One-pot functionalization of graphene with porphyrin through cycloaddition reactions

Article abstract of DOI:10.1002/chem.201100980

Two types of graphene-based hybrid materials, graphene-TPP (TPP=tetraphenylporphyrin) and graphene-PdTPP (PdTPP=palladium tetraphenylporphyrin), were prepared directly from pristine graphene through one-pot cycloaddition reactions. The hybrid materials we

Full text of DOI:10.1002/chem.201100980

FULL PAPER
DOI: 10.1002/chem.201100980
One-Pot Functionalization of Graphene with Porphyrin through
Cycloaddition Reactions
Xiaoyan Zhang,[a, b] Lili Hou,[a] Arjen Cnossen,[a] Anthony C. Coleman,[b]
Oleksii Ivashenko,[c] Petra Rudolf,[c] Bart J. van Wees,[b] Wesley R. Browne,[a] and
Ben L. Feringa*[a]
Abstract: Two types of graphene-based
hybrid materials, graphene-TPP
measurements. The presence of the co-
valent linkages between graphene and
porphyrin was confirmed by FTIR and
Raman spectroscopy and further sup-
ported by control experiments. The
presence of TPP (or PdTPP) in the
hybrid material was demonstrated by
UV/Vis spectroscopy, with TGA results
indicating that the graphene-TPP and
graphene-PdTPP hybrid materials con-
tained approximately 18% TPP and
20% PdTPP. The quenching of fluores-
cence (or phosphorescence) and re-
duced lifetimes suggest excited state
energy/electron transfer between gra-
phene and the covalently attached TPP
(or PdTPP) molecules.
(TPP=tetraphenylporphyrin) and gra-
phene-PdTPP (PdTPP=palladium tet-
raphenylporphyrin), were prepared di-
rectly from pristine graphene through
one-pot cycloaddition reactions. The
hybrid materials were characterized by
thermogravimetric analysis (TGA), by
TEM, by UV/Vis, FTIR, Raman, and
luminescence spectroscopy, and by
fluorescence/phosphorescence lifetime
Keywords: cycloaddition
·
gra-
phene · luminescence · porphyrin ·
Raman spectroscopy
Introduction
phene itself is made difficult by its tendency to aggregate as
a result of the strong p–p interactions between individual
flakes. This aggregation makes fabrication of graphene-
based materials difficult. For this reason, noncovalent[10] and
covalent[11] methods for the functionalization of graphene
have been developed. In this context, graphene-based
hybrid nanomaterials (combining graphene with other func-
tional components) have attracted widespread atten-
tion.[12–14] The hybrid nanomaterials not only combine the
unique optical, electrical, magnetic, and chemical properties
of each component, but also offer the potential to introduce
new properties that can potentially be used in a diverse
range of applications. In the majority of reports on gra-
phene-based hybrid nanomaterials, graphene oxide has been
used as the starting material.[15] However, the oxygen-con-
taining groups in this material greatly change the intrinsic
properties of graphene, therefore potentially affecting the
final properties of graphene-based hybrid nanomaterials.[16]
Consequently, the development of alternative routes is re-
quired.
Recently, preparation of graphene by solvent dispersion
methods, developed by a number of groups,[9] including our
own,[9f] has attracted particular interest because it gives the
advantage of retaining the intrinsic properties of graphene
and at the same time it maintains the dispersibility of gra-
phene in certain solvents. The functionalization of graphene
through cycloaddition on pristine graphene is a promising
choice.[17] In relation to noncovalent methods, covalent func-
tionalization of pristine graphene with other molecules pro-
vides stable and well-defined systems. Furthermore, rational
design of the process can allow for control of the degree of
Graphene, a single-atom-thick layer of graphite, has attract-
ed intense scientific interest because of its extraordinary
properties,[1] such as the existence of massless Dirac fer-
mions,
a room-temperature quantum Hall effect, high
charge carrier mobility, and gas-sensing capability at the mo-
lecular level. It is widely anticipated that graphene will see
applications in supercapacitors,[2] nanoelectronics devices,[3]
chemical sensors,[4] and reinforced composite materials[5] in
the near future.
For such applications, large-scale preparation of high-
purity graphene flakes is essential. Currently, graphene is
typically prepared by one of the following methods: me-
chanical,[6] epitaxial,[7] reduction of graphene oxide,[8] or sol-
vent dispersion of graphite.[9] Further manipulation of gra-
[a] X. Zhang, L. Hou, A. Cnossen, Dr. W. R. Browne,
Prof. Dr. B. L. Feringa
Stratingh Institute for Chemistry, University of Groningen
Nijenborgh 4, 9747 AG Groningen (The Netherlands)
Fax : (+31)050-363-4296
E-mail: b.l.feringa@rug.nl
[b] X. Zhang, Dr. A. C. Coleman, Prof. Dr. B. J. van Wees
Physics of Nanodevices, Zernike Institute for Advanced Materials
University of Groningen, Nijenborgh 4
9747 AG Groningen (The Netherlands)
[c] O. Ivashenko, Prof. Dr. P. Rudolf
Surfaces and Thin Films, Zernike Institute for Advanced Materials
University of Groningen, Nijenborgh 4
9747 AG Groningen (The Netherlands)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201100980.
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functionalization with the retention of the unique properties
of graphene.
Porphyrin, itself a well-known and -studied aromatic mol-
ecule, possesses exceptional optical and electronic properties
and has been used in various fields as diverse as solar cells,
sensors, catalysis, and biology.[18] C60-porphyrin and carbon-
nanotube–porphyrin hybrid materials have been prepared
for a number of applications.[19] Because of the similarity of
graphene, C60, and carbon nanotubes, hybrid materials com-
bining graphene with porphyrin can be envisaged as being
useful for applications such as solar cells, sensors, and catal-
ysis. A distinct difference between C60/carbon nanotubes
and graphene, however, lies in the accessibility of the edge
as well as both faces of the carbon sheet to chemical reac-
tion.
Here we show that graphene-TPP (TPP=tetraphenylpor-
phyrin) and graphene-PdTPP (PdTPP=palladium tetraphe-
nylporphyrin) hybrid materials can be readily prepared
through one-pot cycloaddition reactions. The hybrid materi-
als prepared are stable in solution and have been character-
ized by a number of spectroscopic and microscopy tech-
niques.
Results and Discussion
The method used for the preparation of graphene-TPP and
graphene-PdTPP is shown in Scheme 1. Graphene in ODCB
was prepared by the procedure reported by Hamilton
et al.[9a] TPP-CHO and PdTPP-CHO were synthesized by lit-
erature procedures (see the Experimental Section). In brief,
graphene in ODCB, sarcosine, and TPP-CHO (or PdTPP-
CHO) were treated at 1608C for one week, and the hybrid
materials were purified by multiple filtration/redispersion
cycles.
The presence of TPP (or PdTPP) in the graphene-TPP
(or graphene-PdTPP) hybrid material was confirmed by
UV/Vis spectroscopy. As shown in Figure 1a, TPP-CHO in
DMF shows a strong Soret band at 419 nm, together with
relatively weaker Q bands.[22] The absorption spectrum of
graphene-TPP in DMF also shows a broadened peak at
about 421 nm and weaker Q bands, attributed to the absorp-
Figure 1. a) UV/Vis absorption spectra of TPP-CHO (thick solid line)
and graphene-TPP (thin solid line, baseline corrected) in DMF (Inset:
TPP-CHO (thick solid line), graphene-TPP (thin solid line), and the con-
trol sample (dashed line)). b) UV/Vis absorption spectra of PdTPP-CHO
(thick solid line) and graphene-PdTPP (thin solid line, baseline correct-
ed) in DMF (Inset: PdTPP-CHO (thick solid line), graphene-PdTPP
(thin solid line), and the control sample (dashed line)).
tion of TPP unit in the graphene-TPP hybrid material. The
absorption spectra for PdTPP and for graphene-PdTPP are
shown in Figure 1b. PdTPP-CHO shows a strong Soret ab-
sorption band at 416 nm and also a weaker Q band at
523 nm.[23] For the graphene-
PdTPP hybrid material, two
broadened bands are observed
at 419 and 525 nm and are as-
signed to the absorption of the
covalently tethered PdTPP. In
control reactions for both TPP
and PdTPP, in which the reac-
tions were performed in the ab-
sence of sarcosine (see the Ex-
perimental Section), the UV/
Vis spectra of the recovered
graphene materials did not ex-
hibit the typical porphyrin ab-
Scheme 1. Synthesis of graphene-TPP and graphene-PdTPP hybrid materials.
sorption bands. This supports
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One-Pot Functionalization of Graphene with Porphyrin
FULL PAPER
the conclusion that the porphyrin molecules in the gra-
phene-TPP and graphene-PdTPP hybrid materials are not
physisorbed but attached covalently. In addition, the Soret
band of each porphyrin shows a small redshift when cova-
lently attached to graphene, which suggests there might be
interactions between graphene and porphyrin molecules in
the hybrid materials. Additional confirmation of the success-
ful hybridization of PdTPP with graphene was provided by
the X-ray photoelectron spectrum collected on a dropcast
film of the hybrid material on a polycrystalline copper sub-
strate. Figure 2 shows the Pd 3d core level region, which tes-
which are also apparent from the D/G band ratio in the
Raman spectra, and release of trapped solvent. Graphene-
TPP and graphene-PdTPP show approximately 18 and 20%
weight loss, respectively, relative to graphene, between 250
and 5008C. This weight loss corresponds to the loss of TPP
or PdTPP molecules covalently attached to graphene. Ac-
cordingly, the degree of functionalization was estimated to
be one TPP group per 235 carbon atoms in graphene-TPP
(0.42%) and one PdTPP group per 240 carbon atoms in gra-
phene-PdTPP (0.41%).[25]
Figure 4 shows the FTIR spectra of graphene, graphene-
TPP, and graphene-PdTPP. The spectrum of graphene is
almost featureless, indicative of a low defect content, where-
Figure 2. X-ray photoelectron spectrum of the Pd 3d core level region
measured on a dropcast graphene-PdTPP film on Cu.
Figure 4. FTIR spectra of graphene (top), graphene-PdTPP (middle), and
graphene-TPP (bottom).
tifies to the presence of Pd. The Pd 3d5/2 binding energy of
338.5 eV is in agreement with Pd bonded to N found for
similar compounds.[24]
The loading of TPP or PdTPP in the graphene-TPP and
graphene-PdTPP hybrid materials was determined by TGA.
Figure 3 shows the TGA curves of graphene, graphene-TPP,
and graphene-PdTPP. The weight loss observed for gra-
phene is attributed to the defects caused by sonication,
as for the graphene-TPP and graphene-PdTPP some fea-
tures of TPP and PdTPP molecules were observed in the
1000–1500 cmÀ1 region. The absence of a carbonyl stretch at
about 1700 cmÀ1 for the graphene-TPP and graphene-
PdTPP in relation to the spectra of free TPP-CHO and
PdTPP-CHO (Figure S1 in the Supporting Information) in-
dicates reaction of the aldehyde moieties.
Additional evidence for the functionalization was provid-
ed by Raman spectroscopy. The Raman spectra of graphite,
graphene, graphene-TPP, and graphene-PdTPP are shown in
Figure 5. For the graphitic materials, the typical Raman
bands are: a defect-induced D band at 1350 cmÀ1, an in-
plane vibration of sp2 carbon at 1580 cmÀ1 (G band), and a
two-phonon double-resonance process at ca. 2700 cmÀ1 (2D
band). The D band of graphite is almost invisible, and the
2D band consists of two components with the main peak at
about 2718 cmÀ1.[26] Graphene shows a small D band and a
strong G band at 1580 cmÀ1, with a D/G ratio of 0.22. Both
graphene-TPP and graphene-PdTPP show increased D/G
ratios (0.40 and 0.42 respectively) relative to graphene. The
increased D/G ratio is consistent with the functionalization
of graphene through covalent bonding.[17] It is worth men-
tioning that for graphene and functionalized graphene the
Figure 3. TGA curves of graphene (thick solid line), graphene-TPP (thin
solid line), and graphene-PdTPP (dotted line).
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B. L. Feringa et al.
tron diffraction pattern (Figure 6a, inset: the inner intensity
is stronger than the outer intensity).[9b,f] There are also a
large number of few-layer graphene flakes (Figure 6b). As
expected,[9a] the graphene starting material consists of
single- and few-layer graphene flakes. After the cycloaddi-
tion reactions, little change in the morphologies of gra-
phene-TPP and graphene-PdTPP hybrid materials is ob-
served (Figure 6c–f, Figures S2 and S3 in the Supporting In-
formation).
The interaction between graphene and porphyrin in the
hybrid materials was studied by fluorescence (or phosphor-
escence, in the case of PdTPP-CHO) spectroscopy. The
Soret band absorption intensities in the graphene-porphyrin
hybrids (if allowance is made for the scattering by gra-
phene) were equivalent to those of the free porphyrins for
luminescence measurements. When excited at 410 nm, free
TPP-CHO shows strong fluorescence at 650 and 710 nm
(Figure 7a, solid line). In contrast, for the graphene-TPP
hybrid, the fluorescence is quenched significantly (Figure 6a,
dotted line). The fluorescence quantum yields are reduced
from 4% for TPP-CHO to 0.3% for graphene-TPP. Similar
quenching behavior is observed for the graphene-PdTPP
hybrid material (Figure 7b). The strong phosphorescence
emission at 710 nm for PdTPP-CHO is also quenched in the
hybrid material. The phosphorescence quantum yield is re-
duced from 0.62% for PdTPP-CHO to below 0.01% for
graphene-PdTPP.
Figure 5. Raman spectra of (bottom to top) graphite, graphene, gra-
phene-PdTPP, and graphene-TPP (lexc =532 nm).
2D bands are shifted to 2700 cmÀ1 relative to graphite. Be-
cause of the small size of the graphene flakes and the aggre-
gation of graphene when deposited on the substrate, it is dif-
ficult to distinguish single-layer graphene by Raman spec-
troscopy in the present case. However, by comparing the po-
sitions and shapes of the spectra of graphene and functional-
ized graphene with that of graphite, we assigned the
graphene flakes as a mixture of single- and few-layer gra-
phene.[26,17a]
The morphologies of graphene, graphene-TPP, and gra-
phene-PdTPP were investigated by TEM analysis. Figure 6a
shows a single-layer graphene flake, as indicated by the elec-
The luminescence lifetimes of TPP-CHO (or PdTPP-
CHO) and of graphene-TPP (or PdTPP) show (Figure 8)
that there are strong interactions between the graphene and
TPP (or PdTPP). For graphene-TPP the fluorescence decay
is biexponential, with a short component of <500 ps and a
longer component of 6.2 ns. The short component accounts
for the major part of the decay and confirms that rapid
quenching of the porphyrin singlet excited state occurs. The
biexponential nature of the decay might reflect the location
of the porphyrin on the graphene (that is, on the basal plane
or near the edge). The relatively minor contribution of the
longer-lifetime component is consistent with such an assign-
ment. Comparison of the fluorescence spectra of the cova-
lently modified graphene-TPP both with free TPP-CHO and
with a matched mixture of TPP-CHO and graphene show
that collisional (dynamic) quenching is not significant (Fig-
ure S8 in the Supporting Information).
The phosphorescence decay lifetime of PdTPP-CHO is
44 ms, whereas for the graphene-PdTPP hybrid material the
phosphorescence quantum yield and lifetime are diminished
considerably and are non-exponential (Figure 9). Fitting of
the decay of graphene-PdTPP with a biexponential function
gives the values of 80 and 660 ns, the latter component
being the lesser of the two (Figure S7 in the Supporting In-
formation). The phosphorescence decay lifetime of the pal-
ladium porphyrin is in the microsecond time range and so,
unlike fluorescence, phosphorescence can be quenched both
dynamically and statically. Dynamic quenching is not as effi-
cient as static quenching, however, as shown by matched sol-
utions of PdTPP-CHO mixed with graphene and of gra-
Figure 6. TEM images of a), b) graphene, c), d) graphene-TPP, and
e), f) graphene-PdTPP. The samples were prepared by drop casting onto
holey carbon grids.
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One-Pot Functionalization of Graphene with Porphyrin
FULL PAPER
Figure 8. Fluorescence decay traces for TPP-CHO (solid squares) and
graphene-TPP hybrid material (open squares) in DMF. For fits see Fig-
ure S6 in the Supporting Information.
Figure 7. Top: fluorescence emission spectra of a) TPP-CHO (solid line)
and b) graphene-TPP (dotted line) at lexc =410 nm in DMF. The concen-
tration of porphyrin in both samples (i.e., TPP-CHO and graphene-TPP)
was equivalent (0.7 mm) as determined by the intensity of the Soret band
with the background due to the porphyrin being taken into account (see
Figure S4 in the Supporting Information for the absorption spectra em-
ployed). Bottom: phosphorescence emission spectra of c) PdTPP-CHO
(solid line) and d) graphene-PdTPP (dotted line) at lexc =410 nm. The
concentrations of porphyrin in TPP-CHO and in graphene-TPP were
kept the same (0.6 mm) according to the Soret band absorption intensity
(Figure S5 in the Supporting Information). The absorption spectra were
corrected both for graphene-TPP and for graphene-PdTPP by subtraction
of the scattering of the graphene. * The dips in the spectra are instrumen-
tal artifacts.
Figure 9. Phosphorescence lifetime measurement for a) PdTPP-CHO
(thin line) and b) graphene-PdTPP hybrid material (thick line). The inset
shows an expansion of the figure between 1–3 ms.
Raman spectra for both of the hybrid materials all indicate
the successful functionalization of graphene. From the TGA
data the degrees of functionalization of these two hybrid
materials are relatively low, which allows for extended
patches of undisturbed graphene. The reduced emission life-
times for both of the hybrid materials are consistent with
the fluorescence (or phosphorescence) quenching and the
reduction in emission quantum yield data, indicating that
either energy- or electron-transfer processes between gra-
phene and the attached porphyrin molecules occur.[27]
phene-PdTPP, in which the emission intensity of the cova-
lently attached porphyrin is substantially less than that of
the free PdTPP-CHO/graphene mixture (Figure S9 in the
Supporting Information).
The absence of absorption bands of the porphyrins in
UV/Vis absorption spectra of control samples prepared by
the same procedures as for graphene-TPP and graphene-
PdTPP, the disappearance of aldehyde peaks in the FTIR
spectra of graphene-TPP and graphene-PdTPP, the presence
of Pd revealed by XPS, and the increased D/G ratio in the
Conclusion
Graphene-TPP and graphene-PdTPP hybrid materials have
been successfully prepared through one-pot cycloaddition
reactions. The presence of TPP or PdTPP in the hybrid ma-
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B. L. Feringa et al.
spectrometer (excitation at 532 nm). Raman samples were prepared by
drop casting a few drops of the graphene and functionalized graphene on
clean gold substrates and were then dried under vacuum. Fourier Trans-
form Infrared Spectroscopy (FTIR) was performed with a Perkin–Elmer
Spectrum 400 instrument and a UATR attachment.
terials was confirmed by UV/Vis spectroscopy and XPS.
Fluorescence and phosphorescence quenching is observed
with concomitant decreases in excited state lifetimes. These
observations confirm that energy- and/or electron-transfer
quenching between graphene and the covalently bound por-
phyrin molecules occur. The amounts of TPP or PdTPP
present in the hybrid materials were determined by TGA
and the covalent linkages were confirmed by Raman and
FTIR spectroscopy and further supported by control experi-
ments. TEM images show that the morphologies of the
hybrid materials are not affected by the cycloaddition func-
tionalization processes. The relatively low degrees of func-
tionalization of these two hybrid materials might allow for
retention of the inherent properties of graphene, especially
in comparison with materials prepared, for example, via gra-
phene oxides. In view of the remarkable properties of both
graphene and porphyrin, these two hybrid materials might
have potential applications in a number of areas, such as
solar cells, sensors, and catalysis.[19] With these porphyrin-
modified graphenes, further studies with regard to applica-
tions to exploit the unique properties of the hybrid materials
are underway in our group.
Preparation of graphene: Graphite (100 mg) was sonicated for 2 h in
ODCB (100 mL), and then centrifuged at 3000 rpm for 30 min. The su-
pernatant was decanted to afford graphene in ODCB. The concentration
of graphene in ODCB was 0.01 mgmLÀ1
.
Preparation of graphene-TPP and graphene-PdTPP hybrid materials:
The procedure for the preparation of graphene-TPP and graphene-
PdTPP hybrid materials is shown in Scheme 1. The obtained graphene in
ODCB (50 mL), sarcosine (25 mg), and TPP-CHO (or PdTPP-CHO,
20 mg) were placed in a 100 mL round-bottomed flask and stirred at
1608C under N2 for 1 week. After the reaction was complete, the reac-
tion mixture was filtered through a 0.45 mm nylon membrane. The ob-
tained filter cake was subsequently washed several times with ODCB,
DMF, and CHCl3 with use of repeated redispersion, sonication, and fil-
tration steps. The final suspension in CHCl3 was centrifuged at 5000 rpm
for 30 min. The precipitate was dried under vacuum to afford the desired
graphene-TPP or graphene-PdTPP hybrid material.
Control samples: To confirm the covalent linkages between TPP (or
PdTPP) and graphene, control samples were prepared. The obtained gra-
phene in ODCB (50 mL), and TPP-CHO (or PdTPP-CHO, 20 mg) were
placed in a 100 mL round-bottomed flask, and stirred at 1608C under N2
for 1 week (in the absence of sarcosine). The handling procedures were
as above for the hybrid materials.
Experimental Section
Acknowledgements
Chemicals: Graphite flakes (Sigma–Aldrich) and ortho-dichlorobenzene
(ODCB, 98%, AR, Merck) were used as received without further purifi-
cation. 5-(4-Methylcarboxyphenyl)-10,15,20-triphenylporphyrin (TPP-
COOMe) was obtained by literature procedures.[20] 5-(4-Formylphenyl)-
10,15,20-triphenylporphyrin (TPP-CHO) was synthesized from TPP-
COOMe by a reduction/oxidation method (see the Supporting Informa-
tion). Palladation of TPP-CHO was performed by the general method
published by Lindsey and co-workers.[21]
We acknowledge the financial support from The Zernike Institute for
Advanced Materials (X.Y.Z., O.I.), the Ubbo Emmius Scholarship
(L.H.), Nanoned (A.C.), the Netherlands Organisation for Scientific Re-
search, NWO-Vidi (W.R.B.) and the ERC advanced investigator grant
(no. 227897, B.L.F). We thank Dr I. Meliꢁn-Cabrera for TGA measure-
ments and H. M. M. Hesp for assistance with TCSPC and Raman spec-
troscopy.
Instruments: Sonication was conducted with a low-power sonication bath
(Bransonic, PC 620). Centrifugation was performed with
a Hermle
Z323K centrifuge. Filtration was carried out with a Sintered Micro Filter
holder through a 0.45 mm nylon membrane. UV/Vis spectra were ob-
tained with a JASCO V-630 UV/Vis spectrometer. Fluorescence and
phosphorescence spectra were measured with a JASCO FP-6200 spectro-
fluorimeter. Quantum yield measurements were determined with [Ru-
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ACHTUNGTRENNUNG(bpy)3]ACHTUNGTRENNUNG(PF6)2 in water as a reference. Fluorescence lifetime measure-
ments were performed with a time-correlated single-photon-counting
system with detection by use of a microchannel plate PMT coupled with
a 630 nm long-pass filter. The light source was a Ti:Sapphire laser
(400 nm, 1.9 MHz). Phosphorescence lifetime measurements were ob-
tained with a home-built system consisting of a Zolix Omni-l 300 mono-
chromator coupled with a Zolix PMTH-S1-CR131 PMT detector. Transi-
ents were digitized with the aid of a Tektronix DPO 4032 Digital Phos-
phor Oscilloscope. The light source was an Innolas 400 Nd:YAG laser
(excitation at 532 nm, 10 Hz, 40 mW) with a Si-diode trigger sensor. Solu-
tions for phosphorescence measurements were degassed by at least three
freeze-pump-thaw cycles. X-ray photoelectron spectroscopy (XPS) data
were collected with a dropcast sample of graphene-PdTPP on polycrystal-
line Cu with a Surface Science SSX-100 ESCA instrument and a mono-
chromatic AlKa X-ray source (hn=1486.6 eV). The takeoff angle between
the spectrometer detector and the normal to the surface was 378. Binding
energies (Æ0.1 eV) were referenced to the Cu2p3/2 photoemission line at
a binding energy of 932.7 eV.[28] Thermal gravimetric analysis (TGA) was
performed under N2 with a Mettler Toledo TGA/SDTA851e system.
Transmission electron microscopy (TEM) characterization was carried
out with a PHILIPS CM10 instrument operating at 100 KV. Raman spec-
tra were measured with a JOBIN-YVON model T 64000 triple-grating
8962
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One-Pot Functionalization of Graphene with Porphyrin
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AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
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Chem. Eur. J. 2011, 17, 8957 – 8964