Phthalocyanine-pyrene conjugates: A powerful approach toward carbon nanotube solar cells

Article abstract of DOI:10.1021/ja107131r

In the present work, a new family of pyrene (Py)-substituted phthalocyanines (Pcs), i.e., ZnPc-Py and H₂Pc-Py, were designed, synthesized, and probed in light of their spectroscopic properties as well as their interactions with single-wall carb

Full text of DOI:10.1021/ja107131r

Published on Web 10/25/2010
Phthalocyanine-Pyrene Conjugates: A Powerful Approach
toward Carbon Nanotube Solar Cells
Juergen Bartelmess,† Beatriz Ballesteros,‡ Gema de la Torre,‡ Daniel Kiessling,†
Stephane Campidelli,#,| Maurizio Prato,*,# Toma´s Torres,*,‡,§ and Dirk M. Guldi*,†
Department of Chemistry and Pharmacy and Interdisciplinary Center for Molecular Materials,
Friedrich-Alexander-UniVersity Erlangen-Nuremberg, 91058 Erlangen, Germany, Departamento
de Qu´ımica Orga´nica, UniVersidad Auto´noma de Madrid, Cantoblanco, 28049 Madrid, Spain,
Center of Excellence for Nanostructured Materials and INSTM, Unit of Trieste, Dipartimento di
Scienze Farmaceutiche, UniVersity of Trieste, Piazzale Europa 1, 34127 Trieste, Italy, and
IMDEA Nanociencia, Facultad de Ciencias, Cantoblanco, 28049 Madrid, Spain
Received August 9, 2010; E-mail: tomas.torres@uam.es; dirk.guldi@chemie.uni-erlangen.de
Abstract: In the present work, a new family of pyrene (Py)-substituted phthalocyanines (Pcs), i.e., ZnPc-
Py and H2Pc-Py, were designed, synthesized, and probed in light of their spectroscopic properties as well
as their interactions with single-wall carbon nanotubes (SWNTs). The pyrene units provide the means for
non-covalent functionalization of SWNTs via π-π interactions. Such a versatile approach ensures that the
electronic properties of SWNTs are not impacted by the chemical modification of the carbon skeleton. The
characterization of ZnPc-Py/SWNT and H2Pc-Py/SWNT has been performed in suspension and in thin
films by means of different spectroscopic and photoelectrochemical techniques. Transient absorption
experiments reveal photoinduced electron transfer between the photoactive components. ZnPc-Py/SWNT
and H2Pc-Py/SWNT have been integrated into photoactive electrodes, revealing stable and reproducible
photocurrents with monochromatic internal photoconversion efficiency values for H2Pc-Py/SWNT as large
as 15 and 23% without and with an applied bias of +0.1 V.
Introduction
With regard to novel electron donor-acceptor conjugates/
hybrids, emphasis is placed on carbon nanotubes, in general,
and single-wall carbon nanotubes (SWNTs), in particular.
Carbon-based nanomaterials are currently under active inves-
tigation for producing innovative materials, composites, and
electronic devices of greatly reduced size.6 In particular, SWNTs
are one-dimensional nanowires that readily accept charges,
which can then be transported along their tubular axis. The
electrical conductivity, morphology, and good chemical stability
of SWNTs are promising features that stimulate their integration
into electronic devices.7 To date, SWNTs have been shown to
act as electron acceptors when adequately interfaced with
excited-state electron donors.8 Interfacing SWNTs with electron
Organic photovoltaics has emerged as a major topic in
contemporary research.1 Importantly, the basic modus operandi
for most approaches toward organic photovoltaics is taken from
natural photosynthesis, that is, electron donor-acceptor interac-
tions. To this end, the radical ion pair states, which are generated
upon photoexcitation followed by charge separation, are utilized
to inject charge carriers (i.e., electrons and holes) into electrodes
(i.e., anodes and cathodes) that lead eventually to the production
of electric currents. The realization of organic photovoltaics is
based on, for example, the use of semiconducting polymers
including poly(p-phenylenevinylene) and polythiophene.2 Owing
to the outstanding electron acceptor features of C60 and its
derivatives, strategies have been developed to blend them into
polymer matrices in a bulk heterojunction configuration to
enhance the overall photoactive performance.3 Dyes such as
phthalocyanines (Pcs) and porphyrins have been studied as light-
harvesting units, especially in organic planar heterojunction solar
cells4 and for the sensitization of mesoscopic semiconductors.5
(4) Bottari, G.; de la Torre, G.; Guldi, D. M.; Torres, T. Chem. ReV. 2010,
DOI: 10.1021/cr900254z and references cited therein.
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Chandrasekharam, M.; Lakshmikantam, M. J.; Yum, H.; Kalyana-
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Ed. 2007, 46, 373. (b) Cid, J.-J.; Yum, J.-H.; Jang, S.-R.; Nazeeruddin,
M. K.; Mart´ınez-Ferrero, E.; Palomares, E.; Ko, J.; Gra¨tzel, M.; Torres,
T. Angew. Chem., Int. Ed. 2007, 46, 8358. (c) Imahori, H.; Umeyama,
T.; Ito, S. Acc. Chem. Res. 2009, 42, 1809. (d) Mori, S.; Nagata, M.;
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Chem. Soc. 2010, 132, 4054. (e) Mart´ınez-D´ıaz, M. V.; Torres, T. In
Handbook of Porphyrin Science; Kadish, K. M., Smith, K. M., Guilard,
R., Eds.; Academic Press: San Diego, 2010; Vol. 10, pp 45-141.
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† Friedrich-Alexander-University Erlangen-Nuremberg.
‡ Universidad Auto´noma de Madrid.
# University of Trieste.
§ IMDEA Nanociencia.
| Present address: Laboratoire d’Electronique Moléculaire, CEA Saclay.
(1) Special issue on “Artificial Photosynthesis and Solar Fuels”. Acc.
Chem. Res. 2009, 42, 1859-2029.
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(b) Chen, J.; Cao, Y. Acc. Chem. Res. 2009, 42, 1709.
(3) Dennler, G.; Scharber, M. C.; Brabec, C. J. AdV. Mater. 2009, 21,
1323, and references cited therein.
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R. H. AdV. Mater. 1999, 11, 1281. (b) Tans, S. J.; Verschueren,
A. R. M.; Dekker, C. Nature 1998, 393, 49.
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16202 J. AM. CHEM. SOC. 2010, 132, 16202–16211
10.1021/ja107131r  2010 American Chemical Society

Phthalocyanine-Pyrene Conjugates
A R T I C L E S
Scheme 1. Synthesis of H2Pc-Py and ZnPc-Py
derivatives. Starting from the metal-free, unsymmetrically substi-
tuted Pc bearing a hydroxymethyl moiety (i.e., H2Pc-OH),22 an
esterification reaction was carried out with 1-pyrenebutyric acid in
the presence of dicyclohexylcarbodiimide (DCC) and N,N-dim-
ethylaminopyridine (DMAP) as condensation agents. The resulting
H2Pc-Py is also the precursor of the metalated ZnPc-Py derivative,
which was prepared by treatment of the metal-free conjugate with
donors requires electronic communication between the different
constituents via functionalization. The functionalization of
SWNTs is achieved by either covalent or non-covalent chem-
istry. Covalent functionalization generally includes the oxidation
of SWNTs to remove their caps and to create defect sites (i.e.,
COOH groups) at their ends as well as sidewalls. Defect sites
are useful to add functional entities in subsequent steps via the
formation of covalent bonds with electron donors.9 Ultrasonic
treatment can likewise induce SWNT oxidation.10 All of these
processes have in common that the rather harsh conditions cause
defects on the carbon backbone of SWNTs.11 Undeniably, such
treatments influence the electronic properties of SWNTs due
to partial saturation/destruction of the extended π-system. In
the context of non-covalent functionalization, interactions
between aromatic groups and sidewalls of SWNT are promoted
by means of hydrophobic, π-stacking, or even van der Waals
interactions.8b,12 The major advantage of the non-covalent
functionalization is the fact that the carbon backbone is not
damaged and the properties of SWNTs are largely preserved.
Leading examples include the immobilization of pyrene,13
anthracene,14 and porphyrins.15
In this study, we pursue the non-covalent immobilization of
phthalocyanines as excited electron donors onto SWNTs.
Phthalocyanines show excellent light-harvesting properties in
the visible and NIR region of the solar spectrum as well as high
photostability and unique physical properties.16 Electron
donor-acceptor interactions with phthalocyanines have been
studied in many forms: phthalocyanine/porphyrin conjugates,17
phthalocyanine/fullerene conjugates and hybrids,4,18 as well as
phthalocyanine oligomers.19 Covalently linked Pc-SWNT en-
sembles have also been described.20 The general concept of non-
covalent functionalization was recently extended to probe the
interactions between phthalocyanine-based poly(phenylene vi-
nylene) oligomers and SWNTs.21 In stark contrast to any of
the aforementioned examples, we took advantage of the well-
known features of pyrene and pyrene derivatives to adhere to
SWNTs13 and used them as anchor groups to immobilize metal-
free (H2Pc) as well as zinc phthalocyanines (ZnPc) onto the
surface of SWNTs. The resulting electron donor-acceptor
hybrids form stable suspensions in organic solvents that render
them particularly useful for the production of prototype solar
cells containing SWNT buckypapersa matrix film of individual
and thin bundles of SWNTs.20f
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Synthesis
Scheme 1 summarizes the synthesis of metal-free phthalocyanine-
pyrene (H2Pc-Py) and zinc phthalocyanine-pyrene (ZnPc-Py)
9
J. AM. CHEM. SOC. VOL. 132, NO. 45, 2010 16203

A R T I C L E S
Bartelmess et al.
ZnCl2 in refluxing o-dichlorobenzene (o-DCB)/dimethylformamide
(DMF) (Scheme 1).
Additional experimental details are given in the Supporting
Information.
Results and Discussion
Characterization of H2Pc-Py/ZnPc-Py. The spectroscopic
characterizations of the two lead compounds, namely, H2Pc-Py
and ZnPc-Py, were carried out in the following solvents/solvent
mixtures: tetrahydrofuran (THF) and DMF as well as a mixture
of 25% THF and 75% DMF. Metal-free tetra-tert-butylphtha-
locyanine (H2Pc), the corresponding zinc complex (ZnPc), and
1-pyrenemethanol (Py) were used as internal references.
The absorption spectra of H2Pc-Py and ZnPc-Py in THF show
features of both building blocks, that is, pyrene and H2Pc/ZnPc,
with their corresponding characteristic signatures at 328/345
(Py), 366/680 (ZnPc-Py), and 366/668 and 702 nm (H2Pc-Py).
The H2Pc/ZnPc maxima correlate to the corresponding Soret
and Q-band absorptions.
In light of the strong and long-lived fluorescence features of
H2Pc, ZnPc, and Py moieties, their quantum yields and lifetimes
were determined in the references as well as in H2Pc-Py and
ZnPc-Py. To this end, we focused on the selective excitation
of Py and H2Pc/ZnPc components at 275 and 610 nm,
respectively, in THF. In particular, excitation of the Py reference
leads to a fluorescence spectrum showing maxima in the blue
part of the solar spectrum at 374 and 395 nm, a fluorescence
quantum yield close to unity, and a fluorescence lifetime of
nearly 100 ns.13d Turning to the excitation of the H2Pc and ZnPc
moieties at 610 nm, the following features were recorded in
the red part of the solar spectrum. For ZnPc-Py we found a
maximum at 683 nm and a shoulder at 745 nm, while H2Pc-Py
showed an intense peak at 706 nm with shoulders at 666, 733,
and 777 nm in THF. In ZnPc-Py and H2Pc-Py, excitation of Py
at 275 nm gave rise to strong, almost quantitative quenching of
the pyrene fluorescence features by 99%. A different situation
is observed in the red part of the spectrum. Here, despite the
Py excitation, either the H2Pc- or the ZnPc-centered features
are registered. Note the overlapping absorption features of Py
and H2Pc/ZnPc components in the 250-350 nm region.
Figure 1. Absorption spectra of ZnPc-Py in DMF (black spectrum), H2Pc-
Py in DMF (red spectrum), and H2Pc-Py in THF (blue spectrum).
Nevertheless, such a trend suggests an energy-transfer scenario
from the energetically higher lying Py singlet excited state to
the energetically lower lying H2Pc/ZnPc singlet excited state.
An excitation spectrum, that is, monitoring the Pc fluorescence
features at varied excitation wavelengths, confirms this assump-
tion by revealing features that track the absorption spectra of
H2Pc-Py and ZnPc-Py, including the characteristics of Py and
H2Pc/ZnPc components.
Important for the further characterization is the fact that the
absorption spectra of ZnPc-Py remain nearly unchanged in THF
and DMF, with maxima at 277/345 (Py), 366 (ZnPc Soret band),
and 680 nm (ZnPc Q-band), but notable changes evolve for
H2Pc-Py. In THF, on one hand, the Q-band splits into two
maxima at 668 and 702 nm, while in DMF, on the other hand,
just one maximum is seen at 676 nm (Figure 1). This behavior
has been previously observed23,24 and attributed to the interac-
tion of the basic center of DMF with the pyrrolic protons of
the Pc, which produces a “symmetrization” of the molecule.24
In other words, an electronic structure similar to the dianion-
like speciesscharacteristic for metallic complexes with D4h
symmetrysevolves. A notable but likewise weaker similarity
is seen in the case of H2Pc. Complementary investigations by
means of steady-state and time-resolved fluorescence spectros-
copy helped to confirm our assumption. In THF, ZnPc-Py and
H2Pc-Py revealed typical fluorescence features, namely distinct
maxima at 683 and 706 nm, respectively. In DMF, no ap-
preciable differences were noted for ZnPc-Py, whereas for H2Pc-
Py, in addition to the 706 nm peak, a second fluorescence peak
evolved at 684 nm. The latter coincides with the fluorescence
maximum of ZnPc-Py. This observation is in agreement with
the formation of a dianion-like structure for H2Pc-Py in DMF.
Likewise, for H2Pc, a similar trend is observed.
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Considering that mixtures of THF and DMF were successfully
used to prepare stable suspensions of SWNTs, we carried out a
screening of the spectroscopic behavior of ZnPc-Py and H2Pc-
Py in different ratios of THF and DMF. For example, in a
solvent mixture composed of 25 vol % THF and 75 vol % DMF,
we find an intense Q-band absorption at 674 nm, accompanied
by a weaker band at 702 nm for H2Pc-Py. Mirror-imaging these
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Torres, T.; Guldi, D. M. Chem. Sci. 2010; in press.
(24) Stillman, M. J.; Nyokong, T. in Phthalocyanines: Properties and
Appliocations; Leznoff, C. C., Lever, A. B. P., Eds.; VCH: Weinheim,
1989; p 139.
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Phthalocyanine-Pyrene Conjugates
A R T I C L E S
observations is that the presence of water disrupts the interac-
tions between the basic center of the DMF and the pyrrolic
protons. When, however, more water is added to the solution
of H2Pc-Py in DMF, a broad and blue-shifted band (630 nm)
evolves, which is a signature of aggregates in protic media.
Next, the electrochemical and spectroelectrochemical features
of Py, ZnPc, H2Pc, ZnPc-Py, and H2Pc-Py were explored. All
of the measurements were performed in dichloromethane (DCM)
containing tetrabutylammonium hexafluorophosphate (0.2 M)
as electrolyte. Under oxidative conditions, Py shows a single
oxidation at 1.17 V (vs. Fc/Fc+), while for ZnPc and H2Pc,
oxidations evolved at 0.43/1.13 and 0.52/0.77/1.23 V (vs. Fc/
Fc+), respectively. ZnPc-Py and H2Pc-Py are best described as
the superimposition of their of Py and ZnPc or Py and H2Pc
components, with, however, some minor shifts, which are
probably derived from the different substitution patterns. In
particular, for ZnPc-Py, on one hand, oxidation processes are
registered at 0.61, 1.10, and 1.26 V, and for H2Pc-Py, on the
other hand, the values are 0.8, 0.97, 1.19, and 1.36 V. As a
complement to the electrochemical features, the spectroelec-
trochemical features were determined. To this end, absorption
spectra were recorded without and with applying a potential.
In the case of ZnPc and ZnPc-Py, the potentials were increased
incrementally to 0.7 V. Working with H2Pc required the
application of potentials of up to 0.8 V, and a potential of 1.0
V was finally applied for H2Pc-Py. Notable is that, under
oxidative conditions, the ground-state absorption features fade
away, with the concomitant growth of new bands. The latter
represent the absorptions due to the formation of the one-
electron-oxidized form of either ZnPc or H2Pc. In detail, for
ZnPc and ZnPc-Py, new bands are discernible in the 400-600
nm range and around 840 nm. These are the well-known
fingerprints of the one-electron-oxidized form of ZnPc (see
Supporting Information, Figure S1).26 For H2Pc and H2Pc-Py,
a broad absorption in the 400-600 nm range (i.e., H2Pc, maxima
at 405 and 535 nm; H2Pc-Py, maximum at 490 nm) is
observable; additionally, we can identify a band around 900
nm for both metal-free phthalocyanines.
Figure 2. (Top) Absorption spectra of H2Pc-Pc in DMF (black spectrum)
and after addition of 3 vol % of H2O (red spectrum). (Bottom) Fluorescence
spectra of H2Pc-Py in DMF (black spectrum) and DMF + 3 vol % H2O
(red spectrum).
absorption features, a single peak at 706 nm dominates the
fluorescence spectrum of H2Pc-Py. In addition, the fluorescence
quantum yields and the fluorescence lifetimes (see Supporting
Information, Table S1) change significantly in pure THF and
DMF. Reassuring is the fact that values for the different DMF/
THF mixtures are in between those seen for the pure solvents.
The observations for H2Pc are similar. The recovery of the
splitting upon addition of THF can be rationalized considering
that the presence of this solvent may disturb the interactions
between DMF and the pyrrolic protons. In sharp contrast, ZnPc-
Py as well as ZnPc is not susceptible to any solvent-related
changes, including fluorescence quantum yields that are around
3.0 ns and fluorescence lifetimes that vary marginally between
2.9 and 3.5 ns.
As demonstrated, changes in the solvent environment exert
rather large effects on the H2Pc-Py absorptions. To follow up
on this aspect, we started to perform titration assays with H2Pc-
Py in DMF and variable quantities of water.25 In terms of
absorption, the band at 676 nm decreases steadily and blue-
shifts slightly, with the concomitant formation of a new band
centered at 702 nm. In terms of fluorescence, the intensity of
the peak at 706 nm increases by a factor of almost 3 when 3
vol % of water is present (Figure 2), and the fluorescence
lifetime decreases with a strong reduction of the weight of the
fast component (Table S1). A plausible explanation for these
The aforementioned assays were complemented by transient
absorption measurements. For ZnPc in THF, the following
changes occur upon 387 nm photoexcitation: The singlet excited
state is formed right after the laser pulse. It comprises a broad
transient maximum that is centered at 490 nm, followed by
bleaching between 610 and 685 nm. This transient decays rather
slowly on the time scale of our instrumental detection (i.e., 3.0
ns) due to intersystem crossing to the corresponding triplet
manifold. The main spectral features of the latter are a maximum
at 500 nm and a minimum at 680 nm. At this point we conclude
that, especially in the near-infrared region, the two transients,
namely, the singlet and the triplet excited state, differ largely
in their absorption. Likewise, when exciting ZnPc-Py or H2Pc-
Py at 387 nm in a mixture of 25% THF/75% DMF, the transient
bleach is discernible (Figure 3). To be precise, minima evolve
for ZnPc-Py at 612/679 nm and for H2Pc-Py at 609/677 nm. In
contrast to additional experiments with 660 nm photoexcitation,
a very fast S2-to-S1 conversion is seen, with rate constants
around 1.4 × 1012 s-1 in the case of 387 nm excitation. The
bleach is further accompanied by the formation of several new,
(26) (a) Nyokong, T.; Gasyna, Z.; Stillman, M. J. Inorg. Chem. 1987, 26,
548. (b) El-Khouly, M. E.; Ito, O.; Smith, P. M.; D’Souza, F. J.
Photochem. Photobiol. C 2004, 5, 79. (c) Kahnt, A.; Quintiliani, M.;
Va´zquez, P.; Guldi, D. M.; Torres, T. ChemSusChem 2008, 1, 97.
(25) Important in this context is that the DMF used in the experiments
was from a recently opened, spectrophotometric grade bottle, with
less than 0.15% H2O.
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from thinner and/or thicker bundles. Accordingly, attempts to
map the steady-state fluorescence of SWNTs with white light
were unsuccessful due to low SWNT concentrations. Ap-
preciable SWNT band-gap fluorescence was detected only upon
more intense laser excitation with the second harmonic genera-
tion of a Nd/YAG laser at 532 nm. Under these conditions,
fluorescence maxima at 1120, 1278, 1422, and 1521 nm
correlate well with the aforementioned absorption maxima (see
Supporting Information, Figure S2).
These SWNT suspensions in THF were titrated with variable
amounts of dissolved H2Pc-Py and ZnPc-Py, bringing about
drastic changes in absorption and fluorescence of the Pc
component. First, the presence of SWNTs is manifested as a
significant broadening of the H2Pc and ZnPc features, especially
in the Q-band region. Second, the absorption bands of the latter
red-shift from 684 to 704 nm and from 668/702 to 699/725 nm
for H2Pc-Py in DMF (see Supporting Information, Figure S3).
Third, in steady-state fluorescence experiments, the H2Pc-Py-
and ZnPc-Py-related features are quantitatively quenched until
free, unbound H2Pc-Py/ZnPc-Py remains in solution. Fourth,
the near-infrared absorption transitions, which are associated
with SWNTs, also undergo successive red-shifts of, on average,
4 nm. Finally, SWNTs fluoresce red-shifted (i.e., 1128, 1287,
1434, and 1543 nm) and quenched (vide infra, see also Figure
S2). Taking these observations together, we reach the conclusion
that H2Pc-Py and ZnPc-Py were successfully immobilized onto
SWNT. Regarding H2Pc and ZnPc, the lack of anchor groups
fails to give rise to any notable interactions (see Supporting
Information, Figure S4).
The changes when adding an excess of H2Pc-Py in DMF to
predispersed SWNTs are interesting. In particular, we note that
the Q-band maximum at 676 nm in DMF splits into two maxima
at 671 and 702 nm, a trend that resembles the transformation
seen when going from DMF to THF. Similarly, the fluorescence
of H2Pc-Py is affected, where the long-wavelength transition
at 684 nm disappears and the fluorescence intensity increases,
in line with the results found when going from DMF to THF.
Due to self-absorption of the SWNT suspension and the
quenching of the fluorescence of H2Pc-Py immobilized onto
SWNTs, quantification of this rise is not possible without major
errors. In test experiments with H2Pcslacking Pysthe changes
in absorption and fluorescence were reproduced but without the
aforementioned broadening of the absorption and the strong
fluorescence quenching. Instead, the resulting absorption spectra
are best described as linear combinations of the individual
features (i.e., H2Pc and SWNTs; see Figures S3 and S4).
Figure 3. (Top) Differential absorption spectra (visible and near-infrared)
obtained upon femtosecond flash photolysis (387 nm) of H2Pc-Py in 25%
THF/75% DMF with several time delays between 0 and 3000 ps at room
temperature. (Bottom) Differential absorption spectra (visible and near-
infrared) obtained upon femtosecond flash photolysis (387 nm) of ZnPc-
Py in 25% THF/75% DMF with several time delays between 0 and 3000
ps at room temperature.
broad absorption bands in the ranges from 460 to 530 nm and
from 780 to 960 nm. Py is not excited at either 387 or 660 nm
excitation.
Titrations of SWNT suspensions with H2Pc-Py/ZnPc-Py. The
realization of novel SWNT hybrids, in which SWNTs are
associated with H2Pc/ZnPc by virtue of π-π stacking, is the
major thrust of the current work. To investigate the interactions
between H2Pc-Py/ZnPc-Py and SWNTs, titration experiments
were carried out. First, semistable suspensions of SWNTss
containing finely dispersed SWNTsswere prepared in either
THF or DMF. Here, 0.5 mg of solid SWNTs was added to 3
mL of THF or DMF, followed by a 10 min ultrasonication step.
Three to four drops of this suspension were added to 10 mL of
THF or DMF, followed by an additional 10 min of ultrasoni-
cation. This procedure is repeated up to 10 times to obtain a
light-grayish semistable suspension of SWNTs. Care has been
taken in handling the suspension and maintaining temperature
stability as measures to prevent aggregation/precipitation of
SWNTs. Decisive information for optimizing the dispersion of
SWNTs was derived from absorption and fluorescence spec-
troscopy. As an example, attention was given to registering the
SWNT-associated transitionssas they occur between different
van Hove singularities in the density of statessin the near-
infrared at 1090, 1171, 1318, and 1455 nm. In pure THF or
DMF, it was impossible to perform several sonication and
centrifugation cycles to separate fully individualized SWNTs
Preparation of Stable SWNT Suspensions. Working at higher
concentrations, the association between H2Pc-Py/ZnPc-Py and
SWNTs is too weak to stabilize individual SWNTs and to avoid
bundle formation. To circumvent the issues of instability and
concentration, a mixture of 25 vol % THF and 75 vol % DMF
was utilized since, in these conditions, SWNTs can be readily
dispersed in high concentrations. In the present context, we used
0.5 mg of solid SWNTs that were quantitatively dispersed in
10 mL of the solvent. After ultrasonication, which never
exceeded 20 min, we obtained a homogeneous, dark gray
suspension of SWNTs. Significantly, any excess of SWNTs was
removed by centrifugation for 15 min at 9.6 kG. The resulting
SWNT suspensions did not show appreciable SWNT-based
fluorescence in the NIR, since most likely, debundling is
incomplete. Nevertheless, their overall stability renders them
an ideal starting point for further tests.
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Figure 4. (Left) AFM image of ZnPc-Py/SWNT dispersion, spin-coated on a Si slide (20 × 20 µm). (Right) AFM image of H2P-Py/SWNT dispersion,
spin-coated on a Si slide (5 × 5 µm).
Following the aforementioned procedure, solutions of H2Pc-
Py/ZnPc-Py in 25 vol % THF and 75 vol % DMF afforded
stable suspensions of H2Pc-Py/SWNT or ZnPc-Py/SWNT.
Depending on the ratio of H2Pc-Py to SWNT and ZnPc-Py to
SWNT, features of immobilized and also free H2Pc-Py and
ZnPc-Py were dissected. The earlier are seen in the form of
red-shifted Q-band absorptions at 704 nm for ZnPc-Py/SWNT,
while proof for H2Pc-Py/SWNT is built on maxima at 725 nm.
The red-shifts seen for H2Pc-Py in H2Pc-Py/SWNT are the same
throughout the titration experiments. We believe that both free
and immobilized forms are in equilibrium, but with a slight
tendency to favor the immobilized H2Pc-Py/SWNT. The same
applies to ZnPc-Py and ZnPc-Py/SWNT equilibrium. Regarding
the overall stability of the different suspensions, we could not
derive any differences between ZnPc-Py and H2Pc-Py. The
ZnPc/H2Pc-centered fluorescence in ZnPc-Py/SWNT and H2Pc-
Py/SWNT is quenched by approximately 2 orders of magnitude
when compared to those of ZnPc-Py and H2Pc-Py in the absence
of any SWNTs. Owing to the immobilization onto SWNTs, the
fluorescence is quenched, but competitive light absorption due
to the presence of SWNTs might also contribute.
Atomic force microscopy of the dispersed H2Pc-Py/SWNT
or ZnPc-Py/SWNT was carried out to ensure the homogeneity
of the corresponding suspensions. In Figure 4, typical AFM
images are shown. They confirm, the H2Pc-Py/SWNT and ZnPc-
Py/SWNT throughout the scanned area, the presence of well-
dispersed thin bundles of SWNTs with diameters between 2
and 6 nm and a length of around 5 µm.
More insight into interactions of H2Pc-Py/ZnPc-Py with
SWNTs was gathered by mapping the band-gap fluorescence
of SWNTs upon lamp irradiation at varying wavelengths. First,
a SWNT/ sodium dodecylbenzenesulfonate (SDBS) suspension
in D2O was prepared as an internal standard to be compared
with H2Pc-Py/SWNT or ZnPc-Py/SWNT. Figures 5 -7 compare
the fluorescence spectra of SWNT/SDBS in D2O with those of
H2Pc-Py/SWNT or ZnPc-Py/SWNT in 25 vol % THF and 75
vol % DMF. All suspensions exhibit the same optical density
at different excitation wavelengths. The fact that different
Figure 5. Steady-state fluorescence spectraswith increasing intensity from
blue to green to yellow and to redsof SWNT/SDBS in D2O dispersion.
solvents were used should not influence the overall interpreta-
tion. SWNT/SDBS exhibits the strongest fluorescence maxima,
ascribed to (9,4), (7,6), (8,6), (11,3), (9,5), (10,5), (8,7), and
(9,7).27 A closer look at the fluorescence features following
excitation at 725 nm (see Figure 8) shows that all SWNT-
centered fluorescence of ZnPc-Py/SWNT and H2Pc-Py/SWNT
is red-shifted and strongly quenched (i.e., SDBS/SWNT/D2O,
1055, 1108, 1178, 1267, (1327), 1382, 1484 nm; ZnPc-Py/
SWNT: 1081, 1108, 1213, 1297, 1413, 1535 nm; H2Pc-Py/
SWNT: 1184, 1141, 1219, 1303, 1431, 1543 nm). In other
words, average red-shifts of 33 and 41 nm emerge for ZnPc-
Py/SWNT and H2Pc-Py/SWNT relative to SWNT/SDBS in
D2O. Apparently, larger diameter SWNTs are subject to stronger
red-shifts than smaller diameter SWNTs. These findings further
confirm the successful immobilization.28 When analyzing the
(27) Specific SWNTs were assigned to fluorescence bands by using the
Horiba Jobin Yvon Nanosizer version 2.0.
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A R T I C L E S
Bartelmess et al.
perfect agreement with the extent of red-shifts seen in the
absorption spectra. Laser excitation at 532 nm was carried out
to verify the quenching and red-shifting of the SWNT fluores-
cence bands. Considering the aforementioned, immobilization
of ZnPc-Py or H2Pc-Py onto SWNTs is mainly responsible for
the quenching of SWNT fluorescence.
To probe the electronic and structural influence that the
immobilization of H2Pc-Py/ZnPc-Py onto SWNTs might exert,
Raman measurements were performed with liquid samples as
well as solid samples (i.e., dried on a glass slide) with an
excitation wavelength of 1064 nm. The D-band at 1278 cm-1
,
as a reflection of sp2-hybridization due to rupture in the SWNT
structure or chemical changes of the SWNTs, was quite weak
for all of the samples.29 Moreover, the fact that there is
essentially no difference in intensity between, for example,
SWNTs and H2Pc-Py/SWNT attests to the absence of structural
damage of the SWNT structure as a result of our workup
procedure. This observation is well in line with recently
published studies on the effects of ultrasonication on SWNTs.30
Likewise, no changes are seen when the G-band at 1591 cm-1
and the G′-band at 2548 cm-1 in H2Pc-Py/SWNT and in SWNTs
are compared either in the solid state or in suspensions (i.e.,
25% THF/75% DMF).31 Similar results were gathered for ZnPc-
Py/SWNT.
Figure 6. Steady-state fluorescence spectraswith increasing intensity from
blue to green to yellow and to redsof ZnPc-Py/SWNT in 25% THF/75%
DMF dispersion.
Transient absorption spectroscopy was employed to confirm
the ultrafast singlet excited-state deactivations and, in addition,
to characterize the nature of the photoproducts. The same ZnPc
singlet excited characteristicssdespite the presence of the
electron-accepting SWNTsswere registered upon 387 nm
photoexcitation of ZnPc-Py/SWNT (see Supporting Information,
Figure S5, and Figure 9). This confirms the successful excitation
of ZnPc. In contrast to the slow intersystem crossing which was
noted for ZnPc and ZnPc-Py, the ZnPc singlet excited-state
features now decay rather quickly. The lifetimes (i.e., 3 ps)
reflect the global trends seen in the fluorescence experiments.
Simultaneously with the ZnPc singlet excited-state decay, the
formation of a new transient species evolves with distinct
maxima at 520 and 840, which are attributes of the one-electron-
oxidized ZnPc radical cation. Similarly, the range beyond 1000
nm (i.e., 1000-1600 nm) is important, which immediately after
the photoexcitation is dominated by a negative imprint of the
van Hove singularities of SWNTs. Notable are appreciable blue-
shifts of the transient bleaches, which represent the formation
of a new product, with minima that shift from 1151, 1286, 1448,
and 1582 nm to 1137, 1280 1428, and 1562 nm, respectively.
Figure 7. Steady-state fluorescence spectraswith increasing intensity from
blue to green to yellow and to redsof H2Pc-Py/SWNT in 25% THF/75%
DMF dispersion.
(28) (a) Bachilo, S. M.; Strano, M. S.; Kittrell, C.; Hauge, R. H.; Smalley,
R. E.; Weisman, R. B. Science 2002, 298, 2361. (b) O’Connell, M. J.;
Bachilo, S. M.; Huffman, C. B.; Moore, V. C.; Strano, M. S.; Haroz,
E. H.; Rialon, K. L.; Boul, P. J.; Noon, W. H.; Kittrell, C.; Ma, J.;
Hauge, R. H.; Weisman, R. B.; Smalley, R. E. Science 2002, 297,
593. (c) Weisman, R. B.; Bachilo, S. M. Nano Lett. 2003, 3, 1235.
(d) Hartschuh, A.; Pedrosa, H. N.; Novotny, L.; Krauss, T. D. Science
2003, 301, 1354. (e) Yoon, D.; Kang, S.-J.; Choi, J.-B.; Kim, Y.-J.;
Baik, S. J. Nanosci. Nanotechnol. 2007, 7, 3727.
(29) (a) Eklund, P. C.; Holden, J. M.; Jishi, R. A. Carbon 1995, 33, 959.
(b) Dresselhaus, M. S.; Jorio, A.; Souza Filho, A. G.; Dresselhaus,
G.; Saito, R. Physica B 2002, 15, 15. (c) Chen, G.; Sumanasekera,
G. U.; Pradhan, B. K.; Gupka, R.; Eklund, P. C.; Bronikowski, M. J.;
Smalley, R. E. J. Nanosci. Nanotechnol. 2002, 2, 621. (d) Dresselhaus,
M. S.; Dresselhaus, G.; Saito, R.; Jorio, A. Phys. Rep. 2005, 409, 47.
(e) Lefrant, S.; Baltog, I.; Baibarac, M. Synth. Met. 2009, 159, 2173.
(30) Forney, M. W.; Poler, J. C. J. Am. Chem. Soc. 2010, 132, 791.
(31) (a) Maciel, I. O.; Anderson, N.; Pimento, M. A.; Hartschuh, A.; Qian,
H.; Terrones, M.; Terrones, H.; Campos-Delgado, J.; Rao, A. M.;
Novotny, L.; Jorio, A. Nat. Mater. 2008, 7, 878. (b) Dresselhaus, M. S.;
Jorio, A.; Hofmann, M.; Dresselhaus, G.; Saito, R. Nano Lett. 2010,
10, 751.
Figure 8. Comparison of the NIR fluorescence spectra of SWNT/SDBS
in D2O (black), ZnPc-Py/SWNT in 25% THF/75% DMF (red), and H2Pc-
Py/SWNT in 25% THF/75% DMF (blue) upon lamp excitation at 725 nm.
fluorescence intensity throughout the selected range, we observe
an average quenching of 70% for ZnPc-Py/SWNT and 95%
for H2Pc-Py/SWNT. From the latter we conclude that H2Pc-
Py/SWNT shows the strongest interactions, a trend that is in
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Phthalocyanine-Pyrene Conjugates
A R T I C L E S
695 nm, while the absorption of H2Pc-Py is essentially a mirror
image of that established for the H2Pc-Py in pure dry DMF.
Under the same conditions (i.e., 25 vol % THF, 75 vol % DMF,
and 5 vol % Triton X-100), the fluorescence pattern of H2Pc
reveals two symmetrical fluorescence bands at 671 and 700 nm.
Notably, in the absence of Triton X-100, the band at 700 nm
dominates, and for H2Pc-Py (in the presence of Triton-X-100),
the band at 706 nm disappeared completely, leaving just the
weak fluorescence band at 682 nm. The H2Pc-centered fluo-
rescence quantum yields tend to be much lower in the presence
of Triton X-100. For example, the quantum yields drop for H2Pc
from 0.40 to 0.15 and for H2Pc-Py from 0.18 to 0.03. In
concentration-dependent experiments, in which the Triton X-100
concentration was incrementally increased to a maximum of 5
vol %, the aforementioned changes were further corroborated.
Turning to ZnPc-Py/SWNT, the alterations upon the addition
of Triton X-100 include a decrease of the absorption signature
of the immobilized form (i.e., ZnPc-Py/SWNT), an increase of
the absorption signature associated with the free form (i.e., ZnPc-
Py), and an increase of the overall fluorescence intensity (i.e.,
ZnPc-Py). A similar trend is observed for H2Pc-Py/SWNT in
the presence of Triton X-100 (vide supra). Support for this
notion came from the recovery of the absorption features known
from the free form of H2Pc-Py (in 25 vol % THF, 75 vol %
DMF, and 5 vol % Triton X-100) and that go along with
decreasing intensity of the 706 nm fluorescence and increasing
intensity of the 682 nm fluorescence.32
Finally, we explored the influence of 1,4-diazabicyclo-
[2.2.2]octane (DABCO), a bidendate ligand that coordinates
strongly (105 M-1) to ZnPc, on the equilibrium.33 Crucial for
our assays is that the addition of DABCO to ZnPc or ZnPc-Py
has no profound impact on the fluorescence quantum yield. In
fact, the quantum yields are 0.27 and 0.25 for ZnPc without
and with DABCO, respectively. For ZnPc-Py, the corresponding
values are 0.31 and 0.25. Equally important is the observation
that the fluorescence lifetimes remain constant. Performing the
same titration experiments with SWNT suspensions and addition
of ZnPc-Py until a slight excess of ZnPc-Py is reached (Figure
10), the absorption of immobilized ZnPc-Py intensifies at the
expense of that of free ZnPc-Py upon addition of DABCO. This
indicates the immobilization of additional ZnPc-Py, which is
bound to ZnPc-Py/SWNT by means of DABCO. Also, fluo-
rescence features of the free form of the ZnPc-Py fade due to
connection to the SWNT surface. All these occur exclusively
for ZnPc-Py, since H2Pc-Py/SWNT solutions are resistant owing
to the lack of metal center.
Thin-Film Preparation and Characterization. After our
success in debundling and immobilizing H2Pc-Py/ZnPc-Py onto
SWNTs, solid-state films were prepared. To this end, ZnPc-
Py/SWNT suspensions were prepared in a solvent mixture of
25% THF/75% DMF following the above-described procedure.
Always an excess of either H2Pc-Py or ZnPc-Py was used to
ensure a maximum loading on SWNT. In the first step of the
preparation procedure, an indium tin oxide (ITO) slide was
carefully cleaned in isopropanol and dried. The SWNT suspen-
sions were then filtered by using a 0.2 µm PTFE-supported
membrane filter. During the filtration, the excess of H2Pc-Py
or ZnPc-Py in solution was removed; the filtrate was of greenish
Figure 9. (Top) Differential absorption spectra (extended near-infrared)
obtained upon femtosecond flash photolysis (387 nm) of ZnPc-Py/SWNT
with several time delays between 0 and 10 ps at room temperature. (Bottom)
Time-absorption profiles of spectra shown above at 484, 612, 702, 760,
and 1200 nm, monitoring the charge separation.
Implicit are new conduction band electrons, injected from
photoexcited ZnPc, shifting the SWNT transitions to lower
energies. Thus, we reach the conclusion that photoexcitation
of ZnPc-Py/SWNT is followed by a rapid charge transfer. A
multiwavelength analysis of the newly developed charge-transfer
state reveals its metastable character with decays beyond the
3.0 ns time window of our experimental setup.
The apparent equilibrium between immobilized and free
H2Pc-Py/ZnPc-Py was further tested in a series of follow-up
experiments. First, dilution experiments were the focus of our
investigations. The addition of extra solvent to the suspensions
of H2Pc-Py/SWNT or ZnPc-Py/SWNT led to a notable linear
decrease of the absorption fingerprints of the Pc Q-bands
representing immobilized H2Pc-Py/ZnPc-Py and the concomitant
increase of the absorptions corresponding to free H2Pc-Py/ZnPc-
Py species.
Second, additions of a nonionic surfactant (i.e., Triton X-100)
were tested as a powerful means to replace H2Pc-Py/ZnPc-Py,
which is immobilized onto the surface of SWNTs. As important
reference experiments, ZnPc and ZnPc-Py were found to be
insusceptible to notable changes in terms of their absorption
and fluorescence spectra, fluorescence quantum yield, and
fluorescence lifetimesin a solvent mixture containing 25 vol
% THF, 75 vol % DMF, and 5 vol % Triton X-100. In contrast,
the experiments with H2Pc and H2Pc-Py showed an explicit
influence of Triton X-100 on the characteristics of the system.
The absorption spectrum of H2Pc still reveals the weak band at
(32) Control experiments with SWNTs dispersed with the help of 5 vol %
Triton X-100 failed to provide unambiguous evidence for the im-
mobilization of either ZnPc-Py or H2Pc-Py.
(33) Hunter, C. A.; Meah, M. N.; Sanders, J. K. M. J. Am. Chem. Soc.
1990, 112, 5773.
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Bartelmess et al.
Figure 10. Absorption spectra of SWNTs in 25% THF/75% DMF (black
spectrum), after addition of a small amount of ZnPc-Py solution (gray
spectrum), after a second addition of ZnPc-Py solution (brown spectrum;
note the presence of the absorption band of free ZnPc-Py at 680 nm), and
after addition of 0.1 mg of solid DABCO (red spectrum).
Figure 11. Absorption spectra of SWNT thin film (black spectrum), ZnPc-
Py/SWNT thin film (blue spectrum), and H2Pc-Py/SWNT thin film (red
spectrum), all on cleaned ITO.
color. Owing to the equilibrium between bound and free H2Pc-
Py/ZnPc-Py, we were reluctant to excessively wash the resulting
black films. The resulting SWNT films were carefully transferred
to ITO and dried under low pressure for 20 min at 120 °C,
leading to homogeneous SWNT films on ITO.20f Absorption
spectroscopy was used to determine the optical density of the
thin films and to ensure the presence of H2Pc-Py/SWNT and
ZnPc-Py/SWNT. In the case of ZnPc-Py/SWNT, we observed
an intense Q-band absorption at 705 nm, which corresponds to
ZnPc-Py immobilized onto SWNT. No absorption of free ZnPc-
Py was detectable. Next to ZnPc-Py/SWNT, we also prepared
films with H2Pc-Py/SWNT, DABCO/ZnPc-Py/SWNT, and just
SWNTs, all derived from solvent mixtures of 25% THF/75%
DMF.
In all cases, high-quality films were produced. In DABCO/
ZnPc-Py/SWNT and ZnPc-Py/SWNT, the absorptionssin par-
ticular those of ZnPc-Pyswere quite similar. Importantly, H2Pc-
Py/SWNT shows features (i.e., Q-bands at 699 and 725 nm)
that were established during the titration experiments. A closer
inspection of the features in the NIR reveals a similar basic
structure for ZnPc-Py/SWNT and H2Pc-Py/SWNT, but with
some differences. The most notable exception is that the
absorption bands of H2Pc-Py/SWNT are better resolved and
blue-shifted relative to those of ZnPc-Py/SWNT. Incomplete
SWNT debundling and the lack of resolution lead in the SWNT
films to red-shifted and weak absorption features (Figure 11).
Figure 12. SEM image of H2Pc-Py/SWNT thin film on ITO. 50000×
magnification and an acceleration voltage of 5 kV.
and in the presence of an external bias under white-light
illumination.34 Photoanodic behavior was always observed under
short-circuit conditions, which is indicative of photogenerated
electrons that flow from the film to the ITO electrode. Compar-
ing, however, the photoaction spectra of the different cells, that
is, SWNTs, ZnPc-Py/SWNT, and H2Pc-Py/SWNT, with I-/I2
(0.5 M/0.01 M) electrolyte revealed always the best perfor-
mances for H2Pc-Py/SWNT. In particular, maximum internal
photoconversion efficiency (IPCE) values were 0.4% for SWNT,
0.75% for ZnPc-Py/SWNT, and 2.4% for H2Pc-Py/SWNT for
cells that exhibited the same optical absorption throughout the
visible and near-infrared parts of the spectrum.
In light of the above results, we started to further optimize
the H2Pc-Py/SWNT cells. A major improvement in film quality
came from refluxing the ITO slides in n-propanol for 1 h. This
ensured the optimum hydrophobicity of ITO and, in turn, the
deposition of H2Pc-Py/SWNT. In addition, 0.5 M I- and 0.1 M
Solar Cell Fabrication. In the final part of our investigations,
we manufactured prototype solar cells comprised of ZnPc-Py/
SWNT or H2Pc-Py/SWNT. The latter were deposited onto a
0.2 µm pore size PTFE filter from a suspension in 25% THF/
752% DMF. Variable film thickness and, hence, increasing or
decreasing absorptions are obtained by changing the filtered
volume. Insight into the film morphology came from AFM and
SEM measurements. SEM investigations show that the SWNTs
homogeneously cover the substrate surface and form a membrane-
like film (Figure 12). The SWNTs arrange in intertwined bundles
with widths and lengths of around 10 nm and several hundreds
of nanometers, respectively. AFM images (not shown) cor-
roborate those observations with the presence of thicker bundles
on the surface.
(34) The electrochemical cell consists of SWNT-modified ITO and a Pt-
coated FTO as a counter electrode. To achieve uniform deposition, a
5 mM solution of H2PtCl6 × H2O in dry isopropanol is dispersed onto
the slide. Successive heat treatment at 480 °C removes all additives,
leaving only the pure Pt catalyst on the surface. Subsequently the
SWNT/ITO and the Pt/FTO electrodes are melted together using
Surlyn ionomer. The resulting 50 µm gap is filled with I-/I2 (0.5 M/0.1
M) as a redox mediator.
In the first sets of experiments, photocurrents were recorded
for SWNT, ZnPc-Py/SWNT, and H2Pc-Py/SWNT in the absence
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16210 J. AM. CHEM. SOC. VOL. 132, NO. 45, 2010

Phthalocyanine-Pyrene Conjugates
A R T I C L E S
assembling electron donor-acceptor hybrids with SWNTs.
Owing to the strong ability of pyrene to adhere to SWNT
sidewalls by means of π-π interactions, we have exploited this
polyaromatic anchor to immobilize metal-free (H2Pc) as well
as zinc (ZnPc) phthalocyanines onto the surface of SWNTs.
To this end, the formation of stable ZnPc/SWNT and H2Pc/
SWNT suspensions has been unequivocally confirmed by means
of diverse spectroscopic techniques. Most important in the
characterization of the resulting assemblies are assays with
respect to photoinduced electron-transfer events, from the singlet
excited state of ZnPc or H2Pc to SWNTs, as probed by steady-
state emission studies and transient absorption spectroscopy.
Encouraged by such charge-transfer features, we have utilized
ZnPc/SWNT and H2Pc/SWNT thin films in photoelectrochemi-
cal cells to test their solar energy conversion potential.
Performances have been realized that are much superior to those
of previously reported SWNT conjugates and hybrids.20f In
particular, H2Pc-Py/SWNT thin films reveal stable and repro-
ducible photocurrents with IPCE values as large as 15 and 23%,
without and with an applied bias of +0.1 V, ranking among
the highest reported values for SWNT-based systems. This result
highlights the benefits of non-covalent approaches to the
preparation of electron donor-acceptor hybrids in solar energy
conversion schemes.
Figure 13. IPCE spectra of H2Pc-Py/SWNT thin film in an I-/I2 (0.5 M/0.1
M) sandwich cell.
I2 emerged as the best electrolyte concentrations for H2Pc-Py/
SWNT. Current-voltage measurements were then carried out
in a voltage range between -0.5 V and +0.5 V at a rate of 5
mV/s. Around 26.6 or 53.4 mV for thinner and thicker films of
0.1 and 0.22 au, respectively, an open-circuit voltage sets in,
where no appreciable currents flow, neither in the dark nor under
illumination. Increasing the bias toward the anodic range resulted
in an increase of photocurrent as a consequence of facilitated
electron injection into ITO. These observations are consistent
with the expected photooxidation of H2Pc. At the maximum
applicable biassprior to any irreversible changes in the pho-
toactive layersthe net currents, calculated by subtracting the
dark currents from the photocurrents, were between 79 and 134
µA for 0.1 and 0.22 au, respectively. We noticed that the sign
of the photocurrent reverted when cathodic biases were applied.
Next, the photoaction spectra for H2Pc-Py/SWNT cells under
monochromatic conditions were recorded and the IPCEs deter-
mined. In line with what has been noted in the absorption spectra,
broad and featureless transitions emerge in the photoaction
spectrum between 350 and 720 nm (Figures 11 and 13). The
maximum IPCE values are intriguing, reach from 5 up to 15%
without applied bias. When a bias of +0.1 V was applied, the
derived IPCE values further increased to a maximum of 23%.
Acknowledgment. This work has been supported by the Spanish
MEC (CTQ-2008-00418/BQU and CONSOLIDER INGENIO
2010, CSD2007-00010), the Comunidad de Madrid (MADRISO-
LAR-2, S2009/PPQ/1533), the COST Action D35, the University
of Trieste, the Italian Ministry of Education MIUR (Cofin Prot.
20085M27SS), and Deutsche Forschungsgemeinschaft, Cluster of
Excellence “Engineering of Advanced Materials”, FCI, the Office
of Basic Energy Sciences of the U.S. We are grateful to the
Zentralinstitut fu¨r Neue Materialien und Prozesstechnik in Fu¨rth
for the possibility to perfom AFM measurements.
Supporting Information Available: Experimental section,
fluorescence quantum yields and lifetimes of the different
compounds in varied environments, spectroelectrochemistry of
ZnPc-Py, and spectra of titrations of SWNT suspensions with
H2Pc-Py (absorption and NIR fluorescence) and H2Pc (absorp-
tion). This material is available free of charge via the Internet
at http://pubs.acs.org.
Conclusions
In this work, phthalocyanine-pyrene conjugates, ZnPc-Py
and H2Pc-Py, have emerged as valuable building blocks for
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