Facile decoration of functionalized single-wall carbon nanotubes with phthalocyanines via click chemistry

Article abstract of DOI:10.1021/ja8033262

We describe the functionalization of single-wall carbon nanotubes (SWNTs) with 4-(2-trimethylsilyl)ethynylaniline and the subsequent attachment of a zinc-phthalocyanine (ZnPc) derivative using the reliable Huisgen 1,3-dipolar cycloaddition. The motivation of this study was the preparation of a nanotube-based platform which allows the facile fabrication of more complex functional nanometer-scale structures, such as a SWNT-ZnPc hybrid. The nanotube derivatives described here were fully characterized by a combination of analytical techniques such as Raman, absorption and emission spectroscopy, atomic force and scanning electron microscopy (AFM and SEM), and thermogravimetric analysis (TGA). The SWNT-ZnPc nanoconjugate was also investigated with a series of steady-state and time-resolved spectroscopy experiments, and a photoinduced communication between the two photoactive components (i.e., SWNT and ZnPc) was identified. Such beneficial features lead to monochromatic internal photoconversion efficiencies of 17.3% when the SWNT-ZnPc hybrid material was tested as photoactive material in an ITO photoanode.

Full text of DOI:10.1021/ja8033262

Published on Web 07/29/2008
Facile Decoration of Functionalized Single-Wall Carbon
Nanotubes with Phthalocyanines via “Click Chemistry”
Ste´phane Campidelli,*,† Beatriz Ballesteros,‡ Arianna Filoramo,† David D´ıaz D´ıaz,‡
Gema de la Torre,‡ Toma´s Torres,*,‡ G. M. Aminur Rahman,§ Christian Ehli,§
Daniel Kiessling,§ Fabian Werner,§ Vito Sgobba,§ Dirk M. Guldi,*,§ Carla Cioffi,#
Maurizio Prato,# and Jean-Philippe Bourgoin†
Laboratoire d’Electronique Mole´culaire, DSM/IRAMIS/SPEC (CNRS URA 2464), CEA Saclay,
F-91191 Gif sur YVette Cedex, France, Departamento de Qu´ımica Orga´nica, UniVersidad
Auto´noma de Madrid, Cantoblanco, E-28049 Madrid, Spain, Department of Chemistry and
Pharmacy & Interdisciplinary Center for Molecular Materials, Friedrich-Alexander-UniVersita¨t
Erlangen-Nu¨rnberg, Egerlandstrasse 3, D-91058 Erlangen, Germany, and INSTM, Unit of
Trieste, Dipartimento di Scienze Farmaceutiche, UniVersita` di Trieste, Piazzale Europa, 1,
I-34127 Trieste, Italy
Received May 5, 2008; E-mail: stephane.campidelli@cea.fr; tomas.torres@uam.es
Abstract: We describe the functionalization of single-wall carbon nanotubes (SWNTs) with 4-(2-
trimethylsilyl)ethynylaniline and the subsequent attachment of a zinc-phthalocyanine (ZnPc) derivative using
the reliable Huisgen 1,3-dipolar cycloaddition. The motivation of this study was the preparation of a nanotube-
based platform which allows the facile fabrication of more complex functional nanometer-scale structures,
such as a SWNT-ZnPc hybrid. The nanotube derivatives described here were fully characterized by a
combination of analytical techniques such as Raman, absorption and emission spectroscopy, atomic force
and scanning electron microscopy (AFM and SEM), and thermogravimetric analysis (TGA). The
SWNT-ZnPc nanoconjugate was also investigated with a series of steady-state and time-resolved
spectroscopy experiments, and a photoinduced communication between the two photoactive components
(i.e., SWNT and ZnPc) was identified. Such beneficial features lead to monochromatic internal photocon-
version efficiencies of 17.3% when the SWNT-ZnPc hybrid material was tested as photoactive material in
an ITO photoanode.
composites,7–10 energyconversion,11–17 electronicsandsensing,18–20
or biological applications.21–26 In particular, single-wall carbon
Introduction
Nanotechnologies actually represent a field that stimulates
the imagination of many people (not only scientists but also
economists and sociologists) for the development of new
materials with new structures, functionalities, and applications.1–3
Carbon nanotubes (CNTs)4–6 constitute a relatively new class
of nanostructures composed exclusively of carbon atoms.
Because of their electronic and mechanical properties, carbon
nanotubes appear to be promising materials for polymer
nanotubes (SWNTs) are one-dimensional nanowires that are
either metallic or semiconducting. They readily accept charges,
which can then be transported under nearly ideal conditions
along the tubular SWNT axis.27,28 The electrical conductivity,
morphology, and good chemical stability of SWNTs are
promising features that stimulate their integration into photo-
voltaic systems or electronic devices.
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† DSM/IRAMIS/SPEC (CNRS URA 2464) CEA Saclay.
‡ Universidad Auto´noma de Madrid.
§ Friedrich-Alexander-Universita¨t Erlangen-Nu¨rnberg.
# Universita` di Trieste.
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A R T I C L E S
Campidelli et al.
Scheme 1a
On the other hand, the attachment of phthalocyanines (Pcs)
to nanotubes36–38 and fullerenes39–41 has recently emerged as
an excellent approach to carbon nanostructure Pc-based pho-
tovoltaic and other electronic devices. Phthalocyanines are planar
electron-rich aromatic macrocycles that are characterized by their
remarkably high extinction coefficients in the red/near-infrared
region (which is an important part of the solar spectrum) and
theiroutstandingphotostabilityandsingularphysicalproperties.42–44
These features render them exceptional donor/antenna building
blocks for their incorporation in photovoltaic devices.45–49 The
development of a suitable method to easily graft phthalocyanines
or other photoactive species onto carbon nanotubes is thus an
important objective toward the realization of materials with
improved optoelectronic performances.
a (a) PBr3, CH2Cl2, rt, 2 h, 85%; (b) NaN3, THF/H2O, 70 °C, 4 h, 82%;
(c) ZnCl2, chlorobenzene/DMF, 100 °C, 3 h, 88%.
However, fabrication of nanotube-based molecular assemblies
is still limited because of the difficulty to incorporate highly
engineered molecules on the nanotube surfaces. This problematic
issue can have mainly two origins: incompatibility between the
functionality on the molecules and the conditions required for
nanotube functionalization and/or the fact that nanotube func-
tionalization requires a large excess of reagent which is difficult
or impossible to recycle. There is a real need for simple and
versatile reactions that can easily lead to nanotube-based
functional materials.
Here we describe the functionalization of SWNTs with
4-(trimethylsilyl)ethynylaniline following the procedure devel-
oped by Tour and co-workers50 and the subsequent attachment
of a zinc-phthalocyanine (ZnPc) derivative bearing an azide
group. We show how the Huisgen cycloaddition reaction on
functionalized nanotubes yields new nanotube-based molecular
assemblies and overcomes the problems of functionalization of
carbon nanotubes listed above. The resulting SWNT-ZnPc
conjugate was fully characterized, and its photovoltaic properties
were tested.
The emerging field of “click chemistry” can bring very
elegant solutions to easily achieve nanotube-based functional
materials.29,30 The term “click chemistry”31 defines a chemical
reaction which is versatile and clean, with simple workup and
purification procedures. Among the large collection of organic
reactions, Huisgen cycloaddition, 1,3-dipolar cycloaddition
between azide and acetylene derivatives in the presence of Cu(I)
catalyst, represents the most effective reaction of “click
chemistry”.32–35
Results and Discussion
Synthesis. Synthesis of phthalocyanine 1 is depicted in
Scheme 1. Bromination of metal-free phthalocyanine 237 gave
3, which was converted into 4 by reaction with sodium azide.
Finally, addition of zinc chloride to 4 led to Zn(II) azidoph-
thalocyanine 1. The synthesis of the SWNT-ZnPc conjugate 5
is described in Scheme 2. The SWNT material used in the
present work was produced via laser ablation technique by Dr.
Oliver Jost (University of Dresden).51 The raw material contains
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Functionalized Single-Wall Carbon Nanotubes
A R T I C L E S
Scheme 2a
Figure 1. Raman spectra of the p-SWNTs 6 (black), f-SWNTs 8 (red),
SWNT-ZnPc 5 (blue), and ZnPc 1 (gray) (excitation wavelength at 457.9
and 647.1 nm). The intensities have been normalized (for SWNT derivatives)
with respect to the high frequency side of the G-band (ωG+).
absorption and emission spectroscopy, atomic force and
scanning electron microscopy (AFM and SEM), and ther-
mogravimetric analysis (TGA). The Raman spectra of the
p-SWNTs 6, f-SWNTs 8, SWNT-ZnPc conjugate 5, and
ZnPc 1 are shown in Figure 1. First of all, the Raman analysis
of p-SWNTs 6 presented a very small D-band, indicating
that most of the amorphous carbon was removed from the
nanotube and that the acid treatment was soft enough to not
damage the nanotubes. We do not observe any noticeable
effect of the functionalization on the RBM and G-band
features while we note a significant modification in the
relative intensity of the D-band after the first functionalization
step (f-SWNTs 8). The metallic nanotubes (met-SWNTs),
probed with excitation at 647.1 nm, present a higher
functionalization compared to semiconducting tubes (sem-
SWNTs) (probed with excitation at 457.9 nm, Figure 1). This
behavior is in agreement with the fact that aryldiazonium
derivatives react preferentially with metallic nanotubes.55,56
After reaction with phthalocyanine 1, the spectrum of SWNT-
ZnPc 5, obtained with excitation at 647.1 nm, presents new
features that are attributed to the ZnPc chromophore, while
the spectrum realized at 457.9 nm does not present important
changes. This difference between the spectra of 5 can have
two origins: (1) ZnPc absorbs strongly at 680 nm while it
does not absorb around 450 nm, and the perturbation of the
spectrum at 647.1 nm can be due to absorption and emission
of the ZnPc moieties in 5, and (2) met-SWNTs are more
functionalized than sem-SWNTs (see Raman of 8, red spectra
in Figure 1); the latter would imply that after cycloaddition,
the amount of ZnPc should be higher in met-SWNTs and so
the response of phthalocyanine in the spectrum at 647.1 nm
could be higher. However, the addition of phthalocyanine
does not modify the linkage on the nanotube carbon lattice
and the sp2/sp3 ratio as shown by the absence of change of
the RBM and D-bands with respect to the G-band.
a (a) Isoamyl nitrite, NMP, 60 °C, 48 h; (b) NBu4F, THF/NMP, rt, 1 h;
(c) 1, CuSO4 ·5H2O, sodium ascorbate, NMP, 70 °C, 48 h.
approximately 60-70% of SWNTs and was purified by an
optimized soft acidic treatment.52 Reaction of purified SWNTs
(p-SWNTs) 6 with 4-(2-trimethylsilyl)ethynylaniline 753 in the
presence of isoamyl nitrite in N-methylpyrrolidone (NMP) gave
functionalized nanotubes (f-SWNTs) 8 which were deprotected
by treatment with NBu4F to give 9 and then allowed to react
with azidophthalocyanine 1 in NMP in the presence of CuSO4
and sodium ascorbate to give the nanotube-phthalocyanine
assembly 5.
As we explained before, the problematic issue of function-
alization of carbon nanotubes is the excess of reagent generally
involved in the reaction; functionalization of carbon nanotubes
with diazonium derivatives required usually 4 equiv per carbon
atom or more of amine precursor or diazonium salt.50,54 Here
for example, the functionalization of SWNTs with 4 equiv per
carbon of 4-(2-trimethylsilyl)ethynylaniline is possible since this
compound is synthesized very easily from 4-iodoaniline and
ethynyltrimethylsilane; this kind of reaction cannot be envi-
sioned with high added value molecules such as phthalocya-
nines: for example, the functionalization of 1 mg of SWNTs
with 4 equiv per carbon atom of a hypothetical aminophe-
nylphthalocyanine (containing an amino group linked directly
on the phenyl group, see structure in Figure S1) Via diazonium
reaction would require 520 mg of phthalocyanine. However,
two-step methods introduce more complexity, and they require
the use of an efficient reaction, such as the Huisgen cycload-
dition, for the final coupling.
The thermogravimetric analysis showed a loss of weight of
about 19% for p-SWNTs 6, 27% for f-SWNTs 8, 47% for
SWNT-ZnPc 5, and 43% for ZnPc 1 at 600 °C (Figure 2).
The loss of weight observed for p-SWNTs between 200 and
600 °C may be due to the destruction of the residual amorphous
Characterization. The nanotube derivatives were character-
ized by standard analytical techniques such as Raman,
(55) Strano, M. S.; Dyke, C. A.; Ursey, M. L.; Barone, P. W.; Allen, M. J.;
Shan, H. W.; Kittrell, C.; Hauge, R. H.; Tour, J. M.; Smalley, R. E.
Science 2003, 301, 1519.
(52) Capes, L.; Valentin, E.; Esnouf, S.; Ribayrol, A.; Jost, O.; Filoramo,
A.; Patillon, J.-N. Proc. 2nd IEEE Conf. Nanotechnol., 2002, 439.
(53) Hwang, J.-J.; Tour, J. M. Tetrahedron 2002, 58, 10387.
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Campidelli et al.
Figure 2. Thermogravimetric analysis of ZnPc 1 (black), p-SWNTs 6
(green), f-SWNTs 8 (red), and SWNT-ZnPc 5 (blue) (10 °C/min under
N2).
carbon still present in the nanotubes and to the decarboxylation
of the oxidized species. The corrected weight losses due to the
functional groups on nanotubes were then estimated to be 8%
and 28% for f-SWNTs 8 and SWNT-ZnPc 5, respectively
(weight losses difference of f-SWNTs - p-SWNTs and
SWNT-ZnPc - p-SWNTs). The number of phenylacetylene
functional groups in 8 was then estimated as 1 per 165 carbon
atoms.57 With the same calculation (see ref 57), we estimated
the amount of functional groups as 1 per 365 carbon atoms for
SWNT-ZnPc 5. This number does not represent the reality
because at 600 °C the thermogram of ZnPc 1 presents a loss of
weight of 43%. Considering that the weight loss corresponding
to the clicked ZnPc is about 20% in SWNT-ZnPc 5
(SWNT-ZnPc - f-SWNTs), the amount of clicked ZnPc in 5
may correspond to a real ratio of ca. 47% (20%/43%). Then
the number of functional groups in this case can be estimated
to 1 phthalocyanine per 155 carbon atoms. Considering the error
range of the TGA measurements, we can estimate a similar
functionalization degree for f-SWNTs 8 and SWNT-ZnPc 5.
We believe that all the acetylenic groups have reacted with
azidophthalocyanine moieties, thus pointing out the success of
our approach.
The carbon nanotube derivatives have been investigated by
AFM and SEM. The AFM pictures of f-SWNTs 8 and
SWNT-ZnPc 5 (Figure 4) revealed the presence of individual
or very thin bundles of nanotubes; the typical lengths of the
objects are about 500 nm to 1.5 µm with diameters of about 1
to 3 nm. In SEM pictures of SWNT-ZnPc 5 (Figure 3), one
can see the presence of nanotubes as spaghetti-like aggregates.
Absorption spectra of the nanotubes (performed in SDS/
D2O) are shown in Figure 5. The spectrum of functionalized
nanotubes 8 presents a partial loss of the van Hove transitions
compared to purified SWNTs. After cycloaddition, the
spectrum of SWNT-ZnPc 5 shows two new maxima at 615
and 682 nm, which correspond to the Q-bands of the
phthalocyanine. Considering the electron donor and electron
acceptor character of ZnPc and SWNT, respectively, we
tested the SWNT-ZnPc nanoconjugate 5 in a series of
photophysical measurements. Fluorescence of the ZnPc 1 was
compared to that of SWNT-ZnPc 5 suspended in THF
solutions. Notably, the ZnPc-centered fluorescence is drasti-
Figure 3. SEM pictures of SWNT-ZnPc 5: (a) general view of the surface
with nanotubes (dark gray); (b) close-up of the region delimited with the
black rectangle; (c) picture taken at higher magnification in the center of
the aggregate.
cally quenched in SWNT-ZnPc 5 with quantum yields of
5.5 × 10-5 (compared to 0.3 for 1, see Figure 6). Such
efficient quenching implies a rapid deactivation of the
photoexcited ZnPc. It is important to note in this context
that the overall fluorescence pattern in the nanoconjugate 5
resembles that seen for the ZnPc 1. In other words, despite
being linked to SWNTs the ZnPc keep their electronic
integrity.
The fluorescence assays were complemented by transient
absorption measurements to shed light on the photoproduct that
is associated with the deactivation of the photoexcited ZnPc in
SWNT-ZnPc 5. In the ZnPc 1 the following changes occur
upon 660 nm photoexcitation: the singlet excited state, which
is formed essentially right after the laser pulse, comprises a
broad transient maximum that is centered at 490 nm followed
by bleaching between 610 and 685 nm (see Figure S2). This
transient decays rather slowly on the time scale of our
instrumental detection (i.e., 3.0 ns) by intersystem crossing to
the corresponding triplet manifold. The main spectral feature
of the latter is a maximum at 500 nm and a minimum at 680
nm. Implicit is that, especially in the near-infrared region, the
two transients, namely, the singlet and the triplet excited state,
differ largely in their absorption.
Looking at p-SWNTs 6, which were simply suspended in
THF, a set of transient minima was observed at 1050, 1185,
1310, 1435, and 1555 nm. As Figure S3 shows, all features
decay similarly to recover the ground state with two major
(57) The weight loss was estimated using the following calculation: for
example for f-SWNTs 8, the amount of functional group is (92/MC)/
(8/Mtrimethylsilyl-phenylacetylene unit) ) 165; it means 1 group per about 165
carbon atoms.
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A R T I C L E S
Figure 4. AFM pictures of f-SWNTs 8 (a) and SWNT-ZnPc 5 (b) on polylysine-coated mica. General view on the left with close-up of the surface and
a line profile showing the height of the nanotubes on the middle and on the right.
Figure 5. Absorption spectra of ZnPc 1 (gray) in NMP and p-SWNTs
6 (black), f-SWNTs 8 (red), and SWNT-ZnPc conjugates 5 (blue) in
Figure 6. Steady-state fluorescence spectra of ZnPc reference (gray
spectrum) and SWNT-ZnPc 5 (black spectrum) in THF at room temperature
with matching absorption at the excitation wavelength (i.e., 650 nm). The
spectra were not corrected for different ground state absorption at the
excitation wavelength.
SDS (1.2 CMC)/ D2O.
components (i.e., 1.2 and 520 ps), while no particular shifts
were observable. In fact, this short-lived transient is a mirror
image of the ground-state absorption, revealing a significant
loss in oscillator strength upon excitation.
the photoexcitation is dominated, for example in the case of
SWNTs, by a negative imprint of the van Hove singularities.
These spectral characteristics transform into a new product.
Appreciable blue-shifts of the transient bleaches with minima
that shift from 963 to 940 nm and from 1390 to 1335 nm are
detected; see Figure 7. Implicit are new conduction band
electrons, injected from ZnPc shifting the transitions to lower
energies.58
Photovoltaic Properties. SWNTs, on one hand, and phtha-
locyanines, on the other hand, have been considered indepen-
dently or together for PV applications, owing to their promising
potential as electrodes or charge generation and separation
The same ZnPc singlet excited characteristics, despite the
presence of the electron accepting SWNTs, were registered
upon 660 nm photoexcitation of SWNT-ZnPc 5 (Figure 7
top). This confirms the successful excitation of the ZnPc
component. In contrast to the slow intersystem crossing,
which was noted for the ZnPc 1, the ZnPc singlet excited-
state features now decay rather quickly. The lifetimes (i.e.,
∼20 ps) reflect the global trends seen in the fluorescence
experiments (Figure 7, bottom). Simultaneously with the
ZnPc singlet excited-state decay, the formation of a new
transient species evolves with distinct maxima at 520 and
840 nm, which are attributes of the one-electron oxidized
ZnPc radical cation. Similarly, the range beyond 1000 nm
(i.e., 1000-1400 nm) is important, which immediately after
(58) Herranz, M. A.; Ehli, C.; Campidelli, S.; Gutie´rrez, M.; Hug, G. L.;
Ohkubo, K.; Fukuzumi, S.; Prato, M.; Mart´ın, N.; Guldi, D. M. J. Am.
Chem. Soc. 2008, 130, 66.
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A R T I C L E S
Campidelli et al.
a PTFE membrane filter (Millipore, 200 nm pore size, 37 mm
diameter). As the solvent passed through the pores, the SWNT
samples were trapped on the membrane surface, thereby forming
a homogeneous brown layer. Adjusting the initial concentration
helps to create conductive films of different optical thickness.
In fact, p-SWNTs 6, f-SWNTs 8, and SWNT-ZnPc 5 physical
thicknesses were adjusted to provide similar optical transparen-
cies in the spectral range of interest: ranging from 38% to 60%
between 300 and 800 nm; see Figure S4.
Before transfer of the films onto ITO-covered glass slides,
the latter were first refluxed for 24 h in 2-propanol to render
them hydrophobic. They were then pressed on top of the purified
SWNTs 6, f-SWNTs 8, and SWNT-ZnPc 5 films utilizing
clamps before the whole assembly was placed in an oven at
130 °C for 20 min. After removal of the sample from the oven,
the filter was peeled off, leaving the brown, transparent films
on the ITO substrate.
Insight into the film morphology came from AFM and SEM
measurements. SEM investigations of p-SWNTs 6, f-SWNTs
8, and SWNT-ZnPc 5 show that the SWNTs homogeneously
cover the substrate surface and form a membrane-like film;
Figure S5. The SWNTs arrange in intertwined bundles with
widths and lengths of around 10 nm and several hundreds of
nanometers, respectively. AFM images, not shown, corroborate
those observations and in particular the presence of thicker
bundles on the surface.
Since photooxidation of the ZnPc was expected under
illumination, ascorbic acid was added to the electrolyte solution
(0.1 M Na3PO4) as a sacrificial electron donor. In addition this
should favor anodic currents, because direct injection of
electrons from the ascorbate into the ITO conduction band can
be expected under positive potential. The presence of ascorbate
is essential since in its absence no notable photocurrents arose.
In first sets of experiments, photocurrents were determined
for p-SWNTs 6, f-SWNTs 8, and SWNT-ZnPc 5 in the absence
and presence of an external bias under white-light illumination.
The electrochemical cell consists of three electrodes, SWNT-
modified ITO, Pt wire, and Ag/AgCl electrodes, dipped in an
electrolyte solution of 0.1 M Na3PO4 containing ascorbic acid
5 mM. A simplified representation of the cell is given in Figure
8.
Figure 7. Upper part: differential absorption spectrum (near-infrared)
obtained upon femtosecond flash photolysis (660 nm) of SWNT-ZnPc 5
in nitrogen-saturated THF with time delays between 0.5 ps (black spectrum)
and20ps(yellowspectrum)atroomtemperature.Lowerpart:time-absorption
profiles of the spectra shown above at 840 nm.
layer.11,48 Thus, the PV properties of the SWNT-ZnPc 5
derivative were characterized by using a photoelectrochemical
cell; see Figure 8. For the sake of simplicity the photoelectrode
consisted of an ITO substrate covered with the SWNT derivative
film. The film preparation was handled by filtration and
lamination methods59–64 using suspensions of p-SWNTs 6,
f-SWNTs 8, and SWNT-ZnPc 5. In particular, 0.12 mg of each
sample was suspended in 4.8 mL of o-dichlorobenzene. These
suspensions were diluted to 50 mL before filtering them through
As a first result we observed that the photocurrents were about
30% higher for SWNT-ZnPc 5 than for p-SWNTs 6 and
f-SWNTs 8 under short circuit conditions. A positive photo-
current was always observed in those short circuit conditions,
Figure 8. I-V characteristics of SWNT-ZnPc 5 under white light illumination, gray line, and in the dark, black line. Three-electrode setup, 0.1 M Na3PO4,
1 mM sodium ascorbate, N2 purged. Voltages measured versus a Ag/AgCl reference electrode (0.1 M KCl). On the right: schematic representation of the
photoelectrochemical cell used for the measurements.
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Functionalized Single-Wall Carbon Nanotubes
A R T I C L E S
Chart 1
which is indicative of photogenerated electrons that flow from
the film to the ITO electrode. Current-voltage measurements
were then carried out in a voltage range between -0.6 V and
+0.6 V versus Ag/AgCl (0.1 M KCl). As shown in Figure 8,
around -0.4 V an open circuit voltage sets in, where no
appreciable currents, neither in the dark nor under illumination,
flow. Increasing the bias toward the anodic range resulted in
an increase of photocurrent as a consequence of a facilitated
electron injection into ITO. These observations are consistent
with the expected photooxidation of ZnPc and the presence of
ascorbate in the electrolyte. At the maximum applicable bias,
prior to performing any irreversible changes in the photoactive
layer, the net currents, calculated by subtracting the dark currents
from the photocurrents, were 0.22 mA for SWNT-ZnPc 5. We
noticed that the sign of the photocurrent reverted when applying
cathodic biases below -0.4 V. However, the current level
remained very small corresponding to a very ineffective
photocathodic mechanism. This is very likely due to the use of
a sacrificial electron donor (i.e., ascorbic acid) in the electrolyte
solution, which fail in closing the circuits between the photo-
electrode and the Pt counter electrode. Hypothetically, electron
acceptors such as methylviologen or O2 should render this
pathway much more efficient. The mechanism for the photo-
induced electron transfer in the photoelectrochemical is shown
in Chart 1. Next, the photoaction spectra under monochromatic
conditions were recorded (Figure 9). In line with what has been
noted in the absorption spectra of p-SWNTs 6, f-SWNTs 8, and
SWNT-ZnPc 5 on quartz substrates, broad and featureless
transitions emerge in the photoaction spectrum between 300 and
800 nm (see Figure S4). Contributions in the near-infrared, that
is, operating with a “800 nm” cutoff filter are 13.6%. Finally,
the internal photoconversion efficiencies (IPCE) of SWNT-ZnPc
5 were determined. At +0.6 V we derived IPCE values of 17.3%
for SWNT-ZnPc 5 (the IPCE for f-SWNTs 8 were more than
50% less).
Conclusions
Here we have described the functionalization of carbon
nanotubes with a 4-ethynylbenzene derivative and the subse-
quent attachment of phthalocyanine molecules using the Huisgen
1,3-dipolar cycloaddition.32 The nanotube derivatives were fully
characterized by a combination of analytical techniques. Pho-
tophysical measurements let us identify a photoinduced com-
munication between the two components (i.e., SWNT and ZnPc
moieties). Inspired by such unique charge transfer features, we
integrated SWNT-ZnPc 5 into photoactive electrodes (i.e.,
photoanodes), using ITO as a transparent semiconductor, which
revealed stable and reproducible photocurrents with monochro-
matic IPCE values as large as 17.3%.
We have demonstrated that click chemistry is an elegant
method to functionalize carbon nanotubes. This concept will
be extended to the attachment of other photo- and/or electro-
active moieties on carbon nanotubes. Our interest is to produce
optoelectronic devices based on functionalized carbon nanotube
field effect transistors (fCNT-FETs).20
Acknowledgment. This work was carried out with the partial
support from the University of Trieste, INSTM, and MUR (cofin.
Prot. 2006034372 and Firb RBNE033 KMA), SFB 583, DFG (GU
517/4-1), FCI, and the Office of Basic Energy Sciences of the U.S.
Department of Energy. Funding from MEC (CTQ2005-08933/BQU
and CONSOLIDER-INGENIO 2010 CDS2007-00010 NANO-
CIENCIA MOLECULAR), COST Action D35, and Comunidad
de Madrid (S-0505/PPQ/000225) is acknowledged. We also thank
Dr. Oliver Jost (Dresden University), for providing the SWNTs.
Supporting Information Available: Techniques, experimental
details, and characterization for the synthesis of phthalocyanine
derivatives and functionalization of carbon nanotubes. This
material is available free of charge on the World Wide Web at
http://pubs.acs.org.
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Figure 9. Photoaction spectrum of SWNT-ZnPc 5 recorded with a bias
of 0.6 V versus Ag/AgCl (0.1 M KCl).
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