| 1. |
- Biasin, Elisa, et al.
(author)
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Anisotropy enhanced X-ray scattering from solvated transition metal complexes
- 2018
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In: Journal of Synchrotron Radiation. - 0909-0495. ; 25:2, s. 306-315
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Journal article (peer-reviewed)abstract
- Time-resolved X-ray scattering patterns from photoexcited molecules in solution are in many cases anisotropic at the ultrafast time scales accessible at X-ray free-electron lasers (XFELs). This anisotropy arises from the interaction of a linearly polarized UV-Vis pump laser pulse with the sample, which induces anisotropic structural changes that can be captured by femtosecond X-ray pulses. In this work, a method for quantitative analysis of the anisotropic scattering signal arising from an ensemble of molecules is described, and it is demonstrated how its use can enhance the structural sensitivity of the time-resolved X-ray scattering experiment. This method is applied on time-resolved X-ray scattering patterns measured upon photoexcitation of a solvated di-platinum complex at an XFEL, and the key parameters involved are explored. It is shown that a combined analysis of the anisotropic and isotropic difference scattering signals in this experiment allows a more precise determination of the main photoinduced structural change in the solute, i.e. the change in Pt - Pt bond length, and yields more information on the excitation channels than the analysis of the isotropic scattering only. Finally, it is discussed how the anisotropic transient response of the solvent can enable the determination of key experimental parameters such as the instrument response function.The analysis of time-resolved X-ray scattering patterns collected at an XFEL upon photoexcitation of a di-platinum complex in solution is described. The analysis quantitatively considers the anisotropy of the signal.
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| 2. |
- Kjær, Kasper S., et al.
(author)
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Solvent control of charge transfer excited state relaxation pathways in [Fe(2,2′-bipyridine)(CN)4]2-
- 2018
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In: Physical Chemistry Chemical Physics. - : Royal Society of Chemistry (RSC). - 1463-9076 .- 1463-9084. ; 20:6, s. 4238-4249
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Journal article (peer-reviewed)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.
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| 3. |
- Koroidov, Sergey, et al.
(author)
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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
- 2018
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In: Inorganic Chemistry. - : AMER CHEMICAL SOC. - 0020-1669 .- 1520-510X. ; 57:21, s. 13167-13175
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Journal article (peer-reviewed)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 H-2 generation.
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