Immobilizing water-soluble dendritic electron donors and electron acceptors - Phthalocyanines and perylenediimides - Onto single wall carbon nanotubes

Article abstract of DOI:10.1021/ja100065h

The complementary use of spectroscopy and microscopy sheds light onto mutual interactions between semiconducting single wall carbon nanotubes (SWNT) and either a strong dendritic electron acceptor - perylenediimide - or a strong dendritic electron donor - phthalocyanine. Importantly, the stability of the perylenediimide/SWNT electron donor-acceptor hybrids decreases with increasing dendrimer generation. Two effects are thought to be responsible for this trend. With increasing dendrimer generation we enhance (i) the hydrophilicity and (ii) the bulkiness of the resulting perylenediimides. Both effects are synergetic and, in turn, lower the immobilization strength onto SWNT. Owing to the larger size of the phthalocyanines, phthalocyanine/SWNT electron donor-acceptor hybrids, on the other hand, did not reveal such a marked dependence on the dendrimer generation. Several spectroscopies confirmed that distinct ground- and excited-state interactions prevail and that kinetically and spectroscopically well-characterized radical ion pair states are formed within a few picoseconds.

Full text of DOI:10.1021/ja100065h

Published on Web 04/19/2010
Immobilizing Water-Soluble Dendritic Electron Donors and
Electron AcceptorssPhthalocyanines and
Perylenediimidessonto Single Wall Carbon Nanotubes
Uwe Hahn,† Sarah Engmann,‡ Christian Oelsner,‡ Christian Ehli,‡ Dirk M. Guldi,*,‡
and Tomas Torres*,†
Departamento de Qu´ımica Orga´nica, UniVersidad Auto´noma de Madrid, 28049 Madrid, Spain,
and the Department of Chemistry and Pharmacy and Interdisciplinary Center for Molecular
Materials (ICMM), Friedrich-Alexander-UniVersita¨t Erlangen-Nu¨rnberg, Egerlandstrasse 3,
91058 Erlangen, Germany
Received January 10, 2010; E-mail: tomas.torres@uam.es; dirk.guldi@chemie.uni-erlangen.de
Abstract: The complementary use of spectroscopy and microscopy sheds light onto mutual interactions
between semiconducting single wall carbon nanotubes (SWNT) and either a strong dendritic electron
acceptorsperylenediimidesor a strong dendritic electron donorsphthalocyanine. Importantly, the stability
of the perylenediimide/SWNT electron donor-acceptor hybrids decreases with increasing dendrimer
generation. Two effects are thought to be responsible for this trend. With increasing dendrimer generation
we enhance (i) the hydrophilicity and (ii) the bulkiness of the resulting perylenediimides. Both effects are
synergetic and, in turn, lower the immobilization strength onto SWNT. Owing to the larger size of the
phthalocyanines, phthalocyanine/SWNT electron donor-acceptor hybrids, on the other hand, did not reveal
such a marked dependence on the dendrimer generation. Several spectroscopies confirmed that distinct
ground- and excited-state interactions prevail and that kinetically and spectroscopically well-characterized
radical ion pair states are formed within a few picoseconds.
Introduction
structure give rise to remarkably high extinction coefficients in
the red and near-infrared region. Apart from this, they have been
The considerable interest in carbon nanotubes (CNTs) is
closely related to their intriguing properties including electrical
and thermal conductivities, chemical stability, and mechanical
strength, among others.1 As a result, this relatively novel carbon
allotrope has been proposed for applications such as energy
conversion,2 sensing,3 actuators,4 or biological applications.5
Importantly, the efforts to exploit the favorable CNT properties
in energy conversion2 or electronic devices6 are fundamentally
connected to the ease of CNTs to play the role of an electron
acceptor and/or electron donor. Accordingly, in recent years
carbon nanotube-based nanohybrids/-conjugates formed by
electron donor-acceptor systems have been the focus of
attention.2a Association with electron-donor or -acceptors
thereby assists in modulating the electronic properties of CNTs.
In terms of electron-donating materials, compounds based on
macrocycles such as phthalocyanines represent a class for which
attention emanates from their ability to harvest light and
subsequently occurring energy- or electron transfer processes.7
These planar molecules with an elongated electron-rich aromatic
found to disclose outstanding photostability and singular physi-
cal properties. These features are the origin for phthalocyanines
to be strongly investigated in recent years in various fields such
as for photovoltaic devices.8 The phthalocyanine moiety has
also been in donor-acceptor systems together with fullerenes,9
perylenes,10 or related macrocyclic structures11 to furnish long-
lived and highly emissive charge-separated states. In line with
these observations are species consisting of CNTs and
phthalocyanines.9a,12 On the other hand, perylene derivatives
also represent a class of compounds with high photo- and
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† Universidad Auto´noma de Madrid.
‡ Friedrich-Alexander-Universita¨t Erlangen-Nu¨rnberg.
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6392 J. AM. CHEM. SOC. 2010, 132, 6392–6401
10.1021/ja100065h  2010 American Chemical Society

Dendritic Electron Donors and Acceptors Interacting with CNTs
A R T I C L E S
chemical stability, which in addition shows excellent electron-
acceptor properties.13 As valid as for the phthalocyanines, the
extended aromatic system of perylenes with a fused five-ring
π-system makes them susceptible for π-π stacking onto the
single wall carbon nanotubes (SWNT) surface by noncovalent
interactions. Recently, we have demonstrated the assembly of
perylene-SWNT nanohybrids, thus introducing one of the few
examples in which the SWNT played the role of the electron
donor.14
The solubilization or dispersion of phthalocyanine-CNT
conjugates is crucial when aiming at investigating the materials
properties of such assembled nanohybrids. However, many of
the specimens at the early stage of research on CNTs appeared
to be rather insoluble in common organic solvents. This point
has been addressed in recent years by the development of
synthetic strategies that allowed obtaining soluble derivatives,
while at the same time preserving the tubular CNT structure.15
Apart from the covalent modification of SWNT with phthalo-
cyanine moieties,9a,16 the noncovalent functionalization typically
via π-π-interactions offers a potent means for the construction
of donor-acceptor systems containing SWNT.12a,15,17 Whereas
the former approach most often has a significant impact on the
SWNT materials properties, the latter has been demonstrated
not to disrupt the conjugated electronic structure in nanotubes
with minor influences on the particular SWNT features.
However, a certain effect on the electronic transitions in the
nanotubes has still to be considered. Hence, a viable solution-
phase processability of such nanotube-based functional materials
for example, in the fabrication and deposition into devices
becomes achievable. Both approaches even have the potential
for the development of CNT-based materials being soluble in
water, thus enabling their processing under environmentally
friendly conditions.
Herein, we report the preparation of new dispersible
Pc-SWNT or perylene-SWNT nanohybrids. The employment
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A R T I C L E S
Hahn et al.
Chart 1
of dendritic structures with the hydrophobic phthalocyanine or
perylene core moieties surrounded by an increasing number of
oligoethylene glycol end groups allows for their dispersion and
study in both common organic solvents and aqueous media.
Selecting either a perylenediimide and/or a phthalocyanine offers
the incentive of switching from an electron acceptor to an
electron donor moiety as a potent means to manipulate the
electronic properties of SWNT. The only requirement that the
resulting conjugatessmore specifically their coressmust meet
is their adhesion to the SWNT surface. Notable is that this
balance between hydrophobic and hydrophilic parts induces as
well self-aggregationsvide infra. An in-depth photochemical
study of the charge transfer chemistry of the series of Pc-SWNT
and perylene-SWNT entities together with AFM and TEM
images to disclose their macroscopic structures will be presented.
according to the literature as described by McKeown et al.18
The dendritic wedges of first-to-third generations with hy-
droxymethyl function were employed in aromatic nucleophilic
substitution reactions between these materials and 4-nitrophtha-
lonitrile to produce the required phthalonitriles in good to high
yields of 50-80%. Finally, in each case dendrimer assembly
was accomplished by the cyclotetramerization using lithium
pentoxide in refluxing pentanol, to give PcG1-G3 as a mixture
of four inseparable isomers in 20-35% yield. The dendritic
precursors 1,219 of the first and second generations with tris-
ethylene glycol monomethyl ether end groups and benzylic
bromide functions have also been the initial point for the
synthesis of the preparation of the corresponding dendrons with
amine functions. Consequently, reaction with sodium azide in
(18) (a) Brewis, M.; Helliwell, M.; McKeown, N. B.; Reynolds, S.;
Shawcross, A. Tetrahedron Lett. 2001, 42, 813. (b) Brewis, M.;
Helliwell, M.; McKeown, N. B. Tetrahedron 2003, 59, 3863.
(19) See for example: (a) Smith, D. K. J. Chem. Soc., Perkin Trans. 2
1999, 1563. (b) Rio, Y.; Nicoud, J.-F.; Rehspringer, J.-L.; Nierengarten,
J.-F. Tetrahedron Lett. 2000, 41, 10207. (c) Stephan, H.; Juran, S.;
Born, K.; Comba, P.; Geipel, G.; Hahn, U.; Werner, N.; Vo¨gtle, F.
New J. Chem. 2008, 32, 2016.
Results and Discussion
Synthesis. The dendritic free base phthalocyanines PcG1-
PcG3 (Chart 1) of first-to-third generations have been prepared
9
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Dendritic Electron Donors and Acceptors Interacting with CNTs
A R T I C L E S
Chart 2
water provided quantitatively the corresponding benzylic azide
derivatives 3,4, which were then subjected to Staudinger
reduction using triphenylphosphine in THF in the presence of
traces of water to yield the targeted amines 5,6 as highly viscous
liquids. These amino functions then served for the conjugation
with perylene-3,4,9,10-tetracarboxylic dianhydride (7), which
was accomplished in the presence of zinc acetate by heating a
quinoline solution to 150 °C for 4 h. The target dendrimers
PerG1 and PerG2 (Chart 2) were obtained in high yields of
87 and 89%, respectively, as intense red-colored solids after
purification by column chromatography on silica gel and GPC.
Owing to the ethylene glycol end groups both compounds are
very soluble in common organic solvents such as CH2Cl2,
CHCl3, and THF. In addition, as a result of the high affinity of
the multiple ethylene glycol termini toward water the greatest
dendrimer PerG2 is also readily soluble in aqueous medium.
The increasing amount of hydrophilic surface moieties generates
an amphiphilic environment in which the hydrophobic core is
embedded as a center that is surrounded by the shell of
peripheral oligoethylene glycol surface groups. It should be
noted that PerG2 has to be dissolved in water shortly after
having been evaporated from an organic solution. Once
reorganized in the solid state, this compound becomes margin-
ally soluble in aqueous medium. However, employing the
before-mentioned procedure to evaporating from an organic
solution allows dissolving the substance in water.
adducts with sodium ions as base peaks (see Supporting
Information).
Photophysical Characterization. First the physicochemical
features of the different perylenediimides and phthalocyanines
were tested in various media. Reasonable solubilities were
ensured in toluene/dichloromethane (i.e., nonpolar) and water
(i.e., polar) as the extreme scenarios.
In nonpolar solvents the absorption spectra of all dendritic
perylenediimides PerG1-2 and phthalocyanines PcG1-3 are
best described as monomers. In particular, for the perylenedi-
imides PerG1-2 (Figure 1) the characteristic 0-*2, 0-*1, and
0-*0 transitions were seen in the visible range with increasing
oscillator strength at 460 nm < 490 nm < 529 nm. The
phthalocyanines PcG1-3, on the other hand, reveal the corre-
sponding 0-*2, 0-*1, and 0-*0 transitions at 645, 666, and
703 nm, again with increasing oscillator strength. Notably, the
different dendron generations inflict little if any changes on the
absorption features, and neither does a change in solvent polarity
(i.e., toluene versus THF) lead to any appreciable effects. In all
cases, emission spectra were recorded that mirror-image the
aforementioned absorptions. For the perylenediimides 535 and
575 nm are the *0-0 and *0-1 maxima, while for the
phthalocyanines the *0-0 maximum remained throughout the
generations at 708 nm. Implicit are exceptionally small Stokes
shifts, as the energetic difference between the long wavelength
absorption and short wavelength fluorescence, which are on the
order of 5-6 nm. The latter relate well with the rigid structures
of the chromophores, preventing significant alteration of the
vibrational fine-structure.
Despite these invariable absorption and emission features, a
look at the actual quantum yields of fluorescence reveals that
the perylenediimides must interact electronically. Here, a
reference value of almost unity should be considered. In the
current cases the quantum yields increase with dendrimer
generation and increase with decreasing solvent polarity with
values ranging from 0.8 (i.e., second generation in toluene) to
0.03 (i.e., first generation in dichloromethane). In line with this
general trend are the fluorescence lifetimes that are as long as
The elucidation of all structures was easy, owing to the fact
that the signals corresponding to the different parts of the
dendrimers appear in specific regions. In particular, the protons
of the perylene center appear as two signals located at 8.47 and
8.27 ppm. The NMR shifts for the dendritic part of the molecule
are only significantly influenced at the methylene protons next
to the bisimide function. These can be found at roughly 0.4
ppm downfield shifted when compared to the primary amines.
As expected, the 13C NMR spectra for the two dendrimers
showed analogue shifts for the corresponding carbon atoms. The
dendrimers were also characterized by MALDI-TOF mass
spectrometry giving clean spectra with the target signals or
9
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A R T I C L E S
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Figure 1. (Top) Excitation (i.e., black spectrum: 573 nm emission
wavelength) and emission (i.e., red spectrum: 470 nm excitation wavelength)
spectra of PerG1 (4.3 × 10-6 M) in aerated THF solution at room
temperature. (Bottom) Excitation (i.e., black spectrum: 720 nm emission
wavelength) and emission (i.e., red spectrum: 633 nm excitation wavelength)
spectra of PcG3 (1.7 × 10-6 M) in aerated THF solution at room
temperature.
Figure 2. (Top) Steady-state absorption spectra of PerG1 (3.3 × 10-6 M)
in H2O at room temperature in the absence and presence of variable amounts
of THF (i.e., pure water - black; and pure THF - pink; and THF/water
ratios of 1:4, 0.7:2.3, and 1:3). (Bottom) Emission spectra of PerG1 (3.3
× 10-6 M) in H2O at room temperature upon excitation at 450 nm in the
absence and presence of variable amounts of THF (i.e., pure water - black;
pure THF - pink; and THF/water ratios of 1:4, 0.7:2.3, and 1:3).
2.5 ns and as short as <100 ps (i.e., the time-resolution of our
setup). The phthalocyanine fluorescence, on the other hand,
seems not to be affected at all, with quantum yields of 0.33 (
0.06 and lifetimes of 6.2 ( 0.4 ns in toluene, THF, and
dichloromethane.
800-1300 nm range as a broad transient maximum (Figure S8,
Supporting Information). Interestingly, for THF the singlet
decay, which implies an intersystem crossing to the correspond-
ing triplet manifold, depends on the dendrimer generation.
Overall a stabilizing trend, that is, longer singlet excited-state
lifetimes, evolves as the dendrimer generation increases: PcG1
(630 ps) < PcG2 (940 ps) < PcG3 (1750 ps).
In the corresponding transient absorption measurements,
where we used approximately 150 fs pulse width excitation,
we note the instantaneously formed singlet-singlet features of
the perylenediimides (Figure S7, Supporting Information).20
These include transient minima at 460, 490, and 530 nm of
increasing strength, which indicate consumption of the ground
state, and transient maxima at 720, 875, and 950 nm. A general
feature of perylenediimides is their inefficient intersystem
crossing as the instantaneously formed excited state deactivates
nearly quantitatively via the radiative pathway to the ground
state.21 In this context, the singlet excited-state deactivation
tracks the trend established in the time-resolved fluorescence
measurements with lifetimes as short as 1750 ps: PerG1 in THF.
Likewise, the singlet excited-state features of the phthalocya-
nines evolve in 450-550 nm range as a broad transient
maximum, in the 600-750 nm range as sharp bleaching (645,
665, and 703 nm) of the ground-state transitions and in the
Although all of the tested perylenediimides and phthalocya-
nines were readily soluble in water, their absorption and
fluorescence features differ fundamentally from the conclusions
drawn from experiments in organic solvents. In organic media
just the perylenediimide fluorescence prompts sizable interac-
tions. Now in water perylenediimides and phthalocyanines are
susceptible to changes in absorption as well as in fluorescence
(!). At first glance the absorption spectra tend to reveal
significant broadenings, when compared to those in, for example,
toluene. A closer look sheds more light onto the underlying
phenomena. In particular, in water the 0-*2, 0-*1, and 0-*0
transitions of the perylenediimides are red-shifted and reversed
in oscillator strength, a clear indication of perylenediimide
aggregation through the formation of electronically interacting
J-aggregates, Figure S9, Supporting Information. In a decisive
titration experiment the solvent polarity was changed more
steadily, from pure THF step-by-step to that of almost utter
water, Figure 2. A THF solution of perylenediimide (10-6 M)
with 488 and 523 nm maxima was taken, and the water content
was increased gradually, while keeping the concentration
(20) Weiss, E. A.; Ahrens, M. J.; Sinks, L. E.; Gusev, A. V.; Ratner, M. A.;
Wasielewski, M. R. J. Am. Chem. Soc. 2004, 126, 5577.
(21) (a) Margineanu, A.; Hofkens, J.; Cotlet, M.; Habuchi, S.; Stefan, A.;
Qu, J.; Kohl, C.; Müllen, K.; Vercammen, J.; Engelborghs, Y.; Gensch,
T.; De Schryver, F. C. J. Phys. Chem. B 2004, 108, 12242. (b) Hayes,
R. T.; Wasielewski, M. R.; Gosztola, D. J. Am. Chem. Soc. 2000,
122, 5563. (c) Kaletas¸, B. K.; Dobrawa, R.; Sautter, A.; Würthner,
F.; Zimine, M.; De Cola, L.; Williams, R. M. J. Phys. Chem. A 2004,
108, 1900.
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Dendritic Electron Donors and Acceptors Interacting with CNTs
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constant. Throughout these experiments an isosbestic point
developed at 448 nm. Moreover, an overall weakening of the
absorption developed by approximately a factor of 4, a
significant broadening that extended to ∼650 nm, and a red-
shift in water of all of the transitions evolved. Concurrently the
monomer fluorescence, when exciting at the isosbestic point at
448 nm, underwent changes. This is in agreement with what
has been seen in numerous cases for perylenediimide ag-
gregates.22 In fact, the rather sharp/strong maxima in the blue
(i.e., maxima at 535 and 575 nm) are replaced by a broad/weak
maximum in the red (i.e., maxima at 660 nm for PerG1 or at
680 nm for PerG2). Nevertheless, a significant reduction in
fluorescence quantum yields, 0.8 in organic media and 0.001
(PerG1)/0.002 (PerG2) in water, is associated with this
transformation. In parallel with the absorption an isosbestic
point, that is, a wavelength of equal fluorescence, emerges at
645 nm. Importantly, in time-resolved fluorescence measure-
ments the aggregated forms of PerG1 and PerG2 give rise to
lifetimes of 240 and 630 ps, respectively. In summary,
fluorescence quantum yields track the fluorescence lifetimes
reasonably well.
Finally, when contrasting the features of phthalocyanines in
water with those in toluene, a similar trend, namely red-shifted
and weakened absorptions, is discernible (Figure S10, Support-
ing Information). Please compare maxima for the aggregates at
601, 622, 696, and 745 nm with those of the monomers at 645,
666, and 703 nm. Fluorescence (i.e., quantum yields ,10 -4),
on the other hand, was not seen at all for the phthalocyanines
in water. This encouraged us to attempt the disaggregation by
adding a surface-active surfactant, sodium dodecylbenzene-
sulfonate (SDBS), from which we expected to form a more
homogeneous shield around the phthalocyanine. Indeed, features
like those found in toluene as they evolve in the absorption
and fluorescence spectra corroborate our assumption, that is,
the quantitative recovery of what is assumed to be the monomer
absorption and fluorescence spectra.
Transient absorption changes associated with the perylene-
diimides in water are quantitatively similar to our findings in
nonpolar media. Specifically, we note minima at 490/530/575
nm and maxima at 680/810/1040 nm. It is, however, the relative
intensities of these minima and maxima that are drastically
different. To illustrate this, the minima intensities drop in the
following order: 490 > 530 > 575 nm. Again, this finding
suggests the formation of aggregates in water. The lifetimes of
the perylenediimide singlet excited states are also affected by
the aqueous environment. The values are as short as 200 ps for
PerG1.
In the corresponding femtosecond experiments with the
phthalocyanines, we note a trend that strongly resembles that
seen in THF, Figure 3. In particular, the singlet excited state
features with maxima in the 400-475 and 775-1300 nm
regions and minima at 610 and 700 nm transform with lifetimes
that depend on the dendrimer generation into those associated
with the triplet manifold.
In summary, we have developed a series of spectroscopic
features that assist in distinguishing between truly monomeric
forms of perylenediimides and phthalocyanines and their
corresponding aggregates. Important is their distinct tendency
to adhere to each other, which leads to electronically interacting
Figure 3. (Top) Differential absorption spectra (visible and near-infrared)
obtained upon femtosecond flash photolysis (387 nm) of PcG3 (4.0 × 10-5
M) in D2O with several time delays between 0 and 3000 ps at room
temperature; see figure legend for details about the time progression.
(Bottom) Time-absorption profiles of the femtosecond spectra at 532, 625,
and 890 nm, monitoring the intersystem crossing.
aggregates. This criterion is thought to be beneficial for the
immobilization of the perylenediimides and phthalocyanines
onto CNTs in general, vide infra. In the following, the
aforementioned fingerprints are exploited as a way to character-
ize perylenediimide/SWNT and phthalocyanine/SWNT.
First, titration experiments with perylenediimides and SWNT,
Figure 4, brought the following spectroscopic changes to light.
In the visible range the perylenediimide-centered transitions (i.e.,
483 and 548 nm) shift gradually to the red (i.e., 480, 520, and
560 nm) reflecting interactions between the perylenediimides
and SWNT. It is notable that, when inspecting the relative ratios
of the perylenediimide transitions at the beginning (483 nm >
548 nm) and at the end of the titration (520 nm < 560 nm), a
trend is deduced, suggesting that individual perylenediimides
have been exfoliated out of the initially formed aggregates and
immobilized onto SWNT. We must assume that perylenediim-
ides/SWNT interactions made up by contributions from π-π
stacking, hydrophobic forces, and charge transfer character
prevail over those that just govern perylenediimide/perylene-
diimide interactions. Still, a comparison with the data taken in
THF and/or an aqueous environment containing SDBS implies
a significant red-shift, 25 nm, which relates to the adhesion onto
SWNT. In the near-infrared range, the typical van Hove
singularities of SWNT emerge. However, in contrast to a
previous report the shifts are (i) to longer wavelengths and (ii)
to moderate wavelengths (i.e., ∼4 nm). As a matter of fact,
ground-state interactions as a reflection of partial reduction of
the perylenediimides and the simultaneous p-doping of SWNT
(22) For example: (a) Würthner, F.; Chen, Z.; Dehm, V.; Stepanenko, V.
Chem. Commun. 2006, 1188. (b) Jancy, B.; Asha, S. K. Chem. Mater.
2008, 20, 169.
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Figure 5. AFM image of PerG2/SWNT dispersion (D2O).
Figure 4. (Top) Steady-state absorption spectra of PerG2 and PerG2/
SWNT by stepwise increasing (black, gray, brown, and red) the SWNT
concentration in D2O. The concentration of PerG2 is kept constant for all
experiments (6.0 × 10-6 M). (Bottom) Complementary emission spectra
of PerG2 and PerG2/SWNT upon excitation at 480 nm.
are weak. This trend is also seen in the stability of the
corresponding suspensions.
Insights into perylenediimide/SWNT interactions came from
RAMAN experiments. Here, the two most important signatures,
that is, RBM- and G-modes, reveal upshift from 266 and 1590
cm-1 to 270 and 1592 cm-1 in D2O suspensions of SDBS/
SWNT and perylenediimides/SWNT, respectively, as well in
the solid without evident loss in resonance. This indicates the
fact that π-π interactions are operative. Augmentation through
ground-state charge transfer/p-doping plays, however, only a
minor role, if any.
Figure 6. Steady-state fluorescence spectra, with increasing intensity from
blue to green to yellow and to red, of PerG1/SWNT in D2O dispersion.
SDBS concentrations, varying from 0.0 to 0.1 M, confirms the
quantitative removal of the perylenediimides from the SWNT
surface.
Ensuring the homogeneity of perylenediimide/SWNT is critical.
Atomic force and transmission electron microscopies are important,
since they provide important insights into this aspect. Throughout
the scanned areas predominantly short (i.e., 500 µm) and thin
bundles (i.e., 10 nm) are discernible; Figure 5.
Three-dimensional (3D) fluorescence landscapes were gener-
ated for SWNT/SDBS resembling those seen in previous reports.
SWNT/SDBS exhibits the strongest fluorescence maxima from
SWNT that are ascribed to (9,4), (7,6), (8,6), (11,3), (9,5), (10,5),
(8,7), and (9,7), not shown. For PerG1/SWNT, Figure 6, a
fluorescence pattern evolves with bathochromic changes when
compared to SWNT/SDBS. For example, the 1108, 1180, and
1266 nm maxima shift to 1173, 1231, and 1313 nm, respectively.
Importantly, considering the 1200-1600 nm range, the SWNT
fluorescence is on the order between 10 and 14% when
compared to that of SWNT/SDBS.
Next, we turned to fluorescence as a complement to the
aforementioned absorption assays. The most important observa-
tion is that the aggregate emission in the 550-800 nm range
fades away gradually in the presence of SWNT without
revealing sizable monomer fluorescence. It is safe to assume
that in perylenediimide/SWNT the strong perylenediimide
fluorescence is quenched due to thermodynamically favored
charge- or energy transfer deactivations. Monomer fluorescence
is only seen when SDBS is finally added. In this context, SDBS
plays a key role in recovering the intrinsic fluorescence of the
perylenediimides. Similarly, the original absorption features of
perylenediimides and SWNT are progressively recovered. As
a matter of fact, plotting the absorption changes at 557 nm (i.e.,
maximum changes due to the disappearance of perylenediimide/
SWNT) and 530 nm (i.e., maximum changes due to the
appearance of perylenediimides plus SWNT) as a function of
Transient absorption spectroscopy was employed to confirm
the ultrafast singlet excited state deactivations and, in addition,
to characterize the nature of the photoproducts. In measurements
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Dendritic Electron Donors and Acceptors Interacting with CNTs
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with perylenediimide/SWNT (i.e., PerG1/SWNT and PerG2/
SWNT) 387, 560, and 775 nm were selected as excitation
wavelengths; 387 nm is rather unselective owing to the
overlapping absorption of perylenediimides and SWNT in this
range, while 560 and 775 nm enable the nearly selective
excitation of perylenediimides and SWNT, respectively. Im-
portant is the detection of the rapidly formed singlet excited
states of perylenediimides and SWNT. For the perylenediimides
it is reassuring to see the 480, 520, and 560 nm characteristics,
correlating with the red-shifted ground-state absorption of perylene-
diimide/SWNT rather than those at 460, 490, and 530 nm, due to
the absorption of monomeric perylenediimides. SWNT, on the other
hand, are distinguished by 950, 1160, 1318, 1465, and 1555 nm
transitions. Both singlet excited states are, however, very short-
lived as they deactivate in perylenediimide/SWNT within the first
5 ps following photoexcitation. Simultaneous with the excited-state
decays, evolution of a new product is registered that in all of the
cases includes perylenediimide- and SWNT-centered features. It
is striking that these evolve despite the selective perylenediimide
(i.e., 560 nm) or SWNT (i.e., 775 nm) excitation. This per se attests
to the formation of what could be an energy transfer or a charge
transfer product. A closer look at the features assists in identifying
the nature of the photoproduct. At a delay time of 5 ps the following
characteristics dominate the differential absorption changes. In the
visible range these are a minimum at 555 nm and a maximum at
660 nm, which are clear attributes of the one-electron reduced form
of perylenediimides as generated electrochemically and pulse
radiolytically.14 In the near-infrared region, appreciable blue-shifts
of the transient bleaches with minima at 880 (i.e., from 900 nm),
958 (i.e., 962 nm), 1145 (i.e., from 1160 nm), 1285 (i.e., from
1318 nm), 1435 nm (i.e., from 1465 nm), and 1550 nm (i.e., from
1555 nm) are detected during the deactivation. We imply here the
oxidized form of SWNT, in which the corresponding transitions
are shifted to higher energies. In other words, we have established
unambiguous evidence for charge separation that occurs from either
photoexcited perylenediimides or photoexcited SWNT. In line with
closely spaced entities the charge separated states are relatively
short-lived. From a multiwavelength analysis we derived lifetimes
of 120 and 160 ps for PerG1/SWNT and PerG2/SWNT, respec-
tively. Figure 7 documents the growth and decay kinetics of the
charge separated state.
In contrast, evidence for phthalocyanines that are interacting
with SWNT was first gathered in THF. When THF was added
to suspended SWNT solid probes of phthalocyanines, the
successive growth of the phthalocyanine-centered transitions was
observed at 668 and 703 nm. When inspecting the near-infrared
section of the spectrum, where the SWNT-centered features
appear, the underlying van Hove singularities further resembled
that trend in the form of a red-shift. This corroborates the
spontaneous immobilization of phthalocyanines onto SWNT.
In water, on the other hand, phthalocyanines (10-5 M) were
blended with solid SWNT. Throughout this process, the broad
features of the phthalocyanine aggregates disappeared between 500
and 800 nm and gave way to a 730 nm maximum. We tentatively
assigned this sharp peak to that of monomeric phthalocyanines,
albeit its location is in a range that is dominated by the van Hove
singularities of SWNT. An unusually strong red-shift (i.e., 705 to
730 nm) that inflicts strong electronic interactions between phtha-
locyanines and SWNT driven by π-π and charge transfer forces
would be implicit. Confirmation for this assignment came from
attempts to replace the phthalocyanines by SDBS. The correspond-
ing experiments at room temperature were unsuccessful, however,
owing to the strong adhesion forces that necessitate a significant
Figure 7. (Top) Differential absorption spectra (visible and near-infrared)
obtained upon femtosecond flash photolysis (387 nm) of PerG1/SWNT in
D2O with several time delays between 0 and 200 ps at room temperature;
see figure legend for details about the time progression. (Center) Differential
absorption spectra (extended near-infrared) obtained upon femtosecond flash
photolysis (387 nm) of PerG1 /SWNT in D2O with several time delays
between 0 and 200 ps at room temperature; see figure legend for details
about the time progression. (Bottom) Time-absorption profiles of the
femtosecond spectra at 825, 920, 975, and 1150 nm, monitoring the charge
separation and charge recombination.
activation barrier. This trend is also exemplified by the remarkable
stability of the phthalocyanine/SWNT suspensions that even after
months failed to give rise to any detectable changes. To overcome
the apparent activation barrier for desorbing the phthalocyanines
from SWNT, heat (i.e., up to 60 °C) was applied. Indeed, under
these conditions the known maxima of phthalocyanines in THF
and/or water plus SDBS, vide supra, evolve at 665 and 705 nm.
The transformation (i.e., 730 to 665/705 nm) is, however, incom-
plete and is indicative of an equilibrium that even at elevated
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A R T I C L E S
Hahn et al.
Figure 9. AFM image of PcG3/SWNT dispersion (D2O).
Figure 8. Raman spectra of SWNT/SDBS (gray spectrum), PerG1/SWNT
(black spectrum), and PcG3/SWNT (green spectrum).
temperatures is not fully pushed back to the side of free phthalo-
cyanines plus SWNT.
Raman experiments (see Figure 8) confirm that, for phtha-
locyanine/SWNT, as in the case of perylenediimide/SWNT,
π-π interactions are decisive. In particular, relative to SDBS/
SWNT, the RBM- and G-modes upshift in perylenediimides/
Figure 10. Steady-state fluorescence spectra, with increasing intensity from
blue to green to yellow and to red, of PcG3/SWNT in D2O dispersion.
SWNT from 266 and 1590 cm-1 to 269 and 1592 cm-1
,
respectively (i.e., D2O suspensions and in the solid). Ground-
state charge transfer/n-doping, on the other hand, seems to play
no significant role.
AFM and TEM confirm the presence of SWNT in phthalo-
cyanine/SWNT. Representative images reveal objects with high
aspect ratio that appear throughout the scanned regions, Figure
9. The mean length of these objects is typically on the order of
several micrometers, and their diameters range between a few
nanometers and several tens of nanometers.
Charge transfer processes evolving from either photoexcited
phthalocyanines or SWNT were probed in steady-state fluorescence
experiments in THF as solvent. To this end, the fluorescence of
phthalocyanines at 708 nm was compared in the absence and
presence of SWNT. Of relevance is a significant quenching of
phthalocyanine fluorescence in phthalocyanine/SWNT. Filter effects
(i.e., SWNT absorption, etc.) should not be ruled out as a sensible
contribution to the quantitative quenching.
For phthalocyanine/SWNT a fluorescence pattern evolves
with bathochromic changes, when compared to SWNT/SDBS,
which parallel those seen in the absorption spectrum, Figure
10. For example, the 1108 and 1180 nm maxima shift to 1140
and 1210 nm, respectively. An additional characteristic is that
in the 1000 to 1550 nm range, the SWNT fluorescence is on
the order between 14 and 23% when compared to that of SWNT/
SDBS.23 In line with what we have seen in the absorption
measurements, addition of SDBS led to the expected blue-shift
of the fluorescence peaks. Additionally, the intrinsic fluorescence
intensity of SWNT/SDBS is partially recovered.
Finally, transient absorption measurements were performed
with phthalocyanine/SWNT, see Figure 11. Here, 660 and 775
nm excitation wavelengths were used in addition to 387 nm to
address the phthalocyanines and SWNT individually. Sufficient
stability was, however, only observed for PcG2/SWNT and
PcG3/SWNT, which, in turn, omitted investigations with PcG1/
SWNT. Depending on the excitation wavelength we monitored
how either the phthalocyanine or SWNT or both singlet excited
states transform into the same photoproduct. For example, upon
660 nm the PcG3 excited-state features are discernible in the
400-550 nm region. These decay, however, instantaneously (i.e.,
in less than 2 ps) to afford the charge transfer product. Spectro-
scopic proof for a charge transfer product was lent from the
differential changes in the visible spectrum. Maxima at 530 and
(23) It is notable that the phthalocyanine fluorescence supersedes the SWNT
fluorescence in some parts.
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Dendritic Electron Donors and Acceptors Interacting with CNTs
A R T I C L E S
nm). A global analysis of the 400-1300 nm region resulted in a
lifetime of this newly generated state of about 250 ps.
Conclusions
In summary, we have documented through the complementary
use of spectroscopy and microscopy mutual interactions between
semiconducting SWNT and either a strong electron acceptor,
perylenediimide, or a strong electron donor, phthalocyanine. One
milestone is the establishment of a versatile methodology to achieve
water-soluble SWNT for processing under environmentally friendly
conditions. Importantly, the stability of the perylenediimide/SWNT
electron donor-acceptor hybrids decreases with increasing den-
drimer generation. Two effects are thought be responsible for this
trend. With increasing dendrimer generation we enhance(i) the
hydrophilicity and (ii) the bulkiness of the resulting perylenedi-
imides. Both effects are synergetic and, in turn, lower the
immobilization strength onto SWNT. On the other hand, in
phthalocyanine/SWNT electron donor-acceptor hybrids the larger
size of the phthalocyanines relative to that of the perylenediimides
seem to overcome, in part, the dendron bulkiness. Another
milestone is that a wide range of complementary spectroscopies
confirmed that distinct ground- and excited-state interactions prevail
and that kinetically and spectroscopically well-characterized radical
ion pair states are formed. Ground-state interactions are limited to
π-π stacking as evidenced by Raman spectroscopy without giving
rise to either p- or n-doping.
Experimental Section
Syntheses. The synthesis of dendritic branches 1-219 and free
base phthalocyanines PcG1-G3 has been accomplished according
to literature procedures.18 Synthetic details and analytical data of
the perylene dendrimers PerG1-G2 under study here are provided
in the Supporting Information.
Spectroscopy and Microscopy. Steady-state UV/vis/NIR ab-
sorption spectroscopy was performed by a Lambda 19 (Perkin-
Elmer) or a Cary 5000 spectrometer (VARIAN). Transient absorp-
tion spectroscopy was performed with a Clark MRX fs-Lasersystem
CPA 2101 (1 kHz, 150 fs pulse width, 775 nm) and the TAPPS
system Helios from Ultrafast systems. Steady-state fluorescence
spectra were taken from samples by a FluoroMax3 spectrometer
(HORIBA) in the visible detection range and by a FluoroLog3
spectrometer (HORIBA) with a IGA Symphony (512 mm × 1 mm
× 1 µm) detector in the NIR detection range. Emission lifetimes
were determined via time-correlated single-photon counting (TC-
SPC) by a FluoroLog3 spectrometer with a MCP detector (R3809U-
58). The Raman spectra were run with a FT-Raman spectrometer
RFS100 from Bruker with an excitation wavelength of 1064 nm
and a liquid nitrogen-cooled Germanium detector. AFM images
were taken by an AFM microscope SolverPro M from NT-MDT.
Figure 11. (Top) Differential absorption spectra (visible and near-infrared)
obtained upon femtosecond flash photolysis (387 nm) of PcG3/SWNT in
D2O with several time delays between 0 and 200 ps at room temperature;
see figure legend for details about the time progression. (Center) Differential
absorption spectra (extended near-infrared) obtained upon femtosecond flash
photolysis (387 nm) of PcG3/SWNT in D2O with several time delays
between 0 and 200 ps at room temperature; see figure legend for details
about the time progression. (Bottom) Time-absorption profiles of the
femtosecond spectra at 565, 625, 955, 1005, and 1135 nm, monitoring the
charge separation and charge recombination.
Acknowledgment. Funding from MEC (CTQ2008-00418/BQU,
CONSOLIDER-INGENIO 2010 CDS2007-00010 NANOCIENCIA
MOLECULAR, PLE2009-0070), MICINN (FOTOMOL, PSE-
120000-2009-008), ESF-MEC (MAT2006-28180-E, SOHYDS),
COST Action D35, Comunidad de Madrid MADRISOLAR-2,
S2009/PPQ/1533), and Deutsche Forschungsgemeinschaft (SFB
583) is acknowledged. U.H. thanks the MEC (Spain) for a “Ramo´n
y Cajal” research contract.
840 nm resemble the fingerprints of phthalocyanines that are one-
electron oxidized by means of electrochemistry and pulse radioly-
sis.24 Minima at 630 and 730 nm further accompany these maxima.
Important also is the range beyond 1000 nm, where features at
1140, 1270, 1395, and 1530 nm evolve. In the case of 387, 660,
and 775 nm excitation the latter are formed at the expense of the
decaying SWNT excited state (i.e., 1130, 1170, 1460, and 1535
Supporting Information Available: Syntheses and spectro-
scopic data of perylene dendrimers PerG1-2 as well as
photophysical characterization of PerG1-2/PcG1-3 without
SWNT. This material is available free of charge via the Internet
at http://pubs.acs.org.
(24) Quintillani, M.; Kahnt, A.; Wölfle, T.; Hieringer, W.; Vázquez, P.;
Goerling, A.; Guldi, D. M.; Torres, T. Chem.sEur. J. 2008, 14, 3765.
JA100065H
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