Synthesis and mesomorphic behaviour of new discotic liquid crystalline compounds containing triphenylamine as a core moiety via Sonogashira coupling

Article abstract of DOI:10.1016/j.tetlet.2009.11.066

Design and synthesis of cholesterol based disk-like liquid crystalline compounds using triphenylamine as a core moiety have been achieved by Pd-catalyzed cross-coupling reaction. The newly synthesized compounds exhibit a cholesteric phase with fingerprint texture as well as oily texture. In the low temperature region, there is a signature of smectic B with characteristic dendritic and mosaic textures. The mesogenic properties were characterized by polarizing microscopy, differential scanning calorimetry and HRXRD studies.

Full text of DOI:10.1016/j.tetlet.2009.11.066

Tetrahedron Letters 51 (2010) 521–524
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Synthesis and mesomorphic behaviour of new discotic liquid crystalline
compounds containing triphenylamine as a core moiety via
Sonogashira coupling
*
K. C. Majumdar , Shovan Mondal, Nirupam De, Randhir Kumar Sinha, Nilasish Pal, B. Roy
Department of Chemistry, University of Kalyani, Kalyani 741 235, WB, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Design and synthesis of cholesterol based disk-like liquid crystalline compounds using triphenylamine as
a core moiety have been achieved by Pd-catalyzed cross-coupling reaction. The newly synthesized com-
pounds exhibit a cholesteric phase with fingerprint texture as well as oily texture. In the low temperature
region, there is a signature of smectic B with characteristic dendritic and mosaic textures. The mesogenic
properties were characterized by polarizing microscopy, differential scanning calorimetry and HRXRD
studies.
Received 20 October 2009
Revised 12 November 2009
Accepted 15 November 2009
Available online 3 December 2009
Keywords:
Discotic liquid crystal
Triphenylamine
Ó 2009 Elsevier Ltd. All rights reserved.
Cholesterol
Sonogashira coupling reaction
Discotic liquid crystals (DLCs) are unique nanostructures with
remarkable electronic and optoelectronic properties. Mesophase
formed by disc-shaped molecules is primarily of two types:
nematic and columnar. A few discotic molecules are also re-
ported to form a discotic lamellar mesophase.1,2 In the discotic
nematic phase, there is an orientationally ordered arrangement
of discs with no long-range transitional order, while in the
columnar phase, the discs are stacked one on top of another to
form columns. Two attractive intermolecular interactions are
most probably the main factors responsible for the observation
of mesogenic properties. These are core–core attraction (e.g., dis-
persion forces) and a hydrophobic interaction between aliphatic
chains.
Molecules containing triphenylamine (TPA) moiety have been
widely investigated as active materials for hole-transport and elec-
troluminescence.3The amorphous character of these materials of-
fers possibilities to develop active materials for solar cells4 with
isotropic optical and charge-transport properties. The use of TPA-
based materials for photovoltaic conversion has been scarcely con-
sidered.5 They also play an important role in photorefractive (PR)
materials, which have attracted much interest in recent years be-
cause of their potential applications in holographic optical data
storage and real time imaging processes. Therefore, in continuation
of our studies towards the design and synthesis of liquid crystalline
compounds,6 we became interested in synthesizing a new class of
disc-like molecules with TPA and cholesterol via Sonogashira reac-
tion. The palladium and copper co-catalyzed coupling of terminal
alkynes with various organic halides is the most straightforward
and powerful method for the construction of C (sp2)-C (sp) bonds.7
This method has been widely applied to diverse areas such as
natural product synthesis and material science.8 In our previous
Letter9, we have reported a disc-shaped molecule containing tri-
phenylamine as core moiety with highly branched terminal alkyl
chains which self-assembled (the polar and non-polar regions) to
form the columnar mesophase. Now we have undertaken a study
to design and synthesize a new class of cholesterol-containing tri-
meric mesogens for studying their mesomorphic behaviour. Herein
we report the results.
The methodology for the synthesis of new discotic liquid crys-
tals (9a,b) is depicted in Scheme 1.
The required precursor 4 was synthesized by deprotection of
the TMS group of compound 3 using KF-MeOH. Compound 3 was
in turn prepared by Sonogashira coupling of the compound 2 with
trimethylsilylacetylene. Compound 2 was prepared by the iodin-
ation of triphenylamine (1) with I2 in the presence of HgO in
EtOH10 (Scheme 2).
The other precursors 8a,b were synthesized from naturally
occurring cholesterol. Esterification of cholesterol (5) with the
bromoalkanoylchlorides (6a,b) was carried out in THF in the
presence of pyridine to afford the compounds 7a,b. The cholesteryl
4-iodo alkyl esters (8a,b) were prepared by the reaction of
compounds 7a,b with 4-iodophenol in refluxing acetone in the
11
presence of anhydrous K2CO3 (Scheme 3).
Finally the target compounds (9a,b12
)
were successfully
* Corresponding author. Tel.: +91 33 2582 7521; fax: +91 33 25828282.
E-mail address: kcm_ku@yahoo.co.in (K.C. Majumdar).
obtained by Sonogashira coupling between compound 4 and
0040-4039/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2009.11.066

522
K. C. Majumdar et al. / Tetrahedron Letters 51 (2010) 521–524
compounds 8a,b by using Pd(PPh3)2Cl2 (10 mol %) as catalyst, CuI
(12 mol %) as co-catalyst, and DIPEA as base in THF.
The phase transitions of the new discotic liquid crystalline
materials (9a,b) were determined by differential scanning calorim-
etry (DSC) at a heating rate of 5 °C minÀ1. The transition tempera-
tures and associated enthalpies are shown in Table 1.
H
O
N
H
H
I
O
+
( )n
O
8a,b
a, n = 5
, n = 10
Textural analysis was carried out with the help of a polarizing
optical microscope (POM). Compounds 9a,b exhibited the enantio-
tropic phase sequence of crystal?Smectic B?TGB?chole-
steric?isotropic, when the sample was placed in a glass slide
and sandwiched by a cover slip at homogeneous heating condi-
tions. When the compound 9a was allowed to heat up to isotropic
temperature (116.1 °C) and kept for 5 min in isotropic phase and
was allowed to cool slowly, the reappearance of the texture was
observed at 113.1 °C with four brush cholesteric spherulites
(Fig. 1a) which turned to a pellet-like texture at around 112.8 °C
(Fig. 1b). This pellet-like texture then transformed to characteristic
long pitch cholesteric droplets with different colours correspond-
ing to different twists (Fig. 1c). After lowering the temperature
slightly, the coalescence of the cholesteric droplets occurred during
the phase ordering process. At around 112.2 °C, the cholesteric
fingerprint texture was observed (Fig. 1d). A TGB phase was also
observed at a very short range (0.2–0.5 deg, Fig. 1e). When the
sample was allowed to cool, there was a sudden change in texture
to dendritic form which is often observed in smectic B phase
( Fig. 1f). The dendritic texture persisted till room temperature.
The sample was heated slowly to observe the textural pattern from
dendritic texture, and this texture continuously changed the colour
but without any change in texture as the temperature was in-
creased and turned to cholesteric fingerprint texture before trans-
forming to isotropic phase. The textural appearances of the
compound 9a are shown in Figure 1.
4
Sonogashira
Coupling
b
(i)
O
O
O
O
( )n
( )n
R
R
N
H
9a,b
a, n = 5, 82%
, n = 10, 80%
H
H
b
R =
O
O
O
( )n
R
Scheme 1. Synthetic route for the synthesis of new discotic liquid crystals 9a,b.
Reagent and conditions: (i) Pd(PPh3)2Cl2, CuI, DIPEA, THF, rt, 12 h.
I
I
N
I
N
(i)
79%
(ii)
80%
2
1
TMS
TMS
The powder X-ray diffraction pattern was performed on com-
pound 9a at different temperatures to investigate the supramolec-
ular arrangement of molecules in the liquid crystalline phases.
Careful observation of the X-ray diffraction pattern reveals that
the compound forms highly ordered liquid crystalline phase. The
Debye–Scherrer pattern for the compound 9a at 95 °C and 105 °C
was discussed here as these temperatures corresponds to the
smectic B mesophase. The pattern shows two distinct sharp small
angle peak at both the temperatures where 2h = 2.08° and 2.54° for
95 °C indicating the layer spacing in smectic layer d = 42.47 Å and
34.82 Å, respectively. At 105 °C the 2h = 2.048° and 2.497° and the
layer spacing is pretty close to the previous temperature 95 °C
(d = 43.13 Å and 35.38 Å). In the wide angle region, the scarcity
of diffuse peak indicates the presence of long-range order in the
smectic layer which indicates that the smectic phase is not fluid.
This is quite possible in smectic B type phase. The appearance of
two sharp peaks at small angles may probably be due to first and
second order transitions. In the smecitc B phase, the constituent
molecules pack in a hexagonal array with the molecular long axes
perpendicular to the layer planes. The molecules are assumed to
rotate and the layer is free to slide over one another.13 On the basis
of the above discussion and shape of the newly synthesized mole-
cules 9a,b, we have constructed a supramolecular arrangement of
the molecule in smectic B phase. The proposed model is supposed
to be hexagonal where each corner of the hexagon is occupied by a
hand of the two trihand molecules by free rotation (Fig. 2).
In conclusion, we have succeeded in designing and synthesizing
a new class of disc-like mesogens containing both cholesteryl and
triphenyl amine moieties via Sonogashira coupling as a key step.
We have also characterized the mesophases. It is worth mention-
ing that generally cholesterol-based dimers show Sm A/Sm C phase
(among smectic phases)14 and usually the disc-like molecules with
flexible branched alkyl chains self assembles to form columnar
phase.15 At the present instance, the cholesteryl moiety is con-
N
N
(iii)
80%
3
4
TMS
Scheme 2. Synthesis of precursor 4. Reagent and conditions: (i) HgO, I2, EtOH, rt,
12 h; (ii) trimethylsilylacetylene, Pd(PPh3)2Cl2, CuI, DIPEA, THF, rt, 12 h; (iii) KF,
MeOH, 2 h.
H
O
Br
+
H
H
( )n
Cl
HO
6 (a,b)
(i)
a, n = 5
5
b
, n = 10
H
O
7 (a,b)
a, n = 5, 83%
, n = 10, 81%
H
H
Br
( )n
O
b
(ii)
H
O
8 (a,b)
H
H
I
O
a, n = 5, 84%
( )n
O
b
, n = 10, 81%
Scheme 3. Synthesis of precursors 8a,b. Reagent and conditions: (i) THF, pyridine,
rt, 12 h; (ii) p-iodophenol, K2CO3, acetone, reflux, 12 h.

K. C. Majumdar et al. / Tetrahedron Letters 51 (2010) 521–524
523
Table 1
Phase transition temperatures (°C) associated enthalpies (D
H, KJ molÀ1) of the compounds in the heating and cooling cycles are summarized below
Compound
Phase Transition (Heating cycle)
Temperature
Enthalpy
Phase Transition (cooling cycle)
Temperature
Enthalpy
*
9a
Cr–SmB
SmB–TGB
87.9
104.8
105.1
116.2
90.5
139.0
139.6
145.9
71.9
—
66.5
7.7
36.3
—
I–N
*
113.1
111.3
111.4
rt
142.2
140.3
138.1
rt
72.8
—
75.9
—
6.5
4.6
10.1
—
N –TGB
TGB–SmB
SmB–Cr
I–N*
N*–TGB
TGB–SmB
SmB–Cr
*
TGB–N
*
N –I
9b
Cr–SmB
SmB–TGB
TGB–N*
N*–I
5.4
6.5
Cr = Crystalline phase, SmB = Smectic phase, TGB = Twist Grain Boundary phase.
— = Not detected in DSC.
nected at the terminal position of the triphenyl amine core via al-
kyne linkages (relatively rigid) to study the structure-property
relationship and a higher order Sm B phase has been observed.
We are continuing this work and a full account will be communi-
cated in due course.
Acknowledgements
The financial support from the Department of Science and Tech-
nology through SERC Project No. SR/S1/OC-44/2005 is gratefully
acknowledged. We also thank CSMCRI, Bhavnagar for providing
Powder XRD facility. Four of us are grateful to the DST, New Delhi
(S.M.), CSIR, New Delhi (N.P. & R.K.S.) and UGC, New Delhi (N.D.) for
their fellowships.
References and notes
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Figure 1. Polarizing micrographs of compound 9a.
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12. General procedure for the synthesis of discotic molecules 9a,b via Sonogashira
coupling: Nitrogen gas was purged through a solution of compound 4 (50 mg,
0.16 mmol), 8a (278.3 mg, 0.55 mmol) and DIPEA (4 mL) in dry THF (10 mL) for
30 min. Then catalyst Pd(PPh3)2Cl2 (16.6 mg, 0.02 mmol) and co-catalyst CuI
(6.00 mg, 0.157 mmol) were added and stirred for 12 h at rt. THF was removed
followed by extraction with CHCl3 (3 Â 30 ml) and the extract was washed
with H2O (2 Â 20 ml) followed by brine (10 ml) and dried (Na2SO4), and the
solvent was evaporated to give the crude product which was purified by
column chromatography over silica gel by EA/PE (1:9) as eluent to afford the
compound 9a. Similar treatment of 8b with compound 4, afforded the
compound 9b.Compound 9b: Yellow solid, yield 80%. IR (KBr, cmÀ1
) mmax:
2931, 2214, 1735, 1513; 1H NMR (400 MHz, CDCl3): dH = 0.66–2.31 (m, 183 H),
3.92 (t, J = 6.1 Hz, 6H), 4.59–4.62 (m, 3H), 5.36 (d, J = 4.0 Hz, 3H), 6.84 (d,
Figure 2. Supramolecular arrangement of compound 9a,b.

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K. C. Majumdar et al. / Tetrahedron Letters 51 (2010) 521–524
J = 8.6 Hz, 6H), 7.03 (d, J = 8.5 Hz, 6H), 7.38 (d, J = 8.6 Hz, 6H), 7.41 (d, J = 8.5 Hz,
13. Gray, G. W.; Goodby, J. W. G. Smectic Liquid crystals: Textures and Structures;
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6H). 13C NMR (100 MHz, CDCl3): dC = 18.7, 19.2, 21.0, 22.5, 22.8, 23.8, 24.3,
25.0, 27.8, 28.0, 28.2, 29.2, 29.3, 29.4, 29.5, 31.8, 31.9, 34.7, 35.8, 36.2, 36.6,
37.0, 38.2, 38.7, 39.5, 39.7, 42.3, 50.0, 56.1, 56.6, 68.1, 73.7, 87.8, 89.3, 114.5,
115.3, 118.3, 122.6, 124.0, 132.5, 133.0, 139.7, 146.4, 159.1, 173.2. Anal. Calcd
for C156H219NO9: C, 83.19; H, 9.80, N, 0.62. Found: C, 83.44; H, 9.92, N, 0.78.