Effects of different terminal substituents on the mesomorphic behavior of some azo-Schiff base and azo-ester-based liquid crystals

Article abstract of DOI:10.1080/15421406.2010.504632

In order to investigate the influence of the terminal substitution on mesomorphism, two new series of azo-Schiff base and azo-ester liquid crystals having the following structures have been synthesized. All the compounds possess mesomorphic properties. In series A compounds A1 and A3 exhibit only a nematic mesophase, whereas compounds A2, A4, A5, A6, A7, A8, A9, and A10 exhibit a smectic as well as a nematic phase. In series B compounds B1 to B9 exhibit only a nematic mesophase, and compounds B10 and B11 exhibit a smectic as well as a nematic phase, but compound B12 exhibits only a smectic phase. All these compounds were characterized by elemental analyses and spectroscopic techniques (Fourier transform infrared [FTIR], 1H nuclear magnetic resonance [NMR], and mass spectroscopy). Their mesomorphic properties were measured by optical polarized light microscopy and differential scanning calorimetry (DSC). Copyright Taylor & Francis Group, LLC.

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Effects of Different Terminal
Substituents on the Mesomorphic
Behavior of Some Azo-Schiff Base and
Azo-Ester-Based Liquid Crystals
B. T. Thaker a , J. B. Kanojiya a & R. S. Tandel a
a Department of Chemistry , Veer Narmad South Gujarat University ,
Surat, India
Published online: 20 Oct 2010.
To cite this article: B. T. Thaker , J. B. Kanojiya & R. S. Tandel (2010) Effects of Different Terminal
Substituents on the Mesomorphic Behavior of Some Azo-Schiff Base and Azo-Ester-Based Liquid
Crystals, Molecular Crystals and Liquid Crystals, 528:1, 120-137, DOI: 10.1080/15421406.2010.504632
To link to this article: http://dx.doi.org/10.1080/15421406.2010.504632
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Mol. Cryst. Liq. Cryst., Vol. 528: pp. 120–137, 2010
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421406.2010.504632
Effects of Different Terminal Substituents on the
Mesomorphic Behavior of Some Azo-Schiff Base
and Azo-Ester-Based Liquid Crystals
B. T. THAKER, J. B. KANOJIYA, AND
R. S. TANDEL
Department of Chemistry, Veer Narmad South Gujarat University,
Surat, India
In order to investigate the influence of the terminal substitution on mesomorphism,
two new series of azo-Schiff base and azo-ester liquid crystals having the following
structures have been synthesized. All the compounds possess mesomorphic proper-
ties. In series A compounds A1 and A3 exhibit only a nematic mesophase, whereas
compounds A2, A4, A5, A6, A7, A8, A9, and A10 exhibit a smectic as well as a
nematic phase. In series B compounds B1 to B9 exhibit only a nematic mesophase,
and compounds B10 and B11 exhibit a smectic as well as a nematic phase, but com-
pound B12 exhibits only a smectic phase. All these compounds were characterized
by elemental analyses and spectroscopic techniques (Fourier transform infrared
1
[FTIR], H nuclear magnetic resonance [NMR], and mass spectroscopy). Their
mesomorphic properties were measured by optical polarized light microscopy and
differential scanning calorimetry (DSC).
Keywords Azo; azo benzene; ester; mesophase; nematic; smectic
Address correspondence to B. T. Thaker, Department of Chemistry, Veer Narmad South
Gujarat University, Udhana-Magdalla Road, Surat 395 007, India. E-mail: btthaker1@
yahoo.co.in
120

Azo-Schiff Base and Azo-Ester Liquid Crystals
121
Introduction
Liquid-crystalline materials, either low molar mass or polymeric in nature, contain-
ing an azo group in the mesogenic core, are often studied from the point of view of
their interesting optical properties, which enable application in, for example, optical
switching, holography, and optical storage devices [1–6]. Azo dyes are also being
used in liquid-crystal display devices for the guest–host int0eraction [7].
=
Aromatic azo-containing compounds (ArꢀN NꢀAr ) can undergo a much
=
easier photoinduced trans-cis isomerization than the C C bond and thus can gener-
ate more interesting photoactive liquid crystals (LCs). Among these photoactive
mesogenic units, azobenzene derivatives are most extensively investigated. The rigid
rod-like structure of azobenzene molecules makes them suitable candidates for exhi-
biting liquid crystallinity [8,9]. On the other hand, the unique characteristics of
azobenzene molecules provide the possibility of molecular motion in response to
light or heat and thus offer many opportunities in photonic applications. In such
azo-containing liquid-crystalline materials, the azo-containing moiety is usually
incorporated into the side chain of the polymer backbone to induce the formation
of LCs [10–15].
Azo compounds have the following advantages over substances with other lin-
kages such as ester, tolane, or Schiff’s base. Azo compounds are thermally very
stable and are attractive from the point of view of studying photoinduced effects [16].
For potential commercial applications, the existence of mesophases at lower
temperatures is of very high importance. Lateral substitution by a methyl group
was used for some of these azo compounds in order to decrease the phase transition
temperatures [17].
Most of the studies have been on Schiff bases or esters containing benzene or
biphenyl units; comparatively few studies have been done on the influence of a
naphthalene core on mesomorphism. Gray [18] has reported that 6-n-alkoxy-
2-naphthanoic acids are mesomorphic, whereas 1,4- and 1,5-alkoxy naphthoic acids
are non-mesomorphic. Dave and coworkers [19–21] have synthesized a number of
4-n-alkoxy-1-naphthylidene Schiff bases and cholesteryl naphthoates and studied
their mesomorphism. Interest in naphthalene LC cores has revived in the last decade,
as indicated by a significant number of research papers [22–30]. Recently, Prajapati
et al. have reported few mesogenic homologous series of Schiff’s base ester [31–33]
and azo-esters [34–36] containing naphthalene moiety.
Demus and Sackmann [37] have reported mesomorphic azo ester dyes having a
biphenyl core. Arora and Fergason [38] and Dave and Menon [39] have reported the
azo ester dyes with different terminal substitutions. Vora and Dixit [40] have
reported a homologous series containing an azo ester linking group and lateral sub-
stitution. In recent years Bubnov et al. [41] have reported azo-ester-based new ferro-
electric liquid-crystalline compounds derived from chiral S-lactic acid as a terminal
´
moiety. Prasad and Jakli [42] have synthesized azo-ester-based liquid crystals of
achiral bent core and observed photoinduced effects in antiferroelectric tilted smec-
tic mesophase. Long and Lin [43] also have synthesized and evaluated azo dye com-
pounds and show variation of mesogenic behavior with the length of alkyl chains.
Liquid crystal polymers that contain an azo-ester linkage in the main chain have
also been synthesized [44].
Therefore, in the present work an attempt has been made to study the thermal
stabilities of azomethine-azo and ester-azo central linkages. This led us to prepare

122
B. T. Thaker et al.
more such compounds that would help further in understanding the effect of
substitution of lateral and terminal groups on mesomorphism. In the present work
we have selected two alkoxy chains such as octyl and hexadecyl, viz. 4-(40-n-alkoxy
benzylidene amino)-naphthalene-1-azo-benzene (series A) and 4-(40-n-alkoxy ben-
zoyloxy)-3-methyl-1-400 substituted-1-azobenzene (series B) comprising azomethine-
=
=
=
(-CH N)-azo(-N N) and ester (-COO)-azo(-N N) linkages for synthesis. These
two types of compounds also contain a lateral methyl (-CH3) group at the 20-position
and 20,30-position fused ring system and they also consist of terminal chloro (-Cl),
bromo (-Br), methyl(-CH3), methoxy (-O CH3), and nitro (-NO2) groups. We have
synthesized these compounds to study the influence of the lateral, as well as terminal
group and also central linkages on mesomorphism and thermal stabilities of these
compounds.
Experimental
Reagents and Technique
4-Hydroxy benzaldehyde, and a-naphthylamine were obtained from Merck,
Germany. Benzoic acid, alkyl bromide (Lancaster, England). 4-Hydroxy o-Cresol,
aniline, p-toludine, p-anisidine, p-chloroaniline, p-bromoaniline, and p-nitroaniline
were provided by Nova Dyes Stuff [P] Ltd., Surat and used without further purifi-
cation. The solvents were used after purification using the standard methods
described in the literature [45].
Elemental analyses (C, H, N) were performed at Central Drugs Research Insti-
tute (CDRI) Lucknow. Infrared spectra were recorded with a Perkin-Elmer 2000
Fourier transform infrared (FTIR) spectrophotometer (S.A.I.F), Punjab University,
Chandigarh in the frequency range 4000–400 cmꢀ1 with samples embedded in KBr
discs. 1H nuclear magnetic resonance (NMR) spectra of the compounds were
recorded with a JEOL-GSX-400 using CDCl3 as a solvent and TMS as an internal
reference, and mass spectra (EI) of the compounds at Sofisticated Analytical Instru-
ment Facilities (SAIF), IIT Madras, Chennai. Thin-layer chromatography analyses
were performed by using aluminum-backed silica-gel plates (Merck 60 F524) and
examined under short-wave ultraviolet (UV) light.
The phase-transition temperatures were measured using a Shimadzu DSC-50 at
heating and cooling rates of 5ꢁC minꢀ1. The optical microscopy studies were carried
out with a Leitz Loborlux 12 POL (Wetzler, Germany)polarizing microscope
equipped with a Mettler FP52 hot stage. The textures of the compounds were
observed using polarized light with crossed polarizers with the sample in a thin film
sandwiched between a glass slide and coverslip.
Synthesis
Synthesis of 4-alkoxy benzaldehydes. 4-Alkoxy benzaldehydes were prepared by a
reported method [46–49]. The m.p. of these compounds was compared with
reported values and they are very similar.
Synthesis of 4-amino-naphthalene-400-substituted-1-azobenzene. These dyes were
prepared by condensing a-naphthylamine with diazonium salts of substituted
aniline. First the substituted anilines were diazotized with dilute HCl and NaNO2

Azo-Schiff Base and Azo-Ester Liquid Crystals
123
at 0–5ꢁC and coupled with a-naphthylamine. Then the reaction mixture was
neutralized with diluted NaOH, and the obtained precipitate was collected by
filtration and then air dried. The crude compounds were purified by repeated
recrystallizations using ethanol as solvent [50–51].
Synthesis of 4-(40-n-alkoxy benzylidene amino)-naphthalene-400-substituted-1-azo-
benzene (series A). A mixture of 1 mmol 4-alkoxy benzaldehyde and 4-amino-
naphthalene-400-substituted-1-azobenzene and three drops of acetic acid in absolute
ethanol were heated at reflux for 4 h. The reaction mixture was allowed to cool and
was stirred at room temperature overnight. The solid was collected and recrystallized
from technical ethanol. The product was purified by column chromatography using
a mixture of hexanes=ethyl acetate (7=1) as eluent. The above procedure was
adopted for the synthesis of mesogenic dyes having terminal substitutions such as
methyl, methoxy, chloro, and flouro azobenzene [52–54].
A5: Molecular formula: C31H32ON3Br. Elemental analysis, calculated for
C 68.63% H 5.90%, and N 7.75%; found: C 68.95%, H 5.78%, and N 7.94%,
EI-MS m=z (rel.int%): 541 (M)þ FTIR (in KBr)ꢀ, 12864, 2925 cmꢀ1 (C-H aliphatic),
ꢀ1
=
=
1602 cm (-C N of azomethine), 1575–1515 cm (C C aromatic ring stretching),
1470 cmꢀ1 (CꢀH bending of CH2), 1166 cmꢀ1 (CꢀOꢀC); 1025 cmꢀ1 (CꢀO stretch-
1
ing); H-NMR (CDCl3, d, ppm): 0.87–0.92 (3H, t, CH3, alkyl chain), 1.24–1.87 (m,
-(CH2)n, alkyl chain), 4.03–4.07 (2H, t, -CH2O, alkoxy chain), 7.03–7.98 (m, phenyl
=
protons), 8.49 (s, CH N, aldehydic), 8.40–8.43 (d), and 8.95–8.98 (d) aromatic ortho
proton of first ring).
A8: Molecular formula: C40H51O2N3. Elemental analysis, calculated for C
79.33%, H 8.40%, and N 6.94%; found: C 79.32%, H 8.40%, and N 6.91%. EI-MS
m=z (rel.int%): 606 (M þ 1)þ FT-IR (in KBr), ꢀ21858, 2911 cmꢀ1 (CꢀH aliphatic),
1602 cmꢀ1(-C N of azomethine), 1575–1514 cm (C C aromatic ring stretching),
1480 cmꢀ1 (CꢀH bending of CH2), 1166 cmꢀ1 (C-O-C), 1030 cmꢀ1 (CꢀO stretching);
1H-NMR (CDCl3, d, ppm): 0.87–0.92 (3H, t, CH3, alkyl chain), 1.24–1.87 (m,
-(CH2)n, alkyl chain), 3.91 (s, terminal ꢀOCH3 proton attached to the aromatic
ring), 4.03–4.07 (2H, t, -CH2O, alkoxy chain), 7.03–7.98 (m, phenyl protons), 8.49
=
=
=
(s, CH N azomethine), 8.40–8.43 (d), and 8.95–8.98 (d) (aromatic ortho proton
of first ring).
Synthesis of 4-n-Alkoxy Benzoic Acids. 4-n-Alkoxy benzoic acids were prepared as
reported by Dave and Vora [55]. The m.p. of these compounds was compared
with the reported values and they are very similar.
Synthesis of 4-n-Alkoxy Benzoyl Chlorides. 4-n-alkoxy benzoyl chlorides were
prepared by the reported method [55]. The m.p. of these compounds was
compared with the reported values and they are very similar.
Synthesis of 4-hydroxy-3-methyl-1-azobenzene. A well-stirred mixture of aniline
(0.01 mol, 0.93g) and concentrated HCl (0.03 mol, 3.6 mL) was cooled below 0–5ꢁC
and a solution of NaNO2 (0.01 mol, 0.7 g) in water (5 mL) was added dropwise in
such a way that the temperature of the mixture was in the range 0–5ꢁC. The cold,
dark solution was added dropwise to a cold mixture of o-cresol (0.01 mol, 1.08g),
NaOH (20% w=v) during which the temperature of the mixture was maintained
below 0–5ꢁC. Acidification with aqueous HCl furnished the crude product, which
was collected by filtration, dried in air, and crystallized several times from ethanol [56].

124
B. T. Thaker et al.
Synthesis of 4-(40-n-alkoxy benzoyloxy)-3-methyl-1-azobenzene (series B).
4-Hydroxy-3-methyl-1-azobenzene (0.01 mol, 1.98 g) was dissolved in dry pyridine
(10 mL) and was added dropwise with occasional stirring into ice-cold 4-n-alkoxy
benzoyl chloride (0.01 mol, 2.69 g) in a round-bottom flask. Then the mixture was
refluxed in a hot water bath for 2 h and allowed to stand overnight. The mixture
was acidified with cold 1:1 diluted hydrochloric acid to precipitate the product.
The solid obtained was filtered, washed with water, saturated sodium bicarbonate
solution, and again water and thus the crude azo dye obtained. The solid ester
azo dye was recrystallized from acetic acid until constant transition temperature
was obtained [55]. The above procedure was adopted for the synthesis of
mesogenic dyes having terminal substituents such as methyl, methoxy, chloro,
flouro, and nitro azobenzene.
B1: Molecular formula: C28H32O3N2. Elemental analysis: calculated for C
75.68%, H 7.10%, and N 6.31%; found: C 75.96%, H 7.45%, and N 6.58%. EI-MS
m=z (reꢀl.1int%): 443 (M-1)þ FTIR (in KBꢀr)1, 2864, 2925 cmꢀ1 (CꢀH aliphatic),
=
=
(C C aromatic ring stretching),
1737 cm (-C O of ester), 1575–1515 cm
1470 cmꢀ1 (CꢀH bending of CH2); 1273 cmꢀ1 (CꢀO stretching of ester), 1172 cmꢀ1
1
(CꢀOꢀC); H-NMR (CDCl3, d, ppm): 0.87–0.91 (3H, t, CH3, alkyl chain), 1.26–
1.87 (m, -(CH2)n, alkyl chain), 2.33 (s, lateral ꢀCH3 proton attached to the aromatic
ring), 4.03–4.07 (2H, t, -CH2O, alkoxy chain), 6.97–8.20 (m, phenyl protons).
B7: Molecular formula: C36H48O3N2. Elemental analysis: calculated for C
77.70%, H 8.63%, and N 5.04%; found: C 77.95%, H 8.85%, and N 5.33%. EI-MS
m=z (reꢀl.1int%): 556 (M)þ FTIR (in KBrꢀ),1 2864, 2932 cmꢀ1 (CꢀH aliphatic),
=
=
(C C aromatic ring stretching),
1730 cm (-C O of ester), 1575–1515 cm
1480 cmꢀ1 (CꢀH bending of CH2); 1273 cmꢀ1 (CꢀO stretching of ester), 1179 cmꢀ1
(CꢀOꢀC); 1H-NMR (CDCl3, d, ppm): 0.87–0.91 (3H, t, CH3, alkyl chain),
1.26–1.87 (m, -(CH2)n, alkyl chain), 2.33 (s, lateral ꢀCH3 proton attached to the aro-
matic ring), 4.03–4.07 (2H, t, ꢀCH2O, alkoxy chain), 6.97–8.20 (m, phenyl protons).
Synthesis
SCHEME 1
Preparation of 4-alkoxy benzaldehydes
Diazotization of aniline

Azo-Schiff Base and Azo-Ester Liquid Crystals
Synthesis of 4-amino-naphthalene-1-azobenzene
125
Synthesis of 4-(40-n-alkoxy benzylidene amino)-naphthalene-1-azo-benzene
(Series A)
SCHEME 2
Synthesis of 4-n-alkoxy benzoic acids
Synthesis of 4-n-alkoxy benzoyl chlorides

126
B. T. Thaker et al.
Diazotization of Aniline
Synthesis of 4-hydroxy-3-methyl-1-azobenzene
Synthesis of 4-(40-n-alkoxy benzoyloxy)-3-methyl-1-azobenzene (Series B)
Results and Discussion
In order to investigate the influence of the terminal substitution on mesomorphism
two new series, viz. 4-(40-n-alkoxy benzylidene amino)-naphthalene-400-substituted-
1-azo-benzene (series A) containing azo-schiff base as a central linkage and 4-(40-
n-alkoxy benzoyloxy)-3-methyl-1-400 substituted-1-azobenzene (series B) containing
azo-ester as a central linkage and different terminal substitutions, were synthesized
and their mesomorphic properties were studied. The common structural features
of the compounds are that they consist of the same terminal substitutent at one
end. The mesomorphic properties of all the synthesized compounds have been char-
acterized by differential scanning calorimetry (DSC) and polarizing optical
microscopy (POM) attached with a Mettler hot stage.
The transition temperatures of both series are given in Tables 1 and 2. In series A
compounds A1 and A3 exhibit only a nematic mesophase, whereas compounds A2,

Azo-Schiff Base and Azo-Ester Liquid Crystals
Table 1. Transition temperatures of Series A
127
Transition temperature (ꢁC)
Compounds
R ¼ n-Alkoxy
Octyloxy
Octyloxy
Octyloxy
Octyloxy
Octyloxy
Hexadecyloxy
Hexadecyloxy
Hexadecyloxy
Hexadecyloxy
Hexadecyloxy
X
S
N
I
A1
A2
A3
A4
A5
A6
A7
A8
A9
A10
-H
-CH3
-OCH3
-Cl
-Br
-H
-CH3
-OCH3
-Cl
—
111
—
115
108
65
68
92
82
80
103a
126
107
122
130
74
108
175
205
179
174
94
140
163
149
150
82
112
128
131
-Br
aMonotropic mesophase.
A4, A5, A6, A7, A8, A9, and A10 exhibit a smectic as well as a nematic phase. In series
A compounds A1 is monotropic, whereas the rest of the compounds are enantiotro-
pic in nature. In series B compounds B1 to B9 exhibit only a nematic mesophase, and
compounds B10 and B11 exhibit a smectic as well as a nematic phase, but compound
B12 exhibits only a smectic phase.
DSC is a valuable method for the detection of phase transitions. It yields quan-
titative results; therefore, we may draw conclusions concerning the nature of the
phases that occur during the transition. In the present study, enthalpies of two deri-
vatives of series A and series B were measured by DSC. DSC data for series A and
series B are recorded in Tables 3 and 4, which helps to further confirm the mesophase
type. Table 3 shows the phase transition temperatures, associated enthalpy (DH),
and molar entropy (DS) for compounds of series A (A2, A4, A5, and A7) and
Table 2. Transition temperatures of Series B
Transition temperature (ꢁC)
Compounds
R ¼ n-Alkoxy
Octyloxy
Octyloxy
Octyloxy
Octyloxy
Octyloxy
Octyloxy
Hexadecyloxy
Hexadecyloxy
Hexadecyloxy
Hexadecyloxy
Hexadecyloxy
Hexadecyloxy
X
S
N
I
B1
B2
B3
B4
B5
B6
B7
B8
B9
B10
B11
B12
-H
-CH3
-OCH3
-Cl
-Br
-NO2
-H
-CH3
-OCH3
-Cl
-Br
-NO2
—
—
—
—
—
—
—
—
—
78
91
86
67
85
96
77
81
111
63
76
81
117
106
—
91
161
198
170
160
201
83
134
154
140
136
161

128
B. T. Thaker et al.
Table 3. DSC data for Series A compounds
Peak temp. (microscopic
temp.) (ꢁC)
DH
DS
Series Compound Transition
(Jgꢀ1
)
(Jgꢀ1Kꢀ1
)
A
A2
A4
A5
A7
A9
A10
Cr-S
S-N
N-I
Cr-S
S-N
N-I
Cr-S
S-N
N-I
Cr-S
S-N
N-I
Cr-S
S-N
N-I
Cr-S
S-N
N-I
112 (111)
124.29 (126)
175 (175)
115 (115)
125.54 (122)
179 (179)
108 (108)
131 (130)
174 (174)
72 (68)
81.37 (82)
137.59 (140)
81.96 (82)
128 (128)
145 (149)
85 (80)
97.80
14.57
2.10
—
0.2547
0.0367
0.0047
—
70.66
1.94
7.69
64.36
1.35
6.28
32.81
1.55
47.55
—
0.1773
0.0043
0.0202
0.1593
0.0030
0.0182
0.0926
0.0038
0.1339
—
1.45
92.98
—
0.0034
0.2597
—
132 (131)
144.57 (150)
1.40
0.0034
Table 4 shows the phase transition temperatures, associated enthalpy (DH), and
molar entropy (DS) for compounds of series B (B2, B4, B5, B8, and B10). Enthalpy
values of the various transitions agree well with the existing related literature values
[57]. The DSC curves of representative compounds are shown in Figs. 1 to 4. Micro-
scopic transition temperature values are very similar to the DSC data.
A number of homologues were synthesized constituting a p-phenylene unit to
study the effect of chemical constitution on liquid-crystalline properties. There is a
very delicate balance between the chemical constitution and liquid-crystalline
Table 4. DSC data for Series B compounds
Peak temp. (microscopic
temp.) (ꢁC)
DH
DS
Series Compound Transition
(Jgꢀ1
)
(Jgꢀ1Kꢀ1
)
B
B2
B4
B5
B8
B10
Cr-N
N-I
Cr-N
N-I
Cr-N
N-I
Cr-N
N-I
83.51 (85)
162.6 (161)
82 (77)
165 (170)
78.5 (81)
158 (160)
78.2 (76)
130.7 (134)
81 (78)
49.76
3.17
78.47
1.94
78.64
3.17
35.11
2.11
33.63
—
0.1396
0.0073
0.2210
0.0044
0.2237
0.0074
0.1000
0.0052
0.1000
—
Cr-S
S-N
N-I
118 (117)
136.66 (140)
1.67
0.0041

Azo-Schiff Base and Azo-Ester Liquid Crystals
129
Figure 1. DSC curve of compound A4.
properties. Generally the introduction of a lateral substituent is different for all types
of mesophases. In the quest of obtaining low melting liquid crystals with broad
mesophase range, it was noted that a lateral substituent like chloro (-Cl) or methyl
(-CH3) is quite effective in reducing crystal mesophase temperature to give low
melting liquid crystals [58–60].
In light of this, it was of interest to introduce lateral methyl and terminal hal-
ogens (-Cl, -Br), methyl, methoxy, and nitro substituents on the benzene ring at
Figure 2. DSC curve of compound A9.

130
B. T. Thaker et al.
Figure 3. DSC curve of compound B2.
one end of the p-phenylene system to evaluate the effect of substituents on
mesomrphic behavior.
In series A 10 compounds were synthesized, out of which A1-A5 have an octy-
loxy (OC8H17) chain and A6-A10 have a hexadecyloxy (-OC16H33) chain, in addition
to different terminal subsituents like ꢀH, -CH3, -OCH3, -Cl, and -Br. In series B 12
compounds were synthesized, of which B1-B6 have an octyloxy (OC8H17) chain and
B7-B12 have a hexadecyloxy (-OC16H33) chain; in addition to different terminal sub-
situents like ꢀH, -CH3, -OCH3, -Cl, -Br, and -NO2.
Figure 4. DSC curve of compound B10.

Azo-Schiff Base and Azo-Ester Liquid Crystals
131
Thermal Stability
The average thermal stability of the compounds of series A, that is, azomethine-based
azo dyes, is higher than compounds of series B, that is, ester-based azo dyes. Because
both series have the same terminal subsituents at one end and of the other end a
n-alkoxy chain. The two types of compounds differ only in the central linkage and
=
lateral substitution. Compounds of series A have an azomethine (-CH N)-
=
azo(-N N) central linkage and a lateral substituted benzene ring that is closed at
the 20,30-position of the central unit, whereas compounds of series B have an ester
=
(-COO)-azo(-N N) central linkage and a lateral substituted methyl group at the
30-position of the central unit.
The presence of a naphthalene unit as the central unit in series A shows that the
naphthalene compounds give rise to higher melting points, more stable mesophases,
and wider liquid-crystalline phase ranges in comparison with their phenyl analogues.
These properties are a direct consequence of the molecular arrangement in both the
solid and the liquid-crystal phases. As mentioned earlier, the phenyl and naphthyl
central core units differ mainly in their length and extension of the electronic conju-
gation. The increased transition temperatures can clearly be attributed to the
increased length and higher polarizability of the naphthalene structure that allows
both stronger and wider-ranging molecular interaction.
The introduction of the lateral methyl group in series B compounds changes the
molecular conformation and broadens the molecules, which decreases, the transition
temperature of all phases. This factor is a direct result of an increase in the breath or
thickness of the molecule [61]. Thus, the introduction of the lateral methyl group
results in decreased transition temperature of all phases in the series B compounds.
This has been attributed to the broadening of the molecules as well as the change in
molecular conformation. The increased dissymmetry resulting from the lateral
methyl group leads to less effective packing in the crystal lattice and therefore lowers
the crystal to mesophase transition temperature as shown in Tables 1 and 2.
=
As mentioned earlier, series A has the central azomethine (-CH N-) group,
whereas series B has the central carboxy (-COO-) group. The oxygen atom of this
central carboxy group in the molecules of series B will bump into the nonbonded side
of the adjacent ꢀH of the aromatic ring, thereby causing considerable strain on the
molecule. This will cause some twist around the C-O bond and force the benzene ring
out of the plane of the molecule and broaden the molecule [58] as shown below.
=
The H atom of the central azomethine (-CH N-) group will also behave in the
same way and bring about the twist in the molecules of series A, but the twist in this
case will be much less than in the molecules of series B. Hence, the molecules of series
B will be more non-coplanar than those of series A. This effect is observed in the
present case where the thermal stabilities of series B compounds are less than those
of series A compounds.

132
B. T. Thaker et al.
The N-I average thermal stability order with respect to the terminal group in
series A compounds is observed as
OCH3 > ꢀCl > ꢀBr > ꢀCH3 > ꢀH
ð101ꢁCÞ
ð164ꢁCÞ
ð162ꢁCÞ
ð184ꢁCÞ
ð158ꢁCÞ
In the above order, it shows that N-I average thermal stability of the ꢀOCH3 group
is higher than that of other terminal groups. This could be explained that in ꢀO-CH3
group, the lone pair of electrons of oxygen is shielded by an insulator-like methyl
group. The repulsive forces involving the oxygen lone pairs are thereby substantially
reduced and allow a close approach of the neighboring molecules, increasing bonding
forces. This leads to an increase in the N-I transition temperature. The N-I thermal
stability of ꢀCH3 is less than that of ꢀCl and ꢀBr groups. This is because the ꢀCH3
group is less polar, whereas the ꢀCl and ꢀBr groups will endow the molecule with
higher polarity, high terminal attraction, and hence higher N-I average thermal
stability. The same trend was observed by Gray et al. [18,62] and Coates [63].
In series A mesogenic compounds, the S-N average thermal stability of terminal
group is observed only in both the chain as it shows that is series-A compounds
A1-A5 having octyloxy chain and compounds A6-A10 having hexadecyloxy chain.
In both octyloxy and hexadecyloxy chain the terminal substituents are ꢀBr, ꢀCl,
& CH3 in which average smectic-nematic thermal stabilities is given in order.
ꢀBr > ꢀðC12l5ꢁCÞ> ꢀCH3
ð130:5ꢁCÞ
ð104ꢁCÞ
The above order shows that terminal -CH3 substituted compounds are less
mesomorphic than those with ꢀCl and ꢀBr units. This is because -CH3 less polar,
whereas the ꢀCl and ꢀBr groups endow the molecules with high polarity, greater
terminal attraction, and hence higher S-N thermal stability. Enhanced smectic ten-
dencies have been found by Gray et al. [18] for the 40-bromo and 40-chloro deriva-
tives relative to the unsubstituted compound.
Gray [18] has given the order of decreasing smectic thermal stability of the
compounds in terms of the terminal substituents ꢀX- as
This order indicates no simple relationship between the strength of the dipole and
smectic thermal stability. Thus, the nitro group with its strong dipole acting along the
long axis of the molecule yields a smectic mesophase that is less thermally stable than
=
=
that of X Cl. Also, with no terminal dipole X H, the smectic mesophase is thermally
more stable than for X¼-OCH3, for which the dipole acts at an angle across the long
axis of the molecule. This fact suggests that terminal dipoles contribute to the ter-
minal intermolecular attractions, either by polarization or dipole–dipole interactions
as well as the lateral attractions. If such terminal attraction occurs, the contribution of
the dipole to the lateral attraction will be decreased and the balance of the lateral and

Azo-Schiff Base and Azo-Ester Liquid Crystals
133
Figure 5. Plot of the mesomorphic behavior of compounds A1 to A10.
terminal cohesion will be affected. The determination of the smectic characteristics of
the molecules is therefore complicated by the possible dual role of these dipoles in
affecting both terminal and lateral attractions. The relative span of temperature of
various p-substituted groups of series A and series B compounds is shown in Figs. 5
and 6. The mesophase textures observed are shown in Fig. 7.
Commencement of Smectic Mesophase
In series B compounds, the smectic phase commences late at the hexadecyl deriva-
tives of alkyl chain, whereas it commences at the octyl and hexadecyl derivative
Figure 6. Plot of the mesomorphic behavior of compounds B1 to B12.

134
B. T. Thaker et al.
Figure 7. (a) Texture of nematic phase of A4 of series A at 122ꢁC on cooling. (b) Thread-like
texture of the nematic phase of A10 of series A at 131ꢁC on cooling. (c) Marble texture of the
nematic phase of B1 of series B at 67ꢁC on cooling. (d) Marble texture of the nematic phase of
B9 of series B at 81ꢁC.
respectively in series A. It is rather difficult to postulate the exact commencement of
the smectic mesophase in the compounds of series A because it depends on a number
of factors like polarizability, breath (thickness), and geometry of the molecules.
The difference between the two types of compounds is in the central groups. In
=
series A compounds have an azomethine (-CH N-) group and naphthalene nucleus,
which make the molecules longer and more polarized. Thus, early commencement of
a smectic phase in series A can be understood. Series B has a central ester (-COO)
group and lateral substituted ꢀCH3 group. In series B the lateral ꢀCH3 group,
which is ortho to ꢀCOO- linkage, will have twofold effects: (i) increasing the breath
(thickness) of the molecules and (ii) reducing the coplanarity of the molecules. Both
these effects explain the late commencement of the smectic mesophase and lower
thermal stability of the compounds of series B.
From the above results it can be seen that not only the geometry of the molecules
but also length, breath (thickness), and polarizability of the molecules play an impor-
tant role in the mesomorphic behavior of compounds.
Conclusion
In this article we have presented the synthesis and characterization of two new
mesogenic series, viz. 4-(40-n-alkoxy benzylidene amino)-naphthalene-400-substituted-
1-azo-benzene (series A) containing azo-Schiff base as a central linkage and 4-(40-n-
alkoxy benzoyloxy)-3-methyl-1-400 substituted-1-azobenzene (series B) containing

Azo-Schiff Base and Azo-Ester Liquid Crystals
135
azo-ester as a central linkage and different terminal substitutions. The presence of a
naphthalene nucleus as a central unit in series A shows that the naphthalene
compounds give rise to higher melting points, higher mesophase stability, and wider
liquid-crystalline phase ranges in comparison with their phenyl analogues. These
properties are a direct consequence of the molecular arrangement in both the solid
and the liquid-crystal phases. The phenyl and naphthyl central core units differ
mainly in their length and extension of the electronic conjugation. The increased
transition temperature can clearly be attributed to the increased length and higher
polarizability of the naphthalene structure that allows both stronger and
wider-ranging molecular interactions. The introduction of the lateral methyl group
in series B compounds changes the molecular confirmation and broadening of the
molecules, which decreases the transition temperatures of all phases. The increased
dissymmetry resulting from the lateral methyl group leads to less effective packing
in the crystal lattice and therefore lowers the crystal mesomorphic transition
temperature.
Acknowledgment
We are thankful to Solvay Chemicals Ltd., Ankleshwar (Gujarat), for providing
thermal studies (DSC) and the Department of Applied Chemistry, Faculty of Tech-
nology and Engineering, M. S. University of Baroda, Vadodara, for providing us
with an optical polarizing microscope for mesophase studies.
References
[1] Ruslim, C., & Ichimura, K. (1999). J. Mater. Chem., 9, 673.
[2] Belmar, J., Parra, M., Zuniga, C., Perez, C., Munoz, C., Omenat, A., & Serrano, J. L.
(1999). Liq. Cryst., 26, 389.
[3] Lai, L. L., Wang, E., Liu, Y. H., & Wang, Y. (2002). Liq. Cryst., 29, 871.
[4] Prajapati, A. K. (2001). Mol. Cryst. Liq. Cryst., 364, 769.
[5] Komitov, L., Ichimura, K., & Strigazzi, A. (2000). Liq. Cryst., 27, 51.
[6] Komitov, L., Ruslim, C., Matsuzawa, Y., & Ichimura, K. (2000). Liq. Cryst., 27, 1011.
[7] Matsui, M., Nakagaway, Y. H., Joglekary, B., Shibatay, K., Muramatsuy, H., Abe, Y.,
& Kanekos, M. (1996). Liq. Cryst., 21(5), 669–682.
[8] Ichimura, K. (2000). Chem. Rev., 100, 1847–1873.
[9] Ikeda, T. (2003). J. Mater. Chem., 13, 2037–2057.
[10] Ikeda, T., & Wu, Y. (1999). Pure Appl. Chem., 71, 2131–2136.
[11] Eich, M., Wendorff, J. H., Reck, B., & Ringsdorf, H. (1987). Makromol. Chem. Rapid
Comm., 8, 59–63.
[12] Eich, M., & Wendorff, J. H. (1987). Makromol. Chem. Rapid Comm., 8, 467–471.
[13] Anderle, K., Birenheide, R., Werner, M. J. A., & Wendorff, J. H. (1991). Liq. Cryst., 9,
691–699.
[14] Kato, T., Mizoshita, N., & Kishimoto, K. (2006). Angew. Chem. Int. Ed., 45, 38–68.
[15] Ikeda, T., Mamiya, J.-I., & Yu, Y. (2007). Angew. Chem. Int. Ed., 46, 506–528.
[16] Prasad, V. (2001). Liq. Cryst., 28(1), 145–150.
´
´
[17] Kaspar, M., Hamplova, V., Pakhomov, S. A., Stibor, I., Sverenya, K. H., Bubnov, A. M.,
´
ˇ
Glogarova, M., & Vanek, P. (1997). Liq. Cryst., 22, 557.
[18] Gray, G. W. (1962). Molecular Structure and the Properties of Liquid Crystals, Academic:
London.
[19] Dave, J. S., Kurian, G., Prajapati, A. P., & Vora, R. A. (1971). Mol. Cryst. Liq. Cryst.,
14, 307.

136
B. T. Thaker et al.
[20] Dave, J. S., & Prajapati, A. P. (1975). Pramana, 1, 435.
[21] (a) Dave, J. S., & Kurian, G. (1975). Pramana, 1, 427; (b) Dave, J. S., & Kurian, G.
(1977). Mol. Cryst. Liq. Cryst., 42, 193.
[22] Wedler, W., Demus, D., Zaschke, H., Mohr, K., Schafer, K. W., & Weissflog, W. (1991).
J. Mater. Chem., 1, 347.
[23] Mochizuki, A., Motoyoshi, K., & Nakatsuka, M. (1991). Ferroelectrics, 122, 37.
[24] Sierra, T., Serrano, J. L., Rose, M. B., Ezcurra, A., & Zubia, J. (1992). J. Am. Chem. Soc.,
114, 7645.
[25] Marcos, M., Omenat, A., Serrano, J. L., Sierra, T., & Ezcurra, A. (1992). Adv. Mater., 4,
285.
[26] Hird, M., Toyne, K. J., & Gray, G. W. (1993). Liq. Cryst., 14, 741.
[27] Ganzalez, Y., Ros, M. B., Serrano, J. L., & Perezjubind, A. (1995). Liq. Cryst., 18, 751.
[28] Suste, A., Moslavac, D., & Sunjic, V. (1996). Liq. Cryst., 21, 383.
[29] Sadashiva, B. K., & Gropalakrishnan, C. R. (1998). Macromol. Rapid Comm., 19, 97.
[30] Wu, S.-L., & Hsieh, W.-J. (1999). Chem. Mater., 11(4), 852.
[31] Vora, R. A., & Prajapati, A. K. (1998). Liq. Cryst., 25(5), 567.
[32] Prajapati, A. K. (2000). Mol. Cryst. Liq. Cryst., 348, 65.
[33] Vora, R. A., & Prajapati, A. K. (1996). Proceedings of the Fifth National Seminar on
Liquid Crystals, Patiala: India, p. 97.
[34] Prajapati, A. K. (2000). Liq. Cryst., 27(8), 1017.
[35] Jadav, N. D., Prajapati, B. A., & Prajapati, A. K. (2003). Mol. Cryst. Liq. Cryst., 399,
53–60.
[36] Prajapati, A. K., & Pandya, H. M. (2003). Mol. Cryst. Liq. Cryst., 393, 31–39.
[37] Demus, D., & Sackmann, H. (1963). Z. Phys. Chem., 222, 127.
[38] Arora, S. L., & Fergason, J. L. (1971). Chem. Soc. Faraday Div. Symp., 5, 97.
[39] Dave, J. S., & Menon, M. (2000). Bull. Mater. Sci., 23(3), 237–238.
[40] Vora, R. A., & Dixit, N. (1980). Mol. Cryst. Liq. Cryst., 59, 63.
[41] Bubnov, K. M. A., Hamplova, V., Pirkl, S. R., & Glogarova, M. (2004). Liq. Cryst.,
31(6), 821–830.
´
[42] Prasad, V., & Jakli, A. (2004). Liq. Cryst., 31(4), 473–479.
[43] Long, L. L., & Lin, H. C. (2000). Liq. Cryst., 27(5), 707–709.
[44] Tian, Y., Ren, Y., Sun, R., Zhao, Y., Tang, X., Huang, X., & Shiquan, X. (1997). Liq.
Cryst., 22(2), 177–183.
[45] Saleh, A. A., Pleune, B., Fettinger, J. C., & Poli, R. (1997). Polyhedron, 16, 1391.
[46] Gray, G. W., & Jones, B. (1954). J. Chem. Soc., 1467.
[47] Hildesheimer, A. (1901). Manatsh. Chem., 22, 487.
[48] Stoermer, R., & Wodarg, F. (1928). Chem. Berg., 61B, 2323.
[49] (a) Weygard, C., & Gabler, R. (1940). J. Prakt Chem., 155, 332; (b) Weygard, C., &
Gabler, R. (1938). J. Prakt. Chem., 151, 215.
[50] Keller, P., & Libert, L. (1978). Solid State Phys., 14, 19.
[51] Karaer, H. (1997). Doctoral thesis, Department of Chemistry, Ondokur Mayis University,
Institute of Natural and Applied Science, Samsun: Turkey.
[52] Pinol, R., Ros, M. B., Serrano, J. L., & Sierra, T. (2004). Liq. Cryst., 31, 1293.
[53] Yeap, G.-Y., Ha, S.-T., Lim, P.-L., & Boeg, P.-L. (2006). Mol. Cryst. Liq. Cryst.,
452, 63.
[54] Dec, M., Pina, C., & Serrano, J. L. (2003). Liq. Cryst., 30, 651.
[55] Dave, J. S., & Vora, R. A. (1970). Liquid Crystals and Ordered Fluids. In: Liquid Crystals
and Ordered Fluids, Johnson, J. F. & Porter, R. S. (Eds.), Plenum Press: New York,
p. 477.
[56] Vogel, A. I. (1989). Text Book of Practical Organic Chemistry, 5th ed. ELBS and
Longmann Group Ltd.: London.
[57] Marzotko, D., & Demus, D. (1975). Dramana, 1, 189.
[58] Arora, S. L., Fergason, J. L., & Taylor, T. R. (1970). J. Org. Chem., 35, 4055.

Azo-Schiff Base and Azo-Ester Liquid Crystals
137
[59] Gardland, Z. G., Curtier, R. J., Smith, G. W., Johnson, J. F., & Porter, R. S. (1973).
Liquid Crystals and Ordered Fluids, Plenum Press: New York.
[60] Dubois, J. C., Zann, A., & Begain, A. (1977). Mol. Cryst. Liq. Cryst., 42, 139.
[61] Collings, P. J., & Hird, M. (1998). Introduction to Liquid Crystals Chemistry and Physics,
Taylor and Francis, London.
[62] Aaron, C., Bayron, D. J., & Gray, G. W. Unpublished work.
[63] Coates, D. (1990). Liquid Crystals Applications and Uses, World Scientific: London.