t, 2H, -CH2-O-), 7.12–7.69 (m,
7H, o-substituted phenyl ring), 7.91–7.93 (q, 1H, in phenyl ring ortho to –N N– group),
8.04–8.13 (m, 4H, in phenyl ring ortho to –COO– and –N N– group). The NMR data are
reliable with the molecular structure.

Novel Azoester Liquid Crystals
169
−1
IR in cm for Pentyloxy Derivative. 2868, 2941 (C-H Str. of (-CH2-)n group of –OC5H11),
423, 1579 (aromatic C C Str.), 3066, 3097 (C-H Str. of aromatic ring), 1022, 1280 (C-O
Str. of –COO– group), 1730 (C O, Str. of –COO– group), 1139 (C-O-C-), 690 (C-Br Str.),
50, 848 (para- and ortho-substituted phenyl ring), 1604 (–N N– Str.). The IR data are
consistent with the molecular structure.
1
7
−1
IR in cm for Ethyloxy Derivative. 2875, 2933 (C-H Str. of (-CH2-)n group of –OC2H5),
492, 1585 (aromatic C C Str.), 3061, 3095 (C-H Str. of aromatic ring), 1047, 1191 (C-O
Str. of –COO– group), 1732 (C O, Str. of –COO– group), 1141 (C-O-C-), 688 (C-Br Str.),
50, 850 (para- and ortho-substituted phenyl ring), 1604 (–N N– Str.). The IR data are
consistent with the molecular structure.
1
7

170
V. C. Kotadiya et al.
+
Mass of Hexyloxy Derivative. m/z (rel. int%): 482 (M) , 205, 121, 168, 104.
+
Mass of Octyloxy Derivative. m/z (rel. int%): 510 (M) , 233, 121, 168, 104.

Novel Azoester Liquid Crystals
Texture by Miscibility Method.
171
•
•
Heptyloxy derivative-Nematic-Threaded
Hexadecyl derivative-Nematic-Schlieren
Result and Discussion
n-Alkoxy benzoic acids are dimeric because their two molecules are bonded through hydro-
ꢀ
gen bonding. 4-Hydroxy phenyl azo-2 -bromobenzene azo dye (B) is nonmesogenic. How-
ever, mesogenic character is induced by linking it with 4-n-alkoxy benzoic acid molecule
by esterification through acid chlorides of corresponding 4-n-alkoxy benzoyl chlorides.
The methyl to butyl homologue derivatives of novel series are nonmesogenic, pentyl ho-
mologue derivative is monotropic nematic, and the rest of the homologue derivatives
are enantiotropic nematic. None of the homologue derivatives of the novel series exhibit
smectogenic mesophase formation. Transition temperatures (Table 2) of homologues as de-
termined by polarizing microscopy are plotted versus the number of carbon atoms present
in the n-alkyl chain of the left n-alkoxy terminal end group. A phase diagram (Fig. 1) shows
the phase behavior of the novel series. The solid-isotropic or solid-nematic transition curve
partly adopts a zigzag path of rising and falling values in an irregular manner. The nematic-
isotropic (or vice versa) transition curve follows an overall falling tendency as the series is
ascended, showing alternatively rising and falling behavior at the difference of every two
methylene units of left n-alkoxy group. Thus, the nematic-isotropic transition curve appears
like electromagnetic wave for even numbered mesogenic homologues. Thus, the nematic-
isotropic transition curve slightly deviates from a normal behavior. An odd–even effect is
observed for the nematic-isotropic (or vice versa) transition curve. Alternation of transition
temperatures is also observed for the homologue derivative from the phase diagram. The
odd–even effect disappears from and beyond the heptyl homologue derivative.
◦
Table 2. Transition temperatures in C of series-1
◦
R = n-alkyl
group
Transition temperature in C
Compound
no.
CnH2n+1
Sm
Nm
Isotropic
1
2
3
4
5
6
7
8
9
1
2
3
4
5
6
7
8
10
12
14
16
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
(78)
80
76
78
85
80
68
70
143
124
110
121
95
91
90
87
91
1
1
1
0
1
2
89
90
88
Note: () indicate monotropy.

172
V. C. Kotadiya et al.
Figure 1. Phase behavior of series.
The esterification process of dimeric n-alkoxy benzoic acids breaks the hydrogen
bonding through acid chloride formation. Azo dye (B) is nonmesogenic, however linking
of one phenyl ring with n-alkoxy flexible terminal end group through -COO- central
group balances suitable magnitudes of anisotropic forces of intermolecular attractions [4]
as a result of favorable net rigidity in combination with flexibility, which commences
mesogenic property from a pentyl derivative of a novel series. Nonmesogenic behavior of
methyl to butyl derivatives of a series is attributed to their high crystallizing tendency and
relatively shorter n-alkyl chain length as well as positioning of –Br functional group at the
lateral position. These, three effects do not allow for suitable magnitudes of anisotropic
forces of intermolecular attractions. Consequently, the crystal lattices are abruptly broken
without showing up of mesophase formation. The enthalpy (ꢀH) value and transition
temperatures of the homologues (methyl to butyl) are raised sufficiently but melt sharply
at their melting point without exhibition of any kind of mesophase. The entropy value
(
ꢀS) suddenly reaches a value at which molecular disorder occurs to induce nonmesogenic
character. Exhibition of nematogenic character by pentyl to hexadecyl derivatives of the
series is attributed to the proper proportion of molecular end to end attractions that occur
due to the suitable magnitudes of anisotropic forces of intermolecular adhesion, as a
consequence of molecular flexibility [1–3] irrespective of ortho-substituted –Br end group.
The laterally ortho-substituted –Br end group increases intermolecular distance and may
cause reduction in the magnitudes of anisotropic forces of attractions. However, increased
molecular width may cause an opposing force, which operates at a time for the reason
of increased polarizability due to the laterally substituted –Br group [4]. Thus, the net
effect of intermolecular adhesion from two opposing forces operating at a time depends
upon the major effect, out of two opposing effects. The net magnitudes of intermolecular

Novel Azoester Liquid Crystals
173
attractions as a consequence of molecular rigidity in combinations with molecular flexibility
is sufficient to maintain statistically parallel orientational order of molecules in floating
condition to occur nematogenic mesophase formation, but it is insufficient to prolong
nematic mesophase for longer range of liquid crystallinity. Moreover, the magnitudes
of net intermolecular forces of attractions are insufficient to cause lamellar packing of
molecules in their crystal lattices. Therefore, none of the homologues showed smectogenic
character prior to nematogenic mesophase formation. Thus, absence of lamellar packing of
molecules has hindered smectogenic mesophase formation. An odd–even effect observed
for nematic-isotropic transition curve is attributed to the presence of odd and even numbered
of methylene units in n-alkyl chain of left n-alkoxy terminal end group. Disappearance of
odd–even effect from and beyond the heptyl homologue derivative is due to the flexing
or bending or coupling of the n-alkyl chain to the main axis of the core. The wave-like
appearance of a nematic-isotropic transition curve may be due to the alternative variations
of molecular rigidity and flexibility at the interval of every two methylene units. The average
◦
◦
thermal stability is 89.4 C and mesophase length varies from a minimum of 6 C at the
◦
decyl homologue derivative to a maximum of 22 C at the tetradecyl homologue derivative.
Thus, homologue derivatives of the novel series are nematogenic only, without exhibition
of any smectic property, and of a low melting type and relatively shorter range of liquid
crystallinity.
The mesogenic properties of the novel homologous series are compared with struc-
turally similar other known isomeric and nonisomeric series [14,15] X and Y as shown in
Fig. 2.
Novel homologous series-1 and series X and Y are similar or identical with respect
to their molecular structures, consisting of three phenyl rings bonded through –COO– and
–N N– central bridges, which contributes to the molecular rigidity [1–3]. Series-1 and X
are isomeric, but they differ from each other by positional substitution of flexible –Br group
bonded to third phenyl ring keeping –OR terminal end group unchanged, which contributes
to the molecular flexibility. Series-1 and Y are identical with respect to molecular rigidity
Figure 2. Structurally similar isomeric/nonisomeric series.

174
V. C. Kotadiya et al.
Table 3. Average thermal stability in C
◦
Series
[1]
—
[X]
[Y]
Smectic-isotropic or
smectic-nematic
commencement of
smectic phase
149.7
[C6–C16]
C6
114.0
[C10–C16]
C10
—
Nematic–isotropic
commencement of
nematic phase
89.4
[C6–C16]
C5
231.2
[C1–C10]
C1
145.0
[C1–C8]
C1
but differ with respect to molecular flexibility due to replacement of –H (series Y) by
Br (series-1). The difference of overall molecular rigidity in combination with molecular
–
flexibility in series-1, X, and Y causes difference in the suitable magnitudes of anisotropic
forces of intermolecular attraction, which are reflecting in the mesogenic behaviors from
series to series and homologue to homologue in the same series. The positional difference of
–Br in series-1 and X causes difference in length to breadth ratio of the molecular polarity
to polarizability. Hence, the exhibition of type of mesophase or mesophases, degree of
mesomorphism, commencement of mesophase, extent of mesophase length, mesophase
thermal stability, etc., and many other properties related to mesophase formation and
behaviors have sensitively direct relation with even minute variations of molecular structure.
Table 3 represents average thermal stabilities for mesophase for the series-1, X, and Y.
From Table 3, it clearly indicates that
•
•
•
•
Novel series-1, exhibits only nematogenic character with absence of smectic prop-
erty.
Series X and Y under comparison exhibited smectic and nematic mesophases within
definite range of temperature.
Nematic mesophase commences from pentyl homologue of series-1, while it com-
mences from first member, i.e., methyl homologue of series X and Y.
Nematic–isotropic thermal stability of series-1 is the lowest (89.4) and of series X
is the highest (231.2) among the series under comparison.
Nematic thermal stability of series Y is more than a series-1 and less than series X.
Mesogenic phase length of series X and Y selected for comparison is relatively
wider than a novel series-1.
•
•
Homologous series X and Y selected for comparison are linear and lath-like, while
novel series-1 is nonlinear. Hence, intermolecular end-to-end attractions and lateral attrac-
tion of series X and Y are more than a series-1. The bromo functional group substituted
as lateral position widens the molecule, while same bromo functional group positioned as
terminal end group in isomeric series X increases the length to breadth ratio. Of course,
molecular polarizability factor operating in series-1 is weaker than the series X. Hence,
length to breadth ratio for series X has exceeded to cause relatively stronger intermolecular
anisotropic forces of attraction as a consequence of molecular rigidity and flexibility, this is
also the case with series Y and series-1, but the magnitudes of intermolecular adhesion for
series Y is a little bit less than for series X and more than for series-1. Moreover, suitable
magnitudes of anisotropic forces of intermolecular adhesion are sufficient to cause lamellar
packing of molecules within the crystal lattices of the molecules facilitated belonging to

Novel Azoester Liquid Crystals
175
series X and Y causing formation of smectic phase for particular temperature range, and
then intermolecular forces of adhesion cause formation of statistically parallel orientational
order of molecules in floating condition. Thus, smectic mesophase formation in addition
to nematic mesophase is facilitated to occur. But, such intermolecular force of adhesion
among the molecules of novel series-1 is insufficient to form smectic mesophase in addi-
tion to nematic mesophase. Smectic mesophase commences from hexyloxy and decyloxy
homologue derivatives for series X and Y, respectively, while it does not commences till the
last hexadecyloxy homologue derivative of a series-1, because early or late commencement
of smectic phase depends upon the extent of noncoplanarity caused by a molecule [13].
The variations of mesogenic properties from homologue to homologue in the same series
are attributed to the sequential addition of methylene unit, keeping right terminal or lateral
end group unchanged. Similarly, mesogenic properties vary from series to series for same
homologue (same –OR group) are attributed to the varying right terminal/lateral end group
of different polarities keeping left terminal end group unchanged of a molecule [4,10,13].
Conclusions
•
•
•
Titled novel homologous series is nematogenic without exhibition of any smecto-
genic character; even in the monotropic condition.
Presently investigated series is of short range of liquid crystallinity and low-ordered
melting type.
A suitable magnitude of anisotropic forces of intermolecular attractions as a conse-
quence of favorable molecular rigidity and flexibility governs the formation of liquid
crystal mesophase.
•
•
Absence of smectogenic character indicates absence of lamellar packing of
molecules.
Slight variation in molecular shape, size, rigidity, flexibility, position of same or dif-
ferent functional groups and their polarity, molecular length or/and breadth, and ratio
of the molecular polarity to polarizability can change the degree of mesomorphism
and mesogenic behavior to a very large extent.
•
•
Group efficiency order derived on the basis of thermal stability for smectic and
nematic is as under.
Smectic : para-Br > H > ortho-Br.
Nematic : para-Br > H > ortho-Br.
Group efficiency order derived on the basis of early commencement of smectic and
nematic mesophase is as under.
Smectic : para-Br > H > ortho-Br.
Nematic : para-Br = H > ortho-Br.
Acknowledgments
Authors acknowledge thanks to the Department of Chemistry (DST-FIST Funded & UGC-
SAP Sponsored), Saurashtra University, Rajkot, for research work. Authors are also thankful
to Dr. A. V. Doshi, Ex. Principal M.V.M. Science and Home Science College Rajkot, for
his valuable co-operation and suggestions during present investigation as and when needed.
Also thanks are due to the National Facility for Drug Discovery through New Chemical
Entities (NCEs), for analysis of samples.

176
V. C. Kotadiya et al.
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