Single-wall carbon nanotubes bearing covalently linked phthalocyanines - Photoinduced electron transfer

Article abstract of DOI:10.1021/ja068240n

HiPco single-walled carbon nanotubes (SWNTs) have been sidewall- functionalized with phthalocyanine addends following two different approaches: a straightforward Prato reaction with N-octylglycine and a formyl-containing phthalocyanine, and a stepwise approach that involves a former Prato cycloaddition to the double bonds of SWNTs using p-formyl benzoic acid followed by esterification of the derivatized nanotubes with an appropriate phthalocyanine molecule. The two materials obtained by these routes comprise different carbon/Pc-addenda ratios, as evidenced by Raman, TGA, and photophysical studies. The occurrence of electron transfer from photoexcited phthalocyanines to the nanotube framework in these ZnPc-SWNT ensembles is observed in transient absorption experiments, which confirm the absorption of the one-electron oxidized ZnPc cation and the concomitant bleaching of the van Hove singularities typical from SWNTs. Charge-separation (i.e., 2.0 × 10¹⁰ s^-1) and charge-recombination (i.e., 1.5 × 10⁶ s^-1) dynamics reveal a notable stabilization of the radical ion pair product in dimethylformamide.

Full text of DOI:10.1021/ja068240n

Published on Web 03/31/2007
Single-Wall Carbon Nanotubes Bearing Covalently Linked
Phthalocyanines - Photoinduced Electron Transfer
Beatriz Ballesteros,† Gema de la Torre,† Christian Ehli,‡ G. M. Aminur Rahman,‡
F. Agullo´-Rueda,§ Dirk M. Guldi,*,‡ and Toma´s Torres*,†
Contribution from the Departamento de Qu´ımica Orga´nica, UniVersidad Auto´noma de Madrid,
Cantoblanco 28049-Madrid, Spain, UniVersity of Erlangen, Institute of Physical and Theoretical
Chemistry and Interdisciplinary Center for Molecular Materials, Egerlandstrasse 3,
91058 Erlangen, Germany, and Instituto de Ciencia de Materiales de Madrid, CSIC,
Cantoblanco 28049-Madrid, Spain
Received November 17, 2006; E-mail: tomas.torres@uam.es; guldi@chemie.uni-erlangen.de
Abstract: HiPco single-walled carbon nanotubes (SWNTs) have been sidewall-functionalized with phtha-
locyanine addends following two different approaches: a straightforward Prato reaction with N-octylglycine
and a formyl-containing phthalocyanine, and a stepwise approach that involves a former Prato cycloaddition
to the double bonds of SWNTs using p-formyl benzoic acid followed by esterification of the derivatized
nanotubes with an appropriate phthalocyanine molecule. The two materials obtained by these routes
comprise different carbon/Pc-addenda ratios, as evidenced by Raman, TGA, and photophysical studies.
The occurrence of electron transfer from photoexcited phthalocyanines to the nanotube framework in these
ZnPc-SWNT ensembles is observed in transient absorption experiments, which confirm the absorption of
the one-electron oxidized ZnPc cation and the concomitant bleaching of the van Hove singularities typical
from SWNTs. Charge-separation (i.e., 2.0 × 1010 s-1) and charge-recombination (i.e., 1.5 × 106 s-1
)
dynamics reveal a notable stabilization of the radical ion pair product in dimethylformamide.
Introduction
Functionalizing CNT with electron-donor antenna chro-
mophores is expected to lead to the development of novel
nanoconjugate systems that bear great promise for major
breakthroughs in converting solar energy into electricity.6,7 Thus,
the combination of a light-harvester/electron-donor molecule,
like porphyrins, together with an electron acceptor, the all-carbon
framework of CNT, might lead to highly efficient photovoltaic
cells. Recently, it was shown that examples of covalent8 and,
to a similar extent, noncovalent9,10 porphyrin-SWNT nanoas-
semblies exhibit, in fact, the much anticipated electron-transfer
properties.
Phthalocyanine-nanotube hybrids also emerged as effective
targets for photovoltaic devices. Phthalocyanines (Pcs)11,12 are
planar electron-rich aromatic macrocycles that are characterized
by their remarkably high extinction coefficients in the red/near-
infrared region, an important part of the solar spectrum, and
their outstanding photostability and singular physical proper-
ties.13 These features render them exceptional donor/antenna
The outstanding properties of single-walled carbon nanotubes
(SWNTs) have aroused tremendous interest to implement this
novel carbon allotrope into practical applications such as
molecular electronics, gas storage, sensing, field-emission, etc.1
Ultimately, attention has even been directed to prepare electron-
donor-acceptor nanohybrids/nanoconjugates that are built on
carbon nanotubes (CNT).2 Hereby, the covalent attachment of
electron donors that range from pyrene3 and ferrocene4 to
tetrathiafulvalene (TTF)5 to the nanotube surface is at the
forefront of investigations. Implicit in this concept is that
SWNTs accept electrons with ease, which, in turn, might be
transported under nearly ballistic conditions along the tubular
axis.
† Universidad Auto´noma de Madrid.
‡ Universita¨t Erlangen.
§ Instituto de Ciencia de Materiales de Madrid.
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J. AM. CHEM. SOC. 2007, 129, 5061-5068
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Ballesteros et al.
building blocks. Incorporating them into donor-acceptor sys-
tems together with, for example, fullerenes,14-16 perylenes,17
anthraquinones,18 or related metallomacrocycles19 gives rise to
long-lived and highly emissive charge-separated states. In this
context, we have recently demonstrated that anchoring a novel
titanium phthalocyanine to nanocrystalline TiO2 films affords
electron injection from the titanium phthalocyanine to the TiO2.20
Some previous work has reported on the preparation of CNT-
phthalocyanine composites21 or nanowires.22 These hybrid
materials show enhanced photoconductivity, as a result of
photoinduced charge transfer from the Pc excitons to CNT. On
the other hand, covalent functionalization of carbon nanotubes
with Pc moieties has also been reported by us23 and others24,25
through a reaction of the terminal carboxylic acid groups of
shortened chemically etched SWNT with amino-functionalized
phthalocyanines. However, the resulting materials proved to be
nearly insoluble in common organic solvents. The solubility/
dispersability of Pc-CNT material emerged as an indispensable
task to be resolved for a ready manipulation and a feasible
solution-phase processing of such nanohybrids into devices.
Moreover, most of the important photophysical analyses (i.e.,
identifying promising hybrids/conjugates for solar energy
conversion) are performed in condensed media.
The development of chemical strategies, aimed at solubilizing
SWNT, has lately driven the research in this area.26 In particular,
organic covalent functionalization at the sidewalls of carbon
nanotubes usually leads to very soluble materials.27,28 Deriva-
tization based on 1,3-dipolar cycloaddition of azomethine ylides,
generated by condensation of R-aminoacids and aldehydes, has
been shown to be a powerful methodology for functionalizing
and solubilizing CNT.26,29 This widely applicable approach
affords functionalized SWNT, in which the tubular structure is
preserved. Moreover, they are soluble enough to facilitate
manipulation and solution studies.
In the current work, we report on the preparation of new
dispersable ZnPc-SWNT hybrids through the Prato reaction of
HiPco SWNT, N-octylglycine, and adequately functionalized
ZnPc molecules, bearing six solubilizing groups and an alde-
hyde. Alternatively, a stepwise approach has been explored to
achieve a high degree of CNT functionalization at minimum
synthetic costs. Importantly, we complement our work with a
detailed photophysical investigation on ground- and excited-
state Pc-nanotube interactions.
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Results and Discussion
In general, two possible routes toward functionalizing SWNT
with ZnPc, see Scheme 1, could be considered, both following
the Prato protocol. The first route (route 1, Scheme 1) would
involve the former reaction of N-octylglycine30 and 4-formyl-
benzoic acid with SWNTs and subsequent esterification reaction
of the derivatized nanotube material with 2,3,9,10,16,17-hexa-
tert-butylphenoxy-2-hydroxymethylphthalocyaninate Zn(II) (1),
see Scheme 2. This approach is fairly advantageous, because,
as previously described,27-29 chemical modification of CNT
usually requires a large excess of the reactants. In this particular
case, one of them is the inexpensive 4-formylbenzoic acid.
Subsequent esterification reaction of the pyrrolidine-SWNT
bearing pendent COOH moieties and zinc(II) phthalocyanine 1
would proceed at nearly stoichiometric conditions (Scheme 1).
Pthalocyanine 1 was prepared by condensation of 4,5-tert-
butylphenoxyphthalonitrile31 and 4-hydroxymethylphthaloni-
trile18 in the presence of Zn(OAc)2 in 11% yield (Scheme 2).
The compound was characterized by standard spectroscopic
techniques. It is worth pointing out that the room-temperature
1H-NMR spectrum of this compound reveals an unusual
shielding of some of the aromatic signals (ca. 6.5 ppm in CDCl3,
6.9 in C2D4Cl4). In particular, those are affected that correspond
to the hydroxymethyl-containing isoindole units of the Pc core,
while the remaining aromatic protons appear at average chemical
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D´ıaz, M. V.; de la Escosura, A.; Torres, T.; Da Ros, T.; Prato, M. Chem.
Commun. 2002, 2774. (c) Ballesteros, B.; de la Torre, G.; Hug, G. L.;
Rahman, G. M. A.; Guldi, D. M. Tetrahedron 2006, 62, 2097. (d) Torres,
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Scheme 1. Chemical Routes Followed To Functionalize the Sidewalls of SWNTs with Pc Molecules: Preparation of ZnPc-SWNT-(1) and
ZnPc-SWNT-(2)
shifts (8-9 ppm). However, when the experiments are carried
out at higher temperatures (i.e., 60 °C), the signal at 6.8 ppm
in deuterated tetrachloroethane decreases and a new signal
centered at 7.7 ppm grows-in. At 90 °C, the former signal has
fully disappeared. A likely rationale for this behavior stems from
possible aggregations between individual Pc, mainly through
coordination between the CH2OH groups and the Zn centers.32
tized SWNT material was reacted with an excess (ca. 3 equiv)
of phthalocyanine 1 in THF/DMF dispersions in the presence
of the EDC/HOBT condensation mixture (Scheme 1). After 4
days of reaction, the ZnPc-SWNT hybrid material (ZnPc-
SWNT-(1)) was filtered through a PTFE membrane and washed
with several solvents.
A set-back of this route lies in the fact that it is very difficult
to control the number of -COOH moieties that are subject to
esterification with phthalocyanines. For this reason, we explored
a second strategy (route 2, Scheme 1), that is, preparing a
formyl-containing phthalocyanine 2 (Scheme 2). The benefit
of this strategy is that a formyl-containing phthalocyanine 2 is
straightforwardly attached to CNT through the formation of the
corresponding azomethine ylide. In this case, the esterification
reaction of phthalocyanine 1 and 4-formylbenzoic acid using
standard condensation conditions leads to phthalocyanine 2 in
93% yield (Scheme 2). In the reaction with 4-formylbenzoic
acid and SWNTs, we observed a large amount of degraded
We performed the modification of the nanotube sidewalls
(Scheme 1) using, as previously described, 1.2 equiv of
N-octylglycine and 4-formylbenzoic acid per carbon atom of
the nanotube structure. In our hands, refluxing o-dichloroben-
zene instead of DMF at 130 °C29 affords larger amounts of
functionalized COOH-SWNT material. Clear evidence of the
introduction of COOH moieties in the material comes from the
IR spectrum (see below). The concentration of pyrrolidine-
COOH moieties in the SWNT was estimated to be ca. 7% by
acid-base titration33 (see Experimental Section). This deriva-
(32) (a) Senge, M. O.; Speck, M.; Wiehe, A.; Dieks, H.; Aguirre, S.; Kurreck,
H. Photochem. Photobiol. 1999, 70, 206. (b) Teo, T.-L.; Vetrichelvan, M.;
Lai, Y.-H. Org. Lett. 2003, 5, 4207.
(33) Hu, H.; Bhowmik, P.; Zhao, B.; Hamon, M. A.; Itkis, M. E.; Haddon, R.
C. Chem. Phys. Lett. 2001, 345, 25.
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Ballesteros et al.
Scheme 2. Synthesis of Phthalocyanines 1 and 2
Figure 2. TGA profiles of ZnPc-functionalized SWNT materials and
starting HiPco nanotubes. The heating process was carried out under
nitrogen.
Additional evidence for the successful functionalization of
SWNT is deduced from Raman and IR spectroscopies and
thermogravimetric analysis. Figure 2 shows the thermal behavior
of the two materials together with that of the starting HiPco-
SWNT. The weight loss between 250 and 500 °C is around
30% for ZnPc-SWNT-(1). Considering we had ca. a 7%
functionalization (from titration, see above) in the starting
COOH-SWNT material, a 23% weight loss should correspond
to the hydroxy-functionalized Pc moiety. Thus, the calculated
molar ratio of Pc-pyrrolidine/pyrrolidine-COOH moieties linked
to the nanotubes is 1.7, which allows one to estimate that the
material still holds a 3% weight of the pyrrolidine-COOH group,
and therefore a 27% weight of the Pc-pyrrolydine. Therefore,
the Pc-addenda/carbon ratio was calculated to be approximately
1:380. However, the ZnPc-SWNT-(2) shows 20% weight loss
in TGA, which leads us to conclude that the Pc-addenda/carbon
ratio in this material is 1:580. The relatively low functional-
ization degree of these materials may well be favorable because
it preserves the intrinsic electronic properties of nanotubes. It
is important to point out that the residue did not decompose up
to 800 °C in nitrogen atmosphere. This behavior is consistent
with the presence of entire SWNTs in all of the functionalized
samples.
material arising from the organic reactants. Therefore, consider-
ing that formyl-Pc 2 is a highly elaborated compound, we
decided to employ a lower excess of 2. Thus, we used 1 equiv
of Pc 2 every 25 carbon atoms for the 1,3-dipolar cycloaddition
with SWNT. After 5 days of reaction, the green color fully
disappeared and a dark-brownish mixture was instead present.
The functionalized nanotube material ZnPc-SWNT-(2) was
filtered off and accordingly washed with several organic
solvents.
Chemically modified ZnPc-SWNT-(1) and ZnPc-SWNT-
(2) form stable dispersions in DMF. Both materials were
characterized by analytical and spectroscopic methods: all data
are consistent with a successful functionalization of SWNT with
Pc molecules.
Representative TEM images of SWNT material are shown
in Figure 1. Pristine SWNTs, as received from CNI (Figure 1a),
are typically aggregated in bundles, and the presence of metallic
(iron) nanoparticles is revealed as black spots. Images of ZnPc-
SWNT-(1) and ZnPc-SWNT-(2) (Figure 1b and c, respectively)
reveal less bundled materials than what is typically found for
pristine SWNTs. This observation is consistent with SWNT
functionalization with phthalocyanines, where Pc rings may
disrupt the strong interactions between individual SWNTs.
Raman spectroscopy provides evidence that functionalization
is covalent to the nanotube sidewalls. Figure 3 shows the Raman
spectra for the pristine sample (SWNT) and for the phthalo-
cyanine-funtionalized samples ZnPc-SWNT-(1) and ZnPc-
SWNT-(2). The fluorescence background increases according
to the degree of functionalization, partially due to enhancement
of Pc-related fluorescence. The Raman spectra are characteristic
Figure 1. TEM images (on carbon-coated grids) of (a) HiPco SWNT (as received from CNI), (b) ZnPc-SWNT-(1), and (c) ZnPc-SWNT-(2).
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SWNTs Bearing Covalently Linked Phthalocyanines
A R T I C L E S
Figure 4. IR spectra of COOH- and Pc-containing nanotubes matched up
to those of the starting HiPco material and hydroxymethylphthalo-
cyanine 1.
Figure 3. Raman spectra of pristine SWNTs and two nanotube materials
functionalized with phthalocyanines: ZnPc-SWNT-(2) and ZnPc-SWNT-
(1). Spectra have been offset vertically for clarity. The colored area under
each spectrum marks the zero level. The intensities on the left panel (RBM
peaks) are multiplied approximately by ×5 relative to the right panel (D
and G bands).
of single-wall nanotubes, with several radial breathing modes
(RBM) between 186 and 270 cm-1, corresponding to nanotube
diameters34 between 0.92 and 1.33 nm, a D band at about 1355
cm-1, due to disorder on the carbon hexagonal lattice on the
nanotube sidewalls, and several tangential modes (TM) or G
bands between 1400 and 1600 cm-1. We have not found clear
evidence of any peak characteristic of the functionalizing
moieties, because functionalization is weak. However, func-
tionalization does induce an enhancement of the D band,35 with
respect to the pristine nanotube, the effect being larger for
sample ZnPc-SWNT-(2). The enhancement proves that the
functionalization is through covalent bonding with the sidewall
carbon atoms, which convert some sp2 bonding into sp3.34
More evidence to support covalent functionalization of
SWNTs is found in the IR spectra (Figure 4). Comparing pristine
nanotubes with COOH-SWNT material, the presence of COOH
moieties in the latter is principally confirmed by the occurrence
of a strong band at 1715 cm-1 corresponding to the symmetric
CdO stretching. The phthalocyanine-functionalized nanotube
material ZnPc-SWNT-(1) displays the same band but at 1728
cm-1. This shift matches the transformation of the COOH
moiety of COOH-SWNT into an ester function after reaction
with the hydroxyphthalocyanine 1. Finally, the IR spectrum of
ZnPc-SWNT-(1) also shows other bands that are coincident
with those displayed by phthalocyanine 1. The IR spectrum of
ZnPc-SWNT-(2) displays bands that are similar to those of
ZnPc-SWNT-(1).
Figure 5. Ground-state absorption spectra of ZnPc-SWNT-(1) and ZnPc-
SWNT-(2) in DMF at room temperature.
Figure 6. Steady-state fluorescence spectra of 1, ZnPc-SWNT-(1), and
ZnPc-SWNT-(2) in DMF at room temperature with matching absorption
at the excitation wavelength (i.e., 650 nm). Curves for ZnPc-SWNT-(1)
and ZnPc-SWNT-(2) have been amplified by a factor of 20.
which is dominated by SWNT-related features (i.e., van Hove
singularities). At first glance, the ground-state absorption spectra,
which reveal the features of both constituents, that is, ZnPc and
SWNT, are good mirror reflections of the results that were
gathered in the TGA experiments. In particular, depending on
the degree of functionalization, that is, comparing ZnPc-SWNT-
(2) and ZnPc-SWNT-(1), two trends are noteworthy, see Figure
5. First, the overall intensity of the Pc-centered transitions
increased from ZnPc-SWNT-(2) to ZnPc-SWNT-(1). Second,
the SWNT characteristics deintensified concomitantly. Consid-
ering the intensity of the 680 nm absorption maximum, we
estimate a ZnPc content of around 10% and 20% in ZnPc-
SWNT-(2) and ZnPc-SWNT-(1), respectively. Nevertheless,
Photophysics. When discussing the ground-state absorption
of ZnPc-SWNT, we wish to point out two regions of interest:
the visible range, where we mainly find the Pc-centered
transitions (i.e., Soret- and Q-bands), and the near-infrared range,
(34) Dresselhaus, M. S.; Dresselhaus, G.; Saito, R.; Jorio, A. Phys. Rep. 2005,
409, 47.
(35) (a) Chiu, P. W.; Duesberg, G. S.; Hahn, O.; Dettlaff-Weglikowska; Roth,
S. Appl. Phys. Lett. 2002, 80, 3811. (b) Bahr, J. L.; Tour, J. M. J. Mater.
Chem. 2002, 12, 1952. (c) Dyke, C. A.; Tour, J. M. Chem.-Eur. J. 2004,
10, 812. (d) Marcoux, P. R.; Schreiber, J.; Batail, B.; Lefrant, S.; Renouard,
J.; Jacob, G.; Albertini, D.; Mevellec, J.-Y. Phys. Chem. Chem. Phys. 2002,
4, 2278. (e) Me´nard-Moyon, Y. C.; Izard, N.; Doris, E.; Mioskowski, C. J.
Am. Chem. Soc. 2006, 128, 6552.
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A R T I C L E S
Ballesteros et al.
Figure 8. Differential absorption spectrum (near-infrared) obtained upon
femtosecond flash photolysis (660 nm) of ZnPc-SWNT-(2) in nitrogen-
saturated dimethylformamide with time delays between 0 and 1000 ps at
room temperature.
Figure 7. Differential absorption spectrum (visible and near-infrared)
obtained upon nanosecond flash photolysis (532 nm) of ∼1.0 × 10-5
M
solutions of 1 in nitrogen-saturated dimethylformamide with a time delay
of 50 ns at room temperature. The spectral changes reflect the triplet excited-
state features of 1.
700 nm, no appreciable features are registered for the oxygen-
sensitive triplet state.
a notable red-shift of 2 nm (i.e., 676 to 678 nm) is discernible
for ZnPc in the corresponding ZnPc-SWNT conjugates. Such
shifts point to some electronic communication between the
electroactive components.
Initially, upon photoexciting both ZnPc-SWNT conjugates,
the ZnPc singlet fingerprints, vide supra, are notable. In the
visible range, transient maxima and transient minima centered
around 680 and 470 nm, respectively, attest to the successful
excitation of the ZnPc constituent in ZnPc-SWNT on the fem-
tosecond time scale. However, instead of the slow intersystem
crossings, the singlet excited states decay with rates of about
1010 s-1 (vide supra), from which we deduce electronic coup-
lings between both electroactive elements of 50 cm-1. Consider-
ing approximate absorption cross sections of the ZnPc and
SWNT constituents at the 660 nm excitation wavelength in a
1:9 ratio, a significant fraction of the light photoexcites the
SWNT. Therefore, we notice the instantaneous bleaching of the
van Hove singularities. However, exciton recombination, which
depends on the SWNT diameter, leads to rather fast deactivation.
Such fast deactivation dynamics are outside of the detection
range of our time-resolved fluorescence measurements.
More decisive tests about interactions between the ZnPc and
SWNT constituents are based on fluorescence experiments,
especially considering the prominent ZnPc fluorescence features
(i.e., quantum yield, 0.3; lifetime, 3.1 ns). Photoexciting all
ZnPc-SWNT conjugates and the ZnPc reference (1) in the range
of either the Soret- (i.e., ∼300-350 nm) or the Q-bands (i.e.,
600-680 nm) led to a set of fluorescing features with a strong
maximum and a shoulder at 685 and 750 nm, respectively. This
confirms unmistakably the presence of the ZnPc constituents
in all ZnPc-SWNT samples. Figure 6 demonstrates that a
qualitative comparison (i.e., photoexciting around 650 nm)
reveals, however, vastly different fluorescence quantum yields.
Relative to 1, the following quenching factors were deter-
mined: ZnPc-SWNT-(2) (425) and ZnPc-SWNT-(1) (40);
notably these values have not been corrected for competitive
ground-state absorption at the excitation wavelength (vide
supra). In reference experiments, where 1 was titrated with
variable SWNT concentrations, the lack of appreciable changes
suggests no excited-state interactions, see Figure S1.
At the end of the intrinsically fast ZnPc singlet excited-state
decay, the following differential absorption changes occur for
ZnPc-SWNT-(2) in the visible and near-infrared regions
(Figures 8 and S3 in the Supporting Information); in the visible,
its mainly the bleaching of the Q-bands that is appreciable. This
is further accompanied by a broad absorption in the 425-575
nm range. In the near-infrared, on the other hand, we see a set
of maxima at 840 and 930 nm and a set of minima at 820, 890,
980, 1090, 1150, 1200, and 1310 nm. The maxima, especially
the one at 840 nm, are known to reflect the transient spectrum
of the one-electron oxidized ZnPc radical cation. The same holds
for the 425-575 nm transition. Quite the opposite, the minima
relate to the van Hove singularities seen in the ground-state
spectrum that bleach upon electron doping. Implicit are new
conduction band electrons, injected from photoexcited ZnPc,
shifting the transitions to lower energies. Spectroscopic support
for this assumption came from determining the absolute
spectrum of the product and comparing it with that of the ground
state. As Figure S2 illustrates, the van Hove singularities shift
indeed to the red. Thus, we reach the conclusion that photoex-
citation of ZnPc-SWNT conjugates is followed by a rapid charge
transfer. Figures 8 and S3 illustrate the transformation of the
ZnPc singlet excited state into the radical ion pair state for ZnPc-
SWNT-(2) (i.e., 2.0 × 1010 s-1).
Not surprising, the corresponding time-resolved fluorescence
experiments failed to shed light onto the intraconjugate deac-
tivation of the ZnPc singlet excited state. The time resolution,
100 ps, is mainly thought to be responsible for the lack of
detectable ZnPc fluorescence deactivation.
To confirm the aforementioned observations, we turned to
femto- (i.e., 660 nm excitation) and nanosecond (i.e., 532 nm
excitation) resolved transient absorption spectroscopy. For 1,
which lacks the SWNT, we see the rapid formation of the singlet
excited-state characteristics. These include transient bleach in
the 600-700 nm range, which mirror images the ground-state
absorption (not shown). In addition, in the blue and red regions,
new transients are registered. Please note that the one in the
blue, at 470 nm, will be of particular importance when testing
excited-state interactions in the ZnPc-SWNT conjugates. The
fate of the ZnPc singlet excited state is an intersystem crossing
((3.3 ( 0.5) × 108 s-1) that affords the corresponding triplet
manifold. An illustration of the triplet spectrum of 1 is given
in Figure 7. Please note that in the red region, that is, above
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SWNTs Bearing Covalently Linked Phthalocyanines
A R T I C L E S
TGQ500 analyzer under nitrogen using Hi-Res Dynamic method (scans,
50 °C/min; resolution setting, 4). Raman spectra were taken at room
temperature with a Renishaw Ramascope 2000 microspectrometer and
an argon ion laser operating at a wavelength of 514.5 nm as the
excitation source. Photophysics: Femtosecond transient absorption
studies were performed with 660 nm laser pulses (1 kHz, 150 fs pulse
width) from an amplified Ti:Sapphire laser system (Clark-MXR, Inc.).
Nanosecond laser flash photolysis experiments were performed with
532 nm laser pulses from a Nd:YAG laser (5 ns pulse width) in a front
face excitation geometry. Fluorescence lifetimes were measured with
a Laser Strope fluorescence lifetime spectrometer (Photon Technology
International) with 337 nm laser pulses from a nitrogen laser fiber-
coupled to a lens-based T-formal sample compartment equipped with
a stroboscopic detector. Details of the Laser Strobe systems are
described on the manufacturer’s web site. Emission spectra were
recorded by using a FluoroMax-3 (Horiba Co.). The experiments were
performed at 20 °C.
Similar changes were noted with ZnPc-SWNT-(1), Figure
S4. Important is that the charge-transfer kinetics are virtually
identical to the values determined for ZnPc-SWNT-(2).
The decay kinetics, on the nanosecond scale, reflect the return
of the radical ion pair state to the electronic ground state. The
lifetime of the newly formed radical ion pair state, as derived
by analyzing several wavelengths under unimolecular conditions,
is, for example, in ZnPc-SWNT-(2) 1.5 × 106 s-1 in dimeth-
ylformamide.
Conclusions
Phthalocyanine-SWNT ensembles have been satisfactorily
prepared following the Prato protocol, via 1,3-dipolar cycload-
dition of appropriate formyl derivatives with N-octylglycine.
A straightforward approach and a stepwise one have been
explored; the former cycloaddition of a large excess of p-formyl
benzoic acid (comercial) and the glycine derivative with HiPco
SWNT, followed by esterification reaction between the deriva-
tized nanotube material and the hydroxymethylphthalocyanine
(1), gives a hybrid material (ZnPc-SWNT-(1)) largely func-
tionalized (30 wt %), at lower synthetic cost. In addition to that,
a second approach has been followed to synthesize ZnPc-
SWNT-(2) by means of a 1,3-dipolar cycloaddition reaction
with phthalocyanine 2, early containing a formyl group. This
route solves the difficulty in controlling the degree of esterifi-
cation and is presented as a one-step reaction that leads to the
same nanohybrid system, developing a lower degree of func-
tionalization (20 wt %). The transient absorption data show that,
as previously observed in other donor-SWNT ensembles,3-5,8-10
SWNTs serve as the electron-acceptor component in such hybrid
materials. The estimated number of ZnPc units per carbon atom
is quite low (1:380 and 1:580, respectively), which helps, on
the other hand, to preserve the electronic structure of SWNTs,
as proved by different ground-state and transient absorption
spectroscopies. Taken this, together with the light harvesting
features of the used phthalocyanines, into concert, our approach
toward versatile nanoconjugates opens the way to novel
chemical and light driven systems.
2,3,9,10,16,17-Hexa-tert-butylphenoxy-23-hydroxymethylphtha-
locyaninate Zn(II) (1). A mixture of 4-hydroxymethylphthalonitrile18
(0.20 g, 1.25 mmol), 4,5-tert-butylphenoxyphthalonitrile31 (1.60 g, 3.75
mmol), and Zn(OAc)2 (0.30 g, 1.60 mmol) was refluxed in N,N-
dimethylaminoethanol (10 mL) for 18 h under argon atmosphere. After
cooling, the solvent was evaporated, and a mixture of methanol/water
(3:1) was added to the residue. The green solid was filtered, washed
with methanol, and then subjected to column chromatography on silica
gel using CHCl3 as eluent. Trituration with hot methanol yielded 0.21
(11%) of phthalocyanine 1. mp > 300 °C. 1H NMR (500 MHz, C2D4-
Cl4, 25 °C, TMS): δ ) 8.7-8.3 (m, 7H, arom H), 7.4-7.0 (m, 24H,
phenyl H), 6.9 (bs, 2H, arom H), 4.7 (bs, 2H, CH2O), 3.0 (bs, 1H,
OH), 1.4-1.2 (m, 54H, C(CH3)3) ppm. 1H NMR (500 MHz, C2D4Cl4,
90 °C, TMS): δ ) 8.60-8.20 (m, 7H, arom H), 7.64 (s, 2H, arom H),
7.35, 7.15 (2xm, 24H, phenyl H), 4.83 (s, 2H, CH2O), 3.09 (s, 1H,
OH), 1.30 (m, 54H, C(CH3)3) ppm. IR (KBr): ν ) 2960, 2903, 2867
(CH), 1603 (CdN), 1508, 1488, 1452, 1269, 1216, 1177, 1090, 1029,
893, 828, 744, 722 cm-1; UV/vis (CHCl3): λmax (log ꢀ) 355 (4.9), 614
(4.6), 653 (4.5), 680 nm (5.4). MALDI-TOF MS: m/z ) 1495 [M +
H]+, 1494 [M+]. Anal. Calcd for C93H90N8O7Zn‚2H2O: C, 72.85; H,
6.18; N, 7.31. Found: C, 72.24; H, 6.27; N, 7.09.
2,3,9,10,16,17-Hexa-tert-butylphenoxy-23-(4-formylbenzoyloxym-
ethyl)phthalocyaninate Zn(II) (2). 4-Formylbenzoic acid (0.010 g,
0.067 mmol), dicyclohexylcarbodiimide (0.013 g, 0.067 mmol), and
dimethylaminopyridine (0.008 g, 0.067 mmol) were dissolved in dry
THF (5 mL) under argon atmosphere. The solution was cooled to 0 °C
in an ice bath and stirred for 20 m. Next, a solution of phthalocyanine
1 (0.1 g, 0.0067 mmol) in 5 mL of dry THF was added dropwise. The
mixture was stirred for 1 h at room temperature and for 24 h at room
temperature. The resulting reaction crude was filtered through a glass
frit to remove the insoluble urea byproduct, and the filtrate was then
evaporated and washed with water and methanol. The solid residue
was purified by column chromatography in silica gel (hexane/THF,
Experimental Section
General Methods and Materials. In the experiments, we have used
SWNTs prepared by the HiPco process and provided by Carbon
Nanotechnologies Inc. The raw material was treated with concentrated
HCl to remove the iron content.36 SWNT derivatives were filtered over
0.45 pore-size PTFE membranes (Sartorius). Chemicals were purchased
from commercial suppliers and used without further purification.
N-Octylglycine,30 4,5-tert-butylphenoxyphthalonitrile,31 and 4-hy-
droxymethylphthalonitrile18 were prepared using described procedures.
Column chromatography was carried out on silica gel Merck-60 (230-
400 mesh, 60 Å), and TLC was carried out on aluminum sheets
precoated with silica gel 60 F254 (E. Merck). PTFE filters (pore size
0.45 µm) were used. Melting points were determined in a Bu¨chi
504392-S equipment and are uncorrected. IR spectra were recorded
on a Bruker Vector 22 spectrophotometer using KBr disks. MALDI-
TOF-MS spectra were determined on a Bruker REFLEX III instrument
equipped with a nitrogen laser operating at 337 nm. NMR spectra were
recorded with a Bruker AC-300 instrument. UV/vis spectra were
recorded with a Hewlett-Packard 8453 instrument. Transmission
electron microscopy (TEM) images were obtained on a JEOL JEM1010
(100 kV) system equipped with a Gatan Bioscan camera for digital
imaging. Thermogravimetric analyses (TGA) were recorded on a TA
1
2:1) to give 0.1 g (93%) of 2. mp > 300 °C. H NMR (300 MHz,
CDCl3, 25 °C, TMS): δ ) 10.02 (s, 1H, CHO), 8.90 (m, 2H, arom
H), 8.70-8.55 (m, 6H, arom H), 8.26 (AA′BB′, 2H), 7.95 (m, 1H,
arom H), 7.32, 7.12 (2xm, 24H, phenyl H), 5.78 (s, 2H, CH2O), 1.40-
1.20 (m, 54H, C(CH3)3) ppm. IR (KBr): ν ) 2960, 2903, 2866 (CH),
1727 (COO), 1709 (CHO), 1604 (CdN), 1508, 1489, 1452, 1402, 1269,
1216, 1177, 1090, 1029, 935, 828, 759, 722 cm-1. UV/vis (THF): λmax
(log ꢀ) 355 (4.84), 614 (4.47), 656 (4.48), 680 nm (5.34). MALDI-
TOF MS: m/z ) 1627 [M + H]+, 1626 [M+]. Anal. Calcd for
C101H94N8O9Zn‚H2O: C, 73.64; H, 5.87; N, 6.80. Found: C, 73.49;
H, 5.88; N, 6.98.
COOH-SWNT. Purified HiPco nanotubes (0.04 g) were sonicated
for 15 min in o-DCB (40 mL). 4-Formylbenzoic acid (0.64 g) was
added to the suspension, and the mixture was heated at 180 °C.
N-Octylglycine (0.80 g) was added in portions (4 × 0.20 g every 24
h), and the reaction was stopped after 5 days. The crude was filtered
(36) Va´zquez, E.; Georgakilas, V.; Prato, M. Chem. Commun. 2002, 2308.
9
J. AM. CHEM. SOC. VOL. 129, NO. 16, 2007 5067

A R T I C L E S
Ballesteros et al.
over a PTFE membrane to separate the carbon-based material, which
was subsequently sonicated in DMF and centrifugated. The supernatant
was separated, the solvent evaporated, and the solid residue was washed
consecutively with CHCl3, ethyl acetate, acetone, acetonitrile, methanol,
and diethyl ether to give 0.026 g of COOH-SWNTs. The concentration
of carboxylic acid groups was determined by acid-base titration.33
COOH-SWNTs (5 mg) were stirred in 10 mL of 5 × 10-4 M aqueous
NaOH solution under argon for 48 h. The mixture was filtered over a
PTFE membrane, and the solid residue was washed with deionized
water until the filtrate was neutral. The combined filtrate and washings
were titrated with 7.3 mL of 5 × 10-4 M aqueous HCl solution to
reach pH 7.0, as monitored by a pH meter. The estimated weight ratio
of organic addenda in this material was ca. 7% (1.35 × 10-3 mmol).
ZnPc-SWNT-(1). To a stirred solution of phthalocyanine 1 (26 mg,
0.017 mmol) and EDC (2.7 mg, 0.017 mmol) in THF (10 mL) under
argon atmosphere was added HOBT (2.3 mg, 0.017 mmol). Subse-
quently, a suspension of COOH-SWNTs (20 mg) in dry DMF (8 mL)
was added, and the resulting mixture was stirred for 4 days. The crude
was filtered off, and the black solid was washed with several portions
of water, acetone, methanol, ethyl acetate, and THF to give 24 mg of
ZnPc-SWNT-(1).
membrane, and the black solid was washed with o-DCB and subse-
quently sonicated in DMF and centrifugated. The supernatant was
separated, the solvent evaporated, and the solid residue was washed
with ethyl acetate, acetonitrile, and methanol, to give 16 mg of ZnPc-
SWNT-2 was collected.
Acknowledgment. We are grateful for financial support from
the Ministerio de Educacio´n y Ciencia (CTQ-2005-08933-BQU),
Comunidad de Madrid (S-0505/PPQ/000225), and the ESF/MEC
(SOHYDS), SFB 583, DFG (GU 517/4-1), FCI and the Office
of Basic Energy Sciences of the U.S. Department of Energy
(NDRL 4716). We would also like to thank Dr. Eva M. Maya
for TGA analyses.
Supporting Information Available: Absorption and fluores-
cence spectra of 1 titrated with variable concentrations of
SWNT; absolute spectra of ZnPc-SWNT-(2) in the ground and
reduced states; differential absorption spectra of ZnPc-SWNT-
(2) in the visible and NIR upon femtosecond flash photolysis
(660 nm) and their time-absorption profiles at different
wavelengths; differential absorption spectra of ZnPc-SWNT-
(1) in the visible and NIR upon femtosecond flash photolysis
(660 nm); and full Raman spectra of pristine and functionalized
nanotubes. This material is available free of charge via the
Internet at http://pubs.acs.org.
ZnPc-SWNT-(2). Purified HiPco nanotubes (20 mg) were sonicated
for 15 min in o-DCB (40 mL). Phthalocyanine 2 (26 mg) and
N-octylglycine (6 mg) were added to the suspension, and the mixture
was heated at 180 °C. Sucessive portions of 2 (3 × 26 mg every 24 h)
and N-octylglycine (3 × 6 mg every 24 h) were added, and the reaction
was stopped after 5 days. The mixture was filtered over a PTFE
JA068240N
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5068 J. AM. CHEM. SOC. VOL. 129, NO. 16, 2007