The synthesis and mesomorphic properties of a novel homologous series: α-4-[4′-n-alkoxy benzoyloxy] benzoyl-β-2″-nitro phenyl ethylenes

Article abstract of DOI:10.1080/15421406.2012.720860

The synthesis and mesomorphic properties of novel homologous series are reported. All 11 members of the series, except the methoxy and ethoxy derivatives, are enantiotropically mesogenic. The propoxy to tetradecyloxy homologues exhibits smectognic amd nematogenic behavior, while the hexadecyloxy homologue shows only nematogenic behavior. An odd-even effect is observed for the smectic-nematic and the nematic-isotropic transition curve in the phase diagram. Analytical data support the structure of molecules. The textures of the nematic mesophase are threaded or Schlieren, and that of smectic mesophase are of type-A. The transition temperatures of homologues and other mesomorphic properties are determined using optical polarizing microscopy. The smectic and nematic thermal stabilities are 138.0°C and 163.8°C respectively. The novel series is predominantly nematogenic and partly smectogenic with middle-ordered melting type. The nematic and smectic phase ranges vary between 16°C and 38°C and 6°C and 22°C respectively. The mesogenic properties of the novel series are compared with other structurally similar homologous series.

Full text of DOI:10.1080/15421406.2012.720860

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The Synthesis and Mesomorphic
Properties of a Novel Homologous Series:
α-4-[4′-n-Alkoxy benzoyloxy] Benzoyl
-β-2″-nitro Phenyl Ethylenes
H. N. Chauhan a b & A. V. Doshi a
a Jagdish Prasad Jhabarmal Tibrewala University, Jhunjhunu,
Rajasthan, India
b K. K. Shah Jarodwala Maninagar Science College, Ahmedabad,
Gujarat, India
Published online: 22 Jan 2013.
To cite this article: H. N. Chauhan & A. V. Doshi (2013) The Synthesis and Mesomorphic Properties
of a Novel Homologous Series: α-4-[4′-n-Alkoxy benzoyloxy] Benzoyl -β-2″-nitro Phenyl Ethylenes,
Molecular Crystals and Liquid Crystals, 570:1, 92-100, DOI: 10.1080/15421406.2012.720860
To link to this article: http://dx.doi.org/10.1080/15421406.2012.720860
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Mol. Cryst. Liq. Cryst., Vol. 570: pp. 92–100, 2013
Copyright © Taylor & Francis Group, LLC
ISSN: 1542-1406 print/1563-5287 online
DOI: 10.1080/15421406.2012.720860
The Synthesis and Mesomorphic Properties of a
Novel Homologous Series: α-4-[4ꢀ-n-Alkoxy
benzoyloxy] Benzoyl -β-2ꢀꢀ-nitro Phenyl Ethylenes
H. N. CHAUHAN1,2,∗ AND A. V. DOSHI1
1Jagdish Prasad Jhabarmal Tibrewala University, Jhunjhunu, Rajasthan, India
2K. K. Shah Jarodwala Maninagar Science College, Ahmedabad, Gujarat, India
The synthesis and mesomorphic properties of novel homologous series are reported. All
11 members of the series, except the methoxy and ethoxy derivatives, are enantiotrop-
ically mesogenic. The propoxy to tetradecyloxy homologues exhibits smectognic amd
nematogenic behavior, while the hexadecyloxy homologue shows only nematogenic be-
havior. An odd-even effect is observed for the smectic–nematic and the nematic–isotropic
transition curve in the phase diagram. Analytical data support the structure of molecules.
The textures of the nematic mesophase are threaded or Schlieren, and that of smectic
mesophase are of type-A. The transition temperatures of homologues and other meso-
morphic properties are determined using optical polarizing microscopy. The smectic
and nematic thermal stabilities are 138.0◦C and 163.8◦C respectively. The novel se-
ries is predominantly nematogenic and partly smectogenic with middle-ordered melting
type. The nematic and smectic phase ranges vary between 16◦C and 38◦C and 6◦C and
22◦C respectively. The mesogenic properties of the novel series are compared with other
structurally similar homologous series.
Keywords Liquid crystal; mesogens; mesophase; nematic; smectic
Introduction
Materials with two phenyl rings and central linking groups ( CH CH CO or
CO CH CH ) have been synthesized previously [1a]. However, liquid crystal
mesophase formation did not feature for this type of compound. Chauhan and Doshi
[1b] continued research along similar lines by increasing molecular length and aromaticity
by linking three phenyl rings by two central groups, including CH CH CO or/and
CO CH CH as one of the central groups. Doshi and Chauhan [1b] and Doshi and Patel
[1c] independently observed mesophase formation using COO or CH CH COO
as the central group along with a second CH CH CO central group. Doshi and
Chauhan [2], Doshi and Suthar [3], and Doshi and Chaudhari [4] extended their re-
search work using CH CH CO as one of the central groups linking three phenyl
rings. The resulting molecular rigidity and flexibility [5,6] from three phenyl rings bonded
through two central groups and a lateral/terminal substituent group or groups cause suit-
able magnitudes of anisotropic intermolecular forces of attraction to induce mesophase
formation in a substance. The novel homologous series consists of three phenyl rings
∗Address correspondence to H. N. Chauhan, K. K. Shah Jarodwala Maninagar Science College,
Ahmedabad, Gujarat, India. E-mail: shreeyashailee@yahoo.in
92

Novel Liquid Crystalline Phenylethylenes
93
bridged through two central groups ( COO and CO CH CH ) as the rigid core and
n-alkoxy (–OR) and nitro ( NO2) as flexible units of a molecule. Thereby, the molecular
structural effect on liquid crystallinity in relation to thermal stability, commencement of
mesophase, mesophase range, and the types of mesophase are evaluated and discussed
in terms of molecular polarity and polarizability, length to breadth ratio, and molecular
shape.
Experimental
Synthesis
4-Hydroxybenzoic acid was alkylated by suitable alkylating agents (R-X) to form
4-n-alkoxybenzoic acids, which are converted into the corresponding acid chlorides
(A) using freshly distilled thionyl chloride by the method of Dave and Vora [7]. 4-
Hydroxyacetophenone was treated with 2-nitrobenzaldehyde by the known method [1,7] to
form α-4-hydroxy benzoyl-β-2ꢀ-nitro phenyl ethylene (B). Components (A) and (B) were
condensed in ice-cooled pyridine [2,8] to form final products. The synthetic route to the
series is shown in Scheme 1.
Scheme 1. Synthetic route to the series.

94
H. N. Chauhan and A. V. Doshi
Table 1. Elemental analysis for butoxy, hexyloxy, and dodecyloxy derivatives
Elements found (% calculated)
Sr. No.
Molecular formula
C
H
N
1
2
3
C26H23O6N
C28H27O6N
C34H39O6N
70.31 (70.07)
71.64 (71.02)
73.65 (73.26)
5.47 (5.14)
5.11 (5.74)
7.10 (6.98)
3.35 (3.13)
2.66 (2.98)
2.82 (2.49)
Characterization
Representative members of the novel series were characterized by elemental analysis
(Table 1), infra-red (IR), and Hydrogen-1 Nuclear Magnetic Resonance (1HNMR) spec-
troscopy. Microanalysis was performed on Perkin-Elmer PE2400 CHN analyser. IR spectra
1
were recorded on Perkin-Elmer GX spectrometer, and HNMR spectra were recorded on
Bruker instrument using CDCl3 as a solvent. The textures of the nematic and smectic
mesophases were determined by a miscibility method using optical polarizing microscopy.
Analytical Data
Spectral Data
NMR in ppm for decyloxy derivative
0.99 ( CH3 of OC10H21), 1.17 ( (CH2)n of OC10H21), 3.75 ( OCH2 of OC10H21),
6.73 & 6.63 ( CO CH CH ), 7.53, 7.65, and 8.01 (phenyl ring). The data support the
structure.
NMR in ppm for tetradecyloxy derivative
0.94 ( CH3 of OC14H29), 1.43 ( (CH2)n of OC14H29), 3.23 ( OCH2 of OC14H29)
,
3.91 ( CH CH ), 6.97 ( CO CH CH ), 7.51,7.77, and 8.20 (phenyl ring). The data
support the structure.
IR in cm−1 for propoxy derivative
700 ( (CH2)n of OC3H7), 840, 880 (phenyl ring), 1160 ( CO of OC3H7), 1250,
1600, and 1700 ( COO ), 1430 ( NO2), 970 ( CH CH ). The data support the
structure.
IR in cm−1 for ethoxy derivative
690 ( (CH2)n of OC2H5), 825 (phenyl ring), 1160 ( CO of OC2H5), 1250,
1600, and 1680 ( COO ), 1395, 1430 ( NO2), 930 ( CH CH ). The data support the
structure.
Texture by Miscibility Method
• Dodecyl–Nematic–Threaded
• Octyl–Smectic–Smectic-A Type
• Hexadecyl–Nematic–Schlieren

Novel Liquid Crystalline Phenylethylenes
95
Table 2. Transition temperatures (in ◦C) of series
Transition temperatures in ◦C
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
8
10
12
14
16
–
–
–
–
190.0
213.0
182.0
165.0
160.0
161.0
164.0
164.0
160.0
162.0
156.0
156.0
122.0
112.0
112.0
120.0
124.0
120.0
128.0
–
162.0
140.0
122.0
124.0
132.0
142.0
142.0
140.0
140.0
10.
11.
Note: Sm: smectic; N: nematic.
Results and Discussion
Of the 11 members of the novel homologous series, α-4-[4ꢀ-n-alkoxy benzoyloxy] benzoyl
β-2ꢀꢀ-nitro phenyl ethylenes, the methoxy and ethoxy derivatives are non-mesomorphic,
while the propoxy to hexadecyloxy derivatives are enantiotropic liquid crystals. The hex-
adecyloxy derivative exhibits only the nematic mesophase without the exhibition of any
smectogenic character. The remaining mesogenic homologues exhibit smectic and nematic
mesomorphism. The transition temperatures of the homologues, as recorded in Table 2, are
plotted versus the number of carbon atoms present in the n-alkoxy terminal chain of the
members of the series to obtain a phase diagram. Like points are joined to draw the transition
curve to understand the phase behavior of the series as shown in Fig. 1. The solid-isotopic or
solid-mesomorphic transition curve follows a zigzag path of rising and falling values with
an overall falling nature and behaves in a normal manner. The smectic–nematic transition
curve initially adopts descending tendency and then rises, passes through a maximum, and
finally falls gradually up to the tetradecyloxy derivative. The curve is extrapolated to the
non-smectogenic hexadecyloxy derivative to predict a latent transition temperature (LTT)
[9] for the smectic phase, which coincides with the solid-nematic transition point (140◦C).
The nematic–isotropic transition curve follows a descending tendency as the series is as-
cended and behaves in normal expected manner. An odd-even effect is observed for the
smectic–nematic and nematic–isotropic transition curves with a narrow temperature range.
Both curves for the odd and even members merge into each other at the seventh member of
the series. The odd-even effect diminishes for higher homologues from seventh homologue
because the longer n-alkyl chain may coil, bend, or flex or couple to lie in the line of major
axis of the core [10]. The nematogenic phase range varies from a minimum of 16◦C at the
hexadecyloxy derivative to a maximum of 38◦C at the pentyloxy derivative. Similarly, the
smectogenic mesophase range varies from a minimum of 6◦C at the propoxy derivative
to a maximum of 22◦C at the dodecyloxy derivative. The non-mesomorphic behavior of
methoxy and ethoxy derivatives of the series is attributed to the high crystallizing tendency
of homologues that arises from their relatively stronger intermolecular attractions due to

96
H. N. Chauhan and A. V. Doshi
Figure 1. Phase behavior of series.
the presence of highly polar and shorter n-alkyl chains, which causes unsuitable molecu-
lar rigidity and flexibility [5,6]. Therefore, both methoxy and ethoxy derivatives sharply
transform into isotropic liquid from solid state without passing through a liquid crystal
phase. The exhibition of the nematic mesophase from the propoxy to hexadecyloxy deriva-
tives is attributed to the dis-alignment of molecules on gradual heating at an angle less
than 90◦, withstanding the exposed thermal vibrations which results in suitable magnitudes
of anisotropic intermolecular forces of end-to-end attractions and allows the molecules
to float with statistically parallel orientational order in two-dimensional array within a
definite temperature range reversibly. The smectogenic behavior showed by propoxy to
tetradecyloxy homologues within a definite range of temperature reversibly is attributed to
the presence of lamellar packing of molecules in their crystal lattices which forms a sliding

Novel Liquid Crystalline Phenylethylenes
97
Table 3. Average thermal stability in ◦C
Series
Series-1
Series-X
Series-Y
125.6 (C6–C14)C6
Smectic–nematic or isotropic 138.0 (C3–C14)C3
—
commencement of smectic
phase
Nematic–isotropic
commencement of nematic
phase
163.8 (C3–C16)C3 171.5 (C5–C10)C5 154.6 (C3–C16)C3
layered arrangement of molecules in a floating condition, resisting the exposed thermal
vibrations due to suitable magnitudes of anisotropic intermolecular forces of attractions.
The absence of smectogenic character shown by the hexadecyloxy derivative indicates the
absence of lamellar packing of molecules in its crystal lattices. The odd-even effect for the
smectic–nematic and nematic–isotropic transition curve, as well as variation in mesomor-
phic properties observed from homologue to homologue in the same series, is due to the
sequentially added methylene unit in the n-alkoxy terminal chain [10]. Liquid crystal prop-
erties of the novel homologous series, including average thermal stabilities of the smectic
and nematic mesophases, are compared (Table 3) with structurally similar/isomeric series
as shown in Fig. 2.
All the homologous series under comparison contain three phenyl rings bridged
through COO or CH CH COO and CO CH CH or CH CH CO cen-
tral groups. The n-alkoxy ( OR) terminal end group is commonly present in all homologous
series, i.e., series-1, series-X [4], and series-Y [3], and the NO2 group features in either
Figure 2. Structurally similar/isomeric series.

98
H. N. Chauhan and A. V. Doshi
terminal or lateral position. Therefore, molecular rigidity and flexibility [5,6] vary from
series to series due to varying n-alkoxy terminal end group and varying positional differ-
ence of NO2 group as well as variation arising from central groups from series to series
except molecular aromaticity. However, molecular aromaticity can also vary with molecu-
lar polarity and polarizability, length to breadth ratio, and hence the suitable or unsuitable
magnitudes of anisotropic forces of intermolecular attractions [8,10] which are directly
related with the tendency of inducing or not inducing liquid crystal mesophase in a sample
substance under examination. As mentioned above, even minor structural variations affect
molecular rigidity and flexibility [5,6] and hence cause variation in mesogenic properties
in terms of the degree of mesomorphism, thermal stability, commencement of mesophase
formation (Table 3), transition temperatures, and mesophase range.
Homologous series-1 and series-Y are structurally identical with respect to n-alkoxy(-
OR) terminal end group, three phenyl rings, and COO central groups, but differ in
their substitutional position of NO2 and the linking of the CO group to the phenyl
ring in series-1 and series-Y. The observed difference of smectic and nematic thermal
stability as well as the commencement of mesophases can be linked with the structural
difference of series-1 and series-Y. The molecules of series-1 are broader and shorter
than those of series-Y. Therefore, the length to breadth ratio and the ratio of molecular
polarity to polarizability cause suitable magnitudes of anisotropic intermolecular forces
of adhesion. Broader molecules of series-1 with relatively higher molecular width than
the molecular width of series-Y exerts two opposing effects of intermolecular attractions
at a time, as (i) higher molecular width decreases intermolecular closeness and reduces
intermolecular adhesion on the one hand, and (ii) increases molecular polarizability and
enhances intermolecular adhesion on the other hand. The resultant effect depends upon
the predominating effect of the two opposing effects, i.e., (i) and (ii) [10]. Thus, effect
(ii) due to molecular polarizability is a predominating effect for series-1 of the present
investigation for the smectic and nematic average thermal stabilities, which are higher
than series-Y. This indicates that though intermolecular closeness of the linearly shaped
molecules of series-Y whose intermolecular adhesion are expected to be more than series-
1, but, molecular polarizability of series-1 exceeds the magnitudes of anisotropic forces
of intermolecular attractions. On comparing series-1 with series-X, both differ in their
one central group, i.e., COO and CH CH COO respectively, but the rest of the
structure is identical. However, series-X is only nematogenic, while series-1 is smectogenic
in addition to nemtogenic mesophase, with average nematic isotropic thermal stabilities
of 171.5◦C (series-X) and 163.8◦C (series-1) respectively. Therefore, the difference in
thermal stability can be explained on the basis of the effect due to carboxy COO and
vinylcarboxylate CH CH COO central groups. Thus, in all other respects, except
central bridges, which are COO and CH CH COO , series-1 and series-X under
discussion are closely similar to each other. Both these central bridges are comparable,
although the vinylcarboxylate CH CH COO has greater length, by the CH CH
unit, increased conjugation by the CH CH , which increases the molecular rigidity
[5,6] and lateral attractions and hence the intermolecular adhesion increases, while in the
case of series-Y, the COO central bridge is shorter. Consequently, the stereochemistry
of molecule does, however, preserve the linearity, but results in less thermally stable
nematogenic mesophase. Thus, thermal stability for nematic of series-1 (163.8◦C) is less
than that of series-X (171.5◦C). Moreover, the vinylcarboxylate being longer than COO
causes more non-coplanarity due to a twist caused by interference of the vinylcarboxylate
group and the adjacent hydrogen atoms of aromatic rings. On account of the difference
caused in the extent of non-coplanarity [4,8], the commencement of smectic phase takes
place from the third member in series-1, it commences from the sixth member of series-Y,

Novel Liquid Crystalline Phenylethylenes
99
and does not commence until hexadecyloxy homologue in series-X. This suggests that
the lamellar packing of molecules in crystal lattices of homologues of series-1 enabled
to facilitate from propyloxy derivative to tetradecyloxy derivative, while it is facilitated
from hexyloxy derivative to tetradecyloxy derivative in the case of series-Y. Variations in
mesogenic properties for the same homologue from series to series is attributed to the
NO2 group of fixed polarity substituted at the ortho or para position.
Conclusions
1. Molecular rigidity and flexibility can vary by the same functional group of fixed
polarity if substituted at different positions (ortho, meta, para) of the same phenyl
ring.
2. Magnitudes of anisotropic intermolecular forces of attraction vary with functional
group of the same polarity if substituted at ortho, meta, or para position of the same
phenyl ring.
3. The group efficiency order derived for the smectic and nematic mesophases on the
basis of thermal stability is as follows:
Smectic: ortho NO2 (series-1) > para NO2 (series-Y) with.
with CO-CH CH
CH CH-CO
Nematic: ortho NO2 > ortho NO2 > para NO2
with with with
CH CH-COO
(series-X)
COO
(series-1)
COO
(series-Y)
4. Group efficiency order derived on the basis of the early commencement of the
smectic and nematic phases is as follows:
Smectic: ortho NO2 > para NO2 > ortho NO2
with with with
COO
COO- CH CH-COO
(series-1) (series-Y) (series-X)
Nematic: ortho NO2 para NO2 > ortho NO2
with with with
COO
COO
CH CH-COO
(series-X)
(series-1) (series-Y)
5. Homologous series is predominantly nematogenic and partly smectogenic.
6. Molecular rigidity and flexibility are important factors for the construction of geo-
metrical shape of a novel molecule to induce liquid crystalline property.
Acknowledgments
Authors acknowledge thanks to the principal, Dr. R.R. Shah, and Dr. R. B. Patel, head
chemistry department, K. K. Shah Jarodwala Maninagar Science College, Ahmedabad, and
the authorities of S.P.T. Arts and Science College, Godhra, for their valuable co-operation.
Authors also thank Ankita A. Doshi, department of biochemistry, M.S. University of
Baroda, for her valuable support in this work. Thanks are due to Vaibhav laboratory for
analytical services.

100
H. N. Chauhan and A. V. Doshi
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