Synthesis and superior optical-limiting properties of fluorene-thiophene- benzothiadazole polymer-functionalized graphene sheets

Article abstract of DOI:10.1002/smll.201000981

A polymer based on fluorene, thiophene, and benzothiadazole as the donor-spacer-acceptor triad is covalently coupled to reduced graphene oxide (rGO) sheets via diazonium coupling with phenyl bromide, followed by Suzuki coupling. These polymer-graphene hybrids show good solubility in organic solvents, such as chloroform, tetrahydrofuran (THF), toluene, dichlorobenzene, and N,N-dimethylformamide (DMF), and exhibit an excellent optical-limiting effect with a 532-nm laser beam. The optical-limiting threshold energy values (0.93 J cm² for G-polymer 1 and 1.12 J cm² for G-polymer 2) of these G-polymer hybrids are better than that of carbon nanotubes (3.6 J cm²). Copyright 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Full text of DOI:10.1002/smll.201000981

full papers
Graphene
Synthesis and Superior Optical-Limiting Properties of
Fluorene-Thiophene-Benzothiadazole
Polymer-Functionalized Graphene Sheets
Anupam Midya, Venkatesh Mamidala, Jia-Xiang Yang, Priscilla Kai Lian Ang,
Zhi-Kuan Chen, Wei Ji, and Kian Ping Loh*
A
polymer based on fluorene, thiophene, and benzothiadazole as the donor–
spacer–acceptor triad is covalently coupled to reduced graphene oxide (rGO)
sheets via diazonium coupling with phenyl bromide, followed by Suzuki coupling.
These polymer–graphene hybrids show good solubility in organic solvents, such
as chloroform, tetrahydrofuran (THF), toluene, dichlorobenzene, and N,N-
dimethylformamide (DMF), and exhibit an excellent optical-limiting effect with a
−2
5
32-nm laser beam. The optical-limiting threshold energy values (0.93 J cm for
−2
G–polymer 1 and 1.12 J cm for G–polymer 2) of these G–polymer hybrids are
better than that of carbon nanotubes (3.6 J cm ).
−2
graphene oxide (GO) by chemical exfoliation of graphite. The
1
. Introduction
two-dimensional nanoscale nature of these solution-dispersible
Graphene is a single-atom-thick sheet of hexagonally
[8]
GO sheets allow organic–graphene or inorganic–graphene
2
arrayed sp -bonded carbon atoms. Its unique properties of
hybrids with unique properties to be fabricated by cova-
[
1]
[2]
high charge-carrier mobility, mechanical stiffness, and
broadband nonlinear optical limiting[3] have attracted intense
attention recently. Solution-processed graphene is emerging
as an important class of materials because of its amena-
bility to process in bulk-scale for the formation of nanoscale
hybrids, organic–inorganic composites, and polymer blends.
These new materials are applicable in wide fields ranging
[9]
[10]
[11]
lent coupling, π–π stacking,
or electrostatic assembly.
Such GO–organic hybrids can exhibit multifunctional proper-
ties. For example, the hybrids can be dispersed in a wide range
of solvents when compared to pure graphene. In addition,
[12]
it can form donor–acceptor charge-transfer complexes,
and also exhibit enhanced nonlinear optical (NLO)
[13]
properties.
[
4]
[5]
from optical elements in lasers and optical sensors to solar
cells. The Hummers’ method is widely used to produce
Conjugated polymeric systems have emerged as a prom-
[6]
[7]
[14]
ising class of NLO
materials. Structural modifications of
these polymers can lead to optimized properties for spe-
[15]
cific applications. Cassano et al.
reported the incorpora-
tion of alternative electron-acceptor and -donor units in
the polymer backbone, where the enhancement of the NLO
properties was due to increased hyperpolarizability of the
donor–acceptor systems. NLO effects have potential appli-
cations in the protection of optical sensors, human eyes, and
optical elements from high-intensity laser beams. In this Full
Paper, a donor–spacer–acceptor triad based on fluorene, thi-
ophene, and benzothiadazole moieties form the backbone of
a conjugated polymer system, which is covalently attached
to reduced GO sheets via Suzuki polymerization procedure.
These graphene–polymer hybrids show excellent solubility
in different types of solvents and exhibit superior optical-
limiting performance.
A. Midya, J.-X. Yang, P. K. L. Ang, Prof. K. P. Loh
Department of Chemistry
National University of Singapore
3
Science Drive 3, Singapore, 117543
E-mail: chmlohkp@nus.edu.sg
V. Mamidala, W. Ji
Department of Physics
National University of Singapore
3
Science Drive 2, Singapore, 17542
Z. Chen
Institute of Material Science and Engineering
3
Research Link, Singapore, 117602
DOI: 10.1002/smll.201000981
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Polymer-Functionalized Graphene Sheets
formed. Figure 1a and b show an atomic force microscopy
AFM) image and line-corrugation profile of the nanosized
GO sheets, respectively. The step height of these GO sheets
2
. Results and Discussion
(
2.1. Synthesis and Characterization
is typically <1 nm, which attests to the monolayer dispersion.
Synthetic procedures for the fluorene-, thiophene-, and Chemical reduction of the water-dispersed GO nanoflakes
benzothiadazole-based copolymers grafted to rGO (G– with hydrazine will lead to their irreversible agglomeration
polymer 1 and G–polymer 2), Polymer 1 and Polymer 2, are and precipitation due to π–π stacking. To avoid aggrega-
presented in Scheme 1. Water-soluble GO flakes were syn- tion during reduction, a stable aqueous dispersion of rGO
thesized from graphite powder using Hummers’ method. nanoplatelets was prepared by the reduction of GO with
The micrometer-sized GO flakes were cut to nanometer NaBH4 in the presence of an anionic surfactant, sodium
sizes by sonication in concentrated nitric acid. A stable dodecyl benzene sulfonate (SDBS). 4-bromophenyl groups
aqueous dispersion of these nanosized GO sheets can be were covalently coupled onto the rGO sheets to yield
Scheme 1. Synthesis of polymers and G–polymers: a) lithiumdiisopropylamide (LDA), trimethylchlorostannane, 0 °C; b) Pd(PPh ) , DMF; c) N-bromo
3
4
succinamide (NBS), DMF; d) Pd(PPh ) , Tetrabutyl ammoniumbromide(TBAB), K CO 1 or 3, 90 °C, DMF, THF; e) i) NaBH , 5% aq. Na CO , 80 °C,
3
4
2
3,
4
2
3
pH = 10, ii) 4-bromobenzene diazonium tetrafluoroborate, 0 °C.
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A. Midya et al.
5
5
4
2.7x10
1.8x10
9.0x10
Br 3p
0.0
7
50
600
450
300
150
0
Binding Energy (eV)
Figure 2. XPS survey scan of rGO–PhBr. The presence of the Br 3p peak
proves the successful grafting of the bromophenyl group.
that the area ratio of the C–O (C1s electron binding energy,
BE = 286.8 eV) to the C–C (BE = 285.1 eV) component
decreased from a ratio of 1:1 in GO before reduction to
1
:2.2 after reduction. Grafting of the phenylbromide group
to rGO improves the dispersion of rGO sheets in polar
aprotic solvents, such as N,N-dimethylformamide (DMF),
−1
where a stable dispersion can be formed up to 1 mg mL
with minimal sedimentation.
The intermediate rGO–PhBr (5) was used as a synthon
for Suzuki coupling. 5 was coupled with 2-(2-(1,3,2-diox-
aborinan-2-yl)-9,9-dihexyl-9H-fluoren-7-yl)-1,3,2-dioxa-
borinane (4) by Suzuki coupling. Subsequently, an in situ
Suzuki polymerization reaction allows the copolymer
chains to be grafted to the rGO scaffold in the presence of
4
and 4,7-dibromobenzo[c][1,2,5]thiadiazole (1) or 4,7-bis(5-
bromo-4-hexylthiophen-2-yl) benzo [c] [1,2,5] thiadiazole
3) together in tetrahydrofuran (THF). Compound 3 was
(
obtained by bromination of intermediate 2. 2 was obtained
by the Stille coupling procedure from 1 and (4-hexylthi-
ophen-2-yl)trimethylstannane; the latter was prepared from
3
-hexyl thiophene. After attachment of the fluorene unit to
the phenyl bromide of GO-intermediate 5, the solubility of
the product increases to favor further polymerization proc-
esses in THF and DMF solvents.
After polymerization, the GO–polymer composites were
purified by the precipitation technique of polymer purifi-
cation from methanol-and-water-mixture solvents with a
Figure 1. A) AFM images of GO. B) Section analysis shows the sheet
height of 0.9 nm. C) Digital image of a) Polymer 1, b) G–polymer 1,
c) rGO, d) G–polymer 2, and e) Polymer 2 dispersions in chloroform.
1
0:1 ratio. Physically adsorbed free polymer molecules
and impurity particles were removed by filtration through
.22-μm polyterafluoroethylene (PTFE) filter paper for five
0
intermediate 5 (rGO–PhBr). The radical-initiated cou- cycles. The precipitated powders can be dispersed in different
pling reaction of the 4-bromophenyl radical with conduc- types of organic solvents.
tive rGO is driven by spontaneous electron transfer from
For control studies, Polymer 1 and Polymer 2 were syn-
[
16]
rGO to the diazonium salt. The presence of a significant thesized by the Suzuki coupling reaction, similar to that of
amount of bromine in the X-ray photoelectron spectros- the G–polymers, by reacting 4 with 1 or 3, respectively. The
copy (XPS) spectrum of 5 confirms the successful grafting energy gap of the pure polymers was evaluated from first
of the bromophenyl group (Figure 2; Br 3p peak intensity oxidation and reduction potential in cyclic voltammetry
relative to C1s found to be 11%). From the narrow scan (see the Supporting Information). The dispersion of G–
C1s region (see the Supporting Information), it was found polymers in chloroform solution compared with rGO and
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Polymer-Functionalized Graphene Sheets
2
1
1
0
0
0
.0
.6
.2
.8
.4
.0
(
b)
(
a)
Polyemer2
Polymer1
G-Polymer2
G-Polymer1
rGO-PhBr
rGO-PhBr
3
00
400
500
600
700
800300
400
500
600
700
800
Wavelength (nm)
Wavelength (nm)
Figure 3. UV-vis spectra of a) G–polymer 1 and Polymer 1 in toluene and rGO–PhBr in DMF and b) G–polymer 2 and Polymer 2 in toluene and
rGO–PhBr in DMF.
polymers is displayed in Figure 1C. The dispersion of GO Polymer 2 has similar absorption behavior with the maximum
in polar solvents is mainly governed by enthalpy interac- absorption peak at 361 nm and the ICT band at 507 nm. The
tions between the hydrophilic surface and polar solvents. ICT band has redshifted in Polymer 2 due to the incorpo-
In contrast, rGO nanosheets are hydrophobic in nature ration of the thiophene unit into the monomer units, which
and have a high interlayer cohesive energy (van der Waals reduced the bandgap energy in Polymer 2 (E = 2.06 eV).
g
interaction) and thus rGO cannot be readily dispersed in The UV–vis absorption spectrum of rGO–PhBr shows
nonpolar solvents (Figure 1C). Grafting of polymer onto broad absorption from 280 to 800 nm without any absorp-
rGO sheets significantly reduced the enthalpic interactions. tion maxima. This can be explained by the dominant broad-
Furthermore, dispersion of polymer-grafted rGO in non- band absorption of rGO (or graphene) since the molecule
polar solvents is favored by the increase in the total entropy phenyl bromide does not absorb in this range. In Figure 3a,
(
Figure 1C).
the absorption spectrum of G–polymer 1 can be said to be
The UV-vis spectra of graphene–polymer hybrids and an overlap of the absorption peaks of Polymer 1 (318 and
pure polymers were recorded in toluene and that of rGO– 443 nm) and the broadband absorption of graphene. G–
PhBr in DMF as it is almost insoluble in toluene. The UV- polymer 2 showed similar spectral behavior, which matched
absorption spectra of these hybrids are shown in Figure 3, with Polymer 2 in Figure 3b and the broadband absorption
where comparisons between pure polymer and graphene– of graphene.
polymer hybrid can be seen. Polymer 1 is characterized by
The fluorescence spectra of G–polymers and polymers
two absorption peaks. The absorption maximum at 318 nm is measured in toluene are compared in Figure 4 by normal-
∗
assigned to the π–π transition band of highly delocalized π izing the π–π∗ absorption peak. G–polymer 2 and Polymer 2
electrons and the peak at 443 nm is assigned to the intramo- were excited at a 532-nm wavelength whereas Polymer 1 and
lecular charge transfer (ICT) transition band. In Figure 3b, G–polymer 1 were excited at 450 nm. It was found that the
5
4
3
2
1
00
00
00
00
00
0
80
a
b
6
4
2
0
0
0
0
Polymer 2
Polymer 1
G-Polymer 2
G-Polymer 1
500
600
700
800
600
700
800
Wavelength(nm)
Wavelength(nm)
Figure 4. Fluorescence spectra of a) G–polymer 1 and Polymer 1 excited at 450 nm and b) G–polymer 2 and Polymer 2 excited at 532 nm in toluene.
The fluorescence is effectively quenched by G.
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A. Midya et al.
GO
1
00
G-Polymer 1
rGO
1248
1416
1053
Polymer 1
G-Polymer 2
Polymer 2
1
732
1
618
8
6
4
0
0
0
rGO-PhBr
G-Polymer1
1
690
2850
1720
2921
G-Polymer2
20
0
2
00
400
600
800
1000
4
00 800 1200 1600 2000 2400 2800 3200
o
Temperature ( C)
-
1
Wavenumber (cm )
Figure 6. TGA curves of rGO, Polymer 1, Polymer 2, G–polymer 1, and
G–polymer 2 at a heating rate of 10 °C min in air.
−1
Figure 5. FTIR spectra of GO, rGO–PhBr, G–polymer 1, and G–polymer 2.
G–polymer hybrids exhibit strong fluorescence quenching. the polymer. It can be estimated from the weight loss in the
G–polymer 1 quenches 97% of the 522-nm emission of thermogram that at least 50% of the weight content of the
Polymer 1 and G–polymer 2 quenches 83% of the 615-nm G–polymer hybrids is due to the polymer constituents. Anal-
emission of Polymer 2, which indicates the occurrence of ogously to the greater thermal stability of Polymer 1 versus
photoinduced charge or energy transfer from polymer to Polymer 2, G–polymer 1 also shows higher stability than G–
graphene.
polymer 2, as the latter contains additional thiophene units,
The evidence of successful functionalization following the which are readily oxidized at high temperature in the presence
covalent coupling of the polymer on rGO can be obtained of air.
from the Fourier-transform infrared (FTIR) spectra, as shown
The morphology of the G–polymer flakes was examined
in Figure 5. The FTIR spectrum of GO is characterized by by transmission electron microscopy (TEM).TheTEM images
vibrational bands corresponding to the C=O stretching in Figure 7 shows that G–polymer flakes have dimensions
−1
at 1732 cm , the benzenoid C=C-stretching vibration at greater than 200 nm and are decorated by polymer nano-
−
1
−1
1
618 cm , the O–H-deformation vibration at 1416 cm , particles (3–5 nm). It suggests that the polymerization has
−1
the C–O (hydroxyl) stretching at 1242 cm , and the
−1
C–O (epoxy) stretching at 1053 cm .After partial reduction of
GO,the intensity of the C–O-stretching frequency at 1053 cm−1
decreases. Aromatic in-plane C–H-bending vibration in
rGO–PhBr can be seen at 1149, 1064, and 1007 cm . In addi-
tion, the presence of the phenylbromide groups is verified by
peaks at 603, 831, and 930 cm , which can be assigned to the
aromatic out-of-plane C–H-bending vibrations of the phenyl-
bromide group. In G–polymer 1 and G–polymer 2, the vibra-
tional frequencies are almost similar. The unique aliphatic
−1
−1
−1
asymmetric C–H stretch at 2921 cm and symmetric C–H
stretch at 2850 cm of the alkyl groups were observed in the
spectrum.
−1
The thermal behaviors of the rGO, polymers, and G–
polymers were investigated by thermogravimetric analysis
(
TGA). The TGA thermograms were recorded with a heating
−1
rate of 10 °C min in air and are displayed in Figure 6. T h e
TGA of the G–polymers show a decomposition profile that is
derivative of the following constituents – polymer and rGO.
rGO shows two sharp decomposition onsets at 200 and 600 °C,
which correspond to the loss of bound solvent and oxygen
functional groups, respectively. The decomposition onsets
of G–polymer hybrids occur at higher temperatures and
Figure 7. a) TEM image of G–polymer. b) Magnified view of the rectangle
show a decrease of almost 70 wt% by 600 °C, which is due part of the single sheet in (a). Confocal fluorescence images of polymer-
to the loss of oxygen groups from GO and decomposition of coated graphene sheets: c) G–polymer 1 and d) G–polymer 2.
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Polymer-Functionalized Graphene Sheets
proceeded from the initial grafting point on the basal plane. contained in a 1-cm-path-length quartz cuvette with a spot
Elemental mapping of these clusters in scanning transmission size of 165 μm. Figure 9 shows the nonlinear transmittance of
electron microscopy (STEM) found only the elements C, N, the G–polymers in toluene. Carbon nanotubes (CNTs) were
O, and S, which agrees with the elemental composition of the used as the benchmark material for comparing the optical-
polymers (see the Supporting Information).
limiting performance. It was found that the transmittance of
−2
AFM analysis of the G–polymer surface reveals that G–polymers starts to drop at 0.1 J cm , which is smaller than
polymer nanoparticles decorate the entire basal plane of rGO. rGO and CNTs, indicating a greatly enhanced optical-limiting
Each diazonium and Suzuki-coupled site acts as a nucleation performance. The limiting threshold is defined as the inci-
site for polymer growth. Figure 8 shows AFM images of the dent fluence at which the transmittance falls to 50% of the
polymer-coated graphene. The contrast in roughness between normalized linear transmittance. The limiting thresholds are
−2
the polymer-decorated rGO surface and the clean SiO2 0.93, 1.2, and 3.6 J cm for G–polymer 1, G–polymer 2, and
surface shown in Figure 8b is very clear. With longer growth CNTs, respectively. It is clear that the optical-limiting per-
time, the thickness of the polymer can increase to 7 nm. The formance of the G–polymer hybrids is better than that of the
thickness of the polymer layer is readily controlled by the rGO or polymer alone.
growth time and concentration of monomer, which indicates
To understand the mechanisms responsible for the strong
that this is an efficient method to design polymer–graphene optical-limitingactivityoftheseG–polymersolutionsat532nm,
hybrids. (see S3a–c in the Supporting Information).
we also performed fluence-dependent scattering experi-
ments on G–polymers 1 and 2 and Polymers 1 and 2 in
toluene and rGO and CNTs in water using 532-nm laser
pulses by using a setup similar to the one used by Sun et
2.2. Optical-Limiting Measurement
[17]
al.
The laser-fluence-dependent scattering signal was
The optical-limiting properties of the G–polymers were collected at an angle of 10° to the propagation axis of the
characterized by fluence-dependent transmittance measure- transmitted laser beam at 532 nm and is shown in Figure 10.
ments with 532-nm laser pulses with a pulse duration of 7 ns, For both the G–polymer and the CNT, the scattering
which was frequency doubled from a 1064-nm laser with a rep- signals were dominated by linear scattering at low input flu-
etition rate of 10 Hz (Q-switched neodymium-doped yttrium ences and deviated from linear behavior as the input fluence
aluminium garnet (Nd:YAG) laser, Spectra Physics DCR3). increased. The contribution from nonlinear scattering became
The laser beam was focused onto the G–polymer solutions dominant at high pump fluences. These results suggest that
the nonlinear scattering plays an important
role in the observed optical-limiting activity.
The magnitude of the scattered signal for
the G–polymer solutions is higher than
the scattering signal from CNTs at 532 nm,
consistent with the better optical-limiting
performance of the G–polymer solutions
compared to CNTs. In addition, it was
observed that the pure polymer solutions
showed a much smaller scattering signal
compared to that of G–polymer solutions.
The nonlinear scattering usually arises
from the formation of two types of scat-
tering center after photoexcitation of the
G–polymer solutions. At high pump flu-
ence, the excitation energy absorbed by
the polymer is transferred to the graphene
and then dissipated to the surrounding sol-
vents. Consequently, the solvent is heated
up and the bubbles are formed, which act
as the scattering centers. The initial bub-
bles quickly expand due to the large pres-
sure difference at the interface between
the vapor and G–polymer solution. When
the size of bubbles becomes comparable to
the magnitude of the wavelength of the
incident light, scattering of the incident
beam by the bubble clouds sets in. Alterna-
tively, the thin G–polymer flakes could also
be directly ionized by the high-intensity
laser pulses, forming microplasmas. Fast
Figure 8. AFM images of Polymer-2-coated graphene, grown from micrometer-sized rGO on
SiO substrate. a) Magnified image after a growth time of 20 h, top view showing polymer
2
grains on rGO. b) image of G-polymer 2 on rGO after 20h growth time, indicating clear
difference between G-Polymer 2 and SiO , c) Standard image of G-polymer 2 after growth
2
time of 60 h. G-polymer 2 thickness increases to 6.7 nm after 60 h, which has been shown
in left part of (c).
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A. Midya et al.
1
.1
.0
.9
.8
.7
.6
.5
.4
.3
.2
different types of solvents, such as chloroform, toluene, and
dichlorobenzene. The advantage of this Suzuki polymeriza-
tion coupling procedure is that a wide variety of copolymers
can be grafted by choosing the suitable monomers and this is
potentially useful for developing organic–graphene devices for
optoelectronic applications. The donor–acceptor-type polymer-
grafted rGO displays an enhanced optical-limiting property for
a 532-nm laser beam that is more superior to that of CNTs, due,
perhaps, to more-efficient nonlinear scattering.
1
0
CNT
rGO
Polymer 1
Polymer 2
G-Polymer 1
G-Polymer 2
0
0
0
0
0
4. Experimental Section
0
0
Materials and Characterization: All reagents and chemicals
were purchased from Aldrich. THF was distilled from sodium ben-
zophenone prior to use. DMF was dried and distilled from CaH2.
Other chemicals and reagents were used as received without fur-
0.01
0.1
1
10
2
Input fluence(J/cm )
Figure 9. Optical-limiting response of CNT and rGO in water and Polymer ther purification.
1
7
, Polymer 2, G–polymer 1, and G–polymer 2 in toluene measured using
-ns pulses at 532 nm. The linear transmittances of all the solutions
Characterization: The 1H nuclear magnetic resonance (NMR)
spectrawererecordedinsolutionofCDCl onaBrukerDPX(400MHz)
3
were adjusted to 65%.
NMR spectrometer with tetramethylsilane (TMS) as the internal
standard. UV-vis absorption spectroscopy was studied using a
Shimadzu UV-3101 PC spectrometer at room temperature. Electro-
chemical measurements were performed using an Autolab potenti-
ostat (model PGSTAT30) by Echochimie. FTIR spectra were recorded
on a Varian 3100 FTIR instrument and measured in KBr discs.
Matrix-assisted laser-desorption-ionization time-of-flight (MALDI-
TOF) analysis was performed on a Bruker Autoflex II spectrometer
using dithranol as a matrix. All reactions were performed under N2
atmosphere unless otherwise stated.
expansion of these microplasmas creates scattering centers,
which attenuate the incident beam. The efficiency of scat-
tering is largely dependent on the cluster formations of the
G–polymer hybrids and skeleton structure of the polymer.
The donor–spacer–acceptor-triad system may facilitate for-
mation of microplasmas due to the hyperpolarizabilty effect.
Synthesis of GO: GO was prepared from graphite powder using
a modification of Hummers and Offeman’s method. In a typical
reaction, 1.5 g of graphite, 1.5 g of NaNO , and 69 mL of H SO
3
. Conclusion
In this work, we synthesized polymer-grafted rGO hybrids
3
2
4
were taken in a round-bottom flask (rbf) and cooled in an ice bath.
based on fluorene, thiophene, and benzothiadazole structures
using the Suzuki coupling procedure. This is a facile method
to obtain rGO–polymer hybrids that can be solubilized in
Next, 9 g of KMnO was slowly added to the mixture at 0 °C. After
4
mixing, the solution was transferred to a 40 °C water bath and
stirred for about 1 h, forming a thick paste. After addition of 120 mL
of water, the solution was stirred for 30 min while the tempera-
ture was raised to 95 °C. Finally, the reaction mixture was diluted
with 300 mL of water. The color of the solution turned from dark
brown to yellow after slow addition of 9 mL of H O (30%). The
4
1.4x10
CNT
rGO
4
2
2
1.2x10
warm solution was then filtered and washed with 300 mL of water.
The filter cake was then dispersed in water by mechanical agita-
tion. All visible particles were removed (about 3–5 times) from
the precipitates by repeated low-speed centrifugation for 2 min at
Polymer 1
Polymer 2
G-Polymer 1
G-Polymer 2
4
3
3
3
3
1
.0x10
.0x10
.0x10
.0x10
.0x10
.0
8
1
000 rpm. After that, small GO pieces were removed by two more
6
high-speed centrifugation steps of the supernatant solution of the
previous step at 10 000 rpm for 15 min. Inorganic anions and other
water-soluble impurities were removed through ten washing cycles
that included centrifugation, discarding the supernatant liquid,
and resuspending the solid in ultrapure water and ultrasonication.
This microsized GO was cut into small pieces of nanosized GO by
ultrasonication in presence of 70% HNO (10 mL HNO /100 mg of
4
2
0
0.1
1
10
3
3
2
Input fluence(J/cm )
microsized GO) for 4 h in a 60 °C temperature bath. After ultrasoni-
cation, the concentrated acid solution was diluted 20 times with
deionized water and small-sized GO was collected by high-speed
centrifugation (15 000 rpm). The nanometer-sized graphene was
further purified with ethanol solvent by five cycles of washing as
mentioned above.
Figure 10. Nonlinear scattering signals (at an angle of 10° to the
propagation axis of the transmitted laser beam) for the G–polymers and
polymers in toluene and rGO and CNTs in water using 532-nm (7-ns)
laser pulses. The linear transmittances of all solutions were adjusted
to 65%.
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Polymer-Functionalized Graphene Sheets
Synthesis of rGO–PhBr: In a two-step process, 100 mg of small was then washed with brine and dried over anhydrous sodium
GO was first dispersed in 1 wt% of 100 mL of aqueous SDBS in a sulfate. The solvent was removed at a reduced pressure and recrys-
round-bottom flask. After the solution was homogenized by 1 h of talization gave the product as a red solid (3.47 g, yield: 87%); mp
1
ultrasonication, the pH was adjusted to 9–10 with 5 wt% sodium 101–104 °C. H NMR (CDCl , 400 MHz): δ (ppm) = 7.76 (s, 2H),
3
carbonate solution. The resulting GO dispersion was reduced with 7.74 (s, 2H), 2.63 (t, J = 7.6 Hz, 4H), 1.68–1.61 (m, 4H), 1.41–1.32
0
.8 g of sodium borohydride at 80 °C for 1.5 h. After the reduc- (m, 12H), 0.90 (t, J = 6.8Hz, 6H). 13C NMR (100 MHz, CDCl ): δ
3
tion of GO the reaction mixture was cooled to 0 °C and 1 g of (ppm) = 152.11, 143.03, 138.49, 128.02, 125.17, 124.65, 111.59,
-bromophenylbenezene diazonium tetrafluroborate was added in 31.65, 29.73, 29.69, 28.99, 22.62, 14.09. MS (MALDI-TOF, m/z)
situ. After stirring the reaction mixture for 2 h at room temperature, calculated for C H Br N S : 623.9938; found : 624.035.
4
2
6
30
2 2 3
rGO–PhBr was precipitated by adding acetone. The crude rGO–
Synthesis of Polymer: The synthesis of two alternating
PhBr was purified by ten cycles of washing with different solvents, copolymers was carried out using well-known palladium-cat-
which involved centrifugation, discarding supernatant liquid, and alyzed Suzuki coupling between 9,9-dihexylflourene dibo-
resuspending the solid in a particular solvent and ultrasonication. ronatepropanoate ester (750 mg, 1.49 mmol) and monomer 1
The extent of purification was confirmed by thin-layer chromatog- (438 mg, 1.49 mmol) or monomer 3 (930 mg, 1.49 mmol). In
raphy (TLC) and UV–vis spectra of the supernatant liquid.
a two-necked rbf, two monomers, potassium carbonate (617
Synthesis of (4-Hexylthiophen-2-Yl) trimethylstannane: To mg, 4.47 mmol), Pd(PPh ) (86 mg, 0.075 mmol), and tetrabutyl
3
4
a solution of 3-hexylthiophene (5 g, 29.87 mmol) in anhydrous ammoniumbromide (100 mg) were taken and 20 mL of THF and
THF (50 mL) at −10 °C, lithiumdiisopropylamide (14.9 mL, DMF solvent mixture (1:1) was added to the mixture under N2
29.87 mmol, 2 M in heptane) was added in a dropwise manner and atmosphere. The reaction mixture was stirred and heated under
the mixture was stirred at this temperature under N for 2 h. Next, nitrogen at 100 °C for 72 h. At the end of polymerization, the
2
trimethylchlorostannane (30 mL, 30 mmol, 1 M in THF) was added. terminal boronic ester and bromine groups were end capped by
The cooling bath was removed and the mixture was allowed to sequentially adding excess bromobenzene (15.7 mg, 0.1 mmol)
reach room temperature and kept there for an additional 2 h. The and phenylboronic acid (12.2 mg, 0.1 mmol) and refluxing for
mixture was poured into saturated aqueous sodium hydrogen 12 h, respectively. The reaction mixture was cooled to ≈ 50 °C
carbonate and the organic phase was separated and washed and slowly added to a vigorously stirred mixture of methanol and
with saturated aqueous brine and then dried over an anhydrous water (10:1). The precipitate was collected by filtration was fur-
sodium sulfate. The solvent was removed at a reduced pressure. ther purified by Soxhlet extraction in acetone for 2 d to remove
The residue as a light-yellow oil was used in the next step without oligomers and catalyst residues.
further purification.
Synthesis of G–polymer: rGO–PhBr (30 mg) was dispersed in
Synthesis of 4,7-Bis(4-hexylthiophen-2-Yl)benzo[c][1,2,5]thia- 30 mL DMF solvent in a two-necked rbf equipped with a magnetic
diazole: In a glove box, a 20 mL glass vial equipped with a stirrer bar. Sodium carbonate (412 mg, 3 mmol) 9,9’-dihexylflourene
bar was charged with 4-hexylthiophen-2-yl)trimethylstannane (pre- diboronate propanoate ester (300 mg, 0.59 mmol) and tetrabutyl
pared as above and used directly), 4,7-dibromo-2,1,3-benzothi- ammoniumbromide (100 mg) were added to the dispersion. The
adiazole (2.5 g, 8.5 mmol), Pd(PPh ) (200 mg, 0.17 mmol), and reaction mixture was degassed by purging with nitrogen and evac-
3
4
anhydrous DMF (12 mL). The glass vial was securely sealed inside uating sequentially for five times. Pd(PPh ) (34 mg, 0.03 mmol)
3
4
the glove box and was placed in a microwave reactor and heated at in THF solution (10 mL) was injected into the reaction mixture.
50 °C for 15 min with stirring. The reaction mixture was cooled to The reaction mixture was refluxed for 2 h at 100 °C. After that, a
1
room temperature and extracted with water and ethyl acetate. The mixture of 1 or 3 (0.75 equivalent to monomer 4) with 9,9-dihexyl-
organic part was washed with brine solution and dried over Na SO . fluorene diboronatepropanoate ester (200 mg, 0.39 mmol) in THF
2
4
The crude product was obtained after evaporating the solvent in (20 mL) solution was injected to the reaction mixture and refluxed
vacuum. The orange-color solid (3.38 g, 85% yield) was purified for 72 h at the same temperature in a N atmosphere. At the end
2
from the crude by silica-gel column chromatography with hexane of polymerization, the terminal boronic ester and bromine groups
as the eluent after evaporation of the solvent. 1H NMR (CDCl3, were end capped by sequentially adding excess bromobenzene
4
2
0
1
3
00 MHz): δ (ppm) = 7.97 (s, 2H), 7.82 (s, 2H), 7.03 (s, 2H), (15.7 mg, 0.1 mmol) and phenylboronic acid (12.2 mg, 0.1 mmol)
.69 (t, J = 7.6 Hz, 4H), 1.74-1.66 (m, 4H), 1.41-1.31 (m, 12H), and refluxing for 12 h, respectively. The reaction mixture was
.90 (t, J = 6.8Hz, 6H). 13C NMR (100 MHz, CDCl ): δ (ppm) = cooled to ≈50 °C and slowly added to a vigorously stirred 200 mL
3
52.65, 144.34, 139.02, 129.01, 126.05, 125.49, 121.49, 31.70, mixture of methanol and water (10:1). The precipitate was col-
0.66, 30.46, 29.04, 22.61, 14.06. Mass Spectrometry (MS): lected by filtration from solvent mixture. The G–polymers were fur-
(
MALDI-TOF, m/z) calculated for C H N S : 468.1728; found: ther purified by sequentially precipitating in a methanol and water
26 32 2 3
4
68.243.
Synthesisof4,7-Bis(5-bromo-4-hexyl-2-thienyl)-2,1,3-benzothi- and catalyst residues. Finally, the G–polymers were dissolved in
adiazole (3): 4,7-di(4-hexyl-2-thienyl)-2,1,3-benzothiadiazole(3 g, chloroform and filtered through 0.2-μm PTFE filter paper to remove
.4 mmol) was added to chloroform under nitrogen flow. After the all physically adsorbed polymer-molecule particles. These steps,
mixture (10:1) after dissolving in chloroform to remove oligomers
6
soliddissolvedcompletely,N-bromosuccinimide(2.28g,12.8mmol) dispersion in chloroform and filtering through 0.22-μm PTFE filter
solution in DMF was added dropwise at 0 °C. The reaction mix- paper, were repeated five times until the filtrate did not contain
ture was stirred at room temperature for 2 h and hexane was any free polymer, which was checked by UV-vis spectroscopy. The
added into the mixture. The white precipitate formed was filtered final G–polymers were dried under vacuum to yield G–polymer 1
off and the filtrate was extracted with ether. The organic layer as 42 mg and G–polymer 2 as 51 mg.
small 2010, 6, No. 20, 2292–2300
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.small-journal.com 2299

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Received: June 8, 2010
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small 2010, 6, No. 20, 2292–2300