Synthesis, mesophase behavior and thermal stability of liquid crystals based on different central linkages with lateral substitution and terminal heterocyclic moieties

Article abstract of DOI:10.1080/15421400903054501

The synthesis and evaluation of thermal behavior of two new mesogenic homologous series of liquid crystalline compound containing 1,2,4-triazole and isonicotinic acid ring at the terminus of the molecule have been reported, viz. 3-hydroxy-4-[(4-1,2,4-triazol-4-ylimino)methyl] phenyl 4-alkoxybenzoate (Series-I) and 3-hydroxy-4 (isonicotinoyl carbonohydrazonoyl) phenyl 4-alkoxy benzoate (Series-II). The compounds of both the series have been characterized by elemental Analysis, FT-IR, mass spectrometry, ¹H-NMR and ¹³C-NMR. Phase transition temperatures and the thermal parameters were obtained from differential scanning calarimetry (DSC). The texture observation was performed under polarizing optical microscopy (PMO) attached with Mettler hot stage. All the derivatives are mesomorphic in nature showing nematic phase as well as higher members of both the series show smectic phase. The use of triazole and isonicotinic acid as a terminal group has a very dramatic effect on the melting and clearing points. Similarly lateral-OH substitution on central phenyl ring shows higher clearing temperature due to intermolecular hydrogen bonding. The mesomorphic behavior has been analyzed in terms of structural property relation. Copyright Taylor & Francis Group, LLC.

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Synthesis, Mesophase Behavior
and Thermal Stability of Liquid
Crystals Based on Different
Central Linkages with Lateral
Substitution and Terminal
Heterocyclic Moieties
B. T. Thaker a & Pranay Patel a
a Department of Chemistry , Veer Narmad South
Gujarat University , Surat, India
Published online: 05 Oct 2009.
To cite this article: B. T. Thaker & Pranay Patel (2009) Synthesis, Mesophase Behavior
and Thermal Stability of Liquid Crystals Based on Different Central Linkages with
Lateral Substitution and Terminal Heterocyclic Moieties, Molecular Crystals and Liquid
Crystals, 509:1, 173/[915]-185/[927], DOI: 10.1080/15421400903054501
To link to this article: http://dx.doi.org/10.1080/15421400903054501
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Mol. Cryst. Liq. Cryst., Vol. 509, pp. 173=[915]–185=[927], 2009
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421400903054501
Synthesis, Mesophase Behavior and Thermal
Stability of Liquid Crystals Based on Different
Central Linkages with Lateral Substitution and
Terminal Heterocyclic Moieties
B. T. Thaker and Pranay Patel
Department of Chemistry, Veer Narmad South Gujarat University,
Surat, India
The synthesis and evaluation of thermal behavior of two new mesogenic
homologous series of liquid crystalline compound containing 1,2,4-triazole and
isonicotinic acid ring at the terminus of the molecule have been reported, viz.
3-hydroxy-4-[(4-1,2,4-triazol-4-ylimino)methyl] phenyl 4-alkoxybenzoate (Series-I)
and 3-hydroxy-4 (isonicotinoyl carbonohydrazonoyl) phenyl 4-alkoxy benzoate
(Series-II).
We are thankful to I.I.T.Bombay, CDRI Lucknow and Garda chemicals Ltd.,
Ankleshwar (Gujarat) to providing an elemental analysis, FT-IR, 1H-NMR,13C NMR
Mass and Thermal studies.
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
173=[915]

174=[916]
B. T. Thaker and P. Patel
The compounds of both the series have been characterized by elemental Analysis,
FT-IR, mass spectrometry, 1H-NMR and 13C-NMR. Phase transition temperatures
and the thermal parameters were obtained from differential scanning calarimetry
(DSC). The texture observation was performed under polarizing optical microscopy
(PMO) attached with Mettler hot stage. All the derivatives are mesomorphic in nat-
ure showing nematic phase as well as higher members of both the series show
smectic phase. The use of triazole and isonicotinic acid as a terminal group has
a very dramatic effect on the melting and clearing points. Similarly lateral –OH
substitution on central phenyl ring shows higher clearing temperature due to
intermolecular hydrogen bonding. The mesomorphic behavior has been analyzed
in terms of structural property relation.
Keywords: ester; isonicotinic acid hydrazide; lateral group; mesophase; nematic phase;
Schiff base; smectic phase; triazole
1. INTRODUCTION
Thermotropic liquid crystals are of great technological importance [1].
Many series of liquid crystalline compounds containg heterocyclic
groups have been synthesized because of their interesting properties [2].
There have been a variety of compounds reported with liquid
crystalline properties, but heterocyclic moieties [3,4] are less explored,
compared to homocyclic moieties. The introduction of heteroatom
causes considerable changes in chemical and physical properties and
influences the type of liquid crystal phase, also phase transition
temperatures and other properties of the mesogens [5]. Usually five
or six membered heterocycles are involved, and they form part of the
core in rod-shape (calamitic), bent-shape or disc-shape (discotic)
molecules. However, few mesogenic five membered and six membered
heterocycles have been reported.
In our present work we introduce 4-amino-1,2,4-triazole and
Isonicotinic acid hydrazide as terminal groups and synthesized two
homologues series, viz., 3-hydroxy-4-[(4-1,2,4-triazole-4-ylimino)-
methyl] phenyl 4-alkoxy benzoate (Series-A) and 3-hydroxy-4-(isonico-
tinoyl carbonohydrazonoyl) phenyl 4-alkoxy benzoate (Series-B).
1,2,4-triazoles have been reported as potential biologically active
agent [6,7]. 1,2,4-triazole and their derivatives constitute an impor-
tant class of organic compounds with diverse agricultural, industrial
and biological activities [8,9,10], including anti inflammatory [11].
Acid hydrazide and hydrazones are useful chelating agents and are
of biological importance [12]. Isonicotinic acid hydrazide (inh) and
its derivatives are antibacterial agents that have been used to treat
tuberculosis, interacting with microbial cell walls [13,14].

Synthesis, Mesophase Behavior and Thermal Stability
175/[917]
In our recent work series-A contains triazole ring as terminal group
and due to triazole ring melting point of compound are increases. The
entire compound in series-A show mesomorphic properties. In each
case an enantiotropic nematic mesophase is observed in lower member
where higher member of series -A (A12, A14, A16) shows an enantiotro-
pic smectic C phase and nematic phase whereas in series -B isonico-
tinic acid hydrazide as terminal group is shows nematic phase as
well as smectic C phase for heptyl derivative and smectic A phase
for decyl and tetradecyl derivatives. All the compound of both the
series have been characterized by elemental analysis, FT-IR,
1H-NMR, 13C-NMR, Mass spectrometry. The liquid crystalline beha-
viors of these compounds were observed by DSC study and polarizing
microscope.
2. EXPERIMENTAL
2.1. Reagents and Technique
The solvents were used after purification using the standard method
described in literature [15]. 4-hydroxy benzoic acid, 2,4-dihydroxy
benzaldehyde and DMAP (4-dimethylaminopyridine) were obtain from
Merck (Germany). 4-amino-1,2,4-triazole, Isonicotinic acid hydrazide
and DCC (Dicyclohexylcarbodiimide) was purchased from Fluka
Chemie (Switzerland). Elemental analyses (C, H, N) were performed
at CDRI, Lucknow. Infrared spectra were recorded with a Perkin-
Elmer2000 FT-IR spectrophotometer in the frequency range 4000–
400 cmꢀ1 with samples embedded in KBr discs. 1H-NMR spectra of
the compound were recorded with JEOL-GSX-400 using CDCl3 as a
solvent and TMS as an internal reference at SAIF, IIT Madras,
Chennai. 13C NMR spectra of the compound were recorded with BRU-
KER AVANCE II 400 NMR Spectrometer, SAIF, Chandigarh. Mass
spectra (EI) of the compounds were recorded at SAIF, IIT Madras,
Chennai. Thin-layer chromatography analyses were performed by
using aluminium-backed silica-gel plates (Merck 60 F524) and
examined under short-wave UV light.
The phase-transition temperatures were measured using a shi-
madzu DSC-50 at heating and cooling rates of 5ꢁC minꢀ1, respectively.
The optical microscopy studies were carried out with a Carl Zeiss
polarizing microscope equipped with a Mettler FP52 hot stage. The
textures of the compounds were observed using polarized light with
crossed polarizers with sample in thin film sandwiched between a
glass slide and cover slip.

176/[918]
B. T. Thaker and P. Patel
2.2. Synthesis
2.2.1. Synthesis of 4-n-alkoxy benzoic acid [16]
These were prepared by the reported method. A mixture of
4-hydroxy benzoic acid (1 mmole) and KOH (1.5 mmol) in 250 ml of
Methanol or Ethanol was stirred and heated at 70ꢁC while 1-bromo
alkane (1.02 mmol) was added slowly (1–2 hrs), after which mixture
was refluxed for 11–13 hrs. The reaction mixture was cooled to room
temperature and poured into acidic crushed ice. The product obtained
was filtered off and purified as reported. The molecular structure of
these acids were confirmed by spectroscopic analysis and found to be
in agreement with the reported data.
2.2.2. Synthesis of 4-formyl-3-hydroxyphenyl
4-alkoxy benzoate [17,18]
To a mixture of 4-n-alkoxy benzoic acid (1 mmol) and p-hydroxy
benzaldehyde (1 mmol) in 150 ml of distilled dichloromethane were
added 0.1 mmol of dimethyl aminopyridine (DMAP) under an argon
atmosphere. The mixture was cooled in an ice=water bath and, after
10 min, 1 mmol of dicyclohexylcarbodiimide (DCC) were added under
an argon atmosphere. The mixture was stirred overnight at room tem-
perature, the salts were filtered off and the solvent was evaporated.
The crude product was purified by flash chromatography using a
mixture of hexanes=ethyl acetate (7=1) as eluent.

Synthesis, Mesophase Behavior and Thermal Stability
177/[919]
Data
Compound B
Yield 79%. Found: C, 75.12; H, 7.68; Calc. for C24H30O4; C, 75.39; H,
7.85%. IR (KBr): Vmax=cmꢀ1 2953, 2919, 2850 cmꢀ1(C–H aliphatic),
1729 cmꢀ1(C¼O of ester), 1702 (C¼O of aldehyde), 3441 cmꢀ1(OH).
2.2.3. Synthesis of Compound 3A and 3B [17,19]
2.2.3A. Synthesis
of
3-hydroxy-4-[(4-1,2,4-triazol-4-ylimino)
methyl] phenyl 4-alkoxy benzoate. A mixture of 1 mmol 4-formyl-3-
hydroxyphenyl 4-alkoxybenzoate and 1 mmol of 4-amino-1,2,4-
triazole, and three drops of acetic acid in absolute ethanol was 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.
Data
Compound A10
Yield 78%. M.P. 188ꢁC. Found: C, 67.29; H, 6.90; N, 12.06. Calc. for
C26H32O4N4; C, 67.24; H, 6.89; N, 12.06%. EI-MS m=z (rel.int%):
464(2) (M)þ. IR (KBr): Vmax=cmꢀ1 3425 cmꢀ1(OH), 2948, 2853 cmꢀ1
(C–H aliphatic), 1729 cmꢀ1(C¼O of ester), 1619 cmꢀ1 (CH¼N),

178/[920]
B. T. Thaker and P. Patel
1600 cmꢀ1(C¼C aromatic), 1276 cmꢀ1(C–O), 1589, 1600 cmꢀ1(C¼N,
triazole), 1622 cmꢀ1(N–N¼C, triazole).1H NMR (CDCl3): d 0.91–
0.93 ppm (CH3), 1.23–1.47 ppm (CH2), 4.03–4.07 ppm (OCH2),
6.89–8.07 ppm (Ar–H), 9.10–9.30 ppm (CH¼N), 8.2 ppm (triazole-H).
12.82 ppm (OH). 13C NMR (CDCl3), 14.40 ppm (CH3), 23.00–
34.70 ppm (CH2), 110.00–162.25 ppm (Ar–C), 161.21 ppm (C¼N),
171 ppm (C¼O ester), 142.00–160.00 ppm (triazole-C).
2.2.3B. Synthesis of 3-hydroxy-4 (isonicotinoyl carbonohydrazo-
noyl) phenyl 4-alkoxy benzoate. A mixture of 1 mmol 4-formyl-3-
hydroxyphenyl 4-alkoxybenzoate and 1 mmol of isonicotinic acid
hydrazide, and three drops of acetic acid in absolute ethanol was
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.
Data
Compound B10
Yield 91%. M.P.183ꢁC. Found: C, 69.67; H, 6.77; N, 8.14. Calc.
for C30H35O5N3; C, 69.64; H, 6.76; N, 8.12%. EI-MS m=z (rel.int%):
517(2) (M)þ. IR (KBr): Vmax=cmꢀ1 3434 cmꢀ1 (OH), 2947, 2919,
2847 cmꢀ1(C–H aliphatic), 1738 cmꢀ1 (C¼O of ester), 1614 cmꢀ1
(CH¼N), 1602 cmꢀ1 (C¼C aromatic), 1271 cmꢀ1(C–O), 1667 cmꢀ1
(C¼O of hydrazone linkage), 3179–3240 cmꢀ1 (NH), 1556 cmꢀ1 (C¼N,

Synthesis, Mesophase Behavior and Thermal Stability
179/[921]
1
pyridyl ring), H NMR (CDCl3): d 0.90–0.92 ppm (CH3), 1.30–2.61 ppm
(CH2), 4.03–4.07 ppm (OCH2), 6.68–7.35 ppm (Ar–H), 8.58–8.90 ppm
(CH¼N), 12.82 ppm (OH), 10.98 ppm (NH), 8.7–8.9 ppm (pyridyl-H)
13C NMR (CDCl3), 14.28 ppm (CH3), 22.92–34.86 ppm (CH2),
110.00–163.00 ppm (Ar-C),158.99 ppm (C¼N), 172.05 ppm (C¼O, ester).
155.00–165.00 ppm (–C¼O–NH), 148.44, 149.25 ppm (pyridyl-C).
3. RESULTS AND DISCUSSION
In our recent work, 12 homologous from each of the two series,
3-hydroxy-4-[(4-1,2,4-triazol-4-ylimino) methyl] phenyl 4-alkoxy
benzoate (series-A) and 3-hydroxy-4 (isonicotinoyl carbonohydrazo-
noyl) phenyl 4-alkoxy benzoate (series-B) are synthesized and their
mesomorphic properties are studied. The common structural features
of the compounds are that they consist of two phenyl rings and
linkages with different terminal groups at one end. The mesomorphic
properties of all the synthesized compounds have been characterized
by differential scanning calorimetry (DSC) and polarizing optical
microscope (PMO) attached with Mettler hot stage.
In series-A, it is observed that the lower n-alkoxy derivatives.
i.e., From n-methyloxy to n-propenyloxy members, are gives sharp
melting point and directly convert to isotropic mesophase whereas
n-butyloxy to n-decyloxy members are nematogens. The higher
n-alkoxy derivatives viz. n-decyloxy and n-hexadecyloxy, exhibit an
additional smectic C phase along with nematic mesophase, which
can be attributed to the increase in alkoxy chain length at one end.
It is consistent with the assignment of each mesophase type, using
the classification systems reported by Sackmann and Demus [20],
Gray and Goodby [21]. In series -B, where pyridyl ring as terminal
group and (–CH¼N–NH–CO–) linkage as spacer group with ester link-
age. Compound of series-B are mesomorphic in nature. n-heptyloxy
derivative shows Sm C mesophase and n-dodecyloxy and n-tetradecy-
loxy derivatives shows Sm A mesophase along with nematic phase.
A nematic marble texture was first observed when it was cooled down
from the isotropic phase and upon further cooling the texture charac-
teristic of nematic phase has gradually changed to a phase identified
as Sm A phase. All the compounds exhibit enantiotropic behavior.
The appearance of Sm A phase is found to conform to earlier reported
Schiff base – ester [22]. From n-methyloxy to n-butyloxy derivatives
melting peaks are sharp and no mesomorphism. In both the series
melting point decreasing with increasing length of alkoxy chain.
One of the noticeable features is that the clearing temperature of
compounds of series-A are considerably higher than compounds of

180/[922]
B. T. Thaker and P. Patel
series-B. This information indicates that the introduction of hydroxy
group at the ortho position in the aldehyde fragment increases the
degree of anisotropy of the molecular polarizability of compounds in
both the series and hence increases the degree of molecular order,
causing the smectic phase to be more stable [23]. Structural feature
which has been varied in the dimeric architecture is the type of linking
group between the spacer and mesogenic units including ethers [24],
esters [25], thioesters [26] and thioethers [27]. The family of Schiff’s
bases exhibiting liquid crystalline behavior offers wide scope [28,29].
In series-B the presence of an additional amino group in the hydra-
zides causes delocalization of electrons with subsequent changes in
energy level associated with the carbonyl oxygen which acts as donor
in acid amides. The hydrazone derivatives are used as fungicides and
in the treatment of diseases such as tuberculosis, leprosy and mental
disorders [30]. The remarkable biological activity of acid hydrazides,
R–CO–NH–NH2, their corresponding hydrazones R–CO–NH–
N¼CH–R0 present in the living system have offered significant inter-
est in recent years [12]. The remarkable biological activity of acid
hydrazides R–CO–NH–NH2, a class of Schiff base, their corresponding
aroylhydrazones R–CO–NH–N¼CH–R0 and the dependence of their
mode of chelation with transition metal ions present in the living sys-
tem have been of significant interest in the past [31,32]. In the past
many compounds containg ester-Schiff’s base central linkage shows
mesomorphism [18]. Ester derivatives are more stable compound to
Schiff’s base derivatives.
TABLE 1 Transition Temperature of Series-A
Transition temperature ꢁC
Compounds
R ¼ n alkoxy
SmC
N
I
A1
A2
A3
A4
A5
A6
A7
A8
A10
A12
A14
A16
Methyl
Ethyl
Propyl
Butyl
Pentyl
Hexyl
Heptyl
Octyl
–
–
–
–
–
–
–
–
–
–
–
–
289
285
279
263
247
233
217
199
188
173
160
146
202
189
177
166
156
147
141.14
132
124.92
Decyl
Dodecyl
Tetradecyl
Hexadecyl
109
86
72
Note: Sm C, smectic C; N, nematic; I, isotropic.

Synthesis, Mesophase Behavior and Thermal Stability
181/[923]
FIGURE 1 The phase behavior of series-A compounds.
In series-A twelve compounds have been synthesized and their
mesogenic properties are evaluated. All the compounds synthesized
exhibit enantiotropic smectic C and nematic mesophase. For series-A
transition temperature are recorded in Table 1 and plot of transition
temperatures against the number of carbon atom in the alkoxy chain
(Fig. 1) show steady fall in N-I and Sm C-N transitions.
In series-B all the twelve members synthesized exhibit enantio-
tropic nematic mesophase. Smectic-C mesophase appears for the
TABLE 2 Transition Temperature of Series-B
Transition temperature ꢁC
Compounds
R ¼ n alkoxy
Sm
N
I
B1
B2
B3
B4
B5
B6
B7
B8
B10
B12
B14
B16
Methyl
Ethyl
Propyl
Butyl
Pentyl
Hexyl
Heptyl
Octyl
–
–
–
–
–
–
129y
116
99ꢂ
85
–
–
–
–
183
174
161
149
139
102
83
238
231
227
215
206
198
192
187
183.59
180
Decyl
Dodecyl
Tetradecyl
Hexadecyl
77ꢂ
62
178.01
162
70
Note: y, Sm C; ꢂSm A; N, nematic; I, isotropic.

182/[924]
B. T. Thaker and P. Patel
FIGURE 2 The phase behavior of series-B compounds.
n-heptyloxy derivative, but n-decyloxy and n-tetradecyloxy derivative
shows Smectic A mesophase as an enantiotropic phase. The transition
temperatures are recorded in Table 2. The entire homologous series-B
exhibit mesomorphism. The plot of transition temperature against the
number of carbon atom in the alkoxy chain (Fig. 2) shows a smooth
falling tendency for nematic-isotropic transition temperatures
throughout the series. Series-B also exhibit falling tendency of Sm-N
TABLE 3 DSC Data for Series–A and B Compounds
Peak Temp.
(Microscopic temp.) ꢁC
Series
A
Compound
A10
Transition
DH Jgꢀ1
DS Jgꢀ1Kꢀ1
Cr-N
N-I
Cr–Sm C
Sm C-N
N-I
Cr–Sm A
Sm A-N
N-I
Cr–Sm A
Sm A–N
N–I
147
173
72
124.92
147
99
139
183.59
77
83
32.04
83.88
6.82
0.076
0.188
0.019
0.029
0.033
0.128
0.162
0.193
0.106
0.113
0.191
A16
B10
B14
11.85
13.94
47.83
67.15
88.41
37.33
40.24
86.30
B
178.01
Note: Cr, crystal; SmA, smactic A; SmC, smectic C; N, nematic; I, isotropic.

Synthesis, Mesophase Behavior and Thermal Stability
183/[925]
TABLE 4 Different Transition Temperatures and Range of Mesophases
Observed in Series A and B
Sm-N mesophase
range (ꢁC)
N-I mesophase
range (ꢁC)
Series
Compound
SmC
N
I
A
B
n ¼ 14
n ¼ 14
86
77
132
83
160
178.01
46
6
28
104.01
transition temperatures for higher homologous. Table 3 shows the
phase transition temperatures, associated enthalpy (DH) and molar
entropy (DS) for compound of series-A (A10, A16) and series-B
(B10,B14). Enthalpy values of the various transitions agree well with
the existing related literature values [33].
Table 4 shows the comparison of Sm-N and N-I transition tempera-
ture of compound n ¼ 14, series A, and structurally related compounds
n ¼ 14, series B. The Sm-N mesophase range of compound n ¼ 14
(series A) is higher by 40ꢁC, respectively, when compared with
compound n ¼ 14 (series B). The N-I transition temperature of com-
pound, series-B, is lower by 76.01ꢁC, respectively, when compared
with compound 14 (n ¼ 14, series B). The two series differs by terminal
heterocyclic ring (series-A contain 1,2,4-triazole and series-B contain
pyridyl ring) as well as linking group. Ester linkage is common in both
the series but another linkage is different (series -A contain Schiff base
linkage and series -B contain hydrazone linkage). The thermal stabi-
lity of series-B is lower than of series -A due to hydrazone linkage
and terminal pyridyl ring.
4. CONCLUSION
In this article we have presented the synthesis and characterization of
two new mesogenic homologous series of ester-azomethine derivatives
containing 1,2,4-triazole and isonicotinic acid hydrazide heterocyclic
ring as terminal group and -OH group as lateral substitution. Ester
derivatives are more stable compound to Schiff’s base derivatives.
Compound of series -A shows Sm C mesophase along with nematic
phase as it is a short two phenyl rings system, where as series-B
exhibit nematic as well as Sm C and Sm A mesophases due to the
presence of –CH¼N–NH–CO– linkage and terminal pyridyl ring.
From the study of transition temperature of both the series we can
assume that the introduction of hydroxy group at the ortho position
in the aldehyde fragment increases the degree of anisotropy of the

184/[926]
B. T. Thaker and P. Patel
molecular polarizability of compounds in both the series. The
compound exhibit mesomorphic properties with good thermal stabili-
ties if properly designed.
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