Synthesis, characterization and mesomorphic properties of new rod-like thiophene based liquid crystals

Article abstract of DOI:10.1080/10426507.2012.673943

Two new mesogenic homologous series of Schiff base esters, 2-[4-(4-n-Alkoxy benzoyloxy) benzylidenamino] 3-cyno thiophine (Series-A) and Schiff base cinnamates, 2-[4-(4-n-alkoxy cinnamoyloxy) benzylidenamino] 3-cyano thiophene (Series-B), comprising a thiophene moiety were synthesized. Structural elucidation was carried out using elemental analysis and spectroscopic techniques such as FT-IR, ¹H-NMR and ¹³C-NMR, and mass spectrometry. The mesomorphic properties and thermal stabilities of the title compounds were studied by using differential scanning calorimetry and optical polarizing microscopy. All the derivatives are mesomorphic in nature showing the nematic phase, and the higher members of Series-A show a smectic C phase whereas Series-B exhibits only the nematic mesophase. The mesomorphic properties of the present series are compared with other structurally related compounds. Taylor and Francis Group, LLC.

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Synthesis, Characterization and
Mesomorphic Properties of New Rod-like
Thiophene Based Liquid Crystals
B. T. Thaker a , B. S. Patel a , Y. T. Dhimmar a , D. B. Solnki a , N. J.
Chothani a , N. B. Patel a , K. B. Patel a & U. Makavana a
a Department of Chemistry , Veer Narmad South Gujarat University ,
Surat , Gujarat , India
Published online: 30 Jul 2012.
To cite this article: B. T. Thaker , B. S. Patel , Y. T. Dhimmar , D. B. Solnki , N. J. Chothani , N. B.
Patel , K. B. Patel & U. Makavana (2012) Synthesis, Characterization and Mesomorphic Properties of
New Rod-like Thiophene Based Liquid Crystals, Molecular Crystals and Liquid Crystals, 562:1, 98-113,
DOI: 10.1080/10426507.2012.673943
To link to this article: http://dx.doi.org/10.1080/10426507.2012.673943
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Mol. Cryst. Liq. Cryst., Vol. 562: pp. 98–113, 2012
Copyright © Taylor & Francis Group, LLC
ISSN: 1542-1406 print/1563-5287 online
DOI: 10.1080/10426507.2012.673943
Synthesis, Characterization and Mesomorphic
Properties of New Rod-like Thiophene Based
Liquid Crystals
B. T. THAKER,∗ B. S. PATEL, Y. T. DHIMMAR, D. B. SOLNKI,
N. J. CHOTHANI, N. B. PATEL, K. B. PATEL,
AND U. MAKAVANA
Department of Chemistry, Veer Narmad South Gujarat University, Surat,
Gujarat, India
Two new mesogenic homologous series of Schiff base esters, 2-[4-(4ꢀ-n-Alkoxy benzoy-
loxy) benzylidenamino] 3-cyno thiophine (Series-A) and Schiff base cinnamates, 2-[4-
(4ꢀ-n-alkoxy cinnamoyloxy) benzylidenamino] 3-cyano thiophene (Series-B), compris-
ing a thiophene moiety were synthesized. Structural elucidation was carried out using
elemental analysis and spectroscopic techniques such as FT-IR, 1H-NMR and 13C-NMR,
and mass spectrometry. The mesomorphic properties and thermal stabilities of the title
compounds were studied by using differential scanning calorimetry and optical polar-
izing microscopy. All the derivatives are mesomorphic in nature showing the nematic
phase, and the higher members of Series-A show a smectic C phase whereas Series-B
exhibits only the nematic mesophase. The mesomorphic properties of the present series
are compared with other structurally related compounds.
Keywords 3-cyno thiophene; cinnamates; ester; nematic; schiff base; smectic C
1. Introduction
The field of liquid crystals (LCs) has incorporated numerous different organic systems
in both low and high molecular weight materials [1–3]. Although classical thermotropic
liquid crystals are commonly composed of rod-like molecules, many other types of low
molecular mass compounds with unconventional molecular structures have been shown to
exhibit liquid crystalline properties [4].
Many series of liquid crystalline compounds containing heterocyclic groups have been
synthesized due to their potentially wide range of applications, such as in the optical,
electrical, biological, and medical fields [5–9]. During the last decades a large number
of mesomorphic compounds containing heterocyclic units were synthesized and evaluated
[10,11]. Interest in these compounds arises because the inclusion of heteroatoms can cause
∗Address correspondence to B. T. Thaker, Department of Chemistry, Veer Narmad South
Gujarat University, Surat-395007,, Gujarat, India. Tel.: (+91) 9228377618. E-mail: btthaker1@
yahoo.co.in
98

Thiophene Liquid Crystals
99
large changes in the type of mesophase present and/or in the physical properties of the
materials.
Heterocycles are of great importance as core units in thermotropic liquid crystals due
to their ability to impart lateral and/or longitudinal dipoles combined with changes in
the molecular shape. These materials hold great potential for use in spatial light modula-
tion [12], all-optical signal processing, optical information storage [13], organic thin-film
transistors [14,15], fast switching ferroelectric materials [16], fluorescent probes for the
detection and analysis of biomolecules etc. [17]. Thiophene in particular has emerged as
a core unit that is receiving increasing attention. Sulfur-based heterocycles are also being
used to elucidate the structures of complex mesophases. Thiophenes played a major role
in the synthesis of systems displaying supramolecular chirality when dissolved in solvents
where dissolution is not strongly favored [18]. Five-membered heterocycles have potential
for flexoelectric applications such as found in bistable nematic displays. A number of thio-
phenes [19] have already been evaluated for such applications, and other bent heterocycles
may have equal promise.
There are relatively less examples of LC materials incorporating the thiophene ring.
This is despite the fact that thiophene-based LC materials (a) have lower melting points
than the 1,4-phenylene analogues, (b) promote negative dielectric anisotropy, and (c) have a
tendency to generate a range of different liquid crystalline phases [20–26]. Five-membered
rings provide materials of low melting point and viscosity, large optical anisotropy, and fast
switching times [27].
There has been a continuing interest in the study of heterocyclic-based liquid crys-
tal compounds owing to the great variety of their structures. Thiophene-based calamitic
liquid crystals are currently the subject of intensive study [28–34]. Their applications as
ferroelectric materials as well as potential materials for molecular electronic devices, such
as organic field effect transistors, are of special interest. Heterocyclic compounds such
as five-membered thiadiazole or thiophene rings can be incorporated into the principal
structure of calamitic mesogens [35–39]. Sulfur containing heterocycles are important syn-
thetic intermediates and have found a variety of applications in medicinal, agricultural,
and materials chemistry [40–41]. LCs containing heterocyclic cores, such as thiophene, are
of particular interest due to their slightly bent structure, which leads to features including
a reduced packing ability, a medium to strong lateral dipole and high anisotropy of the
polarizability.
The mesomorphic properties of aromatic Schiff base esters arising from substituents
varying in their polarities have been reported by Ha et al. [42]. Many mesogenic ho-
mologous series contain two central linkages, one of which may be ester and the other
azomethine [43,44]. Vora and Rajput [45] reported binary mixtures of cinnamate ester
exhibit wide rang of smectic and nematic mesophase. Previously we have reported two
mesogenic homologous series of cinnamate-azomethine [46] containing thiophene and fu-
ran heterocycles. 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 longitudinal polarizability and extends the molecular length maintaining
linearity of the molecule. Recently, there has been a continuing interest in study the effect
of an ethylene linking group and thiophene moiety on the mesomorphic properties of such
molecules.
The present investigation concerns the synthesis, characterization, and mesomorphic
properties of two new liquid crystalline homologous series with a common central linkage
(azomethine) with a differing central linkage (cinnamate-ester) with a terminal heterocyclic
moiety such as 2-amino-3-cyanothiophene.

100
B. T. Thaker et al.
NC
RO
C
O
CH
N
S
O
2-[4-(4´-n-alkoxy benzoyloxy)benzylidenamino] 3-cyano thiophine
(Series-A)
NC
RO
CH
CH
C
O
O
CH
N
S
2-[4-(4´- n-alkoxy cinnamoyloxy)benzylidenamino] 3-cyano thiophine
(Series-B).
Where, R=CnH2n+1, n=1 to 8,10,12,14,16,18.
2. Experimental Details
2.1 Materials
4-Hydroxy benzoic acid and 4-hydroxy benzaldehyde were obtained from Merck (Ger-
many). Alkyl bromide (Lancaster, England). 2-amino 3-cyano thiophine and malonic acid
were purchased from Fluka Chemie (Switzerland). N,Nꢀ-dicyclohexylcarbodiimide (DCC)
were purchased from Acros Organics (USA). DMAP (N,N-dimethylaminopyridine) was
purchased from Merck (Germany). Pyridine, piperidine, anhydrous potassium carbon-
ate, acetone, ethanol, methanol, acetic acid, ethyl acetate, HCl, KOH, NaOH etc. were
used as received. Column chromatography was performed using Acme’s Silica Gel (100–
200 mesh). Solvents were dried and distilled prior to use.
2.2 Measurements
The C, H, and N contents of selected mesogenic samples was estimated by G.N.F.C. (Gujarat
Narmda Valley Fertilizer Company Ltd., Bharuch). Infrared spectra were recorded with
a THERMO SCIENTIFIC NICOLET iSO-10 spectrophotometer in the frequency range
4000–400 cm−1 with samples embedded in KBr discs at our department. High resolution
(400 MHz) NMR spectra of the mesogenic compounds were recorded at room temperature
as 15%–20% solution in CDCl3 using TMS as internal standard on a BRUKER AVANCE
II 400 NMR spectrometer at SAIF (Sophisticated Analytical Instrument Facilities), Panjab
University, Chandigarh. Mass spectra (TOFMSES+) of the compounds wererecorded using
Finnegan MAT-8230 Mass Spectrometer at SAIF (Sophisticated Analytical Instrument
Facilities), Panjab University, Chandigarh. Thin-layer chromatography (TLC) analyses
were performed using aluminium-backed silica-gel plates (Merck60 F524) and examined
under shortwave UV light. Thermal (DSC) analyses of the liquid crystalline compounds
were carried out from Atul Industries Ltd. P-P site Atul. DSC analyses were performed on
METTELER M-3 thermo balance (Switzerland) with microprocessor TA-300 instrument
at a heating rate of 10◦C/min in N2 atmosphere. The optical microscopy studies were

Thiophene Liquid Crystals
101
determined by using polarizing microscope NICON ECLIPSE 50i POL (Japan) equipped
with Linkam Analysa-LTS420 hot stage (London) at our department. 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 cover slip.
2.3 Synthesis of Series-A and Series-B Compounds
2.3.1 4-n-alkoxy benzoic acid. Number of methods is known for alkylation of 4-hydroxy
benzoic acid. However, in the present study, the method devloped by Dave and Vora [47]
was followed. The clearing point of these compounds was compared with the reported one
and they are almost similar to reported values [48,49].
2.3.2 4-(4ꢀ-n-alkoxy benzoyloxy) benzaldehydes. The compound has been prepared by
etherification of the appropriate 4ꢀ-n-alkoxy acid (2.02 mmol) and 4-hydroxy benzaldehy-
des (0.246 g, 2.02 mmol), dicyclohexylcarbodiimide (0.457 g, 2.22 mmol) and dimethy-
laminopyridine (0.002 g, 0.2 mmol) in dry CH2Cl2 (20 mL) was stirred at room temperature
for 24 h. The ensuing white precipitate was isolated by Buchner filtration and discarded,
while the filtrate was evaporated to dryness in vacuo. The resultant crude residue was pu-
rified by column chromatography on silica gel eluting with dichloromethane, followed by
repeated recrystallization from ethanol until constant transition temperatures were achieved
[50–52].
2.3.3 2-[4-(4ꢀ-n-alkoxy benzoyloxy) benzylidenamino] 3-cyano thiophene. A mixture of
4-(4ꢀ-n-alkoxy benzoyloxy) benzaldehydes (10 mmol) and 2-amino 3-cyano thiophene
(1.241g, 10 mmol) and three drops of acetic acid in absolute ethanol (10 mL) were refluxed
for 4 h. The reaction mixture was allowed to cool and was stirred at room temperature
overnight. The residue obtained on removal of solvent was chromatographed on silica
gel (100–200 mesh) using petroleum ether (60◦C–80◦C) ethyl acetate mixture (80:20) as
eluant. Removal of solvent from the eluant afforded a solid material, which was crystallized
repeatedly from ethanol until constant transition temperatures were obtained. The purity
of these compounds was checked by thin layer chromatography (Merck silica gel 60 F254
precoated plates).
Data:
A6: Yield 82%. Clearing Point (C.P.) 94◦C, UV-Visible (CHCl3) λmax: 354 nm, 278
nm, Found C, 70.55; H, 6.16; N, 6.08; Calc. for C29H32N2SO3 (488 gm/mole); C, 70.43;
H, 6.08; N, 6.08;%. IR (KBr) υmax cm−1 3079 (C H Str. aromatic), 2932, 2858 (C H
Str. aliphatic), 1731 (C O Str. ester), 1641 (CH N, Str. azomethine), 2228 ( N C). 1H
NMR (400 MHz, CDCl3): ꢀ/ppm 0.86–0.89 (t, CH3), 1.25–1.84 (m, CH2), 4.03–4.06(t,
OCH2), 6.63–8.12 (m, Ar-H), 8.54 (s, CH N). 13C NMR (CDCl3): ꢀ/ppm 14.15 (CH3),
23.71–31.94 (CH2) 68.04 (OCH2), 114.45–163.15 (Ar-C), 159.95( CH = N), 164.41
( C O ), 115.63 ( N C) TOF MS ES+ m/z (rel.int%): 488.5 (M)+m/z.
2.3.4 4-n-alkoxy benzaldehydes. These were synthesized by alkylation of 4-hydroxy ben-
zaldehyde using the reported method of Vyas and Shah [53]. The clearing points of these
compounds were compared with the reported one and they are almost similar to reported
values.
2.3.5 4-n-alkoxy cinnamic acid. 4-n-alkoxy cinnamic acid were prepared by the method
of Gray and Jones [54].

102
B. T. Thaker et al.
2.3.6 4-(4ꢀ-n-alkoxy cinnamoyloxy) benzaldehydes. The compound has been prepared by
esterification of a mixture of the appropriate 4ꢀ-n-alkoxy acid and the appropriate 4-
hydroxy benzaldehydes (0.246 g, 2.02 mmol), dicyclohexylcarbodiimide (0.457g, 2.22
mmol), dimethylaminopyridine (0.002g, 0.2 mmol) and dry CH2Cl2 (20 mL) was stirred at
room temperature overnight. The ensuing white precipitate was isolated by Buchner filtra-
tion and discarded, while the filtrate was evaporated to dryness in vacuo. The resultant crude
residue was purified by column chromatography on silica gel eluting with dichloromethane,
followed by repeated recrystallization from ethanol until constant transition temperatures
were achieved [50–52].
2.3.7 2-[4-(4ꢀ-n-alkoxy cinnamoyloxy) benzylidenamino] 3-cyano thiophene. A mixture of
4-(4ꢀ-n-alkoxy cinnamoyloxy) benzaldehydes (10 mmol) and 2-amino 3-cyano thiophene
(1.241g, 10 mmol) and three drops of acetic acid in absolute ethanol (10 mL) was refluxed
for 4 h. The reaction mixture was allowed to cool and was stirred at room temperature
overnight. The residue obtained on removal of solvent was chromatographed on silica
gel (100–200 mesh) using petroleum ether (60◦C–80◦C) ethyl acetate mixture (80:20) as
eluant. Removal of solvent from the eluant afforded a solid material, which was crystallized
repeatedly from ethanol until constant transition temperatures were obtained. The purity
of these compounds was checked by thin layer chromatography (Merck silica gel 60 F254
precoated plates).
Data:
B6: Yield 80%. Clearing Point (C.P.) 90◦C, UV-Visible (CHCl3) λmax: 322 nm,
Found C, 72.44; H, 6.59; N, 5.41; Calc. for C31H34N2SO3 (514 gm/mol); C72.37; H,
6. 61; N, 5.44;%. IR (KBr) υmax cm−13096 (C H Str. aromatic), 2970, 2884 (C H
Str. aliphatic), 1725 (C O Str. ester), 1630 (CH N, Str. azomethine), 2232 ( N C).1H
NMR (400 MHz, CDCl3): ꢀ/ppm 0.86–0.89(t, CH3), 1.25–1.78 (m, CH2), 4.03–4.07
(t, OCH2), 6.63–8.12 (m, Ar-H), 8.53 (s, CH N). 13C NMR (CDCl3): ꢀ/ppm 14.21
(CH3), 22.77–32.00 (CH2) 68.47 (OCH2), 114.52–163.22 (Ar-C), 160.02 ( CH N),
164.48 ( C O ), 114.49, 147.21 ( CH CH ), 114.70 ( N C) TOF MS ES+ m/z
(rel.int%): 514.4 (M)+ m/z.
3. Results and Discussion
The synthetic route used for the preparation of Series-A and B is shown in Scheme 1.
1
All compounds were characterised by elemental analysis, H-NMR, 13C-NMR, FT-IR
spectroscopy. The mesomorphic properties of all the synthesized compounds have been
investigated by differential scanning calorimetry (DSC) and polarizing optical microscope
(PMO) attached with a Linkam hot stage.
3.1 The phase Behavior of Series A and B
All the thirteen members of Series-A exhibit an enantiotropic nematic phase. The SmC
mesophase commences from the n-dodecanoyloxy derivative along with the nematic phase.
The transition temperatures are recorded in Table 1 and a plot of transition temperatures
against the number of carbon atoms in the alkoxy chain is given in Fig. 1. It can be noticed
that the nematic-isotropic transition temperature shows a smooth falling tendency and
does not exhibit an odd–even effect. It also exhibits a tendency for rising smectic-nematic
transition temperatures in the ascending Series-A.
All the compounds synthesized in Series-B exhibit enantiotropic nematic phase. The
transition temperatures are recorded in Table 2 and a plot of transition temperatures against

Thiophene Liquid Crystals
103
(i)
(ii)
HO
COOH
RO
COOH
RO
C
O
CHO
O
(A)
(B)
(iii)
NC
RO
C
O
O
CH
N
S
(v)
(iv)
RO
CH
CH
COOH
CHO
HO
CHO
RO
CHO
(E)
C
(D)
(vi)
RO
CH
CH
O
O
(F)
(vii)
NC
RO
CH
CH
C
O
O
CH
N
S
Where, R=C H2n+1, n=1 to 8,10,12,14,16,18.
n
Scheme 1. Synthetic route to Series-A and B. Reagents and conditions: (i) R-Br, KOH, Ethanol; (ii)
DCC, DMAP, CH2Cl2, 4-Hydroxy benzaldehyde stirred at 0◦C for 1 h, stirred at room temperature
for 24 h; (iii) Ethanol, 2 to 3 drop AcOH reflux for 4 h; (iv) RBr, K2CO3, Dry acetone; (v) Malonic
acid, Piperidine reflux for 6–8 h; (vi) DCC, DMAP, CH2Cl2, 4-Hydroxy benzaldehyde stirred at 0◦C
for 1 h, stirred at room temperature for 24 h; (vii) Ethanol, 2 to 3 drop AcOH reflux for 4 h.
the number of carbon atoms in the alkoxy chain is given in Fig. 2. It can be noticed that
the crystal to mesophase transition temperatures increase with the usual old-even effect
for lower members. The nematic-isotropic transition temperatures also show no odd–even
effect.
All the compounds of Series-A and Series-B exhibit mesomorphism. On cooling the
isotropic liquid of Series-A the compounds form small droplets that coalesce to classical
Schlieren textures characteristic of the nematic phase. On further cooling, higher members
show the focal-conic texture characteristic of the SmC mesophase. For Series-B on cooling
the isotropic liquid, all the members exhibit the Schlieren texture of the nematic phase,
and no smectic mesophase is observed even in higher homologues. It is consistent with the
assignment of each mesophase type using the classification system reported by Sackmann
and Demus [55] and Gray and Goodby [56].
DSC is a valuable method for the detection of phase transition. It yields quantitative
results; therefore, we may draw concerning the nature of the phase that occurs during
the transition. The phase transition temperatures and corresponding enthalpy changes of

104
B. T. Thaker et al.
Table 1. Transition temperatures (◦C) data of 2-[4-(4ꢀ-n-alkoxy benzoyloxy) benzylide-
namino] 3-cyano thiophene (Series-A)
Transition temperatures ◦C
Compounds
R = n alkoxy
Cr
SmC
N
I
A1
A2
A3
A4
A5
A6
A7
A8
A10
A12
A14
A16
A18
Methyl
Ethyl
Propyl
Butyl
Pentyl
Hexyl
Heptyl
Octyl
Decyl
Dodecyl
Tetradecyl
Hexadecyl
Hexadodecyl
•
•
•
•
•
•
•
•
•
•
•
•
•
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
•
•
•
•
126
118
121
114
110
105
94
78
69
72
74
•
•
•
•
•
•
•
•
•
•
•
•
•
165
162
160
156
153
150
127
110
94
91
88
86
83
•
•
•
•
•
•
•
•
•
•
•
•
•
–
(49)∗
47
52
54
67
64
()∗ monotropic.
compounds A6, A10, B6, B10 were determined using a DSC. The data obtained from the
DSC analysis and from POM are summarized in Table 3, which helps to further confirm the
mesophase type. Table 3 shows the phase transition temperatures, associated enthalpy (ꢀH)
and molar entropy ꢀS for compounds of Series-A (A6, A10) and Series-B (B6, B10). The
DSC curves of representative compounds are shown in Figs. 3–6. Microscopic transition
temperature values are almost similar to DSC data. 7
180
160
140
120
Cr-SmC
100
SmC-N
80
N-I
60
40
20
0
0
2
4
6
8
10
12
14
16
18
20
Number of carbon atom
Figure 1. Transition temperature curve of Series-A.

Thiophene Liquid Crystals
105
Table 2. Transition temperatures (◦C) data of 2-[4-(4ꢀ-n-alkoxy cinnamoyloxy) benzylide-
namino] 3-cyano thiophene (Series-B)
Transition temperatures ◦C
Compounds
R = n alkoxy
Cr
Sm
N
I
B1
B2
B3
B4
B5
B6
B7
B8
B10
B12
B14
B16
B18
Methyl
Ethyl
Propyl
Butyl
Pentyl
Hexyl
Heptyl
Octyl
Decyl
Dodecyl
Tetradecyl
Hexadecyl
Hexadodecyl
•
•
•
•
•
•
•
•
•
•
•
•
•
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
148
151
154
143
147
139
116
95
81
77
74
72
•
•
•
•
•
•
•
•
•
•
•
•
•
197
202
191
180
185
171
153
134
99
96
95
92
88
•
•
•
•
•
•
•
•
•
•
•
•
•
69
3.2 Mesomoxrphic Behavior
In Series-A, as the length of the carbon chain increased, an enantiotropic smectic C phase
was observed from the A14 derivative. In fact, the smectic phase observed as monotropic
on cooling for the compound A12. The crystal to nematic mesophase transition temperature
gradually decreased from the C4 members. Clearing points descended with the increase in
180
160
140
120
Cr-SmC
100
80
SmC-N
N-I
60
40
20
0
0
2
4
6
8
10 12 14 16 18 20
Number of carbon atom
Figure 2. Transition temperature curve of Series-B.

106
B. T. Thaker et al.
Table 3. DSC data of the Series-C and D
Compound
No.
Peak/POM
ꢀH
J/g
ꢀS
Series
A
Transition
Temp. ◦C
J/g◦K
A6
Cr-N
N-I
Cr-N
N-I
104.67(105)
150.67(150)
69.31(69)
32.05
11.87
12.70
4.93
0.3062
0.0787
0.1832
0.0523
A10
94.17(94)
B
B6
Cr-N
N-I
Cr-N
N-I
139.18(139)
170.80(171)
80.81(81)
36.29
15.47
10.37
24.93
0.2607
0.0905
0.1283
0.2510
B10
99.29(99)
length of the carbon chain due to the dilution of the mesogenic core resulting from the flex-
ibility provided by the terminal alkanoyloxy chain. Generally, short-chain members favor
nematic formation, whereas the smectic phase is more favorable in long-chain members
[57]. This general trend was obeyed by the Series-A depicted in Figure 1 in which the
nematic phase range reduced as the length of the terminal chain increased.
In Series-B, all synthesized compounds show only the nematic mesophase, the nematic
transition temperature does not show the odd–even effect in the short-chain members (n =
1 to 6). Then Clearing points descended with the increase in length of the carbon chain due
to the dilution of the mesogenic core resulting from the flexibility provided by the terminal
alkanoyloxy chain [58].
Figure 3. DSC thermogram for compound A6 (Series-A).

Thiophene Liquid Crystals
107
Figure 4. DSC thermogram for compound A10 (Series-A).
3.3 Chemical Structure-Mesomorphic Property Relationship
There is close relation between mesomorphism and molecular constitution of organic
compounds. Hence, transition temperatures and mesophase range as measures of meso-
morphism can be correlated with the molecular constitution of the compounds. Table 4
summarizes the average thermal stabilities, mesophase range, and comparative geometry
Figure 5. DSC thermogram for compound no B6 (Series-B).

108
B. T. Thaker et al.
Figure 6. DSC thermogram for compound no B10 (Series-B).
of the present Series-A, B, and structurally related Series-I [59], II [60], III, and IV [61]
reported in the literature. The average nematic mesophase range of Series-B is higher
by 0.32◦C and the N-I mesophase thermal stability is higher by 19.84◦C compared to
the respective mesophase ranges of Series-A. This is understandable, as the molecules of
Series-B are longer and more polarizable compared to the molecules of Series-A due to the
presence of additional cinnamoyloxy ( CH CH COO ) central linkage. The molecules
of Series-A and B differ only at the central linkages. The molecules of Series- B have
cinnamoyloxy ( CH CH COO ) central linkage, while Series-A have ester ( COO )
central linkage.
Gray [62] has explained that the addition of double bond in the system increases
the polarizability and length of the rod-like molecules. Therefore, the greater mesophase
thermal stability of the present Series-B must be explained in terms of the greater molecular
Table 4. The mesophase range and thermal stabilities of Series-A, B, and structurally
related series-I to IV
Thermal stabilities
Mesophase range (◦C)
(◦C)
Commencement
of Smectic phase
Series
Smectic
Nematic
Sm-N
N-I
A
I
18.75
89.0
31.76
39.58
93.23
181.33
125.00
221.00
C12
C12
II
Nonmesogenic
B
III
IV
—
16.0
—
32.08
69.0
82.28
—
133.69
—
144.84
202.23
212.08
—
C10
—

109

110
B. T. Thaker et al.
length and polarizability of the molecule resulting from additional CH CH units in
the central linkage.
Series-A exhibits both smectic as well as nematic mesophases (texture of smectic and
nematic phases shown in Figure-7), where as Series-B exhibits only the nematic mesophase
(nematogenic). There is only one difference between Series-A and B. Series-A containing
ester-azomethine central likage while Series-B having cinnamate-azomathine linkage. The
ester group is more conducive for mesophase then cinnamate group. Therefore, Series-A
exhibit both mesophase, i.e., smectic at higher homologus and nematic mesophase from
methoxy group until the end. In case of length to breath ratio is higher, which shows nematic
phase only. It can be seen that Series-B having higher length to breath ratio than Series-A.
As a result of this, Series B exhibit only nematic phase (textures of compound B6 and B7
are shown in Figure 7) where as Series A exhibit both smetic as well nematic mesophase.
Comparison of Series-A with Series-I and Series-II, respectively, gives insight on the
role played by the terminal ring. The structural difference between the series is one of the
NC
RO
RO
RO
C
O
O
C
H
N
S
(Series-A)
N
C
O
C
N
N
N
O
H
(Series-I)
C
O
O
C
H
N
(Series-II)
NC
RO
CH
CH
C
O
O
C
H
N
S
(Series-B)
N
S
C
H
N
CH
CH
O
RO
C
O
F
(Series-III)
C
O
CH
CH
O
C
RO
N
H
(Series-IV)

Thiophene Liquid Crystals
111
terminal rings. The compounds of Series-A and Series-I are mesogenic, whereas Series-II
is nonmesogenic in nature because of the presence of the cyclohexane ring.
Series-A is compared with related Series-I. All the members of Series-A exhibit an
enantiotropic nematic phase. The SmC mesophase commences from the n-dodecyloxy
derivative as a monotropic phase. n-Tetradecyloxy to n-octadecyloxy members exhibit
an enantiotropic SmC phase, whereas in Series-I the n-dodecyloxy to n-hexadecyloxy
derivatives exhibit the SmC phase along with an enantiotropic nematic phase. The lone
pairs of electrons on the nitrogen atoms act to broaden the molecule and also introduce
attractive forces, which aid smectic formation. Reference to Table 4 indicates that the
nematic mesophase length and N-I phase thermal stability of Series-I are higher by 7.82◦C
and 96.0◦C, respectively, than that of present Series-A. Both the series differ only at one
terminus. Series-A has a 3-cyano thiophene ring at the terminus instead of the 1,2,4-
triazole ring of Series-II. Owing to the inherent nature of thiophene, it nonlinearly reduces
the efficiency of packing and thus lowers the mesophase thermal stability of members of
Series-A than Series-I.
Reference to Table 4 indicates that the nematic mesophase length and N-I phase thermal
stability of Series-B are lower by 36.92◦C and 57.39◦C, respectively, than that of present
Series-III, similarly Series-B are lower by 50.2◦C and 67.24◦C, respectively, than that
of present Series-IV. Both the series differ only at one terminus. Series-A has a 3-cyano
thiophene ring at the terminus instead of the 6-fluro benzothiazole ring of Series-III and
benzene ring of Series-IV. The fact that thermal stabilities of Series-III and IV are higher
than that of Series-B (of present work) suggests that even though Series-B contains a five-
membered thiophene ring, which normally imparts nonlinearity, the essential attracting
forces are similar to one present in Series-IV benzene analogues. Oh [63] has reported
that all the transition temperatures of pyridine analogues were lower compared to the
benzene analogues. In the present study also shows that the thiophene derivatives have
lower transition temperatures. The hetero atom has high electro negativity and, therefore,
withdraws electron from the other atoms of the ring system, rendering the ring deactivated
related to benzene.
4. Conclusion
In this paper, we have presented the synthesis and characterization of new mesogenic ho-
mologous series viz 2-[4-(4ꢀ-n-alkoxy benzoyloxy) benzylidenamino] 3-cyano thiophene
(Series-A) which containing ester-azomethine central likage and 2-[4-(4ꢀ-n-alkoxy cin-
namoyloxy) benzylidenamino] 3-cyano thiophene (Series-B), which contain a cinnamate-
azomethine central linkage. Series-A with an ester-azomethine central linkage has lower
thermal stabilites compared with Series-B with a cinnamate-azomethine central linkage.
The ester central linkage is more conducive to conferring mesophases in the materials than
the cinnamate central linkage. Series-A exhibits both smectic and nematic mesophases
while Series-B exhibits only the nematic mesophase. The mesophase range of the Series-B
analogues is higher than those of Series-A, which is attributed to the high polarizability
of the molecules. Members of series-B with a cinnamoyloxy central linkage are more sta-
ble compared with the Series-A members containing ester-azomethine central linkage due
to greater molecular length and polarizability of the molecule resulting from additional
CH=CH units in the central linkage. Moreover, the terminal thiophene derivatives have
lower transition temperatures due to high electro negative “S” atom. There is no much
effect have been observed on the transition temperature by the presence of CN group at
lateral position in heterocyclic ring.

112
B. T. Thaker et al.
Acknowledgment
The authors are thankful to Gujarat Narmada Valley Fertilizer Company Ltd. (G.N.F.C.),
Bharuch, for providing facilities of elemental analysis, to Atul Industries Ltd. Atul, for DSC
1
analysis, and also to SAIF Chandigarh for providing facilities of FT-IR, H-NMR, 13C-
NMR, and Mass spectral analysis.
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