Processable hybrids of ferrocene-containing poly(phenylacetylene)s and carbon nanotubes: Fabrication and properties

Article abstract of DOI:10.1021/jp801892t

A group of ferrocene-containing poly(phenylacetylene)s (PPAs) with different alkyl spacers were synthesized by using organorhodium complexes [Rh(diene)Cl]₂ and Rh⁺(nbd)[C₆H ₅B^-(C₆H₅)₃] as catalysts. With the aid of π-π interactions between the walls of carbon nanotubes (CNTs) and the PPA skeleton together with the ferrocene pendants, the polymer (P1, P2(5) and P2(10)) chains effectively wrapped round the shells of both single-walled carbon nanotubes (SWNTs) and multiwalled carbon nanotubes (MWNTs). The additive effect of the PPA skeleton and the ferrocene pendants in dispersing the SWNTs and MWNTs resulted in the generation of highly soluble hybrids. The solubilities of P1-functionalized SWNTs and MWNTs in tetrahydrofuran (THF) are up to 633 mg/L and 967 mg/L, respectively. They are much higher than the solubilities of M1-modified SWNTs and MWNTs, which are only 167 mg/L and 133 mg/L in THF. The results indicate the existence of a powerful polymer effect on dispersing CNTs. The high solubilities of the hybrids in organic solvents allowed us to fabricate high-quality and large-area films. Meanwhile, the desirable loading of ferrocene-containing PPAs onto the CNTs offered polymer/CNTs hybrids with multiple redox centers and ferrocene-featured electrochemical properties. The P1/MWNT hybrid exhibits evident optical-limiting properties. At high incident laser fluence, the optical-limiting power of P1/MWNT is higher than that of C₆₀, a well-known optical limiter. Thermal analyses indicate that the decomposition temperatures (Td, the temperature at which a sample loses its 5% weight) for P1 and P1/MWNT are 342 and 346°C, respectively, much higher than that for PPA (225°C). Thus the attachment of a ferrocene pendant to a PPA backbone, followed by hybridization with CNTs, improved the thermal stability. Upon pyrolysis, both the polymer and the polymer/CNTs hybrid gave rise to superparamagnetic ceramics; the saturation magnetizations (M_s) of the ceramics derived from P1 and P1/MWNT are 29.9 and 26.9 emu/g, respectively. The latter datum is in the list of the best results reported for the magnetic nanocomposites obtained by the attachment of magnetic nanoparticles onto CNTs.

Full text of DOI:10.1021/jp801892t

8896
J. Phys. Chem. B 2008, 112, 8896–8905
Processable Hybrids of Ferrocene-Containing Poly(phenylacetylene)s and Carbon
Nanotubes: Fabrication and Properties
Wang Zhang Yuan,† Jing Zhi Sun,*,† Jian Zhao Liu,‡ Yongqiang Dong,† Zhen Li,§
Hai Peng Xu,† Anjun Qin,† Matthias Ha¨ussler,‡ Jia Ke Jin,† Qiang Zheng,† and
Ben Zhong Tang*,†,‡
Department of Polymer Science & Engineering, Key Laboratory of Macromolecular Synthesis and
Functionalization of the Ministry of Education, Zhejiang UniVersity, Hangzhou 310027, China, Department of
Chemistry, The Hong Kong UniVersity of Science & Technology, Clear Water Bay, Kowloon, Hong Kong,
China, and Department of Chemistry, Wuhan UniVersity, Wuhan 430072, China
ReceiVed: March 4, 2008
A group of ferrocene-containing poly(phenylacetylene)s (PPAs) with different alkyl spacers were synthesized
by using organorhodium complexes [Rh(diene)Cl]2 and Rh+(nbd)[C6H5B-(C6H5)3] as catalysts. With the aid
of π-π interactions between the walls of carbon nanotubes (CNTs) and the PPA skeleton together with the
ferrocene pendants, the polymer (P1, P2(5) and P2(10)) chains effectively wrapped round the shells of both
single-walled carbon nanotubes (SWNTs) and multiwalled carbon nanotubes (MWNTs). The “additive effect”
of the PPA skeleton and the ferrocene pendants in dispersing the SWNTs and MWNTs resulted in the generation
of highly soluble hybrids. The solubilities of P1-functionalized SWNTs and MWNTs in tetrahydrofuran (THF)
are up to 633 mg/L and 967 mg/L, respectively. They are much higher than the solubilities of M1-modified
SWNTs and MWNTs, which are only 167 mg/L and 133 mg/L in THF. The results indicate the existence of
a powerful polymer effect on dispersing CNTs. The high solubilities of the hybrids in organic solvents allowed
us to fabricate high-quality and large-area films. Meanwhile, the desirable loading of ferrocene-containing
PPAs onto the CNTs offered polymer/CNTs hybrids with multiple redox centers and ferrocene-featured
electrochemical properties. The P1/MWNT hybrid exhibits evident optical-limiting properties. At high incident
laser fluence, the optical-limiting power of P1/MWNT is higher than that of C60, a well-known optical limiter.
Thermal analyses indicate that the decomposition temperatures (Td, the temperature at which a sample loses
its 5% weight) for P1 and P1/MWNT are 342 and 346 °C, respectively, much higher than that for PPA (225
°C). Thus the attachment of a ferrocene pendant to a PPA backbone, followed by hybridization with CNTs,
improved the thermal stability. Upon pyrolysis, both the polymer and the polymer/CNTs hybrid gave rise to
superparamagnetic ceramics; the saturation magnetizations (Ms) of the ceramics derived from P1 and P1/
MWNT are 29.9 and 26.9 emu/g, respectively. The latter datum is in the list of the best results reported for
the magnetic nanocomposites obtained by the attachment of magnetic nanoparticles onto CNTs.
Introduction
In the past decade, many efforts have been made to endow
the CNTs with good solubility in organic solvents or aqueous
media. To date, two approaches, covalent7 and noncovalent17–20
functionalizations, have been widely used to improve the
macroscopic processability of CNTs. Compared with the
covalent approach, the noncovalent method enjoys the superior-
ity of not disturbing the electronic structure of CNTs, which is
crucial in the fabrication of high-quality optoelectronic devices.
Noncovalent functionlization of CNTs with conjugated polymers
is regarded as promising in creating new functional nanohybrids
since it not only provides a chance to combine the advantages
of both, but also affords an opportunity to generate novel
characteristics.18,21
Since their discovery, carbon nanotubes (CNTs) have attracted
great attentions in scientific research and technological innova-
tion because of their unique electronic, physicochemical, and
mechanical properties, which render them ideal candidates for
the fabrication of new-generation optoelectronic devices.1–7
There are many successful examples showing that their remark-
able electronic and optical properties have promised much
potential for such high-tech applications as nanosensors, nano-
electrodes, quantum wires, molecular diodes, and photoelec-
trochemical devices.8–16 Individual CNTs, however, because of
their high molecular weights, strong hydrophobicity, π-π
stacking, and van der Waals attraction, are tightly held together
in bundles. This makes CNTs insoluble in any solvents, and
thus hampers not only the research of their solution properties,
but also their practical applications.
As a prototypical conjugated polymer, polyacetylene and its
substituted derivatives, particularly the derivatives of poly-
(phenylacetylene) (PPAs), exhibit a variety of functions.22,23 One
research interest in our group is to create new macroscopically
processable and functional PPA/CNT materials with combined
advantages of the two components. Previously, we successfully
obtained PPA/CNT nanohybrids via the noncovalent approach.18a,b
A PPA/multiwalled carbon nanotube (MWNT) nanocomposite18a
was prepared by in situ polymerization, and hybrids of pyrene-
* Corresponding authors. Tel: +86-571-8795-3797; fax: 86-571-8795-
3734; e-mail: sunjz@zju.edu.cn (J.Z.S.). Tel: +852-2358-7375; fax: +852-
2358-1594; e-mail: tangbenz@ust.hk (B.Z.T.).
† Zhejiang University.
‡ The Hong Kong University of Science & Technology.
§ Wuhan University.
10.1021/jp801892t CCC: $40.75
2008 American Chemical Society
Published on Web 07/02/2008

Hybrids of Ferrocene-Containing PPAs and CNTs
J. Phys. Chem. B, Vol. 112, No. 30, 2008 8897
SCHEME 1
SCHEME 2
Experimental Section
Materials. Tetrahydrofuran (THF) was distilled under normal
pressure from sodium benzophenone ketyl under argon im-
mediately prior to use. Dichloromethane (DCM) was distilled
under normal pressure over calcium hydride under argon before
use. Triethylamine (TEA) was distilled and dried over potassium
hydroxide. Butane-1,4-diol (4), diethyl ether and methanol
(analytical grade) were used directly. MWNTs (90% purity,
Strem Chemicals) and SWNTs (Strem Chemicals) were used
directly. Ferrocenecarboxylic acid (3) was purchased from
Yixing Weite Petrochemical Additives Plant (China). p-Tolu-
enesulfonic acid monohydrate (TsOH), N,N′-dicyclohexylcar-
bodiimide (DCC), 4-dimethylaminopyridine (DMAP) were all
purchased from Aldrich and used as received without further
purification. Rhodium (I) complexes [Rh(nbd)Cl]2 (nbd ) 2,5-
norbornadiene), [Rh(cod)Cl]2 (cod ) 1,8-cyclooctadiene), and
Rh+(nbd)[(η6-C6H5)B-(C6H5)3] were prepared by literature
methods.27,28 4-Ethynylbenzonic acid (6), 6-(4-ethynyl-phe-
noxy)hexanoic acid [7(5)] and 11-(4-ethynylphenoxy)unde-
canoic acid [7(10)] were synthesized according to our previously
published procedures.18b
containing PPA/MWNTs18b were generated through physical
blending. All the hybrids show good functionalities such as
optical-limiting property, light-emitting behavior, and efficient
photoinduced charge transfer performance.
In this contribution, we present facile synthetic routes to PPAs
bearing ferrocene motifs (see Scheme 2, P1, P2(5), and P2(10))
and the preparation of hybrids of P1/single-walled carbon
naotube (SWNT) and P1/MWNT. It is well known that
ferrocene is a famous mediator in electrochemical detections
and biosensors.24–26 It can be used in the detection of DNA,
dopamine, ascorbic acid, hydrogen peroxide,24,25 and selective
recognition of anions.26a It is also a very good electron donor,
which can be applied in the construction of donor/acceptor
ensembles for photovoltaic devices.3 Chemically modified CNTs
by ferrocene-bearing small molecules have been reported by
Prato and co-workers; therein, intramolecular electron transfer
was observed.15 Herein, a group of ferrocene-containing PPAs
were synthesized, and their structures and properties were
characterized and evaluated by gel permeation chromatography
(GPC), IR, NMR, thermogravimetric analysis (TGA), UV-
visible spectroscopy (UV-vis), and cyclic voltammetry (CV).
Through simple physical mixing, ferrocene-containing conju-
gated polymer/CNT nanohybrids were successfully obtained.
The hybrids showed high solubility in common organic solvents,
good film forming ability, evident optical-limiting property, and
desirable redox activity. Electron transfer between P1 and CNTs
was also detected by CV analysis, indicating potential applica-
tions in photovoltaic devices. Because of the existence of
ferrocene motifs, the pyrolytic products of both P1 and
P1/MWNT displayed excellent superparamagnetic behavior.
1
Instrumentation. H and 13C NMR spectra were measured
on a Bruker ARX 500 NMR spectrometer using chloroform-d
as solvent and tetramethylsilane (TMS; δ ) 0 ppm) as the
internal standard. IR spectra were recorded on a Bruker
VECTOR 22 spectrometer. UV-vis absorption spectra were
measured on a Varian CARY 100 Bio UV-visible spectropho-
tometer. The thermal stability of the polymers was evaluated
on a Perkin-Elmer Pyris thermogravimetric analyzer TGA 6.
Molecular weights (Mw and Mn) and polydispersity indexes (Mw/
Mn) of the polymers were estimated THF by a Waters Associates
GPC system. A set of monodisperse polystyrene standards
covering molecular weight range of 103-107 was used for
molecular weight calibration.
Scanning electron microscope (SEM) images of the polymer-
wrapped CNTs were taken on a JEOL 6300L field emission
SEM operating at an accelerating voltage of 5 kV, while their
transmission electron microscope (TEM) images were recorded
with a JEOL/JEM-200 CX TEM at an accelerating voltage of
160 kV. The X-ray photoelectron spectroscopy (XPS) experi-
ments were conducted on a PHI 5600 spectrometer (Physical
Electronics), and the core level spectra were measured using a
monochromatic Al KR X-ray source (hV ) 1486.6 eV). The
analyzer was operated at 23.5 eV pass energy, and the analyzed
area was 800 µm in diameter. The binding energies were
referenced to the adventitious hydrocarbon C 1s line at 285.0

8898 J. Phys. Chem. B, Vol. 112, No. 30, 2008
Yuan et al.
TABLE 1: Polymerization of Ferrocene-Containing
eV, and the curve fitting of the XPS spectra was performed
using the least-squares method. The energy-dispersive X-ray
(EDX) analyses were performed on a Philips XL30 SEM system
with quantitative elemental mapping and line scan capacities
operating at an accelerating voltage of 15 kV. Optical-limiting
experiments were carried out at 532 nm, using 8 ns optical
pulses generated from a frequency-doubled Q-switched Nd:YAG
laser (Quanta Ray GCR-3) operating in a near Gaussian
transverse mode with a repetition rate of 10 Hz. The pulsed
laser beam was focused onto a 1 mm square quartz cell. The
incident and transmitted energies were measured by an OPHIR
detector (30-A-Diff-SH). Cyclic voltammograms (CVs) were
recorded on a Princeton Applied Research model 273A poten-
tiostat. The working and reference electrodes were glassy carbon
and Ag/AgCl (0.1 M in acetonitrile), respectively. Potentials
were reported with reference to ferrocenium/ferrocene couple
(Cp2Fe+/0). The magnetization measurements were carried out
using a Quantum Design vibrating sample magnetometer (VSM,
LakeShore 7037/9509-P) at fields ranging from -9 to 9 kOe at
room temperature.
Monomer Synthesis. The synthetic routes to ferrocenyl-
bearing monomers 1 and 2(m) (m ) 5, 10) are shown in Scheme
1. The detailed procedures are given in the Supporting Informa-
tion.
Polymer Synthesis. All the polymerization reactions and
manipulations were carried out under a nitrogen atmosphere
using Schlenk techniques in a vacuum-line system except for
the purification of the resulting polymers, which was done in
an open atmosphere. The synthetic routes of P1 and P2(m) are
shown in Scheme 2. Typical experimental procedures are given
in the Supporting Information.
Acetylene Monomersa
yield
(%)
b
b
no.
catalyst
solvent
Mw
Mw/Mn
1
c
1
2
3
[Rh(cod)Cl]2
[Rh(cod)Cl]2
[Rh(nbd)Cl]2
[Rh(nbd)Cl]2
Rh+(nbd)
DCM/Et3N 96.0 242 200
THF/Et3N 96.3 182 500
DCM/Et3N 97.8 193 200
3.0
3.3
3.5
3.5
4.1
c
4
THF/Et3N
THF
99.6 646 800
98.1 391 500
5d
[C6H5B-(C6H5)3]c
2(5)
6
7
[Rh(cod)Cl]2
[Rh(nbd)Cl]2
[Rh(nbd)Cl]2
Rh+(nbd)
DCM/Et3N 24.7
DCM/Et3N 22.5
42 600
29 200
69 600
87 500
1.5
1.4
1.9
5.7
8
THF/Et3N
THF
45.6
30.8
9d
[C6H5B-(C6H5)3]
2(10)
10 [Rh(cod)Cl]2
11 [Rh(nbd)Cl]2
12 [Rh(nbd)Cl]2
13d Rh+(nbd)
DCM/Et3N 23.3
DCM/Et3N 26.6
60 000
27 700
96 300
42 100
1.7
2.0
2.1
4.4
THF/Et3N
THF
40.6
20.8
[C6H5B-(C6H5)3]
a Carried out under nitrogen at room temperature for 24 h. [M]0
) 0.2 M, [cat.] ) 2 mM. b Determined by GPC in THF on the basis
of
a
polystyrene
calibration.
c Abbreviations:
cod
)
1,5-cyclooctadiene; nbd ) 2.5-norborndiene. The structures of the
catalysts are shown in the Supporting Information. d Carried out at
60 °C.
known to work as effective catalysts for the polymerizations of
monosubstituted acetylenes.
Polymer/CNT Hybridization. Taking the preparation of P1/
MWNT hybrid for example, into a tube were added 10 mg of
P1, 5 mg of MWNTs, and 3 mL of THF. After sonication for
15 min (or vigorous stirring for 1 h), the mixture was filtered
through a disposable glass Pasteur pipet filled with absorbent
cotton to remove insoluble MWNTs. The filter was dried in
vacuo at 100 °C to a constant weight. The concentration of
MWNTs in THF (c) was calculated by eq 1:
To polymerize the monomers, organorhodium complexes
[Rh(diene)Cl]2 and
a
zwitterionic catalyst Rh+(nbd)-
[C6H5B-(C6H5)3] were used. The polymerization results are
listed in Table 1. For M1, in DCM/Et3N or THF/Et3N, both
[Rh(cod)Cl]2 and [Rh(nbd)Cl]2 are efficiently enough to initiate
the polymerization. In each case, polymeric product was
obtained in high yield (in the case of no. 4, up to 99.6%) with
high molecular weight (Mw up to 646 800). When the zwitte-
rionic catalyst was chosen, the polymerization was carried out
in pure THF, and the result was also quite satisfactory, with
Mw being as high as 391 500 at a yield of 98.1%. For M2(5)
and M2(10), under the same conditions as M1, it seems that
the catalysts also worked well; however, the molecular weights
and yields were obviously lower than those of P1. The lowered
yields of P2(5) and P2(10) should be ascribed to the formation
of some low molecular weight polymers during the polymeri-
zation. Generally speaking, these polymers with low molecular
weights were stably suspended in methanol, and were not
sufficiently collected in the resulting products. This is also why
P2(5)andP2(10)showedrelativelynarrowmoleculardistributions.
Structure Characterization. The polymeric products were
characterized by spectroscopic methods. All the polymers gave
satisfactory analysis data corresponding to their expected
molecular structures (see Supporting Information for details).
An example of the IR spectrum of P1 is shown in Figure 1; the
spectrum of its monomer M1 is also given in the same figure
for comparison. The monomer exhibits three characteristic
absorption bands at 3253, 2107, and 1708 cm-1, which are
assigned to the stretching vibrations of t CH, Ct C and CdO,
respectively. The first two bands associated with acetylenic
absorptions completely disappear in the spectrum of P1, while
that of CdO keeps intact, indicating that the triple bond of M1
has been fully consumed by the polymerization reaction.
WCNT - (WF - WF,0)
c )
(1)
VS
where WCNT is the original weight of the MWNTs, WF and WF,0
are the weights of the cotton filter after and before filtration,
respectively, and VS is the volume of the solvent. (WF - WF,0
)
is thus the weight of the insoluble MWNTs retained by the filter,
and WCNT - (WF - WF,0) is the weight of the soluble MWNTs
that passed through the filter. To learn the effect of the ferrocene-
containing PPAs on the solubilization of CNTs, the content of
CNTs in the CNT/polymer hybrid (c′, %) was calculated by eq
2:
WCNT - (WF - WF,0)
WP + WCNT - (WF - WF,0)
c' )
× 100
(2)
where WP is the weight of the polymer added into the solution.
Results and Discussion
Polymer Syntheses. As shown in Scheme 1, ferrocene-
containing monomers 1 (M1) and 2(m) (m ) 5, 10) were
synthesized by two-step esterification reactions. Each reaction
went smoothly, and desired monomers were obtained in high
yields (∼83-87%). To transform the monomers to polymers,
we tried to use three catalysts based on rhodium (I), which are

Hybrids of Ferrocene-Containing PPAs and CNTs
J. Phys. Chem. B, Vol. 112, No. 30, 2008 8899
Figure 1. IR spectra of (A) monomer M1 and (B) its corresponding
polymer P1.
Figure 3. 13C NMR spectra of (A) monomer M1 and (B) its polymer
P1 in chloroform-d at room temperature. The solvent peaks are marked
with asterisks.
Figure 2. 1H NMR spectra of (A) monomer M1 and (B) its polymer
P1 in chloroform-d at room temperature. The solvent peaks are marked
with asterisks.
NMR spectroscopy provides valuable information about the
1
structure of the polymers. As can be seen from the H NMR
spectrum of P1, no resonance peak can be found at δ 3.2, which
is associated with the acetylene proton (Figure 2B). Meanwhile,
a new peak due to the resonances of olefinic protons appears at
δ 5.7, which is absent in the spectrum of its monomer M1. It
suggests that the polymerization reaction successfully transforms
the acetylenic triple bond of M1 to an olefinic double bond of
polyene. This also upfield-shifts the resonances of the phenyl
protons, which now occur at δ 7.6 and 6.7 (Figure 2B, peaks c
and b), while its monomer’s peaks exist at δ 8.0 and 7.5,
respectively. Like IR spectra, the 1H NMR spectra confirm that
the triple bond of the monomer has been thoroughly transformed
to a double bond during the polymerization process.
Figure 4. Absorption spectra of M1, P1, P2(5), and P2(10) in THF at
room temperature. Concentration ) 1 × 10-4 M. The inset is the
magnified curve for M1 in the range of 300-600 nm.
intensities should be due to the polymer main chains whose
absorptions are overlapped with the characteristic absorption
of ferrocene pendants. Such a wide absorption region is also
reasonable when taking into account that conjugated polymers
generally possess segments with different lengths of electronic
conjugation. Another thing that should be pointed out is the
difference of the absorptions between P1 and P2 (m) (m ) 5,
10): P1 has a much higher and wider absorption beyond 360
nm when compared with that of the other two polymers. This
could be ascribed to the effect of the carbonyl group (CdO),
which extends the effective conjugation length of P1. Mean-
while, CdO is also an auxochrome group, which is helpful to
enhance the absorption of the polymer.
Polymer/CNT Hybridization. Hybrids of ferrocene-based
compounds and CNTs have potential applications in photovol-
taic devices due to the effective electron transfer between them.
However, to date, only a few works have focused on ferrocene
or its derivative functionalized CNTs.15,30 Herein, we tried to
use ferrocene-containing PPAs to fabricate conjugated polymer/
CNT hybrid nanomaterials. Since PPA main chains can wrap
CNTs via physical adsorption,18a and ferrocene pendants can
also interact with CNTs through π-π interactions,30 it is
expected that ferrocene-containing PPAs/CNT hybrids with high
Figure 3 shows the 13C NMR spectra of P1 and M1. Whereas
the acetylenic carbon atoms of M1 resonate at δ 80.5 and 83.1,
these peaks are completely absent in the spectrum of P1. Instead,
two new peaks are observed at δ 139.0 and 145.9, which are
assignable to the resonances of the polyene carbons of the
polymer. This once again proves that the polymerization is
realized through the transformation of the triple bonds of the
monomer to the double bonds of the polymer.
Absorption Spectra. The absorption spectra of the polymers
and M1 are given in Figure 4. As can be seen from the inset,
M1 has a wide absorption region and shows a peak at around
445 nm, which should be ascribed to the featured absorption of
ferrocene.29 However, the absorption of M1 is very weak beyond
300 nm. In contrast, all the polymers have moderate absorbance
at ∼300 nm and well extend to >500 nm. The enhanced

8900 J. Phys. Chem. B, Vol. 112, No. 30, 2008
Yuan et al.
Figure 5. TEM (A,B) and SEM (C,D) images of MWNTs encapsulated
by P1. The scale bars in both C and D are 1 µm.
solubilities in common organic solvents and high loading amount
of CNTs can be obtained. Fortunately, as expected, both SWNTs
and MWNTs wrapped by P1 were highly soluble in common
solvents, such as DCM, THF, DMSO, and CHCl3. For example,
as shown in Figure S1, P1/SWNT and P1/MWNT dispersed
well in THF to form homogeneous black solutions. The hybrid
solutions were stable and could stand for months without
precipitation. Additionally, their dried powders could be redis-
persed in common organic solvents.
Figure 6. HRTEM for (A) P1/SWNT, (B) P1/MWNT, and (C,D) larger
area TEM images for P1/MWNT. In panel C, the sample was directly
used without removing free polymers, and in panel D, the samples were
filtered and washed through a micropore filter membrane (pore size ∼
0.22 µm).
Morphologies of the Hybrids. Direct observation of the
morphology of the hybrids is not only a powerful technique to
study the microstructure of the composites, but also a direct
method to illuminate the interactions between different com-
ponents. Morphological images of P1-functionalized CNTs are
shown in Figure 5 and Figure S2 (see Supporting Information
for details). From the TEM images (Figure 5A,B), it is clear
that the MWNTs have been wrapped by thick polymer shells.
The SWNTs were also densely wrapped by P1, as can be seen
in Figures S2(A) and S2(B). The strong association between
CNTs and polymer chains leads to the high solubility of the
hybrid nanomaterials in organic solvents. The SEM images
(Figure 5C,D) of P1/MWNT hybrid nanomaterials show that
the MWNTs are embedded in the polymer matrix, and the
MWNTs are uniformly dispersed. Both TEM and SEM mea-
surements imply that, with the assistance of P1, the CNTs were
separated from their bundles, and enjoyed a good dispersity in
organic media. The thick wrapping of the polymer chains onto
CNTs and the good dispersity of the hybrids in the polymer
matrix provide a chance to study the properties of one-
dimensional functional materials. In addition, it endows the
composite with integrated merits of the ferrocene-containing
PPAs and CNTs. The electron donor/acceptor (ferrocene and
polymer main chain/CNTs) assembly not only has a potential
application in photovoltaic devices, but is also suitable to apply
in sensors, electrodes, and so on.3,15,24–26
Figure 7. Solubilities of M1-, P1-, and PPA-functionalized SWNTs
and MWNTs in THF.
6D, no obvious polymer shells can be observed on the surfaces
of MWNTs. However, they still can be redispersed in organic
solvents, and the suspension can stand for hours, even to 1 day.
The difference in stabilities between the samples is evidence
of the positive effect of the polymers in the hybrids. And their
image contrast confirms the inherent physical adsorption feature
of noncovalent functionalization of CNTs.
Polymer Effect on Solvating Power. Solvating power of
the compound in dispersing CNTs is an important parameter
for fabricating soluble hybrids. To quantitatively evaluate the
solvating powers of the polymers (P1 and PPA) and M1, we
measured the solubilities of CNTs in THF by using eq 1. All
the measurements were carried out in identical conditions to
ensure the data can be compared with each other. The
experimental results are displayed in Figure 7. The data revealed
that, with the aid of P1, the solubilities of SWNTs and MWNTs
in THF are as high as 633 and 967 mg/L, respectively. The
high performance should be ascribed to the “additive effect” of
the PPA skeleton and ferrocene pendants.
High-resolution transmission electron microscopy (HRTEM)
of the hybrids and TEM images at higher magnifications are
displayed in Figure 6. In Figure 6A,B, the lattices of SWNTs
and MWNTs are clearly shown. Figure 6C exhibits the TEM
image of P1/MWNT in a much larger area, and the polymer
shell and MWNT core can easily be distinguished. However,
when the hybrid was filtrated and washed through a filter
membrane with a pore-size of 0.22 µm, it is observed that not
only free polymers but also some of the polymers attached to
MWNTs were removed from the hybrids. As shown in Figure
More important, we found it is quite a positive “additive
effect”; that is to say, an enhanced “synergistic effect” was

Hybrids of Ferrocene-Containing PPAs and CNTs
J. Phys. Chem. B, Vol. 112, No. 30, 2008 8901
CHART 1
Figure 8. Photographs of (A) P1/SWNT powder and its corresponding
film cast on quartz substrate, and (B) transparent hybrid film of P1/
SWNTs.
observed. As can be seen from the photographs in Figure S1,
both M1 and P1 can help dissolve CNTs in THF, giving
homogeneous hybrids solutions. However, it is not difficult to
find out the difference between M1- and P1-functionalized CNTs
solutions from the evident contrast of their colors. For M1/CNT
hybrids, the solutions are light, whereas those of P1/CNTs are
dark colored. The photographs indicate that much more CNTs
are involved in the P1/CNT hybrids than in the M1/CNTs. This
is confirmed by the solubility data presented in Figure 7. Under
the same conditions, M1-modified CNTs showed much lower
solubility in THF, only 167 and 133 mg/L for SWNTs and
MWNTs, respectively. This observation suggests that there may
be a profound “polymer effect” on the solvating power, and
this effect may be related to the polymers’ PPA skeletons, which
can effectively wrap on CNTs through π-π interactions. To
illuminate the effect of PPA skeleton, the solubilities of PPA-
functionalized SWNTs and MWNTs were checked by the
physical blending of PPA with SWNTs and MWNTs in THF.
Under the same molar concentration as that of P1, it is found
that the solvating power of PPA is much less than that of P1,
only 133 and 100 mg/L for SWNTs and MWNTs in THF,
respectively. In contrast, the solubilities of P1-modified SWNTs
and MWNTs in THF are as high as 633 and 967 mg/L,
respectively. It means that P1 has much higher solvating powers
than the sum of those for PPA and M1. The greatly enhanced
solubilities of CNTs in THF after “conjugation” with P1 is
evidence of the “synergistic effect” between PPA skeleton and
ferrocene pendants.
Mechanism of CNT Solubilization. On the basis of the
above results, the solubilization of CNTs may be realized
through the following mechanisms: For M1, the molecules are
attracted to the CNT shells because of the electronic interactions
between ferrocene motifs and the CNT surfaces; for PPA, the
polymer chains can wrap onto the CNT surfaces through π-π
interactions between the phenyls and CNTs; for P1, both PPA
skeletons and ferrocene pendants take effects, and the affinity
of the polymer chains with the solvent molecules brings the
polymer-wrapped CNTs into the liquid media, leading to
the dissolution of CNTs in the organic solvents. Additionally,
the polymer chains attached to CNT surfaces may attract other
polymer chains via pendant-pendant electronic interaction,
resulting in a thick coating of polymer chains around the CNT
shells. Two factors, i.e., polymer wrapping and the π-π
interactions between CNTs and ferrocene pendants, are con-
sidered to play important roles in dissolving CNTs in solvents.
A schematic illustration is shown in Chart 1.
Figure 9. Optical-limiting response of THF solutions of P1, P1/MWNT
hybrid, and toluene solution of C60 to 8-ns, 10 Hz pulses of 532-nm
laser light. Light path length: 1 mm. The hybrid was prepared according
to the above manipulations.
formability. To check this point, we took P1/SWNT hybrid as
a model to conduct the film fabrication. As expected, when a
solution of P1/SWNT in chloroform was cast onto quartz
substrate, a thin film was formed after a mild evaporation at
room temperature. As displayed in Figure 8, the pristine hybrid
is a black-brown powder, and its cast thin film is uniform and
transparent, which indicated good dispersion of CNTs in
polymer matrix and desirable film forming ability of the hybrid.
This intrinsic property makes the hybrids easy to be processed
into high-quality films with tunable shape and thickness. The
good solubilities, good film forming abilities, and long-term
stabilities pave the way for the applications of ferrocene-
containing PPAs/CNTs hybrids.
Optical-Limiting Property. The optical-limiting behaviors
of the THF solutions of P1 and P1/MWNT hybrid were
investigated by employing the experimental setup used in our
previous studies.31 For comparison, the optical-limiting behavior
of C60 in toluene was also studied. In order to compare the
optical-limiting powers among them, their transmittances were
set to the same level by concentration adjustments. As shown
in Figure 9, their transmittances were set at 0.9 in the linear
region. When the THF solution of P1 was shot by 8-ns pulses
of 532-nm laser light, the transmitted fluence linearly increased
with the increase of the input fluence. This polymer, like many
other polyacetylenes, is thus nonlinear optically inactive. It is
reasonable that there are no bulky effective conjugated systems
in P1. Even though its main chain is conjugated and ferrocene
units are usually used to construct optical-limiting active
materials, the effective conjugation length is short, and ferrocene
pendants are isolated from the conjugated centers without
forming an expanded conjugated system. However, when
Film Forming Ability of the Nanohybrid. A unique
advantage of the polymer-based materials over small molecules
and inorganic materials is their solution processability and good
film formability. Thanks to their good solubilities in organic
solvents, P1/CNT hybrids are expected to show good film

8902 J. Phys. Chem. B, Vol. 112, No. 30, 2008
Yuan et al.
(the temperature at which the sample loses 5% of its weight)
for P1, P2(5), P2(10), and P1/MWNTs is 342, 320, 330, and
346 °C, respectively. These are much higher than that of PPA,
whose Td is only 225 °C.33 The thermal analysis results suggest
that the ferrocene pendants can improve the thermal stability
of the PPA derivatives. This effect should be ascribed to the
“jacket effect” of the ferrocene pendants surrounding the polyene
backbone.18b,22,23
It is worth noting that, during the course of temperature
increasing, two platforms are observed on the TGA curves.
Before 340 °C, the polymers lost little of their weights; however,
sharp decreases of weight were found between 340 to ∼400
°C, and then leveled off when heated to about 487 °C. The
second platform occurred between ∼490-610 °C, and little
weight loss was recorded during this temperature region,
indicating that the polymers had been ceramized in the pyrolysis
process. Further weight loss of the polymers was found when
heating the samples to much higher temperatures, as shown in
Figure 11. The ceramic yields of the polymers at 500 and 900
°C are summarized in Table 2. The data at 500 °C for P1, P2(5),
and P2(10) are 52.8%, 42.6% and 31.6%, respectively. It is
reasonable, when we take into account the structure differences
between the polymers, that the polymers with lower content of
alky spacers should have higher ceramic yields. It seems that
the pyrolytic behaviors became complicated at higher temper-
atures. When the temperature reached 900 °C, the ceramic yield
of P2(10) was even higher than that of P2(5). The pyrolytic
behavior of P1/MWNT hybrid is also shown in Figure 11; it
acted in a way similar to P1. Nevertheless, because of the
incorporation of MWNTs, the hybrid showed higher transition
temperatures than those of P1.
Compositions and Morphologies Study. As reported else-
where, linear or hyper-branched ferrocene-containing polymers
can be utilized as precursors to magnetic ceramics.34 Thus we
checked whether our ceramics were magnetic by attaching a
piece of magnet to the resulting products of TGA tests. It is
found that all of the resulting powders could be attracted to the
magnet, evidencing that the products are really magnetic. To
further explore the magnetic properties of ceramics, we newly
prepared the ceramics by putting P1 and P1/MWNTs into an
argon-filled furnace at 1000 °C for 1 h. The ceramic yields for
P1 and P1/MWNTs are 30.9% and 44.3%, respectively. X-ray
diffraction (XRD) measurements were conducted on the ceramic
products. Figure 12 shows the XRD patterns of the ceramics
obtained from pure P1 and P1/MWNTs hybrid. In comparison
with the pattern of the γ-Fe2O3 standard, the presence of γ-Fe2O3
in both samples was clearly confirmed. Therefore, it is unam-
biguous that the magnetism of the ceramics is due to the
existence of γ-Fe2O3 in the resulting products.
Figure 10. CVs of ferrocene-containing polymers and the hybrid in
DCM containing 0.5 M [(n-Bu)4N]PF6. Scan rate: 100 mV/s. Concen-
tration (mM): P1: 7.7; P2(5): 5.0; P2(10): 4.6; P1/MWNT: 7.7 for P1.
MWNTs are incorporated, the hybrid of P1/MWNT acts in quite
a different way; it exhibits an optical-limiting property at high
input flow capacity. As shown in Figure 9, its optical-limiting
power is evidently higher than C60. For example, at incident
fluence of 976 mJ/cm2, the transmitted fluence for C60 is 797
mJ/cm2, while, for P1/MWNTs, the transmitted fluence is only
739 mJ/cm2 at the identical incident fluence. In consideration
that C60 is a famous optical-limiting material, the hybrids of
ferrocene-containing PPA derivatives and MWNTs are promis-
ing optical-limiting materials.
Electrochemical Property. Ferrocene is a representative
redox active compound.24–26 Thus, the resulting polymers and
the hybrids are also envisioned as being electrochemically active.
To check this idea, the reversibility and oxidation potentials of
the redox processes of the polymers and P1/MWNT hybrid were
characterized by CV in DCM solutions. Figure 10 shows the
CVs of the polymers and the hybrid. The oxidation potential of
P1, P2(5), P2(10), and P1/MWNT are summarized in Table 2.
The CV curves and the extracted data offer the following
information. First, it is found that all the samples exhibit only
one single wave in the range from -0.3 to 1.2 V (vs Ag/AgCl),
which is assigned to the oxidation of the ferrocene motifs,
implying that the electrochemical activity of the conjugated
polyene backbone is undetectable under our measurement
conditions. Second, different polymers, i.e., P1, P2(5), and
P2(10) display similar electrochemical behavior in DCM. These
results indicate the absence of direct interactions between the
iron centers, just as observed for many ferrocene-containing
dendrimers and side-chain polyferrocenes.32 Meanwhile, these
results also suggest the lack of electronic coupling between the
ferrocene motifs and the conjugated polyene backbones. When
the data of P1 were compared with that of P1/MWNT hybrid,
it was found that the Epc of P1/MWNT is higher than that of
P1 by 38 mV, while the Epa of P1/MWNT is lower than that of
P1 by 19 mV. It suggests that the P1/MWNT hybrid mainly
keeps the redox property of ferrocenyl pendants. Nevertheless,
the small but detectable increase in the oxidation potential may
be ascribed to the electronic interactions between MWNTs and
ferrocene pendants of the conjugated polymer chains. Because
of the electronic interaction between them, electrons can be
transferred from the ferrocene donors to the MWNTs acceptors.
Thermal Stability and Pyrolytic Ceramization. The ther-
mogravimetric analysis results of the polymers and the P1/
MWNT hybrid are shown in Figure 11. It is found that the Td
We also used XPS and EDX techniques to analyze the
ceramic products in order to learn their chemical compositions.
As shown in Figure 13, both the ceramics prepared from P1
and P1/MWNT are composed of iron (Fe), oxygen (O), and
carbon (C) species. The EDX analyses presented in Figure 14C
and 14F further confirm the existence of Fe element as revealed
by XPS data. The SEM images of ceramics from P1 and P1/
MWNT in the furnace at 1000 °C under argon are given in
Figure 14A,B and Figure 14D,E. As shown in Figure 14 A,
though most of the ceramic particles from P1 are attached
together to form bulky aggregates, some particles with diameters
of around several hundred nanometers still remain isolated on
the surface. For the hybrid product, the ceramic forms in bulk
matrix, and the CNTs are well dispersed in it. The SEM images
for ceramic obtained from P1/MWNT hybrid thus further

Hybrids of Ferrocene-Containing PPAs and CNTs
J. Phys. Chem. B, Vol. 112, No. 30, 2008 8903
TABLE 2: Thermal and Electrochemical Properties of the Polymers and Hybrids
sample
Td (°C)a
ceramic yield (500 °C, %)b
ceramic yield (900 °C, %)b
Epc (V)c
Epa (V)c
E1/2 (V)d
∆Ep (mV)e
P1
P2(5)
P2(10)
P1/MWNT
342
320
330
346
52.8
42.6
31.6
57.1
34.1
8.3
16.0
47.5
0.556
0.546
0.540
0.594
0.651
0.633
0.635
0.632
0.604
0.588
0.588
0.613
95
87
95
38
a Temperature for 5% weight loss determined by TGA under nitrogen. b Obtained from TGA results. c Determined by CV analysis, where Epa
is the potential of the anodic peak current and Epc is the potential of the cathodic current. d Formal redox potential E1/2 ) (Epa + Epc)/2. e ∆Ep
) Epa - Epc.
Figure 11. TGA thermograms of P1, P2(5), P2(10), MWNTs, and
P1/MWNTs under nitrogen at a heating rate of 10 °C/min.
Figure 13. XPS spectra of (a) ceramic product from P1 and (b) ceramic
product from P1/MWNT hybrid under argon at 1000 °C for 1 h.
very small hysteresis loop with a coercive force (Hc) of 32 Oe
is recorded. However, for the ceramic obtained from P1/MWNT
hybrid (curve b), a much more obvious hysteresis loop can be
observed, and a coercive force of 304 Oe was recorded. Another
characteristic observed in Figure 15 is the difference in
saturation magnetization (Ms) between the two samples. The
Ms values of the ceramics derived from P1 and P1/MWNT
hybrid are 29.9 and 26.9 emu/g, respectively. At first sight, it
seems that the ceramic from P1 has a higher Ms than that from
the hybrid product. In fact, the difference may be largely
ascribed to the relative content of γ-Fe2O3 in these two samples.
In the case of P1/MWNTs hybrid, the component of MWNTs
should also be taken into account. The weight content of
MWNTs in original P1/MWNTs hybrid is 22.5% (see Figure
7), and based on the data of TGA analysis and the ceramic yields
of P1 and P1/MWNTs, after pyrolysis, there is no less than
18.0% MWNTs pyrolytic product in the resulting powders.
Since MWNTs component has no contribution to the magne-
tization at all, the net Ms rendered from the component of P1
in P1/MWNT hybrid is about 32.8 emu/g, which is over 9.7%
higher than that of P1. Even taking the residual iron catalyst
embedded in the MWNTs into account, because of the trace
amount, it still can not give rise to such an evident increase.
Yet, the origin of this magnetization increase has not been
elucidated and needs further investigation.
Figure 12. XRD patterns of the ceramics obtained from P1 and P1/
MWNTs. The vertical lines are the reference data for the γ-Fe2O3
standard taken from the Power Diffraction File 39-1346, International
Centre for Diffraction Data (ICDD).
confirm the uniform dispersion of the functionalized CNTs in
polymer matrix.
Magnetic Property. As mentioned above, the ceramic
particles are magnetic and can be attracted by a magnet bar.
Thus we used a Quantum Design VSM to investigate their
magnetization behaviors in externally applied magnetic field
with a field strength ranging from -9 to 9 kOe. The magnetiza-
tion as a function of magnetic field strength is shown in Figure
15. For both ceramic samples derived from P1 and P1/MWNT
hybrid, the magnetization rapidly increased with the increase
of the field strength and became leveled off at ∼8.8 or -8.8
kOe. These behaviors indicate that they are superparamagnetic
materials. For both of the samples, hysteresis behaviors are
observed. For the ceramic generated from P1 (curve a), only
Conclusions
Ferrocene-containing PPAs have been synthesized by using
organorhodium complexes [Rh(diene)Cl]2 and Rh+(nbd)-
[C6H5B-(C6H5)3] as catalysts. Their structures have been
thoroughly characterized by a combination of IR, 1H NMR, 13
C
NMR, and UV techniques. Attachment of ferrocene pendants
onto PPA backbone conferred a few noticeable properties on
the resultant polymers. First, the polymers (P1, P2(5), and

8904 J. Phys. Chem. B, Vol. 112, No. 30, 2008
Yuan et al.
Figure 14. (A,B and D,E) SEM graphs of ceramic products from P1 and P1/MWNT hybrid under argon at 1000 °C for 1 h, respectively; (C,F)
EDX spectrum of ceramic products from P1 and P1/MWNT hybrid, respectively.
the best results reported for the magnetic nanocomposites
obtained by the attachment of magnetic nanoparticles onto
CNTs.35 With a combination of excellent solubility, good film
formability, and improved stability of the hybrids, our approach
offers a versatile route to large-area, redox-active substrates,
optical-limiting matrices, and magnetic films, which may find
an array of high-tech applications.
Acknowledgment. The work reported in this paper was
partially supported by the Ministry of Science & Technology
of China (Project No. 2002CB613401), the National Natural
Science Foundation of China (Project Nos. 20634020 and
50573065), and the Research Grants Council of Hong Kong
(Project Nos. 602706, HKU2/05C, 603505, and 603304). B.Z.T.
thanks the Cao Guangbiao Foundation of the Zhejiang Univer-
sity for support.
Supporting Information Available: Figures showing supple-
mentary TEM and SEM images for the P1/SWNT hybrid and
absorption and emission spectra of P1, CNTs, and their hybrids
in THF. This material is available free of charge via the Internet
at http://pubs.acs.org.
Figure 15. Magnetization curves for the ceramic products obtained
from (a) P1 and (b) P1/MWNT hybrid at room temperature (∼25 °C).
P2(10)) can efficiently wrap around both SWNTs and MWNTs
to generate hybrids, which are highly soluble in common organic
solvents. The solubilities of P1-functionalized SWNTs and
MWNTs in THF are up to 633 and 967 mg/L, respectively. In
comparison with M1, P1 showed a profound “polymer effect”.
Additionally, the solvating powers of P1 are much higher than
the sum of those for PPA and M1. We ascribed the high
solvating powers to the “synergistic effect” of the PPA skeleton
and ferrocene pendants.
The second merit of P1/MWNT is its noticeable optical-
limiting property. At high incident laser fluence, the optical-
limiting power of P1/MWNT is higher than that of C60, which
is considered as a representative optical-limiting material. Third,
the CV measurement results demonstrated that the polymers
and the hybrids inherited the unique redox activity of ferrocene
pedants. The π-π interactions between CNTs and ferrocene
motifs induced the significant wrapping of polymer chains onto
CNTs. Therefore, the loading of CNTs into soluble ferrocene-
containing PPAs provides a route to immobilization of multiple
redox active centers on tractable one-dimensional CNTs.
Finally, after pyrolysis, the polymers and hybrids were
changed into ceramics. All these ceramic samples are super-
paramagnetic materials as revealed by the measurement results
on a Quantum Design VSM. The saturation magnetization (Ms)
of the ceramics derived from P1 and P1/MWNT hybrid is 29.9
and 26.9 emu/g, respectively. The latter datum is in the list of
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