Measuring Spin-Allowed and Spin-Forbidden d–d Excitations in

Article pubs.acs.org/IC

Measuring Spin-Allowed and Spin-Forbidden d−d Excitations in Vanadium Complexes with 2p3d Resonant Inelastic X‑ray Scattering Benjamin E. Van Kuiken,† Anselm W. Hahn,† Dimitrios Maganas,† and Serena DeBeer*,†,‡ †

Max Planck Institute for Chemical Energy Conversion, Stiftstr. 34−36, D-45470 Mülheim an der Ruhr, Germany Department of Chemistry and Chemical Biology, Cornell University, Ithaca, New York 14853, United States

‡

S Supporting Information *

ABSTRACT: Spectroscopic probes of the electronic structure of transition metal-containing materials are invaluable to the design of new molecular catalysts and magnetic systems. Herein, we show that 2p3d resonant inelastic X-ray scattering (RIXS) can be used to observe both spin-allowed and (in the VIII case) spin-forbidden d−d excitation energies in molecular vanadium complexes. The spin-allowed d−d excitation energies determined by 2p3d RIXS are in good agreement with available optical data. In V(acac)3, a previously undetected spin-forbidden singlet state has been observed. The presence of this feature provides a ligand-field independent signature of VIII. It is also shown that d−d excitations may be obtained for porphyrin complexes. This is generally prohibitive using optical approaches due to intense porphyrin π-to-π* transitions. In addition, the intensities of chargetransfer features in 2p3d RIXS spectroscopy are shown to be a clear indication of metal−ligand covalency. The utility of 2p3d RIXS for future studies of complex inorganic systems is highlighted.

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VIVO(acac)2, and VIVO(TPP) (where acac = acetylacetonate and TPP = tetraphenylporphyrin). The RIXS spectra contain information about ligand field strength, oxidation state, and metal−ligand covalency, providing a means to experimentally determine the ligand field parameters. The correlation of the 2p3d RIXS data to existing optical data is highlighted. The particle/hole picture of the soft X-ray 2p3d RIXS process for a vanadyl complex in C4v symmetry is outlined in Figure 1. In the first step, the complex absorbs an X-ray photon, creating an intermediate state with a metal 2p core hole and an additional electron in a 3d orbital. This core hole is then filled

INTRODUCTION The ability to determine the energy of low-lying excited states in transition metal complexes is essential to our understanding of a range of fundamental processes, including the factors that govern molecular magnetism to the properties that enable twostate reactivity in catalytic systems.1,2 Ultimately, one would hope, via targeted spectroscopic experiments, to map out all eigenstates of an inorganic complex and use this knowledge to guide the rational design of single molecule magnets or catalysts. The challenge, however, is that d−d transitions are formally dipole-forbidden, placing constraints on the ability to detect these transitions optically. Further, transitions to lowlying excited states of a different multiplicity than the ground state are spin-forbidden, meaning that, even when d−d excitations may be observed with UV/vis, circular dichroism (CD), or magnetic circular dichroism (MCD) spectroscopy, states of different spin multiplicity remain spectroscopically silent. This motivates the need for spectroscopic probes, which can elucidate all possible d−d transitions. Herein, we demonstrate the ability of 2p3d resonant inelastic X-ray scattering (RIXS) to experimentally determine both the spin-allowed and the spin-forbidden transitions in a series of molecular vanadium complexes. While 2p3d RIXS has seen growing interest within the condensed matter physics and catalysis communities,3−7 there have been comparatively few molecular applications of 2p3d RIXS spectroscopy within the chemistry community.8−10 In this contribution, we present RIXS spectra for a vanadium series comprising VIII(acac)3, © 2016 American Chemical Society

Figure 1. Illustration of the electronic configurations involved in the RIXS process for a d1 transition metal complex in C4v symmetry. Received: August 23, 2016 Published: October 12, 2016 11497

DOI: 10.1021/acs.inorgchem.6b02053 Inorg. Chem. 2016, 55, 11497−11501

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Inorganic Chemistry

Figure 2. Experimental L3-edge XAS (top) and RIXS (bottom) spectra of (a, d) VO(acac)2, (b, e) VO(TPP), and (c, f) V(acac)3. All XAS spectra are normalized to the maximum of the L3 absorption. The incident energies for RIXS measurements are indicated by dotted lines in the XAS spectra and written in the RIXS plot. All RIXS data are plotted on the energy transfer axis, and spectra have been normalized to the maximum of the CT feature located between 5 and 10 eV.

by an electron from the valence shell, resulting in a final state that is either the same (elastic scattering) or higher (inelastic scattering) in energy than the initial state. The energy difference, called energy transfer (ET), between the incident and outgoing photons gives the excitation energy between the final and initial states. Because 2p → 3d excitations are dipoleallowed for metal L-edges, d−d excitations are the dominant features in RIXS spectra in the 0−5 eV ET range. Since d−d transitions are often symmetry-forbidden and/or obscured by intense charge-transfer or ligand-centric bands in UV/vis spectra, soft X-ray RIXS is a uniquely powerful tool for the interrogation of the electronic structure of transition metal complexes.

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emission spectrometer was calibrated using a series of elastic scattering peaks across the emission energy range. All data are plotted on the ET axis, which is simply the difference between the incident and emitted photon energy. Spectra are converted to ET space by fitting a Gaussian to the elastic scattering feature and setting this peak to 0 eV. The axis is also inverted so the ET becomes a positive value. Ignoring interference effects, the ET is the energy by which a final state lies above the ground state. In these experiments, we encountered the well-known challenge that molecular systems damage quickly when exposed to soft X-ray radiation.12 These effects have been combatted by two approaches. First, all samples were measured at low temperature. They were affixed to a copper plate that was brought into contact with the coldfinger of a liquid He cryostat. All samples were cooled to ∼30 K for measurement. Second, the samples were continually moved at a rate of 50 μm/s, so fresh sample was always exposed to the X-ray beam. The movement speed was determined by performing a series of RIXS scans at the maximum of the L3-edge. By comparing scans performed at 0, 5, 10, 20, 50, and 100 μm/s, it was determined that sample damage was absent at 50 μm/s. This corresponds to a residency time of 0.5 μs of X-ray exposure time per point on the sample. This is determined by the 25 μm vertical spot size set by the monochromator exit slit width. We note that this technique has been previously used to obtain damage free RIXS spectra on a vanadyl phthalocyanine complex.9 A second concern in soft X-ray emission measurements is that selfabsorption of emitted X-rays can distort spectra, giving inaccurate intensitites.13 The self-absorption of X-ray radiation is sensitive to the attenuation length of ingoing and outgoing X-ray beams, and consequently, the amount of self-absorption can be modified by altering the sample geometry chosen in the experiment. In the data presented below, an incident angle of 10° between the sample and incoming beam was used. This “near-grazing” geometry should mitigate effects of self-absorption by decreasing the absorption depth relative to normal incidence. Spectra were also collected at 45°, and no significant differences were observed due to sample geometry. Finally, RIXS spectra have been previously reported for cases where significant self-absorption was observed in the XAS, but the RIXS spectra appear unaltered.14 While it is possible the spectra presented below are affected by self-absorption, this should only modulate relative peak intensities. Interpretations of the data based on the energetic positions of spectral features are independent of any self-absorption.

EXPERIMENTAL SECTION

Materials. All samples were purchased from Sigma-Aldrich and used without further purification, with the exception of V(acac)3. V(acac)3 was recrystallized to remove a VIV impurity, the absence of which was verified by EPR spectroscopy. For X-ray measurements, samples were measured as either powders or thin films. In the case of powder samples, the powder was spread onto carbon tape. For thin films, samples were dissolved in dichloromethane and spin-coated onto Si wafers. VO(TPP) and V(acac)3 were prepared by this approach. X-ray Measurements. All measurements were performed using the X-ray emission spectroscopy (XES) endstation of the REIXS beamline (10ID-2) at the Canadian Light Source (CLS). Monochromatic soft X-rays were provided by an undulator and grating monochromator. All XES measurements were performed using circular polarization to maximize the X-ray flux. For X-ray emission measurements, the X-rays were spectrally dispersed by a grating onto a microchannel plate (MCP) array detector containing 1024 × 64 pixels. The grating for these measurements has a theoretical resolving power of 1000, yielding a resolution of 0.5 at 500 eV. The final experimental resolution is a convolution of incident beam monochromator and the spectrometer resolutions, and it was determined to be ∼0.8 eV from the fwhm of a Gaussian function fit to spectral elastic lines. The absolute energy axis was determined by calibrating X-ray absorption spectroscopy (XAS) measurements to previously published L3-edge XAS data for these complexes, where the reference energy is the maximum of V2O5 L3-edge at 518.9 eV.11 The 11498

DOI: 10.1021/acs.inorgchem.6b02053 Inorg. Chem. 2016, 55, 11497−11501

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Inorganic Chemistry For XAS measurements, the absorption was determined from the total electron yield (TEY) measured by the direct current from the sample. The spectra were recorded at a single sample spot. This was due to the fact that the XAS spectra show large variations when the sample is moved. This results from sample inhomogeneity, which can have a large effect on the TEY. However, much lower X-ray intensities are needed for XAS spectra than for RIXS. Consequently, the undulator was detuned for XAS measurements, reducing the X-ray flux by 2 orders of magnitude. This resulted in spectra that compared well with previously reported data.11 Ligand Field Multiplet Simulations. For the simulations of the XAS and RIXS spectra of V(acac)3, the XClaim program was used.15 Octahedral symmetry was assumed. Charge-transfer effects were not taken into account; however, these should not greatly affect the d−d region of the spectrum. The input parameters for the calculation were the vanadium oxidation state, a reduction factor for the 2-electron integrals, and the ligand field splitting magnitude (10Dq). The 2electron integrals were reduced from their atomic values to account for the effects of covalency. The simulations reproduce all of the features in the experiment with reasonable intensities and energies. For the simulations, both Lorentzian and Gaussian widths were added to discrete features. Widths of 0.5 eV are used in the XAS spectra, and widths of 0.7 eV are used for the emission spectra.

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The d−d spectra for all three molecules exhibit a pronounced dependence on the incident X-ray energy. In the d−d spectrum of VO(acac)2, features are present at 1.8 and 3.2 eV. At lower excitation energies (514−516 eV), the 1.8 eV feature dominates the d−d excitation region of the spectrum. However, as the incident energy reaches the maximum of the L3-edge, the second feature grows in intensity. This dependence is even more clearly evident in the emission spectra of VO(TPP) shown in Figure 2e. When excited at 515.5 eV, VO(TPP) exhibits a single strong d−d excitation at 2.3 eV. However, exciting above 517 eV preferentially induces emission to a state which is ∼3.6 eV above the ground state. Finally, in the spectra of V(acac)3, there is also a dependence on the incident energy, as evidenced by the intense 3.5 eV feature in the 518 eV spectrum that has the low intensity in the 514.3 eV spectrum. This phenomenon can be explained by the fact that exciting to a higher-lying 2p53dn+1 intermediate state allows for the possibility of emission to a higher energy d−d excited state. For example, in the 1-electron picture presented in Figure 1, an excitation from 2p to the a1 (dz2) orbital, followed by electron decay from the b2 (dxy) orbital down to the core hole, would result in a final state involving the b2 → a1 valence excitation. Conversely, if the incident photon does not have sufficient energy to promote the 2p electron to an intermediate state that has significant a1 orbital occupancy, emission to the 2A1 state will not be observed. Excitation to lower energy orbitals results in emission to lower energy d−d excited states. This picture certainly oversimplifies the complex many-electron multiplet structure of the intermediate states. Nevertheless, our previous theoretical studies on these complexes have shown that higherlying orbitals have greater contributions to higher energy 2p excited states, and these orbitals are preferentially occupied in states giving rise to features located at 517−518 eV.11 In order to understand the electronic structure information contained in RIXS spectra, it is useful to compare our results to available optical spectra of the d−d excitations in these complexes. The vanadyl complexes possess C4v symmetry, and the d orbitals are split into the arrangement depicted in Figure 1. Consequently, three transitions are expected, 2B2 → 2 E, 2B2 → 2B1, and 2B2 → 2A1. In VO(acac)2, the 2B2 → 2E transition appears at 1.8 eV. The excitation to the 2B1 state is found around ∼2.1 eV, and the 2B2 → 2A1 excitation appears as a shoulder on the charge-transfer band at ∼3.1 eV.20 In the RIXS data, we observe features at 1.7 and 3.2 eV. The 3.2 eV feature may be assigned to the 2B2 → 2A1 excitation. The 1.7 eV feature presumably has contributions from both 2E and 2B1 excited states. While the individual excitation energies cannot be resolved, the RIXS data are in good agreement with optical measurements. In the case of VO(TPP), there are also two RIXS features, but they appear at higher energy than the corresponding features in the acac complex. This result is expected because the porphyrin ring imposes a stronger ligand field on the VO2+ than the acac ligands. It should be noted that the electronic structure of vanadyl complexes has been thoroughly studied by a broad range of spectroscopic techniques.20−22 Ballhausen and Gray (BG) proposed a model for the electronic structure of vanadyl complexes that took into account the strong covalent VO bonding,21 but their interpretation led to some controversy. Low temperature UV/vis spectra led Selbin and co-workers to argue that all d−d excitations were found below 20 000 cm−1 (2.5 eV) and higher-lying excitations were due to CT.20,23 Ultimately, the BG model was shown to be correct by single-

RESULTS AND DISCUSSION

Figure 2 shows the RIXS spectra for each of the three complexes along with the corresponding L3-edge XAS spectra. The XAS spectra (Figure 2a−c) show several low intensity features, followed by a strong absorption near 517 eV. These XAS spectra have previously been analyzed in detail utilizing ab initio quantum chemical approaches.11 The RIXS spectra were collected at five different incident energies across the L3-edge, as indicated by the vertical lines superimposed on the spectra. These spectra are shown in the lower panel of Figure 2, and each spectrum contains three basic types of features. At 0 eV ET, the elastic scattering line appears. Next, the features between 0 and 5 eV are d−d transitions as depicted in Figure 1. These features will be discussed in detail below. Finally, between 5 and 10 eV, there is a broad band. Features in this energy range typically originate from charge-transfer (CT) and normal fluorescence processes. In this context, CT refers to a process where an electron is transferred from a ligand-based orbital to the metal in the presence of a core hole. This results from strong mixing of 2p53dn+1 and 2p53dn+2L configurations in the intermediate state where L signifies a hole in a ligand orbital. The emission process yields a final state with a 2p63dn+1L configuration. In previous RIXS studies of VIII, VIV, and VV oxides, CT features were identified in this energy range and attributed to hybridization of metal 3d and O 2p orbitals.16−19 In comparing the RIXS spectra shown in Figure 2 d−f, it is evident that there are clear differences in the magnitude of the CT features. Since the spectra have all been normalized to the maximum of the CT feature, only the relative amplitude of the d−d bands to the CT bands is important. In the cases of the vanadyl complexes (Figure 2d,e) the maximum of the CT band is comparable to or greater than the intensity of the d−d bands, whereas, in the case of V(acac)3 (Figure 2f), the intensity of the d−d bands is significantly greater than the CT band. This observation is not surprising due to the strong covalency of the VO moiety. The mixing of metal and ligand orbitals promotes charge transfer from the ligand to the metal. Consequently, one can use the intensity of CT in RIXS spectroscopy as a marker of covalency and an indicator of speciation patterns in complex systems. 11499

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Inorganic Chemistry crystal absorption measurements.24,25 Still, it is worth noting that the presence of the features at 3.2 eV in VO(acac)2 and at 3.6 in VO(TPP) spectra provides additional evidence of higherlying d−d excited states in support of the BG model. Furthermore, our measurements provide the first experimental values for the d−d excitation energies in VO(TPP), since intense ring based absorption features dominate both UV/vis and MCD spectroscopies.26 In both scenarios, the selectivity of 2p3d RIXS for the local transition metal electronic structure provides unique insights, which can be difficult to obtain with other techniques. Now we consider the d−d spectra of V(acac)3 shown in Figure 2f. Octahedral d2 complexes have a 3T1 ground state, and spin-allowed excitations are expected to 3T2 and 3T1 states. The d−d excitation energies have been reported for this complex based on analysis of optical MCD spectra. 27 Excitations appear at 2.1, 2.3, 3.4, and 3.7 eV. Where the 0.2 eV (0.3 eV) splitting in the first (last) two excitation energies is explained by the trigonal distortion from octahedral symmetry induced by the bidentate acac ligands. This distortion splits each of the triplet states into a 3A and 3E term. Features are identified at ∼2.1 eV for the 3T2 and 3.2−3.6 eV for the 3T1 state depending on excitation energy (see the SI). Hence, the RIXS d−d excitation energies are in good agreement with the optical data given the resolution of the RIXS experiment (∼0.8 eV). Lastly, in the RIXS spectra, we identify a third low energy feature at ∼1.3 eV that is most evident in the 515.7 eV spectrum. In order to understand the origin of the feature found at 1.3 eV in V(acac)3, ligand field multiplet calculations have been used to simulate the RIXS spectrum observed with an incident energy of 515.7 eV.6,15,28 The L3-edge XAS spectrum is wellreproduced with a 10Dq of 2.3 eV and a 35% reduction in the 2-electron interactions (see the SI for additional details). This 10Dq value is in good agreement with previous values ranging from 2.2 to 2.7 eV based on optical measurements.27 Diagonalizing the ligand field Hamiltonian reveals that there are two singlet states at ∼1.3 eV, and these states give rise to intensity at 1.3 eV as shown in the simulations presented in Figure 3. Excitations to these states are spin-forbidden in optical spectroscopy, but the strong spin−orbit coupling in the RIXS intermediate states allows for spin-flip transitions. In fact, features arising from excited states possessing a spin state that differs from the ground state have previously been observed in the RIXS spectra of various materials, most notably, manganese

oxide for which all d−d excitations from the 6A ground state are spin-forbidden.3,29 Examination of the Tanabe−Sugano diagram for d2 systems shows that the energies of the lowest-lying singlet states (1T2 and 1E) are independent of the 10Dq value. The simulations plotted in Figure 3 for 10Dq between 2 and 3 eV clearly show that the 1.3 eV feature is constant with respect to changes in the 10Dq value while the energy of the triple states systematically increase. The presence of a ligand field strength independent feature should provide a marker of VIII in a wide variety of systems. Further, we note that the observation of formally spin-forbidden excitations in these spectra enables the entire excited state spectrum to be experimentally determined.

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CONCLUSION The results presented in this article show that 2p3d RIXS spectroscopy is a powerful tool for investigating the electronic structure of molecular systems. The ability to measure d−d excitation energies, separated from MLCT, LMCT, and ligandbased excitations, provides a local probe of transition metal electronic structure. This technique has some distinct advantages over optical spectroscopies that are exemplified by our measurements. First, d−d excitation energies can be obtained regardless of the complexity and spectroscopy properties of the ligand system. The porphyrin complex studied here is a classic example where ligand base excitations dominate the UV/vis and MCD spectra, but RIXS still reports the d−d excitation energies. Second, the observation of spinforbidden excitations provides numerous opportunities for RIXS to be used as a probe for understanding magnetic properties of molecular systems while also providing a robust signature of VIII. Ongoing efforts in our laboratories should enable RIXS spectroscopy to play a more significant role in the field of transition metal catalysis.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b02053. Details and results of fits of the experimental RIXS spectra used to determine energies of spectral features. A comparison of the experimental and simulated XAS spectra of V(acac)3 (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the Max Planck Society for funding. Beamtime was awarded by the Canadian Light Source under proposals 21-6672 and 21-7280. Finally, the authors would like to thank David Muir and Teak Boyko for technical assistance with the RIXS measurements.

Figure 3. Ligand field multiplet simulations of the 515.7 eV feature of V(acac)3 for various 10Dq values. From left to right black lines mark dependence of the 10Dq value on the excitation energies to the 1 T2/1E, 3T2, and 3T1 states, respectively. 11500

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