All Publications


  • Chemical control of competing electron transfer pathways in iron tetracyano-polypyridyl photosensitizers CHEMICAL SCIENCE Kunnus, K., Li, L., Titus, C., Lee, S., Reinhard, M. E., Koroidov, S., Kjaer, K. S., Hong, K., Ledbetter, K., Doriese, W. B., O'Neil, G. C., Swetz, D. S., Ullom, J., Li, D., Irwin, K., Nordlund, D., Cordones, A., Gaffney, K. J. 2020; 11 (17): 4360–73

    View details for DOI 10.1039/c9sc06272f

    View details for Web of Science ID 000532365500005

  • Vibrational wavepacket dynamics in Fe carbene photosensitizer determined with femtosecond X-ray emission and scattering. Nature communications Kunnus, K., Vacher, M., Harlang, T. C., Kjar, K. S., Haldrup, K., Biasin, E., van Driel, T. B., Papai, M., Chabera, P., Liu, Y., Tatsuno, H., Timm, C., Kallman, E., Delcey, M., Hartsock, R. W., Reinhard, M. E., Koroidov, S., Laursen, M. G., Hansen, F. B., Vester, P., Christensen, M., Sandberg, L., Nemeth, Z., Szemes, D. S., Bajnoczi, E., Alonso-Mori, R., Glownia, J. M., Nelson, S., Sikorski, M., Sokaras, D., Lemke, H. T., Canton, S. E., Moller, K. B., Nielsen, M. M., Vanko, G., Warnmark, K., Sundstrom, V., Persson, P., Lundberg, M., Uhlig, J., Gaffney, K. J. 2020; 11 (1): 634

    Abstract

    The non-equilibrium dynamics of electrons and nuclei govern the function of photoactive materials. Disentangling these dynamics remains a critical goal for understanding photoactive materials. Here we investigate the photoinduced dynamics of the [Fe(bmip)2]2+ photosensitizer, where bmip=2,6-bis(3-methyl-imidazole-1-ylidine)-pyridine, with simultaneous femtosecond-resolution Fe Kalpha and Kbeta X-ray emission spectroscopy (XES) and X-ray solution scattering (XSS). This measurement shows temporal oscillations in the XES and XSS difference signals with the same 278fs period oscillation. These oscillations originate from an Fe-ligand stretching vibrational wavepacket on a triplet metal-centered (3MC) excited state surface. This 3MC state is populated with a 110fs time constant by 40% of the excited molecules while the rest relax to a 3MLCT excited state. The sensitivity of the Kalpha XES to molecular structure results from a 0.7% average Fe-ligand bond length shift between the 1s and 2p core-ionized states surfaces.

    View details for DOI 10.1038/s41467-020-14468-w

    View details for PubMedID 32005815

  • Orbital-specific mapping of the ligand exchange dynamics of Fe(CO)(5) in solution NATURE Wernet, P., Kunnus, K., JOSEFSSON, I., RAJKOVIC, I., Quevedo, W., Beye, M., Schreck, S., Gruebel, S., Scholz, M., Nordlund, D., Zhang, W., Hartsock, R. W., Schlotter, W. F., Turner, J. J., Kennedy, B., Hennies, F., de Groot, F. M., Gaffney, K. J., Techert, S., Odelius, M., Foehlisch, A. 2015; 520 (7545): 78-81

    Abstract

    Transition-metal complexes have long attracted interest for fundamental chemical reactivity studies and possible use in solar energy conversion. Electronic excitation, ligand loss from the metal centre, or a combination of both, creates changes in charge and spin density at the metal site that need to be controlled to optimize complexes for photocatalytic hydrogen production and selective carbon-hydrogen bond activation. An understanding at the molecular level of how transition-metal complexes catalyse reactions, and in particular of the role of the short-lived and reactive intermediate states involved, will be critical for such optimization. However, suitable methods for detailed characterization of electronic excited states have been lacking. Here we show, with the use of X-ray laser-based femtosecond-resolution spectroscopy and advanced quantum chemical theory to probe the reaction dynamics of the benchmark transition-metal complex Fe(CO)5 in solution, that the photo-induced removal of CO generates the 16-electron Fe(CO)4 species, a homogeneous catalyst with an electron deficiency at the Fe centre, in a hitherto unreported excited singlet state that either converts to the triplet ground state or combines with a CO or solvent molecule to regenerate a penta-coordinated Fe species on a sub-picosecond timescale. This finding, which resolves the debate about the relative importance of different spin channels in the photochemistry of Fe(CO)5 (refs 4, 16 - 20), was made possible by the ability of femtosecond X-ray spectroscopy to probe frontier-orbital interactions with atom specificity. We expect the method to be broadly applicable in the chemical sciences, and to complement approaches that probe structural dynamics in ultrafast processes.

    View details for DOI 10.1038/nature14296

    View details for Web of Science ID 000352027700040

    View details for PubMedID 25832405

  • Quantifying the Steric Effect on Metal-Ligand Bonding in Fe Carbene Photosensitizers with Fe 2p3d Resonant Inelastic X-ray Scattering. Inorganic chemistry Kunnus, K., Guo, M., Biasin, E., Larsen, C. B., Titus, C. J., Lee, S. J., Nordlund, D., Cordones, A. A., Uhlig, J., Gaffney, K. J. 1800

    Abstract

    Understanding the electronic structure and chemical bonding of transition metal complexes is important for improving the function of molecular photosensitizers and catalysts. We have utilized X-ray absorption spectroscopy (XAS) and resonant inelastic X-ray scattering (RIXS) at the Fe L3 edge to investigate the electronic structure of two Fe N-heterocyclic carbene complexes with similar chemical structures but different steric effects and contrasting excited-state dynamics: [Fe(bmip)2]2+ and [Fe(btbip)2]2+, bmip = 2,6-bis(3-methyl-imidazole-1-ylidine)pyridine and btbip = 2,6-bis(3-tert-butyl-imidazole-1-ylidene)pyridine. In combination with charge transfer multiplet and ab initio calculations, we quantified how changes in Fe-carbene bond length due to steric effects modify the metal-ligand bonding, including sigma/pi donation and pi back-donation. We find that sigma donation is significantly stronger in [Fe(bmip)2]2+, whereas the pi back-donation is similar in both complexes. The resulting stronger ligand field and nephelauxetic effect in [Fe(bmip)2]2+ lead to approximately 1 eV destabilization of the quintet metal-centered 5T2g excited state compared to [Fe(btbip)2]2+, providing an explanation for the absence of a photoinduced 5T2g population and a longer metal-to-ligand charge-transfer excited-state lifetime in [Fe(bmip)2]2+. This work demonstrates how combined modeling of XAS and RIXS spectra can be utilized to understand the electronic structure of transition metal complexes governed by correlated electrons and donation/back-donation interactions.

    View details for DOI 10.1021/acs.inorgchem.1c03124

    View details for PubMedID 35029978

  • Capturing Atom-Specific Electronic Structural Dynamics of Transition-Metal Complexes with Ultrafast Soft X-Ray Spectroscopy. Annual review of physical chemistry Jay, R. M., Kunnus, K., Wernet, P., Gaffney, K. J. 1800

    Abstract

    The atomic specificity of X-ray spectroscopies provides a distinct perspective on molecular electronic structure. For 3d metal coordination and organometallic complexes, the combination of metal- and ligand-specific X-ray spectroscopies directly interrogates metal-ligand covalency-the hybridization of metal and ligand electronic states. Resonant inelastic X-ray scattering (RIXS), the X-ray analog of resonance Raman scattering, provides access to all classes of valence excited states in transition-metal complexes, making it a particularly powerful means of characterizing the valence electronic structure of 3d metal complexes. Recent advances in X-ray free-electron laser sources have enabled RIXS to be extended to the ultrafast time domain. We review RIXS studies of two archetypical photochemical processes: charge-transfer excitation in ferricyanide and ligand photodissociation in iron pentacarbonyl. These studies demonstrate femtosecond-resolution RIXS can directly characterize the time-evolving electronic structure, including the evolution of the metal-ligand covalency. Expected final online publication date for the Annual Review of Physical Chemistry, Volume 73 is April 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

    View details for DOI 10.1146/annurev-physchem-082820-020236

    View details for PubMedID 34985923

  • Reduction of Electron Repulsion in Highly Covalent Fe-Amido Complexes Counteracts the Impact of a Weak Ligand Field on Excited-State Ordering. Journal of the American Chemical Society Larsen, C. B., Braun, J. D., Lozada, I. B., Kunnus, K., Biasin, E., Kolodziej, C., Burda, C., Cordones, A. A., Gaffney, K. J., Herbert, D. E. 2021

    Abstract

    The ability to access panchromatic absorption and long-lived charge-transfer (CT) excited states is critical to the pursuit of abundant-metal molecular photosensitizers. Fe(II) complexes supported by benzannulated diarylamido ligands have been reported to broadly absorb visible light with nanosecond CT excited state lifetimes, but as amido donors exert a weak ligand field, this defies conventional photosensitizer design principles. Here, we report an aerobically stable Fe(II) complex of a phenanthridine/quinoline diarylamido ligand, Fe(ClL)2, with panchromatic absorption and a 3 ns excited-state lifetime. Using X-ray absorption spectroscopy (XAS) and resonant inelastic X-ray scattering (RIXS) at the Fe L-edge and N K-edge, we experimentally validate the strong Fe-Namido orbital mixing in Fe(ClL)2 responsible for the panchromatic absorption and demonstrate a previously unreported competition between ligand-field strength and metal-ligand (Fe-Namido) covalency that stabilizes the 3CT state over the lowest energy triplet metal-centered (3MC) state in the ground-state geometry. Single-crystal X-ray diffraction (XRD) and density functional theory (DFT) suggest that formation of this CT state depopulates an orbital with Fe-Namido antibonding character, causing metal-ligand bonds to contract and accentuating the geometric differences between CT and MC excited states. These effects diminish the driving force for electron transfer to metal-centered excited states and increase the intramolecular reorganization energy, critical properties for extending the lifetime of CT excited states. These findings highlight metal-ligand covalency as a novel design principle for elongating excited state lifetimes in abundant metal photosensitizers.

    View details for DOI 10.1021/jacs.1c06429

    View details for PubMedID 34851636

  • Revealing the bonding of solvated Ru complexes with valence-to-core resonant inelastic X-ray scattering CHEMICAL SCIENCE Biasin, E., Nascimento, D. R., Poulter, B. I., Abraham, B., Kunnus, K., Garcia-Esparza, A. T., Nowak, S. H., Kroll, T., Schoenlein, R. W., Alonso-Mori, R., Khalil, M., Govind, N., Sokaras, D. 2021; 12 (10): 3713–25

    View details for DOI 10.1039/d0sc06227h

    View details for Web of Science ID 000630166700031

  • Short-lived metal-centered excited state initiates iron-methionine photodissociation in ferrous cytochrome c. Nature communications Reinhard, M. E., Mara, M. W., Kroll, T., Lim, H., Hadt, R. G., Alonso-Mori, R., Chollet, M., Glownia, J. M., Nelson, S., Sokaras, D., Kunnus, K., Driel, T. B., Hartsock, R. W., Kjaer, K. S., Weninger, C., Biasin, E., Gee, L. B., Hodgson, K. O., Hedman, B., Bergmann, U., Solomon, E. I., Gaffney, K. J. 2021; 12 (1): 1086

    Abstract

    The dynamics of photodissociation and recombination in heme proteins represent an archetypical photochemical reaction widely used to understand the interplay between chemical dynamics and reaction environment. We report a study of the photodissociation mechanism for the Fe(II)-S bond between the heme iron and methionine sulfur of ferrous cytochrome c. This bond dissociation is an essential step in the conversion of cytochrome c from an electron transfer protein to a peroxidase enzyme. We use ultrafast X-ray solution scattering to follow the dynamics of Fe(II)-S bond dissociation and 1s3p (Kbeta) X-ray emission spectroscopy to follow the dynamics of the iron charge and spin multiplicity during bond dissociation. From these measurements, we conclude that the formation of a triplet metal-centered excited state with anti-bonding Fe(II)-S interactions triggers the bond dissociation and precedes the formation of the metastable Fe high-spin quintet state.

    View details for DOI 10.1038/s41467-021-21423-w

    View details for PubMedID 33597529

  • Origin of core-to-core x-ray emission spectroscopy sensitivity to structural dynamics. Structural dynamics (Melville, N.Y.) Vacher, M., Kunnus, K., Delcey, M. G., Gaffney, K. J., Lundberg, M. 2020; 7 (4): 044102

    Abstract

    Recently, coherent structural dynamics in the excited state of an iron photosensitizer was observed through oscillations in the intensity of Kalpha x-ray emission spectroscopy (XES). Understanding the origin of the unexpected sensitivity of core-to-core transitions to structural dynamics is important for further development of femtosecond time-resolved XES methods and, we believe, generally necessary for interpretation of XES signals from highly non-equilibrium structures that are ubiquitous in photophysics and photochemistry. Here, we use multiconfigurational wavefunction calculations combined with atomic theory to analyze the emission process in detail. The sensitivity of core-to-core transitions to structural dynamics is due to a shift of the minimum energy metal-ligand bond distance between 1s and 2p core-hole states. A key effect is the additional contraction of the non-bonding 3s and 3p orbitals in 1s core-hole states, which decreases electron-electron repulsion and increases overlap in the metal-ligand bonds. The effect is believed to be general and especially pronounced for systems with strong bonds. The important role of 3s and 3p orbitals is consistent with the analysis of radial charge and spin densities and can be connected to the negative chemical shift observed for many transition metal complexes. The XES sensitivity to structural dynamics can be optimized by tuning the emission energy spectrometer, with oscillations up to ±4% of the maximum intensity for the current system. The theoretical predictions can be used to design experiments that separate electronic and nuclear degrees of freedom in ultrafast excited state dynamics.

    View details for DOI 10.1063/4.0000022

    View details for PubMedID 32665965

  • Chemical control of competing electron transfer pathways in iron tetracyano-polypyridyl photosensitizers. Chemical science Kunnus, K., Li, L., Titus, C. J., Lee, S. J., Reinhard, M. E., Koroidov, S., Kjær, K. S., Hong, K., Ledbetter, K., Doriese, W. B., O'Neil, G. C., Swetz, D. S., Ullom, J. N., Li, D., Irwin, K., Nordlund, D., Cordones, A. A., Gaffney, K. J. 2020; 11 (17): 4360-4373

    Abstract

    Photoinduced intramolecular electron transfer dynamics following metal-to-ligand charge-transfer (MLCT) excitation of [Fe(CN)4(2,2'-bipyridine)]2- (1), [Fe(CN)4(2,3-bis(2-pyridyl)pyrazine)]2- (2) and [Fe(CN)4(2,2'-bipyrimidine)]2- (3) were investigated in various solvents with static and time-resolved UV-Visible absorption spectroscopy and Fe 2p3d resonant inelastic X-ray scattering (RIXS). This series of polypyridyl ligands, combined with the strong solvatochromism of the complexes, enables the 1MLCT vertical energy to be varied from 1.64 eV to 2.64 eV and the 3MLCT lifetime to range from 180 fs to 67 ps. The 3MLCT lifetimes in 1 and 2 decrease exponentially as the MLCT energy increases, consistent with electron transfer to the lowest energy triplet metal-centred (3MC) excited state, as established by the Tanabe-Sugano analysis of the Fe 2p3d RIXS data. In contrast, the 3MLCT lifetime in 3 changes non-monotonically with MLCT energy, exhibiting a maximum. This qualitatively distinct behaviour results from a competing 3MLCT → ground state (GS) electron transfer pathway that exhibits energy gap law behaviour. The 3MLCT → GS pathway involves nuclear tunnelling for the high-frequency polypyridyl breathing mode (hν = 1530 cm-1), which is most displaced for complex 3, making this pathway significantly more efficient. Our study demonstrates that the excited state relaxation mechanism of Fe polypyridyl photosensitizers can be readily tuned by ligand and solvent environment. Furthermore, our study reveals that extending charge transfer lifetimes requires control of the relative energies of the 3MLCT and the 3MC states and suppression of the intramolecular distortion of the acceptor ligand in the 3MLCT excited state.

    View details for DOI 10.1039/c9sc06272f

    View details for PubMedID 34122894

    View details for PubMedCentralID PMC8159445

  • Excited state charge distribution and bond expansion of ferrous complexes observed with femtosecond valence-to-core x-ray emission spectroscopy. The Journal of chemical physics Ledbetter, K. n., Reinhard, M. E., Kunnus, K. n., Gallo, A. n., Britz, A. n., Biasin, E. n., Glownia, J. M., Nelson, S. n., Van Driel, T. B., Weninger, C. n., Zederkof, D. B., Haldrup, K. n., Cordones, A. A., Gaffney, K. J., Sokaras, D. n., Alonso-Mori, R. n. 2020; 152 (7): 074203

    Abstract

    Valence-to-core x-ray emission spectroscopy (VtC XES) combines the sample flexibility and element specificity of hard x-rays with the chemical environment sensitivity of valence spectroscopy. We extend this technique to study geometric and electronic structural changes induced by photoexcitation in the femtosecond time domain via laser-pump, x-ray probe experiments using an x-ray free electron laser. The results of time-resolved VtC XES on a series of ferrous complexes [Fe(CN)2n(2, 2'-bipyridine)3-n]-2n+2, n = 1, 2, 3, are presented. Comparisons of spectra obtained from ground state density functional theory calculations reveal signatures of excited state bond length and oxidation state changes. An oxidation state change associated with a metal-to-ligand charge transfer state with a lifetime of less than 100 fs is observed, as well as bond length changes associated with metal-centered excited states with lifetimes of 13 ps and 250 ps.

    View details for DOI 10.1063/1.5139441

    View details for PubMedID 32087640

  • Finding intersections between electronic excited state potential energy surfaces with simultaneous ultrafast X-ray scattering and spectroscopy. Chemical science Kjar, K. S., Van Driel, T. B., Harlang, T. C., Kunnus, K., Biasin, E., Ledbetter, K., Hartsock, R. W., Reinhard, M. E., Koroidov, S., Li, L., Laursen, M. G., Hansen, F. B., Vester, P., Christensen, M., Haldrup, K., Nielsen, M. M., Dohn, A. O., Papai, M. I., Moller, K. B., Chabera, P., Liu, Y., Tatsuno, H., Timm, C., Jarenmark, M., Uhlig, J., Sundstom, V., Warnmark, K., Persson, P., Nemeth, Z., Szemes, D. S., Bajnoczi, E., Vanko, G., Alonso-Mori, R., Glownia, J. M., Nelson, S., Sikorski, M., Sokaras, D., Canton, S. E., Lemke, H. T., Gaffney, K. J. 2019; 10 (22): 5749–60

    Abstract

    Light-driven molecular reactions are dictated by the excited state potential energy landscape, depending critically on the location of conical intersections and intersystem crossing points between potential surfaces where non-adiabatic effects govern transition probabilities between distinct electronic states. While ultrafast studies have provided significant insight into electronic excited state reaction dynamics, experimental approaches for identifying and characterizing intersections and seams between electronic states remain highly system dependent. Here we show that for 3d transition metal systems simultaneously recorded X-ray diffuse scattering and X-ray emission spectroscopy at sub-70 femtosecond time-resolution provide a solid experimental foundation for determining the mechanistic details of excited state reactions. In modeling the mechanistic information retrieved from such experiments, it becomes possible to identify the dominant trajectory followed during the excited state cascade and to determine the relevant loci of intersections between states. We illustrate our approach by explicitly mapping parts of the potential energy landscape dictating the light driven low-to-high spin-state transition (spin crossover) of [Fe(2,2'-bipyridine)3]2+, where the strongly coupled nuclear and electronic dynamics have been a source of interest and controversy. We anticipate that simultaneous X-ray diffuse scattering and X-ray emission spectroscopy will provide a valuable approach for mapping the reactive trajectories of light-triggered molecular systems involving 3d transition metals.

    View details for DOI 10.1039/c8sc04023k

    View details for PubMedID 31293761

  • Solvatochromic and Ligand Effects in Ultrafast Intramolecular Electron Transfer in Fe-based Molecular Photosensitizers Kunnus, K., Li, L., Reinhard, M., Koroidov, S., Kjaer, K. S., Ledbetter, K., Hong, K., Biasin, E., Cordones-Hahn, A., Gaffney, K. J., Cerullo, G., Ogilvie, J., Kartner, F., Khalil, M., Li, R. E D P SCIENCES. 2019
  • Hot Branching Dynamics in a Light-Harvesting Iron Carbene Complex Revealed by Ultrafast X-ray Emission Spectroscopy. Angewandte Chemie (International ed. in English) Tatsuno, H. n., Kjaer, K. S., Kunnus, K. n., Harlang, T. C., Timm, C. n., Guo, M. n., Chàbera, P. n., Fredin, L. A., Hartsock, R. W., Reinhard, M. E., Koroidov, S. n., Li, L. n., Cordones, A. A., Gordivska, O. n., Prakash, O. n., Liu, Y. n., Laursen, M. G., Biasin, E. n., Hansen, F. B., Vester, P. n., Christensen, M. n., Haldrup, K. n., Németh, Z. n., Sárosiné Szemes, D. n., Bajnóczi, É. n., Vankó, G. n., Van Driel, T. B., Alonso-Mori, R. n., Glownia, J. M., Nelson, S. n., Sikorski, M. n., Lemke, H. T., Sokaras, D. n., Canton, S. E., Dohn, A. O., Møller, K. B., Nielsen, M. M., Gaffney, K. J., Wärnmark, K. n., Sundström, V. n., Persson, P. n., Uhlig, J. n. 2019

    Abstract

    Iron N-heterocyclic carbene (NHC) complexes have received a great deal of attention recently because of their growing potential as light sensitizers or photocatalysts. We present a sub-ps X-ray spectroscopy study of an FeII NHC complex that identifies and quantifies the states involved in the deactivation cascade after light absorption. Excited molecules relax back to the ground state along two pathways: After population of a hot 3 MLCT state, from the initially excited 1 MLCT state, 30 % of the molecules undergo ultrafast (150 fs) relaxation to the 3 MC state, in competition with vibrational relaxation and cooling to the relaxed 3 MLCT state. The relaxed 3 MLCT state then decays much more slowly (7.6 ps) to the 3 MC state. The 3 MC state is rapidly (2.2 ps) deactivated to the ground state. The 5 MC state is not involved in the deactivation pathway. The ultrafast partial deactivation of the 3 MLCT state constitutes a loss channel from the point of view of photochemical efficiency and highlights the necessity to screen transition-metal complexes for similar ultrafast decays to optimize photochemical performance.

    View details for DOI 10.1002/anie.201908065

    View details for PubMedID 31602726

  • The nature of frontier orbitals under systematic ligand exchange in (pseudo-)octahedral Fe(II) complexes PHYSICAL CHEMISTRY CHEMICAL PHYSICS Jay, R. M., Eckert, S., Fondell, M., Miedema, P. S., Norell, J., Pietzsch, A., Quevedo, W., Niskanen, J., Kunnus, K., Fohlisch, A. 2018; 20 (44): 27745–51

    Abstract

    Understanding and controlling properties of transition metal complexes is a crucial step towards tailoring materials for sustainable energy applications. In a systematic approach, we use resonant inelastic X-ray scattering to study the influence of ligand substitution on the valence electronic structure around an aqueous iron(ii) center. Exchanging cyanide with 2-2'-bipyridine ligands reshapes frontier orbitals in a way that reduces metal 3d charge delocalization onto the ligands. This net decrease of metal-ligand covalency results in lower metal-centered excited state energies in agreement with previously reported excited state dynamics. Furthermore, traces of solvent-effects were found indicating a varying interaction strength of the solvent with ligands of different character. Our results demonstrate how ligand exchange can be exploited to shape frontier orbitals of transition metal complexes in solution-phase chemistry; insights upon which future efforts can built when tailoring the functionality of photoactive systems for light-harvesting applications.

    View details for DOI 10.1039/c8cp04341h

    View details for Web of Science ID 000450660400065

    View details for PubMedID 30211412

  • Probing the Electron Accepting Orbitals of Ni-Centered Hydrogen Evolution Catalysts with Noninnocent Ligands by Ni L-Edge and S K-Edge X-ray Absorption. Inorganic chemistry Koroidov, S., Hong, K., Kjaer, K. S., Li, L., Kunnus, K., Reinhard, M., Hartsock, R. W., Amit, D., Eisenberg, R., Pemmaraju, C. D., Gaffney, K. J., Cordones, A. A. 2018

    Abstract

    The valence electronic structure of several square planar Ni-centered complexes, previously shown to catalyze the hydrogen evolution reaction, are characterized using S K-edge and Ni L-edge X-ray absorption spectroscopy and electronic structure calculations. Measurement of the atomic Ni 3d and S 3p contributions enables assessment of the metal-ligand covalency of the electron accepting valence orbitals and yields insight into the ligand-dependent reaction mechanisms proposed for the catalysts. The electron accepting orbital of the Ni(abt)2 (abt = 2-aminobenzenethiolate) catalyst is found to have large ligand character (80%), with only 9% S 3p (per S) character, indicating delocalization over the entire abt ligand. Upon two proton-coupled reductions to form the Ni(abt-H)2 intermediate, the catalyst stores 1.8 electrons on the abt ligand, and the ligand N atoms are protonated, thus supporting its role as an electron and proton reservoir. The electron accepting orbitals of the Ni(abt-H)2 intermediate and Ni(mpo)2 (mpo = 2-mercaptopyridyl- N-oxide) catalyst are found to have considerably larger Ni 3d (46-47%) and S 3p (17-18% per S) character, consistent with an orbital localized on the metal-ligand bonds. This finding supports the possibility of metal-based chemistry, resulting in Ni-H bond formation for the reduced Ni(abt-H)2 intermediate and Ni(mpo)2 catalyst, a critical reaction intermediate in H2 generation.

    View details for PubMedID 30351073

  • Solvent control of charge transfer excited state relaxation pathways in [Fe(2,2 '-bipyridine)(CN)(4)](2-) PHYSICAL CHEMISTRY CHEMICAL PHYSICS Kjaer, K. S., Kunnus, K., Harlang, T. B., Van Driel, T. B., Ledbetter, K., Hartsock, R. W., Reinhard, M. E., Koroidov, S., Li, L., Laursen, M. G., Biasin, E., Hansen, F. B., Vester, P., Christensen, M., Haldrup, K., Nielsen, M. M., Chabera, P., Liu, Y., Tatsuno, H., Timm, C., Uhlig, J., Sundstom, V., Nemeth, Z., Szemes, D., Bajnoczi, E., Vanko, G., Alonso-Mori, R., Glownia, J. M., Nelson, S., Sikorski, M., Sokaras, D., Lemke, H. T., Canton, S. E., Warnmark, K., Persson, P., Cordones, A. A., Gaffney, K. J. 2018; 20 (6): 4238–49

    Abstract

    The excited state dynamics of solvated [Fe(bpy)(CN)4]2-, where bpy = 2,2'-bipyridine, show significant sensitivity to the solvent Lewis acidity. Using a combination of optical absorption and X-ray emission transient spectroscopies, we have previously shown that the metal to ligand charge transfer (MLCT) excited state of [Fe(bpy)(CN)4]2- has a 19 picosecond lifetime and no discernable contribution from metal centered (MC) states in weak Lewis acid solvents, such as dimethyl sulfoxide and acetonitrile.1,2 In the present work, we use the same combination of spectroscopic techniques to measure the MLCT excited state relaxation dynamics of [Fe(bpy)(CN)4]2- in water, a strong Lewis acid solvent. The charge-transfer excited state is now found to decay in less than 100 femtoseconds, forming a quasi-stable metal centered excited state with a 13 picosecond lifetime. We find that this MC excited state has triplet (3MC) character, unlike other reported six-coordinate Fe(ii)-centered coordination compounds, which form MC quintet (5MC) states. The solvent dependent changes in excited state non-radiative relaxation for [Fe(bpy)(CN)4]2- allows us to infer the influence of the solvent on the electronic structure of the complex. Furthermore, the robust characterization of the dynamics and optical spectral signatures of the isolated 3MC intermediate provides a strong foundation for identifying 3MC intermediates in the electronic excited state relaxation mechanisms of similar Fe-centered systems being developed for solar applications.

    View details for PubMedID 29364300

  • Metalloprotein entatic control of ligand-metal bonds quantified by ultrafast x-ray spectroscopy SCIENCE Mara, M. W., Hadt, R. G., Reinhard, M., Kroll, T., Lim, H., Hartsock, R. W., Alonso-Mori, R., Chollet, M., Glownia, J. M., Nelson, S., Sokaras, D., Kunnus, K., Hodgson, K. O., Hedman, B., Bergmann, U., Gaffney, K. J., Solomon, E. I. 2017; 356 (6344): 1276-+

    Abstract

    The multifunctional protein cytochrome c (cyt c) plays key roles in electron transport and apoptosis, switching function by modulating bonding between a heme iron and the sulfur in a methionine residue. This Fe-S(Met) bond is too weak to persist in the absence of protein constraints. We ruptured the bond in ferrous cyt c using an optical laser pulse and monitored the bond reformation within the protein active site using ultrafast x-ray pulses from an x-ray free-electron laser, determining that the Fe-S(Met) bond enthalpy is ~4 kcal/mol stronger than in the absence of protein constraints. The 4 kcal/mol is comparable with calculations of stabilization effects in other systems, demonstrating how biological systems use an entatic state for modest yet accessible energetics to modulate chemical function.

    View details for PubMedID 28642436

    View details for PubMedCentralID PMC5706643

  • Quantifying covalent interactions with resonant inelastic soft X-ray scattering: Case study of Ni2+ aqua complex CHEMICAL PHYSICS LETTERS Kunnus, K., JOSEFSSON, I., Schreck, S., Quevedo, W., Miedema, P. S., Techert, S., de Groot, F. M., Foehlisch, A., Odelius, M., Wernet, P. 2017; 669: 196-201
  • Density functional simulation of resonant inelastic X-ray scattering experiments in liquids: acetonitrile PHYSICAL CHEMISTRY CHEMICAL PHYSICS Niskanen, J., Kooser, K., Koskelo, J., Kaambre, T., Kunnus, K., Pietzsch, A., Quevedo, W., Hakala, M., Foehlisch, A., Huotari, S., Kukk, E. 2016; 18 (37): 26026-26032

    View details for DOI 10.1039/c6cp03220f

    View details for Web of Science ID 000385172600026

  • Anti-Stokes resonant x-ray Raman scattering for atom specific and excited state selective dynamics NEW JOURNAL OF PHYSICS Kunnus, K., Josefsson, I., Rajkovic, I., Schreck, S., Quevedo, W., Beye, M., Greubel, S., Scholz, M., Nordlund, D., Zhang, W., Hartsock, R. W., Gaffney, K. J., Schlotter, W. F., Turner, J. J., Kennedy, B., Hennies, F., Techert, S., Wernet, P., Odelius, M., Foehlisch, A. 2016; 18
  • Viewing the Valence Electronic Structure of Ferric and Ferrous Hexacyanide in Solution from the Fe and Cyanide Perspectives JOURNAL OF PHYSICAL CHEMISTRY B Kunnus, K., Zhang, W., Delcey, M. G., Pinjari, R. V., Miedema, P. S., Schreck, S., Quevedo, W., Schroeder, H., Foehlisch, A., Gaffney, K. J., Lundberg, M., Odelius, M., Wernet, P. 2016; 120 (29): 7182-7194

    Abstract

    The valence-excited states of ferric and ferrous hexacyanide ions in aqueous solution were mapped by resonant inelastic X-ray scattering (RIXS) at the Fe L2,3 and N K edges. Probing of both the central Fe and the ligand N atoms enabled identification of the metal- and ligand-centered excited states, as well as ligand-to-metal and metal-to-ligand charge-transfer excited states. Ab initio calculations utilizing the RASPT2 method were used to simulate the Fe L2,3-edge RIXS spectra and enabled quantification of the covalencies of both occupied and empty orbitals of π and σ symmetry. We found that π back-donation in the ferric complex is smaller than that in the ferrous complex. This is evidenced by the relative amounts of Fe 3d character in the nominally 2π CN(-) molecular orbital of 7% and 9% in ferric and ferrous hexacyanide, respectively. Utilizing the direct sensitivity of Fe L3-edge RIXS to the Fe 3d character in the occupied molecular orbitals, we also found that the donation interactions are dominated by σ bonding. The latter was found to be stronger in the ferric complex, with an Fe 3d contribution to the nominally 5σ CN(-) molecular orbitals of 29% compared to 20% in the ferrous complex. These results are consistent with the notion that a higher charge at the central metal atom increases donation and decreases back-donation.

    View details for DOI 10.1021/acs.jpcb.6b04751

    View details for Web of Science ID 000380730300016

    View details for PubMedID 27380541

  • Identification of the dominant photochemical pathways and mechanistic insights to the ultrafast ligand exchange of Fe(CO)5 to Fe(CO)4EtOH. Structural dynamics Kunnus, K., JOSEFSSON, I., RAJKOVIC, I., Schreck, S., Quevedo, W., Beye, M., Weniger, C., Grübel, S., Scholz, M., Nordlund, D., Zhang, W., Hartsock, R. W., Gaffney, K. J., Schlotter, W. F., Turner, J. J., Kennedy, B., Hennies, F., de Groot, F. M., Techert, S., Odelius, M., Wernet, P., Föhlisch, A. 2016; 3 (4): 043204-?

    Abstract

    We utilized femtosecond time-resolved resonant inelastic X-ray scattering and ab initio theory to study the transient electronic structure and the photoinduced molecular dynamics of a model metal carbonyl photocatalyst Fe(CO)5 in ethanol solution. We propose mechanistic explanation for the parallel ultrafast intra-molecular spin crossover and ligation of the Fe(CO)4 which are observed following a charge transfer photoexcitation of Fe(CO)5 as reported in our previous study [Wernet et al., Nature 520, 78 (2015)]. We find that branching of the reaction pathway likely happens in the (1)A1 state of Fe(CO)4. A sub-picosecond time constant of the spin crossover from (1)B2 to (3)B2 is rationalized by the proposed (1)B2 → (1)A1 → (3)B2 mechanism. Ultrafast ligation of the (1)B2 Fe(CO)4 state is significantly faster than the spin-forbidden and diffusion limited ligation process occurring from the (3)B2 Fe(CO)4 ground state that has been observed in the previous studies. We propose that the ultrafast ligation occurs via (1)B2 → (1)A1 → (1)A' Fe(CO)4EtOH pathway and the time scale of the (1)A1 Fe(CO)4 state ligation is governed by the solute-solvent collision frequency. Our study emphasizes the importance of understanding the interaction of molecular excited states with the surrounding environment to explain the relaxation pathways of photoexcited metal carbonyls in solution.

    View details for DOI 10.1063/1.4941602

    View details for PubMedID 26958587

    View details for PubMedCentralID PMC4752567

  • Identification of the dominant photochemical pathways and mechanistic insights to the ultrafast ligand exchange of Fe(CO)(5) to Fe(CO)(4)EtOH STRUCTURAL DYNAMICS Kunnus, K., JOSEFSSON, I., RAJKOVIC, I., Schreck, S., Quevedo, W., Beye, M., Weniger, C., Gruebel, S., Scholz, M., Nordlund, D., Zhang, W., Hartsock, R. W., Gaffney, K. J., Schlotter, W. F., Turner, J. J., Kennedy, B., Hennies, F., de Groot, F. M., Techert, S., Odelius, M., Wernet, P., Foehlisch, A. 2016; 3 (4)

    View details for DOI 10.1063/1.4941602

    View details for Web of Science ID 000383880700006

  • Free-electron laser based resonant inelastic X-ray scattering on molecules and liquids JOURNAL OF ELECTRON SPECTROSCOPY AND RELATED PHENOMENA Kunnus, K., Schreck, S., Foehlisch, A. 2015; 204: 345-355
  • From Ligand Fields to Molecular Orbitals: Probing the Local Valence Electronic Structure of Ni2+ in Aqueous Solution with Resonant Inelastic X-ray Scattering JOURNAL OF PHYSICAL CHEMISTRY B Kunnus, K., Josefsson, I., Schreck, S., Quevedo, W., Miedema, P. S., Techert, S., de Groot, F. M., Odelius, M., Wernet, P., Foehlisch, A. 2013; 117 (51): 16512-16521

    Abstract

    Bonding of the Ni(2+)(aq) complex is investigated with an unprecedented combination of resonant inelastic X-ray scattering (RIXS) measurements and ab initio calculations at the Ni L absorption edge. The spectra directly reflect the relative energies of the ligand-field and charge-transfer valence-excited states. They give element-specific access with atomic resolution to the ground-state electronic structure of the complex and allow quantification of ligand-field strength and 3d-3d electron correlation interactions in the Ni(2+)(aq) complex. The experimentally determined ligand-field strength is 10Dq = 1.1 eV. This and the Racah parameters characterizing 3d-3d Coulomb interactions B = 0.13 eV and C = 0.42 eV as readily derived from the measured energies match very well with the results from UV-vis spectroscopy. Our results demonstrate how L-edge RIXS can be used to complement existing spectroscopic tools for the investigation of bonding in 3d transition-metal coordination compounds in solution. The ab initio RASPT2 calculation is successfully used to simulate the L-edge RIXS spectra.

    View details for DOI 10.1021/jp4100813

    View details for Web of Science ID 000329331800016

    View details for PubMedID 24304205

  • L-Edge X-ray Absorption Spectroscopy of Dilute Systems Relevant to Metalloproteins Using an X-ray Free-Electron Laser JOURNAL OF PHYSICAL CHEMISTRY LETTERS Mitzner, R., Rehanek, J., Kern, J., Gul, S., Hattne, J., Taguchi, T., Alonso-Mori, R., Tran, R., Weniger, C., Schroeder, H., Quevedo, W., Laksmono, H., Sierra, R. G., Han, G., Lassalle-Kaiser, B., Koroidov, S., Kubicek, K., Schreck, S., Kunnus, K., Brzhezinskaya, M., Firsov, A., Minitti, M. P., Turner, J. J., Moeller, S., Sauter, N. K., Bogan, M. J., Nordlund, D., Schlotter, W. F., Messinger, J., Borovik, A., Techert, S., de Groot, F. M., Foehlisch, A., Erko, A., Bergmann, U., Yachandra, V. K., Wernet, P., Yano, J. 2013; 4 (21): 3641-3647

    Abstract

    L-edge spectroscopy of 3d transition metals provides important electronic structure information and has been used in many fields. However, the use of this method for studying dilute aqueous systems, such as metalloenzymes, has not been prevalent because of severe radiation damage and the lack of suitable detection systems. Here we present spectra from a dilute Mn aqueous solution using a high-transmission zone-plate spectrometer at the Linac Coherent Light Source (LCLS). The spectrometer has been optimized for discriminating the Mn L-edge signal from the overwhelming O K-edge background that arises from water and protein itself, and the ultrashort LCLS X-ray pulses can outrun X-ray induced damage. We show that the deviations of the partial-fluorescence yield-detected spectra from the true absorption can be well modeled using the state-dependence of the fluorescence yield, and discuss implications for the application of our concept to biological samples.

    View details for DOI 10.1021/jz401837f

    View details for Web of Science ID 000326845200010

    View details for PubMedCentralID PMC3901369

  • Ab Initio Calculations of X-ray Spectra: Atomic Multiplet and Molecular Orbital Effects in a Multiconfigurational SCF Approach to the L-Edge Spectra of Transition Metal Complexes JOURNAL OF PHYSICAL CHEMISTRY LETTERS Josefsson, I., Kunnus, K., Schreck, S., Foehlisch, A., de Groot, F., Wernet, P., Odelius, M. 2012; 3 (23): 3565-3570

    View details for DOI 10.1021/jz301479j

    View details for Web of Science ID 000312170600029

  • Dissecting Local Atomic and Intermolecular Interactions of Transition-Metal Ions in Solution with Selective X-ray Spectroscopy JOURNAL OF PHYSICAL CHEMISTRY LETTERS Wernet, P., Kunnus, K., Schreck, S., Quevedo, W., Kurian, R., Techert, S., de Groot, F. M., Odelius, M., Foehlisch, A. 2012; 3 (23): 3448-3453

    View details for DOI 10.1021/jz301486u

    View details for Web of Science ID 000312170600009

  • A setup for resonant inelastic soft x-ray scattering on liquids at free electron laser light sources REVIEW OF SCIENTIFIC INSTRUMENTS Kunnus, K., Rajkovic, I., Schreck, S., Quevedo, W., Eckert, S., Beye, M., Suljoti, E., Weniger, C., Kalus, C., Gruebel, S., Scholz, M., Nordlund, D., Zhang, W., Hartsock, R. W., Gaffney, K. J., Schlotter, W. F., Turner, J. J., Kennedy, B., Hennies, F., Techert, S., Wernet, P., Foehlisch, A. 2012; 83 (12)

    Abstract

    We present a flexible and compact experimental setup that combines an in vacuum liquid jet with an x-ray emission spectrometer to enable static and femtosecond time-resolved resonant inelastic soft x-ray scattering (RIXS) measurements from liquids at free electron laser (FEL) light sources. We demonstrate the feasibility of this type of experiments with the measurements performed at the Linac Coherent Light Source FEL facility. At the FEL we observed changes in the RIXS spectra at high peak fluences which currently sets a limit to maximum attainable count rate at FELs. The setup presented here opens up new possibilities to study the structure and dynamics in liquids.

    View details for DOI 10.1063/1.4772685

    View details for Web of Science ID 000312834300010

    View details for PubMedID 23277974

  • Intrinsic deviations in fluorescence yield detected x-ray absorption spectroscopy: the case of the transition metal L-2,L-3 edges JOURNAL OF PHYSICS-CONDENSED MATTER Kurian, R., Kunnus, K., Wernet, P., Butorin, S. M., Glatzel, P., de Groot, F. M. 2012; 24 (45)

    Abstract

    Fluorescence yield (FY) detected x-ray absorption spectra (XAS) of 3d transition metal ions are calculated from the integrated 2p3d resonant x-ray emission spectra. The resulting FY-XAS spectra are compared with the normal XAS spectra corresponding to the absorption cross section and significant deviations between the two spectra are found. This implies that the assumption that the FY-XAS spectrum identifies with the XAS spectrum is disproved. Especially for the early transition metal systems the differences between the FY-XAS and XAS are large, due to the opening of inelastic decay channels from selected x-ray absorption final states. The theoretical calculations show that the difference between FY detection and XAS is largest for the detection in depolarized geometry. The calculations are compared with experimental spectra for oxides and coordination compounds for Fe(2+), Co(2+) and Ni(2+) systems. The implications for the sum rules in XAS and magnetic circular dichroism experiments are discussed.

    View details for DOI 10.1088/0953-8984/24/45/452201

    View details for Web of Science ID 000310571800001

    View details for PubMedID 23060535