Photo-induced electron transfer in a pyrenylcarbazole containing polymer-multiwalled carbon nanotube composite

Article abstract of DOI:10.1039/c3nj41086b

A polymer incorporated with a pyrenylcarbazole pendant poly(12-(3-(pyren-1- yl)-carbazol-9-yl)dodecyl methacrylate, denoted as PCP) was synthesized and applied in the functionalization of multi-walled carbon nanotubes (MWCNTs) by noncovalent π-π interaction. The PCP-MWCNT hybrids were isolated and characterized by SEM, TEM, and UV-visible absorption and emission spectroscopies. The strong interaction between PCP and the MWCNT in a 1,1,2,2-tetrachloroethane (TCE) solution was studied. It demonstrated an effective quenching effect on emission from PCP by the MWCNTs. DFT calculations showed electron delocalization between the pyrene and carbazole moieties. The LUMO of PCP is mainly located on the pyrene moiety while the LUMO + 1 is predominantly positioned on the carbazole moiety. Femtosecond transient absorption (TA) experiments determined the characteristic TA peaks of the excited states, which have contributions from both the pyrene and carbazole moieties. The excited state lifetime of the polymer PCP was measured to be 659 ps and the photo-excited electrons were injected into the MWCNTs very effectively on a time scale of 420 fs.

Full text of DOI:10.1039/c3nj41086b

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Photo-induced electron transfer in a pyrenylcarbazole
containing polymer–multiwalled carbon nanotube
composite†
Cite this: NewJ.Chem., 2013,
37, 1833
a
a
Lihong Yu,z Kin Cheung Lo,z Jingyu Xi,b David Lee Phillips*a and Wai Kin Chan*a
A polymer incorporated with a pyrenylcarbazole pendant poly(12-(3-(pyren-1-yl)-carbazol-9-yl)dodecyl
methacrylate, denoted as PCP) was synthesized and applied in the functionalization of multi-walled
carbon nanotubes (MWCNTs) by noncovalent p–p interaction. The PCP–MWCNT hybrids were isolated
and characterized by SEM, TEM, and UV-visible absorption and emission spectroscopies. The strong
interaction between PCP and the MWCNT in a 1,1,2,2-tetrachloroethane (TCE) solution was studied. It
demonstrated an effective quenching effect on emission from PCP by the MWCNTs. DFT calculations
showed electron delocalization between the pyrene and carbazole moieties. The LUMO of PCP is mainly
located on the pyrene moiety while the LUMO + 1 is predominantly positioned on the carbazole
moiety. Femtosecond transient absorption (TA) experiments determined the characteristic TA peaks of
the excited states, which have contributions from both the pyrene and carbazole moieties. The excited
state lifetime of the polymer PCP was measured to be 659 ps and the photo-excited electrons were
injected into the MWCNTs very effectively on a time scale of 420 fs.
Received (in Montpellier, France)
27th November 2012,
Accepted 30th April 2013
DOI: 10.1039/c3nj41086b
www.rsc.org/njc
have been widely used as electron acceptors in BHJ systems and
power conversion efficiencies (PCEs) as high as 6 to 7% have
Introduction
been achieved.10–12 However, the PCE of polymer solar cells is
still significantly lower (due to the extremely short exciton
diffusion length13 in polymers) than that of their inorganic
counterparts, such as Si,14–16 CdTe,17,18 and CIGS19–22 (copper
indium gallium selenide). The use of carbon nanotube (CNT)
derivatives as charge transport materials in OPV has been
explored in recent years. CNTs have drawn much attention
because their electronic properties make them ideal candidates
for optoelectronic applications.23–25 The introduction of a one-
dimensional (1D) carbon nanotube into solar cells not only
provides charge carriers with highly conductive pathways and
high carrier mobility, but also offers a broad absorption band
that extends into the near-infrared range due to the small
energy bandgap of the CNTs.26–32 However, the lack of solubility
and the poor dispersibility of pristine CNTs in aqueous solutions
arising from their strong hydrophobicity and van der Waals attrac-
tions have impeded their applications.33,34 Different functional
groups can be introduced onto the CNT surface by covalent
functionalization, which involves the chemical modification of
the graphene surface of CNTs, which usually results in the
degradation of the mechanical and electrical performance of the
CNTs.35–37 Recently, we have demonstrated the functionalization
of a CNT surface by a multifunctional block copolymer consisting
Considering the need for energy sustainability and environmental
protection, solar energy is the largest carbon-neutral energy source
to be explored and utilized more extensively than before.1 The
organic photovoltaic (OPV) materials and devices are envisioned to
exhibit advantages such as low cost, high device flexibility, and
fabrication from highly abundant materials to provide vital alter-
natives to their inorganic counterparts.2 Most of the OPVs reported
today are based on conjugated polymers as the active materials,
which have been fabricated into different device architectures.3–7
The most widely studied polymer solar cell is based on solution
processed bulk-heterojunction (BHJ)8 systems utilizing the concept
of photoinduced charge transfer from a conjugated polymer donor
to a fullerene acceptor.9 In these devices, the collection and
transport of charges remains one of the most critical issues, which
greatly affects the device performance. Fullerene and its derivatives
a Department of Chemistry, The University of Hong Kong, Pokfulam Road,
Hong Kong, P. R. China. E-mail: phillips@hku.hk, waichan@hku.hk
b Laboratory of Advanced Power Sources and Key Laboratory of Thermal
Management Engineering and Materials, Graduate School at Shenzhen,
Tsinghua University, Shenzhen 518055, China
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
c3nj41086b
‡ LY and KCL contributed equally to this work.
c
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of a pyrene containing block and a photosensitizing ruthenium transient absorption (TA) spectroscopy. Our findings reveal that
complex block. The copolymer functions as both a CNT dispersant the pyrenylcarbazole containing polymer may act as an electron
and a photosensitizer. Upon the formation of the polymer–CNT donor and CNTs may act as an electron accepting material in
composite, an enhancement in the photocurrent by the ruthenium CNT based OPVs.
complex was observed upon light irradiation.38
In this paper, we report the synthesis of a polymer (PCP)
functionalized with carbazole substituted units. Carbazole is a well
known electron donor unit in molecules that exhibit intramolecular
Results and discussion
Synthesis
electronic energy transfer processes. The pyrenylcarbazole moieties The synthetic route of the target polymer PCP is shown in
in the polymer served as the anchoring units. In order to Scheme 1. 1-Bromopyrene was synthesized by in situ oxidation
understand the fundamental photophysical processes of the followed by bromination with hydrobromic acid/hydrogen
polymer–CNT composite and the electronic interactions peroxide.39 1-Bromopyrene was converted to the corresponding
between the pyrenylcarbazole and the CNT surface, the polymer boronic ester 1 by lithiation.40 3-Bromocarbazole was obtained
was studied by various ultrafast spectroscopic techniques. The by mono-bromination of carbazole,41 and the product was
photophysical processes regarding the charge generation subjected to an alkylation reaction by 12-bromododecan-1-ol.
and the transport dynamics were monitored by femtosecond Monomer 4 was obtained by the subsequent nucleophilic acyl
Scheme 1 Synthesis of PCP.
c
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substitution reaction between compound 3 and methacryloyl image of PCP–MWCNT composite obtained from the TCE
chloride. Methacrylic monomers are considered to be activated solution. After dispersion with PCP, the diameter of the nano-
monomers in reversible addition-fragmentation chain transfer tubes obtained increases significantly due to the presence of a
(RAFT) polymerization,42 and the chain transfer agent 5 is a layer of amorphous polymeric material on the surface of the
commonly used dithioester that has a good control of the RAFT MWCNTs (Fig. 1b). The TEM image observed has an irregular
polymerization towards methacrylic monomers.43–46 Polymeriza- surface due to the presence of the polymer, and it is easy to
tion of 4 was conducted in chlorobenzene and the monomer to recognize the dense plaques around the regular MWCNT
initiator ratio was kept at 9. From the GPC chromatogram of the surface. Such an observation is similar to what was observed in
polymer PCP obtained (Fig. S1, ESI†), the number average mole- other polymer–MWCNT composites.38 The dispersion obtained
cular weight and polydispersity were calculated to be 6470 and from the TCE solution has a black color, and was stable for more
1.10, respectively, which correspond to a number averaged degree than one week without the formation of a precipitate. However,
of polymerization of B8–9. This clearly shows a good molecular if the dispersion process was done in a chloroform solution, the
weight control in the RAFT polymerization.
dispersion formed was highly unstable, and the aggregates
formed almost immediately (see Fig. S3 in ESI†). Fig. 1c shows
the images of different solutions of MWCNTs and PCP/MWCNT
dispersed in CHCl3 and TCE. For the vial with chloroform
solution, only a very light color solution was observed after
removal of the precipitate formed. For the vial with TCE
solution, the dark color persisted for a long time after disper-
sion, and no precipitate was observed at the bottom of the vial.
The results suggest a good solvation of PCP/MWCNT in TCE,
which formed a stable dispersion. The stability of the disper-
sion may be due to the stronger solvent–polymer–MWCNT
interactions, but the reason is not clear. In view of the stability
of the solution, all the measurements on the photophysical
properties were conducted in TCE solution.
Fig. 2a shows the UV-vis absorption spectra of PCP and PCP/
MWCNT in different solvents. Pyrene shows intense absorption
peaks at 340 and 275 nm, and carbazole shows strong absorp-
tion at ca. 290 nm and weaker absorption bands at 310–320 and
330–350 nm.51–54 In the absorption spectrum of PCP, the peaks
at 269, 280 and 350 nm are assigned to the absorption due to
the pyrene and carbazole moieties, and some of the weaker
absorptions are observed as shoulders. On the other hand, the
absorption spectrum of PCP/MWCNT in TCE is dominated by
the characteristic absorption of the MWCNTs, which displays a
very broad and intense absorption in the range between 300 nm
and 800 nm.55–57 Nevertheless, the absorption due to the pyrene
and carbazole moieties can still be identified by the presence of a
peak and a shoulder at ca. 270 and 370 nm, respectively. The shift
in the absorption baseline for PCP/MWCNT in TCE indicates the
rich presence of MWCNTs in the solution, and may also be
contributed from undispersed MWCNTs.
Characterization
The 1H NMR spectrum of PCP (Fig. S2, ESI†) shows similar
spectral features as an analogous spectrum of monomer 4 but
the bands are generally broadened. The broad bands from
7 ppm to 9 ppm were assigned to the protons on the pyrene
and the carbazole moieties, while the peaks at ca. 4 to 4.5 ppm
and 1 to 1.8 ppm are assigned to the protons of the methylene
units. The absence of peaks from 5 to 7 ppm implies the absence
of vinylene protons due to the monomer after purification.
PCP can be used as an effective dispersant for MWCNTs due
to the strong p–p interaction between the pyrene moiety and
MWCNTs.47–50 Upon mixing PCP and MWCNTs (10 : 1 wt ratio)
in 1,1,2,2-tetrachloroethane (TCE) solution, most of the
MWCNTs were dispersed and a black color solution was
obtained. The morphology of the polymer–MWCNT composite
formed was studied by TEM. The length and diameter of the
commercially available MWCNTs before treatment with PCP
are 5–20 mm and 15 nm respectively. Fig. 1a shows the TEM
image of a MWCNT without PCP and Fig. 1b shows the TEM
The thermal stability of the polymers and composites was
investigated by thermal gravimetric analysis (TGA) under a
nitrogen atmosphere. In the TGA thermograms shown
(Fig. 2b), pristine MWCNTs have an initial decomposition
temperature at 550 1C, while the onset of decomposition of
PCP is observed at 400 1C and it completely decomposes at
500 1C with almost no residue left. The weight loss below 400 1C
may be due to the evaporation of residual solvents. For the
PCP–MWCNT composite, the initial decomposition tempera-
ture is the same as PCP, but a weight residue of 20% was
obtained at 700 1C. From these data, it can be estimated that
Fig. 1 (a) TEM image of a pristine MWCNT. (b) TEM image of PCP/MWCNT
obtained from TCE solution. (c) Image of MWCNTs in TCE; and PCP/MWCNT in the content of PCP in the composite PCP–MWCNT is
TCE (1.75 mg mlꢀ1) and CHCl3.
approximately 75%.
c
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Fig. 2 (a) UV-vis absorption spectra of PCP (0.025 mg mlꢀ1) and PCP/MWCNT
(MWCNTs 0.02 mg mlꢀ1) in TCE solutions. (b) TGA analysis of MWCNTs, PCP, and
PCP/MWCNT respectively, under a nitrogen atmosphere, with a speed of 10 1C per min.
Fig. 3 Emission spectra of (a) pyrene and carbazole; (b) PCP, PCP/MWCNT, and
compound 3 in 1,1,2,2-tetrachloroethane (excitation wavelength = 266 nm).
Fluorescence quenching effects
geometry optimization and vibrational frequency calculations. Only
the 12-acetyloxydodecyl substituted pyrenylcarbazole segment in
PCP was used as the model compound in the calculation to
estimate the electronic energy levels and electron density, because
this is the unit that interacts strongly with the MWCNT. Fig. 4
shows the frontier orbitals and energy levels of the 12-acetoxydo-
decyl pyrenylcarbazole unit obtained from the DFT calculations.
The standard orientation of the unit is presented in Table S1 (ESI†).
As expected, the long acetyloxydodecyl chain has no contribution to
the frontier orbitals HOMO, LUMO and LUMO + 1 in the molecule.
The HOMO is located both on the pyrene and carbazole units. It
can be seen that the pyrene and carbazole rings form a dihedral
angle of 601 and they have some degree of conjugation in the
HOMO electron distribution. On the other hand, the LUMO is
mainly located on the pyrene unit, while the LUMO + 1 is
predominantly on the carbazole unit. This may partially explain
in Fig. 3 that the emission band of PCP is similar to that of pyrene,
while the emission features of carbazole are not observed.
Fig. 3 shows the fluorescence spectra of PCP and PCP/MWCNT
collected in TCE solutions. The fluorescence spectra of pure pyrene,
carbazole and compound 3 are also shown for comparison.
Both pyrene and carbazole show very strong emission bands at
ca. 380–400 and 320–370 nm, respectively.58,59 PCP shows one
single broad emission band centered at 430 nm, which is substan-
tially red-shifted compared to those of carbazole and pyrene. The
emission spectrum of PCP is similar to that of compound 3 in both
dilute and concentrated solutions, which suggests that the pyrene
moieties in PCP do not form excimers. The sharp emission bands
from pyrene or carbazole moieties are not observed in the emission
spectrum of PCP. This may be due to the conformational change
of the pyrene and carbazole units in PCP. After forming the PCP–
MWCNT composite, the emission was almost completely quenched
compared to that from PCP (Fig. 3, bottom). This suggests the
presence of a strong electronic interaction between the excited state
of PCP and MWCNTs.
DFT calculations
Electrochemical properties
The electronic structures of PCP were studied by DFT calculations, The electrochemical properties of PCP were studied by cyclic
which were performed at the B3LYP/6-311G** level of theory for the voltammetry (CV), and its HOMO and LUMO levels were
c
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Fig. 4 Frontier orbitals (left) and energy levels (right) of pyrenylcarbazole model compound obtained from the DFT calculations.
estimated by comparing the redox potential observed with levels of PCP obtained by CV and by DFT calculations. In general,
that of ferrocene acting as the internal standard. The cyclic the experimental and theoretical results agree very well with each
voltammogram is shown in Fig. 5. The onset oxidation other, except that the LUMO level obtained by the DFT calculations
potential (0.52 V) was used to estimate the HOMO level of is higher than that obtained from CV. This discrepancy may be
PCP, which was calculated by comparing this potential with the attributed to the fact that when calculating the HOMO–LUMO
ferrocene/ferrocenium (Fc/Fc+) couple using the relationship: gap directly from the optical bandgap, factors such as Coulomb
onset
EHOMO = ꢀ(E
+ 4.8)60 and ELUMO = EHOMO + Optical Band and exchange integral between HOMO and LUMO are not taken
oxi
Gap.61 The optical bandgap was estimated from the absorption into account. In addition, in the calculations where only one
band edge of PCP. Two irreversible anodic waves at 0.8 and repeating unit of PCP was involved, some approximations in
1.4 V are observed, which are assigned to the oxidation of the the DFT calculations, such as Coulombic and exchange inter-
pyrenylcarbazole units. Table 1 summarizes the HOMO/LUMO actions, and vibronic coupling, have also been adopted.5,62
Fig. 5 Cyclic voltammogram of PCP.
c
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Table 1 HOMO–LUMO levels of PCP estimated from cyclic voltammetry and
Transient absorption spectroscopy and electron transfer
kinetics
from DFT calculations (based on the model compound shown in Fig. 4)
LUMO level (eV)
HOMO level (eV)
Bandgap (eV)
Fig. 6 shows the temporal evolution of the femtosecond TA
spectra for PCP and PCP–MWCNT. The TA spectra of pyrene,
carbazole, and pure TCE are also shown for comparison. In
Fig. 6a, pyrene shows two strong TA peaks at 370 nm and
480 nm and with a weak shoulder at ca. 520 nm. This is
DFT
CV and Eop
ꢀ1.69
ꢀ2.15
ꢀ5.29
ꢀ5.32
3.60
3.17
Compared with our previously reported pyrene-containing polymer, consistent with the literature reported spectra.64 Carbazole
PCP has a higher HOMO level (ꢀ5.32 vs. ꢀ5.83 eV),38 which may be shows a strong and broad TA band centered at 625 nm. From
due to the presence of a partially conjugated system. The conjuga- Fig. 6b, it can be seen that pure TCE solvent demonstrates no
tion also results in a smaller bandgap of PCP (3.17 eV) than that of TA band in the region between 500–650 nm, which has no
poly(N-vinylcarbazole) (3.53 eV).63
overlap with the region which we studied. In Fig. 6c, PCP shows
Fig. 6 (a) Femtosecond TA spectra of pyrene and carbazole. (b) Femtosecond TA spectra of pure TCE. (c) and (d) Temporal evolution of TA spectra for PCP. (e) and (f)
Temporal evolution of TA spectra for PCP/MWCNT. All measurements were done in 1,1,2,2-tetrachloroethane solutions and the excitation wavelength was 266 nm.
c
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the growth of a strong and broad TA band centered at 610 nm
and a negative TA band centered at 430 nm. By comparing
Fig. 6a and c, the TA band at 610 nm may be assigned to the
contribution from the carbazole moieties in PCP. The negative
TA band at 430 nm may be due to the induced emission of PCP.
A shoulder is also noticed at 490 nm in Fig. 6c. It grows to a
maximum at 1.9 ps and disappears rapidly within 30 ps. The
shoulder at 490 nm may be due to some intermediate states
between the higher excited state and the final longer living S1
state, which is supported by the observation of an isosbestic
point between the absorption of this intermediate and the S1
state at 520 nm. This intermediate is not observed in the TA
decay profile of PCP in Fig. 6d. Fig. 6e and f show the TA growth
and decay profile of the PCP–MWCNT composite. The disordered
spectra before 500 nm are assigned to the temporal evolution of
TA spectra of TCE, which agrees well with the spectra shown in
Fig. 6b. In Fig. 6e and f, only the characteristic bands due to the
carbazole moieties are observed, while the TA bands due to
the pyrene units are not detected. Besides, in the TA spectra of
PCP/MWCNT, no negative band of emission is observed. This
result suggests that the excited state of the pyrene unit does not
undergo decay through emission. Instead, electrons may be
transferred to the MWCNT and such an electron transfer process
is too rapid to be detected on the femtosecond time scale. By
comparing Fig. 6d and f, it can be seen that the TA decay of PCP is
three orders of magnitude more rapid in the presence of the
MWCNT. The decay process of PCP lasts for approximately 3 ns,
while for PCP/MWCNT it is only 3 ps. This strongly suggests
that there is a rapid electron transfer from the carbazole
localized excited state to the MWCNT.
Fig. 7 (a) TA kinetics of PCP and PCP/MWCNT in 1,1,2,2-tetrachloroethane
solutions, (b) the same profiles in a shorter period. Wavelength of the pump
beam = 266 nm (power = 30 mW), wavelength of the probe beam = 610 nm.
The electron transfer from the excited states of PCP to the
MWCNT may be explored by the TA kinetics. Fig. 7 shows the
TA kinetics of PCP and PCP/MWCNT in both a longer and a
shorter time window. From Fig. 7a, the TA of PCP has a slow
decay profile and it does not decay to zero within 2000 ps, while
for PCP/MWCNT, the TA decays very fast and it cannot be
observed precisely in the longer time window. From Fig. 7b, it can
be seen that the TA of PCP/MWCNT decays completely in less
than 5 ps. These TA kinetics are both fitted well by multi-
exponential functions that include growth and decay processes.
The multi-exponential function used is defined to be
Table 2 Fitting results of TA kinetics for PCP and PCP/MWCNT calculated from
Fig. 7
Time constant
tg (ps)
td1 (ps)
td2 (ps)
t (ps)
t1/2 (ps)
PCP
PCP/MWCNT
7.78
—
108
0.40
992
16.2
659
—
—
0.42
from excited PCP to the MWCNTs (0.42 ps). The time constant
of the TA growth process for PCP/MWCNT is too short to be
I = Ag exp(ꢀt/tg) + Ad1 exp(ꢀt/td1) + Ad2 exp(ꢀt/td2
)
where I, A, t, tg, td represent TA intensity, constants, decay time, calculated due to a rapid decay, while for PCP it is calculated to
time constant for the growth process, and time constant for the be 7.8 ps. This suggests that the electron transfer to the MWCNTs
decay process, respectively. The lifetime of the excited state t is may occur from a higher excited state of PCP before relaxation to
defined by
the S1 state, since the S1 state has a longer growing time than the
electron injection time (7.8 ps vs. 0.42 ps).
t = td1[Ad1/(Ad1 + Ad2)] + td2[Ad2/(Ad1 + Ad2)].
The electron injection process from excited PCP to the
MWCNTs may be quantified by the half-lifetimes (t1/2), which
Conclusions
is defined as the time when the amplitude of the decay curve A pyrenylcarbazole containing polymer (PCP) was synthesized
decreases to half of the initial value.65 The fitting results are and used as the dispersing agent for MWCNTs. The polymer
summarized in Table 2. The lifetime of the excited state of PCP could disperse MWCNTs efficiently by the formation of p–p
is 659 ps, which is much longer than the electron injection time stacking between the pyrenylcarbazole and the MWCNTs surface
c
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in a TCE solution. DFT calculations showed that the HOMO of the were collected using a Philips Tecnai G2 20 S-TWIN scanning
polymer spreads around the whole aromatic ring, while the LUMO transmission electron microscope. Samples for TEM were pre-
and LUMO + 1 of the polymer are mainly located on the pyrene and pared by dispersing the composite in TCE onto a carbon-
carbazole units, respectively. Femtosecond transient absorption coated 400-mesh hexagonal copper grid at room temperature.
(TA) spectra demonstrate the characteristic TA peaks of excited TGA thermograms were collected using a thermogravimetric
states of PCP contributed from the pyrene and the carbazole analyzer (NETZSCH STA 449F3) under a nitrogen atmosphere at
moieties. In the PCP–MWCNT hybrids, the excited state electrons a heating rate of 10 1C per min. Cyclic voltammetry was
are rapidly transferred to the MWCNTs, and the fluorescence of performed on an eDAQ EA161 potentiostat. A 3 mm glassy
PCP was effectively quenched. The lifetime of the excited state of carbon electrode was used as the working electrode, a platinum
PCP is 659 ps and the electron injection time to the MWCNTs is foil was used as the reference electrode and a platinum wire
420 fs. Based on the results obtained in this work, it is possible was used as the auxiliary electrode. 0.1 M tetrabutylammonium
to design new polymeric materials such as block copolymers for hexafluorophosphate and deoxygenated HPLC grade dichloro-
light harvesting applications. Other light harvesting units may be methane was used as the supporting electrolyte and solvent,
incorporated into a PCP main chain, and the resulting materials respectively.
may serve as potential candidates for photosensitizers and electron
DFT calculations
acceptors in polymer solar cells.
The geometry optimization, vibrational frequency and DFT
calculations were done using the B3LYP method with a
6-311G** basis set for the singlet state. All of the density
functional theory calculations presented in this work made
use of the Gaussian 03 program suite66 installed on the High
Performance Computing cluster at Computer Centre in The
University of Hong Kong.
Experimental section
The synthetic procedures for making the monomer 4, the chain
transfer agent 5, and other intermediates are described in ESI.†
Synthesis of poly(12-(3-(pyren-1-yl)-9H-carbazol-9-yl)dodecyl
methacrylate) (PCP)
Monomer 4 (0.592 g, 0.955 mmol), 2-cyanoprop-2-yl dithio-
benzoate 5 (0.021 g, 0.106 mmol) and AIBN (8 mg, 0.0053 mmol)
were dissolved in chlorobenzene (2.3 ml) in a 10 cm3 ampoule
under a nitrogen atmosphere. The solution was degassed by
three freeze–pump–thaw cycles and sealed under vacuum. The
mixture was heated at 60 1C in the dark for 24 hours. The crude
product was precipitated in methanol (250 cm3) and the solid
was obtained by filtration. The solid was re-dissolved in a
minimum amount of chloroform and reprecipitated in metha-
nol again. The precipitation procedure was repeated twice. The
purified polymer was dried under vacuum and appeared as a
pink solid. Yield: 0.491 g (80%).
Transient absorption measurements
The femtosecond TA experiment was carried out using a
femtosecond Ti:sapphire regenerative amplified Ti:sapphire
laser system (Spectra Physics, Spitfire-Pro) and an automated
data acquisition system (Ultrafast Systems, Helios). The pump
laser was of 267 nm (the third harmonic of the fundamental
800 nm from the regenerative amplifier). The probe pulse was a
white-light continuum (300–800 nm) generated by a CaF2
crystal using approximately 5% of the original output from
the Spitfire. Before passing through the sample the probe pulse
was split into two beams: one beam travels through the sample,
and the other is sent directly to the reference spectrometer that
monitors the fluctuation of the probe beam intensity. The instru-
ment response function is evaluated to be 150 fs. The signals of all
samples TA experiments were reproducible, and therefore the
accumulation of photo-catalytic products was negligible.
Preparation of the PCP–MWCNT composite
MWCNTs (0.5 mg) and PCP (10 mg) were dispersed in 6 ml of
solvent (TCE or chloroform), and the mixture was ultrasonicated
for 1 h at room temperature. The sediment was removed by a
filter paper and the light black sediment on the filter paper was
mainly undispersed MWCNTs which were discarded. The
resulting solution obtained was filtered using a PTFE membrane
filter (0.2 mm) to remove any excess polymer from solution. The
PCP/MWCNT remaining on the surface of the membrane filter
was then redispersed in TCE solution by ultrasonication. The
resulting PCP/MWCNT dispersion in TCE was stable at room
temperature for more than one week.
Acknowledgements
The work described in this paper was substantially supported
by the University Grants Committee of the Hong Kong Special
Administrative Region, China (Project No. T23-713/11, HKU
7039/07P, HKU 7003/11P, N_HKU705/10, Special Equipment
Grant SEG-HKU-07). J. Xi acknowledged the support from the
National Natural Science Foundation of China (20973099).
Measurements
UV-vis absorption spectra were collected on a U-2910 spectro-
meter (Hitachi, Japan). Fluorescence spectra were collected on
an F-7000 fluorescence spectrophotometer (Hitachi, Japan).
SEM images were collected using a field emission scanning
electron microscope (FE-SEM, Hitachi S-4800) and TEM images
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