Synthesis, structural and mesophase characterization of three ring based thiophene liquid crystals

Article abstract of DOI:10.1080/15421406.2013.858013

Thiophene based calamitic mesogens are receiving paramount importance due to their applications in functional organic materials. The insertion of thiophene in the core unit favours a large change in mesophase characteristics as well as application properties in contrast to those mesogens with phenyl ring core alone. In this work, we report the structural as well as mesophase characterization of six mesogens which are built with a core of three phenyl rings and thiophene ring. The thiophene ring is placed at one end of the core and varied the other end with alkyl/alkoxy chains to investigate the mesophase characteristics. The molecular structures of representative mesogens are confirmed by means of FT-IR and Two-dimensional solution NMR techniques. An enantiotropic nematic phase is observed in all the cases as supported by HOPM and DSC techniques. The mesophase characteristics such as melting and clearing temperatures and phase stability are discussed. A dramatic increase in nematic phase stability for the synthesised mesogens is noticed in contrast to two phenyl ring based thiophene mesogens reported in literature.

Full text of DOI:10.1080/15421406.2013.858013

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Synthesis, Structural and Mesophase
Characterization of Three Ring Based
Thiophene Liquid Crystals
M. Kesava Reddya, K. Subramanyam Reddya, B. V. N. Phani Kumarb &
T. Narasimhaswamyb
a Department of Chemistry, S. V. University, Tirupati 517 502, India
b Chemical Physics Lab and Polymer Lab, CSIR-Central Leather
Research Institute, Adyar, Chennai 600 020, India
Published online: 27 May 2014.
To cite this article: M. Kesava Reddy, K. Subramanyam Reddy, B. V. N. Phani Kumar & T.
Narasimhaswamy (2014) Synthesis, Structural and Mesophase Characterization of Three Ring
Based Thiophene Liquid Crystals, Molecular Crystals and Liquid Crystals, 593:1, 1-24, DOI:
10.1080/15421406.2013.858013
To link to this article: http://dx.doi.org/10.1080/15421406.2013.858013
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Mol. Cryst. Liq. Cryst., Vol. 593: pp. 1–24, 2014
Copyright © Taylor & Francis Group, LLC
ISSN: 1542-1406 print/1563-5287 online
DOI: 10.1080/15421406.2013.858013
Synthesis, Structural and Mesophase
Characterization of Three Ring Based Thiophene
Liquid Crystals
M. KESAVA REDDY,1 K. SUBRAMANYAM REDDY,1
B. V. N. PHANI KUMAR,2 AND T. NARASIMHASWAMY2
1Department of Chemistry, S. V. University, Tirupati 517 502, India
2Chemical Physics Lab and Polymer Lab, CSIR-Central Leather Research
Institute, Adyar, Chennai 600 020, India
Thiophene based calamitic mesogens are receiving paramount importance due to their
applications in functional organic materials. The insertion of thiophene in the core unit
favours a large change in mesophase characteristics as well as application properties
in contrast to those mesogens with phenyl ring core alone. In this work, we report the
structural as well as mesophase characterization of six mesogens which are built with a
core of three phenyl rings and thiophene ring. The thiophene ring is placed at one end of
the core and varied the other end with alkyl/alkoxy chains to investigate the mesophase
characteristics. The molecular structures of representative mesogens are confirmed
by means of FT-IR and Two-dimensional solution NMR techniques. An enantiotropic
nematic phase is observed in all the cases as supported by HOPM and DSC techniques.
The mesophase characteristics such as melting and clearing temperatures and phase
stability are discussed. A dramatic increase in nematic phase stability for the synthesised
mesogens is noticed in contrast to two phenyl ring based thiophene mesogens reported
in literature.
Keywords Mesogens; Thiophene; Calamitic; Nematic; NMR; Chemical Shifts
1. Introduction
Calamitic mesogens based on thiophene are increasingly gaining importance owing to
their wide range of applications [1–3]. Insertion of thiophene in the mesogenic core not
only influences the mesophase transition temperatures considerably but also contributes
for interesting properties enabling them to be classified as functional materials [4–6]. The
replacement of phenyl ring with thiophene in the core unit of calamitic mesogens results
in variation in overall molecular shape [7,8]. The contemporary approach of designing of
thermotropic liquid crystals clearly emphasizes the role of molecular shape in varying the
mesophase characteristics [9,10]. Usually, the thiophene ring can be introduced in mesogens
either at the end or at the center of the core depending upon the position of the linking
unit [11–13]. For instance, substitution at 2- or 3-position of thiophene results in calamitic
mesogens with thiophene moiety at end of the molecule while 2,5- substitution favours bent-
core mesogens [14]. In continuation of our earlier work on thiophene mesogens [15–18], in
∗Address correspondence to T. Narasimhaswamy, Chemical Lab & Polymer Lab, CSIR-Central
Leather Research Institute, Adyar, Chennai 600020, India. E-mail: tnswamy99@hotmail.com
1

2
M. K. Reddy et al.
this paper, 2- and 3-substituted thiophene mesogens built with three phenyl ring core and
terminal alkyl/alkoxy chains are studied. For the construction of the core, ester and imines
are used as linking units while alkyl/alkoxy chains served as flexible terminal unit [19,20].
Earlier work on two phenyl ring core based thiophene mesogens revealed enantiotropic
nematic mesophase with low mesophase stabilities [15, 21, 22]. In order to investigate the
influence of additional phenyl ring on mesophase stabilities, in the present work, the core
is built with three phenyl rings along with thiophene. Further, the influence of terminal
chain on mesophase transition temperatures is studied by varying the length of the terminal
group. Since thiophene and its oligomers are finding interesting applications in functional
organic materials [23–25], the examination of influence of substitution position and core
length of thiophene mesogens is of considerable interest. Further, probing basically simple
mesogens proposed in the present work would enable to establish the structure-property
correlations which can be extended to structurally complex mesogens that are employed
for advanced applications [1,3,4].
2. Experimental Details
2.1 Materials
4-hydroxy benzaldehyde, 2-thiophene carboxylic acid, 3-thiophene carboxylic acid, dode-
cyl benzoic acid, 4-nitro phenol, N-BOC-4-amino phenol, N,Nꢀ-dicyclohexylcarbodiimide
(DCC) and 4-dimethylamino pyridine (DMAP) were purchased from Aldrich (USA)
and used without further purification. SnCl2, triethylamine, thionyl chloride (SOCl2),
N, Nꢀ-dimethylformamide (DMF), tetrahydrofuran (THF), ethanol and methanol (SD
Fine, Mumbai) were used as received. Trifluoroacetic acid, 4-hydroxy methyl benzoate,
dichloromethane, ethyl acetate, diethyl ether, n-hexane, n-heptane, acetone, isopropanol,
acetonitrile, ethyl methyl ketone (EMK), potassium hydroxide (pellets), anhydrous potas-
sium carbonate, anhydrous sodium sulphate, sodium chloride, calcium chloride, sodium
hydroxide, sodium bicarbonate and hydrochloric acid were obtained from Merck (India) and
used as received. 4-hexyloxy benzoic acid and 4-dodecyloxy benzoic acid were synthesized
in laboratory.
2.2 Instrumental Details
FT-IR spectra of all the compounds were recorded on ABB BOMEM MB3000 spectrom-
eter using KBr pellets. The nature of the mesophase and the temperature of occurrence
were determined with an Olympus BX50 hot-stage optical polarizing microscope (HOPM)
equipped with a Linkam THMS 600 stage with a TMS 94 temperature controller. The
photographs were taken using an Olympus C7070 digital camera. Differential scanning
calorimetry (DSC) traces were recorded using a DSC Q200 instrument with a heating rate
of 10◦C/minute in nitrogen atmosphere. The data obtained from second heating and cooling
is used for discussion. All NMR experiments were run on a JEOL ECA-500 MHz high res-
olution FT-NMR spectrometer operating at frequencies 500.16MHz (1H) and 125.77 MHz
(13C). An amount of 50 mg of compound was dissolved in 0.6 ml of CDCl3. Tetramethylsi-
lane was used as an internal reference and the spectra were recorded at 25◦C. JEOL-Delta
software package (version 5.0.2) was used for the purpose of NMR pulse sequences and
data processing.

Three Ring Based Thiophene Liquid Crystals
3
The experimental parameters chosen for 1D 1H/13C NMR were, spectral width
15/250 ppm, number of data points 16384/32768, number of scans 8/200, acquisition
time 2.18/1.04 s, relaxation delay 5/2 s, and 90◦ pulse width 13/10.2 μs. Two-dimensional
correlation spectra were recorded in order to track 1H-1H and 1H-13C correlations. A relax-
ation delay of 1.5 s was used in all the 2D experiments.1H-1H COSY spectra were obtained
using the gradient version of DQF-COSY (homonuclear double quantum filtered correla-
tion spectroscopy) pulse sequence. The spectra resulted from 1024 (F2) × 256 (F1) data
matrix size with one scan per t1 increment. A spectral width of 10 ppm was used in both
F1 and F2 dimensions. 13C-1H HETCOR (heteronuclear correlation) spectra were recorded
using the standard hetcor pulse program. The acquisition parameters were as follow: spectra
resulted from 1024 × 128 data matrix size with eight scans per t1 increment. A spectral
1
width of 200 ppm in F2 and 10 ppm in F1 was recorded. The gradient version of H-13C
HMBC (heteronuclear multiple-bond correlation) experiments was recorded in order to
sketch the long-range (two and three bonds) 1H-13C correlations. The long-range coupling
constant (nJCH) optimized was 8 Hz. The spectra resulted from 2048 × 256 data matrix
size with eight scans per t1 increment. A spectral width of 10 ppm in F2 and 200 ppm in F1
was recorded.
2.3 Synthesis of Mesogens
Preparation of 4-hexyloxy benzoic acid
2.3.1. Synthesis of 4-hexyloxybenzoic acid (1a). It was prepared by two step process.
In a representative experiment, 4-hydroxy methylbenzoate (3.8 g, 0.025 mol) was
placed in a 250 ml three necked round bottom flask equipped with stirrer and thermometer.
To that, DMF (100 ml) and potassium carbonate (5.18 g, 0.0375 mol) were added. The re-
sulting mixture was stirred while maintaining the temperature at 90◦C, then n-bromohexane
(3.52 ml, 0.025 mol) was added through a pressure equalizing dropping funnel over a pe-
riod of 30 minutes and the stirring was continued for about 4 hours and then the reaction
mixture was allowed to cool to room temperature, poured into a two liter beaker [26].
The contents were diluted with water (75 ml) and then transferred to 250 ml separating
funnel and diethyl ether was added. The ether layer collected was washed twice using 10%
potassium hydroxide solution and followed by distilled water. Then the organic layer was
dried over anhydrous sodium sulphate and upon evaporation of ether, liquid 4-hexyloxy
methyl benzoate was collected.
4-hexyloxy methyl benzoate (4.73 g, 0.02 mol) was placed in 250 ml single neck round
bottom flask equipped with double wall water condenser. Ethanol (100 ml) and potassium
hydroxide (3 g, 0.05mol) dissolved in distilled water (75 ml) were added to the flask. The
solution was refluxed for two hours and allowed to cool to room temperature and neutralized
with 10% hydrochloric acid to get a white precipitate [26]. The compound was purified by
recrystallizing from methanol.
Above procedure was followed for synthesizing other compound namely dodecyloxy
benzoic acid.
4-hexyloxy benzoic acid (1a). Yield: 84.7%, m.p-112◦C, FT-IR (KBr, cm−1): 2941,
2867 (C Hstr), 2549 (O-Hstr.of carboxylic acid), 1683 (C Ostr of carboxylic acid),
1604, 1511 (C Cstr aromatic), 1467, 1426 (C Hben) 1297, 1253, 1166, 1121, (C O C
1
of ether); H NMR ppm (CDCl3): 8.06 (d, 2H), 6.93 (d, 2H), 4.01 (t, 2H),
asym&symstr
1.78 (m, 2H), 1.46 (m, 2H), 1.33 (m, 4H), 0.90 (t, 3H); 13C NMR ppm (CDCl3): 172.38,
163.79, 132.43, 121.48, 114.26, 68.37, 31.64, 29.15, 25.76, 22.69 and 14.13.

4
M. K. Reddy et al.
4-dodecyloxy benzoic acid (1b). Yield: 80%, m.p-96.5◦C, FT-IR (KBr, cm−1): 2921,
2851 (C Hstr), 2558 (O Hstr.of carboxylic acid), 1681 (C Ostr of carboxylic acid), 1605,
1512 (C Cstr aromatic), 1467, 1427 (C Hben), 1255, 1167 (C O C asym & sym str ether);
1H NMR ppm (CDCl3): 8.05 (d, 2H), 6.93 (d, 2H), 4.01 (t, 2H), 1.79 (m, 2H), 1.45 (m,
2H), 1.34 (m, 16H), 0.87 (t, 3H); 13C NMR ppm (CDCl3): 171.91, 163.86, 132.42, 120.91,
114.27, 68.38, 32.02, 29.76, 29.73, 29.69, 29.65, 29.45, 29.17, 26.06, 22.79 and 14.23.
2.3.2 Synthesis of 4-aminophenyl 4-hexyloxybenzoate (2a, 3a). It was synthesized by two
step process
Step.1: Synthesis of 4-nitrophenyl 4-(hexyloxy) benzoate
In a representative experiment, hexyloxy benzoic acid (5g, 0.02 mol) and 4-nitro
phenol (2.78 g, 0.02 mol) were placed in a conical flask. To this, dichloromethane (100 ml)
was added and the solution was stirred at room temperature with magnetic stirrer. 4-
Dimethylamino pyridine (0.24 g, 0.002 mol) was added as a catalyst to the solution. After
10 minutes, dicyclohexylcabodiimide (5.15 g, 0.025 mol) dissolved in dichloromethane was
added to the flask and the solution was allowed to stir for 12 hrs [27]. The precipitated N,N’-
dicyclohexyl urea was filtered off and washed with excess of dichloromethane (100 ml).
The combined organic solution was taken into separating funnel and then washed twice
with 5% KOH solution followed by distilled water. The yellow solid obtained was purified
by recrystallization from isopropyl alcohol.
For the synthesis of 4-nitrophenyl 4-(dodecyloxy) benzoate, the above procedure was
followed.
4-nitrophenyl 4-(hexyloxy) benzoate (2a). Yield: 72.4%, m.p-68ºC, FT-IR (KBr,
cm−1): 2934, 2865 (C Hstr), 1742 (C Ostr), 1605 and 1516 (C Cstr aromatic), 1487,
1319 (C Hben), 1343 (-NO2str), 1255, 1164 (C O C asym&symstr of ester and ether respec-
1
tively); H-NMR ppm (CDCl3): 8.28 (d, 2H), 8.12 (d, 2H), 7.39 (d, 2H), 6.98 (d, 2H),
4.03 (t, 2H), 1.86 (m, 2H), 1.47 (m, 2H), 1.34 (m, 4H), 0.90 (t, 3H); 13C NMR ppm
(CDCl3): 164.14, 164.03, 156.06, 145.26, 132.59, 125.27, 122.76, 120.40, 114.59, 68.53,
31.63, 29.12, 25.74, 22.69 and 14.13.
4-nitrophenyl 4-(dodecyloxy) benzoate (2b). Yield: 68%, m.p-66.5◦C, FT-IR (KBr,
cm−1): 3116(aromatic C Hstr), 2916, 2850(C Hstr), 1732(C Ostr), 1606, 1524(C Cstr
aromatic), 1468(C Hben), 1345( NO2str), 1222, 1172, 1110(C O C asym&symstr of ester
and ether respectively);1H-NMR ppm (CDCl3): 8.30(d, 2H), 8.13(d, 2H), 7.39(d, 2H),
6.99(d, 2H), 4.04(t, 2H), 1.82(m, 2H), 1.47(m, 2H), 1.36(m, 16H), 0.87(t, 3H);13C-NMR
ppm (CDCl3): 164.15, 164.04, 156.06, 145.31, 132.61, 125.30, 122.76, 120.45, 114.60,
68.53, 32.02, 29.75, 29.74, 29.69, 29.65, 29.45, 29.15, 26.06, 22.79 and 14.23.
Step.2: Reduction of 4-nitrophenyl 4-(hexyloxy) benzoate
In this experiment, 4-nitrophenyl 4-(hexyloxy) benzoate (3.21 g, 0.00936 mol)
and SnCl2 (10.53 g, 0.046 mol) were placed in a single necked round bottom flask
along with ethanol and refluxed for two hrs [28]. Then it was cooled to room tem-
perature, neutralized with 10% NaHCO3 solution then filtered and solid obtained
was dried in vacuum oven. The solid was treated with 100 ml EMK for 1 hr and
the solvent was evaporated to get light brown solid which was recrystallized from
heptane.
The above procedure was used to synthesise 4-aminophenyl 4-dodecyloxy benzoate.
4-aminophenyl 4-hexyloxybenzoate (3a). Yield: 76.1%, m.p-91◦C, FT-IR (KBr,
cm−1): 3407, 3329 (NH2str), 3212 (aromatic C Hstr), 2934, 2861 (C Hstr), 1721 (C Ostr),
1607, 1512 (C Cstr aromatic), 1467, 1420, 1395 and 1315 (C Hben), 1261, 1196, 1166

Three Ring Based Thiophene Liquid Crystals
5
(C O C asym&symstr of ester and ether respectively); 1H-NMR ppm (CDCl3): 8.13 (d, 2H),
6.96 (m, 4H), 6.69 (d, 2H), 4.03 (t, 2H), 3.54 (s, 2H), 1.81 (m, 2H), 1.47 (m, 2H), 1.35
(m, 4H), 0.92 (t, 3H); 13C-NMR ppm (CDCl3): 165.68, 163.47, 144.31, 143.21, 132.28,
122.46, 121.88, 115.80, 114.31, 68.38, 31.66, 29.17, 25.77, 22.72 and 14.18.
4-aminophenyl 4-dodecyloxybenzoate (3b). Yield: 71.5%, m.p-96◦C, FT-IR (KBr,
cm−1): 3460, 3372(NH2str), 2954, 2919, 2850(C Hstr), 1715(C Ostr), 1629(NHben), 1605,
1579, 1513(C Cstr aromatic), 1471, 1420(C Hben), 1258, 1191, 1167, 1120(C O C
of ester and ether respectively);1H-NMR ppm (CDCl3): 8.13(d, 2H), 6.98(m,
asym&symstr
4H), 6.70(d, 2H), 4.02(t, 2H), 3.65(s, 2H), 1.81(m, 2H), 1.46(m, 2H), 1.34(m, 16H), 0.89(t,
3H); 13C-NMR ppm (CDCl3): 165.66, 163.46, 144.26, 143.23, 132.27, 122.47, 121.88,
115.78, 114.30, 68.38, 32.01, 29.67, 29.59, 29.54, 29.48, 29.46, 29.44, 29.21, 26.09, 22.81
and 14.26.
2.3.3 Synthesis of 4-aminophenyl 4-dodecylbenzoate (4, 5). It was synthesized by two step
process:
Step.1: Synthesis of 4-[(tert-butoxycarbonyl) amino] phenyl 4-dodecylbenzoate (4)
In this experiment, 4-dodecyl benzoic acid (5.80 g, 0.02 mol) and N-BOC-4-amino
phenol (4.18 g, 0.02 mol) were placed in a 250 ml conical flask. To this, dichloromethane
(150 ml) was added and the solution was stirred at room temperature with magnetic stirrer.
4-dimethylamino pyridine (0.24 g, 0.002 mol) was added as a catalyst to the solution. After
10 minutes, dicyclohexylcarbodiimide (5.15 g, 0.025 mol) dissolved in dichloromethane
was added to the flask and the solution was allowed to stir for 15 hours. The precipitated N,
N’-dicyclohexyl urea was filtered off and washed with excess of dichloromethane (100 ml).
The combined organic solution was taken into separating funnel and then washed with
5% KOH solution (2×100 ml), 10% HCl solution(2×100 ml) and followed by distilled
water (3×100 ml).The organic layer collected was dried over anhydrous Na2SO4. Upon
evaporation of solvent, the white solid resulted was purified by recrystallization from
isopropyl alcohol.
4-[(tert-butoxycarbonyl) amino] phenyl 4-dodecylbenzoate (4). Yield: 65.1%,
m.p-134◦C, FT-IR (KBr, cm−1): 3372(N-Hstr), 2923, 2851(C Hstr), 1727(C Ostr),
1704(C Ostr), 1609, 1527(C Cstr aromatic), 1465, 1411(C Hben), 1274, 1241,
1172(C O C
of ester);1H-NMR ppm (CDCl3): 8.08(d, 2H), 7.40(d, 2H),
asym&symstr
7.28(d, 2H), 7.11(d, 2H), 6.55(s, 1H), 2.68(t, 2H), 1.64(m, 3H), 1.51(m, 8H), 1.31(m, 18H),
0.87(t, 3H);13C NMR ppm (CDCl3): 165.55, 152.85, 149.45, 146.40, 136.09, 130.31,
128.73, 127.01, 122.23, 119.51, 36.18, 32.02, 31.24, 29.76, 29.73, 29.70, 29.66, 29.56,
29.45, 29.34, 28.42, 22.79 and 14.23.
Step.2: Synthesis of 4-aminophenyl 4-dodecylbenzoate (5)
In a typical experiment, 4-[(tert-butoxycarbonyl)amino]phenyl dodecylbenzoate
(4.67 g, 0.0097 mol) was placed in a 250 ml two necked round bottom flask which was
kept in an ice bath. Dichloromethane (150 ml) was added to the flask while maintaining
0◦C. Then trifluoroacetic acid (13.7 ml, 0.185 mol) was added and the reaction mixture
was stirred for two hours [29]. Later it was taken out from the ice bath; saturated sodium
bicarbonate solution (50 ml) was added and stirred for 20 minutes. The contents were
transferred to a 500 ml separating funnel and dichloromethane was added. The organic
layer was washed twice with 1% potassium hydroxide and distilled water respectively.
Upon evaporation of dichloromethane, solid 4-aminophenyl 4-dodecylbenzoate was
resulted. It was purified by recrystallization from heptane.

6
M. K. Reddy et al.
4-aminophenyl 4-dodecylbenzoate (5). Yield: 68.7%, m.p-94◦C, FT-IR (KBr, cm−1):
3400, 3326(NH2str), 2921, 2852(C Hstr), 1726(C Ostr), 1611(NHben), 1510(C Cstr
aromatic), 1465(C Hben), 1278 and 1193(C O C
of ester);1H-NMR ppm
asym&symstr
(CDCl3): 8.12(d, 2H), 7.31(d, 2H), 6.98(d, 2H), 6.70(d, 2H), 3.66(s, 2H), 2.69(t, 2H),
1.66(m, 2H), 1.33(m, 18H), 0.90(t, 3H); 13C-NMR ppm (CDCl3): 165.93, 149.24, 144.41,
143.18, 130.28, 128.71, 127.34, 122.41, 115.77, 36.19, 32.05, 31.29, 29.78, 29.71, 29.65,
29.60, 29.50, 29.38, 22.83 and 14.27.
2.3.4 Synthesis of 2 or 3- thiophene carbonyl chloride (6). 2 (or) 3- thiophene carboxylic
acid (3 g, 0.0234 mol) which was placed in a 100 ml single neck round bottomed flask fitted
with a double wall reflux condenser and a calcium chloride guard tube. Thionyl chloride
(2.13 ml, 0.0292 mol) and a few drops of DMF were added to the flask. The reaction
mixture was refluxed for about one hour and the excess SOCl2 was removed by vacuum
pump.
2.3.5 Synthesis of 4-formylphenyl thiophene-2-carboxylate or 4-formylphenyl thiophene-
3-carboxylate (7, 8). In an experiment, 4-hydroxy benzaldehyde (2.85 g, 0.0234 mol)
dissolved in ethyl methyl ketone was placed in a 500 ml two necked round bottomed flask
and was kept in an ice bath. Triethylamine (3.3 ml, 0.0234 mol) solution in EMK was added
to it. 2 or 3- thiophene carbonyl chloride was added slowly through a pressure equalizing
dropping funnel over a period of 30 minutes while maintaining temperature at 0◦C and
stirring was continued for 4 hours. Then, the precipitated triethylamine salt was filtered off
and washed with excess of EMK. The solvent was removed under reduced pressure. The
solid so obtained was washed twice with 2% potassium hydroxide solution followed by
distilled water, dried and recrystallized from isopropyl alcohol.
4-formylphenyl thiophene-2-carboxylate (7). Yield: 72.5%, m.p-132.7◦C, FT-
IR (KBr, cm−1): 3100(aromatic C Hstr), 2814, 2721, 2069(C Hstr), 1725(C Ostr),
1692( HC O),1593, 1500(C Cstr aromatic), 1409, 1356, 1304(C Hben), 1261, 1206,
1
1153(C O C
of ester), H-NMR ppm (CDCl3): 9.99(s, 1H), 7.99(m, 1H),
asym&symstr
7.93(d, 2H), 7.69(m, 1H), 7.39(d, 2H), 7.18(m, 1H); 13C-NMR ppm (CDCl3): 191.03,
159.92, 155.33, 135.33, 134.30, 134.18, 132.20, 131.35, 128.33 and 122.53.
4-formylphenyl thiophene-3-carboxylate (8). Yield: 71%, m.p-138◦C, FT-IR (KBr,
cm−1): 3109(aromatic C Hstr), 2928, 2816, 2721(C Hstr), 1728(C Ostr), 1693( HC O),
1593, 1504(C Cstr aromatic), 1403, 1304(C Hben), 1258, 1161(C O C
of
asym&symstr
ester); 1H-NMR ppm (CDCl3): 9.97(s, 1H), 8.32(m, 1H), 7.95(d, 2H), 7.69(d, 1H), 7.38(d,
2H), 7.37(s, 1H);13C NMR ppm (CDCl3): 191.11, 160.36, 155.49, 134.78, 134.10, 132.23,
131.38, 128.25, 126.83 and 122.62.
2.3.6 Synthesis of (E)-4-((4-(4-(dodecylbenzoyloxy)] phenylimino) methyl) phenyl
thiophene-2-carboxylate (or) (E)-4-((4-(4-(dodecyloxy) benzoyloxy)] phenylimino) methyl)
phenyl thiophene-2-carboxylate (9a, 9b). In a representative experiment, equimolar 4-
formylphenyl thiophene-2-carboxylate (0.75 g, 0.0032 mol) and 4-aminophenyl 4-dodecyl
benzoate (1.17 g, 0.0032 mol) (or) 4-aminophenyl 4-dodecyloxy benzoate (1.2 g,
0.0032 mol) were taken in a 100 ml conical flask. A few drops of ethanol were added
and the flask was placed in a microwave oven (power: 40 W) for ten minutes. Then it was
allowed to cool to room temperature, washed with methanol and recrystallized twice from
acetonitrile.

Three Ring Based Thiophene Liquid Crystals
7
Similarly the synthesis of (E)-4-((4-(4-(hexyloxy) benzoyloxy)] phenylimino) methyl)
phenyl thiophene-2-carboxylate was accomplished.
(E)-4-((4-(4-(dodecylbenzoyloxy)] phenylimino) methyl) phenyl thiophene-2-
carboxylate (2-DBPPTC) (9a). Yield: 71.3%, m.p-131ºC, FT-IR (KBr, cm−1):
2921, 2850(C Hstr), 1729(C Ostr), 1620(-C Nstr), 1505(C Cstr aromatic), 1469,
1359(C Hben), 1256, 1202(C O C asym&symstr of ester); 1H-NMR ppm (CDCl3): 8.49(s,
1H), 8.14(d, 2H), 8.01(d, 1H), 7.98(d, 2H), 7.68(d, 1H), 7.39(d, 2H), 7.36(d, 2H), 7.29(d,
2H), 7.26(d, 2H), 7.20(t, 1H), 2.69(t, 2H), 1.63(m, 2H), 1.26(m, 18H), 0.89(t, 3H); 13C-NMR
ppm (CDCl3):165.47, 160.74, 159.31, 153.04, 149.57, 149.53, 149.32, 135.09, 134.05,
133.97, 132.61, 130.37, 130.18, 128.78, 128.25, 126.97, 122.53, 122.25, 121.95, 36.20,
32.02, 31.26, 29.75, 29.67, 29.57, 29.47, 29.36, 22.80 and 14.25.
(E)-4-((4-(4-(dodecyloxy) benzoyloxy)] phenylimino) methyl) phenyl thiophene-2-
carboxylate (2-DoBPPTC) (9b). Yield: 70.9%, m.p-124◦C, FT-IR (KBr, cm−1): 2920,
2850(C Hstr), 1725(C Ostr), 1606, 1509(C Cstr aromatic), 1468, 1360(C Hben), 1269,
1193, 1167(C O C
of ester and ether respectively);1H-NMR ppm (CDCl3):
asym&symstr
8.48(s, 1H), 8.16(d, 2H), 7.99(d, 1H), 7.96(d, 2H) 7.68(d, 1H), 7.34(d, 2H), 7.28(d, 2H),
7.24(d, 2H), 7.19(m,1H), 6.98(d, 2H), 4.04(t, 2H), 1.83(m, 2H), 1.46(m, 2H), 1.31(m, 16H),
0.89(t, 3H); 13C-NMR ppm (CDCl3): 165.14, 163.66, 160.30, 159.22, 153.03, 149.44,
149.39, 135.08, 134.08, 133.96, 132.62, 132.39,130.16, 128.25, 122.57, 122.23, 121.93,
121.55, 114.41, 68.43, 32.01, 29.67, 29.52, 29.48, 29.43, 29.41, 29.20, 26.09, 22.80 and
14.26.
(E)-4-((4-(4-(hexyloxy) benzoyloxy)] phenylimino) methyl) phenyl thiophene-2-
carboxylate (2-HoBPPTC) (9c). Yield: 74.1%, m.p-134ºC, FT-IR (KBr, cm−1): 2944,
2869 (C Hstr), 1720 (C Ostr), 1625 ( C Nstr), 1606, 1579, 1511 (C Cstr aromatic),
1471, 1360 (C Hben), 1220, 1192, 1161, 1100 (C O C
of ester and ether
asym&symstr
respectively); 1H-NMR ppm (CDCl3): 8.47 (s, 1H), 8.15 (d, 2H), 8.0 (d, 1H), 7.97(d, 2H),
7.67 (d, 1H), 7.35 (d, 2H), 7.26 (d, 2H), 7.22 (d, 2H), 7.17 (m, 1H), 6.98 (d, 2H), 4.03
(t, 2H), 1.81 (m, 2H), 1.48 (m, 2H), 1.35 (m, 4H), 0.92 (t, 3H); 13C-NMR ppm (CDCl3):
165.05, 163.61, 160.21, 159.11, 153.0, 149.40, 134.98, 134.04, 133.85, 132.61, 132.31,
130.09, 130.04, 128.15, 122.49, 122.14, 121.84, 121.55, 114.36, 68.43, 31.66, 29.17, 25.78,
22.72 and 14.18.
2.3.7 Synthesis of (E)-4-((4-(4-(dodecylbenzoyloxy)] phenylimino) methyl) phenyl
thiophene-3-carboxylate (or) (E)-4-((4-(4-(dodecyloxy) benzoyloxy)] phenylimino) methyl)
phenyl thiophene-3-carboxylate (10a, 10b). In a typical experiment, a mixture of 4-
formylphenyl thiophene-3-carboxylate (0.5 g, 0.0021mol) and 4-aminophenyl 4-dodecyl
benzoate (0.5 g, 0.0021mol) (or) 4-aminophenyl 4-dodecyloxy benzoate (0.82 g,
0.0021mol) in equimolar was taken in a 100 ml conical flask. A few drops of ethanol
were added and the flask was placed in a microwave oven (power: 40 W) for ten minutes.
Then it was cooled to room temperature, washed with methanol and recrystallized twice
from EMK.
Similarly the synthesis of (E)-4-((4-(4-(hexyloxy) benzoyloxy)] phenylimino) methyl)
phenyl thiophene-3-carboxylate was carried out.
(E)-4-((4-(4-(dodecylbenzoyloxy)] phenylimino) methyl) phenyl thiophene-3-
carboxylate (3-DBPPTC) (10a). Yield: 70.7%, m.p-148.8ºC, FT-IR (KBr, cm−1): 3102(aro-
matic C Hstr), 2918, 2849(C Hstr), 1730(C Ostr), 1606, 1508 (C Cstr aromatic), 1465,
1413(C Hben), 1267, 1205(C O C
of ester);1H-NMR ppm (CDCl3): 8.48(s,
asym&symstr
1H), 8.33(d, 1H), 8.13(d, 2H), 7.98(d, 2H), 7.68(d, 1H), 7.39(m, 1H), 7.32(m, 8H), 2.69(t,

8
M. K. Reddy et al.
2H), 1.63(m, 2H), 1.26(m, 18H), 0.88(t, 3H);13C-NMR ppm (CDCl3): 165.46, 160.74,
159.33, 153.19, 149.55, 149.50, 149.31, 134.46, 133.97, 132.63, 130.37, 130.18, 128.78,
128.31, 126.98, 126.64, 122.53, 122.30, 121.95, 36.21, 32.03, 31.27, 29.75, 29.68, 29.57,
29.47, 29.37, 22.80 and 14.25.
(E)-4-((4-(4-(dodecyloxy) benzoyloxy)] phenylimino) methyl) phenyl thiophene-3-
carboxylate (3-DoBPPTC) (10b). Yield: 72.8%, m.p-141ºC, FT-IR (KBr, cm−1): 3105(aro-
matic C Hstr), 2920, 2853(C Hstr), 1735(C Ostr), 1608, 1510(C Cstr aromatic), 1469,
1408(C Hben), 1280, 1207(C O C
of ester and ether respectively);1H-NMR
asym&symstr
ppm (CDCl3): 8.48(s, 1H), 8.34(d, 1H), 8.15(d, 2H), 7.98(d, 2H), 7.67(d, 1H), 7.40(m,
1H), 7.34(d, 2H), 7.26(d, 2H), 7.24(d, 2H), 6.98(d, 2H), 4.04(t, 2H), 1.82(m, 2H), 1.47(m,
2H), 1.31(m, 16H), 0.89(t, 3H);13C-NMR ppm (CDCl3): 165.17, 163.66, 160.75, 159.30,
153.17, 149.45, 149.38, 134.46, 133.9, 132.62, 132.39,130.17, 128.31, 126.64, 122.58,
122.29, 121.92, 121.53, 114.40, 68.43, 32.02, 29.76, 29.74, 29.70, 29.66, 29.47, 29.20,
26.08, 22.80 and 14.25.
(E)-4-((4-(4-(hexyloxy) benzoyloxy)] phenylimino) methyl) phenyl thiophene-3-
carboxylate (3-HoBPPTC) (10c). Yield: 74.3%, m.p-148.3ºC, FT-IR (KBr, cm−1):
3106(aromatic C Hstr), 2929, 2864(C Hstr), 1729(C Ostr), 1606, 1508(C Cstr aromatic),
1406(C Hben), 1260, 1193(C O C
of ester and ether respectively);1H-NMR
asym&symstr
ppm (CDCl3): 8.48(s, 1H), 8.33(d, 1H), 8.14(d, 2H), 7.98(d, 2H), 7.67(d, 1H), 7.39(d, 1H),
7.34(d, 2H), 7.28(d, 2H), 7.22(d, 2H), 6.98(d, 2H), 4.03(t, 2H), 1.83(m, 2H), 1.48(m, 2H),
1.35(m, 4H), 0.92(t, 3H);13C-NMR ppm (CDCl3): 165.90, 165.14, 163.66, 160.74, 159.28,
153.18, 149.44, 149.38, 134.46, 133.98, 132.39, 130.17, 128.30, 126.65, 122.57, 122.29,
121.93, 121.55, 114.41, 68.42, 31.65, 29.16, 25.77, 22.71 and 14.16.
3. Results and Discussion
The molecular structures of synthesised mesogens are shown in Figures (1 & 2). They are
made by varying the substitution position at thiophene with a common core. The core unit
is built with three phenyl rings linked by ester and azomethine with a terminal alkyl/alkoxy
chain. These are classified into 2-mesogens and 3-mesogens indicating the substitution
position at thiophene. Further, the terminal units are varied to have hexyloxy, dodecyloxy,
dodecyl chains and accordingly six mesogens are realized. The synthesis is accomplished as
outlined in Scheme 1. Consequently, 4-alkoxy benzoic acids are converted to two ring nitro
compounds which are reduced to get amino intermediates [28]. Thiophene 2/3-carboxylic
acids are converted to aldehyde intermediates using 4-hydroxy benzaldehyde. Upon con-
densation of thiophene aldehyde with two ring amines, the target three phenyl ring based
thiophene mesogens are obtained. The main focus of the work is to find the mesophase
characteristics, evaluate the phase and thermal stabilities and compare the data with two
phenyl ring based thiophene mesogens reported in literature [12,16]. In the design of the
mesogen, the thiophene is placed at one end of the mesogen to investigate its influence
on mesophase formation. Further, the molecular structures of representative mesogens are
accomplished by solution (1D & 2D) NMR and the nature of the mesophase and transition
temperatures are examined by HOPM and DSC.
In FT-IR, 3-DBPPTC shows absorptions at 2918, 2849 cm−1 for CH stretching vibra-
tions arising from terminal dodecyl chain, 1730 cm−1 and 1615 cm−1 for ester carbonyl
as well as imine CH stretching of core unit, 1606 and 1508 cm−1 for ring skeletal C Cstr
and 1267 and 1205 cm−1 for C O Cstr of ester respectively. The appearance of one ester
carbonyl stretching (1730 cm−1) despite the presence of two ester linking units indicates
that the absorption frequencies are same. The final structural confirmation of representative

Three Ring Based Thiophene Liquid Crystals
9
O
S
O
O
O
N
O
A
C6H13
O
O
S
N
O
O
O
B
C
H
12 25
O
O
S
O
N
C
H
12 25
C
O
Figure 1. Molecular Structures of 2-HoBPPTC (A), 2-DoBPPTC (B) and 2-DBPPTC (C)
mesogens namely 2-DBPPTC and 3-DBPPTC is carried out using 1H, 13C NMR (Figures 3
& 4) and 2D experiments such as 1H-1H DQF-COSY, 13C 1H HETCOR and 1H-13C HMBC
NMR techniques (Figures 5–7). The structural assignment by 1H and 13C NMR in solution
is advanced by first identifying the chemical shift values of different units of the mesogen.
O
O
S
O
O
N
O
A
B
C6H13
O
O
S
O
N
O
C12H25
O
O
O
S
O
N
C
H
12 25
C
O
Figure 2. Molecular Structures of 3-HoBPPTC (A), 3-DoBPPTC (B) and 3-DBPPTC (C)

10
M. K. Reddy et al.
NH
O
2
NO2
NO2
CH
O
CH
O
3
3
O
R
O
OH
O
SnCl2/C2H5OH
RBr/K2CO3
DMF, 90oC
C2H5OH/KOH sol
Reflux
OH
DCC/DMAP OoC
O
O
O
Reflux
O
OH
O
R
R=C6H13, C12H25
(1a - 1b)
O
R=C6H13, C12H25
O
1
R
R
2
(2a - 2b)
(3a - 3b)
3
O
H C
3
CH
3
O
NH
O
H
C
2
3
CH
HN
O
O
3
CH
3
O
OH
HN
O
CH
3
DCC, DMAP
CF3COOH
OoC
+
DCM, RT
O
O
R
OH
R=C12H25
R
R
4
5
8
(10a - 10c)
10
O
O
O
O
O
S
R
O
O
OH
N
O
Cl
O
R
H
2
N
O
O
S
S
S
O
SOCl2/DMF
O
OH
TEA
(or)
(or)
(or)
(or)
R=C12H25 , OC12H25, OC6H13
0oC
Reflux
O
C2H5OH
O
O
O
Microwave
S
S
S
R
OH
Cl
O
N
O
O
O
6
7
S
(9a - 9c)
9
Scheme 1. Synthetic strategy for the Target Mesogens
The final assignment, however, is accomplished by making use of 2D spectral data. For
2-DBPPTC, the 1H NMR spectrum showed 10 peaks for the core as well as linking units
and four peaks for the terminal chain. The chemical shift values of core unit protons are
noticed in the range 7.2-8.49 ppm and based on multiplicity as well as peak integration,
the number of protons associated with each peak is arrived. The 13C NMR of 2-DBPPTC
showed 19 sharp lines in the range 121-166 ppm for all the core unit carbons. For the termi-
nal chain, 10 peaks are noticed in the range 14-37 ppm. The aromatic CH carbons showed
high intensity while quaternary carbons exhibited lowest intensity. The methine carbons of
thiophene moiety, on the other hand, showed medium intensity. Table 1 lists the 1H and 13
C
chemical shifts along with carbon numbering. By using 1H-1H DQF-COSY data, the proton
pairs arising from each phenyl ring are identified. For azomethine, no correlation could be
found as it is not directly connected to any phenyl ring protons. The 13C 1H HETCOR data

Three Ring Based Thiophene Liquid Crystals
11
Figure 3. 1H NMR Spectra of 2-DBPPTC (A) and 3-DBPPTC (B)
enabled to identify the 13C chemical shifts of corresponding proton pairs that have been
assigned from 1H-1H DQF-COSY. This exercise facilitated the identification of phenyl ring
carbons. For thiophene unit, three protons showed 12 signals characteristic of three spin
system. By using the scalar coupling constants (J Hz), the individual protons of thiophene
ring are assigned. The 13C 1H HETCOR data also provided the assignment of CH carbons
of thiophene from proton chemical shifts. The final confirmation, however, is achieved by
Figure 4. 13C NMR Spectra of 2-DBPPTC (A) and 3-DBPPTC (B)

12
M. K. Reddy et al.
Table 1. DQF-COSY and HSQC data of 2-DBPPTC
17
O
18
16
13
12
15
19
14
7
8
10
S
O
11
9
3
2
6
O
O
N
1
4
C
H
12 25
5
Carbon Number
1H
13C
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
—
7.31
8.13
—
—
—
7.25
7.27
—
8.48
—
7.99
7.36
—
149.3
128.8
130.3
126.9
165.4
149.5
122.5
121.9
149.5
159.3
134.0
130.1
122.1
153.0
160.3
132.6
135.0
128.1
133.9
—
—
8.01
7.20
7.69
1
making use of H 13C HMBC data where proton chemical shifts and the corresponding
13C NMR chemical shift correlations arising from two as well as three bond couplings
are noticed. In other words, HMBC provided information about long range couplings of
protons with neighbouring carbons. The whole exercise facilitated the complete assignment
of core unit of the mesogen. Similar approach is extended to 3-DBPPTC and Tables 1 & 2
list the assigned NMR data from 1D and 2D experiments. A comparison of 13C chemical
shifts of 2-DBPPTC and 3-DBPPTC mesogens revealed that except C-17, all other carbons
of the core unit showed similar chemical shifts indicating alike chemical environment. A
9 ppm variation in chemical shift values of C-17 carbons, on the other hand, is attributed to
effect of position of substitution. In other words, the higher chemical shift value for C-17
carbon of 2-DBPPTC is ascribed to the presence of sulphur as a neighbouring atom.
All the synthesised mesogens are examined by HOPM with a heating rate of 10◦C/m.
The samples are sandwiched between 12 mm cover slips and are heated to isotropic tem-
perature and the mesophase transitions are noted upon cooling. The HOPM investigations
revealed the existence of enantiotropic nematic phase for all the cases (Figure 8) and the
appearance of nematic phase is confirmed by observing birefringent droplets on cooling the
isotropic liquid [30, 31]. On further cooling, the droplets coalesced to give either threaded

Three Ring Based Thiophene Liquid Crystals
13
Figure 5. 2D 1H-1H DQF-COSY Spectra of 2-DBPPTC (A) and 3-DBPPTC (B)
texture or marble texture characteristic of nematic phase. The enantiotropic nature of the
phase is further confirmed by DSC measurements (Figures 9–11). The DSC scan of 3-
DBPPTC (Figure 11 B) shows two peaks in which high intense peak is observed at low
temperature whereas the high temperature peak is detected with low intensity. Accordingly,
the transition enthalpy values are found to be high for the low temperature peak and lower
for high temperature transition. These are assigned to Cr-N and N-I transitions respectively

14
M. K. Reddy et al.
Table 2. DQF-COSY and HSQC data of 3-DBPPTC
O
O
18
16
17
13
12
19
15
10
14
7
8
S
11
9
3
2
6
O
O
N
4
1
C
H
12 25
5
Carbon Number
1H
13C
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
—
7.32
8.13
—
—
—
7.24
7.29
—
8.48
—
7.99
7.31
—
149.3
128.7
130.3
126.9
165.4
149.5
122.5
121.9
149.5
159.3
133.9
130.1
122.3
153.1
160.7
132.6
126.6
128.4
134.4
—
—
7.39
7.68
8.34
[32]. In other words, the high transition enthalpy value indicates the melting temperature
(Cr N) while the second transition is attributed to isotropic transition (N-I). The cooling
scan also exhibited two peaks associated with I-N and N-Cr. The appearance of two peaks
both in heating as well as cooling cycles is clear signature of enantiotropic nature of the
mesophase. For 2-HoBPPTC and 3-HoBPPTC isomers, the isotropic transition tempera-
tures are found to be very high (>300◦C) (Figure 9). In order to find the thermal stabilities
of these two mesogens, thermogravimetric analysis has been carried out in nitrogen atmo-
sphere. Accordingly, initial degradation is found to be 320◦C which suggests that even at
300◦C, the mesogens are thermally stable exhibiting nematic phase. Tables 3 & 4 show
the transition temperatures, enthalpy values and mesophase stabilities for all the mesogens.
The typical enthalpy values for Cr-N transition are found to be ∼ 10 k cal/mol whereas
for N-I transition, the values are found to be ∼ 0.25 k cal/mol [32]. These values are in
consistent with literature data of nematogens made up of rod-like core units [32].
The mesophase as well as thermal stability values of all the mesogens are found to
be high in contrast to the reported two phenyl ring based thiophene mesogens [12, 16].
For instance, the 2-thiophene based two phenyl ring mesogens with a dodecyloxy terminal
chain showed phase stability ∼18◦C whereas for three phenyl ring mesogen (2-DoBPPTC),

Three Ring Based Thiophene Liquid Crystals
15
Figure 6. 2D 13C- 1H HETCOR Spectra of 2-DBPPTC (A) and 3-DBPPTC
the value is ∼150◦C. Further, the thermal stability of two ring based mesogen is ∼160◦C
and for 2-DoBPPTC, the value is ∼270◦C. Such large mesophase and thermal stabilities
for three phenyl ring based mesogens are ascribed to lengthy rod-like core in contrast to
two ring based mesogens (Figure 12). Further, the high thermal stabilities indicate that the
mesogens possess high molecular anisotropic polarizability. The existence of four rings i.e.

16
M. K. Reddy et al.
Figure 7. 2D 1H-13C HMBC Spectra of 2-DBPPTC (A) and 3-DBPPTC (B)
three phenyl rings and a thiophene ring besides providing rigidity to the core, enhances the
molecular anisotropic polarizability. The thiophene unit in the core further increases the
molecular polarizability in view of sulfur atom through the dipole moment. A comparison
of mesophase transitions of 2- and 3-series is attempted by taking into account of terminal

Three Ring Based Thiophene Liquid Crystals
17
Figure 8. HOPM photographs of mesogens while cooling from isotropic phase (A): 2-DBPPTC
nematic phase at 247.6◦C, (B): 3-DBPPTC nematic phase at 231◦C, (C): 2-DoBPPTC nematic phase
at 276◦C (D): 3-DoBPPTC nematic phase at 259.5◦C, (E): 2-HBPPTC nematic phase at 296.2◦C and
(F): 3-HBPPTC nematic phase at 155◦C
chains. For 2-series of thiophene mesogens, the N-I temperatures decreased with increasing
chain length. Among the mesogens with dodecyloxy and dodecyl chains, the mesogen with
dodecyl unit (2-DBPPTC) showed lower N-I transition. For 3-series also the decrease of
N-I transition with an increase in terminal chain length is observed. As for as the N-I
transition values of dodecyloxy and dodecyl mesogens of 3-series, the difference is found
to be very less.
The overall comparison of mesophase characteristics of 2- and 3- series of thiophene
mesogens discloses that the Cr-N transitions are higher for 3-series than 2-series. The N-I
transitions, on the other hand, are found to be higher for 2-series over 3-series. As a result,
the 2-series of thiophene mesogens exhibits better mesophase stability. The influence of

18
M. K. Reddy et al.
Figure 9. DSC II cycle scans of 2-HoBPPTC (A) and 3-HoBPPTC (B)
terminal chain on mesophase transition temperatures is examined for the both the series. For
3-series, 3-DBPPTC and 3-HoBPPTC mesogens showed very close N-I transitions even
though for 2-series, a variation of ∼24◦C is clearly noticed. The careful examination of
mesophase data further reveals that an addition of phenyl ring in the present case, substantial

Three Ring Based Thiophene Liquid Crystals
19
Figure 10. DSC II cycle scans of 2-DoBPPTC (A) and 3-DoBPPTC (B)
increase in Cr-N transition is noticed in contrast to two ring based thiophene mesogens [12,
16]. However, a dramatic increase is observed in N-I transition temperature thus enhancing
thermal window of the mesogens. Further, despite the addition of third phenyl ring which
boosts the (length) aspect ratio of core unit, the appearance of only nematic phase similar
to two phenyl ring based thiophene mesogens advocates that nematic phase dominates if

20
M. K. Reddy et al.
Table 3. Transition temperatures and enthalpy values of 2-series
Temperature
(◦C)
Mesophase stability
Code
Transition
ꢀH(k.cal/mole)
(ꢀT) (◦C)
2-HoBPPTC#
Cr-N
N-I
Cr-N
N-I
Cr1-Cr2
Cr2-N
N-I
Cr-N
N-I
134.21
—
8.59
—
39.2
0.8
3.96
9.89
0.23
7.5
—
—
9
—
—
149.62
—
18
—
118.15
—
2-TMAPH∗
123.0
132.0
86.69
124.51
274.13
99.1
117.1
131.24
249.39
2-DoBPPTC
2-DDCPTC∗
0.1
8.99
0.24
2-DBPPTC
Cr-N
N-I
# 2-HoBPPTC (Clearing is above 300◦C)
∗
Thiophene with two phenyl ring mesogens from Refs: [12b &16a]
thiophene ring is placed at terminal position in the core unit [15, 16, 33,34]. If thiophene
moiety is placed at the center of the core unit, on the other hand, both nematic and smectic
mesophases can be realised [8, 11, 13]. Further, for calamitic mesogens, usually the smectic
mesophases are noticed when the linear core is connected with lengthy flexible alkyl/alkoxy
chains at both ends [35]. Thus, in a homologous series, for a common core, the nematic phase
is often observed when the terminal chain length is short whereas smectic mesophases are
seen with increase in terminal chain length [35]. This behaviour is rationalised by taking
into account of aspect ratio of the mesogen [35]. For a homologues series, as the core
length is constant and length of the chain is variable, the higher homologues are endowed
Table 4. Transition temperatures and enthalpy values of 3-series
Temperature
(◦C)
Mesophase stability
Code
Transition
ꢀH (k.cal/mole)
(ꢀT)(◦C)
3-HoBPPTC#
Cr-N
N-I
Cr-N
N-I
Cr1-Cr2
Cr2-N
N-I
Cr-N
N-I
148.35
—
9.29
—
37.4
1.1
6.04
10.09
0.27
10.2
0.3
—
—
32
—
—
113.35
—
9.4
—
103.13
—
3-TMAPH∗
113.0
145.0
116.07
141.34
254.69
113.3
122.7
148.86
251.99
3-DoBPPTC
3-DDCPTC∗
3-DBPPTC
Cr-N
N-I
9.93
0.25
# 3-HoBPPTC (Clearing is above 300◦C)
∗
Thiophene with two phenyl ring mesogens from Ref: [12b &16a]

Three Ring Based Thiophene Liquid Crystals
21
Figure 11. DSC II cycle scans of 2-DBPPTC (A) and 3-DBPPTC (B)
with high aspect ratio. Consequently, the lateral interactions are dominant over terminal
interactions; thus favouring smectic mesophase [35]. Yet another way of realising smectic
mesophase is to introduce polar terminal groups like nitro, cyano etc. by replacing one of the
terminal chains [35]. In the present work, however, all the mesogens showed only nematic
mesophase despite lengthy core and terminal chains. As deliberated above, the absence

22
M. K. Reddy et al.
17
S
O
O
18
19
16
13
12
15
14
7
8
10
11
9
3
2
6
O
O
N
1
4
C
H
12 25
5
A
O
18
16
13
12
19
15
10
14
7
8
O
S
17
11
9
3
2
6
O
N
4
1
C
H
12 25
5
O
B
Figure 12. Molecular Structures and space filled models of 2-DBPPTC (A) and 3-DBPPTC (B)
of smectic mesophases in these mesogens can be argued since the mesogens have only
one terminal chain as against two terminal chains often required for smectogens. Further,
the presence of less symmetric thiophene unit at one end of the mesogen also disfavours
layer ordering leading to predominantly nematic mesophase. The influence of lengthy core
as well as terminal chain of these mesogens, on the other hand, is very well reflected on
nematic phase stability as against to those thiophene mesogens in which only two phenyl
rings are present in the core (Tables 3 and 4) [12, 16].
The thiophene mesogens reported in this work are expected to have significant impor-
tance as 3-substituted mesogens are susceptible for polymerization and can yield thiophene
based polymeric liquid crystals. Such polymers are known to exhibit polarized emission
paving the way for their use in organic/polymeric light emitting diodes [15, 36]. The studies
can be further extended to incorporate bithiophene, terthiophene at one end of the molecule
to examine the mesophase as well as photo-physical properties. Additionally, the solid state
13C NMR experiments of these mesogens in nematic phase are underway which would
provide the orientational order for the phenyl rings as well as thiophene moiety and the

Three Ring Based Thiophene Liquid Crystals
23
information can be supportive in understanding the orientational constraints and its impact
on material properties.
4. Conclusions
Six thiophene based calamitic mesogens were synthesized by multistep route. The molecu-
lar structureswereconfirmedbyFT-IRspectroscopyinsolidstateandbyNMRspectroscopy
1
1
in solution. The assignment was carried by H, 13C, H-1H DQF-COSY, 13C 1H HSQC
and H 13C HMBC techniques. For the assignment of thiophene ring, proton coupling
1
constants (J Hz) were additionally used. The HOPM and DSC measurements confirmed
the existence of enantiotropic nematic phase in all the mesogens. The influence of posi-
tion of substitution on mesophase characteristics reveals that 2-substituted thiophene based
mesogens showed low melting. The appearance of only nematic phase was attributed to
the presence of thiophene at terminal position. The mesogens based on hexyloxy terminal
chains (2-HoBPPTC and 3-HoBPPTC) showed high clearing temperatures and were found
to undergo decomposition at 320◦C. For all the mesogens, high clearing temperatures (N-I)
were attributed to four rings in the core which enhances the molecular anisotropic polar-
izability. Further, a dramatic increase in mesophase stabilities for three ring mesogens in
contrast to two phenyl ring based thiophene mesogens reported in literature demonstrates
that core length (aspect ratio) as well as the molecular anisotropic polarizability played
major role.
Acknowledgments
MKR and TNS are thankful to Dr. A. B. Mandal, Director, CSIR-CLRI for his keen interest
and encouragement for our work. MKR expresses thanks to Mr. D. Muralidharan, Ms. N.
Priya and Mr. L. Edwin Paul, CLRI for their help in recording DSC and TGA measurements.
The partial financial support from NWP-23 project is duly acknowledged.
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