Functionalization of pristine graphene with conjugated polymers through diradical addition and propagation

Article abstract of DOI:10.1002/asia.201200520

Hanging on: Pristine graphene was grafted with conjugated polymers through addition and propagation of diradicals generated from Bergman cyclization of enediyne-containing dendrimers. The surface functionalization was confirmed with TGA, FTIR and Raman spectroscopy, and AFM analysis. The sp² network of graphene is only slightly destroyed, as revealed by conductivity measurements. Copyright

Full text of DOI:10.1002/asia.201200520

DOI: 10.1002/asia.201200520
Functionalization of Pristine Graphene with Conjugated Polymers through
Diradical Addition and Propagation
Xiaowei Ma, Fei Li, Youfu Wang, and Aiguo Hu*[a]
Graphene, a single-atom-thick honeycomb-like allotrope
of carbon, has attracted intense interest since it was uncov-
ered in 2004.[1] The tendency of individual graphene sheets
to form tight aggregates through p–p interaction, however,
makes the fabrication of graphene-based materials difficult,
which presents considerable challenges for the modification
of graphene and introduction of functionalities onto gra-
phene. In recent years, extensive efforts have been devoted
to this end, with most of them relaying on modification of
graphene oxide (GO).[2] GO is exfoliated from graphite
oxide, which is mainly prepared by (modified) Hummers[3]
methods, including oxidation of graphite in the presence of
strong acids and oxidants. This oxidation process, however,
is uncontrollable and extensive, leading to loss of conductiv-
ity[4] and defects of structure.[5] The oxygen-containing
groups greatly change the intrinsic properties of graphene,
therefore potentially affecting the final properties of gra-
phene-based nanomaterials. Reduction of GO by chemical,
thermal, or electrochemical treatments, which are capable of
removing a majority of the oxygen functionality and partial-
ly recovering the electronic property of graphene, is always
needed.[2c]
Discrete reports on covalent functionalization of pristine
graphene have emerged lately. Most of them involve reac-
tive species, such as diazonium salts,[6] nitrene radicals pro-
duced with azidotrimethylsilanes, and azides,[7] azomethine
ylides,[8] and arynes.[9] Although all these methods are able
to introduce organic functionalities onto pristine graphene,
it is still challenging to introduce conjugated polymers onto
graphene, because conjugated polymers are typically synthe-
sized through transition-metal-catalyzed cross-coupling reac-
tions,[10] which are incompatible with above-mentioned
methods. Functionalization of pristine graphene with conju-
gated polymers may introduce electronic conducting anten-
nas onto the conducting graphene basal plane and generate
novel nanostructure for applications in catalyst supports,
sensors, fuel cells, and electrode materials. Herein, we
report our work on functionalization of pristine graphene
with conjugated polymers through thermal triggered Berg-
man cyclization of enediyne-containing molecules.
Bergman cyclization[11] is a cycloaromatization of ene-
diyne-containing compounds that proceeds through a diradi-
cal intermediate. These highly reactive diradicals show great
promise in the development of novel anticancer and antitu-
mor agents[12] and attract tremendous interest in pharma-
ceutical field. The diradicals generated through Bergman
cyclization are also able to couple with each other to form
conjugated polymers without addition of any catalyst.[13] Re-
cently, Bergman cyclization of various enediynes was uti-
lized to directly attack sp2 carbon atoms on the surface of
carbon nano-onions and carbon nanotubes, leading to solu-
ble and processible functionalized carbon nanomaterials.[14]
To the best of our knowledge, there has been no attempt to
introduce conjugated polymers onto chemically inert pris-
tine graphene through Bergman cyclization.
The asymmetric enediyne moiety chosen for this study
consists of a terminal alkyne group and an internal alkyne
group that links to a low-generation dendron to improve the
solubility of final products (Scheme 1). In our previous
work,[15] we found that this kind of asymmetric enediyne un-
derwent thermally triggered Bergman cyclization at an
onset temperature of 1508C. The reaction can be accelerat-
ed at even higher temperature, like in boiling N-methylpyr-
rolidinone (NMP, b.p. ca. 2068C) in this work.
Commercial available microcrystalline graphite (500 mg)
was sonicated in NMP for two hours.[16] The resultant sus-
pension, which contains single-layer graphene, few-layers
graphene, and unexfoliated graphite flakes, was centrifuged
at 12000 rpm for 5 min.[17] The collected homogeneous dis-
persion of graphene in NMP was degassed under high
vacuum for 30 min. Then nitrogen was bubbled in and the
dispersion was heated to reflux. To this hot suspension, ene-
diyne G1 or G2 (100 mg) in NMP (5 mL) was added with
a peristaltic pump over 4h, and the reaction mixture was
further heated for 12 h. After it was cooled to room temper-
ature, the suspension was filtered through a 0.22 mm poly(te-
trafluoroethylene) (PTFE) membrane and thoroughly
washed with tetrahydrofuran (THF) to remove the unat-
tached polymeric products. The obtained filter cakes can be
dispersed (>1 mgmLÀ1) in a variety of solvents, including
NMP, N,N-dimethylformamide, 1,2-dichlorobenzene, THF,
[a] X. Ma, F. Li, Y. Wang, Prof. A. Hu
Shanghai Key Laboratory of Advanced Polymeric Materials
School of Materials Science and Engineering
East China University of Science and Technology
Shanghai, 200237 (China)
Fax : (+86)21-64253037
E-mail: hagmhsn@ecust.edu.cn
Homepage: http://hagmhsn.ecust.edu.cn
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/asia.201200520.
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bonded ones. The weight loss of
graphene is attributed to de-
fects introduced by sonication
and some trapped or intercalat-
ed solvent molecules,[8] which
should be deducted from the
TGA analysis.
The grafting of polymers on
graphene is also evidenced by
FTIR analysis. Figure 2 shows
FTIR spectra of graphene, G1-
graphene and G2-graphene.
The spectrum of graphene is
almost featureless; the broad
band around 1100 cmÀ1 proba-
Scheme 1. Functionalization of pristine graphene with enediyne molecules.
À
chloroform, toluene, and ethyl acetate, to form homogenous
suspensions, which are stable for months.
bly arises from C O bonds introduced by sonication. In con-
trast, in the spectra of G1-graphene and G2-graphene, fea-
À
Thermal gravimetric analysis (TGA) was applied to deter-
mine the amount of organic components that were grafted
on graphene. The oven-dried samples were heated under ni-
trogen at a rate of 108CminÀ1. Figure 1 shows clear weight
loss steps (350–4508C for G1-graphene and 300–4008C for
G2-graphene), which are due to the decomposition of poly-
mers that bonded to the graphene basal plane. The maximal
weight loss rates appear at 4248C and 3548C for G1-gra-
phene and G2-graphene, respectively. The decomposition of
polymers was also evidenced by differential scanning calo-
rimetry (Figure S1). The weight losses of these two samples
are 4.0% and 6.0%, which correspond to estimated func-
tionalities of over one enediyne unit per 940 and 980 carbon
atoms, respectively. The peripheral groups in these dendrim-
ers guarantee good dispersibility of functionalized graphene
in a variety of solvents (see above) with such low grafting
densities. To differentiate chemical bonding from physical
adsorption of polymers to graphene, control experiments
were conducted by mixing hot solutions of Bergman cycliza-
tion products of G1 or G2[18] in NMP with exfoliated gra-
phene for 24h and then subjecting them to a similar work-
up procedure. TGA curves of these samples show almost no
weight loss as compared to G1-graphene and G2-graphene
in the same region, which indicates that nearly all the physi-
cal adsorbed polymers are removed during rinsing with
THF, while the leftovers are primarily the chemically
tures of various functionalities are observed, including C H
(uCÀH at around 2900 cmÀ1, dCÀH at 1450 cmÀ1), carbonyl
group (uC at 1630 cmÀ1 for G1-graphene and 1650 cmÀ1
=
O
for G2-graphene), and C O C (uas at 1260 cmÀ1, us at
À À
À1
À
1080 cm ). The vibration peaks of C C and fingerprint
peaks of mono- and trisubstituted benzene rings are clearly
seen at 1580, 800, and 700 cmÀ1. The appearance of these
peaks clearly shows the presence of polymers originating
from dendrimers G1 and G2 in the functionalized graphene.
To further investigate the graphene with grafted polymer,
Raman spectroscopy with a laser excitation of 514 nm was
used. The Raman spectra of graphite, graphene, G1-gra-
phene, and G2-graphene are shown in Figure 3. There are
several well-defined bands in all these samples, which are
Figure 2. FTIR spectra of graphene (a), G1-graphene (b), and G2-gra-
phene (c).
Figure 1. TGA curves of graphene (b), G1-graphene (d), G2-graphene
(e), and graphene flakes that were first physical adsorbed with G1-poly-
mers (c) or G2-polymers (a) and then rinsed with THF.
Figure 3. Raman spectra of graphite (d), graphene (c), G1-graphene (b),
and G2-graphene (a).
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Functionalization of Pristine Graphene with Conjugated Polymers
typically denoted as D (ca. 1350 cmÀ1), G (ca. 1580 cmÀ1)
and 2D (ca. 2700 cmÀ1) bands of graphitic materials, and D’
(ca. 1590 cmÀ1) band for graphene and functionalized gra-
phene. G bands are related to the in-plane vibration mode
of the benzenoid framework, D bands are attributed to
defect-induced sp3 hybridized carbon atoms, and 2D bands
correspond to two-phonon double-resonance processes. The
intensity ratio of D and G bands (ID/IG) is typically used as
an index of the extent of disorder in graphene. The values
of ID/IG for these samples are 0.03, 0.28, 0.23, and 0.23, re-
spectively. After the Bergman-cyclization-mediated modifi-
cations, the D/G ratios of the graphene samples do not
change very much, which indicates that the functionalization
of graphene with conjugated polymers through Bergman
cyclization does not bring much disruption to the sp2 net-
work of graphene. Additionally, the D’ band, which occurs
through an intravalley double-resonance process in the pres-
ence of defects, does not significantly change after introduc-
tion of these polymeric functionalities. This result indicates
that the growth of conjugated polymers on graphene does
not introduce more defect sites to the exfoliated graphene
sheets.
The grafted polymers on graphene are visualized through
atomic force microscopy (AFM) with tapping mode
(Figure 4). Plenty of tiny particles are found on the basal
plane of graphene with heights in a range of 1–7 nm (see the
Supporting Information). Considering the theoretical height
of the repeating unit in the conjugated polymers (primarily
naphthylene and indenylmethylene)[15] is around 0.6 nm, the
degree of polymerization of these polymeric particles is cal-
culated to be in the range of 1–12. As characteristic of Berg-
man cyclization, each enediyne-containing molecule gener-
ates a 1,4-aryl diradical intermediate. After one side of the
diradical attacks graphene, the other side may attack anoth-
er enediyne molecule or link to other diradicals to grow
a polymeric chain.[19] The propagating chains can be termi-
nated by abstracting hydrogen atoms from solvent mole-
cules, thus leaving low-molecular-weight conjugated poly-
mers on graphene basal plane. The height of each grafted
graphene sheet is about 2.0 nm (Figure 4c and 4e), which
consists of two parts: the height of the graphene sheet
(single or few layers) plus the height of conjugated polymers
underneath (Figure 4d).
The exfoliated and polymer-grafted graphene samples are
pressed into paper-like specimens for four-probe conductivi-
ty measurements. The conductivity of the exfoliated gra-
phene sample is around 100 ScmÀ1, whereas the conductivity
of G1-graphene and G2-graphene are 25 and 10 ScmÀ1, re-
spectively. The decreases of conductivity may be due to the
presence of fewer conducting polymers between the neigh-
boring graphene sheets. Nevertheless, the grafted graphene
samples are still highly conductive thanks to the functionali-
zation with diradicals generated from Bergman cyclization,
which only introduces limit number of defects to the gra-
phene sp2 network (see above).
Using the diradicals generated from the Bergman cycliza-
tion of enediyne-containing molecules, pristine graphene is
grafted with conjugated polymers. The modified graphene
samples exhibit good solubility in a variety of organic sol-
vents and show good electric conductivity. The structural
variability of enediyne molecules allows a wide range of
functionalities to be attached onto pristine graphene under
similar conditions, which provides a new way for one-pot
synthesis of graphene/polymer composite materials.
Acknowledgements
The support of the National Natural Science Foundation of China
(20704013, 91023008), New Century Excellent Talents in University, the
Fundamental Research Funds for the Central Universities, and Shanghai
Leading Academic Discipline Project (B502) is gratefully acknowledged.
A.H. thanks Prof. Shouwu Guo for his kind help with AFM.
Keywords: conjugation · cyclization · graphene · polymers ·
radical reactions
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Received: June 12, 2012
Revised: July 14, 2012
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Graphene
Xiaowei Ma, Fei Li, Youfu Wang,
Aiguo Hu*
&&&& — &&&&
Functionalization of Pristine Graphene
with Conjugated Polymers through
Diradical Addition and Propagation
Hanging on: Pristine graphene was
grafted with conjugated polymers
through addition and propagation of
diradicals generated from Bergman
cyclization of enediyne-containing den-
drimers. The surface functionalization
was confirmed with TGA, FTIR and
Raman spectroscopy, and AFM analy-
sis. The sp2 network of graphene is
only slightly destroyed, as revealed by
conductivity measurements.
Chem. Asian J. 2012, 00, 0 – 0
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemasianj.org
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These are not the final page numbers! ÞÞ