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.
Programme doctoral Photonique
Faculté des sciences de base
Institut des sciences et ingénierie chimiques
Laboratoire de spectroscopie ultrarapide
Record created on 2010-10-21, modified on 2016-08-08