| 1. |
- Jay, Raphael, et al.
(author)
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Capturing Atom-Specific Electronic Structural Dynamics of Transition-Metal Complexes with Ultrafast Soft X-Ray Spectroscopy
- 2022
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In: Annual review of physical chemistry (Print). - : Punctum Books. - 0066-426X .- 1545-1593. ; 73, s. 187-208
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Research review (peer-reviewed)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.
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| 2. |
- Kunnus, Kristjan, et al.
(author)
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Chemical control of competing electron transfer pathways in iron tetracyano-polypyridyl photosensitizers
- 2020
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In: Chemical Science. - : ROYAL SOC CHEMISTRY. - 2041-6520 .- 2041-6539. ; 11:17, s. 4360-4373
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Journal article (peer-reviewed)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 (MLCT)-M-1 vertical energy to be varied from 1.64 eV to 2.64 eV and the (MLCT)-M-3 lifetime to range from 180 fs to 67 ps. The (MLCT)-M-3 lifetimes in 1 and 2 decrease exponentially as the MLCT energy increases, consistent with electron transfer to the lowest energy triplet metal-centred ((MC)-M-3) excited state, as established by the Tanabe-Sugano analysis of the Fe 2p3d RIXS data. In contrast, the (MLCT)-M-3 lifetime in 3 changes non-monotonically with MLCT energy, exhibiting a maximum. This qualitatively distinct behaviour results from a competing (MLCT)-M-3 -> ground state (GS) electron transfer pathway that exhibits energy gap law behaviour. The (MLCT)-M-3 -> GS pathway involves nuclear tunnelling for the high-frequency polypyridyl breathing mode (h nu = 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 (MLCT)-M-3 and the (MC)-M-3 states and suppression of the intramolecular distortion of the acceptor ligand in the (MLCT)-M-3 excited state.
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| 3. |
- Kunnus, Kristjan, et al.
(author)
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Quantifying the Steric Effect on Metal-Ligand Bonding in Fe Carbene Photosensitizers with Fe 2p3d Resonant Inelastic X-ray Scattering
- 2022
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In: Inorganic Chemistry. - : American Chemical Society (ACS). - 0020-1669 .- 1520-510X. ; 61:4, s. 1961-1972
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Journal article (peer-reviewed)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 σ/πdonation and πback-donation. We find that σ donation is significantly stronger in [Fe(bmip)2]2+, whereas the πback-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.
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| 4. |
- Kunnus, Kristjan, et al.
(author)
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Vibrational wavepacket dynamics in Fe carbene photosensitizer determined with femtosecond X-ray emission and scattering
- 2020
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In: Nature Communications. - : Springer Nature. - 2041-1723. ; 11:1
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Journal article (peer-reviewed)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 Kα and Kβ 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 278 fs 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 110 fs time constant by 40% of the excited molecules while the rest relax to a 3MLCT excited state. The sensitivity of the Kα XES to molecular structure results from a 0.7% average Fe-ligand bond length shift between the 1 s and 2p core-ionized states surfaces.
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| 5. |
- Reinhard, Marco, et al.
(author)
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Ferricyanide photo-aquation pathway revealed by combined femtosecond Kβ main line and valence-to-core x-ray emission spectroscopy
- 2023
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In: Nature Communications. - 2041-1723. ; 14:1
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Journal article (peer-reviewed)abstract
- Reliably identifying short-lived chemical reaction intermediates is crucial to elucidate reaction mechanisms but becomes particularly challenging when multiple transient species occur simultaneously. Here, we report a femtosecond x-ray emission spectroscopy and scattering study of the aqueous ferricyanide photochemistry, utilizing the combined Fe Kβ main and valence-to-core emission lines. Following UV-excitation, we observe a ligand-to-metal charge transfer excited state that decays within 0.5 ps. On this timescale, we also detect a hitherto unobserved short-lived species that we assign to a ferric penta-coordinate intermediate of the photo-aquation reaction. We provide evidence that bond photolysis occurs from reactive metal-centered excited states that are populated through relaxation of the charge transfer excited state. Beyond illuminating the elusive ferricyanide photochemistry, these results show how current limitations of Kβ main line analysis in assigning ultrafast reaction intermediates can be circumvented by simultaneously using the valence-to-core spectral range.
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| 6. |
- Sopena Moros, Arturo, et al.
(author)
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Tracking Cavity Formation in Electron Solvation : Insights from X-ray Spectroscopy and Theory
- 2024
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In: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 0002-7863 .- 1520-5126. ; 146:5, s. 3262-3269
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Journal article (peer-reviewed)abstract
- We present time-resolved X-ray absorption spectra of ionized liquid water and demonstrate that OH radicals, H3O+ ions, and solvated electrons all leave distinct X-ray-spectroscopic signatures. Particularly, this allows us to characterize the electron solvation process through a tool that focuses on the electronic response of oxygen atoms in the immediate vicinity of a solvated electron. Our experimental results, supported by ab initio calculations, confirm the formation of a cavity in which the solvated electron is trapped. We show that the solvation dynamics are governed by the magnitude of the random structural fluctuations present in water. As a consequence, the solvation time is highly sensitive to temperature and to the specific way the electron is injected into water.
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| 7. |
- Tatsuno, Hideyuki, et al.
(author)
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Hot Branching Dynamics in a Light-Harvesting Iron Carbene Complex Revealed by Ultrafast X-ray Emission Spectroscopy
- 2020
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In: Angewandte Chemie - International Edition. - : Wiley. - 1433-7851 .- 1521-3773. ; 59:1, s. 364-372
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Journal article (peer-reviewed)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 FeIINHC 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 3MLCT state, from the initially excited 1MLCT state, 30 % of the molecules undergo ultrafast (150 fs) relaxation to the 3MC state, in competition with vibrational relaxation and cooling to the relaxed 3MLCT state. The relaxed 3MLCT state then decays much more slowly (7.6 ps) to the 3MC state. The 3MC state is rapidly (2.2 ps) deactivated to the ground state. The 5MC state is not involved in the deactivation pathway. The ultrafast partial deactivation of the 3MLCT 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.
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| 8. |
- Vacher, Morgane, et al.
(author)
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Origin of core-to-core x-ray emission spectroscopy sensitivity to structural dynamics
- 2020
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In: Structural Dynamics. - : AIP Publishing. - 2329-7778. ; 7:4
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Journal article (peer-reviewed)abstract
- Recently, coherent structural dynamics in the excited state of an iron photosensitizer was observed through oscillations in the intensity of K alpha 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.
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