Synthesis and mesomorphic behavior of novel liquid-crystalline thiophene derivatives

Article abstract of DOI:10.1080/15421400903568112

A new class of thiophene-based cholesteryl and ferrocenyl derivatives was synthesized and characterized by common analytical techniques. Liquid-crystalline properties were investigated by polarizing optical microscopy (POM) and differential scanning calorimetry (DSC). Thiophene-based cholesteryl derivatives show homeotropic liquid-crystalline phase sequence crystal → smectic A → cholesteric → isotropic, whereas the ferrocenyl derivative is nonmesomorphic. The effects of thiophene ring, terminal group, and length of the polymethylene spacers on the mesomorphic behavior are discussed. Copyright Taylor & Francis Group, LLC.

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Synthesis and Mesomorphic Behavior
of Novel Liquid-Crystalline Thiophene
Derivatives
a
a
a
K. C. Majumdar , Shovan Mondal & Tapas Ghosh
a
Department of Chemistry , University of Kalyani , Kalyani, West
Bengal, India
Published online: 06 Jul 2010.
To cite this article: K. C. Majumdar , Shovan Mondal & Tapas Ghosh (2010) Synthesis and Mesomorphic
Behavior of Novel Liquid-Crystalline Thiophene Derivatives, Molecular Crystals and Liquid Crystals,
524:1, 17-25, DOI: 10.1080/15421400903568112
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Mol. Cryst. Liq. Cryst., Vol. 524: pp. 17–25, 2010
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421400903568112
Synthesis and Mesomorphic Behavior of Novel
Liquid-Crystalline Thiophene Derivatives
K. C. MAJUMDAR, SHOVAN MONDAL, AND
TAPAS GHOSH
Department of Chemistry, University of Kalyani, Kalyani,
West Bengal, India
A new class of thiophene-based cholesteryl and ferrocenyl derivatives was
synthesized and characterized by common analytical techniques. Liquid-crystalline
properties were investigated by polarizing optical microscopy (POM) and differen-
tial scanning calorimetry (DSC). Thiophene-based cholesteryl derivatives show
homeotropic liquid-crystalline phase sequence crystal ! smectic A ! cholesteric !
isotropic, whereas the ferrocenyl derivative is nonmesomorphic. The effects of
thiophene ring, terminal group, and length of the polymethylene spacers on the
mesomorphic behavior are discussed.
Keywords Cholesterol; ferrocene; Sonogashira coupling; thiophene-based liquid
crystal
Introduction
Thiophene derivatives have been used widely as conducting materials, which are
chemically more stable than other aromatic compounds [1]. In particular, 2,5-thieny-
lene derivatives are structurally planar and electrically conducting. The use of
p-conjugated polymers and oligomers has stimulated enormous interest in recent
years for molecular electronic, photonic, and electroluminescent devices [2]. Seed
et al. synthesized the polarizable thiophene derivatives for use in nonlinear optical
applications and showed that thiophene-containing materials, which are highly bire-
fringent, can be used in electrooptic devices [3]. In their earlier studies they showed
that the incorporation of thiophene into the molecular core can substantially
increase the optical anisotropy and provide molecules with relatively low melting
points [4,5]. Hiyama et al. synthesized and explored the liquid-crystalline (LC)
properties of thiophene derivatives. They observed a nematic phase when azoben-
zene moiety is in trans form, whereas no LC phase was exhibited by the cis confor-
mer [6]. The electrical and structural properties of thiophene derivatives have been
explored extensively by other groups. The photoresponsive properties were also
explored with an intension to develop photonic materials by using electron polariza-
tion of p-conjugated units [7].
Address correspondence to K. C. Majumdar, Department of Chemistry, University of
Kalyani, Kalyani, Nadia, West Bengal 741235, India. E-mail: kcm_ku@yahoo.co.in
17

1
8
K. C. Majumdar et al.
The simplest and most technologically important liquid-crystalline phase is the
nematic phase [8]. Thermotropic nematic liquid crystals are generally optically uni-
axial materials composed of rigid rods or discs that can undergo rapid rotation, to
give a uniformly disrotated orientation along one axis (Fig. 1(a)). In an effort to
achieve new nematic phases with potential biaxial optical properties, we became
interested in bent-rod structures with lateral dipole (Fig. 1(b)). Rotation of a
bent-rod structure about its long axis will require a larger displacement of the neigh-
boring molecules and thereby increase the rotational activation barrier relative to a
rod mesogen. The addition of lateral dipoles also has the potential to produce
head-to-head ferroelectric organization, as in a MacMillan type smectic C structure
(Fig. 1(c)) [9] or arrange in a antiferroelectric dimerized structure (Fig. 1(d)).
The structures and interactions necessary to create stable liquid-crystalline
phases from bent-rod systems are not fully understood. It is generally found that
materials with severely bent structures display liquid-crystalline phases with lower
thermodynamic stability than their straight-rod relatives [10]. To produce a bent-rod
structure with liquid-crystalline property, Swager et al. [11] synthesized nematic
liquid crystals with a range of lateral dipoles with the thiophene cores. In continu-
ation of our recent work in cholesterol-based liquid crystals [12], we decided to focus
on materials with a thiophene as a central core of the mesogens. Here we report our
results.
Figure 1. Different dipolar organization of dimers.

Liquid-Crystalline Thiophenes
19
Results and Discussion
Synthesis
The synthetic route for the new class of bent-rod liquid crystals (9a, b), is depicted in
Scheme 1.
The required precursor 4 was synthesized by deprotection of the trimethylsilane
(TMS) group of compound 3 using KF-MeOH. Compound 3 was in turn prepared
by Sonogashira coupling of compound 2 with trimethylsilylacetylene. Compound 2
was prepared by the bromination of thiophene with slow addition of bromine in
CHCl following a standard literature procedure [13] (Scheme 2).
3
The other precursors 8a, b were synthesized from naturally occurring choles-
terol. Esterification of cholesterol (5) with the bromoalkanoylchlorides (6a, b) was
carried out in tetrahydrofuran (THF) in the presence of pyridine to afford the com-
pounds 7a, b. The cholesteryl 4-iodo alkyl esters (8a, b) were prepared by the reac-
tion of compounds 7a, b with 4-iodophenol in refluxing acetone in the presence of
anhydrous K CO [14] (Scheme 3).
2
3
Finally, the target compounds (9a, b) were successfully obtained by Sonogashira
coupling between compound 4 and compounds 8a, b by using Pd(PPh ) Cl
2
3
2
(
(
10 mol%) as catalyst, CuI (12 mol%) as co-catalyst, and N,N-Diisopropylethylamine
DIPEA) as base in THF.
The methodology for the synthesis of thiophene derivatives containing ferroce-
nyl moiety is depicted in Scheme 4.
Ferrocene (10) was first converted to 6-bromohexyloyl ferrocene (11) by treat-
ment with acid chloride (6a) in dry chloroform in the presence of anhydrous alu-
minium chloride. Compound 11, on treatment with zinc amalgam in the presence
of concentrated hydrochloric acid in benzene-water under refluxing condition,
3 2 2
Scheme 1. Reagent and conditions: (i) Pd(PPh ) Cl , CuI, DIPEA, THF, rt, 12 h.

2
0
K. C. Majumdar et al.
Scheme 2. Reagent and conditions: (i) Br , CHCl , rt, 6 h (ii) Trimethylsilylacetylene,
2
3
3 2
Pd(PPh ) Cl, CuI, DIPEA, THF, rt, 12 h (iii) KF, MeOH, 2 h.
afforded compound 12 [12c]. Compound 12 on reaction with 3,4,5-trihydroxyethyl-
benzoate and anhydrous potassium carbonate in refluxing acetone followed by sub-
sequent hydrolysis of the ester function with ethanolic potassium hydroxide afforded
the acid derivative 14. The acid derivative 14 on esterification with p-iodophenol
afforded the iodo derivative 15. The target compound 16 was successfully synthe-
sized by Sonogashira coupling of compound 4 and compound 15 by using
Pd(PPh ) Cl (15 mol%) as catalyst, CuI (20 mol%) as co-catalyst, and DIPEA as
3
2
2
base in THF.
Mesomorphic Behavior
Thiophene derivatives containing cholesteryl moiety (9a, b) show mesomorphic
behavior, whereas thiophene derivative 16 containing ferrocenyl moiety did not exhi-
bit any mesomorphic behavior. The phase transitions of the bent-rod liquid crystals
9
a, b were measured using differential scanning calorimetry (DSC) at a heating rate
ꢁ1
ꢀ
of 5 C min . The transition temperatures and associated enthalpies are shown in
Table 1.
Textural analysis was carried out with the help of a polarizing optical micro-
scope (POM). Compound 9a showed enantiotropic phase sequence of crystal -
!
smectic A ! cholesteric ! isotropic. When the hexamethylene spacer of 9a was
replaced by an undecamethylene spacer, compound 9b also displayed the same phase
sequence. On cooling the isotropic phase of compound 9a, an oily streak texture
appeared, which instantly transformed to a fan-like texture, the characteristic texture
ꢂ
of N phase (Fig. 2(a)). On further cooling the sample, the homeotropic texture of
Scheme 3. Reagent and conditions: (i) THF, pyridine, rt, 12 h (ii) p-Iodophenol, K CO ,
2
3
acetone, reflux, 12 h.

Liquid-Crystalline Thiophenes
21
Scheme 4. Reagents and Conditions, (i) 6-bromohexanoyl chloride, AlCl
Zn-Hg, Conc. HCl, benzene-water, reflux, 36–48 h (iii) acetone, K CO , reflux, 12 h (iv)
3
, CHCl
3
, rt, 2 h (ii)
2
3
C
2
H
5
OH, KOH, reflux, 2 h (v) p-iodophenol, DCC, DMAP, CHCl
CuI, DIPEA, THF, rt, 12 h.
3 3 2
, rt, 2 h (vi) Pd(PPh ) Cl,
SmA phase, as characterized by the focal conic texture (Fig. 2(b)), appeared. Com-
pound 9b also showed similar phase transitions as indicated in Fig. 2(c) (cholesteric
droplets) and Fig. 2(d) (focal conic texture of SmA phase). But compound 16 did not
show any mosomorphic behavior even when placed in a thin cell with a cell gap of
d ¼ 5 ꢃ 0.2 mm with homogeneous planar boundary conditions.
In conclusion, we have synthesized a new class of bent-shaped liquid crystals
containing thiophene as a core moiety. Compounds containing cholesteryl moiety
ꢂ
show N phase over a wide temperature range along with SmA phase. When ferro-
cenyl moiety was introduced in place of the cholesteryl moiety, the resulting
compound becomes nonmesomorphic.

2
2
K. C. Majumdar et al.
ꢀ
ꢁ1
)
Table 1. Phase transition temperatures ( C) and associated enthalpies (DH, KJ mol
of the compounds in the heating and cooling cycles are summarized below
Phase
transition
Phase
transition
(cooling
cycle)
(
heating
cycle)
Compound
Temperature Enthalpy
Temperature Enthalpy
ꢂ
9
a
Cr-SmA
ꢂ
113.4
169.4
226.5
114.2
171.5
226.4
34.9
I-N
ꢂ
225.5
159.1
91.1
225.9
159.4
91.2
2.3
0.5
26.7
2.5
0.87
28.1
SmA-N
ꢂ
0.68 N -SmA
N -I
Cr-SmA
2.7
35.7
1.1
SmA-Cr
ꢂ
9
b
I-N
ꢂ
ꢂ
SmA-N
ꢂ
N -SmA
SmA-Cr
N -I
2.8
Cr ¼ Crystalline phase, Sm ¼ Smectic phase, N ¼ nematic phase, I ¼ isotropic phase.
Figure 2. Polarizing micrographs of compound 9a,b.
Experimental
General
All the chemicals were procured from either Sigma Aldrich Chemicals Pvt. Ltd. or
Spectrochem, India. Silica gel (60–120 mesh) was used for chromatographic

Liquid-Crystalline Thiophenes
23
separation. Silica gel G (E-Merck, Mumbai, India) was used for thin layer chroma-
ꢀ
tography (TLC). Petroleum ether refers to the fraction boiling between 60 C and
ꢀ
8
in cm ) on KBr disks. H-NMR spectra were determined for solutions in CDCl
0 C. IR spectra were recorded on a Perkin-Elmer L 120-000A spectrometer (n
max
ꢁ1
1
3
with TMS as internal standard on a Bruker DPX-400 and Bruker DPX-300.
1
3
C-NMR spectra were determined for solutions in CDCl on a Bruker DPX-400.
3
Carbon hydrogen nitrogen (CHN) was recorded on 2400 series II CHN analyzer
Perkin Elmer) from the Chemistry Department of Kalyani University. The
(
liquid-crystalline properties were established by thermal microscopy (Nikon polariz-
ing microscope LV100POL attached to an Instec hot and cold stage HCS302, with
STC200 temperature controller configured for HCS302) and the phase transitions
were confirmed by differential scanning calorimetry (Perkin-Elmer Diamond DSC
Pyris1 system).
Synthesis of Compound 3. Nitrogen gas was purged through a solution of compound
2
(1 g, 2.50 mmol), trimethylsilylacetylene (0.54 g, 5.50 mmol), and DIPEA (4 mL) in
dry THF (10 mL) for 30 min. Then catalyst Pd(PPh ) Cl (175.6 mg, 0.250 mmol) and
3
2
2
cocatalyst CuI (57.2 mg, 0.300 mmol) were added and stirred for 12 h at room
temperature. THF was removed followed by extraction with CHCl (3 ꢄ 30 mL)
3
and the extract was washed with H O (2 ꢄ 20 mL) followed by brine (10 mL),
2
dried (Na SO ), and the solvent was evaporated to give the crude product, which
4
2
was purified by column chromatography over silica gel by EA:PE (1:99) as elutant
to afford compound 3.
ꢁ
1
1
Yellow solid, yield 80%. IR (KBr, cm ) nmax: 2151; H NMR (400 MHz,
1
3
CDCl ): d ¼ 0.26 (s, 18 H). C NMR (100 MHz, CDCl ): d ¼ 0.0, 95.4, 105.9,
3
H
3
C
þ
1
C H Br SSi : C, 38.71; H, 4.18%. Found: C, 38.85; H, 4.23%.
21.5, 129.4. MS: (m=z) ¼ 432 (M ), 434 (M þ 2), 436 (M þ 4). Anal. Calcd. for
1
4
18
2
2
Synthesis of Compound 4. A mixture of compound 3 (1 g, 2.30 mmol) and KF
1.34 g, 23.0 mmol) in MeOH (10 mL) was stirred for 2 h at room temperature.
After removal of the solvent, the reaction mixture was extracted with CHCl3
(
(
(
3 ꢄ 20 mL) and the extract was washed with H O (2 ꢄ 10 mL) followed by brine
2
10 mL), dried (Na SO ), and the solvent was evaporated to give compound 4.
2
4
ꢁ1
1
Yellow solid, yield 80%. IR (KBr, cm ) n : 2101; H NMR (400 MHz,
max
þ
CDCl ): d ¼ 3.67 (s, 2 H). MS: (m=z) ¼ 288 (M ), 290 (M þ 2), 292 (M þ 4). Anal.
3
H
Calcd. for C H Br S: C, 33.14; H, 0.70%. Found: C, 33.23; H, 0.76%.
8
2
2
General Procedure for the Synthesis of Bent-Rod Liquid Crystals 9a,b via
Sonogashira Coupling
Nitrogen gas was purged through a solution of compound 4 (100 mg, 0.345 mmol),
8a (385 mg, 0.759 mmol), and DIPEA (4 mL) in dry THF (10 mL) for 30 min. Then
catalyst Pd(PPh ) Cl (25 mg, 0.035 mmol) and cocatalyst CuI (8 mg, 0.041 mmol)
3
2
2
were added and stirred for 12 h at room temperature. THF was removed followed
by extraction with CHCl3 (3 ꢄ 30 mL) and the extract was washed with H O
2
(
2 ꢄ 20 mL) followed by brine (10 mL), dried (Na SO ), and the solvent was evapo-
2
4
rated to give the crude product, which was purified by column chromatography over
silica gel by EA:PE (1:99) as elutant to afford compound 9a. Similar treatment of 8b
with compound 4 afforded compound 9b.

2
4
K. C. Majumdar et al.
ꢁ
1
Compound 9a. White solid, yield 82%. IR (KBr, cm ) nmax: 2935, 2202, 1731, 1518;
1
H NMR (400 MHz, CDCl ): d ¼ 0.60–2.27 (m, 102 H), 3.92 (t, J ¼ 6.4 Hz, 4 H),
3
H
4
.51–4.59 (m, 2 H), 5.31 (d, J ¼ 4.0 Hz, 2 H), 6.80 (d, J ¼ 8.7 Hz, 4 H), 7.41 (d,
3 C
1
3
J ¼ 8.4 Hz, 4 H). C NMR (100 MHz, CDCl ): d ¼ 11.9, 18.7, 19.3, 21.0, 22.6,
2
3
1
2.8, 23.8, 24.3, 24.7, 25.6, 27.8, 28.0, 28.2, 28.8, 29.7, 31.8, 31.9, 34.6, 35.8, 36.2,
6.6, 37.0, 38.2, 39.5, 39.7, 42.3, 50.0, 56.1, 56.7, 67.8, 73.9, 79.9, 98.9, 113.8,
14.7, 118.3, 121.3, 122.7, 133.3, 133.5, 139.7, 159.9, 173.0. Anal. Calcd. for
C H Br O S: C, 71.74; H, 8.26%. Found: C, 71.87; H, 8.32%.
6
8
6
118
2
ꢁ
1
Compound 9b. Yellow solid, yield 80%. IR (KBr, cm ) nmax: 2929, 2203, 1734,
1
1
4
(
2
3
9
519; H NMR (300 MHz, CDCl ): d ¼ 0.67–2.29 (m, 122 H), 3.98 (t, J ¼ 6.4 Hz,
3
H
H), 4.53–4.61 (m, 2 H), 5.37 (d, J ¼ 4.0 Hz, 2 H), 6.89 (d, J ¼ 8.7 Hz, 4 H), 7.48
1
3
d, J ¼ 8.4 Hz, 4 H). C NMR (100 MHz, CDCl ): d ¼ 11.9, 18.7, 19.3, 21.0,
3
C
2.6, 22.8, 23.9, 24.3, 25.1, 26.0, 27.8, 28.0, 28.2, 29.0, 29.1, 29.2, 29.3, 29.4, 31.9,
4.7, 35.8, 36.2, 36.6, 37.0, 38.2, 39.5, 39.7, 42.3, 50.0, 56.1, 56.7, 68.1, 73.7, 79.9,
8.9, 113.8, 114.7, 118.3, 121.3, 122.7, 133.3, 133.5, 139.7, 160.0, 173.3. Anal.
Calcd. for C H Br O S: C, 72.98; H, 8.80%. Found: C, 73.09; H, 8.88%.
6
9
6
138
2
Procedure for the Synthesis of Compound 16
Nitrogen gas was purged through a solution of compound 4 (100 mg, 0.345 mmol),
15 (893 mg, 0.759 mmol), and DIPEA (4 mL) in dry THF (10 mL) for 30 min. Then
catalyst Pd(PPh ) Cl (36.5 mg, 0.052 mmol) and cocatalyst CuI (13.2 mg,
3
2
2
0
.069 mmol) were added and stirred for 12 h at room temperature. THF was
removed followed by extraction with CHCl (3 ꢄ 30 mL) and the extract was washed
3
with H O (2 ꢄ 20 mL) followed by brine (10 mL), dried (Na SO ), and the solvent
2
2
4
was evaporated to give the crude product, which was purified by column chromato-
graphy over silica gel by EA:PE (1:99) as elutant to afford compound 16.
ꢁ
1
Compound 16. Reddish solid, yield 80%. IR (KBr, cm ) nmax: 2931, 2214, 1735,
1
1
513; H NMR (400 MHz, CDCl ): d ¼ 1.39–1.55 (m, 36 H), 1.82–1.85 (quin,
3
H
J ¼ 7.3 Hz, 12 H,), 2.33 (t, J ¼ 7.8 Hz, 12 H), 4.02–4.04 (m, 36 H), 4.08 (s, 30 H),
.22 (d, J ¼ 8.7 Hz, 4 H), 7.39 (s, 4 H), 7.62 (d, J ¼ 8.6 Hz, 4 H). Anal. Calcd. for
Br Fe O S: C, 65.40; H, 5.83%. Found: C, 65.51; H, 5.88%.
7
C
H
1
30 138
2
6
10
Acknowledgments
The financial support from the Department of Science and Technology through
SERC project No. SR=S1=OC-44=2005 is gratefully acknowledged. Two of us are
grateful (SM and TG) to the CSIR (New Delhi) for fellowships.
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