Carbon nanotubes in triphenylene and rufigallol-based room temperature monomeric and polymeric discotic liquid crystals

Article abstract of DOI:10.1039/b802965b

In this article, after a brief account of the work carried out in the area of dispersion of carbon nanotubes in liquid crystals, we present the dispersion of functionalized single-wall carbon nanotubes in novel room temperature liquid crystalline discotic

Full text of DOI:10.1039/b802965b

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PAPER
www.rsc.org/materials | Journal of Materials Chemistry
Carbon nanotubes in triphenylene and rufigallol-based room temperature
monomeric and polymeric discotic liquid crystals†
Hari Krishna Bisoyi and Sandeep Kumar*
Received 20th February 2008, Accepted 8th April 2008
First published as an Advance Article on the web 6th May 2008
DOI: 10.1039/b802965b
In this article, after a brief account of the work carried out in the area of dispersion of carbon nanotubes
in liquid crystals, we present the dispersion of functionalized single-wall carbon nanotubes in novel
room temperature liquid crystalline discotic monomers and polymers. The effect of nanotubes on phase
behavior of electron-rich triphenylene derivatives and electron-deficient anthraquinone derivatives has
been investigated. Mesophase behavior of the composites with nanotubes has been studied by
polarizing optical microscopy, differential scanning calorimetry and X-ray diffractometry. Inclusion of
carbon nanotubes into the columnar matrix does not destroy the mesophase of the host compounds but
the columnar to isotropic transition temperatures decrease with increase in the quantity of carbon
nanotubes in both monomeric and polymeric discotic liquid crystals.
Introduction
approaches to modify the physical properties of LCs important
4
for optoelectronic applications. Lee and Chiu initiated the work
The serendipitous discovery of carbon nanotubes (CNTs) by
1
Iijima in 1991 generated tremendous activity in most areas of
on doping of CNTs in LCs in 2001. They observed self-diffrac-
5
tion by gratings in nematic LCs doped with multiwalled CNTs.
science and technology as a result of their unprecedented phys-
ical and chemical properties. CNTs, one-dimensional carbon
allotropes, are well ordered all-carbon hollow cylinders of
graphite with a high aspect ratio. CNTs can be either metallic or
semiconducting depending on the sheet direction about which
In the same year, on the basis of continuum-based density
functional theory, Somoza et al. proposed that CNTs should
form a columnar phase or lyotropic LC phase in the presence and
6
absence of van der Waals interactions. Lynch and Patrick ach-
ieved a high degree of CNT alignment along the nematic director
field by taking advantage of the self-assembly properties of
2
the graphite sheet is rolled to form a nanotube cylinder. The
combination of superlative mechanical, thermal and electronic
properties displayed by CNTs makes them ideal for a wide range
of applications, such as conductive and high-strength compos-
ites, catalyst supports in heterogeneous catalysis, energy-storage
and energy-conversion devices, field emitters, transistors,
sensors, gas storage media, tips for scanning probe microscopy
and molecular wires. Synthesis, characterization, physical
properties and applications of CNTs have been extensively
7
nematic LCs. Later the electro-optically induced photo-
8
refractive effect, reduction of the dc driving voltage, voltage-
9
dependent transmittance and capacitance under ac and dc
10
electric field of LC-CNT mixtures were studied. Similarly the
electrooptical characteristics of TN-LCD cells filled with positive
and negative dielectric anisotropy LCs and CNTs were investi-
11
gated. Dierking et al. have studied the alignment and reor-
ientation of doped CNTs in LC cells under both electric and
magnetic fields along with the change in the conductivity of the
LC cell upon reorientation of the CNTs with the LC director
2
covered in several reviews. Despite the extraordinary promise of
CNTs, their realistic application as one-dimensional conductors
or semiconductors has been restricted because of difficulties in
aligning them in the desired direction. Recent studies have
proved that the alignment of CNTs plays a critical role in the
12
field. In contrast to alignment of CNTs with LCs, the alignment
13
of LCs has also been achieved by CNTs. There are a few
14
examples of inclusion of CNTs into lyotropic LCs also.
3
properties of nanotube based materials. However, processing
Preparation of CNTs typically requires high temperature or
high pressure. Moreover, the orientation of the graphene layers
in the nanotubes is important from their application point of
view. To form ordered graphite nanostructures without metal
catalysts and with desired graphene layer orientations, one
possible approach is carbonization within a discotic columnar
mesophase. From this point of view, the columnar structures of
DLCs are worthy precursors as the molecules possess a nano-
graphene subunit as the aromatic core and these molecules stack
one on top of one another to build up columns which in turn
materials with well-controlled CNT alignment still remains
a challenge.
There has been growing interest in the field of dispersion of
CNTs in both thermotropic and lyotropic liquid crystal (LC)
4
phases besides mesophase behavior of CNTs themselves and
synthesis of CNTs from discotic liquid crystals (DLCs). Mixing
CNTs into LC hosts has become one of the attempted
Raman Research Institute, C.V. Raman Avenue, Sadashivanagar,
Bangalore, 560 080, India. E-mail: skumar@rri.res.in; Fax: +91 80
15
organize themselves in different two-dimensional lattices.
2
3610492; Tel: +91 80 23610122
This paper is part of a Journal of Materials Chemistry theme issue on
Upon carbonization under a controlled heating process the
preorganized ordered columnar superstructures can be converted
†
Liquid Crystals Beyond Display Applications. Guest editor: Carsten
Tschierske.
16
into nanotubes. Mullen et al. have produced nanotubes from
3
032 | J. Mater. Chem., 2008, 18, 3032–3039
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1
7
thermotropic DLCs whereas Crawford et al. have produced
nanotubes from both thermotropic and lyotropic DLCs.
Synthesis of 1,5-dihydroxy-2,3,6,7-tetrakis(3,7-
dimethyloctyloxy)-9,10-anthraquinone, 3
In general, LCs formed by disc-shaped molecules have been
attracting growing interest because the supramolecular order of
their columnar phases is of fundamental importance not only as
models for the study of energy and charge migration in self-
organized systems but also as functional materials for many
device applications, such as one-dimensional conductors, pho-
toconductors, light emitting diodes, photovoltaic solar cells, field
effect transistors and gas sensors. The functional capabilities of
these materials are due to their easier processability, spontaneous
alignment between electrodes and self-healing of defects owing to
To a solution of rufigallol 2 (1.52 g, 5 mmol), 3,7-dimethyl
octylbromide (4.86 g, 22 mmol) in DMSO (10 mL) was added
NaOH (0.8 g, 20 mmol) and the mixture was stirred for about
ꢁ
2
0 h at 90 C. After cooling the reaction mixture, water was
added and the aqueous solution was extracted with chloroform.
After solvent evaporation the product was precipitated from
chloroform into methanol thrice to obtain the pure product 3
1
1.51 g, yield 35%). H NMR: dH 12.76 (s, 2 H, Ar-OH), 7.4 (s, 2
(
H, Ar-H), 4.23–4.13 (m, 8 H, ArOCH ), 1.9–1.1 (m, 40 H,
2
aliphatic CH
2
and -CH), 0.98 (d, 6 H, J ¼ 6.3 Hz, -CH-CH
.95 (d, 6 H, J ¼ 6.6 Hz, CH ), 0.88 (d, 12 H, J ¼ 4.5 Hz, CH
); C NMR: d
3
),
),
15,18
their dynamic nature.
0
0
1
2
2
1
3
3
The insertion (dispersion) of CNTs in the supramolecular
order of room-temperature discotic liquid crystalline monomers
and polymers may lead to novel materials with interesting
properties useful for device applications. With this in mind, we
have initiated a research program to disperse functionalized
CNTs into the matrix of room-temperature liquid crystalline
discotic monomers and polymers. Here, in continuation of our
13
.85 (d, 12 H, J ¼ 4.5 Hz, CH
3
C
186.4, 158.1,
57.3, 141.2, 128.9, 111.8, 104.8, 72.1, 67.8, 39.3, 37.3, 36.1, 31.1,
9.8, 29.7, 28.8, 28.0, 24.7, 22.6, 19.6; FT-IR: (nmax) 2922,
854, 1616, 1569, 1506, 1456, 1427, 1365, 1315, 1280, 1226,
ꢀ1
138, 1095, 1045, 950, 864, 802, 723 cm . Elemental analysis:
requires C, 74.96; H,
Found: C, 75.0; H, 10.71. C54
0.25%.
88 8
H O
1
19
work on dispersion of functionalized nanomaterials in DLCs,
we present a study of the mesophase behavior of composites of
functionalized single-wall carbon nanotubes (SWNTs) with
rufigallol and triphenylene-based room temperature monomeric
and polymeric DLCs.
Polymerization of 1,5-dihydroxy-2,3,6,7-tetrakis(3,7-
dimethyloctyloxy)-9,10-anthraquinone
A solution of tetraalkoxy anthraquinone 3 (0.865 g, 1 mmol),
1
,12-dibromododecane (0.328 g, 1 mmol) and caesium carbonate
ꢁ
Experimental
(1.3 g, 4 mmol) in o-dichlorobenzene (5 mL) was stirred at 90 C
under argon for 10 days. The reaction mixture was then cooled
and 20 mL of chloroform was added to it. The organic layer was
separated and washed with water and dried. The solvent was
removed under reduced pressure to yield the crude polymer. It
General information
Chemicals and solvents (AR quality) were used without any
purification. Column chromatographic separation was per-
1
formed on silica gel (100–200 mesh). H NMR spectra and
13
was further purified by repeated precipitation from chloroform
C
1
into methanol. Yield 60%. H NMR: d
H
7.6 (s, 2 H, Ar-H), 4.03–
and
181.2, 157.4, 153.9,
47.0, 132.6, 120.4, 107.0, 74.7, 72.4, 67.5, 45.2, 39.2, 37.3, 36.1,
2.8, 30.4, 29.7, 27.9, 27.3, 27.1, 26.1, 24.7, 22.7, 22.6, 19.6; FT-
NMR spectra were recorded in CDCl on a 400 MHz (Bruker
3
4
.18 (m, 12 H, ArOCH
2 2
), 1.1–2.0 (m, 60 H, aliphatic -CH
AMX 400) spectrometer. All chemical shifts are reported in
d (ppm) units down field from tetramethylsilane (TMS) and J
values are reported in Hz. FT-IR spectra were recorded as KBr
discs on a Shimadzu FTIR-8400. Elemental analysis was per-
formed on a Carlo-Erba Flash 1112 analyser. Gel permeation
chromatography (GPC; Shimadzu-Japan) analysis of the poly-
mer samples was carried out in THF solution calibrated with
a polystyrene standard and having a RI detector. Transition
temperatures were observed using a Mettler FP82 HT hot stage
and FP90 central processor in conjunction with an OLYMPUS
BX51 polarizing optical microscope. Transition temperatures
and associated enthalpies were measured by differential scanning
13
-
CH), 0.8–1.0 (m, 36 H, -CH ); C NMR: d
3 C
1
3
IR:(nmax) 2924, 2854, 1666, 1572, 1462, 1377, 1319, 1282, 1132,
094, 771, 721. Mol. wt. ꢂ 13691; PDI ¼ 1.44.
1
Synthesis of 2,6-bis(3,7-dimethyloctyloxy)-3,7,10,11-
tetrakis(pentyloxy)-triphenylene, 9
To a solution of 2,6-dihydroxy triphenylene 8 (0.605 g, 1 mmol)
in methyl ethyl ketone, MEK (10 mL), 3,7-dimethyloctylbromide
(0.885 g, 4 mmol) and caesium carbonate (1.3 g, 4 mmol) was
added and the reaction mixture was refluxed overnight. After
cooling, the reaction mixture was filtered and solvent evaporated
and the product was purified by column chromatography fol-
ꢁ
calorimetry heating from ꢀ30 C to isotropic temperatures at
ꢁ
ꢀ1
a scan rate of 5 C min (Perkin-Elmer Model Pyris 1D with
Intracooler 2 P cooling system). X-Ray diffraction measure-
˚
ments were carried out using Cu-Ka radiation (l ¼ 1.54 A)
generated from a 4 kW rotating anode generator (Rigaku Ultrax-
lowed by precipitation from dichloromethane into ethanol. Yield
1
62%. H NMR: d
1
8) equipped with a graphite crystal monochromator. Samples
H
7.8 (s, 6 H, ArH), 4.2–4.3 (m, 12 H, ArOCH
2
),
were placed in Hampton research capillaries (0.5 mm diameter)
from the isotropic phase, sealed and held on a heater. For all the
1.15–2.0 (m, 44 H, aliphatic CH and CH), 1.03 (d, 6 H, J ¼ 6 Hz,
2
CH ), 0.98 (t, 12 H, J ¼ 6 Hz, CH ), 0.87 (d, 12 H, J ¼ 5.6 Hz,
3
3
13
samples, X-ray diffraction was carried out at room temperature
ꢁ
CH ); C NMR: d 149.0, 123.6, 107.4, 69.7, 68.0, 39.2, 37.4,
3 C
(
25 C) and diffraction patterns of the mesophase were recorded
36.4, 30.0, 29.1, 28.3, 27.9, 24.7, 22.5, 19.7, 14.0; FT-IR: (nmax)
2922, 2854, 1616, 1514, 1464, 1435, 1379, 1263, 1171, 1053, 976,
870, 837, 771, 729. Elemental Analysis: Found: C, 78.69; H,
on a two-dimensional image plate (Marresearch). Octadecyl-
amine (ODA) functionalized carbon nanotubes were purchased
from Carbon Solutions Inc. (P5-SWNT, Batch 05-124).
11.05, C58
H
92
O
6
requires C, 78.68; H, 10.47%.
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Polymerization of 2,6-dihydroxy-3,7,10,11-tetrakis(pentyloxy)-
triphenylene
property, this could be because the branched chains fill the
necessary space around the core to induce mesomorphism. The
thermal behavior of the compound was investigated by polar-
izing optical microscopy (POM) and differential scanning calo-
rimetry (DSC). This compound shows a columnar hexagonal to
A solution of 2,6-dihydroxy triphenylene 8 (0.423 g, 0.7 mmol),
1
,12-dibromododecane (0.23 g, 0.7 mmol) and caesium carbonate
(
0.326 g, 1 mmol) in N-methylpyrrolidone, NMP (2 mL), was
ꢁ
isotropic transition at 115.7 C while heating and an isotropic to
ꢁ
ꢁ
submerged into a preheated oil bath at 110 C and stirred for 1 h
and cooled. Water was added and the resulting solid repeatedly
washed with water and ethanol. Reprecipitation from dichloro-
columnar hexagonal transition at 114.1 C while cooling, no
other transitions are observed while heating and cooling (scan
ꢁ
ꢀ1
ꢁ
rate 5 C min ) between ꢀ30 C and the isotropic temperature in
the DSC thermogram which supports the POM observation. The
texture of the columnar phase observed under POM for
compound 3 at room temperature is shown in Fig. 1a. The
columnar hexagonal mesophase structure of this compound was
established by X-ray diffraction studies. The intensity vs. q graph
derived from the diffraction pattern of the compound at room
1
methane with ethanol afforded the polymer 10 (295 mg). H
NMR: d 7.8 (s, 6 H, Ar-H), 4.2 (t, 12 H, J ¼ 6 Hz, ArOCH ), 1.94
H
2
(
m, 12 H, OCH CH ) 1.3–1.6 (m, 32 H, aliphatic CH ), 0.98 (t, 12
2
2
2
13
H, J ¼ 6.8 Hz, -CH
3
); C NMR: d
C
149.0, 123.6, 107.4, 69.7, 69.1,
3
2
2.8, 29.7, 29.5, 29.1, 28.3, 26.2, 22.5, 14.0; FT-IR: (nmax) 2924,
852, 1620, 1504, 1456, 1377, 1261, 1169, 1040, 977, 810, 770, 721
ꢀ1
cm . Mol. wt. ꢂ 13515; PDI ¼ 1.92.
Preparation of liquid crystalline nanocomposites with ODA
functionalized carbon nanotubes
Novel nanocomposites (by weight) of monomeric and polymeric
liquid crystalline compounds were prepared as follows: the
commercial octadecylamine (ODA) functionalized SWNTs (25
mg) were dissolved in dichloromethane (10 mL) and the insoluble
portion was filtered off using a PTFE membrane (0.45 mm). The
soluble portion was dried under vacuum to know the exact
amount of soluble SWNTs. It was found to be 10 mg. The soluble
SWNTs were re-dissolved in 20 mL of dichloromethane. A
calculated amount of this solution was added to the liquid crystal
monomers (3 and 9) and main chain polymers (4 and 10). The
solution was sonicated for a few minutes followed by removal of
solvent and drying under vacuum to make different functional-
ized SWNT-DLC composites.
Results and discussion
To investigate the effect of carbon nanotubes on the mesophase
behavior of electron deficient rufigallol derivatives we have
prepared novel room temperature liquid crystalline rufigallol
derivatives. The synthesis of electron deficient novel room-
temperature liquid crystalline monomer 3 and polymer 4 is
outlined in Scheme 1. Commercially available gallic acid 1 was
20
converted to rufigallol 2 as reported. Rufigallol 2 was alkylated
with 3,7-dimethyloctylbromide under mild etherification condi-
21
tions to afford the tetraalkoxy rufigallol 3. Surprisingly the
branched chain substituted tetraalkoxy compound 3 exhibits
liquid crystalline behavior at room temperature, in contrast to its
straight chain analogues which do not show any liquid crystalline
ꢁ
Fig. 1 Optical photomicrographs of (a) 3 and (b) composite 3b at 25 C
on cooling from the isotropic liquid (crossed polarizers, magnification
ꢃ200).
2 4
Scheme 1 Synthesis of room-temperature liquid crystalline anthraquinone monomer and polymer. Reaction conditions and reagents: (i) H SO , MW,
ꢁ
ꢁ
8
0%; (ii) 3,7-dimethyloctylbromide, NaOH, DMSO, 90 C, 35%; (iii) 1,12-dibromododecane, Cs
2
CO
3
, o-dichlorobenzene, 90 C, 60%.
3
034 | J. Mater. Chem., 2008, 18, 3032–3039
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Table 1 Phase transition temperatures (peak C) and associated
ꢁ
ꢀ
1
enthalpies (J g , in parentheses) of novel liquid crystalline compounds
ꢁ
ꢀ1
and their composites with carbon nanotubes (scan rate 5 C min ). Col
hexagonal columnar, Col
from polarized optical microscopy (POM)
h
¼
r
¼ columnar rectangular, I ¼ isotropic, * ¼
Sample
Heating scan
Cooling scan
3
Col
Col
Col
Col
Col
Col
Col
Col
Col
Col
Col
Col
Col
Col
Col
h
h
h
h
r
115.7 (6.4) I
113.0 (6.0) I
112.0 (6.0) I
109.5 (5.5) I
56.3 (3.3) I
47.7 (1.9) I
43.3 (2.0) I
39.2 (1.0) I
69.0 (6.0) I
67.2 (5.1) I
64.4 (3.8) I
63.8 (3.6) I
89 I*
I 114.1 (6.3) Col
h
h
h
h
3
3
3
4
4
4
4
9
9
9
9
1
1
a
b
c
I 110.7 (6.0) Col
I 109.8 (5.9) Col
I 107.6 (5.6) Col
—
—
—
—
a
b
c
r
r
r
h
h
h
h
h
h
h
I 67.0 (6.0) Col
h
h
h
h
a
b
c
0
0a
I 65.6 (5.1) Col
I 61.8 (3.7) Col
I 61.7 (3.6) Col
—
—
—
83 I*
80 I*
Fig. 2 X-Ray diffraction patterns and the derived one-dimensional
ꢁ
10b
intensity vs. q profiles for (a) compound 3 and (b) composite 3b at 25 C.
temperature is shown in Fig. 2a. In the low angle region three
sharp peaks, one strong and two weak, are seen, with d-spacings
in the ratio of 1 : 1/O3 : 1/O4. Identifying the first peak with the
Miller index 100, the ratios conform to the expected values for
a two-dimensional hexagonal lattice. In the wide angle region
two diffused reflections are seen. The broad one centered at
microscope we could observe small aggregates of CNTs which
means the CNTs are not homogeneously dispersed in the liquid
crystal matrix of the composites when the CNTs percentage is
more.
The dihydroxy functionalized molecule 3 was polymerized to
a main chain electron deficient liquid crystalline polyether 4 as
shown in Scheme 1 to study the phase behavior of its nano-
composites with carbon nanotubes. The mesophase behavior of
this polymer was characterized by polarizing optical microscopy
and differential scanning calorimetry. This polymer, which could
be deformed at room temperature shows, under the microscope,
only one transition on heating. When cooled from the isotropic
state it does not exhibit any characteristic texture but a sandy
birefringence texture as shown in Fig. 3a appears slowly from the
isotropic liquid. The DSC thermogram of the compound shows
˚
about 4.5 A corresponds to the liquid like order of the aliphatic
˚
chains. The relatively sharper one seen at about 3.4 A and well
separated from the broad one is due to the stacking of the
molecular cores on top of one another. All the above mentioned
features are characteristics of the Colh phase. Three composi-
tions (by weight) of this compound were made by adding the
dichloromethane solution of ODA functionalized carbon nano-
tubes. Three compositions (3a: 0.5%; 3b: 1%; 3c: 2%) were
prepared and analyzed by DSC, POM and X-ray diffraction. All
the composites 3a–c were also found to be room temperature
liquid crystalline like the pure host compound 3. These
composites, under POM, exhibit classical textures of columnar
mesophase upon cooling from the isotropic phase as shown in
Fig. 1b. Data obtained from the heating and cooling cycles of
ꢁ
only one transition at 56.3 C while heating corresponding to
a columnar to isotropic transition but on cooling it does not
show any discernible transition. The mesophase structure of the
polymer was established to be a rectangular columnar phase with
˚
˚
lattice parameters a ¼ 42.02 A and b ¼ 19.80 A. The intensity vs.
q graph derived from the X-ray diffraction pattern of the poly-
mer at room temperature shows five reflections in the small angle
region as shown in Fig. 4a with two strong reflections at lower
angles. Two strong peaks in the small angle region are charac-
teristic of a rectangular columnar phase and the observed other
three weak peaks in the small angle region can be indexed for
a rectangular lattice. So upon polymerization the columnar
hexagonal structure of monomer 3 is transformed to a more
ordered columnar rectangular phase as has been observed for
ꢁ
ꢀ1
DSC with a scan rate of 5 C min for these composites are
collected in Table 1. The insertion of CNTs decreases the mes-
ophase–isotropic phase transition temperature. With increase in
the percentage of the CNTs the mesophase–isotropic transition
temperature decreases (Table 1). This is logical as the insertion of
CNTs is expected to reduce the ordering of the cores. The X-ray
diffraction pattern of composite 3c at room temperature shows
similar features to that of compound 3 confirming the hexagonal
columnar mesophase of the composites as shown in Fig. 2b.
Since these composites 3a–c contain very little amounts of
functionalized carbon nanotubes we do not observe any signifi-
cant increase in the intercolumnar distance of the pure
compound 3 in these composites. The columnar mesophase
structure is not destroyed by the inclusion of functionalized
carbon nanotubes, only the columnar to isotropic transition
temperature decreases. When we tried to insert more than 2%
CNTs in the columnar liquid crystal those composites still
exhibited liquid crystalline properties but under the polarizing
21
main chain polyethers of rufigallol. As in the case of the
monomer, we have prepared three different compositions (4a:
0.5%; 4b: 1% and 4c: 2%) of the polymer by adding functional-
ized carbon nanotube solution to the polymer followed by
sonication and solvent removal. The mesophase behaviors of all
these three compositions were characterized by POM, DSC and
X-ray diffraction studies. They also exhibit the sandy texture
under the polarizing microscope when cooled from the isotropic
state as shown in Fig. 3b. Data obtained from the heating cycle of
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synthesized novel room-temperature liquid crystalline tripheny-
lene derivatives. The synthesis of novel room-temperature liquid
crystalline triphenylene monomer 9 and polymer 10 is outlined in
Scheme 2. The dihydroxy intermediate 8 was prepared and iso-
22
lated as reported. Upon replacement of two pentyloxy chains
by 3,7-dimethyloctyloxy branched chains compound 9 exhibits
liquid crystalline property at room temperature. This could be
due to the unsymmetrical substitution of alkoxy chains on the
core and stereoheterogeneity introduced by the branched alkoxy
chains. Under polarized optical microscopy compound 9 exhibits
the classical broken fan texture of a hexagonal columnar phase
when cooled from the isotropic phase as shown in Fig. 5a. This
compound shows a columnar hexagonal to isotropic transition at
ꢁ
6
9 C on heating and an isotropic to columnar hexagonal tran-
ꢁ
sition at 67 C while cooling, no other transitions are observed
while heating and cooling between ꢀ30 ꢁC and the isotropic
temperature in the DSC thermogram as shown in Fig. 6. The
mesophase structure of the compound was established with the
help of X-ray diffractometry. In the small angle region there are
two reflections, one strong and one weak. The ratio of the
d-spacings of these two peaks is 1 : 1/O3, which corresponds to
a two-dimensional hexagonal lattice. In the wide angle region
two reflections are observed: one very broad and another rela-
˚
tively sharp peak. The broad peak centered at about 4.7 A is due
to the liquid like order of the alkyl chains surrounding the
˚
aromatic cores; the other peak centered at about 3.6 A is due to
Fig. 3 Optical photomicrograph of (a) polymer 4 and (b) its composite
ꢁ
4
b at 25 C on cooling from the isotropic liquid (crossed polarizers,
the stacking of the aromatic cores on top of one another which
constitute the columns. All these features are characteristic of the
columnar hexagonal phase. Three different mixtures of func-
tionalized carbon nanotubes were made with this room temper-
ature liquid crystalline triphenylene derivative. All these
compositions (9a: 0.2%; 9b: 1% and 9c: 2%) were analysed with
POM, DSC and X-ray diffraction. Under polarizing optical
microscopy these composites develop the classical broken fan
textures of a columnar hexagonal phase as shown in Fig. 5b, just
like the pure host compound 9. As with those of anthraquinone
derivative composites the columnar to isotropic phase transition
temperature decreases with increasing the nanotube quantity and
the transitions are broadened. Data collected during the DSC
heating and cooling cycles of the pure compound and the
composites are collected in Table 1. X-Ray diffraction patterns
of the composites show similar features to that of the pure
compound confirming the columnar hexagonal phase of the
composites as shown in Fig. 7a. In the X-ray diffraction patterns
of these composites also we do not observe any significant change
in the intercolumnar distance as compared to the pure compound
magnification ꢃ200).
9. So the inclusion of CNTs to the columnar matrix destabilizes
the phase to a small extent but retains the mesophase structure.
In order to see the effect of CNTs on triphenylene based main
chain polymers we synthesized polymer 10 as shown in Scheme 2
from the dihydroxy precursor 8. The liquid crystalline phase
behavior of this polymer was studied by POM, DSC and X-ray
diffraction. This highly viscous compound does not show any
peak during heating and cooling in the DSC thermogram, but it
exhibits a birefringence texture like the above mentioned
anthraquinone polymer under the polarizing microscope when
cooled from isotropic state as shown in Fig. 8a. When heated
from room temperature this polymer passes slowly to isotropic
Fig. 4 X-Ray diffraction patterns and the derived one-dimensional
intensity vs. q profiles for (a) polymer 4 and (b) for the composite 4c at
ꢁ
25 C.
DSC for these composites are collected in Table 1. As expected
the isotropic transition temperature of the composites decreases
with increase in the CNT quantity. But the mesophase structure
of these composites remains rectangular as confirmed by X-ray
diffraction of the composites as shown in Fig. 4b.
To investigate the effect of carbon nanotubes on the meso-
phase behavior of electron rich triphenylene derivatives we have
ꢁ
state at 89 C and upon cooling the birefringence texture appears
3
036 | J. Mater. Chem., 2008, 18, 3032–3039
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Scheme 2 Synthesis of room-temperature liquid crystalline triphenylene monomer and polymer. Reaction conditions and reagents: (i) C
CO , MEK; (ii) FeCl , CH Cl ; (iii) Cat-B-Br (B-bromocatecholborane), CH Cl ; (iv) 3,7-dimethyloctylbromide, Cs CO , MEK; (v) 1,12-dibro-
mododecane, Cs CO , NMP, 110 C, 54%.
5
H11Br,
K
2
3
3
2
2
ꢁ
2
2
2
3
2
3
Fig. 6 DSC thermogram for compound 9 on heating and cooling (scan
ꢁ
ꢀ1
rate of 5 C min ).
consider this to be a columnar hexagonal phase. Moreover, the
miscibility of the mesophase of polymer 10 with the mesophase of
compound 9 confirms the columnar hexagonal structure. Two
different nanocomposites (10a: 0.5%; 10b: 1%) of the polymer
with functionalized carbon nanotubes were prepared and their
liquid crystalline properties were studied by POM and X-ray.
Like the pure polymer, these composites do not give any peak in
their DSC thermograms so the transition temperatures were
determined only by POM. The mesophase to isotropic transition
temperatures as observed under polarizing optical microscope of
these composites are listed in Table 1. Here also the mesophase to
isotropic transition temperature decreases and broadens as the
amount of carbon nanotubes increases in the matrix. The sandy
texture exhibited by the composites when cooled from the
isotropic state is shown in Fig. 8b. In all textures some change in
the colour was seen upon doping CNTs. This could be because of
the disturbed alignment of molecules by the CNTs. X-Ray
diffraction patterns of the composites show similar features to
Fig. 5 Optical photomicrographs of (a) compound 9 and (b) its
ꢁ
composite 9c at 25 C on cooling from the isotropic liquid (crossed
polarizers, magnification ꢃ200).
slowly under the polarizing microscope. In order to confirm the
mesophase structure we carried out an X-ray diffraction study of
the polymer. Only one strong reflection is observed in the small
angle region of the X-ray diffraction pattern; though it is difficult
to assign the mesophase structure with only one peak in the small
angle region, nevertheless given the general mesophase structure
23
of triphenylene based discotic monomers and polymers we
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Conclusions
We have synthesized two novel monomeric and two novel
polymeric discotic liquid crystals and studied the effect of
inclusion of carbon nanotubes on their mesophase behavior. The
insertion of carbon nanotubes in small amounts does not affect
the mesophase structure of the pure compounds but brings down
the isotropic transition temperatures. With an increase in the
amount of carbon nanotubes, the isotropic transition tempera-
ture decreases in all the composites. We have previously shown
that in discotic liquid crystal–carbon nanotube composites,
SWNTs align in the hexagonal columnar phase along the
director. These room-temperature liquid crystalline nano-
composites with broad mesophase ranges and different electronic
properties may be important for many device applications, such
as photoconductors, light-emitting diodes, photovoltaic solar
cells, sensors, thin-film transistors, etc.
Fig. 7 X-Ray diffraction patterns and the derived one-dimensional
ꢁ
intensity vs. q profiles for (a) composite 9c and (b) composite 10a at 25 C.
References
1
S. Iijima, Nature, 1991, 354, 56–58; S. Iijima and T. Ichihashi, Nature,
993, 363, 603–605.
Special issue on Carbon Nanotubes: Acc. Chem. Res., 2002, 35, pp.
97–1113; Carbon Nanotubes: Synthesis, Structure, Properties and
1
2
9
applications, ed. M. Dresselhaus, G. Dresselhaus and Ph. Avouris,
Springer-Verlag, Berlin, 2001; D. Tasis, N. Tagmatarchis, A. Bianco
and M. Prato, Chem. Rev., 2006, 106, 1105–1136; A. Hirsch,
Angew. Chem., Int. Ed., 2002, 41, 1853–1859; P. M. Ajayan, Chem.
Rev., 1999, 99, 1787–1799; R. H. Baughman, A. A. Zakhidov and
W. A. deHeer, Science, 2002, 297, 787–792; T.-J. Park, S. Banerjee,
T. Hemraj-Benny and S. S. Wong, J. Mater. Chem., 2006, 16,
1
41–154.
X.-L. Xie, Y.-W. Mai and X.-P. Zhou, Mater. Sci. Eng., R, 2005, 49,
9–112; W. Z. Li, S. S. Xie, L. X. Qian, B. H. Chang, B. S. Zou,
W. Y. Zhou, R. A. Zhao and G. Wang, Science, 1996, 274, 1701–
703; Z. F. Ren, Z. P. Huang, J. W. Xu, J. H. Wang, P. Bush,
3
4
8
1
M. P. Siegal and P. N. Provencio, Science, 1998, 282, 1105–1107;
P. M. Ajayan, O. Stephan, C. Colliex and D. Trauth, Science, 1994,
2
65, 1212–1214.
C. Zakri, Liq. Cryst. Today, 2007, 16, 1–11; C. Zakri and P. Poulin,
J. Mater. Chem., 2006, 16, 4095–4098.
5
6
W. Lee and C.-S. Chiu, Opt. Lett., 2001, 26, 521–523.
A. M. Somoza, C. Sagui and C. Roland, Phys. Rev. B, 2001, 63,
0
81403(R).
7
8
M. D. Lynch and D. L. Patrick, Nano Lett., 2002, 2, 1197–1201.
I. C. Khoo, J. Ding, Y. Zhang, K. Chen and A. Diaz, Appl. Phys.
Lett., 2003, 82, 3587–3589.
9
W. Lee, C.-Y. Wang and Y.-C. Shih, Appl. Phys. Lett., 2004, 85,
5
13–515.
0 I.-S. Baik, S. Y. Jeon, S. H. Lee, K. A. Park, S. H. Jeong, K. H. An
and Y. H. Lee, Appl. Phys. Lett., 2005, 87, 263110.
1
1
1 W. Lee and Y.-C. Shih, J. Soc. Inf. Display, 2005, 13, 743–747;
C.-Y. Huang, C.-Y. Hu, H.-C. Pan and K.-Y. Lo, Jpn. J. Appl.
Phys., 2005, 44, 8077–8081; H.-Y. Chen and W. Lee, Opt. Rev.,
2
2 I. Dierking, G. Scalia, P. Morales and D. LeClere, Adv. Mater., 2004,
005, 12, 223–225.
1
1
2
9
6, 865–868; I. Dierking and S. E. San, Appl. Phys. Lett., 2005, 87,
33507; I. Dierking, G. Scalia and P. Morales, J. Appl. Phys., 2005,
7, 044309.
Fig. 8 Optical photomicrographs of (a) polymer 10 and (b) its
ꢁ
1
3 R. A. Mrozek, B.-S. Kim, V. C. Holmberg and T. A. Taton, Nano
Lett., 2003, 3, 1665–1669; J. M. Russel, S. Oh, I. LaRue, O. Zhou
and E. T. Samulski, Thin Solid Films, 2006, 509, 53–57.
4 G. Scalia, C. von Buhler, C. Hagele, S. Roth, F. Giesselmann and
J. P. F. Lagerwall, Soft Matter, 2008, 4, 570–576, and references
therein.
5 S. Kumar, Chem. Soc. Rev., 2006, 35, 83–109.
6 L. Zhi, J. Wu, J. Li, U. Kolb and K. Mullen, Angew. Chem., Int. Ed.,
2005, 44, 2120–2123; J. Wu, B. E. Hamaoui, J. Li, L. Zhi, U. Kolb and
K. Mullen, Small, 2005, 1, 210–121; L. Gherghe, C. Kubel, G. Lieser,
composite 10a at 25 C on cooling from the isotropic liquid (crossed
polarizers, magnification ꢃ200).
1
that of the pure polymer as shown in Fig. 7b which again shows
that the mesophase is not destroyed by the inclusion of carbon
nanotubes though a large quantity of carbon nanotubes could
not be dispersed homogeneously in the matrix.
1
1
3
038 | J. Mater. Chem., 2008, 18, 3032–3039
This journal is ª The Royal Society of Chemistry 2008

View Article Online
H.-J. Rader and K. Mullen, J. Am. Chem. Soc., 2002, 124, 13130–
3138.
7 C. Chan, G. Crawford, Y. Gao, R. Hurt, K. Jian, H. Li, B. Sheldon,
M. Sousa and N. Yang, Carbon, 2005, 43, 2431–2440; M. E. Sousa,
S. G. Cloutier, K. Q. Jian, B. S. Weissman, R. H. Hurt and
G. P. Crawford, Appl. Phys. Lett., 2005, 87, 173115.
8 S. Laschat, A. Baro, N. Steinke, F. Giesselmann, C. Hagele, G. Scalia,
R. Judele, E. Kapatsina, S. Sauer, A. Schreivogel and M. Tosoni,
Angew. Chem., Int. Ed., 2007, 46, 4832–4887; S. Sergeyev, W. Pisula
and Y. H. Geerts, Chem. Soc. Rev., 2007, 36, 1902–1929; J. Wu,
W. Pisula and K. Mullen, Chem. Rev., 2007, 107, 718–747;
Y. Shimizu, K. Oikawa, K. Nakayama and D. Guillon, J. Mater.
Chem., 2007, 17, 4223–4229; C. Tschierske, Annu. Rep. Prog.
Chem., Sect. C, 2001, 97, 191; N. Boden, R. J. Bushby, J. Clements
and B. Movaghar, J. Mater. Chem., 1999, 9, 2081–2086;
R. J. Bushby and O. R. Lozman, Curr. Opin. Solid State Mater.
Sci., 2002, 6, 569–578; H. Takezoe, K. Kishikawa and E. Gorecka,
J. Mater. Chem., 2006, 16, 2412–2416; K. Ohta, K. Hatsusaka,
M. Sugibayashi, M. Ariyoshi, K. Ban, F. Maeda, R. Naito,
K. Nishizawa, A. M. van de Craats and J. M. Warman, Mol. Cryst.
Liq. Cryst., 2003, 397, 25–45; A. N. Cammidge and R. J. Bushby,
in Handbook of Liquid Crystals, ed. D. Demus, J. Goodby,
G. W. Gray, H.-W. Spiess and V. Vill, Wiley-VCH, Weinheim,
1998, vol. 2B, ch. VII, pp. 693–748.
1
1
1
19 S. Kumar and H. K. Bisoyi, Angew. Chem., Int. Ed., 2007, 46,
1501–1503; S. Kumar, S. K. Pal, P. Suresh Kumar and
V. Lakshminarayanan, Soft Matter, 2007, 2, 896–900; S. Kumar
and V. Lakshminarayanan, Chem. Commun., 2004, 1600–1601.
20 H. K. Bisoyi and S. Kumar, Tetrahedron Lett., 2007, 48,
4399–4402.
21 S. Kumar, Phase Transitions, 2008, 81, 113–128.
22 S. Kumar and M. Manickam, Synthesis, 1998, 1119–1122.
23 S. Kumar, Liq. Cryst., 2004, 31, 1037–1059; S. Kumar, Liq. Cryst.,
2005, 32, 1089–1113.
This journal is ª The Royal Society of Chemistry 2008
J. Mater. Chem., 2008, 18, 3032–3039 | 3039