Synthesis and characterization of two phenyl ring core-based thiophene mesogens

Article abstract of DOI:10.1080/15421406.2013.790635

Novel thiophene mesogens built with two phenyl ring core and alkoxy terminal chains are synthesized by multistep route. The phenyl ring core is connected to thiophene by ester unit at (a) 2-position and (b) 3-position to yield 2-series of mesogens. The structural characterization of them is accomplished by using spectroscopy tools and the mesophase characteristics are evaluated by HOPM and DSC. The ¹³C chemical shifts of representative mesogens calculated from DFT by quantum chemical calculations are compared with experimental ¹³C chemical shifts and further utilized for structural assignment of core unit carbons. A good agreement between calculated and experimental values is noticed. An enantiotropic nematic phase is confirmed for all the mesogens and accordingly threaded nematic texture in HOPM is observed and is supported by DSC transition enthalpy values. The mesophase as well as thermal stabilities of the mesogens are discussed with reference to terminal chain length and position of the linking unit on thiophene.

Full text of DOI:10.1080/15421406.2013.790635

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Synthesis and Characterization of Two
Phenyl Ring Core-Based Thiophene
Mesogens
M. Kesava Reddy a , K. Subramanyam Reddy a , M. Prakash b & T.
Narasimhaswamy b
a Department of Chemistry , Sri Venkateswara University , Tirupati ,
India
b Chemical Lab & Polymer Lab , CSIR-Central Leather Research
Institute , Adyar , Chennai , India
Published online: 03 Oct 2013.
To cite this article: M. Kesava Reddy , K. Subramanyam Reddy , M. Prakash & T. Narasimhaswamy
(2013) Synthesis and Characterization of Two Phenyl Ring Core-Based Thiophene Mesogens, Molecular
Crystals and Liquid Crystals, 582:1, 1-14, DOI: 10.1080/15421406.2013.790635
To link to this article: http://dx.doi.org/10.1080/15421406.2013.790635
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Mol. Cryst. Liq. Cryst., Vol. 582: pp. 1–14, 2013
Copyright © Taylor & Francis Group, LLC
ISSN: 1542-1406 print/1563-5287 online
DOI: 10.1080/15421406.2013.790635
Synthesis and Characterization of Two Phenyl Ring
Core-Based Thiophene Mesogens
M. KESAVA REDDY,1 K. SUBRAMANYAM REDDY,1
M. PRAKASH,2 AND T. NARASIMHASWAMY2,∗
1Department of Chemistry, Sri Venkateswara University, Tirupati, India
2Chemical Lab & Polymer Lab, CSIR-Central Leather Research Institute, Adyar,
Chennai, India
Novel thiophene mesogens built with two phenyl ring core and alkoxy terminal chains
are synthesized by multistep route. The phenyl ring core is connected to thiophene
by ester unit at (a) 2-position and (b) 3-position to yield 2-series of mesogens. The
structural characterization of them is accomplished by using spectroscopy tools and the
mesophase characteristics are evaluated by HOPM and DSC. The 13C chemical shifts
of representative mesogens calculated from DFT by quantum chemical calculations
are compared with experimental 13C chemical shifts and further utilized for structural
assignment of core unit carbons. A good agreement between calculated and experimental
values is noticed. An enantiotropic nematic phase is confirmed for all the mesogens and
accordingly threaded nematic texture in HOPM is observed and is supported by DSC
transition enthalpy values. The mesophase as well as thermal stabilities of the mesogens
are discussed with reference to terminal chain length and position of the linking unit on
thiophene.
Keywords 13C chemical shifts; DFT; mesogen; nematic phase; thiophene
1. Introduction
Designing of thermotropic liquid crystals with heterocyclic units in the core is gaining
prominence and an interest in such mesogens is continuously growing [1]. This could be
attributed to the presence of hetero atom, which strongly influences the mesophase char-
acteristics by bringing considerable change in polarity, polarizability, and geometry of the
molecule [2, 3]. Among the heterocyclic units used for building mesogens, thiophene—a
five membered sulfur containing aromatic ring—has attracted prime attention owing to ap-
plications in functional organic materials [4, 5]. From molecular structure point of view, in-
sertion of thiophene in the mesogenic core can substantially influence the optical anisotropy
and considerably reduces the melting transition temperature [5, 6]. In addition, studies have
revealed that thiophene-based mesogens possess a strong lateral dipole, which promotes
negative dielectric anisotropy. Further, it eliminates the need for lateral cyano and fluoro
substituents in the core, which not only enhances the molecular breadth but also the viscos-
ity [7]. Thus, establishing a relationship between mesogenic structure and the magnitude
of the polarizability anisotropy in thiophene mesogens is very crucial both for control
∗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. Kesava Reddy et al.
and optimization of optical properties [8, 9]. Additionally, thiophene mesogens show great
promise for use in organic light-emitting devices, as macroscopic alignment of the director
in the mesophase favors the generation of anisotropic charge carrier mobility and polarized
electroluminescence emission [10]. In this work, we report 2-series of mesogens in which
thiophene unit is placed at one end of the molecule. Accordingly, 2-substituted as well as
3-substituted thiophene mesogens are designed in which two phenyl rings connected by
ester unit serve as core and alkoxy chains as terminal unit. The main idea in synthesizing
and examining the mesophase properties of these mesogens is to explore the influence of
thiophene substitution pattern on molecular shape as well as molecular dipole and its impact
on mesophase characteristics. For representative mesogens, quantum chemical calculations
based on density functional theory (DFT) method are employed for finding the molecular
shape as well as 13C chemical shift values.
2. Experimental Details
2.1. Materials
2-thiophene carboxylic acid, 3-thiophene carboxylic acid, 4-benzyloxy phenol, 4-hydroxy
methylbenzoate, n-bromoalkanes (C4, C8, C10, and C12), and palladium on charcoal (Pd/C,
10%) were purchased from Aldrich, Inc. (St. Louis, MO, USA) and used without further
purification. Triethylamine, thionyl chloride (SOCl2), N,Nꢀ-dimethylformamide (DMF),
tetrahydrofuran (THF), and methanol (SD Fine Chemicals, Ltd., Mumbai, Maharashtra,
India) were used as received. Ethyl acetate, diethyl ether, n-hexane, acetone, ethyl methyl
ketone (EMK), isopropanol, potassium hydroxide (pellets), anhydrous potassium carbonate,
anhydrous sodium sulfate, Celite-540, and silica gel (100–200 mesh) were obtained from
Merck, Ltd. (Mumbai, Maharashtra, India) and used as received. The 4-alkoxy benzoic
acids were synthesized in laboratory except 4-ethoxy benzoic acid, which was procured
from Aldrich, Inc.
2.2. Instrumental Details
FT-IR spectra of all the compounds were run on ABB BOMEM MB3000 spectrometer
using KBr pellets. 1H and 13C NMR spectra of the compounds in CDCl3 as a solvent were
run on a JEOL 500 MHz instrument at room temperature (r.t.) using tetramethylsilane as an
internal standard. The resonance frequencies of 1H and 13C were 500.15 and 125.76 MHz,
respectively. The nature of the mesophase and the temperature of occurrence were deter-
mined 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 (Olympus Corp., Tokyo, Japan). Dif-
ferential 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.
2.3. Computational Details
The gas phase molecular geometries of 2-DCPTC and 3-DCPTC were optimized by us-
ing DFT-based Becke’s three parameter hybrid exchange functional and Lee−Yang−Parr
correlation functional [11] (B3LYP) method employing 6-31G(d) basis set. The NMR

Two Phenyl Ring Core-Based Thiophene Mesogens
3
chemical shifts were calculated at B3LYP/6-311+G(2d, p) level of theory using the gauge
invariant atomic orbitals (GIAO) method to circumvent the gauge problem using B3LYP/6-
31G(d) geometry [12]. Cheeseman recommended the 6-311+G(2d, p) basis set for NMR
chemical shifts after scrutiny of various basis sets and hence the same was selected for
the present study [13]. The scale factor determined for B3LYP/6-311+G(2d, p)//B3LYP/6-
31G(d) methodology from the previous study was used for the calculation of 13C NMR
chemical shifts [14]. Chemical shifts of tetramethylsilane calculated at the same level of
theory were used as reference. All calculations were carried out using Gaussian 03W suite
of programs [15].
2.4. Synthesis of Mesogens
2.4.1. Preparation of 4-Dodecyloxy Benzoic Acid.
2.4.1.1. Preparation of 4-Dodecyloxy Methyl Benzoate (1). In a representative ex-
periment, 4-hydroxy methyl benzoate (7.6 g, 0.05 mol) was placed in a 500 ml
three-necked round-bottom flask equipped with stirrer and thermometer. To that, N,Nꢀ-
dimethylformamide (150 ml) and potassium carbonate (10.36 g, 0.075 mol) were added.
The resulting mixture was stirred while maintaining the temperature at 90◦C, then n-
bromododecane (12 ml, 0.05 mol) was added through a pressure equalizing dropping
funnel over a period of 30 minutes and the stirring was continued for about 4 hours and
then the reaction mixture was allowed to cool down at r.t. poured into a 2-l beaker [16].
The contents were diluted with water (250 ml) and then transferred to a 500 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. The organic layer was dried
with anhydrous sodium sulfate. Upon evaporation of ether, 4-dodecyloxy methyl benzoate
resulted as a liquid.
2.4.1.2. Hydrolysis of 4-Dodecyloxy Methyl Benzoate (2). 4-dodecyloxy methylben-
zoate (13.3 g, 0.04 mol) was placed in a one liter single-necked round-bottom flask equipped
with double wall water condenser. Ethanol (250 ml) and potassium hydroxide (6 g, 0.1 mol)
dissolved in distilled water (250 ml) were added to the flask. The solution was refluxed for
2 hours and allowed to cool down at r.t. and then neutralized with 10% hydrochloric acid
to get a white precipitate. It was purified by recrystallizing from methanol.
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 Casym & 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.
2.4.2. Synthesis of 4-Hydoxy Phenyl 4-(Dodecyloxy) Benzoate.
2.4.2.1. Synthesis of 4-(Benzyloxy) Phenyl 4-(Dodecyloxy) Benzoate (3a). In an ex-
periment, 4-benzyloxy phenol (5 g, 0.025 mol) was placed in a 500 ml two-necked round-
bottom flask with ethyl methyl ketone as solvent, which was kept in an ice bath. Triethy-
lamine (3 ml, 0.03 mol) was added to the reaction solution on stirring. To this, 4-dodecyloxy
benzoyl chloride—which was prepared from acid by reacting with thionyl chloride—was
slowly added through a pressure equalizing dropping funnel for 30 minutes while maintain-
ing the temperature at 0◦C. The stirring was continued for 4 hours and then the precipitated
triethylamine salt was filtered and the solvent was removed from the solution under reduced

4
M. Kesava Reddy et al.
pressure. The solid obtained was washed with 4% potassium hydroxide solution twice to
remove unreacted acid, phenol, further washed with water, then dried and recrystallized.
Yield: 67.4%, m.p. 78◦C, FT-IR (KBr, cm–1): 2921, 2852 (C Hstr), 1728 (C Ostr),
1683, 1606, 1511 (C Cstr aromatic), 1464, 1333 (C Hben), 1251, 1200, 1170 (C O
Casym & sym str of ester and ether); 1H-NMR ppm (CDCl3): 8.13 (d, 2H), 7.46 (d, 2H), 7.40
(t, 2H), 7.35 (t, 1H), 7.12 (d, 2H), 7.02 (d, 2H), 6.98 (d, 2H), 5.07 (s, 2H), 4.04 (t, 2H),
1.84 (m, 2H), 1.48 (m, 2H), 1.37 (m, 16H), 0.90 (t, 3H); 13C-NMR ppm (CDCl3): 165.42,
163.57, 156.50, 144.83, 136.98, 132.34, 128.72, 128.13, 127.61, 122.70, 121.69, 115.56,
114.35, 70.51, 68.40, 32.05, 29.79, 29.77, 29.76, 29.73, 29.70, 29.50, 29.22, 26.10, 22.83,
and 14.27.
2.4.2.2. Debenzylation of 4-(Benzyloxy) Phenyl 4-(Dodecyloxy) Benzoate (4a). In a
typical experiment, 4-(benzyloxy) phenyl 4-(dodecyloxy) benzoate (4 g, 0.0081 mol) was
placed in a 250 ml single-necked round-bottom flask and tetrahydrofuran (100 ml) was
added. To that, 10% Pd/C (0.4 g) was added while stirring the solution [17]. The hydrogen
atmosphere was maintained by purging the hydrogen gas through balloon. The progress of
the reaction was monitored through TLC. After the completion of reaction, the solid Pd/C
powder was filtered by celite powder. The solution was concentrated to get solid, which
was purified by column chromatography on silica gel using a mixture of 20% ethyl acetate
in hexane as an eluent.
Yield: 65%, m.p. 112.5◦C, FT-IR (KBr, cm–1): 3469 ( OH), 2955, 2918, 2871
(C Hstr), 1709 (C Ostr), 1604, 1511 (C Cstr aromatic), 1443, 1316 (C Hben), 1257,
1187, 1167,1129 (C O Casym & sym str of ester and ether); 1H-NMR ppm (CDCl3): 8.13 (d,
2H), 6.96 (m, 4H), 6.75 (d, 2H), 4.03 (t, 2H), 1.81 (m, 2H), 1.46 (m, 2H), 1.35 (m, 16H), 0.88
(t, 3H); 13C-NMR ppm (CDCl3): 166.25, 163.72, 153.85, 144.15, 132.43, 122.62, 121.43,
116.32, 114.41, 68.44, 32.03, 29.77, 29.75, 29.72, 29.71, 29.68, 29.48, 29.18, 26.08, 22.81,
and 14.26.
2.4.3. Synthesis of 4-{[(4-Dodecyloxyphenyl)carbonyl]oxy} Phenyl Thiophene-2-
Carboxylate (2-DDCPTC) (5a).
It was prepared by two-step process. In a typical experiment, 2-thiophene carboxylic
acid was placed in a 100 ml single-necked round-bottom flask fitted with a double wall
reflux condenser and a calcium chloride guard tube. Thionyl chloride (1.09 ml, 0.0075 mol)
and few drops of N,Nꢀ-dimethylformamide were added to the flask and refluxed for 1 hour.
The excess thionyl chloride was removed by vacuum pump. Then it was dissolved in ethyl
methyl ketone (50 ml) and added dropwise through pressure equalizing dropping funnel
for 45 minutes at 0◦C to the mixture of 4-hydoxy phenyl 4-(dodecyloxy) benzoate (2 g,
0.005 mol) and triethylamine (0.84 ml, 0.006 mol) in ethyl methyl ketone. The reaction
mixture was allowed for stirring for 5 hours. Then the flask was taken out, the triethylamine
salt was filtered, and the solvent was removed from solution under reduced pressure. The
solid obtained was washed twice with 2% potassium hydroxide solution followed by water
and then dried, recrystallized from isopropyl alcohol and methanol.
Similar protocols were used for synthesizing other homologues of the series.
Yield: 64.9%, m.p. 102.1◦C, FT-IR (KBr, cm–1): 2920, 2852 (C Hstr), 1728 (C Ostr),
1608, 1509, (C Cstr aromatic), 1416, 1359 (C Hben), 1270, 1185 (C O Casym & sym str of
ester and ether); 1H-NMR ppm (CDCl3): 8.14 (d, 2H), 7.99 (d, 1H), 7.66 (d, 1H), 7.28 (m,
4H), 7.18 (t, 1H), 6.96 (d, 2H), 4.04 (t, 2H), 1.81 (m, 2H), 1.45 (m, 2H), 1.31 (m, 16H), 0.88
(t, 3H), 13C-NMR ppm (CDCl3): 164.92, 163.71, 160.59, 148.71, 147.96, 139.90, 133.72,
132.81, 132.40, 128.16, 122.83, 122.62, 121.39, 114.42, 68.43, 32.02, 29.75, 29.72, 29.69,
29.65, 29.63, 29.45, 26.18, 26.07, 22.79, and 14.23.

Two Phenyl Ring Core-Based Thiophene Mesogens
5
2.4.3.1. 4-{[(4-Decyloxyphenyl) Carbonyl] Oxy} Phenyl Thiophene-2-Carboxylate
(2-DCPTC) (5b). Yield: 71.5%, m.p. 104◦C, FT-IR (KBr, cm–1): 3084 (aromatic C Hstr),
2922, 2853 (C Hstr), 1728 (C Ostr), 1607, 1507 (C Cstr aromatic), 1469, 1358 (C Hben),
1266, 1178 (C O Casym & sym str of ester and ether); 1H-NMR ppm (CDCl3): 8.15 (d, 2H),
7.98 (d, 1H), 7.65 (d, 1H), 7.25 (m, 4H), 7.17 (t, 1H), 6.97 (d, 2H), 4.03 (t, 2H), 1.82
(m, 2H), 1.48 (m, 2H), 1.35 (m, 12H), 0.89 (t, 3H); 13C-NMR ppm (CDCl3): 164.89,
163.72, 160.57, 148.72, 147.98, 139.89, 133.74, 132.81, 132.40, 128.17, 122.83, 122.62,
121.40, 114.43, 68.44, 32.01, 29.67, 29.48, 29.46, 29.44, 29.20, 26.09, 22.79, and 14.25.
2.4.3.2. 4-{[(4-Octyloxyphenyl) Carbonyl] Oxy} Phenyl Thiophene-2-Carboxylate
(2-OCPTC) (5c). Yield: 73.4%, m.p. 103◦C, FT-IR (KBr, cm–1): 3078 (aromatic C Hstr),
2924, 2955 (C Hstr), 1731 (C Ostr), 1606, 1577, 1502 (C Cstr aromatic), 1467, 1359
1
(C Hben), 1224, 1170 (C O Casym & sym str of ester and ether); H-NMR ppm (CDCl3):
8.13 (d, 2H), 7.99 (d, 1H), 7.67 (d, 1H), 7.26 (m, 4H), 7.17 (t, 1H), 6.98 (d, 2H), 4.03
(t, 2H), 1.82 (m, 2H), 1.47 (m, 2H), 1.34 (m, 8H), 0.89 (t, 3H); 13C-NMR ppm (CDCl3):
164.91, 163.71, 160.59, 148.71, 147.97, 134.90, 133.73, 132.80, 132.41, 128.17, 122.83,
122.62, 121.40, 114.43, 68.44, 31.90, 29.42, 29.33, 29.19, 26.08, 22.75, and 14.2.
2.4.3.3. 4-{[(4-Butoxyphenyl) Carbonyl] Oxy} Phenyl Thiophene-2-Carboxylate (2-
BCPTC) (5d). Yield: 72%, m.p. 140◦C, FT-IR (KBr, cm–1): 3077 (aromatic C Hstr), 2853,
2867 (C Hstr), 1723 (C Ostr), 1604, 1578, 1505 (C Cstr aromatic), 1468, 1386 (C Hben),
1227, 1166 (C O Casym & sym str of ester and ether); 1H-NMR ppm (CDCl3): 8.12 (d, 2H),
7.97 (d, 1H), 7.67 (d, 1H), 7.25 (m, 4H), 7.18 (t, 1H), 6.96 (d, 2H), 4.05 (t, 2H), 1.82 (m,
2H), 1.49 (m, 2H), 0.99(t, 3H); 13C-NMR ppm (CDCl3): 164.91, 163.72, 160.58, 148.71,
147.98, 134.89, 133.73, 132.81, 132.41, 128.16, 122.82, 122.61, 121.41, 114.43, 68.11,
31.21, 19.29, and 13.92.
2.4.3.4. 4-{[(4-Ethoxyphenyl) Carbonyl] Oxy} Phenyl Thiophene-2-Carboxylate (2-
ECPTC) (5e). Yield: 75.1%, m.p. 153◦C, FT-IR (KBr, cm–1): 3093 (aromatic C Hstr),
2982 (C Hstr), 1723 (C Ostr), 1603, 1504 (C Cstr aromatic), 1412, 1359 (C Hben),
1170, 1117 (C O Casym & sym str of ester and ether); 1H-NMR ppm (CDCl3): 8.14 (d, 2H),
7.99 (d, 1H), 7.67 (d, 1H), 7.26 (m, 4H), 7.18 (t, 1H), 6.97 (d, 2H), 4.11 (t, 2H), 1.46 (t, 3H);
13C-NMR ppm (CDCl3): 164.91, 163.49, 160.61, 148.69, 147.97, 134.89, 133.72, 132.79,
132.42, 128.16, 122.82, 122.62, 121.47, 114.39, 63.91, and 14.76.
2.4.4. Synthesis of 4-{[(4-Dodecyloxyphenyl) Carbonyl] Oxy} Phenyl Thiophene-3-
Carboxylate (3-DDCPTC) (6a).
The synthetic approach adopted for synthesizing 2-series is followed for preparing
3-series by using 3-thiophene carboxylic acid as a starting material.
Yield: 67.2%, m.p. 116◦C, FT-IR (KBr, cm–1): 3104 (aromatic C Hstr), 2919, 2951
(C Hstr), 1728 (C Ostr), 1609, 1510, (C Cstr aromatic), 1471, 1359 (C Hben), 1265,
1194 (C O Casym & sym str of ester and ether); 1H-NMR ppm (CDCl3): 8.31 (d, 1H), 8.15
(d, 2H), 7.66 (d, 1H), 7.38 (m, 1H), 7.25 (m, 4H), 6.98 (d, 2H), 4.04 (t, 2H), 1.82 (m, 2H),
1.49 (m, 2H), 1.35 (m, 16H), 0.88 (t, 3H); 13C-NMR ppm (CDCl3): 164.93, 163.71, 161.04,
148.63, 148.10, 134.26, 132.81, 132.41, 128.32, 126.53, 122.83, 122.67, 121.41, 114.42,
68.43, 32.02, 29.76, 29.75, 29.73, 29.70, 29.67, 29.47, 29.19, 26.09, 22.80, and 14.25.
2.4.4.1. 4-{[(4-Decyloxyphenyl) Carbonyl] Oxy} Phenyl Thiophene-3-Carboxylate
(3-DCPTC) (6b). Yield: 70.1%, m.p. 119◦C, FT-IR (KBr, cm–1): 3104 (aromatic C Hstr),
2921, 2850 (C Hstr), 1728 (C Ostr), 1609, 1509, (C Cstr aromatic), 1466, 1418 (C Hben),
1262, 1191 (C O Casym & sym str of ester and ether); 1H-NMR ppm (CDCl3): 8.31 (d, 1H),
8.15 (d, 2H), 7.67 (d, 1H), 7.37 (m, 1H), 7.25 (m, 4H), 6.98 (d, 2H), 4.03 (t, 2H), 1.82
(m, 2H), 1.48 (m, 2H), 1.37 (m, 12H), 0.89 (t, 3H); 13C-NMR ppm (CDCl3): 164.91, 163.71,

6
M. Kesava Reddy et al.
161.02, 148.65, 148.10, 134.24, 132.82, 132.27, 128.32, 126.54, 122.83, 122.66, 121.43,
114.43, 68.44, 67.93, 32.01, 29.67, 29.48, 29.44, 29.21, 26.10, 22.80, and 14.25.
2.4.4.2. 4-{[(4-Octyloxyphenyl) Carbonyl] Oxy} Phenyl Thiophene-3-Carboxylate
(3-OCPTC) (6c). Yield: 73.8%, m.p. 120.5◦C, FT-IR (KBr, cm–1): 3104 (aromatic C Hstr),
2922, 2853 (C Hstr), 1730 (C Ostr), 1610, 1509 (C Cstr aromatic), 1468, 1400 (C Hben),
1269, 1192 (C O Casym & sym str of ester and ether); 1H-NMR ppm (CDCl3): 8.31 (d, 1H),
8.14 (d, 2H), 7.67 (d, 1H), 7.38 (m, 1H), 7.25 (m, 4H), 6.97 (d, 2H), 4.04 (t, 2H), 1.82 (m,
2H), 1.47 (m, 2H), 1.35 (m, 8H), 0.90 (t, 3H); 13C-NMR ppm (CDCl3): 164.92, 163.71,
161.03, 148.64, 148.10, 134.25, 132.82, 132.41, 128.32, 126.54, 122.83, 122.66, 121.42,
114.43, 68.44, 31.91, 29.44, 29.34, 29.19, 26.09, 22.76, and 14.23.
2.4.4.3. 4-{[(4-Butoxyphenyl) Carbonyl] Oxy} Phenyl Thiophene-3-Carboxylate (3-
BCPTC) (6d). Yield: 74.1%, m.p. 147.2◦C, FT-IR (KBr, cm–1): 3110 (aromatic C Hstr),
3077, 2953, 2867 (C Hstr), 1736 (C Ostr), 1609, 1578, 1504 (C Cstr aromatic), 1467,
1
1384, (C Hben), 1249, 1166 (C O Casym & sym str of ester and ether); H-NMR ppm
(CDCl3): 8.32 (d, 1H), 8.14 (d, 2H), 7.67 (d, 1H), 7.38 (m, 1H), 7.25 (m, 4H), 6.98
(d, 2H), 4.05 (t, 2H), 1.79 (m, 2H), 1.51 (m, 2H), 0.99 (t, 3H); 13C-NMR ppm (CDCl3):
164.93, 163.71, 161.04, 148.64, 148.10, 134.25, 132.82, 132.41, 128.32, 126.53, 122.82,
122.66, 121.43, 114.43, 68.11, 31.22, 19.29, and 13.93.
2.4.4.4. 4-{[(4-Ethoxyphenyl) Carbonyl] Oxy} Phenyl Thiophene-3-Carboxylate (3-
ECPTC) (6e). Yield: 72.9%, m.p. 162◦C, FT-IR (KBr, cm–1): 3106 (aromatic C Hstr),
2978 (C Hstr), 1730 (C Ostr), 1605, 1507 (C Cstr aromatic), 1419 (C Hben), 1256, 1167
1
(C O Casym & sym str of ester and ether); H-NMR ppm (CDCl3): 8.31 (d, 1H), 8.13 (d,
2H), 7.66 (d, 1H), 7.38 (m, 1H), 7.25 (m, 4H), 6.97 (d, 2H), 4.11 (t, 2H), 1.45 (t, 3H);
13C-NMR ppm (CDCl3): 164.93, 163.49, 161.06, 148.63, 148.10, 134.26, 132.79, 132.43,
128.32, 126.54, 122.83, 122.67, 121.48, 114.40, 63.93, and 14.77.
3. Results and Discussion
The molecular structures of representative decyloxy mesogens of both the series along
with carbon numbering are shown in Figs. 1 and 2. Scheme 1 depicts the strategy adopted
Figure 1. Molecular structure and energy optimized space filled model of 2-DCPTC.

Two Phenyl Ring Core-Based Thiophene Mesogens
7
Figure 2. Molecular structure and energy optimized space filled model of 3-DCPTC.
for the synthesis of mesogens. Figs. 1 and 2 also show the energy optimized space filled
models of the decyloxy mesogens computed from DFT method of quantum chemical
calculations. The ester linking units are employed for the construction of core with alkoxy
chain (even carbons except C6 homologue) as terminal unit. The target mesogens as well
1
as the intermediates are well characterized by FT-IR, H, and 13C NMR spectroscopy.
The FT-IR spectrum of decyloxy mesogen revealed prominent intense vibrations at 2920,
1728, and 1270 cm–1, which are attributed to C H asymmetric stretching, ester carbonyl
stretching, and C O C asymmetric stretching vibrations, respectively. In proton NMR,
the expected spectral lines with multiplicities are observed accounting for all the protons
of the mesogens. For instance, thiophene ring showed characteristic three-spin pattern,
while the terminal OCH2 is distinctly seen as a triplet at 4.04 ppm, and the other spectral
features are in consistent with molecular structure.
3.1. 13CNMR Spectroscopy
Figure 3 shows the proton decoupled 13C NMR spectra of decyloxy mesogens. The chemi-
cal shift assignment of the signals of the core unit is carried out using DFT-based quantum
chemical calculations. In recent years, DFT-based methods are often employed for struc-
tural assignment of mesogens using 13C chemical shifts, as complexity increases with more
number of carbons [18–20]. Table 1 lists the 13C chemical shifts of experimentally deter-
mined values as well as those computed from DFT method for the core unit of 2-DCPTC and
3-DCPTC. The spectrum shows 14 lines in the region 114–165 ppm for the core unit, while
10 lines are clearly seen in the range 14–69 ppm for the decyloxy chain. A good agreement
is noticed between experimental and calculated 13C chemical shifts with exception to ester
carbonyl and thiophene carbons. For thiophene ring, the carbon chemical shift assignment
is further verified by comparing with the literature data [21]. A close examination of 13C
chemical shifts of 2-DCPTC and 3-DCPTC indicates that, except C14, the chemical shifts
of core as well as terminal chain are alike in view of structural similarity. A 7 ppm variation
noticed for C14 chemical shift between the mesogens is attributed mainly to substitution
pattern of the thiophene ring.

8
M. Kesava Reddy et al.
OH
O
CH
O
CH
O
3
3
O
O
O
R
Cl
O
R
O
R
OH
O
HO
O
SOCl2/DMF
Reflux
RBr/K2CO3
DMF, 90oC
C2H5OH/KOH sol
Reflux
Pd/C , H2
O
THF at RT
TEA
,
O0C
O
R
O
OH
O
O
O
4
2
1
R
R =C12H25, C10H21, C8H17
,
O
3
C4H9, C2H5
(3a-3e)
(4a-4e)
O
O
O
O
S
O
O
OH
O
Cl
O
HO
O
O
O
O
R
S
S
R
SOCl2/ DMF
Reflux
(or)
(or)
(or)
6
(6a-6e)
TEA O0C
R =C12H25, C10H21, C8H17
,
O
O
C4H9, C2H5
S
S
OH
Cl
S
O
O
O
R
5
(5a-5e)
Scheme 1. Synthetic strategy for the mesogens.
Figure 3. Proton decoupled 13C NMR spectra of (a) 2-DCPTC and (b) 3-DCPTC in solution state.

Two Phenyl Ring Core-Based Thiophene Mesogens
9
Table 1. 13C Chemical shifts of 2-DCPTC and 3-DCPTC
3-DCPTC 13C chemical shift
(ppm)
2-DCPTC 13C chemical shift (ppm)
C. No
Experimental
QCC
Experimental
QCC
1
2
3
4
5
6
7
8
9
10
11
12
13
14
163.7
114.4
132.4
121.3
164.9
147.9
122.8
122.6
148.7
160.5
132.8
133.7
128.1
134.9
162.6
116.9
133.3
121.3
161.9
147.8
122.4
120.8
147.9
157.7
138.3
134.4
125.2
136.9
163.7
114.4
132.4
121.4
164.9
148.1
122.8
122.6
148.6
161.0
134.2
132.8
128.3
126.5
163.1
115.6
133.4
121.7
162.0
149.0
122.4
122.2
147.7
158.0
137.0
133.5
128.6
129.3
3.2. Mesophase Transitions
The mesophase characteristics like melting and clearing transitions for both the series are
determined by HOPM and DSC techniques. Tables 2 and 3 list the mesophase temperatures
and transition enthalpy values. All the mesogens showed enantiotropic nematic phase as
confirmed by DSC heating and cooling curves as well microscopy data (Figs. 4 and 5).
In HOPM, the samples upon cooling showed characteristic Schlieren or threaded texture
typical of nematic phase for all the mesogens. Fig. 6 shows the HOPM photographs of
2-OCPTC (91.2◦C) and 3-DDCPTC (120.4◦C) mesogen on cooling the isotropic phase. In
DSC scan (Figs. 4 and 5), two peaks are typically noticed for all the mesogens among which
Table 2. Transition temperatures and enthalpy values of 2-series
Code
Transition
Temperature (◦C)
ꢀH (k.cal/mole)
2-ECPTC
Cr–N
N–I
Cr–N
N–I
Cr–N
N–I
Cr–N
N–I
146.8
160.4
137.5
149.1
100.0
120.1
98.2
117.7
86.8
99.1
8.3
0.1
8.0
0.1
5.9
0.1
7.1
0.1
0.8
7.5
0.1
2-BCPTC
2-OCPTC
2-DCPTC
2-DDCPTC
Cr1–Cr2
Cr2–N
N–I
117.1

10
M. Kesava Reddy et al.
Table 3. Transition temperatures and enthalpy values of 3-series
Code
Transition
Temperature (◦C)
ꢀH (k.cal/mole)
3-ECPTC
Cr–N
N–I
Cr–N
N–I
Cr–N
N–I
Cr1–Cr2
Cr2–N
N–I
157.8
166.7
140.6
150.0
113.0
131.0
79.6
113.2
126.3
86.1
7.25
0.1
6.3
0.1
7.7
0.1
0.6
7.7
0.1
6.5
10.2
0.3
3-BCPTC
3-OCPTC
3-DCPTC
3-DDCPTC
Cr1–Cr2
Cr2–N
N–I
113.3
122.7
crystal–nematic transition is associated with higher enthalpy value (∼6–8 k.cal/mol), while
the nematic–isotropic showed lower values (∼0.1–0.3 k.cal/mol). For higher homologues,
i.e., 2-DCPTC and 3-DCPTC as well as 2-DDCPTC and 3-DDCPTC, crystal–crystal
transitions are observed (Figs. 4 and 5) below crystal–nematic transition. This feature
is typical for molecular crystals in which the terminal unit is constructed with lengthy
alkyl/alkoxy chain [22]. The transition enthalpy values further substantiate the HOPM
Figure 4. DSC Scan of 2-DDCPTC.

Two Phenyl Ring Core-Based Thiophene Mesogens
11
Figure 5. DSC Scan of 3-DDCPTC.
observations. Further, a good agreement is noticed in the transition temperatures measured
by HOPM and DSC.
3.3. Molecular Shape and Mesophase Characteristics
The nematic–isotropic transitions of both the series showed decreasing trend with an
increasing terminal chain length. The crystal–nematic transition also exhibited similar
Figure 6. HOPM photographs of mesogens—cooling from isotropic phase (a) 2-OCPTC at 91.2◦C
nematic Schlieren texture (b) 3-DDCPTC at 120.4◦C threaded nematic texture.

12
M. Kesava Reddy et al.
trend for 2-series, whereas for 3-series, the value is found to be same for 3-OCPTC and
3-DCPTC. Interestingly, the mesophase stability values for both the series are not high
despite two phenyl rings in the core besides thiophene unit. In other words, in contrast
to the phase stability values of three ring core [23–25], the thiophene mesogens exhibited
lower phase stability. The data also suggest that the mesogens of 2-series showed better
phase stability than those of 3-series. A comparison of thermal stability values, on the other
hand, suggests that for lower homologues, i.e., C2 and C4, the values are comparable but
with an increase in terminal chain length; an increase in thermal stability is clearly noticed
for 3-series. These trends can be correlated to the molecular structures of the mesogens and
their dipole moments. Figs. 1 and 2 show the energy optimized space filled models of C10
mesogens of 2- as well as 3-series. It can be seen that both 2-DCPTC and 3-DCPTC are
rod-like typical of calamitic mesogens. For such mesogens, the crystal–nematic transition is
generally found to be high if the core is linear with high polarizability anisotropy [26]. The
dipole moments of the 2-DCPTC and 3-DCPTC calculated by DFT method are 2.74 D and
3.50 D, respectively. The relatively high crystal–nematic transition temperature noticed
for 3-thiophene mesogens could be ascribed to high dipole moment, which influences
the molecular anisotropic polarizability. Any deviation from rod-like shape of the core
is known to reduce the mesophase characteristics [27, 28]. Earlier Nugent et al. also
observed low mesophase stabilities for thiophene and furan-based mesogens constructed
with two phenyl ring core [29]. The thiophene mesogens reported by them too showed
enantiotropic nematic mesophase similar to our results. Further, a clear structural similarity
between those examined by Nugent et al. and 2- as well as 3-series exists and in both
the cases thiophene is positioned at one terminal. The low mesophase stabilities noticed
for both the series in contrast to three phenyl ring-based core could be attributed to very
high anisotropic polarizability values for three ring mesogens [23–25]. For instance, the
mesophase stabilities of 5 (2-series) are ∼12◦C–20◦C, whereas for 6 (3-series), the values
are in the range ∼9◦C–18◦C. For three phenyl ring core-based mesogens with terminal
dimethylamino unit, on the other hand, the values are found to be ∼63◦C–108◦C [23]. For
the case of mesogens with terminal hydroxyhexyloxy chain, the phase stabilities are in
the range ∼85◦C–113◦C [25]. These observations suggest that the replacement of phenyl
ring with thiophene in the core unit dramatically reduces the mesophase stability values.
Additionally, the high symmetry associated with 1,4-disubstituted phenyl ring is lost by the
introduction of irregular pentagon geometry typical for thiophene as reported in literature
[30]. As a result, thiophene is rated as a low terminal efficiency group as it brings molecular
asymmetry to the mesogen [31].
The investigations on thiophene mesogens indicate that the incorporation of thiophene
at terminal position ensures enantiotropic nematic phase and favorably reduces the melting
temperature as reported in literature [1–3]. Further, the present study points out that the 3-
thiophene series can be used for synthesizing polymeric liquid crystals as 2,5-positions are
available for chemical or electrochemical polymerization. Such polymers are anticipated
to exhibit polarized emission as reported earlier for structurally similar systems owing to
extended conjugation [32]. Furthermore, the solid state 13C NMR experiments are underway,
which may provide information about not only the molecular structure in nematic phase
but also the orientation of thiophene with respect to orientation axis as demonstrated earlier
for similar kind of mesogens [33].
4. Conclusion
The synthesized thiophene-based mesogens with ester connecting units and phenyl ring
core showed enantiotropic nematic phase. The phase stabilities for both the series were

Two Phenyl Ring Core-Based Thiophene Mesogens
13
found to be low, which was attributed to insertion of less symmetrical thiophene at the
terminal position. The quantum chemical calculations not only provided the molecular
shape for the representative mesogens but also aided the assignment of core unit carbons
through 13C chemical shifts. The relatively high crystal–nematic transition temperature
noticed for 3-DCPTC was attributed to high dipole moment in contrast to 2-DCPTC. The
studies corroborate that the incorporation of thiophene at terminal position not only results
in nematic phase but also marginally reduces the melting temperature as observed in earlier
investigations [1–3].
Acknowledgments
M. Kesava Reddy and T. Narasimhaswamy sincerely thank Dr. A. B. Mandal, Director,
CSIR-CLRI, for his keen interest and constant support in this work. Thanks are also due
to Dr. B. V. N. Phanikumar and Dr. V. Subramanian, CSIR-CLRI, for their help in solution
NMR as well as quantum chemical calculations. The partial financial support from NWP-23
is duly acknowledged.
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