Electron-donating behavior of few-layer graphene in covalent ensembles with electron-accepting phthalocyanines

Article abstract of DOI:10.1021/ja411830x

We describe herein the first example of highly exfoliated graphene covalently linked to electron accepting phthalocyanines. The functionalization of the nanocarbon surface with alkylsulfonyl phthalocyanines was attained by means of a click chemistry protocol. The new ensemble was fully characterized (thermogravimetric analysis, atomic force microscopy, transmission electron microscopy and Raman, as well as ground-state absorption) and was studied in terms of electron donor-acceptor interactions in the ground and in the excited state. In particular, a series of steady-state and time-resolved spectroscopy experiments demonstrated photoinduced electron transfer from the graphene to the electron-accepting phthalocyanines. This is the first example of an electron donor-acceptor nanoconjugate, that is, few-layer graphene/phthalocyanine, pinpointing the uncommon electron donating character of graphene.

Full text of DOI:10.1021/ja411830x

Article
pubs.acs.org/JACS
Electron-Donating Behavior of Few-Layer Graphene in Covalent
Ensembles with Electron-Accepting Phthalocyanines
Maria-Eleni Ragoussi,† Georgios Katsukis,‡ Alexandra Roth,‡ Jenny Malig,‡ Gema de la Torre,*,†
Dirk M. Guldi,*,‡ and Tomas Torres*,†,§
́
†Departamento de Química Organ
́
ica, Universidad Auton
́
oma de Madrid, Cantoblanco, Madrid 28049, Spain
‡Department of Chemistry and Pharmacy and Interdisciplinary Center for Molecular Materials (ICMM),
Friedrich-Alexander-Universitat Erlangen-Nurnberg, Erlangen 91058, Germany
̈
̈
§IMDEA-Nanociencia, c/Faraday 9, Campus de Cantoblanco, Madrid 28049, Spain
S
* Supporting Information
ABSTRACT: We describe herein the first example of highly
exfoliated graphene covalently linked to electron accepting
phthalocyanines. The functionalization of the nanocarbon surface
with alkylsulfonyl phthalocyanines was attained by means of a
“click” chemistry protocol. The new ensemble was fully
characterized (thermogravimetric analysis, atomic force micros-
copy, transmission electron microscopy and Raman, as well as
ground-state absorption) and was studied in terms of electron
donor−acceptor interactions in the ground and in the excited state.
In particular, a series of steady-state and time-resolved spectroscopy experiments demonstrated photoinduced electron transfer
from the graphene to the electron-accepting phthalocyanines. This is the first example of an electron donor−acceptor
nanoconjugate, that is, few-layer graphene/phthalocyanine, pinpointing the uncommon electron donating character of graphene.
donor character via steady-state and time-resolved spectrosco-
py,8 examples of graphene/electron acceptor hybrid architec-
tures are less common.9 Moreover, the electronic communica-
tion between graphene and electron acceptors was only
recently demonstrated in terms of ground and excited state
features.10
INTRODUCTION
■
Graphene, a single-atom thick nanomaterial with unprece-
dented properties, has generated enormous interest during
recent years due to its tremendous potential in the area of
nanoelectronics.1 As such, graphene is by far considered as the
most promising nanostructured carbon allotrope. However,
practical use of graphene is limited by its high-quality
production on a large scale. Among all the methods described
for the production of graphene, wet chemical exfoliation of bulk
graphite is the best approach for mass production of practical
single to few-layer graphene.
A sheer endless number of studies have focused on the
covalent2 and noncovalent3 functionalization of exfoliated
graphite. The thrust is not only to render it processable and
dispersible in common solvents, but also to afford hybrid
architectures for optoelectronic applications. In particular,
linking photoactive chromophores, such as porphyrins or
phthalocyanines (Pcs),4 to single and few-layer graphene adds
light harvesting, charge transfer and charge transport
capabilities and, therefore, opens the potential for solar energy
conversion schemes. To this date, a fairly large number of
graphene−porphyrin5 but only a few graphene−Pc hybrid
architectures6 have been reported, showing charge transfer
capabilities after photoexcitation of the chromophore.
Pcs usually feature electron-donating properties after photo-
excitation in covalent conjugates and/or noncovalent hybrids
with carbon nanostructures such as fullerenes and carbon
nanotubes.11 Concerning graphene, noncovalent adsorption
and immobilization of metalated Pcs onto epitaxially grown
graphene results in weak interactions in terms of n-doping of
graphene.12 In stark contrast, we have recently reported on a
covalent graphene−Pc nanoconjugate, in which the implemen-
tation of a molecular spacer decouples Pc from few-layer
graphene, and gives rise to through-space, ultrafast charge
separation evolving from the photoexcited Pc to graphene.6a
Nevertheless, tailor-made electron accepting Pcs are also
attainable by placing appropriate substituents at the periphery.
In this regard, alkylsulfonyl groups are ideal electron-with-
drawing substituents for the preparation of electron accepting
Pcs13 since they decrease the energy of the highest occupied
molecular orbital (HOMO) and the lowest unoccupied
molecular orbital (LUMO) of the molecule and render it fairly
soluble in organic solvents.
Single and few-layer graphene is usually employed as an
electron acceptor when combined with a wide facet of electron
donors.7 While in the case of single-walled carbon nanotubes
numerous reports have assisted in demonstrating their electron
Received: November 26, 2013
Published: February 25, 2014
© 2014 American Chemical Society
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Herein, we report on the preparation of a covalent few-layer
graphene/electron-accepting Pc nanoconjugate via copper
catalyzed azide−alkyne cycloaddition (CuAAC) “click” chem-
istry reaction between ethynylphenyl-functionalized graphene
and an appropriate Pc bearing one azide and six electron-
withdrawing alkylsulfonyl groups. Notably, few-layered gra-
phene was obtained by solvent-assisted exfoliation of graphite
following a modification of a procedure recently reported by
Tour and co-workers, which relies on the use of chlorosulfonic
acid.14 In addition, a full-fledged and comprehensive assay
regarding the electronic features of the resulting nanoconjugate
is described.
RESULTS AND DISCUSSION
■
Figure 2. (Top) Raman spectrum of EG. (Bottom) Histogram with
relative counts versus I(2D)/I(G) ratio and the corresponding log-
normal distribution. The sample was drop casted from a DMF
dispersion onto a SiO2 wafer and was excited at 532 nm.
In our previous work on covalent few-layer graphene/electron-
donating Pc conjugates, we followed the well-known Coleman’s
procedure, namely sonication of graphite in N-methylpyrroli-
done (NMP) to obtain dispersions of few-layered graphene.15
In search for a higher degree of exfoliation, we have now
followed a method reported by Tour,14 which relies on the use
of chlorosulfonic acid as the exfoliating agent. This method
affords relatively large flakes with no significant defects and a
high degree of exfoliation, as demonstrated in the following
paragraphs. Therefore, exfoliated graphite (EG) was obtained
by treating graphite flakes with chlorosulfonic acid for 3 days,
followed by centrifugation and quenching of the supernatant
with water (see Supporting Information for experimental
details).
Transmission electron microscopy (TEM) and atomic force
microscopy (AFM) techniques were utilized to shed light onto
the nature of EG. In the previous work, drop casting of the EG
suspension and evaporation of the solvent evoked reagglomera-
tion of the flakes. Nevertheless, individual flakes are discernible
featuring micrometer-squared sizes (Figure 1). To further
corroborate the aforementioned, we performed AFM measure-
ments, which provided additional support for the presence of
highly exfoliated graphite. Likewise, AFM reveals flakes with
heights as low as 2−3 nm (Figure 1), suggesting the presence
of few-layer graphene.
is best fit by a log-normal distribution function, which peaks at
a ratio of 1.1.
For bulk graphite or few-layer-to-multilayer graphene the
intensity ratio is expected to be 0.7 or less (Figure S1 in the
Supporting Information). In the current case, with such a
distribution (Figure 2), 14% of the collected spectra would
reveal bulk graphite/few-layer-to-multilayer graphene character,
while 86% of the collected spectra would correspond to
turbostratic exfoliated graphite/bilayer-to-monolayer gra-
phene,17 but a major presence of turbostratic exfoliated
graphite is more likely. Therefore, in confirmation with TEM
and AFM measurements, we conclude that we are dealing
mainly with a mixture of turbostratic exfoliated graphite as well
as few-layer-to-multilayer graphene. The presence of true
monolayers is, however, statistically inferior with maximum
I(2D)/I(G) intensity ratios of around 3.0.
Following the confirmation of the successful exfoliation of
graphite, we proceeded to its covalent functionalization. First,
Pc 1 bearing six 2-ethylhexylsulfonyl chains was synthesized in
five steps (Scheme 1). Starting from 4,5-dichlorophthalonitrile,
phthalonitrile 3 was prepared by a reaction with 2-ethyl-
hexanethiol, followed by oxidation of the thioether to sulfone
using MCPBA. Statistical condensation of phthalonitriles 3 and
418 afforded Pc 5 in 15% yield. Finally, transformation of the
alcohol moiety first to bromide (Pc 6, 70% yield) and then to
azide (75% yield) afforded compound 1.
The procedure for the covalent functionalization of EG
involved two steps (Scheme 2). To add the required
phenylethynyl groups protected as trimethylsilyl ethers (i.e.,
compound 7), we converted 4-[(trimethylsilyl)ethynyl]aniline
into the corresponding aryl diazonium salt, which reacted in situ
with a dispersion of EG in NMP by means of a microwave-
assisted methodology, recently reported by us.19 After
deprotection of 7, the ethynylphenyl-functionalized conjugate
was subjected to CuAAC conditions in NMP with azide-
terminated Pc 1 to afford 8. Nanoconjugates 7 and 8 were
characterized by a number of analytical techniques, such as
thermogravimetric analysis (TGA), AFM, TEM, Raman
spectroscopy, and steady-state and time-resolved spectroscopic
techniques.
Further insights into the nature of EG came from Raman
spectroscopy performed with laser excitation at 532 nm.16
Figure 2 shows a typical Raman spectrum of EG together with a
statistical distribution of I(2D)/I(G) intensity ratio of 725
measured spectra. Typically, the D-band is discernible at 1340
cm−1, while the G- and 2D-bands are centered around 1580 and
2662 cm−1, respectively. A statistical distribution of I(2D)/I(G)
First, TGA experiments (Figure 3) were performed, which
indicated that the initial functionalization with phenylacetylenes
in 7 leads to a 7% weight loss at 700 °C, upon excluding weight
losses due to adsorbants up to 350 °C. Overall, approximately
Figure 1. TEM image of EG on a lacey carbon grid (left) and tapping
mode AFM image of EG on a SiO2 surface and height profile at
position 1 (right).
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a
one functional group is present per 200 carbon atoms. The
thermogram of 8 shows an additional 13% weight loss, when
taking into consideration that Pc 1 gives rise to an overall
decomposition of 73% at 700 °C. The observed weight loss
corresponds to a functionalization of approximately one Pc
every 900 carbon atoms of the exfoliated material. Considering,
however, the presence of few-layer graphene, the real Pc-to-
carbon ratio at the surface must be assumed to be higher owing
to the fact that the inner layers remain unfunctionalized. But, it
is very difficult to give an accurate value in light of the
heterogeneity of 8. Remarkably, the degree of functionalization
achieved is higher than that reported on the covalent linkage of
electron donating Pcs to NMP-assisted EG.6a We attribute this
fact to the use of efficient CuAAC “click” chemistry method to
attach the Pcs to the functionalized graphene surface.
Scheme 1
In control experiments, 7 was stirred in NMP in the presence
of Pc 1 for 48 h. Under these conditions, covalent attachment is
unfeasible and the presence of Pc molecules in the graphitic
material could only be explained by noncovalent interactions
between 1 and the graphene surface. Importantly, TGA
experiments with the resulting graphitic material revealed no
significant weight loss in the 400 to 700 °C regime. Additional
proof for the absence of Pcs on the surface of graphene came
from UV−vis spectroscopy. Here, no appreciable Pc absorption
was seen. These findings support our initial assumption that the
weight loss in 8 is exclusively due to covalently linked Pcs.
Further characterization of 8 was based on AFM and TEM
(Figure 4). The graphene flakes were about 1−3 nm in height
owing to the presence of folded and intertwined sheets. Along
the same lines, TEM reveals folded as well as regularly stacked
sheets. The lateral sizes, as confirmed in AFM measurements,
are up to 1 μm. As a matter of fact, the reaggregation of the
graphene flakes prior to functionalization (vide supra) was only
partially prevented by introducing Pcs, while size and shape of
the corresponding flakes are marginally impacted.
a
Conditions: (a) 2-ethylhexanethiol, K2CO3, DMA, 90 °C, 8 h; (b)
MCPBA, CH2Cl2, rt, 18 h; (c) 4, o-DCB/DMF, Zn(OAc)2, 135 °C, 18
h; (d) CBr4, PPh3, CH2Cl2, room temp, 24 h; (e) NaN3, THF/H2O,
70 °C, 4 h.
a
Scheme 2
Next, 8 was probed by means of Raman spectroscopy upon
532 nm laser excitation. In Figure 5, a representative Raman
spectrum and a statistical distribution of I(2D)/I(G) intensity
ratio of 2274 measured spectra are displayed. The D-, G-, and
2D-bands of 8 drop casted onto a silicon wafer are centered at
around 1345, 1580, and 2687 cm−1, respectively. The best fit of
the statistical distribution of I(2D)/I(G) by a log-normal
distribution function affords a maximum at 0.84. Interestingly,
the maximum of the log-normal function shifts to lower values,
compared to EG. This observation could be rationalized on the
basis of increasing defects owing to the covalent functionaliza-
a
Conditions: (a) 4-[(trimethylsilyl)ethynyl]aniline, isoamyl nitrite,
NMP, MW, 80 °C, 1.5 h; (b) TBAF, NMP, 0 °C to room temp, 2 h;
(c) Pc 1, CuSO4, sodium ascorbate, 70 °C, 48 h.
Figure 3. TGA curves of graphite (black spectrum), nanoconjugate 7
(brown spectrum), nanoconjugate 8 (red spectrum), and Pc 1 (gray
spectrum).
Figure 4. TEM image of 8 on a lacey carbon grid (left) and tapping
mode AFM image of 8 on a SiO2 surface and height profile at position
1 (right).
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energy and/or electron transfer, it was important to determine
the spectral features of a Pc reference upon reduction or
oxidation. To avoid redox processes associated with the azide
group in the Pc precursor 1, we studied the electrochemical
behavior of Pc 5 by cyclic voltammetry experiments in toluene/
acetonitrile (4:1 v/v) with 0.1 M tetrabutylammonium
hexafluorophophate (TBAPF6) as electrolyte. In addition, a
silver wire, which was calibrated versus the Fc/Fc+ reference
redox couple, was used as quasi-reference electrode. The first
reversible reduction was detected at −1.1 V, whereas a quasi-
reversible oxidation was noted at +0.6 V.
Compared to a (tBu)4ZnPc reference (i.e., a typical electron-
donating counterpart), which reveals its oxidation at +0.1 V and
its reduction at −1.4 V, 5 renders easier to be reduced and
more difficult to be oxidized. These differences originate,
indeed, from the electron-withdrawing character of the sulfonyl
groups. HOMO (−5 eV) and LUMO (−3.3 eV) levels of Pc 5
(and hence, of related hexasulfonylPc derivatives) are
consistent with a plausible electron transfer, upon photo-
excitation of the Pc component in nanoconjugate 8, from the
Fermi level of graphene/exfoliated graphite (−4.5 eV to −5
eV) to the semioccupied HOMO level of the choromophore
(Figure S2 in the Supporting Information). This finding
supports our initial assumption that sulfonyl-susbstituted Pcs
can be electron-accepting components in ensembles with
graphene.
Turning to spectroelectrochemical measurements, a potential
of −1.4 V versus Ag-wire was applied for 2 h to reduce 5 and
determine the spectral characteristics of its reduced form
(Figure 7). New maxima evolve in the region between 400 and
600 nm as well as at 755 nm. These are assigned to the
fingerprints of the one electron reduced form of 5. Additionally,
the spectral features are reversibly transformed to those of the
nonreduced form upon applying a potential of +0.1 V versus
Ag-wire.
Figure 5. (Top) Raman spectrum of 8. (Bottom) Histogram with
relative counts versus I(2D)/I(G) ratio and the corresponding log-
normal distribution. The sample was drop casted from a DMF
dispersion onto a SiO2 wafer and excited at 532 nm.
tion of EG, which has a more profound decrease in intensity of
the 2D band20 in thinner than in thicker flakes.
The ground state features of Pcs in 8 were probed by steady-
state absorption experiments with freshly prepared dispersions
in DMF via ultrasonication. In 1, the typical Pc Q-band
signatures appear in the form of two maxima at 677 and 712
nm, as a consequence of the unsymmetrical functionalization
(Figure 6). In the case of 8, the Soret-band transitions in the
range between 300 and 500 nm are superimposed by features
that correspond to graphene absorptions and light scattering,
but appreciable features that may relate to the distinct and well-
resolved Pc Q-bands at 677 and 712 nm are discernible in the
spectrum (Figure 6). In stark contrast to previously published
Pc−graphene hybrids,6a Pc transitions are not bathochromically
shifted when compared to the reference, namely Pc 1. This
finding can be rationalized on the basis of weak electronic
communication between the basal plane of graphene and Pc
due to the rigid nature of the linker. The solution behavior of 8
rather than light scattering, etc. was confirmed by performing a
series of dilution experiments. In this context, perfect linear
relationships between the uniform dilution steps and the
resulting decrease in optical density are in perfect agreement
with the Lambert−Beer law. The absorption spectrum of 7, on
the other hand, is featureless and is best described as a
monotonically decreasing absorption.
In complementary fluorescence assays, emphasis was placed
on the fluorescent nature of Pc 1, which shows a fluorescence
maximum at 714 nm, a fluorescence quantum yield of 0.22
0.1 (toluene/pyridine (100:1 v/v)), and a fluorescence lifetime
of 2.1 ns (Figure S3 in the Supporting Information). As a
consequence of covalently linking 1 to the basal plane of
graphene, only weak Pc fluorescence, quenched by more than
90%, was noted for 8 in both steady-state and time-resolved
experiments.
Before characterizing the electronic interactions between
graphene and Pc molecules in nanoconjugate 8 in terms of
Figure 7. Differential absorption spectrum (visible) obtained upon
electrochemical reduction of 5 in toluene/acetonitrile (4:1 v/v) with
0.1 M TBAPF6 with an applied potential of −1.4 V versus Ag-wire.
Figure 6. Absorption spectra of 1 (red spectrum) and 8 (black
spectrum) in DMF.
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To gain insights into the product of fluorescence quenching,
we employed femtosecond transient absorption measurements
following excitation at 387 nm with dispersions of EG and 8 in
DMF. In reference experiments with 1, the instantaneous
formation of the Pc singlet excited state is discernible
throughout the visible and near-infrared section of the solar
spectrum (Figure S4 in the Supporting Information). Spectral
attributes of the Pc singlet excited transient are minima around
675 and 710 nm, which are good reflections of the ground state
bleaching, and maxima at 515 and 850 nm. Owing to their
metastability, these features decay rather slowly via intersystem
crossing (i.e., 1.7
0.2 ns) to the energetically lower lying
triplet excited state, showing its signatures for the triplet−triplet
absorption at 460 and 790 nm. In the absence of molecular
oxygen, the lifetime of the Pc triplet excited state is ca. 100 μs.
In complementary reference experiments with EG in DMF we
note, upon excitation at 387 nm, the instantaneous bleach (i.e.,
0.5 0.5 ps) of graphene centered transitions in the range of
800 to 1400 nm (Figure S5 in the Supporting Information).
For the major decay component of this bleaching, we
determined a lifetime of 1.5
0.5 ps corresponding to the
reinstating of the graphene ground state.
Finally, turning to 8, the rapid formation and deactivation of
the singlet excited state characteristics of the Pc is discernible
within a time window of 1.5 ps (Figure 8). In particular, the
spectral range for the Pc centered bleaching extends from 600
to 750 nm, with distinct features at 685 nm that resemble the
ground state. Simultaneously with the Pc singlet excited state
decay, the formation of a new transient species evolves. The
latter maximizes at 570, 636, and 765 nm as well as minimizes
at 450 and 680 nm in good agreement with the spectroelec-
trochemical findings recorded upon one-electron reduction of
the Pc. The range beyond 900 nm (i.e., 900−1500 nm) is
equally important, which, immediately after the photo-
excitation, is dominated by a broad bleaching. Here, new
features were noted during the transient decay with a broad
maximum ranging from 950 to 1300 nm (Figure 8). Implicit are
new valence band holes in graphene. A multiwavelength
analysis affords a short-lived and a long-lived component with
lifetimes of 1.0 0.5 and 650 100 ps, respectively, in DMF.
In line with the aforementioned features we rationalize the
kinetics in terms of charge separation and charge recombina-
tion.
Figure 8. (Top) Differential absorption spectra (visible) obtained
upon femtosecond pump probe experiments (387 nm) of 8 in DMF
with time delays between 1.3 and 20 ps at room temperature (for time
delays see Figure legend of middle part). (Middle) differential
absorption spectra (visible) obtained upon femtosecond pump probe
experiments (387 nm) of 8 in DMF with time delays between 1.3 and
20 ps at room temperature (for time delays see Figure legend).
(Bottom) Time absorption profile of the spectra shown in the upper
part at 510 nm, monitoring the charge transfer.
CONCLUSIONS
■
We present here the first example of an electron-accepting Pc
covalently linked to the basal plane of EG. Full-fledged
microscopic and spectroscopic assays confirm, on one hand,
the structure of the resulting electron donor−acceptor
conjugate and, on the other hand, the electron transfer
evolving from EG to the photoexcited Pc component. In
terms of electron transfer mechanism, charge separation with
1.0 0.5 ps leads us to assume that a through-space pathway
rather than a through bond pathway is operative before charge
delocalization within few-layer graphene takes over. Implicit in
such a mechanism is a close proximity of the Pcs relative to the
basal plane of graphene.
study donor−acceptor interactions with nonmodified exfoliated
graphene.
Covalent functionalization introduces sp3 centers within the
basal plane of EG, which are expected to impact the charge
delocalization and electron donating behavior. Currently, we
are directing out efforts to the preparation of noncovalent
hybrids using specifically tailored electron-accepting Pcs to
ASSOCIATED CONTENT
* Supporting Information
■
S
Experimental section, synthetic procedures, characterization
data, and some selected supplementary figures. This material is
available free of charge via the Internet at http://pubs.acs.org.
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C 2013, 117, 3195. (c) Malig, J.; Jux, N.; Guldi, D. M. Acc. Chem. Res.
2013, 46, 53.
AUTHOR INFORMATION
■
Corresponding Author
tomas.torres@uam.es
(8) (a) Oelsner, C.; Schmidt, C.; Hauke, F.; Prato, M.; Hirsch, A.;
Guldi, D. M. J. Am. Chem. Soc. 2011, 133, 4580. (b) D’Souza, F.;
Sandanayaka, A. S. D.; Ito, O. J. Phys. Chem. Lett. 2010, 1, 2586.
(c) Ohtani, M.; Fukuzumi, S. Chem. Commun. 2009, 4997. (d) Boul, P.
J.; Cho, D.-G.; Aminur Rahman, G. M.; Marquez, M.; Ou, Z.; Kadish,
K. M.; Guldi, D. M.; Sessler, J. L. J. Am. Chem. Soc. 2007, 129, 5683.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
́
́
(e) Ince, M.; Bartelmess, J.; Kiessling, D.; Dirian, K.; Martınez-Dıaz,
M. V.; Torres, T.; Guldi, D. M. Chem. Sci. 2012, 3, 1472.
(9) (a) Kozhemyakina, N. V.; Englert, J. M.; Yang, G.; Spiecker, E.;
Schmidt, C. D.; Hauke, F.; Hirsch, A. Adv. Mater. 2010, 22, 5483.
(b) Pinto, H.; Jones, R.; Goss, J.; Briddon, P. J. Phys.: Condens. Matter
2009, 21, 402001. (c) Chen, W.; Chen, S.; Qi, D. C.; Gao, X. Y.; Wee,
A. T. S. J. Am. Chem. Soc. 2007, 129, 10418. (d) Voggu, R.; Das, B.;
Rout, C. S.; Rao, C. J. Phys.: Condens. Matter 2008, 20, 472204.
(e) Varghese, N.; Ghosh, A.; Voggu, R.; Ghosh, S.; Rao, C. J. Phys.
Chem. C 2009, 113, 16855. (f) Hao, R.; Qian, W.; Zhang, L.; Hou, Y.
Chem. Commun. 2008, 6576.
■
Financial support from the MICINN and MEC, Spain (CTQ-
2011-24187/BQU, PIB2010US-00652, CONSOLIDER-IN-
GENIO 2010 CDS 2007-00010 Nanociencia Molecular,
PLE2009-0070), CAM (MADRISOLAR-2, S2009/PPQ/
1533), and the Deutsche Forschungsgemeinschaft, the
Graduate School of Molecular Science (GSMS), and the
Cluster of Excellence: Engineering of Advanced Materials
(EAM) is acknowledged. We would like to dedicate this paper
to Prof. Fritz Wasgestian on the occasion of his 80th birthday.
(10) Costa, R. D.; Malig, J.; Brenner, W.; Jux, N.; Guldi, D. M. Adv.
Mater. 2013, 25, 2600.
(11) Bottari, G.; de la Torre, G.; Guldi, D. M.; Torres, T. Chem. Rev.
2010, 110, 6768.
(12) (a) Yang, K.; Xiao, W. D.; Jiang, Y. H.; Zhang, H. G.; Liu, L. W.;
Mao, J. H.; Zhou, H. T.; Du, S. X.; Gao, H.-J. J. Phys. Chem. C 2012,
REFERENCES
■
(1) (a) Hirsch, A.; Englert, J. M.; Hauke, F. Acc. Chem. Res. 2013, 46,
87. (b) Georgakilas, V.; Otyepka, M.; Bourlinos, A. B.; Chandra, V.;
Kim, N.; Kemp, K. C.; Hobza, P.; Zboril, R.; Kim, K. S. Chem. Rev.
2012, 112, 6156. (c) Sun, Z.; James, D. K.; Tour, J. M. J. Phys. Chem.
Lett. 2011, 2, 2425. (d) Allen, M. J.; Tung, V. C.; Kaner, R. B. Chem.
Rev. 2010, 110, 132.
(2) (a) Englert, J. M.; Dotzer, C.; Yang, G.; Schmid, M.; Papp, C.;
Gottfried, J. M.; Steinruck, H.-P.; Spiecker, E.; Hauke, F.; Hirsch, A.
Nat. Chem. 2011, 3, 279. (b) Quintana, M.; Montellano, A.; del Rio
Castillo, A. E.; Van Tendeloo, G.; Bittencourt, C.; Prato, M. Chem.
Commun. 2011, 47, 9330. (c) Quintana, M.; Spyrou, K.; Grzelczak, M.;
Browne, W. R.; Rudolf, P.; Prato, M. ACS Nano 2010, 4, 3527.
(3) (a) Castelain, M.; Salavagione, H. J.; Gomez, R.; Segura, J. L.
Chem. Commun. 2011, 47, 7677. (b) Qi, X.; Pu, K. Y.; Li, H.; Zhou, X.;
Wu, S.; Fan, Q.-L.; Liu, B.; Boey, F.; Huang, Q.; Zhang, H. Angew.
Chem., Int. Ed. 2010, 49, 9426. (c) Barja, C.; Garnica, M.; Hinarejos, J.
J.; Vazquez de Parga, A. L.; Martin, N.; Miranda, R. Chem. Commun.
2010, 46, 8198.
116, 14052. (b) Hamalainen, S. K.; Stepanova, M.; Drost, R.; Liljeroth,
̈
̈
̈
P.; Lahtinen, J.; Sainio, J. J. Phys. Chem. C 2012, 116, 20433.
(c) Scardamaglia, M.; Lisi, S.; Lizzit, S.; Baraldi, A.; Larciprete, R.;
Mariani, C.; Betti, M. G. J. Phys. Chem. C 2013, 117, 3019.
(13) Zhang, Y.; Ma, P.; Zhu, P.; Zhang, X.; Gao, Y.; Qi, D.; Bian, Y.;
Kobayashi, N.; Jiang, J. J. Mater. Chem. 2011, 21, 6515.
(14) Behabtu, N.; Lomeda, J. R.; Green, M. J.; Higginbotham, A. L.;
Sinitskii, A.; Kosynkin, D. V.; Tsentalovich, D.; Nicholas, A.; Parra-
Vasquez, G.; Schmidt, J.; Kesselman, E.; Cohen, Y.; Talmon, Y.; Tour,
J. M.; Pasquali, M. Nat. Nanotechnol. 2010, 406.
(15) Hernandez, Y.; Nicolosi, V.; Lotya, M.; Blighe, F. M.; Sun, Z.;
De, S.; McGovern, I. T.; Holland, B.; Byrne, M.; Gun’Ko, Y. K.;
Boland, J. J.; Niraj, P.; Duesberg, G.; Krishnamurthy, S.; Goodhue, R.;
Hutchison, J.; Scardaci, V.; Ferrari, A. C.; Coleman, J. N. Nat.
Nanotechnol. 2008, 3, 63.
(4) The Porphyrin Handbook; Kadish, K. M., Smith, K. M., Guilard,
R., Eds.; Academic Press: San Diego, CA, 2003; Vols. 15−20.
(5) (a) Umeyama, T.; Mihara, T.; Tezuka, N.; Matano, Y.; Stranius,
K.; Chukharev, N. V.; Tkachenko, N. V.; Lemetyinen, H.; Noda, K.;
Matsushige, K.; Shishido, T.; Liu, Z.; Hirose-Takai, K.; Suenega, K.;
Imahori, H. Chem.−Eur. J. 2012, 18, 4250. (b) Karousis, N.;
Sandanayaka, A. S. D.; Hasobe, T.; Economopoulos, S. P.;
Sarantopoulou, E.; Tagmatarchis, N. J. Mater. Chem. 2011, 21, 109.
(c) Zhang, X.; Hou, L.; Cnossen, A.; Coleman, A. C.; Ivashenko, O.;
Rudolf, P.; van Wees, B. J.; Browne, W. R.; Feringa, B. L. Chem.Eur.
J. 2011, 17, 8957. (d) Xu, Y.; Zhao, L.; Bai, H.; Hong, W.; Li, Ch.; Shi,
G. J. Am. Chem. Soc. 2009, 131, 13490. (e) Xue, T.; Jiang, S.; Qu, Y.;
Su, Q.; Cheng, R.; Dubin, S.; Chiu, C.-Y.; Kaner, R.; Huang, Y.; Duan,
X. Angew. Chem., Int. Ed. 2012, 51, 3822. (f) Kiessling, D.; Costa, R.
D.; Katsukis, G.; Malig, J.; Lodermeyer, F.; Feihl, S.; Roth, A.; Wibmer,
L.; Kehrer, M.; Volland, M.; Wagner, P.; Wallace, G. G.; Officer, D. L.;
Guldi, D. M. Chem. Sci. 2013, 4, 3085. (g) Malig, J.; Stephenson, A.
W.; Wagner, P.; Wallace, G. G.; Officer, D. L.; Guldi, D. M. Chem.
Commun. 2012, 48, 8745.
(16) Englert, J. M.; Vecera, P.; Knirsch, K. C.; Schafer, R. A.; Hauke,
F.; Hirsch, A. ACS Nano 2013, 7, 5472.
(17) (a) Ferrari, A. C.; Meyer, J. C.; Scardaci, V.; Casiraghi, C.;
Lazzeri, M.; Mauri, F.; Piscanec, S.; Jiang, D.; Novoselov, K. S.; Roth,
S.; Geim, A. K. Phys. Rev. Lett. 2006, 97, 187401. (b) Green, A. A.;
Hersam, M. C. J. Phys. Chem. Lett. 2009, 1, 544.
̈
(18) Ballesteros, B.; Campidelli, S.; de la Torre, G.; Ehli, C.; Guldi, D.
M.; Prato, M.; Torres, T. Chem. Commun. 2007, 2950.
(19) Ragoussi, M.-E.; Casado, S.; Ribeiro-Viana, R.; de la Torre, G.;
Rojo, J.; Torres, T. Chem. Sci. 2013, 4035.
(20) (a) Niyogi, S.; Bekyarova, E.; Hong, J.; Khizroev, S.; Berger, C.;
de Heer, W.; Haddon, R. C. J. Phys. Chem. Lett. 2011, 2, 2487.
(b) Cancado, L. G.; Jorio, A.; Martins Ferreira, E. H.; Stavale, F.;
Achete, C. A.; Capaz, R. B.; Moutinho, M. V. O.; Lombardo, A.;
Kulmala, T. S.; Ferrari, A. C. Nano Lett. 2011, 11, 3190.
(6) (a) Ragoussi, M. E.; Malig, J.; Katsukis, G.; Butz, B.; Spiecker, E.;
de la Torre, G.; Torres, T.; Guldi, D. M. Angew. Chem., Int. Ed. 2012,
51, 6421. (b) Malig, J.; Jux, N.; Kiessling, D.; Cid, J.-J.; Vazquez, P.;
Torres, T.; Guldi, D. M. Angew. Chem., Int. Ed. 2011, 50, 3561.
(c) Karousis, N.; Ortiz, J.; Ohkubo, K.; Hasobe, T.; Fukuzumi, S.;
Sastre-Santos, A.; Tagmatarchis, N. J. Phys. Chem. C 2012, 116, 20564.
(d) Dirian, K.; Herranz, M. A.; Katsukis, G.; Malig, J.; Rodriguez-
Perez, L.; Romero-Nieto, C.; Strauss, V.; Martin, N.; Guldi, D. M.
Chem. Sci. 2013, 4, 4335.
(7) (a) Bottari, G.; Suanzes, J. A.; Trukhina, O.; Torres, T. J. Phys.
Chem. Lett. 2011, 2, 905. (b) Umeyama, T.; Imahori, H. J. Phys. Chem.
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