Structure and Spin Dynamics of a Metal Complex Studied by Synchrotron Radiation - Victoria KABANOVA -

QUICK INFORMATION
Type
Online PhD Defense
Start Date
18-12-2020 09:00
End Date
18-12-2020 11:30
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Coordinator contact(s)
Eva Jahn

Scientific contact(s)
Michael Wulff


09:00 -10:30  Seminar and questions

You can join the thesis defense online here
https://esrf.zoom.us/j/99130934000?pwd=QmZGSlE2ajZUYUs3cUJVZk8ybllqUT09

Meeting ID: 991 3093 4000
Passcode: 498540

11:00 -11:15   Announcement by jury president

You can join the announcement online here
https://esrf.zoom.us/j/97289810732?pwd=TlUzM1VvVE1BWkZRRTI3UGZvMWJwZz09

Meeting ID: 972 8981 0732
Passcode: 766596


Abstract:

The thesis describes an experimental study of the metal complex [FeII(phen)3]2+ in solution by time-resolved X-ray scattering and emission spectroscopy aimed at monitoring changes in structure and spin during its photocycle. In the photoexcited state, a 3d electron is transferred to the ligand for a fraction of a picosecond. From this so-called metal-to-ligand charge transfer state (MLCT), the electron returns to the metal in an excited high spin state (HS) that in turn decays to the low spin (LS) ground state in 725 ps. The structure and spin of the HS state were measured by X-ray scattering (WAXS) and X-ray emission spectroscopy (XES) respectively with 100 picosecond resolution using single X-ray pulses from the synchrotron. 

Chapter 1 describes the importance of visualising atoms in chemical reactions and transformations. The use of X-rays to gain structural sensitivity is now allowing to visualise photoinduced reactions with 100 picosecond resolution at synchrotrons and lately at 100 femtosecond resolution at XFELs.

In Chapter 2 “Probing Molecular Structure in Solution with X-rays”, the theory of X-ray scattering is presented stressing that when the structure is known, the molecular scattering pattern is readily calculated. Compton scattering dominates the scattering at high q and has to be included in the scaling. The intensity of the scattering from a 0.36 mm water sheet is calculated for a 1.0 × 109 photon pulse at 18 keV.

When a solute is dissolved in a solvent, the atomic positions are described by statistical atom-atom functions gab (r) that can be calculated by MD. The scattering function S(q) is then calculated from    gab (r) for [FeII(phen)3]2+in water using the TIP4P model with LS and HS structures from DFT.

X-ray scattering probes all atom-atom pairs in the solution sample including that of the solvent bulk. In the hydrodynamic scattering theory, the liquid is assumed to be in local thermal equilibrium. The theory for the cooling of hot points is presented and the calculation shows that a solution with 2 mM excited [FeII(phen)3]2+attains local thermal equilibrium 100 ps after the laser excitation.

The end of Chapter 2 gives a summary of X-ray emission spectroscopy (XES). The Ka, Kb and valence-to-core (VtC) emissionlines are presented including their intensity, spin and ligand sensitivity. Kb is the most sensitive probe of the spin state of Fe.

In Chapter 3 the ESRF and the ID09 beamline for pump-probe experiments are shortly described. The details of the Johann (JS) and Von Hamos (VH) spectrometers for XES are described with emphasis on the VH since it was used for the first time at the ESRF. The count rate from Kb is extremely low, typically 0.01 ph/pulse/analyser and the sample has to be exposed for about 1 hour per time delay to get a Kb spectrum with a good S/N ratio.

The WAXS and XES experiments are described in Chapter 4. After photoexcitation to the MLCT state, the electron returns to a metal centred HS state in < 100 fs for then to return to the GS in 725 ps. dS(q,t) are 100 ps snapshots of the average structural change for all pairs of atoms at time t. On short time scales t < 10 ns, the solvent is heated adiabatically at constant volume. The thermal response of water was measured in a dye/water mixture. The solvent corrected WAXS data show that the Fe-N distance increases by 0.19 Å in the HS state and that the HS population returns to the LS in 725 ps. The change in the water cage radius is inferred from the low-q data. It is found to contract by 0.3 Å in the HS state in spite of the 0.19 Å expansion of the Fe-N distance.

The XES line shapes of the Kb lines were measured with the VH spectrometer and compared with Crispy simulations. The simulations confirm that the 725 ps state is the HS  S=2 quintet. Very weak VtC emission, 100 times weaker than Kb, was also observed.