Effect of a lateral chloro group on azomesogens containing the naphthalene moiety

Article abstract of DOI:10.1080/15421400390202908

A homologous series of azomesomesogens, 2″[4-(4′-n- alkoxybenzoyloxy)-3-chloro phenylazo] naphthalene, with lateral chloro group was synthesized. All twelve homologues of the series synthesized exhibit enantiotropic nematic mesophase. The plot of transition temperatures against the number of carbon atoms in the alkoxy chain shows a steady fall in nematic-isotropic transition tempera ture. The mesogenic behavior of the present series is compared with other structurally related series to evaluate the effect of lateral chloro group on mesomorphism.

Full text of DOI:10.1080/15421400390202908

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Effect of a Lateral Chloro
Group on Azomesogens
Containing the Naphthalene
Moiety
a
a
A. K. Prajapati & H. M. Pandya
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 & H. M. Pandya (2003) Effect of a Lateral Chloro
Group on Azomesogens Containing the Naphthalene Moiety, Molecular Crystals and
Liquid Crystals, 393:1, 31-39, DOI: 10.1080/15421400390202908
To link to this article: http://dx.doi.org/10.1080/15421400390202908
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Mol. Cryst. Liq. Cryst., Vol. 393, pp. 31–39
Copyright # 2003 Taylor & Francis
1542-1406/03 $12.00+.00
DOI: 10.1080/15421400390202908
EFFECT OF A LATERAL CHLORO GROUP
ON AZOMESOGENS CONTAINING THE
NAPHTHALENE MOIETY
A. K. Prajapati* and H. M. Pandya
Department of Applied Chemistry, Faculty of Technology
and Engineering, M. S. University of Baroda, P. B. No. 51,
Kalabhavan, Vadodara 390 001, India
00
0
A homologous series of azomesomesogens, 2 [4-(4 -n-alkoxybenzoyloxy)-3-chloro
phenylazo] naphthalene, with lateral chloro group was synthesized. All twelve
homologues of the series synthesized exhibit enantiotropic nematic mesophase.
The plot of transition temperatures against the number of carbon atoms in the
alkoxy chain shows a steady fall in nematic-isotropic transition tempera-
ture. The mesogenic behavior of the present series is compared with other
structurally related series to evaluate the effect of lateral chloro group on
mesomorphism.
Keywords: azobenzene derivative; lateral chloro group; nematic mesophase
INTRODUCTION
A vast number of mesogenic compounds showing nematic or other meso-
phases are reported [1–3] containing naphthalene moiety as core system.
Wiegand [4] reported few mesogenic Schiff base of 2,6-, 1,5- and 1,4-
diaminonaphthalenes. Gray and Jones [5] investigated liquid crystalline
properties of different alkoxy naphthoic acids. Coates and Gray [6] syn-
thesized 4-n-alkyl/alkoxy phenyl esters of 6-n-alkyl/alkoxy naphthalene 2-
carboxylic acid. Dave and coworkers studied a variety of liquid crystalline
compounds exhibiting smectic, nematic, and choleseric mesomorphism
This paper was accepted for presentation at the 19th International Liquid Crystal Confer-
ence 2002 held at Edinburg, UK during 30 June to 5 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 is thankful to
University Grant Commission New Delhi for a minor research projects grant for the year
2
001–2002.
Corresponding author. E-mail: akprajapati@yahoo.co.uk
*
31

3
2
A. K. Prajapati and H. M. Pandya
containing naphthalene moiety, such as alkozybenzoates of 1,5- and 1,4-
dihydroxynaphthalene [7], esters of cholesterol [8] and alkoxy naphthyli-
dene Schiff’s bases [9–11]. Malthete et al. [12] synthesized tetra-acylated
1
‘
,4,5,8-tetrahydroxynaphthalene derivatives, which may be looked upon as
‘conjoined twin’’ mesogens. In the last decade significant number of
research papers on naphthalene LC cores appeared in the literature
13–21]. Previously we have reported [22,23] the mesogenic homologous
[
series of Schiff’s base esters containing naphthalene moiety and studied the
effect of lateral thiol and methoxy substituent on mesomorphism. We have
also reported mesogenic homologous series of azomesogens without lateral
substituent [25] as well as lateral methyl group [25] and evaluated the effect
of lateral methyl group on mesomorphism. As an extension of this work, we
have replaced lateral methyl group by a polar chloro group and prepared a
00
0
0
homologous series of 2 [4(4 -n-alkoxybenzoyloxy) 3 -chloro phenylazo]
naphthalenes to investigate the influence of the lateral chloro group on
mesomorphism.
EXPERIMENTAL
4
-Hydroxy benxoic acid, the appropriate n-alkylhalides, o-chlorophenol,
and 2-aminonaphthalene were used as received. Solvents were dried and
distilled prior to use. Microanalysis of the compounds were performed on a
Coleman carbon-hydrogen analyzer. Infrared (IR) spectra were determined
1
via KBr pellets, using a Schimadzu IR-408 spectrophotometer. H NMR
spectra were obtained with a Perking-Elmer R-32 spectrometer using tet-
ramethylsiland (TMS) as an internal reference standard. The chemical shifts
are quoted as d (parts per million) downfield from the reference. CDCl was
3
used as solvant. UV spectra were recorded on a Hitachi U-2000 spectro-
photometer. Liquid crystalline properties were investigated on a Leitz
Laborlux 12 POL microscope provided with heating stage. The enthalpies of
transitions reported as J/g were determined from thermograms obtained on
ꢀ
a Mettler TA-400 system, adopting a scanning rate of 5 C/min.
The synthetic route to series I is illustrated in scheme 1. Using a con-
ventional method of diazotization and coupling [26], 2(4-Hydroxy-3-chlor-
ophenylazo)naphthalene(A) was synthesized. Also 4-n-Alkoxybenzoic acids
and 4-n-alkoxybenzoyl chlorides(B) were synthesized by the modified
method of Dave and Vora [27].
Synthesis of Series I Compounds
First 2(4-Hydroxy-3-chlorophenylazo)naphthalene (0.02 mol)was dissolved
in dry pyridine (5 ml), and a cold solution of a 4-n-alkoxybenzoyl chloride

Napthyl Azomesogens with Lateral Chloro Group
33
SCHEME 1 Synthetic route to series I compounds.
0.02 mol) in dry pyridine was added slowly with constant stirring in an ice
(
bath. The mixture was allowed to stand overnight at room temperature. It
was acidified with cold 1:1 dilute hydrochloric acid. The solid obtained was
separated, dried, and triturated by stirring for 30 min with 10% aqueous
sodium hydroxide, and was then washed with water.
The insoluble product was thus separated from the reactants; finally, all
the compounds of series I were crystallized from acetic acid until constant
transition temperatures were obtained. The elemental analysis of all the
compounds were found to be satisfactory and have been mentioned in
1
Table I. The ultraviolet (UV), IR, and H 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 compound 8 UV(CHCL )
3
ꢁ
1
lmax : 263, 338 nm; (KBr) nmax/cm : 2900, 1740 ((-COO-), 1610)-N¼N-),
1
600, 1520, 1475, 1410, 1360, 1205, 890, 850, 760, 680 (-C-Cl); : H NMR
1
CDCl : d 0.95(t, 3H, -CH ), 1.35–1.7(m, 12H, 6 x -CH -), 4.1(t, 2H, -OCH -),
3
3
2
2
0
0
6
7
.9(d, J ¼ 9 Hz, 2H at C-3 and C-5 ), 7.25(s, 1H at C-2), 7.40–8.30 (m, 9H,
0
H of naphthalene ring and 2H at C-5 and C-6), 8.5 (d, J ¼ 9 Hz, 2H at C-2
0
and C-6 ).

3
4
A. K. Prajapati and H. M. Pandya
TABLE I Elemental Analysis for Series I Compounds
%
Required (%found)
H
R ¼ -C
n 2nþ1
H
n ¼
Compound
Formula
C
N
1
2
3
4
5
6
7
8
9
1
2
C
24
C
25
C
26
C
27
C
28
C
29
C
30
C
31
C
33
C
35
C
37
C
39
H
17
H
19
H
21
H
23
H
25
H
27
H
29
H
31
H
35
H
39
H
43
H
47
N
2
N
2
N
2
N
2
N
2
N
2
N
2
N
2
N
2
N
2
N
2
N
2
O
3
O
3
O
3
O
3
O
3
O
3
O
3
O
3
O
3
O
3
O
3
O
3
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
69.15(69.28)
69.69(69.58)
70.19(70.06)
70.67(70.55)
71.11(71.42)
71.53(71.26)
71.93(72.17)
72.30(71.95)
72.99(72.73)
73.62(73.46)
74.18(74.28)
74.70(74.60)
4.08(4.28)
4.41(4.48)
4.72(4.92)
5.02(4.86)
5.29(5.38)
5.55(5.58)
5.79(5.84)
6.02(6.37)
6.45(6.66)
6.84(6.85)
7.18(7.26)
7.50(7.25)
6.72(6.55)
6.50(6.58)
6.30(6.21)
6.12(6.37)
5.93(6.06)
5.75(5.92)
5.59(5.54)
5.44(5.24)
5.16(5.18)
4.91(4.82)
4.68(4.77)
4.47(4.42)
3
4
5
6
7
8
10
12
14
16
1
1
1
0
1
2
RESULTS AND DISCUSSION
The transition temperature of the compounds of series I are summarized in
Table II. Calorimetry is a valuable method for the detection of phase
transitions. It yields quantitative results, therefore we may draw conclu-
sions concerning the nature of the phases which occur during the transi-
tions. In the present study, enthalpies of n-decyloxy and n-tetradecyloxy
derivatives were measured by differential scanning calorimetry. Data are
recorded in Table II. Enthalpy values of the nematic phase transitions agree
well with the literature value [28] which has helped in further confirmation
of mesophase type.
All the twelve members of series I synthesized exhibit enantiotropic
nematic (N) mesophase. 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 melting transition temperatures decrease with the
increase in the length of terminal alkoxy chain with the exception in the
case of compounds 5, 7, and 10. The nematic-isotropic (N-Iso) transition
temperatures also decrease with increase in terminal alkoxy tail and exhibit
the usual odd-even effect which is in agreement with the observation
reported for such homologues series.
Table III shows the comparison of N-Iso transition temperature and
range (width) of nematic phase between a representative compound
7
B [25], and C [23] reported in the literature. The nematic range of com-
(n ¼ 7) of the present series I and structurally related compounds A [24],
ꢀ
ꢀ
pound 7 is about 75 C, where as that of compound A is 83 C. However the

Napthyl Azomesogens with Lateral Chloro Group
35
ꢀ
a
TABLE II Transition Temperatures( C) of the Present Series I
Compound
n
Cr
N
Iso
1
2
3
4
5
6
7
8
1
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
202
167
137
122
179
99.5
111
100
96
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
224
218
207
209
196
197
186
181
170
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
2
3
4
5
6
7
8
10
b
9
ꢁ1
ꢁ1
[
DH/Jg ¼ 44.30]
[DH/Jg ¼ 0.89]
ꢁ
1
ꢁ1
ꢁ1 ꢁ1
[
[
DS/Jg
K
¼ 0.1201]
[DS/Jg
K
¼ 0.0020]
1
0
1
12
14
ꢂ
ꢂ
116
94
ꢂ
ꢂ
159
149
ꢂ
ꢂ
ꢂ
b
1
ꢁ
1
ꢁ1
[
DH/Jg ¼ 68.04]
[DH/Jg ¼ 0.92]
ꢁ1
ꢁ1
ꢁ1 ꢁ1
DS/Jg
K
¼ 0.1854]
[DS/Jg
K
¼ 0.0022]
1
2
16
ꢂ
86
ꢂ
136
a
Determined by optical microscopic observation.
b
The calorimetric studies have performed on this sample and therefore enthalpy and entro-
py values have been mentioned.
Cr, Crystal; N, nematic phase; Iso, isotropic liquid state; n, number of carbon atoms in the
alkoxy chain.
N-Iso transition temperature for the compound 7 is much lower when
compared with that of compound A. The molecular structure of 7 differs
from compound A only at the central aromatic core; compound 7 has a
lateral chloro group at the central benzene nucleus, whereas compound A
has no lateral substituent at the central benzene nucleus. The presence of
lateral chloro group on the central benzene ring increases the breadth of
the molecules of compound of series I. Gray [29] has explained that
increase in the breadth of the molecules reduces both nematic and smectic
mesophase stability. It seems that the lateral chloro group not only
increases the breadth of the molecules of series I but also increases the
acoplanarity in the system due to steric interaction. Both these factors
would eliminate the smectogenic tendencies as well as decrease the
mesophase range and N-Iso transition temperature for compounds repor-
ted in the present investigation.
Reference to Table III indicates that the nematic mesophase range and
ꢀ
ꢀ
N-Iso transition temperature of compound 7 are higher by 27 C and 26 C,
respectively, when compared with that of compound B. the difference in
the molecular structure of both the compounds is only in the type of lateral
substituent on the central benzene nucleus. Compound 7 has lateral chloro
group, whereas compound B has lateral methyl group. Probably the

3
6
A. K. Prajapati and H. M. Pandya
FIGURE 1 The phase behavior of series I.
increase in the polarizability of the compound 7 due to polar chloro group
slightly overcomes the increase molecular separation caused by the lateral
substituent and therefore the width of the nematic mesophase, as well as
N-Iso transition temperature of compound 7 becoming a little higher than
that of compound B.
The molecular structural difference between compounds 7 and C again
lies in the lateral substituent at the central benzene nucleus. Compound 7
has lateral chloro group, whereas compound C has lateral methoxy group.
This difference has significant influence on the nematic mesophase range

Napthyl Azomesogens with Lateral Chloro Group
37
ꢀ
TABLE III The Comparison of N-Iso Transition Temperatures ( C) and the Range
of Nematic Phase Between a Representative Compound 7 (C homologue) of the
Present Series I and Structurally Related Compounds Reported in the Literature
7
ꢀ
Transition Temperatures( C)
Mesophase
Range
Commencement
of Smectic Phase
Compound
Cr
N
Iso
7
ꢂ
ꢂ
ꢂ
ꢂ
111
135
112
98
ꢂ
ꢂ
ꢂ
ꢂ
186
218
160
118
ꢂ
ꢂ
ꢂ
ꢂ
75
83
48
20
—
A
B
C
C
10
—
—
Where R ¼ -C
7 15
H
Comparative molecular structure of the compounds 7, A, B, and C.
as well as N-Iso transition temperature, as can be seen in Table III. The
destabilization of nematic phase in the case of compound C is even more
compared to compound B, and perhaps the foregoing discussion would
hold good for this behavior.
From the above discussion the lateral group efficiency order for
the nematic phase thermal stability can be derived for the compounds as
under:
-
H > -Cl > -CH > -OCH :
3 3
It agrees well with the lateral efficiency order for the nematic phase
obtained by Gray [29] for pure nematogenic rod-shaped molecules.

3
8
A. K. Prajapati and H. M. Pandya
CONCLUSION
New mesogenic homologous series of naphthalene derivative with lateral
chloro group was synthesized. The study indicated that the lateral chloro
group adversely affects mesophase thermal stabilities. However, due to the
polar nature of the chloro substituent, the effect is less pronounced com-
pared to the lateral methyl and methoxy substituents.
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