Study of homologous series of azomesogens with lateral aromatic branch

Article abstract of DOI:10.1080/15421400802221904

A homologous series 4-acetyl-2',4'-bis (4-n-alkoxybenzoyloxy) azobenzenes of eleven homologues has been synthesized by fixing 4-acetyl phenyl azo group to resorcinol moiety. Both the terminal and lateral hydroxyl group were esterified by 4-n-alkoxy groups. All the members of the series are monotropic/ enantiotropic nematogens with the exception of the first member. The members of the series were characterized by standard spectroscopic methods, DSC, and optical microscopy. The mesomorphic properties of the present series are compared with other structurally related series.

Full text of DOI:10.1080/15421400802221904

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Study of Homologous Series
of Azomesogens with Lateral
Aromatic Branch
K. J. Ganatra a & U. C. Bhoya a
a M. V. M. Science and Home Sc. College, Rajkot,
Gujarat, India
Version of record first published: 31 Aug 2012.
To cite this article: K. J. Ganatra & U. C. Bhoya (2008): Study of Homologous Series
of Azomesogens with Lateral Aromatic Branch, Molecular Crystals and Liquid Crystals,
487:1, 110-116
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Mol. Cryst. Liq. Cryst., Vol. 487, pp. 110–116, 2008
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421400802221904
Study of Homologous Series of Azomesogens
with Lateral Aromatic Branch
K. J. Ganatra and U. C. Bhoya
M. V. M. Science and Home Sc. College, Rajkot, Gujarat, India
A homologous series 4-acetyl-20,40-bis (4-n-alkoxybenzoyloxy) azobenzenes of eleven
homologues has been synthesized by fixing 4-acetyl phenyl azo group to resorcinol
moiety. Both the terminal and lateral hydroxyl group were esterified by 4-n-alkoxy
groups. All the members of the series are monotropic=enantiotropic nematogens
with the exception of the first member. The members of the series were character-
ized by standard spectroscopic methods, DSC, and optical microscopy. The meso-
morphic properties of the present series are compared with other structurally
related series.
Keywords: enantiopropy; liquid crystal; monotropy; nematic
INTRODUCTION
The vast majority of liquid crystals consists of rod- like molecules
comprising of two or more aromatic and=or aliphatic rings and a small
polar substituent at the lateral or terminal position and a fairly long
straight hydrocarbon chain (usually alkyl or alkoxy) [1]. It is well
known that the lateral substitution in the mesogenic core of the mole-
cules causes a significant depression in the clearing point and hence
diminishing the liquid crystal range, but the effect of lateral aromatic
branch on the type of liquid crystals formed is quite different [2].
A number of homologous series containing aromatic branch at the
lateral position of the mesogenic core have been reported in literature
[3–5].
In the present study, a homologous series containing three rings
in the main core linked through ester and azo central linkages and
substituted by a lateral aromatic branch on the central benzene ring
Address correspondence to K. J. Ganatra, M. V. M. Science and Home Sc. College,
Kalawad Road Under Bridge, Rajkot 360005, Gujarat, India. E-mail: kj_ganatra@yahoo.
co.in
110

Study of Homologous Series of Azomesogens
111
has been synthesized in order to investigate the influence of the lateral
aromatic branch on mesomorphism.
EXPERIMENTAL
Characterization
Some representative homologues of the series were selected for character-
1
ization of the structures by elemental, FT-IR, and H-NMR techniques.
Microanalysis was performed on Perkin Elmer PE 2400 CHN Analyzer.
1
IR spectra were recorded on Perkin Elmer Spectrum GX and H-NMR
was recorded on Bruker using CDCl3 as solvent. The calorimetric study
was carried out DSC 2920 TA Instrument adopting a scanning rate of
5 ꢀC=min. Liquid crystalline properties were investigated using Leitz
Labourlux 12 POL Polarising Microscope with a heating stage.
Synthesis
4-Aminoacetophenone (Lancaster), sodium nitrite, resorcinol, pyri-
dine, thionyl chloride (GSC), 4-hydroxybenzoic acid, n-alkyl halides
(SRL, Lancaster) were used as received. The synthetic route to the
present series is outlined in Scheme 1.
SCHEME 1 &.

112
K. J. Ganatra and U. C. Bhoya
4-Acetyl 20-40-dihydroxyazobenzene (A) was synthesized by using
conventional method of diazotization and coupling [6]. 4-n-Alkoxyben-
zoic acids and 4-n-alkoxybenzoylchlorides were synthesized by the
modified method Dave and Vora [7]. The eleven diesters of the series
were synthesized by condensing two moles of 4-n-alkoxybenzoyl chlor-
ides with one mole of the dye (A) in pyridine. The diesters were crystal-
lized from alcohol till constant transition temperatures were obtained.
Data
IR for Dodceyloxy Derivative
n
max=cmꢁ1
:
2921, 2852(ꢁCꢁH aliphatic), 1738(ꢁC¼O ester),
1686(ꢁC¼O ketone), 1603(ꢁN¼Nꢁ), 1510, 1470(arom. ꢁC¼Cꢁ),
1251(ꢁCꢁOꢁ ester), 1168, 1052, 845, 760.
1H-NMR (CDCl3, Standard TMS, 400 MHz) for Decyloxy
Derivative
d 0.92 (t, 6H, 2 ꢂ ꢁCH3), 1.3–2.6 (m, 32H, 2 ꢂ ꢁCH2), 2.7 (s, 3H
ꢁCOCH3), 4.1 (t, 4H, 2 ꢂ ꢁOCH2), 7.3–8.2 (m, 15H, ArꢁH).
RESULTS AND DISCUSSION
In the present study, 11 homologues the series 4-acetyl-20, 40 bis(4-n-
alkoxybenzoyloxy) azo benzenes were synthesised and evaluated for
their mesomorphic characteristics. The transition temperatures of
the homologous series are recorded in Table 2. All the compounds of
the series, except the first homologue, showed threaded or schlieren
texture on heating the crystalline solid or cooling the isotropic liquid
when viewed under a hot stage polarizing microscope. Ethyl to hexyl
derivatives exhibited nematic mesophase in the monotropic forms
while nematic enantiotropy condition commenced from the octyl
derivative and continued upto the hexadecyl derivative. Figure 1
shows the dependence of the transition temperatures on the number
of carbon atoms in the terminal substitutions. The solid-mesomorphic
TABLE 1 Elemental Analysis for Butyloxy and Octyloxy Derivatives
Elements % found (% calculated)
Molecular formula
C
H
N
C36H36N2O7
69.98 (71.05)
72.79 (73.33)
5.30 (5.92)
6.95 (7.22)
4.10 (4.60)
3.45 (3.88)
C
44H52N2O7

Study of Homologous Series of Azomesogens
113
TABLE 2 Transition Temperatures of the Series
Compound No. R ¼ ꢁCnH2n þ 1
n
Sm
N
I
1
2
3
4
5
6
7
8
9
1
2
3
4
5
6
8
10
12
14
16
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
&
156
(130)a 145
(89) 126
(104) 123
(87) 101
(92) 102
85
89 103
78 102
110 118
105 116
97
10
11
aValues in parenthesis indicate monotropy.
FIGURE 1 Synthetic route to the series.

114
K. J. Ganatra and U. C. Bhoya
TABLE 3 DSC Data for Dodecyloxy Derivative
Peak temp.ꢀC
(microscopic temp.)
Entropy change
Jgꢁ1kꢁ1
Transition
DH=Jgꢁ1
Cr!N
N!I
79.40 (78.0)
103.0 (102.0)
42.83
0.1215
The enthalpy value was not obtained.
or isotropic transition curve follows a zig-zag path of rising and falling
tendency as the series is ascended. The plot shows slight odd–even
effect in the nematic–isotropic transition curve up to the fifth homol-
ogue, then rises upto the fourteenth homologue with a negligible
decline at the twelfth homologue, and finally falls off at the sixteenth
homologue.
In the present study, the enthalpy change of the dodecyloxy deriva-
tive was measured by differential scanning calorimetry; the data is
recorded in Table 3. Table 4 summarizes the average thermal
stabilities and molecular structure of the present series and other
structurally related series A [4] and series B [5] chosen for compara-
tive study.
The molecular geometry of all the homologous series under com-
parison consists of three phenyl rings linked through an ꢁCOOꢁ
and ꢁN¼Nꢁ central linkage with n-alkoxy group at the left terminal
and flexible lateral aromatic branch. The right-end terminal groups
are ꢁCOCH3, ꢁCH3, and ꢁOC12H25 for series 1, A, and B respectively.
The variation in the mesomorphic characteristics can be attributed to
the presence of different right terminal groups which have different
polarity and polarizability for the formation of mesophase.
Comparing the mesomorphic characteristics of series 1 with series
A, it is observed that the homologues of the entire series are mono-
tropic or enantiotropic nematogens, while in the case of series A, the
members of the series exhibit smectic phase monotropically. Molecules
of Series 1 and A differ only in the right terminal groups. In general,
ꢀ
TABLE 4 Average Thermal Stabilities in C
Series
1
–
A
B
Smectic & nematic or isotropic
47.5
(C4ꢁC16
–
54.05 (C12ꢁC16
)
)
Nematic & isotropic
Commencement of smectic phase
88.09
–
71.55 (C4,C8,C12, C16
)
C4
C12

Study of Homologous Series of Azomesogens
115
FIGURE 2 Phase behavior of the series.
large lateral substituents hinder the packing of layered structure;
however, the short methyl group at the terminal position in series A
along with lateral aromatic branch seems to facilitate lamellar pack-
ing required for the generation of the smectic phase. Introduction of
polar acetyl group at the terminal position in series 1 enables consider-
able and significant forces of attraction which serve to stabilize
parallel orientation by end-to-end molecular attractions which are
conducive to the formation of nematic mesophase. This effect persists
even in the homologues with long alkoxy groups. Comparison of the
molecular geometry of the series 1 and series B indicates that series 1
has an acetyl group at the right terminal, while series B has n-dodecy-
loxy terminal group. The long flexible n-dodecyloxy group in series B
aids the formation of smectic mesophase due to enhanced lateral attrac-
tion as compared with terminal attraction, which facilitates lamellar
packing required for the formation of the smectic mesophase [8]. Thus
series B exhibits smectogenic tendency along with the nematic charac-
teristics. The nematic thermal stability of series A being marginally
higher than series B is understandable because of the differences in
terminal attractions arising due to the acetyl and dodecyloxy groups.
CONCLUSION
A new homologous series of mesogen 4-acetyl 20, 40 bis (4-n-alkoxyben-
zoyloxy) azobenzenes with a lateral aromatic branch has been

116
K. J. Ganatra and U. C. Bhoya
synthesized for the study of mesomorphic characteristic. All the
members of the series, except the first homologue, exhibit nematic
phase under monotropic or enantiotropic condition. The study sug-
gests that in spite of a large lateral flexible substituent; a polar sub-
stituent at the terminal position in homologous series with such
geometry gives way for the exhibition of nematic phase.
ACKNOWLEDGMENTS
The authors are thankful to Dr. A. V. Doshi, Ex-Principal, M. V. M.
Science & Home Sc. College, Rajkot and Dr. A. K. Prajapati, Applied
Chem., Dept. M. S. U., Baroda for their valuable guidance. The
authors are also thankful to SICART, Chemistry Department, S. P.
Uni., V. V. Nagar for the analytical service and UGC for providing
financial assistance.
REFERENCES
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Physics, Taylor and Francis: New York.
[2] Imrie, C. T. & Taylor, L. (1989). Liquid Crystals, 6, 1.
[3] Baumeister, U., Kosturkiewicz, Z., Hartung, H., Weissflog, W., & Demus, D. (1990).
Liquid Crystals, 7, 241.
[4] Berdage, P., Bayle, J. P., Ho, M.-S., & Fung, B. M. (1993). Liquid Crystals, 14, 667.
[5] Vora, R. A., Prajapati, A. K., & Kevat, J. B. (2001). Liquid Crystals, 28, 983.
[6] Vogel, A. I. (1989). In: A Textbook of Practical Organic Chemistry, 5th ed., E. L. B. S.
& Longman Group Ltd.: UK, p. 897.
[7] Dave, J. S. & Vora, R. A. (1970). In: Liquid Crystals and Ordered Fluids, Plenum
Press: New York, 477.
[8] Gray, G. W. (1962). In: Molecular Structures and Properties of Liquid Crystal,
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