3090
J. Phys. Chem. C 2007, 111, 3090-3098
Interaction of Cobalt(II) Tetraarylporphyrins with a Ag(111) Surface Studied with Photoelectron Spectroscopy Thomas Lukasczyk, Ken Flechtner, Lindsay R. Merte, Norbert Jux,† Florian Maier, J. Michael Gottfried,* and Hans-Peter Steinru1 ck UniVersita¨t Erlangen-Nu¨rnberg, Lehrstuhl fu¨r Physikalische Chemie II, Egerlandstr. 3, 91058 Erlangen, Germany, and UniVersita¨t Erlangen-Nu¨rnberg, Institut fu¨r Organische Chemie II, Henkestrasse 42, 91054 Erlangen, Germany ReceiVed: August 14, 2006; In Final Form: December 23, 2006
The interaction of cobalt(II) tetraphenylporphyrin (CoTPP) and cobalt(II) tetrakis-(3,5-di-tert-butylphenyl)porphyrin (CoTTBPP) with a Ag(111) surface has been investigated with photoelectron spectroscopy (XPS/ UPS). It is demonstrated that these adsorbed metal complexes are excellent model systems for studying the electronic interaction between a coordinated metal ion and a metal surface. The photoelectron spectra and work function data provide evidence that the electronic interaction between the cobalt ion and the silver surface results in a transfer of electron density from the surface to the ion. The presence of an additional electronic state located ∼1 eV above the singly occupied molecular orbital (SOMO) of the metalloporphyrins is consistent with a molecular orbital (MO) model of the Co-Ag interaction as is the fact that the energetic position of this state depends on the distance between the Co ion and the Ag surface. The adsorbate-induced work function changes for the saturated monolayers amount to -0.72 eV for CoTPP and -0.91 eV for CoTTBPP. For comparison, we also present data of monolayer films of tetraphenylporphyrin and zinc(II) tetraphenylporphyrin.
1. Introduction Metalloporphyrins and other structurally related planar metal complexes are widespread in nature and play a very important role as active centers of enzymes.1 In addition, technical applications, for example, as colorimetric gas sensors2-4 or catalysts,5 have been proposed. Recently, self-assembled layers of metalloporphyrins and metallophthalocyanines have attracted considerable attention, especially with respect to potential applications as photonic wires, optical switches, and lightharvesting systems.6 Supramolecular multiporphyrin arrays are considered as ideal model systems for the study of energytransfer mechanisms and as mimics of the natural photosynthetic system.7 Furthermore, it has been shown that metalloporphyrins can be used as efficient photosensitizers of wide-band gap semiconductors such as TiO2 in dye solar cells (Gra¨tzel cells).8 Ordered monolayers of porphyrins and metalloporphyrins on inert metal surfaces have been used as convenient model systems for studying reactivity and electronic structure of this class of molecules.9-14 An advantage of these thin-film systems is that reactions such as direct metalation11-14 and attachment of additional ligands10 or the interaction with the underlying surface9 can be studied under ultrahigh vacuum (UHV) conditions with surface science techniques such as photoelectron spectroscopy or scanning tunneling spectroscopy. The high thermal stability of the metalloporphyrins allows for film preparation by evaporation deposition, despite the high molecular weight.15 Most previous investigations on vapor-deposited metalloporphyrins and metallophthalocyanines focused on self-assembly * To whom correspondence should be addressed. Phone: +49 9131 8527314; fax: +49 9131 85-28867; e-mail:
[email protected]. † Institut fu ¨ r Organische Chemie II.
and geometric structure.16-30 For example, Hipps and coworkers studied the adsorption of several metalloporphyrins,15,17-19 metallophthalocyanines,20-27 and mixtures of both9,28,29 on metal surfaces in great detail, especially with scanning tunneling microscopy (STM) and tunneling spectroscopy,9,15,17-29 but also with X-ray photoelectron spectroscopy (XPS),18,27 ultraviolet photoelectron spectroscopy (UPS),15,19 and reflection absorption infrared spectroscopy (RAIRS).18 The reversible attachment of axial amine ligands on an adsorbed Zn(II)-porphyrin was investigated by Williams et al. with STM.10 Recently, Auwa¨rter et al. studied the intramolecular conformation of adsorbed tetrapyridyl-porphyrin molecules on Ag(111) with STM at 15 K.30 Less attention has been paid to the interaction of adsorbed metalloporphyrins or metallophthalocyanines with the underlying metal surface. Several photoemission studies have been performed on multilayers15,19 and therefore provide no information about the specific electronic interaction between the complexes and the surface. However, other UPS investigations have focused on adsorption-induced work function changes and energy-level alignment at the metal/(metallo)porphyrin interface.31,32 Furthermore, Barlow et al.9 studied submonolayers of cobalt(II) phthalocyanine (CoPc) and cobalt(II) tetraphenylporphyrin (CoTPP) on Au(111) with STM and STM-based tunneling spectroscopy. For both complexes, strong coupling of the Co(II) ion to the underlying metal surface was observed, despite the fact that the Co-Au distance of adsorbed CoTPP is 0.15 nm larger than that of CoPc. Scudiero et al.15 compared tunneling images of cobalt(II) and nickel(II) tetraphenylporphyrin monolayers on Au(111) and observed characteristic differences, which were linked to the different occupation of the 3dz2 orbital of the metal ion. This interpretation implies that tunneling is partly mediated by this orbital15 and suggests
10.1021/jp0652345 CCC: $37.00 © 2007 American Chemical Society Published on Web 02/01/2007
Interaction of Cobalt(II) Porphyrins with Ag(111)
Figure 1. Space-filling models of Co-tetraphenylporphyrin (CoTPP) and Co-tetrakis(3,5-di-tert-butylphenyl)porphyrin (CoTTBPP). The estimated molecular dimensions are based on X-ray diffraction data34-37,45 in combination with bond lengths and van der Waals radii of the atoms under the assumption that the dihedral angle between aryl rings and porphyrin plane amounts to 60°. In most previous work on adsorbed meso-phenyl substituted porphyrins, it was observed that the phenyl rings are pairwise inclined toward each other rather than forming a pinwheel-like structure in which all four rings are rotated in the same direction.30,47-49 This leads to the here shown 2-fold rotational symmetry of the adsorbed molecules instead of a 4-fold symmetry for a pinwheel conformation.
electronic interaction between substrate and coordinated metal ions. This specific interaction has found considerable interest from theoretical side as well (ref 33 and references therein). In a recent density functional theory (DFT) study of Pd and Mn porphyrins on Au(111), the formation of a covalent or metallic bond between the Mn ion and the Au surface, accompanied by a shift of electron density (partial charge transfer) from Mn 3p and 3d orbitals to Au(111) states, was predicted.33 In the present work, we study the interaction between porphyrin-coordinated Co(II) ions and a Ag(111) surface using photoelectron spectroscopy. We vary the distance and, thus, the strength of this interaction by spacer substituents on the periphery of the porphyrin unit. This approach is inspired by the pioneering work of Barlow et al., who compared the coupling of coordinated Co ions in CoPc and CoTPP to a Au(111) surface.9 As spacer substituents, we use phenyl and 3,5di-(tert-butyl)-phenyl groups attached to the meso-positions of the porphyrin (Figure 1). In the adsorbed state, the actual distance between surface and porphyrin plane depends on the dihedral angle between these aryl spacer groups and the porphyrin plane. Large dihedral angles between 60° and 90° have been observed in the solid phase,34-39 which reflects the sterical repulsion between the aryl groups and the porphyrin at smaller angles. This repulsion results in rotational barriers (∆Grot*) of typically 50-140 kJ/mol per aryl group, depending on substituent, metal center, and solvent.40-44 For angles below 60°, the repulsion becomes so substantial that it can cause outof-plane distortion of the porphyrin ring,45,46 mainly because the ortho-positions of the aryl rings start to interfere with the hydrogens on the neighboring pyrrole units. The onset of substantial repulsion is probably also the reason why dihedral angles around 60° have been observed on adsorbed mesotetraarylporphyrins as, for example, in a recent low-temperature STM study of tetrapyridylporphyrin on Ag(111).30 Similar angles seem also to play a role for adsorbed metallo-tetrakis-
J. Phys. Chem. C, Vol. 111, No. 7, 2007 3091 (3,5-di-tert-butylphenyl)porphyrins (M(II)-TTBPPs).47-49 If we assume that a dihedral angle of ∼60° also applies to our systems, we can estimate the distance between porphyrin plane and surface using bond lengths and van der Waals radii of the atoms involved. With this assumption, the respective distances are 0.3 nm for CoTPP and 0.5 nm for CoTTBPP (Figure 1). These values should be considered as rough estimates, since for some adsorbed metalloporphyrins the dihedral angles deviate considerably from 60°.47-49 Apparently, there is a rather delicate balance between the intramolecular rotational barrier on the one hand and attractive adsorbate-substrate interactions on the other. The well-studied example of CuTTBPP on Cu surfaces shows that, even for the same molecule and the same substrate metal, the dihedral angle can vary between 0° and 90° depending on the surface orientation.49 Moreover, attractive forces between metal center and substrate may shift the Co ion to a position below the porphyrin plane. This type of distortion has recently been observed for Ga phthalocyanine on Cu(111),50 in which the metal center is located 0.5 Å below the N atoms of the phthalocyanine. Sn phthalocyanine with its out-of-plane gas-phase structure (in which the Sn ion protrudes from the molecular plane) adsorbs on Ag(111) in a “Sn-down” geometry,51 which suggests attractive interactions between Sn ion and surface. DFT calculations for Mn porphyrin on Au(111) predicted an out-of-plane displacement of the Mn ion toward the surface by 0.2 Å.33 The actual distances between Co ion and surface in our systems are subject to ongoing experimental and theoretical studies. However, for the qualitative discussion in this paper, it is sufficient to assume that this distance is larger for CoTTBPP than for CoTPP. Because of the presence of the tert-butyl groups, this would be the case even for a dihedral angle of 0°. Only in the extreme and unlikely case of dihedral angles of 90° for CoTPP and 0° for CoTTBPP would the respective distances between porphyrin plane and surface amount to ∼3.2 Å and ∼2.2 Å, respectively, that is, the molecule with the larger substituents would have the smaller distance. 2. Experimental All experiments were performed with a commercial X-ray photoelectron spectrometer (Scienta ESCA-200) equipped with an Al KR X-ray source (1486.6 eV) with monochromator and a hemispherical energy analyzer (SES-200). The overall energy resolution amounts to 0.3 eV. The base pressure in this UHV system is below 1 × 10-10 mbar. In addition to the XP spectrometer, the system is equipped with a differentially pumped gas discharge lamp for UP photoelectron spectroscopy (UPS), LEED optics (ErLEED-1000A), two ion guns for sample cleaning and ion-scattering experiments, a mass spectrometer (Balzers QMS 112), and several evaporators. The porphyrins were evaporated with a home-built Knudsen cell evaporator at a temperature of 638 K, measured at the quartz glass crucible. The resulting fluxes, typically 0.05 Å/s at the sample position, were measured with a quartz crystal microbalance. The sample was a 10 mm × 10 mm square Ag single crystal (purity > 99.999%) with a thickness of 2 mm and a polished (111) surface, which was aligned to 99%), were degassed in vacuo for 24 h at 420 K prior to the evaporation deposition. The coverage θ is defined as the number of adsorbed molecules or atoms divided by the number of substrate atoms, whereas “monolayer” denotes a closed, saturated adlayer of molecules in direct contact with the substrate surface at 300 K. For example, monolayers of 2H-TPP and CoTPP correspond to θ ) 0.037, and the monolayer of CoTTBPP to θ ) 0.023.52 These values were determined using a combination of XPS (which proves the existence of complete monolayers) and lowenergy electron diffraction (which provides the ratio of the lattice constants of adsorbate and substrate in this monolayer) and were confirmed by STM.13 3. Results 3.1. Monolayer Preparation and Thermal Stability. The most important prerequisite for our study is the preparation of well-defined monolayers of the metalloporphyrins. This task can be accomplished either by evaporation deposition of precisely the necessary amount of the porphyrin or, alternatively, by deposition of an excess of the porphyrin and subsequent thermal desorption of the excessive layers. The first procedure requires a precise flux calibration, and the second requires a sufficient thermal stability of the molecules in the first layer to endure the thermal desorption step. To elucidate the thermal stability of the adsorbed porphyrin and to estimate the temperature window between multilayer desorption and monolayer decomposition, we performed temperature-dependent XPS measurements on both CoTPP and CoTTBPP adlayers. Figure 2 displays C 1s XP spectra taken after deposition of 10 monolayers of CoTPP at 300 K and subsequent heating to the indicated temperatures for 30 s. We observe only one
Lukasczyk et al. combined signal at 284.9 eV, because all carbon atoms in the porphyrin are aromatic and show very similar binding energies. The two shakeup satellites at 288.0 and 291.8 eV are typical for organic molecules with extended conjugated π systems.53 Between 500 and 525 K, the main signal loses intensity and shifts slightly (by less than 0.2 eV) toward lower binding energy, which indicates multilayer desorption in this temperature range. This conclusion is supported by a relaxation shift of all porphyrin-related photoemission signals in the valence region (see Figure 5). As further evidence for multilayer desorption above 500 K, we found that the spectrum at 525 K is identical to a spectrum obtained after direct deposition of a monolayer of CoTPP at 300 K. The LEED pattern of the annealed monolayer shows domains of square symmetry with a lattice constant of 1.2 ( 0.2 nm, which is in agreement with the value of 1.4 nm obtained in a previous STM study on ordered CoTPP monolayers18 and with ongoing STM studies13,54 in our laboratory. This lattice constant is in accordance with the diameter of the porphyrin and thus confirms that the molecules adsorb with the porphyrin plane parallel to the surface. At and above 700 K, an additional shift to lower binding energies by 0.25 eV is observed, accompanied by substantial broadening of the peak from 1.1 to 1.3 eV full width at half-maximum (fwhm), but only small changes in intensity. These changes in the spectrum suggest that thermal decomposition of the porphyrin starts between 600 and 700 K. Similar temperature-dependent measurements were performed with multilayers of the other porphyrin, CoTTBPP. Because of the presence of the aliphatic tert-butyl groups, the C 1s signal is broadened by 0.2 eV compared to CoTPP (spectra not shown). All temperature-induced changes are very similar to Figure 2, indicating that multilayer desorption temperature and thermal stability are similar for both Co porphyrins. As is the case for CoTPP, the spectrum taken after heating to 525 K is identical with a spectrum of a monolayer of CoTTBPP directly deposited at 300 K. Our measurements prove that both Co(II) porphyrins are stable up to at least 600 K on the Ag surface and that the temperature window between multilayer desorption and monolayer decomposition has a width of at least 75 K. Therefore, multilayer desorption can safely be used for the preparation of monolayers of both porphyrins. This procedure has previously been employed for the preparation of monolayers of large organic molecules and metal complexes (see, for example, Stadler et al.).51 In a separate set of experiments, we observed that the metal-free meso-tetraphenylporphyrin (2H-TPP) shows very similar multilayer desorption and decomposition temperatures, which implies that monolayers of the free base can be prepared in the same way. This multilayer desorption procedure ensures better reproducible results than the direct deposition of the monolayer, because uncertainties in the flux measurement have no influence on the coverage. 3.2. Co 2p3/2 and N 1s XP Signals. In Figure 3, we compare the Co 2p3/2 XP signals of CoTPP multilayer and monolayer. The multilayer spectrum (A) is in good agreement with previous XPS data from CoTPP multilayer films.18 The main peak, at 780.0 eV, is located at a typical Co(II) position (e.g., 780.2 eV for CoO),55,56 while the multiplet structure of the signal is in agreement with previous measurements on other Co(II) compounds (e.g., cobalt(II) oxide)55 and with theoretical calculations on the free Co2+ ion considering spin-orbit and electrostatic electron-electron interactions.57,58 In the Co 2p3/2 signal of the CoTPP monolayer (Figure 3B), the main peak is located at 778.2 eV, which is close to a typical
Interaction of Cobalt(II) Porphyrins with Ag(111)
Figure 3. Co 2p3/2 XP spectra of CoTPP/Ag(111). (A) Multilayer (seven layers), (B) monolayer. Curve A was recorded in normal emission, curve B in grazing emission (80° with respect to the surface normal) for improved signal-to-noise ratio. The intensity of the shoulder at 778 eV in spectrum A decreases with increasing thickness of the multilayer, which indicates that this shoulder originates from the CoTPP monolayer.
Co(0) position (e.g., 778.1 eV for Co metal).56 This surfaceinduced shift (1.8 eV) is much larger than that of the C 1s peak (
