Mesomorphism Dependence on Terminal and Lateral Polar Groups in the Nonlinear Novel Azoester Series

Article abstract of DOI:10.1080/15421406.2014.990201

The synthesis of a novel azoester homologous series was carried out with a view to understanding and establishing the effect of molecular structure on liquid crystal (LC) behavior of a substance. The novel series consists of eleven homologs, and all are enantiotropically nematogenic without exhibition of any smectic property. Transition and melting temperatures, textures of LC were determined by an optical polarizing microscope equipped with a heating stage. The textures of the nematic phase are threaded or Schlieren. Transition curves of a phase diagram behave in normal manner. The nematic-isotropic transition curve exhibits an odd-even effect. Analytical and spectral data support and confirm the structures of the homologues. Average thermal stability for the nematic is 137.2°C. The mesomorphic phase length ranges from 15°C to 50°C. LC behavior of the novel series is compared with a structurally similar known homologous series. The novel azoester homologous series is entirely nematogenic without exhibition of smectogenic character and of a middle-ordered melting type.

Full text of DOI:10.1080/15421406.2014.990201

Molecular Crystals and Liquid Crystals
ISSN: 1542-1406 (Print) 1563-5287 (Online) Journal homepage: http://www.tandfonline.com/loi/gmcl20
Mesomorphism Dependence on Terminal and
Lateral Polar Groups in the Nonlinear Novel
Azoester Series
H. R. Prajapati & A. V. Doshi
To cite this article: H. R. Prajapati & A. V. Doshi (2015) Mesomorphism Dependence on
Terminal and Lateral Polar Groups in the Nonlinear Novel Azoester Series, Molecular Crystals
and Liquid Crystals, 616:1, 37-45, DOI: 10.1080/15421406.2014.990201
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Date: 03 October 2015, At: 16:40

Mol. Cryst. Liq. Cryst., Vol. 616: pp. 37–45, 2015
Copyright © Taylor & Francis Group, LLC
ISSN: 1542-1406 print/1563-5287 online
DOI: 10.1080/15421406.2014.990201
Mesomorphism Dependence on Terminal and
Lateral Polar Groups in the Nonlinear Novel
Azoester Series
H. R. PRAJAPATI1,∗ AND A. V. DOSHI1,2
1C.U. Shah Science College, Ahmedabad, Gujarat, India
2Matushri Virbaima Mahila Science and Home Science College, Saurashtra
University, Rajkot, Gujarat, India
The synthesis of a novel azoester homologous series was carried out with a view to
understanding and establishing the effect of molecular structure on liquid crystal (LC)
behavior of a substance. The novel series consists of eleven homologs, and all are
enantiotropically nematogenic without exhibition of any smectic property. Transition
and melting temperatures, textures of LC were determined by an optical polarizing
microscope equipped with a heating stage. The textures of the nematic phase are threaded
or Schlieren. Transition curves of a phase diagram behave in normal manner. The
nematic-isotropic transition curve exhibits an odd-even effect. Analytical and spectral
data support and confirm the structures of the homologues. Average thermal stability for
the nematic is 137.2◦C. The mesomorphic phase length ranges from 15◦C to 50◦C. LC
behavior of the novel series is compared with a structurally similar known homologous
series. The novel azoester homologous series is entirely nematogenic without exhibition
of smectogenic character and of a middle-ordered melting type.
Keywords Liquid crystal; mesogen; mesomorphism; nematic, smectic
Introduction
The liquid crystalline state (LC) of matter intermediate to crystalline solid and isotropic
liquid was discovered in 1888 [1]. Chemists, physicists, biologists, technologists, pharma-
cists, and engineers have shared their keen interest in the study and applications of liquid
crystals [2,–5]. Various research groups of different disciplines study the LC state with
different angles, views, aims and objects [6,7]. Chemists design and synthesize, and evalu-
ate LC materials by carrying out changes or minor major alternations in molecular shape,
size, aromaticity, terminal and/or lateral functional groups, and their positional substitu-
tion etc. On changing the above variables, the molecular rigidity and/or flexibility [8–11]
changes and novel LC material are generated. The present investigation is planned with a
view to understanding and establishing the relation between LC behavior of a substance
and the molecular structure [12–14], as a consequence of molecular rigidity and flexibil-
ity. The novel homologous series of azoester molecules have potential applications in the
photochromism and electro optical devices [15,16]. Thus, the present study will include
∗Address correspondence to H. R. Prajapati, C.U. Shah Science College, Ahmedabad, Gujarat,
India. E-mail: drhrprajapati@yahoo.co.in
37

38
H. R. Prajapati and A. V. Doshi
synthesis and characterization (not applications) of novel LC substances, to correlate a phe-
nomena of mesomorphism (LC state) and the molecular structure of a substance. Further
studies, including applications, will be studied by others [17,18].
Synthesis
4-Hydroxy benzoic acid was alkylated with suitable alkylating agents (R-X) to convert
it into the corresponding n-alkoxy benzoic acids by an established method of Dave and
Vora [19]. Azo dye 2-hydroxy napthyl azo 2’-nitro benzene was prepared by a reported
method (M.P. is 175.0◦C and Yield is 75%) [20]. 4-n-Alkoxy benzoic acids and the azo dye
were condensed in ice cooled pyridine to obtain a series of final azo ester products [21].
Final products were decomposed, filtered, washed, dried, and purified until their constant
transition temperatures were obtained.
4-Hydroxy benzoic acid, alkyl halides [R-X], methanol, ethanol, KOH, thionyl chloride,
pyridine, β–naphthol, ortho nitro aniline, HCl, NaNO2, etc. required for synthesis were used
as received except for solvents which were dried and distilled prior to use. The synthetic
route to the series is shown in scheme-1.
Characterization
Some selected members of a novel series were characterized by elemental analysis, po-
1
larizing microscopy, mass spectra, infrared spectra, H-NMR spectra, and texture deter-
mination by miscibility method. Microanalysis of the compounds were performed on a
Perkin Elmer PE 2400 CHN analyzer, IR spectra were performed on Perkin Elmer spec-
1
trometer, and H-NMR spectra were performed on Bruker spectrometer using CDCl3 as
solvent.
Analytical Data
IR Spectra in cm−1: for butyloxy and decyloxy derivatives:
Butyloxy: 2919, 2852(alkyl group),1733,1684,1204(ester group),1559,1360(Ar-NO2),
1163(C-N Str. due to N N ), 758(o-sub.benzene) 848(p-sub benzene).
Decyloxy: 2920,2849(alkyl group),1652,1733(ester group), 1559,1300 (Ar-NO2),
1169 (C-N Str. due to N N ), 740 (o-sub. benzene), 844 (p-sub benzene).
1H- NMR Spectra in CDCl3 in δ ppm: for methoxy and tetradecyloxy derivatives
Methoxy:3.91 (t, 3H, OCH3) 7.02–8.80 (m, 14H, Ar-H)
Tetradecyloxy: 0.88 (t,3H, CH3)1.25 (m, 24H, CH2) 4.053 (t, 2H, OCH2)
6.66–8.45 (m, 14H, Ar-H)
Mass spectra (molecular weight) ethoxy homolog:
Calculated-441
Experimental-441
Textures by miscibility method
Nematic:
Propyloxy homolog—threaded texture
Heptyloxy homolog—threaded texture
Decyloxy homolog—Schlieren texture

Mesomorphism Dependence on Terminal and Lateral Polar Groups
39
Scheme 1. Synthetic route to the series.
Results and Discussion
Azo dye 2-hydroxy napthyl azo 2ꢀ-nitrobenzene is nonmesomorphic (m.p. = 175.0 ◦C).
However on linking it with the 4-n-alkoxy benzoic acids through their acid chlorides
novel nematogenic azoester derivatives are generated. Transition temperatures and melting
temperatures of homologs (Table 2) were plotted versus the number of carbon atoms in
n-alkoxy terminal end group. Like or related points are linked to form solid-nematic and
N-I transition curves to represent the phase behavior of the novel series in a phase diagram
(Fig. 1).
The solid-nematic transition curve follows a zigzag path of rising and falling values
with an overall descending tendency in a normal manner. The N-I transition curve is
descended as the series is ascended except for the last tetradecyloxy homolog, which shows
a negligible deviation of about 5◦ from a normal behavior with the exhibition of odd-even

40
H. R. Prajapati and A. V. Doshi
Table 1. Elemental Analysis for Ethyloxy, Hexyloxy, Octyloxy, and Dodecyloxy
Derivatives
Molecular
Sr. no.
formula
Elements% found (calculated%)
H
C
N
1
2
3
4
C25H19N3O5
C29H27N3O5
C31H31N3O5
C35H39N3O5
68.09(68.02)
70.05(70.02)
70.55(70.85)
72.00(72.28)
4.00(4.30)
5.35(5.43)
5.80(5.90)
6.59(6.71)
9.50(9.50)
8.37(8.45)
7.88(8.00)
7.13(7.22)
effect. Thus, both the transition curves of the phase diagram behave in normal manner.
The N-I transition curves for the odd and even homologs merge into each other at the
octyloxy homolog and from the tetradecyloxy derivative a single transition curve results.
The LC behavior varies from homolog to homolog in the present novel series. The average
thermal stability for nematic is 137.2◦C and mesophase length varies between 15◦C and
50◦C. Thus, the entire novel series is fully nematogenic and of a middle-ordered melting
type.
The induction of LC property in first (C1) and second azoester derivatives is due to
the linking of the azo dye to n-alkoxy benzoic acid molecules which increases molecular
length and width and acquires the suitable magnitudes of anisotropic forces of end to end
intermolecular attractions as a consequence of appropriate molecular rigidity and flexibil-
ity. The molecules of all the members of the novel azoester series, under the influence
of externally exposed thermal vibrations, misalign at an angle less than 90◦ and resist
Table 2. Transition Temperatures in ◦C for 2(4’-n-alkoxy benzoyloxy)-napthyl -1-azo-2”-
nitro Benzenes
Compound
no.
n-alkyl group
Cn H2n+1 (n)
Transition temperatures in ◦C
Sm
N
Isotropic
1
2
3
4
5
6
7
8
1
2
3
4
5
6
7
8
—
94.0
127.0
170.0
149.0
163.0
123.0
147.0
121.0
136.0
126.0
121.0
127.0
—
—
—
—
—
—
—
—
—
—
135.0
134.0
130.0
95.0
99.0
103.0
114.0
89.0
71.0
99.0
9
10
11
10
12
14
Sm, smectic; N, nematic.

Mesomorphism Dependence on Terminal and Lateral Polar Groups
41
Figure 1. Phase behaviors of A series.
exposed thermal vibrations for some temperature difference called the degree of meso-
morphism and then the molecules of a sample under microscopic examination cease to
appear from and beyond isotropic temperature. The molecules of C1–C14 adopt a statis-
tically parallel orientational molecular order within a definite range of temperature prior
to the isotropic temperature. However, the same molecules are randomly oriented in all
possible directions with high order of disorder or high entropy (ꢀS = ꢀH/T) from and
beyond isotropic temperature. The same molecules show the nematic mesophase reversibly
below the isotopic temperature on cooling the melt under microscopic observation. The
absence of a smectic phase is attributed to the absence of lamellar packing of molecules
in their crystal lattices and unfavorable extent of noncoplanarity of the azoester molecules
due to the nonlinear shape and size. The odd-even effect observed for N-I transition curve
is attributed to the sequentially added methylene unit to the n-alkoxy chain bonded to
phenyl ring. The odd-even effect for the N-I transition curve diminishes for higher ho-
mologs as the series is ascended because the n-alkoxy end group may coil or bend or
flex or couple to lie with the major axis of the core structure. Variations in mesomorphic
behavior from homolog to homolog in the same series are attributed to the changing length

42
H. R. Prajapati and A. V. Doshi
Figure 2. Structurally similar series.
of n-alkyl chain, and hence the changing molecular length, molecular polarity and polar-
izability, etc. Some mesomorphic properties of presently investigated homologous series 1
are compared with the structurally similar known homologous series X [22] as shown in
Fig. 2.
Homologous series 1 and X are structurally similar and identical with respect to one
napthyl and two phenyl rings bonded through –COO- and –N = N- central bridges as well
as left n-alkoxy terminal end group for the same homolog from series to series. But they
differ from each other with respect to right handed terminal or lateral end groups ortho
–NO2 or para –COCH3 for the same series 1 and X, respectively. Therefore, the variations
in mesomorphic properties and the degree of mesomorphism depend upon varying lateral
or terminal end group for the same homolog from series to series and from homolog to
homolog in the same series. Table 3 shows the average thermal stability, commencement
of mesophase, phase length for the present series 1 and a series X chosen for comparison.
The common molecular part as described above for the same homolog from series to
series and from homolog to homolog in the same series contributes almost equal molecular
rigidity except for the induced effect of the lateral or terminal end groups of –NO2 or
–COCH3, respectively. The changing parts are the n-alkyl chain for the same series and the
lateral or terminal group for the same homolog from series to series, which contributes to the
molecular flexibility. Thus, the combined effects due to molecular rigidity and flexibility
are responsible for inducing mesomorphism in a substance, and vary from homolog to
homolog in the same series and from series to series for the same homolog.
Table 3. Average Thermal Stabilities in ◦C
Series →
Smectic-nematic or smectic-isotropic
commencement of Smectic
mesophase
1
–
X
–
Nematic-isotropic commencement of
nematic phase
Mesophase range
137.2 (C1–C14)
144.0 (C6–C14)
C1
C6
15◦C–50◦C C3
C12
47◦C–60◦C C8
C12

Mesomorphism Dependence on Terminal and Lateral Polar Groups
Table-3 indicates that
43
Both azoester homologous series under comparison are nematogenic only without exhibi-
tion of any smectogenic character.
A nematogenic mesophase formation commences from very first member (C1) of a presently
investigated novel series 1 but it commences late from sixth (C6) member of a series
X, chosen for comparison.
Average thermal stability of series 1 is (137.2◦C) lower than a series X (144.0◦C).
The mesophase length ranges from 15◦C to 50◦C for a series 1, while it ranges from 47◦C
to 60◦C, i.e., phase length ranges for 50 – 15 = 35◦C and the same is appearing for 60
– 47 = 13◦C for series 1 and X, respectively.
Nematogenic mesophase induced from C1 to C14 in present novel series, while the same is
induced from C6 to C14.
The absence of smectogenic property in both the series under comparison is attributed
to the absence of lamellar packing of molecules in their crystal lattices due to unsuitable
magnitudes of anisotropic forces of intermolecular attractions required to build up three-
dimensional molecular network in crystalline state or two dimensional sliding layered
molecular ordered network in floating condition.
The unfavorable extent of noncoplanarity of molecules also hinders the formation of
smectic phase till the last member of the series 1 and X. Lateral substitution of flexible
group and a part of nonlinearity of molecules of both series 1 and X increases molecular
width. Increased molecular width reduces intermolecular attractions on one hand and
on the other hand it enhances the intermolecular attractions due to increased molecular
polarizability. Thus, two opposing effects are operated at a time for the same (width)
reason. The resultant net effect of intermolecular attractions depends upon the dominant
factors operating at a same time for the same reason. Looking to the thermal stability values
137.2 and 144.0 of series 1 and X, they are nearer to each other with a difference of 8.8
more of series X. This difference of series X destabilizes and hinders even formation of
nematic phase in methoxy to pentyloxy homologues of series X due to para substitution
of highly polar –COCH3 group. The presence of highly polar –NO2 lateral group (ortho
substituted) causes such a suitable magnitudes of anisotropic forces of intermolecular end
to end attractions that, nematic mesophase induces from the very first member of a seris
1, i.e., the nematic mesophase is stabilized and facilitated for homologues of entire series
at enthalpy value(ꢀH) corresponding to 137.2◦C thermal stability. Thus, nematogenic
mesophase formation facilitated earlier for present series 1 as compared to series X. Hence,
the mesomorphic behaviors for the same homolog from series to series vary mainly with
the changing molecular flexibility by ortho –NO2 and para –COCH3 group from series 1
to series X.
Conclusions
Azoester novel homologous series of 11 homologs is entirely nematic with absence of
smectic phase.
The group efficiency order derived for nematic on the basis of (i) thermal stability and (ii)
early commencement of mesophase is as under.
Nematic: para COCH3 > ortho –NO2
Nematic: ortho NO2 > para –COCH3
LC with napthyl derivatives are generally nematogenic.

44
H. R. Prajapati and A. V. Doshi
Suitable magnitudes of anisotropic forces of intermolecular end to end attractions occur as
a consequence of favorable rigidity and flexibility.
Molecular rigidity and flexibility are sensitive and susceptible to molecular structure.
A phenomena of mesomorphism depends upon the molecular structure of a substance
concerned, i.e., molecular shape, size, molecular polarity and polarizability, dispersion
forces, and aromaticity constituting the molecular rigidity and flexibility.
Acknowledgments
Authors acknowledge thanks to the Management of M.V.M. Science and Home Science
College, Rajkot, for their valuable support and cooperation, as and when needed. Thanks
are due to the CSMCRI Bhavnagar for analytical services and Applied Chemistry Dept.,
Faculty of Tech and Engg., The M.S. University of Baroda, Vadodara, for microscopic
facility.
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