New mesogenic homologous series of schiff base cinnamates comprising naphthalene moiety

Article abstract of DOI:10.1080/10587250307066

New mesogenic homologous series of Schiff base cinnamates comprising a naphthalene moiety were prepared by condensing different 4(4'-n-alkoxy cinnamoyloxy) benzaldehydes with 2-amino naphthalene. All the synthesized compounds were characterized by combination of elemental analysis and spectroscopic techniques. All the homologues synthesized exhibit mesomorphism. The mesomorphic properties of the present series are compared with other structurally related series.

Full text of DOI:10.1080/10587250307066

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New Mesogenic Homologous
Series of Schiff Base
Cinnamates Comprising
Naphthalene Moiety
A. K. Prajapati a , Viral Thakkar a & Nilesh Bonde a
a Department of Applied Chemistry, Faculty of
Technology and Engineering , M. S. University of
Baroda , P. B. No. 51, Kalabhavan, Vadodara, 390
001, India
Published online: 18 Oct 2010.
To cite this article: A. K. Prajapati , Viral Thakkar & Nilesh Bonde (2003)
New Mesogenic Homologous Series of Schiff Base Cinnamates Comprising
Naphthalene Moiety, Molecular Crystals and Liquid Crystals, 393:1, 41-48, DOI:
10.1080/10587250307066
To link to this article: http://dx.doi.org/10.1080/10587250307066
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Mol. Cryst. Liq. Cryst., Vol. 393, pp. 41–48
Copyright # 2003 Taylor & Francis
1542-1406/03 $12.00+.00
DOI: 10.1080/15421400390202917
NEW MESOGENIC HOMOLOGOUS SERIES
OF SCHIFF BASE CINNAMATES COMPRISING
NAPHTHALENE MOIETY
A. K. Prajapati, Viral Thakkar, and Nilesh Bonde
Department of Applied Chemistry, Faculty of Technology and
Engineering, M. S. University of Baroda, P. B. No. 51,
Kalabhavan, Vadodara-390 001, India.
E-mail: akprajapati@yahoo.co.uk
New mesogenic homologous series of Schiff base cinnamates comprising a
naphthalene moiety were prepared by condensing different 4(40-n-alkoxy cin-
namoyloxy) benzaldehydes with 2-amino naphthalene. All the synthesized
compounds were characterized by combination of elemental analysis and
spectroscopic techniques. All the homologues synthesized exhibit mesomorph-
ism. The mesomorphic properties of the present series are compared with other
structurally related series.
Keywords: Schiff’s base cinnamates; naphthalene moiety; mesomorphism
INTRODUCTION
The mesomorphic behavior of an organic compound is sensitive to its
molecular architecture; a slight change in the molecular geometry brings
about considerable change in its mesomorphic properties. The detailed
study of many mesogenic homologous series has helped to evolve some
general rules for the effect of chemical constitution in the nematogenic and
smectogenic compounds [1]. Most of these studies have been on Schiff
bases or esters containing benzene or biphenyl nuclei; comparatively few
studies have been done on the influence of naphthalene nucleus on
mesomorphism. Gray [1] has reported that 6-n-alkoxy-2-naphthanoic acids
This paper was accepted for presentation at the 19th International Liquid crystal confer-
ence 2002 held at Edinburgh, UK during 30 June to July 2002.
The authors are thankful to Head of the Applied Chemistry Department and Dean, Faculty
of Technology and Engineering for providing research facilities. A. K. Prajapati and Nilesh
Bonde are also thankful to University Grant Commission New Delhi for a major research pro-
jects grant for the years 2001–2003.
41

42
A. K. Prajapati et al.
are mesomorphic whereas 1,4- and 1,5-alkoxy naphthoic acids core
nonmesomorphic. Dave and coworkers [2, 4] have synthesized a number of
4-n-alkoxy-1-naphthylidene Schiff bases and cholesteryl naphthoates and
studied their mesomorphism. Interest in naphthalene liquid crystal (LC)
cores has revived in the last decade, as indicated by a significant number of
research papers [5–13]. Previously we have reported few mesogenic
homologous series of Schiff’s base ester [14, 15] and azo-esters [16, 17]
containing naphthalene moiety. The ethylene linking group is very useful
structural unit connecting one part of a rigid core with another in calamitic
mesogene molecules. This fully conjugative group enhances the longi-
tudinal polarizability and extends the molecular length maintaining line-
arity of the molecule. The present investigation was undertaken with a view
to study the effect of an ethylene linking group (cinnamoyl linkage) and
naphthalene moiety on the mesomorphic properties of such molecules.
EXPERIMENTAL
Malonic acid, n-alkyl halide, K2CO3, p-hydroxy benzaldehyde, and 2-amino
naphthalene were used as received. Solvents were dried prior to use.
Microanalyses of compounds were performed on Coleman Carbon-Hydrogen
analyzer. Infrared (IR) spectra were determined via KBr pellets using
Shimadzu IR-408 spectrophotometer. 1H NMR spectra were obtained with a
Perkin-Elmer-32 spectrometer, using tetra methyl silane (TMS) as an
internal reference standard. The chemical shifts are quoted as d (parts per
million) downfield from the reference. CDCl3 was used as a solvent.
Ultraviolet (UV) spectra were recorded on a Hitachi U 2000 spectro photo-
meter. Liquid crystalline properties were investigated on a Leitz Laborlux
12 POL microscope provided with heating stage. The synthetic route of
series I is illustrated in Scheme 1.
Synthesis of Series I
First 4-n-Alkoxy benzaldehydes (A) and 4-n-alkoxy cinnamic acids (B)
were prepared by the method of Gray and Jones [18], and 4-n-alkoxy cinna-
moylchlorides were prepared by following the method described in the
literature [19]. Also, 4-(40-n-alkoxy cinnamoyloxy) benzaldehydes (C) were
synthesized by the method of Lohar and Patel [20].
The 12 Schiff’s bases of series I were synthesized by condensing equi-
molar quantities of appropriate 4-(40-n-alkoxy cinnamoyloxy) benzalde-
hydes with 2-aminonaphthalene in boiling ethanol. The Schiff’s bases of the
series I were crystallized from ethanol till the constants’ transition
temperatures were obtained. The elemental analysis of all the compounds

Mesogenic Schiff Base Cinnamates
43
SCHEME 1 Synthetic route to series I.
was found to be satisfactory, and all are listed in Table I. The UV, IR, and 1H
NMR spectral data of all the compounds were found to be consistent with
the proposed structure. As a representative case we provide spectral data
for the compound 6: UV(CHCl3) lmax: 271 and 292 nm; IR nmax /cmꢀ1: 3055,
1
2930, 1724(-COO-), 1633, 1602(-CH¼N), 1510, 1423, 1142, 1174, 814; H
NMR(CDCl3): d 0.95( t, 3H, -CH3), 1.35–1.8(m, 8H, 4 x -CH2-), 3.95(t, 2H,
Ar-O-CH2-), 6.5(d, J ¼ 18.6 Hz, 2H, -CH¼CH-), 6.9(d, J ¼ 9 Hz, 2H, ArH),
7.2–7.4 (m, 2H, ArH), 7.4–7.6(m, 7H of naphthalene ring system), 7.7–
7.9(m, 2H, ArH), 8.0(d, J ¼ 9 Hz, 2H, ArH), 8.6(s, 1H, -CH¼N).
RESULTS AND DISCUSSION
The transition temperatures of the compounds of series I are recorded in
Table II.
All the twelve members exhibit enantiotropic nematic phase. The SmC
phase commences from the n-octyloxy derivative as enantiotropic meso-
phase.

44
A. K. Prajapati et al.
TABLE I Elemental Analysis for Series I Compounds
% Required (Found)
H
Molecular
formula
Sr. No
R ¼ -CnH2nþ1 n –
C
N
1
1
2
C
27H21O2N
82.86 (82.66)
82.96 (82.94)
83.05 (82.89)
83.52 (83.54)
83.97 (83.88)
84.03 (83.94)
84.43 (84.54)
84.95 (84.85)
85.32 (85.38)
86.08 (86.08)
86.77 (86.87)
87.39 (87.48)
5.37 (5.45)
5.67 (5.62)
5.97 (5.80)
6.26 (6.15)
6.55 (6.64)
6.76 (6.83)
7.04 (7.15)
7.25 (7.38)
7.63 (7.78)
7.98 (7.86)
8.29 (8.52)
8.57 (8.55)
3.58 (3.50)
3.46 (3.62)
3.34 (3.32)
3.25 (3.28)
3.16 (3.28)
3.06 (3.15)
2.99 (3.06)
2.90 (2.83)
2.74 (2.73)
2.60 (2.62)
2.47 (2.58)
2.35 (2.31)
2
C28H23O2N
29H25O2N
C30H27O2N
31H27O2N
C32H29O2N
3
3
C
4
4
5
5
C
6
6
7
7
C
33H31O2N
34H33O2N
8
8
C
9
10
12
14
16
C36H37O2N
38H41O2N
C40H45O2N
42H49O2N
10
11
12
C
C
Figure 1 shows the plot of transition temperatures against the number of
carbon atoms in the alkoxy chain, from which it can be noticed that crystal
mesophase transition temperatures decrease with the increase in the
length of terminal alkoxy tail. The nematic-isotropic transition tempera-
tures also decrease with increase in the number of carbon atoms in the
alkoxy chain, whereas it exhibits a tendency for rising smectic C-nematic
transition temperature in ascending series, which levels off slightly in the
higher homologues.
TABLE II Transition Temperatures (ꢁC) for Series I Compounds
Compound
n
Cr
SmC
N
I
1
1
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
131
126
122
117
114
112
110
98
—
—
—
—
—
—
—
ꢂ
—
—
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
265
261
254
249
247
245
236
230
224
215
197
193
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
2
2
3
3
—
—
4
4
5
5
—
—
6
6
7
7
—
8
8
146
161
155
141
138
9
10
12
14
16
93
89
ꢂ
10
11
12
ꢂ
86
85
ꢂ
ꢂ
Cr, crystal; SmC, smectic C phase; N, nematic phase; I, isotropic liquid state; n, number of
carbon atoms in the alkoxy chain.

Mesogenic Schiff Base Cinnamates
45
FIGURE 1 The phase behavior of series I.
Table III shows the average mesophase range of the present series I and
structurally related series A [14], B [14], and C [21] reported in the litera-
ture. The average smectic mesophase range of series I is higher by 66.1ꢁC
and the nematic mesophase range is higher by 66.0ꢁC compared to the
respective mesophase ranges of series A. This is understandable, as
the molecules of series I are longer and more polarizable compared to the
molecules of series A due to the presence of additional cinnamoyloxy
(-CH¼CH-COO-) central linkage and benzene moiety.
Reference to Table III indicates that the average width of smectic and
nematic mesophase of series I is higher by 25.6ꢁC and 14.5ꢁC, respectively
compared to series B. The molecules of series I and series B differ only at
the central linkages. The molecules of series I have cinnamoyloxy
(-CH¼CH-COO-) central linkage, while series B have ester (-COO-) central
linkage. Gray [1] has explained that the addition of double bond in

46
A. K. Prajapati et al.
ꢁ
TABLE III Average Mesophase Range in C
Commencement of
smectic phase
Series
Smectic
Nematic
I(C8–C12
)
67.1
01.0
41.5
69.8
69.0
03.0
54.5
17.5
C8
Only for C12
A(C12
B(C10, C12
C(C8–C12
)
)
C7
C6
)
Comparative Molecular Structure of Series I, A, B, and C.
the system increases the length and polarizability of the rodlike molecules.
Therefore the greater mesophase thermal stability of the present series I
must be explained in terms of the greater molecular length and polariz-
ability of the molecule resulting from additional -CH¼CH- units in the
central linkage.
The naphthalene moiety at the terminus of the present series I has a
pronounced effect on the mesophase transition temperatures when com-
pared with the parent compound of series C having benzene moiety at the
terminus. A comparison of mesophase thermal stabilities of series I and
series C reveals that there is an average decrease of about 1.7ꢁC in smectic
mesophase range and average increase of about 51.5ꢁC in nematic
mesophase range of series I. Compared with molecules of series B, the
molecules of series I are a little longer and more polarizable, but have
increase breadth due to the naphthalene moiety. The reduction of average
smectic mesophase range has been attributed to a thickening effect [1]

Mesogenic Schiff Base Cinnamates
47
caused by the presence of naphthalene moiety. However, this thickening
effect is not so prominent due to 2-substitution in the naphthalene nucleus
of the molecules, making them linear. Therefore, nematic thermal stability
of series I is probably higher. Thus it seems that the smectic mesophase is
more affected by thicking effect than the nematic mesophase. This is in
accordance with Gray’s [1] work on various mesomorphic homologous
series of compounds, which show a similar behavior.
CONCLUSION
In this article we have presented the synthesis and characterization of new
mesogenic homologous series of Schiff’s base cinnamates comprising
naphthalene moiety. All the compounds of the series exhibit enantiotropic
mesomorphism. The mesophase range of present series I is higher than
those of structurally related compounds that has been attributed to the
high polarizability of the molecule. However, the smectic mesophase phase
is more adversely affected than the nematic mesophase by the thickening
effect due to presence of naphthalene moiety.
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