Synthesis of novel mesomorphic homologous series: α-4-[-4′-n- alkoxy cinnamoyloxy] phenyl-β3″ nitrobenzoyl ethylenes

Article abstract of DOI:10.1080/15421406.2012.698234

Eleven novel mesomorphic homologues were synthesized and evaluated for their mesomorphic properties. The methoxy to n-butyloxy homologues are nonmesomorphic, while the other homologues are enantiotropic mesomorphic. The n-pentyloxy, n-hexyloxy, and n-octyloxy homologues are polymesomorphic, i.e., smectic and nematic mesophases are exhibited enantiotropically. Only the nematic mesophase is exhibited by the n-decyloxy to n-hexadecyloxy homologues. Analytical data confirm the structure and textures of molecules. The nematic mesophase shows a threaded-type texture, and that of smectic phase is of smectic A type. The mesomorphic properties are compared with a structurally similar homologous series. The average thermal stability for the smectic and nematic mesophases is 154.6C and 163.8C, respectively, as observed through optical polarizing microscope with heating stage. Copyright Taylor & Francis Group, LLC.

Full text of DOI:10.1080/15421406.2012.698234

Mol. Cryst. Liq. Cryst., Vol. 569: pp. 49–56, 2012
Copyright © Taylor & Francis Group, LLC
ISSN: 1542-1406 print/1563-5287 online
DOI: 10.1080/15421406.2012.698234
Synthesis of Novel Mesomorphic Homologous
Series: α-4-[-4ꢀ-n-Alkoxy cinnamoyloxy]
Phenyl-β-3ꢀꢀ Nitrobenzoyl Ethylenes
R. P. CHAUDHARI1,∗ AND A. V. DOSHI2
1Chemistry Department, K. K. Shah Jarodwala Maninagar Science College,
JJT University Rajasthan, Ahmedabad, Gujarat, India
2JJT University, Jhunjhunu, Rajasthan, India
Eleven novel mesomorphic homologues were synthesized and evaluated for their meso-
morphic properties. The methoxy to n-butyloxy homologues are nonmesomorphic, while
the other homologues are enantiotropic mesomorphic. The n-pentyloxy, n-hexyloxy, and
n-octyloxy homologues are polymesomorphic, i.e., smectic and nematic mesophases are
exhibited enantiotropically. Only the nematic mesophase is exhibited by the n-decyloxy
to n-hexadecyloxy homologues. Analytical data confirm the structure and textures of
molecules. The nematic mesophase shows a threaded-type texture, and that of smectic
phase is of smectic A type. The mesomorphic properties are compared with a struc-
turally similar homologous series. The average thermal stability for the smectic and
nematic mesophases is 154.6◦C and 163.8◦C, respectively, as observed through optical
polarizing microscope with heating stage.
Keywords Liquid crystal; mesogen; mesomorphic; nematic; smectic
Introduction
Various central groups linking two phenyl rings have been widely reported previously,
except for the –CH CH–CO– linking group [1–3]. Lohar and Doshi [1a] tried to exploit
–CH CH–CO– linking group between two phenyl rings, but did not succeed in generat-
ing liquid crystal mesophase formation, largely because of an unsuitable combination of
molecular rigidity and flexibility [1–4].
However, Doshi continued attempts to exploit the –CH CH–CO– linking group in
combination with other groups linking three phenyl rings. Chauhan and Doshi [1b] and
Patel and Doshi [1c] successfully observed mesophase formation in the molecules of a
homologous series, which included the –CH CH–CO– moiety as one of the central groups
in combination with –COO– and –CH CH–COO– central groups, respectively, bridging
three phenyl rings. Thus, suitable magnitudes of molecular rigidity and flexibility have a
direct relation with molecular structure to cause mesophase formation with appropriate
selection of core, terminal, central linking groups, and/or lateral groups.
∗Address correspondence to R. P. Chaudhari, Chemistry Department, K. K. Shah Jarodwala
Maninagar Science College, JJT University Rajasthan, Ahmedabad 380008, Gujarat, India. E-mail:
rpchaudhari@ymail.com
49

50
R. P. Chaudhari and A. V. Doshi
Table 1. Elemental analysis for propyloxy, butyloxy, decyloxy, and dodecyloxy derivatives
Element % found (% calculated)
Molecular formula
C
H
N
C27H23NO6
C28H25NO6
C34H37NO6
C36H41NO6
70.62 (70.89)
73.35 (73.46)
73.35 (73.51)
73.98 (74.09)
5.07 (5.03)
5.33 (5.30)
6.69 (6.66)
7.11 (7.03)
3.02 (3.06)
2.92 (2.97)
2.46 (2.52)
2.36 (2.40)
The novel homologous series reported here consists of three phenyl rings bridged
through –CH CH–COO– and –CH CH–CO– central groups with a left n-alkoxy terminal
end group and a meta-substituted nitro group at the third phenyl ring to broaden the
molecule. Hence, the effect of such molecular structure on mesophase formation can
be correlated in terms of molecular polarity, polarizability, aromaticity, molecular width,
or broadening, and the results are discussed and compared with a structurally similar
homologous series.
Experimental
Characterization
Selected homologues of the novel series were characterized by infrared (IR) and 1H NMR
techniques. Nuclear magnetic resonance (NMR) spectra were recorded using CDCl3 as sol-
vent and IR spectra were recorded using the PerkinElmer GX spectrometer. Microanalysis
was performed on the PerkinElmer PE 2400 CHN analyzer with results shown in Table 1.
The mesogenic properties, transition, and melting temperatures were investigated by using
optical polarizing microscopy. The textures of the smectic and nematic mesophases were
determined by the miscibility method.
Synthesis
4-Hydroxycinnamic acid was synthesized by reacting 4-hydroxybenzaldehyde and malonic
acid. The alkylation of the hydroxy group was performed using the suitable corresponding
alkylating agent. 4-Hydroxybenzaldehyde, malonic acid, alkylating agents, MeOH, KOH,
SOCl2, EtOH, m-nitroacetophenone, etc., required for the synthesis were used as received
(Scheme 1).
α-4-Hydroxy phenyl β-3ꢀ-nitrobenzoyl ethylene (y) was synthesized by the method
of Patel and Doshi [1c]. 4-n-Alkoxycinnamic acids and corresponding acid chlorides
were prepared by a modified method of Dave and Vora [5]. The 11 benzoylesters were
synthesized by condensing equimolar proportions of 4-n-alkoxycinnamoyl chloride (x)
and α-4-hydroxy phenyl β-3ꢀ-nitrobenzoyl ethylene [6,7] (y) in ice-cool dry pyridine.
The products were crystallized from alcohol until constant transition temperatures were
obtained.

Novel Liquid Crystalline Nitrobenzoyl Ethylenes
51
Scheme 1. Synthetic route to the series.
Analytical Data
NMR in ppm for Octyloxy Derivative
0.886 (–CH3 of alkoxy), 1.288 (–CH2– of alkoxy), 1.806 (–OCH2–CH2), 4.019 (–OCH2 of
alkoxy), 6.943, and 8.035 (p-sub. phenyl ring) 7.259 (complex) (m-sub. phenyl ring). NMR
supports the structure.
IR in cm−1 for Hexyloxy Derivative
650 and 760 (m-sub. phenyl ring), 840 (p-sub. phenyl ring), 950 trans (–CH CH–), 1160
(C–O of alkoxy), 1350 and 1410 (–NO2 group), and 1260, 1700 (–COO– group). IR
supports the structure.
IR in cm−1 for Tetradecyloxy Derivative
650 and 770 (m-sub. phenyl ring), 850 (p-sub. phenyl ring), 1150 (C–O of alkoxy), 1250
and 1700 (–COO group), and 1420 and 1500 (–NO2 group). IR supports the structure.

52
R. P. Chaudhari and A. V. Doshi
Table 2. Transition temperature of series in ◦C
Transition temperatures in ◦C
Compound No
R = –CnH2n+1 (n)
Sm
Nm
Isotropic
1
2
3
4
5
6
7
8
9
1
2
3
4
5
6
8
10
12
14
16
–
–
–
–
–
–
–
153.0
171.0
150.0
162.0
176.0
178.0
170.0
160.0
153.0
152.0
158.0
–
138.0
132.0
134.0
–
–
–
–
149.0
158.0
157.0
144.0
140.0
126.0
148.0
10
11
Note: Sm – smectic; Nm – nematic.
Texture
The pentyloxy homologue shows a threaded type texture and the hexyloxy homologue
shows a smectic A type texture.
Results and Discussion
The reported novel homologous series consists of 11 members, which were evaluated
for their mesomorphic characteristics. The transition temperatures of the series under
discussion are recorded in Table 2. The pentyl to hexadecyl members of the series exhibit
threaded or Schlieren textures on heating the solid crystalline or a smectic mesophase
reversibly on heating and cooling to isotropic liquid, while the methyl to butyl homologues
are nonmesomorphic. The pentyl, hexyl, and octyl members of the series show a smectic
A texture on heating the crystalline solid reversibly.
Thus, the pentyl, hexyl, and octyl homologues are enantiotropically polymesomorphic
with the exhibition of smectic and nematic mesophases, while the decyl, dodecyl, tetradecyl,
and hexadecyl derivatives of the series are enatiotropically nematogenic only without ex-
hibition of any smectogenic character even in the monotropic condition. Table 2 represents
the dependence of transition temperatures on the number of carbon atoms in the terminal
n-alkyl chain of molecules. The average thermal stability and other mesomorphic behavior
of the titled homologous series 1 are compared with the structurally similar homologous
series A as shown in Fig. 1.
The phase behavior of the homologues of the series under discussion is shown in
Fig. 2. The solid mesomorphic or isotropic transition curve adopts a zigzag path of rising
and falling tendency as the series is ascended. The plot of the phase diagram (Fig. 2) shows
absence of odd–even effect in the nematic–isotropic and smectic–nematic transition curves.
Smectic–nematic transition curve initially rises, passes through maxima, and then falls
smoothly, and on extrapolation [8] it exactly merges and coincides into the solid–nematic
transition temperature (144◦C) of the decyl derivative indicating and proving disappearance

Novel Liquid Crystalline Nitrobenzoyl Ethylenes
53
Figure 1. Structurally similar homologous series.
of smectogenic character of the present series from and beyond the decyl derivatives of the
series.
The nematic–isotropic transition curve shows a descending tendency as the series is
ascended except for the final derivative, which shows a negligible rise of 6◦C. Thus, all
three curves of the present series behave in a normal manner, i.e., the behavior of the
Figure 2. Phase behavior of series.

54
R. P. Chaudhari and A. V. Doshi
Table 3. Average thermal stability in (◦C)
Series
(1)
(A)
Nematic–isotropic
163.8 (C5–C16)
144.5 (C6–C16)
Commencement of nematic phase
Smectic–isotropic
Commencement of smectic-phase
C5
154.6 (C5–C8)
C5
C6
–
–
nematic–isotropic transition curve suggests that the enthalpy change value of the homo-
logue gradually decreases from the pentyl to tetradecyl derivative and slightly increases in
case of the hexadecyl derivative. Similarly, the behavior of the smectic–nematic transition
curve suggests that the enthalpy change values corresponding to the pentyl, hexyl, and
octyl homologues initially increase and then decrease to reach a minimum for emergence
of the smectic mesophase. Hence, a low and high value of enthalpy corresponds to a low
and a high amount of heat energy. On comparing the molecular geometry and other pa-
rameters affecting intermolecular attractions, series 1 and A differ in respect to the lateral
substitution of the –NO2 group (series 1). Therefore, series A is linear, while series 1 is
nonlinear. The opposing effects [9,10] to the molecular geometry of the meta-substituted
nitro group operates (1) in effect due to the widening of a molecule causing the reduction
in the intermolecular adhesion by increasing the intermolecular distance, and on other hand
(2) increased molecular width increases molecular polarizability causing an increase in
the intermolecular adhesion. Thus, closeness of molecular packing depends upon the net
resultant intermolecular forces of attractions, which depends upon the predominating effect
out of two opposing effects (1) and (2). In the present study, the later effect is the predom-
inating effect. Table 3 summarizes the average thermal stabilities and molecular structure
of the present series and other structurally related series A [1c] chosen for the comparative
study.
The molecular geometry of homologous series 1 and A under comparison consists of
three phenyl rings linked through the –CH CH–COO– and –CH CH–CO– central link-
ages with the n-alkoxy left terminal group, and meta substituted –NO2 and –H on the third
phenyl ring for series 1 and A, respectively. The variation in mesomorphic characteristics
can be attributed to the effect of the presence of the meta substituted –NO2 and –H in the
third phenyl ring, which causes a different magnitude of anisotropic forces of attractions as
a consequence of their molecular rigidity and flexibility [2–4] related to molecular polarity
and polarizability for the occurrence of mesophase formation [8,11]. On comparing the
mesomorphic characteristics of series 1 with series A, it is observed that the homologues
of both the series differing in respect of only a meta substituted nitro group of series 1.
The introduction of the –NO2 group in place of –H causes a considerable and sig-
nificant difference in the magnitude of the anisotropic forces [9,10] for the pentyloxy to
hexadecyloxy homologues, which serve to stabilize a statistically parallel orientation, as the
order of the molecules by end-to-end attractions is conducive to nematogenic mesophase
formation. This effect persists even in the homologues with relatively longer n-alkoxy
groups. Such nematogenic mesophase formation takes place in the case of a series A with a
linear molecular geometry in which the –NO2 group is replaced by a –H from the hexyloxy
to hexadecyloxy homologues. The presence of the highly polar nitro group in the present
series 1 promotes the formation of a smectic mesophase due to enhanced lateral attractions
in addition to the end-to-end terminal attractions, which facilitates the lamellar packing

Novel Liquid Crystalline Nitrobenzoyl Ethylenes
55
required for occurrence of the smectic mesophase formation [5] for the pentyl, hexyl, and
octyl homologues. Thus, series 1 exhibits smectogenic character in addition to nematogenic
character, while smectogenic character is absent in case of series A.
The higher smectic and nematic thermal stabilities of series 1 when compared with se-
ries A are understandable because of the difference in the suitable magnitude of anisotropic
forces of intermolecular attractions related to molecular polarity and polarizability from
the –NO2 and –H groups. The early commencement of smectic mesophase is attributed
to suitable extent of noncoplanarity caused by the molecules of series 1, while it does not
commence until the final hexadecyl derivative of series A. The variation observed in meso-
morphic properties from homologue to homologue in the same series is due to sequential
addition of a methylene unit. Likewise, the variation from series to series is attributed to
the change from the –NO2 group to the –H.
Conclusions
(1) The variation in molecular rigidity caused by the –CH CH–CO– central group
and/or the flexibility caused by the –NO2 group confers variation in liquid crystal
properties of a substance.
(2) The study suggests that the replacement of –H by highly polar –NO2 group though
favorable to induce the nematic mesophase can also induce smectogenic character
in the presence of the –CH CH–CO– central bridge of a molecule.
(3) The present novel series is predominantly nematogenic and partly smectogenic
with a middle order melting behavior and a 10◦C–46◦C mesophase length.
Acknowledgments
The authors acknowledge and thank the chairman and all the members of the management of
Bai Jivkor Lallubhai Trust as well as the Principal Dr. R. R Shah and Head of the chemistry
department Dr. R. B. Patel of K. K. Shah Jarodwala Maninagar Science College for their
high valued co-operation in this work. Also, thanks are due to the Vaibhav Laboratory for
analytical services as well as authorities of Arts & Science college Godhra for their valuable
co-operation.
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