Synthesis and Mesomorphic Phase Behavior of Laterally Substituted Novel Azoesters

Article abstract of DOI:10.1080/15421406.2014.884391

A novel homologous series of 4(4′-n-alkoxy benzoyloxy)phenylazo 2′′-chloro-3′′-methyl benzenes was synthesized and studied for mesophase behavior dependence on molecular structure. The nematogenic mesophase is exhibited from the first to the last member and the smectogenic mesophase from the hexyloxy to the tetradecyloxy member of the series. The nematic mesophase is of a threaded or Schlieren type, and the smectic mesophase is of a focal conic fan of the type A or C. Mesomorphic properties of the present novel homologous series are compared with other structurally similar homologous series.

Full text of DOI:10.1080/15421406.2014.884391

Mol. Cryst. Liq. Cryst., Vol. 605: pp. 32–41, 2014
Copyright © Taylor & Francis Group, LLC
ISSN: 1542-1406 print/1563-5287 online
DOI: 10.1080/15421406.2014.884391
Synthesis and Mesomorphic Phase Behavior
of Laterally Substituted Novel Azoesters
M. D. KHUNT, V. C. KOTADIYA, AND U. C. BHOYA∗
Department of Chemistry, Saurashtra University, Rajkot, Gujarat, India
A novel homologous series of 4(4ꢀ-n-alkoxy benzoyloxy)phenylazo 2ꢀꢀ-chloro-3ꢀꢀ-methyl
benzenes was synthesized and studied for mesophase behavior dependence on molecular
structure. The nematogenic mesophase is exhibited from the first to the last member and
the smectogenic mesophase from the hexyloxy to the tetradecyloxy member of the series.
The nematic mesophase is of a threaded or Schlieren type, and the smectic mesophase
is of a focal conic fan of the type A or C. Mesomorphic properties of the present novel
homologous series are compared with other structurally similar homologous series.
Keywords Liquid crystal; mesomorphic; monotropic; nematic; smectic
Introduction
Looking to the utility of liquid crystalline (LC) materials of definite range of temperature
or concentration in the manufacture of various articles such as pharmaceutical prepara-
tions, medical instruments, engineering, physical and chemical processes, display devices,
biology, and biological systems requires the synthesis and evaluation of novel materi-
als. The formulation of LC materials of definite temperature range or the concentration
range needs the construction of a molecule with a novel geometrical shape, size, polarity
of functional group or groups linked at the lateral and/or terminal positions of a phenyl
ring to maintain suitable magnitudes of anisotropic forces of intermolecular attractions
[1], as a consequence of favorable molecular rigidity and/or flexibility [2–4]. Thus, the
present investigation is planned with a view to synthesize novel LC substances that can
have three phenyl rings bonded through –COO– and –N=N– central bridges as a rigid
core and -OR, -Cl, -CH3 groups as terminal and lateral groups that act as a flexible part
of a molecule. Thus, the role of molecular shape, size, polarity of functional groups,
molecular polarizability, etc., can be correlated with mesophase formation and degree of
mesomorphism. Hence, the present study can evolve an idea to understand and establish
the relation between molecular structure and liquid crystal behavior of a newly constructed
molecule.
∗Address correspondence to U. C. Bhoya, Department of Chemistry, Saurashtra University, Ra-
jkot, 360005 Gujarat, India.Tel: +919824203183 Fax: 0281 2583984; E-mail: drucbhoya@gmail.com
Color versions of one or more of the figures in the article can be found online at
www.tandfonline.com/gmcl.
32

Novel Azoester Liquid Crystals
33
Table 1. Elemental analysis for (1) ethyloxy, (2) propyloxy, and (3) octyloxy derivatives
Elements % found Elements % calculated
Sr. no.
Molecular formula
C
H
N
C
H
N
1
2
3
C22H19ClN2O3
C23H21ClN2O3
C22H19ClN2O3
66.95
62.74
69.58
4.80
5.20
6.79
7.27
6.80
6.73
66.92
67.56
70.21
4.85
5.18
6.52
7.09
6.85
5.85
Experimental
Synthesis
Alkylation of 4-hydroxy benzoic acid was carried out by appropriate alkylating agents
(R-X) to form 4-n-alkoxy benzoic acids [5]. The n-alkoxy benzoic acids were converted
to their corresponding acid chlorides using freshly distilled thionyl chloride [A] and used
directly for further reaction without purification. Azo dye, 4-hydroxy phenyl azo-2ꢀ-chloro-
3ꢀ-methyl benzene [B] of melting point 106◦C was prepared by the usual established method
of diazotization [6]. Component [A], i.e., 4-n-alkoxy benzoyl chloride and component [B],
i.e., the azo dye were condensed in dry cold pyridine to prepare final azoester molecule by
a usual etherification method [7,8]. Final products were individually decomposed, filtered,
washed, dried, and purified until constant transition temperatures were obtained. Transition
temperatures of the homologues were determined by optical polarizing microscopy. The
chemicals, viz., 4-hydroxy benzoic acids, alkyl halides (R-X), methanol, ethanol, KOH,
thionyl chloride, phenol, 2-chloro-3-methyl aniline, pyridine, and NaOH required for
synthesis were used as received excepted solvents that were dried and purified prior to use.
The synthetic route to the series is shown in Scheme 1.
Characterization
Some of members of a novel series as the representative member of a series were char-
1
acterized by elemental analysis (Table 1), infrared spectroscopy, H NMR spectra, mass
spectrometry, and textural identification. Microanalysis was performed on a EuroEA El-
emental Analyzer (Waltham, Massachusetts, U.S.A.). IR spectra were recorded on a Shi-
madzu FTIR-8400 spectrometer (Nishinokyou Kuwabara-cho, Nakagyo-Ku Kyoto, Japan),
1H NMR spectra were recorded on Bruker spectrometer (Billerica, Massachusetts, U.S.A.)
using DMSO-d6 as a solvent, and mass spectra were recorded on a Shimatzu GC-MS
Model No. QP-2010 (Nishinokyou Kuwabara-cho, Nakagyo-Ku Kyoto, Japan). The liquid
crystal behavior and the type of textures were determined by a miscibility method through
microscopic observations. Thermodynamic properties enthalpy (ꢀH) and entropy (ꢀS =
ꢀH/T) are discussed qualitatively.
Analytical Data
Elemental Analysis (Table 1).

34
M. D. Khunt et al.
Scheme 1. Synthetic route to the series
Spectral Data
NMR in ppm for the Butoxy Derivative. 0.93–0.97 (t, 3H, -CH3 of –OC4H9 group),
1.26–1.77 (q, 2H, CH3-CH2-), 1.73–1.77 (t, 2H, -CH2-CH2-O-), 4.06–4.13 (t, 2H, -CH2-
O-), 2.72–2.76 (s, 3H, CH3-Ph), 7.13–7.15 (d, 2H, ortho to –OC4H9 group in phenyl
ring), 7.38–7.41 (t, 1H, para to –Cl group in phenyl ring), 7.52–7.58 (m, 3H, phenyl
ring), 7.66–7.67 (d, 1H, in phenyl ring ortho to –N=N– group), 8.02–8.12 (m, 4H, ortho to

Novel Azoester Liquid Crystals
35
–COO– and –N=N– group in phenyl ring). The NMR data are consistent with the molecular
structure.
NMR in ppm for the Decyloxy Derivative. 0.82–0.85 (t, 3H, -CH3 of –OC10H21 group),
1.24 (m, 12H, CH3-CH2-(-CH2-)6-CH2-CH2-O-), 1.39–1.40 (q, 2H, CH3-CH2-), 1.73–1.77
(t, 2H, -CH2-CH2-O-), 2.71 (s, 3H, CH3-Ph), 4.06–4.13 (t, 2H, -CH2-O-), 7.10–7.12 (d,
2H, ortho to –OC10H21 group in phenyl ring), 7.36–7.40 (t, 1H, para to –Cl group in phenyl
ring), 7.50–7.56 (m, 3H, phenyl ring), 7.64–7.65 (d, 1H, in phenyl ring ortho to –N=N–
group), 8.00–8.10 (m, 4H, ortho to –COO– and –N=N– group in phenyl ring). The NMR
data are consistent with the molecular structure.
IR in cm−1 for the Heptyloxy Derivative. 2852, 2920 (C-H Str. of (-CH2-)n group of
–OC7H15), 1440, 1604 (aromatic C C Str.), 3068 (C-H Str. of aromatic ring), 1010, 1068,
1168, 1267 (C-O Str. of –COO– group), 1728 (C O, Str. of –COO– group), 1139 (C-O-C-),
648 (C-Cl Str.), 750, 848, 698 (phenyl ring), 1604 (–N=N– Str.). The IR data are consistent
with the molecular structure.
IR in cm−1 for the Dodecyloxy Derivative. 2852, 2933 (C-H Str. of (-CH2-)n group of
–OC12H25), 1465, 1506, 1579 (aromatic C C Str.), 3070 (C-H Str. of aromatic ring), 1066,
1170 (C-O Str. of –COO– group), 1732 (C O, Str. of –COO– group), 1143 (C-O-C-), 626
(C-Cl Str.), 750, 842, 692 (phenyl ring), 1604 (–N=N– Str.). The IR data are consistent
with the molecular structure.
Mass Spectra of the Hexyloxy Derivative. m/z (rel. int.%): 450 (M)+, 245, 205, 121, 93.
Mass Spectra of the Tetradecyloxy Derivative. m/z (rel. int.%): 562 (M)+, 345, 425, 317,
121.
Texture Determination by Miscibility Method.
• Propoxy homologue: threaded nematic
• pentyloxy homologue: Schlieren nematic
• octyloxy homologue: smectic-A
• tetradecyloxy homologue: smectic-C
Result and Discussion
4-Hydroxy benzoic acid is dimeric, which on linking through the acid chloride with the non-
mesomorphic azo dye component resulted in the formation of mesomorphic products. The
methoxy and ethoxy homologues are monotropic nematic, the propoxy, butoxy, pentyloxy,
and hexadecyloxy homologues are enantiotropic nematic only, and the rest of the homo-
logues also exhibit smectogenic mesomorphism in addition to nematogenic mesophase in
an enantiotropic manner.
Thus, all the members of the novel series show LC behavior. A phase diagram (Fig. 1)
shows the number of carbon atoms present in n-alkyl chain of the left n-alkoxy terminal end
group versus the transition temperatures (Table 2) of the homologues showing mesophase
behavior of the novel series. A solid-mesomorphic transition curve follows a zigzag path
of rising and falling values with alternation of transition temperatures and behaves in

36
M. D. Khunt et al.
Figure 1. Phase behavior of series.
normal manner. The smectic-nematic transition curve initially rises and then descends after
passing through a maxima, as series is ascended with exhibition of an odd–even effect.
Hence, the smectic-nematic transition curve behaves in a usual manner. The nematic-
isotropic transition curve descends as the series is ascended with a sudden steep rise at
the hexadecyloxy homologue and exhibits an odd–even effect. The curve for odd and even
members of the novel series merges into each other between the nonyloxy and decyloxy
homologues and then a single curve propagated until the last hexadecyloxy homologue
of the series and generally behaves in a normal manner except for the hexadecyloxy
Table 2. Transition temperatures in ◦C of series-1
Transition temperature in ◦C
Compound no.
R = n-alkyl group
Sm
Nm
Isotropic
1
2
3
4
5
6
7
8
9
Methyl
Ethyl
Propyl
Butyl
Pentyl
Hexyl
Heptyl
Octyl
—
—
—
—
—
84.0
78.0
80.0
91.0
83.0
88.0
—
(129.0)
(132.0)
96.0
101.0
96.0
88.0
83.0
90.0
96.0
131.0
138.0
116.0
114.0
105.0
128.0
103.0
111.0
113.0
116.0
115.0
130.0
Decyl
10
11
12
Dodecyl
Tetradecyl
Hexadecyloxy
97.0
95.0
124.0
Note: () indicate monotropy.
Sm – Smectic: Nm – Nematic.

Novel Azoester Liquid Crystals
37
derivative. Thus, the phase diagram as a whole behaves in the usual established manner,
for the novel homologous series under discussion. The nematogenic phase length ranges
from 6◦C to 40◦C and smectogenic phase length varies between 4◦C and 14◦C. The
smectic and nematic average thermal stabilities are 91.5◦C and 113.4◦C, respectively.
All the members of the novel series exhibit nematogenic character and the homologues
from and beyond hexyloxy homologue additionally exhibit smectogenic character except
for the hexadecyloxy homologue. Thus, the novel series under present investigation is
predominantly nematogenic and partly smectogenic and is of a low melting type.
The esterification process breaks hydrogen bonding of dimerized 4-n-alkoxy benzoic
acids through their acid chlorides. The methoxy and ethoxy homologues sharply transform
into isotropic state on heating but on cooling they show a nematic mesophase below their
isotropic temperature in an irreversible manner at the respective monotropic temperature.
The molecules arrange themselves in a statistically parallel orientational order under a float-
ing condition with favorable molecular rigidity and flexibility, which causes the molecular
disalignment on the plane of a surface at an angle less than 90◦ under the influence of suitable
magnitudes of anisotropic forces of intermolecular adhesion resisting thermal environment.
The remaining homologues, i.e., propoxy to hexadecyloxy of the novel homologous series
resist externally exposed thermal vibrations for a definite range of temperature causing
nematic and/or smectic mesophase formation by arranging the molecules as statistically
parallel orientational order of molecular and/or in a sliding layered molecular arrangement
in a floating condition. The butoxy, pentyloxy, and hexadecyloxy homologues do adopt
only parallel orientational order of molecules within definite temperature range giving rise
to only nematic mesophase formation, while the hexyloxy to tetradecyloxy homologues
show a sliding layered arrangement of molecules due to the lamellar packing of molecules
in their crystal lattices at first, for definite temperature range showing smectic mesophase
formation and then they arrange themselves in a statically parallel orientational order in
floating condition for another higher temperature range showing nematogenic mesophase
formation [9,10]. Every substance does have the capacity to resist thermal vibrations ex-
posed upon it up to a definite temperature, limit depending upon energy stored (ꢀH) due to
its mass and characteristics. However, since its heat resisting capacity crosses the limit, the
molecules of a substance under thermal energy exposure loses intermolecular attractions
and molecules are randomly oriented in all possible directions without any ordered manner.
Thus, high order of molecular disorder or molecular randomness or entropy (ꢀS = ꢀH/T)
occurs from and beyond isotropic temperature, i.e., mesophase formation destabilizes and
ceases to appear. Thus, the thermal behavior of a thermotropic liquid crystal substance
varies with temperature giving rise to various stabilized physical states of a substance. An
odd–even effect is observed for the smectic-nematic and the nematic-isotropic transition
curves that are attributed to the sequentially added methylene units at the left n-alkoxy
terminal end group. The odd–even effect for both the transition curves of the phase diagram
diminishes as series is ascended from and beyond a point between the nonyloxy and the
decyloxy homologues because the longer n-alkyl chain may coil, bend, flex, or couple to lie
with major axis of the core [1]. Thus, molecular rigidity and flexibility [2–4] are reduced
or ineffective. The variations in mesomorphic properties from homologue to homologue in
the novel homologous series are attributed to the gradual addition of -CH2- unit at the left
n-alkoxy terminal end group keeping the right side laterally substituted almost equipolar
and flexible groups -Cl and -CH3 that contribute to increased molecular polarizability and
flexibility. Thus, the intermolecular adhering anisotropic forces of attractions are favorably
enhanced to induce mesophase formation right from the first member of the series to the
last member of the series. The mesomorphic behavior of the presently investigated novel

38
M. D. Khunt et al.
Figure 2. Structurally similar homologous series.
series-1 is compared with structurally similar other known homologous series X [11] and
Y [12] as shown in Fig. 2.
All the homologous series 1, X, and Y are structurally similar with respect to three
phenyl rings bonded through –COO– and –N=N– central bridges that contribute to the
molecular rigidity as well as the left n-alkoxy terminal end group that contributes to the
molecular flexibility [2–4]. However, they differ in their position of substituted functional
groups -Cl and -CH3 that are contributing to the total molecular flexibility, keeping rest of
the molecular part unchanged. Thus, variations in mesomorphic properties can be linked
with changing molecular structure, i.e., changing molecular flexibility with unchanged
molecular rigidity. Homologous series 1, X, and Y are isomeric with respect to their
molecular formula and changing and/or unchanging molecular part, but their positional
difference of substitution on the middle and/or third phenyl ring or rings differs and causes
a difference in their mesomorphic behavior. Homologous series 1 and Y are also isomeric,
but differ in the position of substitution in which -Cl is bonded at the ortho position to the
–COO– and the meta position to the –N=N– at the middle phenyl ring as well as the -CH3
at the para position to the –N=N– at the third phenyl ring. Thus, the observed variations
in mesomorphic behavior and the degree of mesomorphism are considered due to varied
molecular flexibility, while keeping molecular rigidity intact for series 1, X, and Y.
Table 3 shows the average thermal stabilities for the smectic and the nematic phases,
the commencement of mesophase, and series to series for the present series-1 and X or Y
chosen for comparison.
Table 3 indicates the following.
(i) The smectic thermal stability of the novel series-1 is lower than that of series X,
but higher than that of series Y because the smectic mesophase does not stabilize
in the case of series Y.

Novel Azoester Liquid Crystals
39
Table 3. Average thermal stability in ◦C
Series
[1]
[X]
[Y]
—
Smectic-nematic
91.5 [C6–C14] 108.0 [C10–C16]
commencement of smectic phase
Nematic–isotropic
commencement of nematic phase
C6 C10
113.4 [C3–C14] 148.8 [C1–C16] 175.75 [C1–C16]
C1 C1 C1
(ii) The smectic mesophase commences earlier from hexyloxy homologue in case of
the novel series-1, but it commences late from the decyloxy homologue in case of
series X and is not exhibited by series Y.
(iii) The nematic thermal stability increases from series-1 to series X to series Y.
(iv) The nematic mesophase appears from very first member of all the series 1, X, and
Y.
(v) Series-1 and X are predominantly nematogenic and partly smectogenic, while se-
ries Y is entirely nematogenic without the exhibition of any smectogenic character,
even in the monotropic condition.
Homologous series 1, X, and Y all possess laterally or terminally substituted chloro
and/or methyl groups, at the third phenyl ring in case of series-1 and at the middle phenyl
ring in case of series X and Y. The terminally substituted end group increases the molecular
length and end-to-end intermolecular attractions, while the lateral group increases molec-
ular width and reduces intermolecular attractions; the same situation increases molecular
polarizability and consequently increases intermolecular attraction. Thus, the length to
breadth ratio varies from series to series. Moreover, an increase in the intermolecular forces
of attraction is caused due to polarizability due to the increased molecular breadth at the
laterally substituted positions. Thus, two opposing forces of intermolecular adhesion oper-
ate in all series 1, X, and Y. The net effect of intermolecular anisotropic forces of attractions
depends upon the predominance of any one force out of the two opposing forces. However,
the magnitudes of net effect of intermolecular attractions differ from series to series due
to the type of functional group attached to middle or/and third phenyl ring or rings, their
position of substitution, and their individual polarity. Thus, the observed variations in the
mesomorphic behavior can be attributed to the varying magnitudes of anisotropic forces of
intermolecular attractions for series-1 to series X to series Y, in spite of their same molecular
formula but different structural formula. Series-1 and X exhibit smectic character but series
Y does not. Smectic mesophase formation commences early from hexyloxy homologue
in case of series-1 but the smectic mesophase commences from decyloxy homologue in
series X, due to their difference in their extent of noncoplanarity [7], because of laterally
substituted -CH3 and -Cl group on same third phenyl ring which differs from series X
and Y. The commencement of the nematic mesophase occurs from very first member of
all the series 1, X, and Y under comparison, which indicates that end-to-end attractions
operate equally irrespective of the position of substitution of the -CH3 and -Cl groups.
Therefore, magnitudes of anisotropic forces of attractions as a consequence of molecular
rigidity and/or flexibility determine the mesomorphic behavior of a molecule [1–4]. Hence,
the mesomorphic behavior from series to series not only depends upon the varied lateral
and/or terminal end groups, but also depends upon their varied substitution on different
positions of same or different phenyl rings, irrespective of the same polarity of the same
functional groups.

40
M. D. Khunt et al.
Conclusions
• The group efficiency order derived on the basis of thermal stabilities is as follows.
Smectic: series X > series-1 > series Y
Nematic: series Y > series X > series-1
• The group efficiency order is derived on the basis of early commencement of the
smectic and the nematic mesophase.
Smectic: series-1 > series X > series Y
Nematic: series-1 = series X = series Y
• The presently investigated novel series is predominantly nematogenic and partly
smectogenic.
• The thermal stabilities for the smectic and the nematic mesophase formation are
lowered by substituting same functional group if positioned on the same phenyl ring
instead of on different phenyl rings of a molecule, keeping the rest of the molecular
part unchanged.
• Molecular rigidity and flexibility play an important role in liquid crystal formation
and LC behavior of a molecule.
• Variations in mesomorphic (LC) behavior of a molecule also depend upon the
positional difference of the substitution of same functional group or groups in
isomeric homologous series.
• Commencement of the smectic mesophase depends upon the position of substitution
either middle or third phenyl ring as lateral and/or terminal end group.
• The commencement of the nematic phase does not depend upon the place of substi-
tution either on middle or third phenyl ring as lateral or terminal end group.
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 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.
References
[1] Marcos, M., Lomenta, A., Serrano, J. L., & Ezcurra, A. (1992). Adv. Mater., 4, 285.
[2] Hird, M., Toyne, K. J., & Gray, G. W. (1993). Liq. Cryst., 14, 741.
[3] Hird, M., Toyne, K. J., & Gray, G. W., Day, S. E., & Mc Donnell, D. G. (1993). Liq. Cryst., 15,
123.
[4] (a) Gray, G. W. (1962). Molecular Structure and the Properties of Liquid Crystal, Academic
Press: London. (b) Gray, G. W., & Windsor, P. A. (1974). Liquid. Crystals and Plastic Crystals,
Vol. 1., Chapter 4., Ellis Horwood: Chichester, UK.
[5] Dave, J. S., & Vora, R. A. (1970). Liquid Crystal and Ordered Fluids, Plenum Press: New York,
477 pp.
[6] Furniss, B. S., Hannaford, A. J., Smith, P. W. G., & Tatchell, A. R. (1989). Vogel’s Textbook of
Practical Organic Chemistry, 5th edition, Pearson Education, Ltd.
[7] (a) Doshi, A. V., & Ganatra, K. J. (1999). Proc. Ind. Acad. Sci. (Chem. Sci.), 4, 11, 562.
(b) Bhoya, U. C., Vyas, N. N., & Doshi, A. V. (2012). Mol. Cryst. Liq. Cryst., 552, 104.

Novel Azoester Liquid Crystals
41
[8] (a) Doshi, A. V., & Makwana, N. G. (2011). Mol. Cryst. Liq. Cryst., 548, 220. (b) Doshi, A. V.,
Odedra, D. A., & Patel, R. B. (2012). Mol. Cryst. Liq. Cryst., 552, 97.
[9] Chauhan, M. L., & Doshi, A. V. (2007). J. Ind. Chem. Soc., 84, 774.
[10] (a) Patel, R. B., & Doshi, A. V. (2011). Der Pharma Chem., 3, 147. (b) Patel, V. R., & Doshi, A.
V. (2010). Der Pharma Chem., 2, 429. (c) Chandhari, R. P., & Doshi, A. V. (2013). Mol. Cryst.
Liq. Cryst., 570-1, 109.
[11] Chauhan, B. C., & Doshi, A. V. (2005). J. Ind. Chem. Soc., 82, 463.
[12] Makwana, N. G., & Doshi, A. V. (2006). Acta Ciencia Indica, XXXII, 391.

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