Homologous series of azomesogens: 4-(4'-n-Alkoxy Benzoyloxy)-3-Chloro phenylazo-2"-chlorobenzenes

Article abstract of DOI:10.1080/15421406.2011.590376

The increasing demand of liquid crystalline materials in various fields of applications has inspired the present investigation. A mesogenic homologous series, 4-(4'-n-alkoxy benzoyloxy)-3-chloro phenylazo-2"-chlorobenzenes, of twelve homologues has been synthesized. All the members of the series are either monotropically or enantiotropically mesogenic except the first member of the series. The nematic mesophase commences from the ethyl derivative and goes to the dodecyl derivative of the series. Tetradecyl and hexadecyl derivatives show a monotropic smectic phase. The nematic mesophase shows a threaded texture, while the smectic mesophase shows a focal-conic-fan-shaped texture. Polymesomorphism is not displayed by any member of the series. The solid-isotropic or solid-mesomorphic transition curve follows a zigzag path of falling and rising tendency as the series is ascended. The usual odd-even effect is observed in the nematic-isotropic transition curve with alternation of transition temperature. Transition temperatures are observed through a polarizing microscope with a heating stage. Analytical data support the structure of the molecules. The average thermal stability and mesomorphic properties of the series are compared with structurally similar other homologous series. Group efficiency order for the smectic and the nematic phases is suggested. Copyright Taylor &Francis Group, LLC.

Full text of DOI:10.1080/15421406.2011.590376

This article was downloaded by: [University of Saskatchewan Library]
On: 19 September 2012, At: 01:31
Publisher: Taylor & Francis
Informa Ltd Registered in England and Wales Registered Number: 1072954 Registered
office: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK
Molecular Crystals and Liquid Crystals
Publication details, including instructions for authors and
subscription information:
http://www.tandfonline.com/loi/gmcl20
Homologous Series of Azomesogens:
4-(4′-n-Alkoxy Benzoyloxy)-3-Chloro
Phenylazo-2″-Chlorobenzenes
A. V. Doshi a & N. G. Makwana b
a Chemistry Department, M. V. M. Science and Home Science
College, Rajkot, Gujarat, India
b General Department, K. D. Polytechnic, Patan, Gujarat, India
Version of record first published: 07 Oct 2011.
To cite this article: A. V. Doshi & N. G. Makwana (2011): Homologous Series of Azomesogens: 4-(4′-
n-Alkoxy Benzoyloxy)-3-Chloro Phenylazo-2″-Chlorobenzenes, Molecular Crystals and Liquid Crystals,
548:1, 220-227
To link to this article: http://dx.doi.org/10.1080/15421406.2011.590376
PLEASE SCROLL DOWN FOR ARTICLE
Full terms and conditions of use: http://www.tandfonline.com/page/terms-and-conditions
This article may be used for research, teaching, and private study purposes. Any
substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing,
systematic supply, or distribution in any form to anyone is expressly forbidden.
The publisher does not give any warranty express or implied or make any representation
that the contents will be complete or accurate or up to date. The accuracy of any
instructions, formulae, and drug doses should be independently verified with primary
sources. The publisher shall not be liable for any loss, actions, claims, proceedings,
demand, or costs or damages whatsoever or howsoever caused arising directly or
indirectly in connection with or arising out of the use of this material.

Mol. Cryst. Liq. Cryst., Vol. 548: pp. 220–227, 2011
Copyright © Taylor & Francis Group, LLC
ISSN: 1542-1406 print/1563-5287 online
DOI: 10.1080/15421406.2011.590376
Homologous Series of Azomesogens: 4-(4ꢀ-n-Alkoxy
Benzoyloxy)-3-Chloro Phenylazo-2ꢀꢀ-Chlorobenzenes
A. V. DOSHI1 AND N. G. MAKWANA2,∗
1Chemistry Department, M. V. M. Science and Home Science College,
Rajkot, Gujarat, India
2General Department, K. D. Polytechnic, Patan, Gujarat, India
The increasing demand of liquid crystalline materials in various fields of applications
has inspired the present investigation. A mesogenic homologous series, 4-(4ꢀ-n-alkoxy
benzoyloxy)-3-chloro phenylazo-2ꢀꢀ-chlorobenzenes, of twelve homologues has been
synthesized. All the members of the series are either monotropically or enantiotropically
mesogenic except the first member of the series. The nematic mesophase commences
from the ethyl derivative and goes to the dodecyl derivative of the series. Tetradecyl
and hexadecyl derivatives show a monotropic smectic phase. The nematic mesophase
shows a threaded texture, while the smectic mesophase shows a focal-conic-fan-shaped
texture. Polymesomorphism is not displayed by any member of the series. The solid-
isotropic or solid-mesomorphic transition curve follows a zigzag path of falling and
rising tendency as the series is ascended. The usual odd–even effect is observed in the
nematic–isotropic transition curve with alternation of transition temperature. Transi-
tion temperatures are observed through a polarizing microscope with a heating stage.
Analytical data support the structure of the molecules. The average thermal stability
and mesomorphic properties of the series are compared with structurally similar other
homologous series. Group efficiency order for the smectic and the nematic phases is
suggested.
Keywords Azoester; liquid crystal; nematic; smectic
Introduction
A vast number of new homologous series have been reported in the literature. Terminal
or lateral substituents play an important role in promoting or depressing mesomorphic
properties in mesogens. Generally the terminal or lateral substituent comprise of either a
homologue alkoxy or alkyl group or a compact unit such as nitro, cyano, and halogen. A
number of homologous series that have a rod-like linear molecular structure were synthe-
sized by researchers [1–14]. In the present study, a new homologous series containing three
phenyl rings in the main core linked through azoester central linkages in addition to n-
alkoxy and chloro terminals, respectively, and substituted by a lateral chloro substituent on
the middle phenyl ring has been synthesized in order to study their effect on mesomorphic
properties.
∗Address correspondence to N. G. Makwana, General Department, K. D. Polytechnic, Nr. North
Gujarat University, Patan 384265, Gujarat, India. E-mail: ngmakwana@yahoo.com
220

Homologous Series of Azomesogens
221
Experimental
Characterization
Microanalysis of the compound was performed on Perkin Elmer PE 2400 CHN analyzer, IR
spectra were recorded on Perkin Elmer spectrum, and 1H-NMR spectra were obtained with
Bruker spectrometer using CDCl3 as solvent. Liquid crystalline properties were investigated
on a Leitz Laborlux 12 POL Polarizing microscope with a heating stage.
Synthesis
2-Chlorophenol, 2-chloroaniline, sodium nitrate, pyridine, thionyl chloride, 4-hydroxy ben-
zoic acid, and n-alkyl halides were used directly as received. The solvents were dried and
distilled before use. The synthetic route to the present series is illustrated in the Scheme 1.
4-n-alkoxy benzoic acid and corresponding 4-n-alkoxy benzoyl chloride (A) was syn-
thesized by the modified method of Dave et al. [15]. 4-Hydroxy-3-chloro phenylazo-2ꢀ-
chlorobenzenes (B) were prepared by a known method [16]. 4-(4ꢀ-n-alkoxy benzoyloxy)-3-
chloro phenylazo-2ꢀꢀ-chlorobenzenes were synthesized by condensing 4-n-alkoxy benzoyl
Scheme 1. Synthetic route to series (1) compounds.

222
A. V. Doshi and N. G. Makwana
Table 1. Elemental analysis for 4-(4ꢀ-n-alkoxy benzoyloxy)-3-chloro phenylazo-2ꢀꢀ-
chlorobenzenes
Calculated %
H
Observed %
H N
R = n-alkyl
Molecular
formula
Sr. no.
chain
C
N
C
1
2
3
4
5
6
7
8
9
Methyl
Ethyl
Propyl
Butyl
Pentyl
Hexyl
Heptyl
Octyl
Decyl
Dodecyl
Tetradecyl
Hexadecyl
C20H14N2O3Cl2 59.85 3.49 6.98 59.72 3.41 6.90
C21H16N2O3Cl2 60.72 3.85 6.75 60.68 3.80 6.72
C22H18N2O3Cl2 61.54 4.19 6.53 61.59 4.10 6.59
C23H20N2O3Cl2 62.30 4.51 6.32 62.38 4.58 6.38
C24H22N2O3Cl2 63.02 4.81 6.13 63.11 4.80 6.19
C25H24N2O3Cl2 63.69 5.09 5.94 63.60 5.19 5.88
C26H26N2O3Cl2 64.33 5.36 5.77 64.30 5.22 5.70
C27H28N2O3Cl2 64.93 5.61 5.61 64.88 5.60 5.68
C29H32N2O3Cl2 66.03 6.07 5.31 66.18 6.10 5.35
C31H36N2O3Cl2 67.03 6.49 5.04 67.11 6.40 5.18
C33H40N2O3Cl2 67.92 6.86 4.80 67.99 6.81 4.69
C35H44N2O3Cl2 68.73 7.20 4.58 68.66 7.90 4.50
10
11
12
chlorides (A) with 4-hydroxy-3-chloro phenylazo-2ꢀ-chlorobenzene (B) in pyridine [8].
The azoesters were crystallized from alcohol until constant transition temperatures were
obtained.
Data
The elemental analysis of compounds found to be satisfactory (see Table 1).
IR for n-pentyloxy derivative (vmax · cm−1): 3066 ( C H aromatic stre.), 2945,
2878, 1464.8, 1385 (alkyl group), 1729.1, 1226.6 (ester group), 1600 ( N N group),
1579.6 ( C C aromatic stre.), 1063 (ether group), 846.7 (p-sub. benzene ring), 763.8
(tri sub.benzene ring), 1120.6 (C Cl aromatic).
IR for n-Decyloxy derivative (vmax · cm−1): 3066 ( C H aromatic stre.), 2933, 2856,
1468.7, 1351 (alkyl group), 1733.9, 1230.5 ( COO group), 1605 ( N N group),
1583.4 ( C C aromatic stre.), 1066.6 ( O ether group), 838 (p-sub. benzene ring),
757 (tri sub. benzene ring), 1118.6 (C Cl aromatic).
1H-NMR (CDCl3, standard tetramethylsilane [TMS]) for n-propyloxy derivative, δ
ppm: 1.07 (t, 3H, CH3), 1.88 (m, 2H, OCH2 CH2 ), 4.03 (t, 2H, OCH2 ), 6.98–8.36
(m, 11H, Ar H).
1H-NMR (CDCl3, standard TMS) for n-butyloxy derivative, δ ppm: 1.0 (t, 3H, CH3),
1.54 (m, 2H, CH2 ), 1.82 (m, 2H, OCH2 CH2 ), 4.07 (t, 2H, OCH2 ), 6.97–8.22
(m, 11H, Ar H).
Results and Discussion
4-Hydroxy-3-chloro phenylazo-2ꢀ-chlorobenzene is not a liquid crystalline substance but
the linking of a phenyl ring bridged through COO and an n-alkoxy terminal chain
increases the length of the molecules favoring lateral intermolecular attractions and

Homologous Series of Azomesogens
223
Table 2. Transition temperatures for 4-(4ꢀ-n-alkoxy benzoyloxy)-3-chloro phenylazo-2ꢀꢀ-
chlorobenzenes
Transition temperature (◦C)
R = n-alkyl
Sr. no.
chain
Smectic
Nematic
Isotropic
1
2
3
4
5
6
7
8
Methyl
Ethyl
Propyl
Butyl
Pentyl
Hexyl
Heptyl
Octyl
—
—
—
—
—
—
—
—
—
138.0
87.0
(84.0)
(90.0)
(76.0)
(74.0)
70.0
152.0
260.0 (d)
141.0
117.0
102.0
104.0
76.0
72.0
9
Decyl
—
—
(52.0)
(64.0)
(73.0)
55.0
—
83.0
73.0
69.0
66.0
10
11
12
Dodecyl
Tetradecyl
Hexadecyl
—
Note: Values in parenthesis indicate monotropy.
polarizability. This results in the molecules of the series, 4-(4ꢀ-n-alkoxy benzoyloxy)-
3-chloro phenylazo-2ꢀꢀ-chlorobenzenes, exhibiting mesomorphic characteristics. Twelve
homologues of the series were synthesized and evaluated for their mesomorphic properties.
The transition temperatures of the present homologous series are recorded in Table 2. All
the members of the series are mesomorphic except the first member of the series, which
is nonmesogenic. The second to the twelfth homologue of the series display a nematic
mesophase, while the tetradecyl and the hexadecyl derivatives of the series display only
a smectic mesophase. The ethyl, propyl, octyl, and dodecyl derivatives are enantiotropic
nematic, while the butyl to the heptyl and the decyl derivatives are monotropic nematic.
The tetradecyl and the hexadecyl derivatives are monotropic smectic. Polymesomorphism
is not displayed by any member of the series. The second member of the series does not
melt sharply at a definite temperature but decomposes at higher temperature. The transi-
tion temperatures are plotted versus the number of carbon atoms in the n-alkoxy chain
of the homologues. The phase diagram is shown in Fig. 1. The solid-mesomorphic or
solid-isotropic transition curve follows a zigzag path of falling and rising tendency as the
series is ascended. The solid-isotropic or solid-mesomorphic transition curve steeply falls
from the first to the third member and rises to the fourth homologue. Thereafter the level
to the fifth homologue again falls up to the eighth homologue with negligible rise at the
sixth homologue and then rises at the tenth homologue of the series. Then it falls at the
twelfth homologue with a rise at the fourteenth homologue. Thereafter it falls to the six-
teenth homologue of the series. The zigzag path of the solid-isotropic or solid-mesomorphic
curve is due to the sequential addition of methylene units in the n-alkoxy chain. This causes
differences in the length and linearity of the molecules and hence the difference in end-to-
end intermolecular forces of attractions. It is directly related to the magnitude of cohesive
forces arising out of odd and even numbers of carbon atoms in the n-alkoxy terminal chain.
Thus overall end-to-end intermolecular forces of attractions are altered from homologue to
homologue in the same series for odd and even member of the series. The nematic–isotropic

224
A. V. Doshi and N. G. Makwana
Figure 1. 4-(4ꢀ-n-alkoxy benzoyloxy)-3-chloro phenylazo-2ꢀꢀ-chlorobenzenes.
(or vice versa) transition curve for odd and even members follows a smooth gradual falling
tendency from the third to the twelfth homologue of the series in the usual manner. Both
curves (odd and even) merge to the main stream of the nematic–isotropic (or vice versa)
transition curve at the eighth homologue. Thus the usual odd–even effect is observed in the
nematic–isotropic (or vice versa) transition curve. The isotropic–smectic transition curve
rises steeply as the series ascended. The nematic phase appeared as a threaded type of
texture as judged directly by visualizing the sample of the homologues in a field of view of
the hot-stage polarizing microscope. The texture of the smectic mesophase is focal conic
fan shaped. Thus the homologous series under present investigation can be considered
a middle ordered melting type with a mesophase range minimum of 2◦C at the eighth

Homologous Series of Azomesogens
225
member and maximum of 122◦C at the second member for the series. The first member of
the homologous series is nonmesogenic because of its high crystallizing tendency and the
incapability of intermolecular cohesion forces to maintain a statistically parallel orienta-
tion or to form an ordered layered arrangement of molecules. Enantiotropic or monotropic
nematic mesomorphism is exhibited by the remaining members of the series due to the
statistically parallel orientations of molecules with end-to-end attractions. Late or early
commencement of the smectic mesophase in the series is related to the extent noncopla-
narity caused by the molecule. Thus, the commencement of the smectic mesophase takes
place later at tetradecyl and hexadecyl derivatives. The nematic–isotropic (or vice versa)
transition curve shows alternation of transition temperatures from the third to the seventh
homologue but alternation diminishes as the series ascended because higher homologues
are of even number, and in the case of higher homologues, the longer n-alkoxy chain may
be coiled or coupled to lie in the line with major axis of the core. Thus, end-to-end contact
would then ultimately be the same for odd and even homologues.
The mesomorphic characteristics of the homologous series (1) are compared with the
structurally similar homologous series (A) and (B) (see Fig. 2). The homologous series
(1), (A), and (B) possess three phenyl rings linked through COO and N N central
bridges, and an n-alkoxy terminal chain at the para position, with chloro unit at the ortho
position to COO group at the middle phenyl ring as common identical features, with
the only uncommon part being another Cl unit, which is positioned at ortho, meta, and
para with respect to N N at the third phenyl ring in the series (1), (A), and (B),
respectively. Therefore, mesogenic behavior and degree of mesomorphism of the series
under comparison is varied due to the variation in the position of the Cl unit at the third
Figure 2. Relative characteristic of series.

226
A. V. Doshi and N. G. Makwana
Table 3. Average thermal stabilities in ◦C
Series
1
A
B
Smectic–Isotropic or
Smectic–Nematic
Commencement of smectic
phase
Nematic–Isotropic or
Isotropic–Nematic
Commencement of nematic
phase
67.5 (C14-C16)
—
75.75 (C10-C16)
C14
114.22 (C2-C12)
C2
—
130.6 (C6-C14)
C6
C10
149.4 (C3-C16)
C3
phenyl ring. The average thermal stabilities of the series (1), (A), and (B) are given in
Table 3. The nematic–isotropic average thermal stability of the homologous series (B) is
the highest amongst the group of the homologous series (1), (A), and (B) (see Table 3).
On comparing the nematic–isotropic average thermal stabilities for the series (1) and (A),
the meta-substituted derivative has stronger intermolecular forces of attractions than the
ortho-substituted derivative, though length to breadth ratio being same. This may be due
to the distance of location of Cl unit at the third phenyl ring from the N N central
bridge. Thus group efficiency order derived for smectic and nematic for the Cl group with
respect to its position is as follows.
Smectic group efficiency order (Terminal substitution): Cl (para) > Cl (ortho) > Cl
(meta)
Nematic group efficiency order (Terminal substitution): Cl (para) > Cl (meta) > Cl
(ortho)
Conclusion
A new homologous series of azoester mesogens with a terminal Cl (ortho) unit has been
synthesized for the study of their mesomorphic properties. The study indicated that the
position of lateral Cl (ortho) unit adversely affects the mesophase thermal stabilities.
Meta-substituted Cl unit does not allow formation of smectic mesophase even in the
monotropic condition.
Acknowledgment
The authors are thankful to Head and teaching staff of the Department of Applied Chemistry,
Faculty of Technology and Engineering, M. S. University of Baroda, Vadodara, Gujarat,
India, for their valuable co-operation in the work. Authors are also thankful to the Central
Salt & Marine Chemicals Research Institute (CSMCRI), Bhavnagar, Gujarat, India, for the
analytical service.
References
[1] Demus, D., & Zaschke, H. (1984). Flussige Kristalle in Tabellen, VEB Deustscher Verlag fur
Grundstoff Industrie: Leipzig, Germany.
[2] de-Gennes, P. G., & Prost, J. (1993). The Physics of Liquid Crystals, Claredon Press: Oxford.

Homologous Series of Azomesogens
227
[3] Vill, V. (1996). Liquid crystal 2.0 – Database of liquid crystalline compounds for personal
computers, Fijisu Kyushu System (FQS) Ltd.: Fukuoka, Japan (LCI: Hamburg, Germany).
[4] Patel, D. H., & Doshi, A. V. (2006). Acta Cienc. Indica, XXXII C(4), 413.
[5] Chauhan, B. C. (2003). Unpublished “Synthesis and Study of Mesogenic Materials” PhD thesis,
Saurashtra University, Rajkot, India.
[6] Gray, G. W., & Winsor, P. A. (1974). Liquid Crystal and Plastic crystals, Eillis Harwood:
Chichester, 1, pp. 103–105.
[7] Doshi, A. V., & Bhoya, U. C. (2002). J. Ind. Chem. Soc., 79, 1–3.
[8] Doshi, A. V., & Ganatra, K. J. (1999). Proc. Ind. Acad. Sci. (Chem. Sci.), 11(4), 562.
[9] Shah, N. H., Vora, R. A., & Jadav, N. D. (1991). Mol. Cryst. Liq. Cryst., 209, 291.
[10] Dave, J. S., Menon, M. R., & Patel, P. R. (2002). Mol. Cryst. Liq. Cryst., 378, 1–11.
[11] Prajapati, A. K., & Pandya, H. M. (2003). Mol. Cryst. Liq. Cryst., 393, 31–39.
[12] Prajapati, A. K., & Pandya, H. M. (2004). Liq. Cryst., 31(6), 889–894.
[13] Ganatra, K. J., & Bhoya, U. C. (2008). Mol. Cryst. Liq. Cryst., 487, 110–116.
[14] Chauhan, M. L., & Doshi, A. V. (2010). J. Inst. of Chemist (India), 82(Part I), 10–17.
[15] Dave, J. S., & Vora, R. A. (1970). In: J. F. Johnson & R. S. Porter (Eds.), Liquid Crystals and
Ordered Fluids, Plenum Press: New York, p. 477.
[16] Furniss, B. S., Hannford, A. J., Smith, P. W. G., & Tatchell, A. R. (1989). Vogel’s Textbook of
Practical Organic Chemistry, 4th ed., Logman: Singapore.