Synthesis and Study of Molecular Structure and Its Relation to Liquid Crystal Behavior in a Novel Azoester Series

Article abstract of DOI:10.1080/15421406.2014.990239

A novel liquid crystal (LC) homologous series of 12 azoesters was synthesized and evaluated, and nine members of the series exhibited enantiotropically or monotropically nematogenic LC behavior. Smectogenic character is totally absent. The methoxy, tetradecyloxy, and hexadecyloxy homologues are non-liquid crystals (NLC). Textures of nematic phase are threaded, or schlieren. The solid-isotropic/nematic and N-I or vice versa transition curves behave in normal manner. The N-I transition curve exhibits an odd-even effect. The average thermal stability for nematic is 113.25°C. Mesomorphic phase length ranges from 21 to 35°C. The textures of nematic mesophase and transition temperatures were determined by an optical polarizing microscope equipped with a heating stage. Analytical and spectral data confirm the molecular structures of homologues. The LC behavior of the present novel series is compared with a structurally similar known series. The present series is predominantly nematogenic with the absence of smectic property, and is of a middle-ordered melting type.

Full text of DOI:10.1080/15421406.2014.990239

Molecular Crystals and Liquid Crystals
ISSN: 1542-1406 (Print) 1563-5287 (Online) Journal homepage: http://www.tandfonline.com/loi/gmcl20
Synthesis and Study of Molecular Structure and
Its Relation to Liquid Crystal Behavior in a Novel
Azoester Series
N. G. Makwana & A. V. Doshi
To cite this article: N. G. Makwana & A. V. Doshi (2015) Synthesis and Study of Molecular
Structure and Its Relation to Liquid Crystal Behavior in a Novel Azoester Series, Molecular
Crystals and Liquid Crystals, 616:1, 55-63, DOI: 10.1080/15421406.2014.990239
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Mol. Cryst. Liq. Cryst., Vol. 616: pp. 55–63, 2015
Copyright © Taylor & Francis Group, LLC
ISSN: 1542-1406 print/1563-5287 online
DOI: 10.1080/15421406.2014.990239
Synthesis and Study of Molecular Structure
and Its Relation to Liquid Crystal Behavior
in a Novel Azoester Series
N. G. MAKWANA1,∗ AND A. V. DOSHI2
1General Department (Chemistry), R. C. Technical Institute, Ahmedabad,
Gujarat, India
2Saurashtra University, Rajkot, Gujarat, India
A novel liquid crystal (LC) homologous series of 12 azoesters was synthesized and
evaluated, and nine members of the series exhibited enantiotropically or monotropi-
cally nematogenic LC behavior. Smectogenic character is totally absent. The methoxy,
tetradecyloxy, and hexadecyloxy homologues are non-liquid crystals (NLC). Textures of
nematic phase are threaded, or schlieren. The solid–isotropic/nematic and N-I or vice
versa transition curves behave in normal manner. The N-I transition curve exhibits an
odd-even effect. The average thermal stability for nematic is 113.25◦C. Mesomorphic
phase length ranges from 21 to 35◦C. The textures of nematic mesophase and transi-
tion temperatures were determined by an optical polarizing microscope equipped with
a heating stage. Analytical and spectral data confirm the molecular structures of ho-
mologues. The LC behavior of the present novel series is compared with a structurally
similar known series. The present series is predominantly nematogenic with the absence
of smectic property, and is of a middle-ordered melting type.
Keywords Azoester; liquid crystals; monotropy; smectic; nematic
Introduction
Since the report [1] of a novel physical state of matter intermediate between crystalline
solid and isotropic liquid, all research groups of various branches of science and technology
have shared their keen interest to exploit the liquid crystalline (LC) state with different
aims and objects because of its dual characteristics of flow as an ordered liquid and an
ability to show optical behavior [2–6]. Chemists have designed and synthesized novel
LC compounds by varying the geometrical shape, size, aromaticity, terminal end group
or groups, central and/or lateral groups, and their position of substitution as ortho, meta,
and para, and studied or evaluated their effects on LC behavior and molecular structure
[7–10] as a consequence of molecular rigidity and/or flexibility. In continuation of the
same, the present investigation is planned to synthesize a novel homologous series of LC
compounds with azo ( N N ) linkages, which are thermally very stable and undergo
a trans-cis isomerism in presence of UV light, and are useful for electrooptical devices
and photochromism [11, 12]. The present investigation will cover only the synthesis of
∗Address correspondence to N. G. Makwana, General Department (Chemistry), R. C. Technical
Institute, Ahmedabad 380060, Gujarat, India. E-mail: ngmakwana@yahoo.com
55

56
N. G. Makwana and A. V. Doshi
Table 1. Elemental analysis for propyloxy, hexyloxy, decyloxy, and tetradecyloxy
derivatives
% Elements found (% calculated)
Sr. No.
Molecular formula
C
H
N
1.
2.
3.
4.
C22H17N3O5Cl2
C25H23N3O5Cl2
C29H31N3O5Cl2
C33H39N3O5Cl2
55.79(55.70)
58.25(58.14)
60.78(60.84)
63.17(63.06)
3.45(3.59)
4.49(4.46)
5.49(5.42)
6.29(6.21)
8.77(8.86)
8.23(8.14)
7.26(7.34)
6.76(6.69)
novel azoester compounds with three phenyl rings, two laterally substituted –Cl groups
at the middle phenyl ring, and left and right terminal end groups of –OR and –NO2. The
novel compounds were characterized by elemental analysis, spectral data, and LC behavior
by an optical polarizing microscope equipped with a heating stage. Their LC behavior
was evaluated in terms of molecular rigidity and flexibility [13–16]. Other scientific and
technological investigations, including applications, will be studied by other groups of
researchers.
Experimental
Synthesis
2,6-dichlorophenol, 4-nitroaniline, sodium nitrite, pyridine, thionyl chloride, 4-hydroxy
benzoic 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 Scheme 1.
4-n-Alkoxy benzoic acid and corresponding 4-n-alkoxy benzoyl chlorides (A) were synthe-
sized by the modified method of Dave and Vora [17]. 4-Hydroxy-3,5-dichlorophenylazo-4ꢀ-
nitrobenzene (B) was prepared by a known method [18, 19]. 4-(4ꢀ-n-Alkoxy benzoyloxy)-
3,5-dichloro phenylazo-4ꢀꢀ-nitrobenzenes were synthesized by condensing 4-n-alkoxy ben-
zoyl chlorides (A) with 4-hydroxy-3,5-dichlorophenylazo-4ꢀ-nitrobenzene (B) in pyridine
[20, 21]. Synthetic route to the series is shown in Scheme 1.
Characterization
Some members of the series were characterized by elemental analysis, infrared (ir) spectra,
Hydrogen-1 nuclear magnetic resonance (1H-NMR) spectra, mass spectra, and texture
determination by a miscibility method. Microanalysis of the representative compounds was
performed on a Perkin Elmer PE 2400 CHN analyzer as shown in Table 1. IR spectra were
performed on Perkin Elmer spectrometer and 1H-NMR spectra were performed on Bruker
spectrometer using CDCl3 as solvent. LC properties were investigated on a Leitz Laborlux
12 POL polarizing microscope with a heating stage.

Novel Azoesters
57
Scheme 1. Synthetic route to series-1.
Scheme Spectral Data
IR for n-Bulyloxy Derivative (in cm−1
)
2948, 2856, 1457.1, 1390 (alkyl group), 1733.8, 1239.2 (ester group), 1599 ( N N
group), 1559.3 ( C C aromatic stre.), 1343.3, 1521.7 (Ar-NO2, nitro group), 1164
(ether group), 849.6 (p-sub. benzene ring), 1043.4 (C Cl aromatic).
IR for n-Dodecyloxy Derivative (in cm−1
)
2921, 2850.6, 1467.7, 1391.1 (alkyl group), 1739.7, 1243 (ester group), 1602.7 ( N N
group), 1560.3 ( C C aromatic stre.), 1342.4, 1528.5 (Ar-NO2, nitro group), 1163
(ether group), 847.7 (p-sub. benzene ring), 1033.8 (C Cl aromatic).

58
N. G. Makwana and A. V. Doshi
Table 2. Transition temperatures (in ◦C) for 4-(4ꢀ-n-alkoxy benzoyloxy)-3,5-dichloro
phenylazo-4ꢀꢀ-nitro benzenes
Compound No.
n-alkyl group –CnH2n+1 (n)
Sm
N
Isotropic
1
2
3
4
5
6
7
8
9
1
2
3
4
5
6
7
8
10
12
14
16
—
—
—
—
—
—
—
—
—
—
—
—
—
171.0
108.0
133.0
135.0
119.0
132.0
126.0
128.0
98.0
(100.0)
(123.0)
109.0
98.0
(130.0)
(114.0)
(118.0)
63.0
10
11
12
68.0
—
—
101.0
123.0
170.0
Notes. Sm: smectic, N: nematic.
Values given in parentheses indicate monotropy.
1H-NMR in CDCl3, δppm for n-Heptyloxy Derivative
0.9 (t, 3H, –CH3), 1.33–1.55 (m, 8H, 4x-CH2-), 1.84 (m, 2H, –OCH2-CH2-), 4.07 (t, 2H,
–OCH2-), 7.0–8.39 (d, 8H, Ar-H), 8.05 (s, 2H, Ar-H).
1H-NMR in CDCl3, δppm for n-Octyloxy Derivative
0.89 (t, 3H, –CH3), 1.31–1.47 (m, 10H, 5x–CH2-), 1.83 (m, 2H, –OCH2-CH2-), 4.06 (t,
2H, –OCH2-), 6.99–8.43 (d, 8H, Ar-H), 8.05 (s, 2H, Ar-H).
Mass Spectra for Pentyloxy Homologue
Theoretical (calculated) = 502
Experimental = 502
Textures of Nematic Phase by Miscibility Method
Butyloxy homologue = threaded.
Decyloxy homologue = schlieren.
Dodecyloxy homologue = schlieren.
Results and Discussion
A novel LC homologous series of azoesters comprising 12 members has been synthesized
and evaluated. Nine members exhibit nematogenic LC property, and only three are not LC.
The ethoxy, propyloxy, hexyloxy, heptyloxy, and octyloxy homologues are monotropic ne-
matic and the rest of the LC homologues are enantiotropic nematic. Transition temperatures
(Table 2) of novel homologues were plotted versus the number of carbon atoms present in
the n-alkoxy terminal end group. Transition curves were drawn by linking similar or related

Novel Azoesters
59
Figure 1. Phase behaviors of a series.
points, constituting a phase diagram (Fig. 1), which shows phase behavior of the series. The
solid–isotropic/nematic transition curve follows a zigzag path of rising and falling values
with a descending tendency up to the dodecyloxy LC homologue derivative. The N-I tran-
sition curve initially rises smoothly, passes through maxima at butyloxy (C4) homologue,
and then descended to dodecyloxy (C12) homologue with an overall descending tendency.
The N-I transition curve exhibits an odd-even effect. The N-I transition curve is extrap-
olated [22–24] to C14 homologue to predict its nematic transition temperature, following
the trend of N-I transition curve. The transition curves for odd and even members merge
into each other at the nonyloxy homologue. The series under discussion is predominantly
nematogenic with the absence of smectogenic character, and is of a middle-ordered melting
type.
Nine members of the novel series (C2 to C8, C10, and C12) out of 12 members ex-
hibited nematogenic mesomorphism in either monotropic (C2,C3,C6,C7,C8) manner or
enantiotropic (C4, C5, C10, C12) manner. Molecules of C1, C14, and C16 homologues are

60
N. G. Makwana and A. V. Doshi
Figure 2. Structurally similar series.
unable to withstand exposed thermal energy due to inadequate and unsuitable magnitudes
of anisotropic forces of end-to-end and lateral attractions, as a consequence of unfavor-
able molecular rigidity and flexibility from their molecular structure. Some molecules
(monotropic), on cooling the isotropic melt, develop suitable magnitudes of anisotropic
forces of intermolecular attractions and show mesophase formation below the isotropic
temperature for a short range of temperatures, depending upon the rate of cooling of melt.
Thus, substances showing mesophase only below isotropic temperature in (i) irreversible
manner are monotropic LC, or in (ii) reversible manner, i.e. on heating and cooling, are
enantiotropic LC, or in (iii) neither reversibly nor irreversibly manner, i.e. does not show
LC phase, are NLC. The molecules which exhibit mesophase formation are disaligned
at an angle less than 90◦ with the plane of a floating surface with statistically parallel
orientational order to exhibit nematic mesophase. The nematogenic and non-mesogenic
homologues of present series bear absence of lamellar packing of molecules in their crystal
lattices due to unsuitable magnitudes of the extent of molecular noncoplanarity, which
hinders the formation of smectogenic mesophase. The diminishing of odd-even effect be-
yond merging of curves for odd and even effect for higher homologues is attributed to the
coiling or bending or flexing, or coupling of n-alkyl chain with the principal axis of the
core structure of a molecule. Variations in LC behavior from homologue to homologue in
the present series are attributed to the sequentially added methylene unit at the n-alkoxy
chain bonded with a phenyl ring. Average thermal stability for nematic calculated on the
basis of enantiotropic homologues (C4, C5, C10, and C12) is relatively low, but the energies
stored by the respective homologue (ꢀH) molecules are sufficient to facilitate and stabilize
nematic mesophase formation.
Extrapolation [25, 26] of N-I transition curve merges into the solid–isotropic transition
temperature, which indicates that the predicted N-I transition temperature of C14 and
solid–isotropic transition temperature are the same. Therefore, the molecules of C14 do
randomly orient from and beyond its isotropic temperature. Thus, there is no possibility of
nematic mesophase formation for C14 and C16 homologues due to unfavorable magnitudes
of anisotropic forces of intermolecular end-to-end attractions as a consequence of unsuitable
magnitudes of molecular rigidity and molecular flexibility.
The LC behaviors of the presently investigated series-1 are compared with the struc-
turally similar known homologous series-X [27] as shown in Fig. 2.

Novel Azoesters
61
The presently investigated homologous series-1 and a structurally similar homologous
series-X chosen for comparison are identical in all respects except for the right-sided termi-
nal end groups of NO2 and Cl. The phenyl rings are bridged by COO and N N
central groups contributing mainly to the molecular rigidity, and a varying terminal end
group –OR for the same homologue from series to series, which contributes some part to the
total molecular flexibility. The changing parts of the same homologue from series to series
are NO2 and Cl substituted at the para position of N N central bridges, which
cause variations in total molecular rigidity and flexibility (including common share by
too laterally substituted Cl group) of both the series under comparison. Thus, variations
observed in LC behavior and the degree of mesomorphism from homologue to homologue
in the same series and from series to series for the same homologue are attributed to the
changing molecular rigidity and flexibility. Table 3 represents some LC behavior of series-1
and the known series-X chosen for comparison.
Table 3 indicates the following:
• Series-1 and series-X under comparison consist of 12 homologues, with nine and
six members respectively showing enantiotropically or monotropically nematogenic
behavior without exhibition of a smectogenic mesophase.
• Only N-I thermal stabilities are calculated on the basis of enantiotropic transition
temperatures of C4, C5, C10, and C12 homologues for series-1, but N-I and I-N
thermal stability together is calculated for series-X.
• Mesophase length (nematic) is presented on the basis of enantiotropic transition
temperatures only.
• Mesophase formation commences from C2 and C4 for series-1 and series-X respec-
tively.
• Series-1 and series-X are predominantly monotropic nematic and partly enan-
tiotropic nematic with low degree of mesomorphism.
The laterally substituted dichloro group in the middle phenyl ring broadens the
molecules. As a result, on one hand the intermolecular distance increases and intermolecular
end-to-end attraction reduces. On other hand, the molecular polarizability due to lateral sub-
stitution increases and the intermolecular attractions increase. Thus, two opposing effects
are operating at the same time in both series-1 and series-X. Early or late commencement
of the mesophase depends upon the extent of noncoplanarity of molecules. The NO2
terminal end group is triatomic and Cl end group is monoatomic, with different group
polarity yields different values of the extent of noncoplanarity of a molecule. Thus, the
nematic mesophase commences from C2 homologue in the present series but it commences
late from the C4 homologue of series-X.
Table 3. Average thermal stability (in ◦C)
Series
1
X
Nematic–isotropic or
Isotropic–nematic
Commencement of nematic phase
Mesophase length range
113.25
(C4-C12)
C2
96.3
(C4-C10)
C4
21◦C to 35◦C
21◦C

62
N. G. Makwana and A. V. Doshi
Conclusions
• Azoester novel series of 12 homologues exhibits monotropic or enantiotropic me-
somorphism of nematic type without exhibition of any smectic property by nine
members of the series.
• Mesomorphism induced by favorable magnitudes of molecular flexibility emerged
by para substituted NO2 or Cl terminal end group or tail group.
• Group efficiency order derived on the basis of (i) thermal stability and (ii) early
commencement of mesophase (N) are as follows:
(i) Nematic: NO2 > Cl
(ii) Nematic: NO2 > Cl
• Molecular rigidity and flexibility are very sensitive and susceptible to molecular
structure.
• Favorable molecular rigidity and flexibility of suitable magnitudes, which induce
LC behaviors in a substance, depend upon the status of n-alkyl chain bonded to
phenyl ring through oxygen atom.
• The phenomena of mesomorphism is related to the molecular shape, size, aromatic-
ity, polarity and polarizability, dispersion forces, position of substitution of the same
or different functional group or groups, etc., i.e. molecular structure is effective to
mesomorphism.
Acknowledgments
The authors are thankful to the head and teaching staff of applied chemistry of Faculty of
Technology and Engineering, M.S. University, Vadodara, for their valuable cooperation in
the work. Authors are also thankful to CSMCRI, Bhavnagar, for the analysis of samples.
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