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2008, 112, 3453-3455 Published on Web 02/14/2008
Interaction of Cerium Atoms with Surface-Anchored Porphyrin Molecules A. Weber-Bargioni,*,† J. Reichert,† A. P. Seitsonen,‡ W. Auwa1 rter,§ A. Schiffrin,† and J. V. Barth*,†,§ Departments of Chemistry and Physics & Astronomy, UniVersity of British Columbia VancouVer, BC V6T 1Z4, Canada, IMPMC, CNRS and UniVersite´ Pierre et Marie Curie, 4 place Jussieu, case 115, F-75252 Paris, France, and Physik Department E20, Technische UniVersita¨t Mu¨nchen, James-Franck Strasse, D-85748 Garching, Germany ReceiVed: August 30, 2007; In Final Form: January 22, 2008
By exposing tetraphenyl-porphyrin (H2-TPP) molecules anchored on a Ag(111) surface to a beam of cerium atoms under vacuum conditions, a selective interaction of the lanthanide with the porphyrin macrocycle is achieved. A novel Ce-TPP species is formed that can be clearly identified in scanning tunneling microscopy data resolving intramolecular features. The electronic structure of the Ce-TPP was characterized by scanning tunneling spectroscopy, identifying molecular resonances that are related to those of coadsorbed Co-TPP. The inhomogenous local electronic density distribution of the dominating Ce-TPP and Co-TPP occupied states was visualized by tunneling spectroscopy mapping. With complementary density functional theory calculations, the geometry and energetics of the Ce-porphyrin bonding was determined.
Cerium and other lanthanide porphyrinates have been synthesized and investigated for more than three decades regarding their functional properties and potential for molecular architecture.1 In contrast to the abundant metalloporphyrins with 3d centers, the large, generally highly coordinated lanthanides cannot be accommodated in the macrocycle plane. Thus, sandwich-type complexes could be realized,2 enabling the design of systems with rotating porphyrin rings,3 whose control on surfaces is a topic of current interest.4 Here, we present a combined scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS) study on the interaction of single cerium atoms with free base tetra-phenyl-porphyrin (H2-TPP) molecules anchored on a Ag(111) surface. Topographic data reveal that cerium is selectively captured by the H2-TPP macrocycle. Tunneling spectroscopy data provide clear evidence for the formation of a novel Ce-porphyrinato complex, whose electronic structure is related to the simultaneously characterized Co-TPP species. STM experiments were performed in a custom-designed ultrahigh vacuum apparatus housing a commercial low-temperature STM.5 The Ag(111) preparation followed procedures outlined earlier.5 To have a metalated reference species, a mix of Co-TPP and H2-TPP was thermally evaporated by organic molecular beam epitaxy at 470 K (flux 0.01ML/min) while keeping the substrate at room temperature. Ce was evaporated thermally,6 whereby the H2-TPP/Co-TPP matrix was held at 300 K and exposed to short periods of a beam with 0.001 ML/ min flux. For the STM/STS measurements, the samples were * To whom correspondence should be addressed. E-mail: (A.W.-B.)
[email protected]; (J.V.B.)
[email protected]. † University of British Columbia Vancouver. ‡ IMPMC, CNRS and Universite ´ Pierre et Marie Curie. § Technische Universita ¨ t Mu¨nchen.
10.1021/jp076961i CCC: $40.75
cooled to 10 K. The indicated bias voltages were applied to the sample (topographic imaging in constant current mode). STS data were obtained via lock-in amplifier technique, using a 10 mV rms modulation amplitude at a frequency of 4.3 kHz. The feedback loop was open for point spectroscopy and closed for STS mapping at constant bias. The chemical structure of both H2-TPP and Co-TPP is depicted in Figure 1a. Upon adsorption on the substrate, they form arrays with a square packing scheme analogous to FeTPP/Ag(111) islands.7 The two different species of the mixed layer shown in Figure 1b are marked R (H2-TPP) and β (CoTPP), respectively. When the occupied states are imaged, the H2-TPP molecules exhibit a central ring, reflecting the porphyrin macrocycle, while the four smaller peripheral protrusions emanate from the phenyl meso substituents. The metalated species (β) exhibits an increased apparent height, dominated by an oval protrusion at the center because of the additional density of states with Co d character,8 similar to the imaging of Fe-TPP.7 A new species with different intramolecular features, labeled γ, appears in the H2-TPP/Co-TPP matrix after Ce exposure. It has a comparable apparent height to Co-TPP, yet is marked by a round central protrusion. The formation of the γ-species is associated with the selective interaction of Ce atoms with the H2-TPP macrocycle. This finding resembles previously observed metalation phenomena of porphyrins with Fe or Co under vacuum conditions7,9 whence the γ-species is the designated Ce-TPP. However, no three-dimensional (3D) analogue of the presently encountered compound is known from the literature, because it has been obtained in vacuo on a metal substrate10 with the Ag substrate spatially confining the molecular reactants, thus preventing from complete Ce coordination. © 2008 American Chemical Society
3454 J. Phys. Chem. C, Vol. 112, No. 10, 2008
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Figure 2. Differentiation of the three species’ electronic structure. (a) STS performed at the molecular centers, showing for H2-TPP no feature in the probed negative bias range (occupied states). Co-TPP has a resonance at -600 mV, whereas that of Ce-TPP is shifted to -950 mV. The unoccupied state of H2-TPP at 800 mV shifts to 950 (600) mV for Co-TPP (Ce-TPP). (b) Topography of the domain used for dI/dV mapping. (c,d) Spectral mapping of the electron distribution of dominant Co-TPP (Ce-TPP) states at -600 (-950) mV.
Figure 1. (a) H2-TPP and Co-TPP. (b) STM image of the CoTPP/H2-TPP mixed array on Ag(111) (data recorded at I ) 0.1 nA, V ) -800 mV). Molecules with an oval central protrusion are CoTPP (β), and the remaining ones are H2-TPP (R). Following exposure to Ce, a third species evolves with a round central protrusion (γ).
To explore the nature of the Ce-TPP complex and to compare its electronic structure with conventional (metallo)porphyrins, STS experiments were carried out. Figure 2a depicts tunneling spectra of H2-TPP (red), Co-TPP (green), and CeTPP (blue) obtained with the tip at the center of the molecule. The peaks in the spectra reflect major contributions to the tunneling current, mediated by molecular orbitals, whereby the negative (positive) bias regime represents the occupied (unoccupied) states. While the H2-TPP spectrum is flat in the range of -1000 mV to the Fermi level (zero bias), Co-TPP shows a strong resonance at -600 mV, originating from additional states with Co d-character.8 Also the new Ce-TPP species reveals a pronounced occupied resonance, which is shifted toward lower voltages (-950 mV). This is a clear indication that a Ce-porphyrinato complex has formed. Regarding the unoccupied states in the positive bias regime, less differences are found: they are comparable in intensity and appear shifted with respect to each other (H2-TPP at 800 mV, Co-TPP at 950 mV, and Ce-TPP at 600 mV). To further characterize the spatial extent of the dominant occupied states, tunneling spectroscopy (dI/dV) mapping at the peak energies was carried out.11 As a reference, Figure 2b shows a topographic image with all three species. The dI/dV map at -600 mV in Figure 2c shows pronounced features of a specific molecular orbital, characterized by two protrusions over the
macrocycle, which arise from Co-TPP, deduced from comparison with the topography image (for a full discussion of the Co-TPP frontier orbitals, see ref 12). Their anisotropy is associated with a conformational adaptation of the porphyrin leading to a symmetry break.7,12 Both Ce-TPP and H2-TPP show no distinct contributions in the dI/dV map and appear thus transparent. The situation is reversed at -950 mV, that is, for the pronounced CeTPP resonance, where the Ce-related contributions dominate (Figure 2d). The charge distribution of the Ce-TPP’s occupied level has the same spatial characteristics compared to Co-TPP, apart of a slight asymmetry, implying the presence of a small perturbation of the cerium compound, which might possibly be a hydrogen ligand (an issue that could be clarified by inelastic tunneling spectroscopy).6 The similarity between the Co-TPP and Ce-TPP electronic structure is remarkable, indicating that the porphyrin macrocycle response to valence electrons of lanthanides and 3d metal centers is closely related. Because of the size of Ce, its influence on the molecular geometry is expected to be different than of metalloporphyrins with 3d centers. To address the molecular structure and elucidate the manner in which Ce interacts with the macrocycle, systematic density functional theory (DFT) calculations of CeTPP in vacuum were performed. Because the treatment of the cerium f electron shell is a nontrivial issue,13 a series of functionals was tested. It turns out that the variation of bond lengths and angles based on these different functionals is minute (cf. the detailed description in the Supporting Information). Figure 3 shows the resulting 3D structure model of an isolated CeTPP species in vacuum. The Ce is coordinated by the N atoms of the macrocycle; however, it cannot be fully incorporated into the macrocycle plane because of its appreciable size. Hence, the cerium is hovering approximately 1 Å above the macrocycle plane. The macrocycle itself adopts a saddle-shaped structure, where two opposing pyrrole rings are facing upward and two
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J. Phys. Chem. C, Vol. 112, No. 10, 2008 3455 electronic structure of the novel species was furthermore assessed by systematic DFT calculations. It is suggested that the presented approach can be applied in general to realize novel rare-earth porphyrinates or phthalocyanides on surfaces, as arrays7 or scaffolds to design supramolecular architectures, notably including multidecker superstructures.
Figure 3. A 3D model of Ce-TPP based on DFT. The Ce resides approximately 1 Å above the macrocycle plane, whereby the Ce-N bond distance amounts to 2.3 Å. The pyrrole rings of the macrocycle adopt a slight saddle shape, where two opposing pyrroles are oriented upward and downward, respectively.
downward. The Ce-N bonding distance amounts to 2.3 Å and thus comes close to values found for Ce-porphyrinato sandwich compounds.13 The electronic energy levels of Ce-TPP were evaluated, showing considerable variation within the series of simulations employing different functionals (cf. Supporting Information). Furthermore, we investigated the stability of an adsorbed hydrogen as a ligand on the Ce-TPP molecule. The adsorption energy of the hydrogen relative to an atom in an H2 molecule in the gas phase is only 80 meV, and thus an H atom is not expected to bind at room temperature. On the other hand, the hydrogen ligand increases the Ce-N distance by 0.12 Å, and the eigenvalue of the highest occupied molecular orbital in Ce-TPP is pushed down, increasing the highest occupied molecular orbital-lowest unoccupied molecular orbital (HOMOLUMO) gap substantially (cf. Supporting Information). Finally, DFT calculations were performed to assess the interaction pathway of Ce atoms with the porphyrin macrocycle. We notably considered the existence of an intermediate sitting atop (SAT) complex recently discussed in the surface-confined metalation of H2-TPP by Zn atoms.14 Indeed, a similar state is possible for the case of Ce on H2-TPP; however, the final Ce-TPP compound is energetically preferred by 0.8 eV over the SAT complex. Because of the known high reactivity of Ce and the similarity of the present findings with the previously studied Fe metalation of porphyrins7 where also a single metalated species was observed, we conclude that the metalligand bonding takes place at room temperature. In conclusion, we presented an atomic-scale investigation of the interaction of single cerium atoms with surfaceanchored porphyrin macrocycles. This work demonstrated the first in vacuo synthesis of a coordinatively unsaturated lanthanide-porphyrin compound. The electronic and structural properties of the realized Ce-TPP species were investigated by tunneling spectroscopy measurements, whereby its relation to 3d analogues could be demonstrated. The geometric and
Acknowledgment. Work supported by Canada Foundation of Innovation, National Science and Engineering Research Council of Canada, and British Columbia Knowledge Development Fund. A.W.-B. and J.R. thank the German Academic Exchange Service and the Deutsche Forschungsgesellschaft for scholarships, respectively. We are particularly grateful to F. Patthey for providing the Ce and W.-D. Schneider for a helpful comment. Supporting Information Available: DFT methods, energetics, geometries, orbital eigenvalues from DFT, and table of bond lengths and angles. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) (a) Wong, C.-P.; Venteicher, R. F.; Horrocks, W. DeW. J. Am. Chem. Soc. 1974, 96, 7149. (b) Tran-Thi, T.-H. Coord. Chem. ReV. 1997, 160, 53. (c) Tsukube, H.; Shinoda, S. Chem. ReV. 2002, 102, 2389. (2) (a) Buchler, J. W.; Kapellmann, H.-G.; Knoff, M.; Lay, K.-L.; Pfeifer, S. Z. Naturforsch., B: Chem. Sci. 1983, 38B, 1339. (b) Buchler, J. W.; De Cian, A.; Fischer, J.; Kihn-Boutlinski, M.; Paulus, H.; Weiss, R. J. Am. Chem. Soc. 1986, 108, 3652. (3) (a) Tashiro, K.; Konishi, K.; Aida, T. Angew. Chem., Int. Ed. 1997, 36, 856. (b) Shinkai, S.; Ikeda, M.; Sugasaki, A.; Takeuchi, M. Acc. Chem. Res. 2001, 34, 494. (4) (a) Otsuki, J.; Kawaguchi, S.; Yamakawa, T.; Asakawa, M.; Miyake, K. Langmuir 2006, 22, 5708. (b) Yoshimoto, S.; Sawaguchi, T.; Su, W.; Jiang, J.; Koboyashi, N. Angew. Chem., Int. Ed. 2007, 46, 1071. (5) Auwa¨rter, W.; Weber-Bargioni, A.; Schiffrin, A.; Riemann, A.; Gro¨ning, O.; Fasel, R.; Barth, J. V. J. Chem. Phys. 2006, 124, 194708. (6) Silly, F.; Pivetta, M.; Ternes, M.; Patthey, F.; Pelz, J. P.; Schneider, W.-D. Phys. ReV. Lett. 2004, 92, 16101. Pivetta, M.; Ternes, M.; Patthey, F.; Schneider, W.-D. Phys. ReV. Lett. 2007, 99, 126104. (7) Auwa¨rter, W.; Weber-Bargioni, A.; Brink, S.; Riemann, A.; Schiffrin, A.; Ruben, M.; Barth, J. V. ChemPhysChem 2007, 8, 250. (8) Liao, M.-S.; Scheiner, S. J. Chem. Phys. 2002, 117, 205. (9) (a) Auwa¨rter, W.; Weber-Bargioni, A.; Riemann, A.; Schiffrin, A.; Barth, J. V. 23rd European Conference on Surface Science, ECOSS 23, Berlin, 2005, p 36. (b) Gottfried, J. M.; Flechtner, K.; Kretschmann, A.; Lukasczyk, T.; Steinru¨ck, H.-P. J. Am. Chem. Soc. 2006, 128, 5244. (10) Barth, J. V. Annu. ReV. Phys. Chem. 2007, 58, 375. Barth, J. V.; Costantini, G.; Kern, K. Nature 2005, 437, 671. (11) Lu, X. H.; Grobis, M.; Khoo, K. H.; Louie, S. G.; Crommie, M. F. Phys. ReV. Lett. 2003, 90, 096802. (12) Weber-Bargioni, A.; Auwa¨rter, W.; Klappenberger, F.; Reichert, J.; Lefrancois, S.; Strunskus, T.; Wo¨ll, C.; Schiffrin, A.; Pennec, Y.; Barth, J. V. ChemPhysChem 2008, 9, 89. Auwa¨rter, W.; Klappenberger, F.; WeberBargioni, A.; Schiffrin, A.; Strunskus, T.; Wo¨ll, Ch.; Pennec, Y.; Riemann, A.; Barth, J. V. J. Am. Chem. Soc. 2007, 129, 11279. (13) Ricciardi, G.; Rosa, A.; Baerends, E. J.; van Gisbergen, S. A. J. J. Am. Chem. Soc. 2002, 104, 12319. (14) Shubina, T. E.; Marbach, H.; Flechtner, K.; Kretschmann, A.; Jux, N.; Buchner, F.; Steinru¨ck, H. P.; Clark, T.; Gottfried, J. M. J. Am. Chem. Soc. 2007, 129, 9476.
