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J. Phys. Chem. C 2010, 114, 10314–10322
Spin-Orbit Coupling and Metal-Ligand Interactions in Fe(II), Ru(II), and Os(II) Complexes Erik M. J. Johansson,*,† Michael Odelius,‡ Stefan Plogmaker,† Mihaela Gorgoi,§ Svante Svensson,† Hans Siegbahn,† and Håkan Rensmo† †
Department of Physics and Astronomy, DiVision of Surface and Interface Science, Uppsala UniVersity, Box 516, SE-751 20 Uppsala, Sweden, ‡ Fysikum, AlbaNoVa UniVersity Center, Stockholm UniVersity, SE-106 91 Stockholm, Sweden, § Helmholtz Zentrum Berlin, BESSY II, Albert-Einstein-Strasse 15, 12489 Berlin, Germany ReceiVed: April 29, 2010
The purpose of the present paper is to experimentally map the energy levels governing the trends observed in oxidation potentials and absorption spectra of M(bpy)32+ complexes (bpy ) 2,2′-bipyridine, M ) Fe(II), Ru(II), and Os(II)). Molecular films of the transition metal complexes were investigated with element specific methods using photoelectron spectroscopy (PES) at high kinetic energy using hard X-rays and by X-ray absorption spectroscopy (XAS). The results were compared to electronic structure calculations on the complexes and the ligand. The approach allows us to experimentally measure and interpret the energy levels in terms of spin-orbit coupling and metal-ligand interactions. Specifically, it was verified that the anomaly in the trend in oxidation potentials could be explained by a large spin-orbit coupling for the Os(bpy)32+. The influence of the different metal ions on the state formed upon light absorption was also investigated by N 1s X-ray absorption, and from the spectra we could determine the relative position of the levels originating from d-σ and π contributions. The results for the occupied and unoccupied electronic levels explain the lower energy of the MLCT transition of the Os(bpy)32+ in comparison to the Ru(bpy)32+. Introduction Ru(bpy)32+ is an extensively investigated metal complex and may be considered as a model compound in inorganic chemistry research. The combination of chemical stability and the electronic properties of the ground and excited state makes it specifically interesting and versatile in the development of many directions in photochemistry and photophysics.1-4 Moreover, similar d6 complexes having Fe and Os metal center have been widely studied, and the Fe, Ru, and Os series of complexes is of specific interest for the general understanding of metal-ligand interaction and the valence electronic structure of such metal complexes.1-9 The valence electronic structure is a most important factor for redox potentials. Specifically, the influence from spin-orbit coupling for a series of Fe, Ru, and Os transition metal complexes has recently been investigated by theoretical methods.5 The spin-orbit coupling and the metalligand interaction in the valence electronic energy levels also determine the absorption spectra. It is therefore important to experimentally map and follow the basic changes in the electronic energy levels for a better understanding of properties such as redox potentials and absorption spectra for these kinds of metal-organic complexes. An important motivation for many studies of metal-polypyridyl complexes is their potential use in photoconversion to energy-rich compounds or to electricity.10-19 For example, some ruthenium-polypyridine complexes such as Ru(dcbpy)2(NCS)2 have resulted in solar cell systems having very high photon to current conversion efficiency,16-19 and other metal complexes have been used in experiments for the conversion of water to oxygen and hydrogen.10-15 Also, similar complexes based on Os and Fe metal centers show promising efficiency20-22 although differences in the absorption and charge separation mechanism * Corresponding author. E-mail:
[email protected].
have been observed. The efficiency of the energy conversion processes is largely linked to the matching of electronic structure of the different molecular components. Many different spectroscopies and other experimental techniques have been used to characterize the electronic structure of these types of complexes.1-9,23-36 Oxidation of the d6 metal-polypyridyl complexes is generally referred to occur on the metal center while reduction is centered on the ligands. In accordance with this, the lowest visible light energy absorption process is considered as a metal-to-ligand charge transfer (MLCT).1-9 At higher energies metal and ligand centered transitions occur. The experimental results are often discussed and analyzed at different levels of basic molecular orbital theory and often include ligand field splitting in the general framework. Several more advanced quantum theory based studies have also been made to theoretically investigate the electronic structure of Ru-polypyridine molecules and dyes.5,27,28,37-47 Generally, the mechanism of the photochemical cycle for the complexes is based on understanding the ground state, light absorption, and the relaxed states formed from different deactivation processes. The present investigation focus on the first two targets, and we show how a combination of high kinetic energy electron spectroscopy (HIKE) and XAS can be used to characterize the occupied and unoccupied valence electronic structure with respect to both relative energies and orbital characters. Specifically, the study will show how the element specificity of the X-ray spectroscopies can be used to obtain detailed information about the valence structure monitoring the effects of spin-orbit coupling and metal-ligand interaction in the series of metal-polypyridine complexes containing the fundamentally important Fe(bpy)32+, Ru(bpy)32+, and Os(bpy)32+.
10.1021/jp103884c 2010 American Chemical Society Published on Web 05/14/2010
M(bpy)32+ Complexes
Figure 1. Molecular structures of the investigated complexes. The metal center is iron, ruthenium, and osmium in the different complexes. Counterions are PF6- for all complexes.
Experimental Section Tris(2,2′-bipyridine)iron(II) hexafluorophosphate (Fe(bpy)32+), tris(2,2′-bipyridine)ruthenium(II) hexafluorophosphate (Ru(bpy)32+), and tris(2,2′-bipyridine)osmium(II) hexafluorophosphate (Os(bpy)32+) were prepared by ion exchange from chloride and thiocyanate salts. [Fe(bpy)32+][NCS-]2 was purchased from Solaronix S.A, and the [Ru(bpy)32+][Cl-]2 was purchased from Sigma-Aldrich. [Os(bpy)32+][PF6-]2 was prepared from Os(bpy)2Cl248 and 2,2′-bpy using microwave heating (Initiator single mode microwave cavity at 2450 MHz, Biotage) in ethylene glycol at 196 °C as described for similar complexes.49 After counterion exchange with NH4PF6, the complex was recrystallized by Et2O diffusion into a CH3CN solution. The molecular structures of the measured complexes are shown in Figure 1. Multilayers of the complexes were prepared by smearing out the dye powder on conducting glass (SnO2:F on glass) using a stainless steel spatula. The formation of multilayers was checked for via the Sn 2p substrate signal in the PES spectra. The photoelectron spectroscopy (PES) measurements at high kinetic energy were performed using hard X-ray synchrotron light at the high kinetic energy electron spectroscopy facility (HIKE) end station on the beamline KMC-1 at BESSY in Berlin.50 The XAS spectra were measured at the beamline BL I4-11 at Maxlab in Lund. All experimental spectra are aligned to the C 1s peak position, which was set to 285.0 eV. The intensity of the core level spectra were normalized versus the intensity of N 1s. In the valence spectra in Figure 6 the intensity is normalized versus the highest occupied electronic structure to facilitate comparison between the three different complexes. In the resonant and nonresonant spectra of Ru(bpy)32+ in Figure 5c, the intensity is normalized versus the electronic structure between 15 and 20 eV binding energy. The energy in the N 1s XAS spectra were calibrated using first- and second-order light. Computational Details Single ion(2+) tris(2,2′-bipyridine) complexes of Os(II), Ru(II), and Fe(II) (Os(bpy)32+, Ru(bpy)32+, and Fe(bpy)32+) were studied theoretically with density functional methods to simulate the hard X-ray photoelectron emission and soft X-ray nitrogen K-edge X-ray absorption spectra. All theoretical results presented for the complexes are based on geometry optimizations and theoretical spectrum simulations performed in the StoBe-deMon code.51 We used the Becke exchange functional and a Perdew correlation functional.52,53 All atoms were described using effective core potentials (ECPs),54-58 except for hydrogen, for which the Huzinaga (311/ 1) basis set59 was used. The valence electrons in carbon and nitrogen were described by (311/211) and (211/21) basis sets,
J. Phys. Chem. C, Vol. 114, No. 22, 2010 10315 respectively. The auxiliary basis sets used were (5,2;5,2) for carbon and nitrogen and (3,1;3,1) for hydrogen, where the nomenclature (NC(s), NC(spd); NXC(s), NXC(spd)) indicates the number of s- and spd-functions used to fit the Coulomb and exchange correlation potentials, respectively. The basic analysis of the evolution of the electronic structure from pyridine, via bipyridine and Mg(bpy)32+, to the systems under study are based on the Kohn-Sham eigenstates in DFT calculations of the electronic ground states. The states in the isolated pyridine and bipyridine molecules are shifted by 5.5 eV for alignment with the metal ion complexes. The total density of states (DOS) of the occupied orbitals is compared with the partial density of states (PDOS) of nitrogen and the metal ion d-orbitals. In the negative direction, the angularly resolved nitrogen Px,Py,Pz PDOS allows us to follow the ligand states. The x direction is perpendicular to the aromatic ring and corresponds to the π system. The σ orbitals are confined to the yz plane, and the z direction is along the molecular axis of pyridine. The half-core-hole transition potential approximation was used to simulate nitrogen K-edge X-ray absorption spectra. A double-basis set procedure is added after SCF convergence to achieve a better description of Rydberg and continuum states.60,61 Core relaxation effects are well described by a core extended IGLO-III basis set of Kutzelnigg et al.62 on the core-excited nitrogen. The energy scale of the whole X-ray absorption spectrum is improved by a delta Kohn-Sham calculation63 of the total energy differences between the lowest core-excited state and the electronic ground state. The discrete calculated photoemission and X-ray absorption spectra are convoluted with a Gaussian distribution with full width at half-maximum of 0.8 and 0.4 eV, respectively. The influence of spin-orbit coupling of the splitting of the t2g states was investigated by multiconfigurational (CAS) calculations in MOLCAS64 including spin-orbit coupling with the restricted active space (RAS) state interaction approach.65 Scalar relativistic calculations, based on the Douglas-Kroll Hamiltonian,66,67 were performed using relativistic ANO-DK3 basis sets.68 The initial and final states in the photoemission process are determined with an orbital active space consisting of the three t2g orbitals. Results and Discussion The Basis of Metal-Ligand Interaction from DFT Calculations. Before presenting the experimental measurements, we will briefly describe the electronic structure in the iron, ruthenium, and osmium complexes from a series of density functional (DFT) calculations. The emphasis is to dissect the underlying contributing atomic character and distributions of the valence occupied and unoccupied energy levels in a way compatible with the experimental map obtained from the spectroscopic results. In Figure 2, the evolution of the electronic structure from pyridine, via bipyridine and Mg(bpy)32+, to the systems under study is depicted and schematically summarized in Figure 3. In pyridine, the highest occupied molecular orbital (HOMO) is the nitrogen lone pair, in plane, and below there are π bonding orbitals that are followed by σ orbitals. The lowest unoccupied states are of π* character. The electronic structure of bipyridine is nearly identical apart from the splitting of the pyridine levels. In the presence of the divalent Mg metal ion in the complexes, the ligand σ orbitals pointing toward the metal ion are strongly affected in two regards. First of all, the former HOMO of pyridine is pulled down in energy by 1.5 eV relative to the π
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Figure 3. Schematic figure of the calculated electronic structure going from pyridine to bipyridine to Mg(bpy)32+ and to Ru(bpy)32+. The levels that have large Ru 4d contribution are marked in black, and levels with a small Ru 4d contribution are marked in gray. The levels with large pyridine nitrogen σ orbital contribution in the direction toward the metal are marked in red, and levels with a small pyridine nitrogen σ orbital contribution in the direction toward the metal are marked in light red. The levels with pyridine nitrogen π contribution are marked in blue.
Figure 2. Calculated total density of states (TDOS) for pyridine, the bipyridine ligand, Mg(bpy)32+, Fe(bpy)32+, Ru(bpy)32+, and Os(bpy)32+ is displayed together with the partial density of states both for the unoccupied electronic structure and the occupied electronic structure. The TDOS is shown together with the partial density of states (PDOS) of nitrogen (N) and the metal ion d-orbitals (M-d). In the negative direction, the angularly resolved nitrogen Nx,Ny,Nz PDOS allows us to follow the ligand states. The x direction is perpendicular to the aromatic ring and corresponds to the π system. The σ orbitals are confined to the yz plane, and the z direction is along the molecular axis of pyridine. The theoretical TDOS and PDOS are based on the Kohn-Sham eigen states in DFT calculations of the electronic ground states. Included in the Fe(bpy)32+, Ru(bpy)32+, and Os(bpy)32+ spectra are also theoretical XAS simulations. The XAS simulations are performed using the transition potential method based on “half-corehole” (HCH) calculation (see Computational Details).
bonding orbitals by the presence of the metal ion. Second, there is an orbital mixing with a lower orbital of the same symmetry which distributes the nitrogen character over two σ levels at 4-5 and 7-8 eV, respectively. In the magnesium complex there are some, but remarkably small, changes in the lowest unoccupied orbitals. Finally, in the transition metal complexes, there are clear and systematic signatures of interactions between the ligand orbitals and metal d-orbitals. In simple terms, the transition metal ions form low-spin d6 electron configurations in strong ligand fields in octahedral symmetry and contain occupied t2g levels and unoccupied eg levels. Together with the ligand orbitals in the magnesium complex, the t2g and eg metal d-orbitals forms a natural reference basis to understand the metal-ligand interaction. At a binding energy around 2 eV, the metal-centered t2g levels are placed, isolated in the ligand “band gap”, and form the HOMOs in the complexes. In the unoccupied levels the energy of the eg levels show a strong dependence on the principal quantum number over the Fe, Ru, and Os series. As
expected, we also see parallel variations in the mixing and shifts of the occupied and unoccupied nitrogen-σ levels in the series of complexes due to the mixing of the metal eg d-levels and the two occupied σ levels. Just as this σ* nitrogen level follows the energy of the eg levels, there is corresponding metal d-character in the two σ levels at 4-5 and 7-8 eV arising from mixing with the eg levels. The unoccupied π* orbitals are only slightly affected but show a similar mixing between the metal t2g and lowest pyridine π* orbitals, which gives rise to metal d-character in the LUMO orbitals. This confirms the traditional picture of “σ bonding” and “π* back-donation” and specifically show that the signature of metal eg-pyridine σ interaction is very clear in both the occupied and the unoccupied valence levels. To be able to show how the X-ray absorption probe at the nitrogen K-edge can, in this context, be used to measure the experimental position of the σ* and π* states, the simulated N 1s spectrum is included in Figure 2. This XAS simulations are performed using the transition potential method based on “halfcore-hole” calculations, whereas all other curves are derived from the electronic ground state, but the σ* state is essentially unperturbed by the presence of the core-hole. In contrast, the lowest π* states are contracted by the core-hole. As shown below, the analysis above will form a powerful framework for the analysis of the element specific experimental X-ray spectroscopy data. In the HIKE PES experiments, we are able to probe the transition metal character in the occupied valence orbitals due to the large cross section for the metal d-levels in ruthenium and osmium (see below). In the N 1s XAS spectra, the corresponding changes in the ligand orbitals are revealed since the nitrogen σ* character is picked up by probing the nitrogen K-edge. With this support from theory the combined information from PES and XAS below will give a comprehensible experimental picture of the valence electronic structure and of the state initially obtained from light absorption in the series of transition metal ion complexes. Core and Valence Level Electron Spectroscopy. In photoelectron spectroscopy (PES) the surface sensitivity depends on the kinetic energy of the emitted electrons, and for high kinetic energies of the electrons the measurement becomes more bulk sensitive than at low kinetic energies (see e.g. ref 69). High kinetic energy electron spectroscopy is therefore useful for
M(bpy)32+ Complexes
Figure 4. (A) Spectra of the C 1s electronic structure of multilayers of the complexes measured with photon energy of 2800 eV. To facilitate comparison of the main peak of the C 1s spectra, we remove the contribution from the Ru 3d3/2 by referencing to the Ru 3d5/2 peak and by using values from a carbon free Ru 3d spectrum to obtain the position, intensity, and shape of the Ru 3d3/2 peak. (B) Spectra of the N 1s electronic structure of multilayers of the complexes measured with photon energy of 2800 eV.
studying the bulk electronic structure, and the bulk sensitivity is particularly useful for measurements of valence structures on molecular films that are difficult to prepare in situ (in a UHV environment) due to difficulties in physical vapor deposition of the material without decomposition. For such ex-situ samples the valence electron spectra may contain contributions from a contamination layer making, for example, the detailed interpretation of traditional soft X-ray spectroscopy measurements more difficult. However, at a high kinetic energy of 2800 eV, as used in this study, the mean free path is in the range of 5 nm, that is, about 5 molecular layers, which means that about 40% of the signal originates from layers buried deeper than 5 nm and that the contribution from the surface contamination in the interpretations therefore is substantially diminished. The C 1s and N 1s core level spectra of the three molecular layers are shown in parts A and B of Figure 4, respectively. Comparing the spectra in Figure 4A of the different complexes, we observe that there are some differences. In the spectra of the Ru(bpy)32+ and the Os(bpy)32+ there are also peaks due to Ru 3d5/2 and Os 4d5/2, which are clearly observed at binding energies below 285 eV. Also, Ru 3d3/2 and Os 4d3/2 occur within the spectral region 4.2 and 14 eV at higher binding energy from the d5/2 peaks, respectively. After the subtraction of the Ru 3d contribution, the C 1s spectra for the three different complexes are rather similar which confirms the low contribution of contamination in the spectra. The spectra largely contain two
J. Phys. Chem. C, Vol. 114, No. 22, 2010 10317 features separated by about 1.0 eV and with an intensity ratio of about 2:3. The general structure may be expected from the pyridine structure with two out of the five carbons bonding to nitrogen although the slightly less resolved structure for Os(bpy)32+ indicates a stronger metal-ligand mixing with the carbon orbitals for this complex. The N 1s spectra for the different complexes are compared in Figure 4B. The widths of the peaks are very similar, but the binding energy varies with respect to the C 1s position at 285.0 eV. The N 1s peak for Fe(bpy)32+ is observed at 399.9 eV and for Ru(bpy)32+ and Os(bpy)32+ the peak is shifted to 400.1 and 400.3 eV, respectively. The binding energies for the N 1s levels in the different molecules were calculated to 409.94, 410.02, and 410.05 eV for Fe(bpy)32+, Ru(bpy)32+, and Os(bpy)32+, respectively. Although the absolute energies differ, mainly since they are referenced versus vacuum and are calculated in their 2+ state, they follow the same trend. Again, the shift is attributed to a different interaction between the nitrogen and the different metal centers, and this interaction will be discussed further in view of the N 1s XAS measurements below. An important parameter for the intensities observed in the experimental valence spectra is the photon energy dependence of the cross section of the different energy levels. The cross section is largely dependent on the combination of atomic orbitals forming the different molecular energy levels. Specifically, in the ruthenium and osmium complexes, the cross section of Ru 4d and Os 5d relative C 2p and N 2p in the valence energy levels will increase considerably when using hard X-rays compared to soft X-rays in the 100 eV regime. It is therefore possible to obtain atom specific information about the molecular orbitals. Some such experimental variations are observed already at photon energies below 800 eV28 but are substantially amplified when measuring with HIKE using photon energies over 2000 eV.27 The relative cross section may also be estimated by the theoretical cross sections from ref 71, indicating that the cross section for Ru 4d and Os 5d at a few keV is about 2 orders of magnitude higher than the cross sections for C 2p and N 2p contributing to the ligands and more than 1 order of magnitude higher than the cross sections for P 3p and F 2p in the counterion. Other theoretically obtained cross sections show a similar trend although the differences are slightly different.72 Thus, for the Os(bpy)32+ and Ru(bpy)32+ complexes the experimental spectra will largely map the metal contribution in the valence spectra. For the iron complex the cross section difference between metal-centered Fe 3d and the ligand levels is somewhat less pronounced. In Figure 5a the experimental valence electronic structure of the three molecules is shown together with calculations of the total density of states. The theoretical partial density of states for the complexes is shown in Figure 2. When comparing the different experimental spectra in the binding energy region above 11 eV, the appearance is very similar. Also, the structures in the region between 4 and 11 eV show large similarities although the general intensity increase in the order Fe(bpy)32+, Ru(bpy)32+, Os(bpy)32+. In the region below 4 eV all experimental spectra mainly display one feature although the intensity is very different with Os(bpy)32+ showing the strongest signal. Energy and Structure of Occupied d-σ and d-π Levels. Comparing the calculated density of states (DOS) with the experimental spectra in Figure 5a, they show large similarities with three structures in the region 4-11 eV if shifting the theoretical by about 1 eV toward higher binding energies. Such a shift may be ascribed to the limitations in modeling of the 2+ complex, neglecting final state effects and interactions with
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Figure 6. Spectra of the highest occupied valence electronic structure of the complexes measured at 2800 eV. The inset shows the spectra with the main peak set to zero binding energy to facilitate comparison of the shape of the peaks.
Figure 5. (a) Spectra of the valence electronic structure of the three complexes measured at 2800 eV (full line) together with the calculated total density of states (broken line). (b) Experimental difference spectra obtained by subtracting the Fe(bpy)32+ spectrum from that of Ru(bpy)32+ and Os(bpy)3 (full line). The difference spectra are compared to the calculated partial density of states of the metal d contribution (broken line). (c) Valence electronic spectra of the ruthenium-based complex measured at nonresonant photon energy 2800 eV, core resonant Ru 2p3/2 photon energy 2841 eV, and after resonance with a photon energy of 2850 eV.
the counterions, and to limitations of the employed exchangecorrelation functional. From the theoretical calculations (Figure 2), we conclude that the valence structure in this region largely originates from the ligand σ and π orbitals and is dominated by the many carbon atoms.
Below 4 eV the calculated total spectra show two smaller peaks while mainly one strong feature is present in the experimental spectra. These discrepancies of the experimental spectra with the theoretical total DOS is explained by the large differences in photoemission cross section for the metal atoms compared to lighter atoms as discussed above. The strong experimental peak below 4 eV therefore reflects the metal d character for the different metal complexes in this region.27 Comparing the experimental spectra to the theoretical DOS and metal-centered partial density of states (M-PDOS), we conclude that the structure experimentally observed below 4 eV largely contain contribution from metal d levels. Minor density of states originating from C π orbitals observed in the calculations between 2.5 and 4 eV are not experimentally observed as expected, due to the lower cross section. The M-PDOS calculation also indicates metal contribution at higher binding energy (mainly between 4 and 10 eV) mixed with the ligand orbitals. Signatures of such character are observed in the experimental spectra. Specifically, at about 5-8 eV binding energy we observe an increase in the intensity comparing the spectra of Fe(bpy)32+ with those of Ru(bpy)32+ and Os(bpy)32+, which directly indicate the presence of metal character in this binding energy region. A more detailed analysis can be accomplished from the experimental difference spectra obtained by subtracting the Fe(bpy)32+ spectrum from that of Ru(bpy)32+ and Os(bpy)32+ (see Figure 5b). Neglecting the Fe contribution in the Fe(bpy)32+ spectrum in the binding region 5-10 eV and assuming that the ligand electronic structure is very similar in the three compounds, such a procedure will remove the ligand contribution in this energy interval. Support for this operation is observed in the theoretical spectra (showing large similarities in the ligand-centered DOS) and from the fact that Fe 3d has substantially lower cross section compared to Ru 4d and Os 5d. In the difference spectra we see a clear contribution in the 5-10 eV binding energy region very similar to the Me-DOS spectra. This confirms the result from the calculation, discussed above and allows us to experimentally determine the two binding d-σ levels in Ru(bpy)32+ and Os(bpy)32+. A complementary experimental method that can be used to further verify the character of a valence structure is resonant PES measurements. In this method valence spectra are measured
M(bpy)32+ Complexes
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TABLE 1: Results on Ionization Energies of t2g States in PES Spectrum Based on Multiconfigurational SCF (CAS) Calculations Including Spin-Orbit Coupling and Explicit Final State Effects (Degeneracy Is Given within Parentheses) metal ion
M-N distance (Å)
Fe2+
ionization energies without spin-orbit coupling (eV)
ionization energies including spin-orbit coupling (eV)
splitting without spin-orbit coupling (eV)
splitting including spin-orbit coupling (eV)
16.865 (4) 16.906 (2)
16.864 (2) 16.865 (2) 16.906 (2)
0.041
0.042
14.317 (2) 14.499 (4)
14.274 (2) 14.495 (2) 14.546 (2)
0.182
0.221 0.272
12.940 (2) 13.150 (4)
12.657 (2) 13.278 (2) 13.306 (2)
0.210
0.621 0.649
1.972 Ru2+ 2.081 Os2+ 2.102
at a photon energy that coincides (resonance) with the excitation energy of a core orbital to an unoccupied orbital in the system.28 With the current setup this can be used to match the Ru 2p3/2 to Ru 4d excitation. In Figure 5c a strong enhancement of the intensity in the spectrum at 2 eV was observed at 2841 eV, which thus verifies the Ru 4d character of this state. In the resonant valence spectrum we also observe increased intensity between 5 and 15 eV, as expected from the metal contribution also in this binding energy region. Comparing the binding energy of the highest occupied electronic structures of the three complexes in Figure 6, we observe that the structure obtained from Ru(bpy)32+ is shifted about 0.35 eV to higher binding energy compared to Fe(bpy)32+. The highest occupied electronic structure of Os(bpy)32+ has a intensity maximum shifted toward higher binding energy compared to Ru(bpy)32+ but is clearly asymmetric. The structure can be deconvoluted with two peaks having an intensity ratio of 2:1 and with binding energy positions of 1.9 and 1.3 eV. The latter is thus shifted about 0.2 eV toward lower binding energy in comparison to the Fe(bpy)32+ peak. The different binding energies of the HOMO levels are closely related to the oxidation (M2+/3+) redox potentials for the three complexes, which have been determined to 1.03, 1.20, and 0.81 V vs SCE for Fe(bpy)32+, Ru(bpy)32+, and Os(bpy)32+ (in acetonitrile), respectively.73 This gives a difference of -0.17 V between the Fe(bpy)32+ and Ru(bpy)32+ complexes and +0.22 eV between Fe(bpy)32+ and Os(bpy)32+. The trend in the redox potentials for the three complexes is therefore very similar to the binding energy trend in the electronic structure measurements. The differences between the two types of measurements may partly be due to the different environment of the complexes. In the redox measurements, there is a liquid environment, whereas the binding energies here are determined for a thick layer of molecules, without any liquid present. The splitting in the lowest binding energy states observed for Os(bpy)32+ deserves special attention, since it relates to the electrochemical properties (as well as the spectroscopical properties see below) of the complex. In the DFT calculations, the outermost occupied electronic structure between 0 and 3 eV is predominantly metal-based with a small ligand contribution. The splitting of the t2g d levels in the D3 symmetry, due to the interaction with the ligands, are calculated for the different complexes. The splitting is calculated to 0.253, 0.231, and 0.277 eV for Fe(bpy)32+, Ru(bpy)32+, and Os(bpy)32+, respectively. The DFT calculations show that the osmium complex has the largest splitting of the t2g set, but it is too small to explain the experimental data and there is no distinct difference between the osmium complex and the other complexes.
As an alternative more accurate approach, the t2g peaks are also derived from total energies by explicit calculations of the initial and final states in the photoelectron emission process. The highest peaks (
