Ultrafast X-ray and Optical Spectroscopy of Binuclear Molecular Complexes

In this thesis we followed the synergetic approach of combining ultrafast optical and X-ray spectroscopies to unravel the electronic and geometric structural dynamics of the solvated binuclear transition metal complex [Pt2(P2O5H2)4] 4- (PtPOP). This molecule belongs to a broader class of d8 – d8 compounds that are known for their interesting photophysical properties and rich photochemical and photocatalytic reactivity. Broadband femtosecond fluorescence up-conversion and transient absorption spectroscopy have revealed the ultrafast vibrational-electronic relaxation pathways following excitation into the 1A2u (σ*dz2 → σpz) excited state for different solvents and excitation wavelengths. Both sets of data exhibit clear signatures of vibrational cooling (∼2 ps) and wave packet oscillations of the Pt-Pt stretch vibration in the 1A2u state with a period of 224 fs, that decay on a 1-2 ps time scale, and of intersystem crossing into the 3A2u state within 10-30 ps. The vibrational relaxation and intersystem crossing times exhibit a clear solvent dependence. We also extract from the transient absorption measurements the spectral distribution of the wave packet at given time delays, which reflects the distribution of Pt-Pt bond distances as a function of time, i.e. the structural dynamics of the system. We clearly establish the vibrational relaxation and coherence decay processes and we demonstrate that PtPOP represents a clear example of an harmonic oscillator that does not comply with the optical Bloch description due to very efficient coherence transfer between vibronic levels. We conclude that a direct Pt-solvent energy dissipation channel accounts for the vibrational cooling in the singlet state. Intersystem crossing from the 1A2u to the 3A2u state is induced by spin-vibronic coupling with a higher-lying triplet state and/or (transient) symmetry breaking in the 1A2u excited state. The particular structure, energetics and symmetry of the molecule play a decisive role in determining the relatively slow rate of intersystem crossing, despite the large spin-orbit coupling strength of the Pt atoms. Ultrafast X-ray absorption spectroscopy (XAS) is a powerful tool to observe electronic and geometric structures of short-lived reaction intermediates. We have measured the photoinduced changes in the Pt LIII X-ray absorption spectrum of PtPOP with picosecondix nanosecond resolution. A rigorous analysis of the time-resolved EXAFS results allowed us to establish the structure of the lowest triplet 3A2u excited state. We found that the Pt atoms contract by as much as 0.31(5) Å due to the formation of a new intermetallic bond. In addition, a significant, though minute, elongation of 0.010(6) Å of the coordination bonds could be derived from the transient X-ray absorption spectrum for the first time. Using state-of-the-art theoretical XAS codes, we were also able to assign photoinduced changes in the XANES spectrum, to changes in Pt d-electron density, ligand field splitting and orbital hybridization in the excited state. Spectral changes in the XANES multiplescattering features were used to derive a semi-quantitative value for the Pt-Pt contraction of ∼0.3 Å, which is in excellent agreement with the time-resolved EXAFS results. Application of ultrafast XAS and the data analysis methods to other chemical and biological systems in liquids offers an exciting perspective; in particular, in view of the recent development of intense free electron laser sources delivering ∼100 fs X-ray pulses, opening new venues in X-ray science that scientists could hitherto only dream of.

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