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pubs.acs.org/JPCC
Following the Metalation Process of Protoporphyrin IX with Metal
Substrate Atoms at Room Temperature
Ruben Gonzalez-Moreno,†,‡ Carlos Sanchez-Sanchez,‡ Marta Trelka,§ Roberto Otero,§ Albano Cossaro,||
ngel Martín-Gago,‡,^
Alberto Verdini,|| Luca Floreano,|| Marta Ruiz-Bermejo,^ Aran García-Lekue,3 Jose A
and Celia Rogero*,†
†
Centro de Física de Materiales (CSIC-UPV/EHU), Materials Physics Center MPC, San Sebastian, Spain
Instituto de Ciencia de Materiales de Madrid (ICMM CSIC), Madrid, Spain
§
Instituto Nicolas Cabrera and Instituto Madrile~
no de Estudios Avanzados en Nanociencia (IMDEA-NANO), Universidad Autonoma
de Madrid, Madrid, Spain
Laboratorio TASC, Istituto Officina dei Materiali (CNR-IOM), Trieste, Italy
^
Centro de Astrobiología (CSIC-INTA), Madrid, Spain
3
Donostia International Physics Center (DIPC), San Sebastian, Spain
)
‡
ABSTRACT: We have studied the in situ coordination reaction
of porphyrin molecules, particularly protoporphyrin IX
(H2PPIX), with copper substrate atoms in ultrahigh vacuum
conditions with a combination of X-ray photoelectron spectroscopy and scanning tunneling microscopy. We show that
these protoporphyrin IX molecules deposited on Cu surfaces, as
Cu(110) and Cu(100), form metalloprotoporphyrin IX
(CuPPIX) by incorporation of Cu atoms from the surface
already at room temperature. We have followed this reaction
as a function of temperature and we have determined intermediate situations at lower temperatures where the physisorbed
macrocycle rings present a tendency to establish hydrogen
bonding between molecules.
’ INTRODUCTION
The interaction of planar metal complexes, such as porphyrins,
phthalocyanines, or corroles (tetrapyrrole molecules), with
surfaces is especially interesting for designing novel catalysts,
sensors, and other devices. Because of their photophysical
properties they are good candidates for the construction of
photonic devices, such as solar cells and organic light diodes.1,2
Depending on the required applications, the properties of these
macrocycles can be tailored, for example, by changing the
functional groups around the central core or the metal in the
center of the core. On the other hand, the chemical reactivity of
the metal core may affect the performance of the application
devices, for example, by rapid oxidation of the metal, which
makes it necessary to work with passivated molecules, or by loss
of the metal by interaction with the surfaces.3 An alternative
procedure for handling with these metalomolecules is to start
with nonmetalated molecules (free molecules) and metalate
them directly on a surface. The incorporation of selected metal
atoms into porphyrins and phthalocyanines on the surface,414
also called metalation process, represents an advantage against
the direct sublimation of metalomolecules.414 The routes
reported in the literature for surface-mediated metalation in
ultrahigh vacuum (UHV) conditions involve evaporation of
r 2011 American Chemical Society
the metal atoms by vapor deposition in the appropriate stoichiometry before or after the molecular deposition, usually followed
by annealing of the system formed by the substratemolecule
metal atoms.414 Interestingly, in none of the reported cases was
the formation of complexes with the substrate metal atoms
detected, although such a mechanism would simplify the surface
synthesis of metalloporphyrins and could likely improve some of
their properties. Moreover, since the metal core and the substrate
atoms are of the same nature, the moleculesubstrate contact
can be enhanced. Metalation with the substrate has been
suggested but it has never been proven so far. In this work we
demonstrate that surface metalation takes place when the protoporphyrin IX (H2PPIX) molecule is deposited on Cu substrates.
Depending on the chosen central metal atom, metalloprotoporphyrin IX molecules form biological complexes that are
essential for life, for example, hemoglobin (Fe), which is responsible for oxygen transport in animals, and chlorophyll (Mg), which
governs energy conversion in the photosynthesis process.15,16
Received: January 18, 2011
Revised:
February 22, 2011
Published: March 22, 2011
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Figure 1. (a) Top and (B) side views of the DFT-optimized ball-andstick model of H2PPIX.
The latter feature is one of the main reasons why this family of
molecules is investigated within the context of photoluminescence applications. The H2PPIX molecule is the nonmetalated
version of this family of molecules, where two hydrogen atoms
replace the central metal one. Its structure is almost planar and
the central macrocycle is surrounded only by short methyl and
ethenyl groups and two propionic acids (Figure 1).
In this work we use X-ray photoelectron spectroscopy (XPS)
to investigate the chemical changes observed in H2PPIX when
the molecule is interacting directly with metal surfaces, as
Cu(110) and Cu(100). These surfaces have been chosen as
models for further studies on more realistic ones for applications,
as they could be metallic oxides. We study the transition from the
physisorbed molecular configuration to the chemisorbed one as a
function of the temperature. Thus, we follow the coordination
reaction with the substrate atoms, demonstrating that, already at
room temperature, H2PPIX molecules adsorbed on Cu surfaces
can form Cu-protoporphyrin IX (CuPPIX) in strongly bound
self-assembled monolayers (SAMs), as confirmed by scanning
tunneling microscopy (STM) images.
’ EXPERIMENTAL DETAILS
Growth of Molecules. The growth of the molecular films is
performed in UHV, with a base pressure of <5 1010 mbar.
Atomically flat crystalline Cu(110) and Cu(100) surfaces were
prepared by standard sputter/anneal procedures (sputter with 2
kV Arþ ions for 15 min followed by annealing to 800 K for
another 15 min), which resulted in large terraces, separated by
monatomic steps as depicted by STM images. Protoporphyrin IX
(3,7,12,17-tetramethyl-8,13-divinyl-2,18-porphinedipropionic
acid), H2PPIX, was purchased by SigmaAldrich (purity
g95%). Molecules were evaporated at a temperature around
600 K by means of both electrically heated tantalum evaporator
and boron nitride crucibles. In general, substrate temperatures
during vapor depositions were maintained at about 200 K or
lower, except in some controlled experiments as indicated. We
used very low deposition rates (in the range of 0.1 Å/min
monitored by a quartz microbalance) in order to avoid the risk
of thermal decomposition. We checked that the powder does not
decompose at a temperature around 600 K by comparing
infrared spectra taken on the molecular powder in the cells
before and after the evaporation experiments.
Reference Samples. A bulk reference sample was prepared by
depositing the H2PPIX molecular powder directly on a carbon
tape and introducing the sample into UHV for analysis with XPS.
Additional bulk films were prepared by casting a drop of a
solution containing the molecule on a polycrystalline Au
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substrate grown on glass. The Au substrate was cleaned with
dichloromethane (DCM) and measured by XPS to check the
cleanness before the drop deposition. A highly concentrated
solution of H2PPIX was prepared in dimethyl sulfoxide
(DMSO) with the molecules as purchased from SigmaAldrich
(without further purification). Solvent evaporates completely at
room temperature (RT), and only molecules remain on the
surface.
X-ray Photoelectron Spectroscopy. XPS experiments on
Cu(100) and on the bulk reference samples were performed in
the Centro de Física de Materiales (CSIC-UPV/EHU) on a
Phoibos photoelectron spectrometer equipped with a nonmonochromatic Al KR X-ray source as incident photon radiation.
The overall resolution was around 0.9 eV. The binding energies
were referred to the Cu 2p core level at 932.9 eV.17 During
measurement of the bulk reference on polycrystalline Au
(performed in Centro de Astrobiología on another Phoibos
analyzer), some charging effects were observed and, as a consequence, the spectra appear shifted. Therefore, we have calibrated a posteriori the XPS spectra to the bulk line of the Au 4f7/2
core level at 84 eV.18
For fitting the spectra, we used the CASA program with a
GaussianLorentzian (product) mix function, and Shirley background subtraction was employed to obtain the XPS spectra. Mix
parameter was 80% and the full width at half-maximum (fwhm)
was 1.6 eV.
High-resolution XPS spectra from films grown on the Cu(110) surface were acquired at the Aloisa beamline at Elettra
(Trieste, Italy).19 The photon energy used for acquiring the N 1s
core level was 500 eV, and 650 eV was used for the O 1s core
level. The overall resolution was 0.2 eV. The binding energies
were referred to the Cu 3p core level (74.9 eV). The photoelectrons were detected at normal emission, while the sample was
kept at grazing incidence of 3 in transverse magnetic polarization (p-polarization). For the fitting, we have used a Voigt profile
and Shirley background subtraction. The characteristic parameters of the fitting are a Lorentzian width of 0.3 eV and a Gaussian
broadening of 0.9 eV.
Scanning Tunneling Microscopy. Low-temperature (LT)
STM images were acquired with an Aarhus-type variable-temperature fast-scanning STM purchased from SPECS, and the
images were analyzed with WSxM.20 W tips were used. The
temperature of the microscope can be tuned from 125 up to
500K. Images shown in the paper were acquired at 170 K. The
typical current was around 0.2 nA, and voltage was around V =
1.5 V (empty states). The images were recorded in the constant
current mode.
’ RESULTS AND DISCUSSION
XPS has proven to be one of the most powerful techniques for
studying the metalation of tetrapyrrole macrocycles on
surfaces.4,68,10,11 Particularly, the fingerprints used as indication
of metalation are the changes in core-level spectra, mainly in the
metal and in the N 1s core levels. For porphyrins it is known that
the two components observed in the N 1s peak of the free
molecule (corresponding to the nonequivalent iminic and pyrrolic N atoms) disappear after metal incorporation, leading to a
single component peak associated with the Nmetal bond4,68,10,11
As we discuss next, we observe these changes in the N 1s core level
when a submonolayer of H2PPIX interacts at RT with both
Cu(110) and Cu(100).
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Figure 2. N1s core-level XPS peaks of (A) bulk reference H2PPIX molecules; (B) H2PPIX evaporated on Cu(100) at 160 K and annealed to RT; (C)
H2PPIX evaporated on Cu(110) at 200 K and annealed to RT; (D) H2PPIX evaporated on Cu(110) at 200 K and annealed at 573 K for 3 min; (E)
H2PPIX evaporated on Cu(110) at 263 K; (F, G) H2PPIX evaporated on Cu(110) at 200 K (F, monolayer; G, low coverage); and (H) reference
molecule in solution in DMCO.
Figure 2 A shows the N1s core level peak of the powder
H2PPIX molecule used as reference and the best fit obtained.
The spectrum shows two equivalent components as expected for
the pristine molecule with the two nonequivalent N atoms,
iminic at 397.8 eV and pyrrolic at 399.9 eV (47% and 53% of
the total intensity, respectively). The line shape of N 1s changes
completely when the molecules are deposited on the copper
substrates. Figure 2 panels B and C show the N 1s core level
measured on Cu(100) and Cu(110) substrates, respectively,
after evaporation of H2PPIX at low temperature and the
subsequent anneal at RT (the same result is obtained directly
evaporating at RT, although with much lower sticking of the
molecule). In both cases the intensity of the component at higher
binding energy clearly decreases and the spectrum presents an
intense component at 398.2 eV, slightly shifted with respect to
the 397.8 eV of the iminic groups.
The origin of this new line shape can be explained as the result
of the reaction of the four N atoms with the Cu substrate atoms
forming a coordination bond; that is, metalation. In fact, this
component is not at the same position of the iminic N in the
reference (we remark that it is not possible to have molecules
with the four N atoms in an iminic conformation). Its energy
position is consistent with the nitrogen binding energy reported
for other copper porphyrins.21 Compared to other bulk
metalloporphyrins,10,22 the energy position for the CuPPIX film
is slightly lower although compatible with them, if we take into
account two effects related to the planar geometry of the
molecule and the submonolayer thickness: (i) substrate charge
transfer and (ii) a more efficient screening of the final core hole.
In some cases, the first stage of the metalation is witnessed by the
appearance of a small component at 399.1 eV. This component is
attributed to deprotonated polypyrroles, that is, macrocycles that
have lost one H atom,23 and it is an intermediate step before the
coordination reaction. No variation of the Cu photoemission
spectra was detected, because of the very low surface density of
incorporated Cu atoms even when our measurements are
performed at grazing incidence.
The feasibility of in situ metalation of preadsorbed metal-free
tetrapyrroles with vapor-deposited metal atoms was initially
demonstrated for Fe13,14 and extended later for other metals,
such as Ce, Zn, Co, or Ni.58,24 For some of them, like Fe and Co,
the reaction proceeds also at RT. For others, like Zn,8 the
reaction requires elevated temperatures (above 500 K). On the
basis of density functional theoery (DFT) calculations of the
activation barriers, Cu and Zn were proposed to react with
porphyrins at elevated temperatures.8 In the present case, we
observed no significant changes either in the position or in the
shape of the core level after an annealing beyond room temperature. Figure 2 D shows the spectrum of the molecules absorbed
on Cu(110) and measured after an annealing of the system at 573
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Figure 3. O 1s core level XPS peaks of H2PPIX, evaporated on (A)
Cu(110) at 200 K and (B) Cu(100) at 160 K.
K (same behavior is observed for the other crystal face). At higher
temperature, the molecule starts to decompose. In fact, the signal
of the O 1s core level disappears and new components in the C 1s
core level appear at lower binding energies that are related to
CuC interactions. Therefore, contrary to the theoretical predictions calculated for added adatoms, the H2PPIX molecules
metalate with the Cu substrate atoms already at RT. Moreover,
we have observed that this process starts at even lower temperatures . When the evaporation is performed with the substrate held
at 263K (see Figure 2 E), the N 1s core level presents one
relevant component centered at 398.2 eV, which is related to the
metalated component rather than to the iminic groups. Another
component is visible at 399.9 eV that might be entirely attributed
to the pyrrolic N. This double component assignment would
suggest an intermediate situation, with coexisting free and
metalated molecules. In fact, similar evidence was reported for
the metalation of tetraphenylporphyrin (H2TPP) on Ag(111) by
evaporation of Zn4,8 where, at RT, the coexistence of three
species was reported: (i) free molecules, (ii) metalloporphyrins,
and (iii) an intermediate species where the Zn atoms were
coordinated although the pyrrolic NH bonds were preserved.
However, such a simplified decomposition of the spectrum in
Figure 2E is in contrast with the molecular stoichiometry, since it
would indicate an excess of the pyrrolic component with respect
to the iminic one for the population of the intact metal-free
molecules.
In order to understand the origin of this apparently unbalanced component, we study the behavior of these molecules
when they just “land” at LT, just after the deposition. Figure 2 F
shows a high-resolution spectrum of N 1s core level after the
evaporation of a submonolayer of H2PPIX on Cu(110) with the
substrate at 200 K. The intensity of the iminic component almost
disappears and the spectrum presents an intense component at
400.1 eV, together with a small deprotonated component at
399.1 eV and an even fainter component at 397.9 eV (this
component is relatively larger for very low coverage). Apart from
the slightly larger binding energy (þ0.2 eV), the energy of the
former component would be compatible with most of the N
atoms in a pyrrolic-like configuration. This might imply that the
moleculesurface interaction mediates the formation of a
Figure 4. (A, B) N 1s and (C, D) O 1s core-level XPS peaks of ZnPPIX,
(A, C) evaporated on Cu(110) at 220 K and (B, D) annealed to RT.
zwitterionic phase of almost all molecules where the H atoms
from the carboxylic acid groups migrate to the center of the
pyrrole rings.3
To check the possible occurrence of this change of molecular
charge state, we measured the XPS spectra from carboxylic
groups of the molecule after evaporation. Figure 3 shows the
O 1s core level spectra measured on both samples, Cu(110) and
Cu(100), at low temperature. Both spectra can be fitted with
three components, centered at 532.9, 531.8, and 530.9 eV. The
components at 532.9 and 531.8 eV correspond to the two
nonequivalent O atoms in the carboxylic acid group, CdO and
C—OH, respectively, and the one at 530.9 eV corresponds to the
oxygen atoms in the deprotonated COO group.2527 Thus,
although we have detected a partial deprotonation of the
carboxylic group at low temperature, its amount is not large
enough to account for the dominant pyrrolic-like component in
the N 1s core level around 400 eV.
The hypothesis of migration of H atoms from the carboxylic
groups to the central core was proposed by Rienzo et al.3 for the
adsorption of zinc protoporphyrin (ZnPtP) on TiO2 (110). In
this work, the authors suggested that the strong interaction of
ZnPtP with the oxide surface induces the ejection of Zn atoms
from the central core and the subsequent occupation with two H
atoms probably comes from the carboxylic anchor groups. In
order to check whether this mechanism can be operative on a
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Figure 5. STM images of H2PPIX, (A) after evaporation on Cu(110) at
200 K (100 100 nm2 with a zoom of an individual molecule) and (B)
after annealing to 300 K (5 5 nm2).
copper substrate, we have performed comparative experiments
with ZnPtP molecules deposited on the Cu(110) surface.
Figure 4 panels A and C show the N 1s and O 1s core levels
measured after evaporation of a submonolayer of ZnPtP on
Cu(110) at LT (220 K). In the N 1s core level at LT, we observe
for Cu(110) the same double peak result that Rienzo et al.3
obtained at RT on TiO2 (110). However, at LT the O 1s core
level presents a very low degree of deprotonation of the
carboxylic group. When the analysis is done after the annealing
to RT, deprotonation of the carboxylic group is completed
(Figure 4 D), although at this temperature we observed only
the metalated molecule, with no signal in the pyrrolic N region
(Figure 4 B). Therefore, even when the deprotonation of the
carboxylic group takes place and it is completed at RT, the high
intensity of the component at around 400 eV in the N 1s core
level cannot be related only to formation of the zwitterionic
phase.
An alternative explanation, which fits all our experimental
evidences, is the formation of hydrogen-bond interactions between H in the carboxylic, methyl and/or ethenyl groups and N
in the tetrapyrrole ring among surrounding molecules. For
isonicotinic acid (where each molecule contains a single N
atom), it has been reported by XPS that two N components
occur in the N 1s core level that are separated by 1.7 eV.28 This
core-level shift is associated with hydrogen bonding between the
carboxylic group of one molecule and the nitrogen of the
pyridine ring of the next (this shift between the H-bonded
nitrogen and the non-H-bonded N atoms is even higher, 1.9
eV, for picolinic acid, which presents the same chemical formula
but a different arrangement of atoms in the molecule).29 This
head-to-tail hydrogen bonding does not yield deprotonation of
the carboxylic group and is known from X-ray crystallography
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studies to connect the isonicotinic acid molecules in infinite
chains in the solid state.2830 This mechanism might also apply
to the present case, as favored by the planar orientation of the
molecules, large size of the hole in the center of the macrocycle,
and flexibility of the bonds of the propionic acids. For comparison, we performed a reference experiment consisting of dissolving H2PPIX in a solvent, particularly DMSO. Figure 2 H shows
the N 1s core level of the reference sample (check the Experimental Details section for specifics). It presents two components at
binding energies of 399.9 and 397.8 eV. Also in this case part of the
iminic N atoms form the H bonding and the carboxylic group is not
deprotonated, according to the double peak structure observed for
the O 1s core level. We may conclude that H bonds between N and
H atoms of adjacent molecules can be formed also by deposition
from solution, in agreement with the mechanism observed by
evaporation in UHV. We can speculate that the very low deposition
rate of H2PPIX favors hydrogen bonding, as was also reported for
the film growth of isonicotinic acid and its isomers.31
Finally, we remark that metalation is possible thanks to the close
planarity of the molecular structure that allows the tetrapyrrole core
to stay very close to the surface. Near-edge X-ray absorption fine
structure (NEXAFS) and STM experiments confirm this assumption (a detailed discussion about the adsorption geometry will be the
object of another work). Figure 5 A shows two STM images
corresponding to H2PPIX deposited at low temperature on Cu(110). After evaporation at 200 K, the molecules aggregates into a
large density of small clusters, where molecules mainly lie parallel to
the surface, as shown in the inset image of an isolated molecule. After
annealing to RT, the metalation reaction is accompanied by selforganization of CuPPIX. Figure 5 B illustrates the organized
structure observed after annealing to RT. In this molecular arrangement, variable-polarization NEXAFS measurements indicate PPIX
molecules to be oriented with the macrocycle almost parallel to the
surface. Porphyrin orientation induced by some kind of coordination reaction with the Cu substrate atoms has been previously
reported.22 Klappenberger et al.23 discuss the formation of an
ordered network of molecules mediated by the coordination
reaction that takes place after evaporation and subsequent annealing
of the tetrapyridylporphyrins (TPyP) on Cu(111).23,32 In that case,
the reaction affected the four peripheral N-pyridyl substituents
instead of the four N atoms in the central macrocycle, and therefore
metalation was not involved.23 The reason is that in TPyP the
pyridyl nitrogen atoms are not coplanar with the tetrapyrrole, and
when the molecules interact with the surface, the central macrocycle
is too far from the surface to react with it.23,32 On the contrary, the
geometry of H2PPIX exhibiting an almost planar configuration
allows the surfacemacrocycle interaction. We remark that direct
metalation with surface atoms has not been reported yet for other
planar tetrapyrroles (like octaethylporphyrin), possibly because they
have been mostly studied on Ag(111),11 where the large size may
result in an additional steric barrier. In fact, in UHV, no previous
work reports such a clear surface metalation of any porphyrin. Only
for gold substrate in a liquid environment a partial surface coordination of tetrapyrrole ring with the substrate was suggested, by
Katsonis et al.,33 who pointed out that some iminic nitrogen atoms
react with Au atoms thanks to the strong distortion that tetradodecylporphyrin suffers on Au(111).
’ CONCLUSIONS
In summary, we show a ready moleculesubstrate coordination reaction at RT leading to metalation of porphyrin molecules
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(H2PPIX) with Cu surface atoms (CuPPIX). From our point of
view, there are several factors that combine to make this reaction
possible: (1) the closely planar structure of H2PPIX, which
allows direct surfacetetrapyrrole ring contact;34 (2) the high
density of Cu adatoms moving on the crystal surfaces that could
be involved in the coordination reaction;10 and (3) the rapid rate
of incorporation of Cu ions by porphyrin macrocycles35 and the
high deprotonation rate at RT. The coordinated porphyrins form
ordered self-assembled monolayers upon annealing. This highly
stable porphyrin network is more suitable for applications than
previously attempts to build SAMs with porphyrins.
’ AUTHOR INFORMATION
Corresponding Author
*Centro de Física de Materiales, Po. Manuel de Lardizabal 5,
Donostia - San Sebastian, Gipuzkoa E-20018, Spain. Tel (þ34)
943015804; fax (þ34) 943015800; e-mail [email protected].
’ ACKNOWLEDGMENT
We acknowledge funding through Spanish research projects
CSD2007-41, MAT2008-1497, PET2008-109, FIS2010-19609C02-00, and BIO 2007-67523 and intramural project 200960I159.
C.S.-S. is grateful to Ministerio de Educaci
on for the AP2005-0433
FPU grant. C.R. and R.G.-M. acknowledge help from and useful
discussions with A. Arnau and Enrique Ortega.
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