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

Ultrafast core-loss spectroscopy in four-dimensional electron microscopy

We demonstrate ultrafast core-electron energy-loss spectroscopy in four-dimensional electron microscopy as an element-specific probe of nanoscale dynamics. We apply it to the study of photoexcited graphite with femtosecond and nanosecond resolutions. The transient core-loss spectra, in combination with ab initio molecular dynamics simulations, reveal the elongation of the carbon-carbon bonds, even though the overall behavior is a contraction of the crystal lattice. A prompt energy-gap shrinkage is observed on the picosecond time scale, which is caused by local bond length elongation and the direct renormalization of band energies due to temperature-dependent electron–phonon interactions

Unusual molecular material formed through irreversible transformation and revealed by 4D electron microscopy

Four-dimensional (4D) electron microscopy (EM) uniquely combines the high spatial resolution to pinpoint individual nano-objects, with the high temporal resolution necessary to address the dynamics of their laser-induced transformation. Here, using 4D-EM, we demonstrate the in situ irreversible transformation of individual nanoparticles of the molecular framework Fe(pyrazine)Pt(CN)4. The newly formed material exhibits an unusually large negative thermal expansion (i.e. contraction), which is revealed by time-resolved imaging and diffraction. Negative thermal expansion is a unique property exhibited by only few materials. Here we show that the increased flexibility of the metal–cyanide framework after the removal of the bridging pyrazine ligands is responsible for the negative thermal expansion behavior of the new material. This in situ visualization of single nanostructures during reactions should be extendable to other classes of reactive systems

Modeling nonequilibrium dynamics of phase transitions at the nanoscale: Application to spin-crossover

In this article, we present a continuum mechanics based approach for modeling thermally induced single-nanoparticle phase transitions studied in ultrafast electron microscopy. By using coupled differential equations describing heat transfer and the kinetics of the phase transition, we determine the major factors governing the time scales and efficiencies of thermal switching in individual spin-crossover nanoparticles, such as the thermal properties of the (graphite) substrate, the particle thickness, and the interfacial thermal contact conductance between the substrate and the nanoparticle. By comparing the simulated dynamics with the experimental single-particle diffraction time profiles, we demonstrate that the proposed non-equilibrium phase transition model can fully account for the observed switching dynamics

Vibrational Relaxation and Intersystem Crossing of Binuclear Metal Complexes in Solution

The ultrafast vibrational-electronic relaxation upon excitation into the singlet (1)A(2u) (d sigma*-> p sigma) excited state of the d(8)-d(8) binuclear complex [Pt-2(P2O5H2)(4)](4-) has been investigated in different solvents by femtosecond polychromatic fluorescence up-conversion and femtosecond broadband transient absorption (TA) spectroscopy. Both sets of data exhibit clear signatures of vibrational relaxation and wave packet oscillations of the Pt-Pt stretch vibration in the (1)A(2u) state with a period of 224 fs, that decay on a 1-2 ps time scale, and of intersystem crossing (ISC) into the (3)A(2u), state. The vibrational relaxation and ISC times exhibit a pronounced solvent dependence. We also extract from the TA measurements the spectral distribution of the wave packet at a given delay time, 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 a 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. ISC from the (1)A(2u) to the (3)A(2u) state is induced by spin-vibronic coupling with a higher-lying triplet state and/or (transient) symmetry breaking in the (1)A(2u) excited state. The particular structure, energetics, and symmetry of the molecule play a decisive role in determining the relatively slow rate of ISC, despite the large spin-orbit coupling strength of the Pt atoms

EXAFS Structural Determination of the Pt2(P2O5H2)44– Anion in Solution

We present the first structural determination of the Pt2(P2O5H2)44– anion in solution by analyzing the extended X-ray absorption fine structure (EXAFS) spectrum of the Pt LIII edge. The data could be fit with a simple model involving single and multiple scattering paths to near and far P-atoms, bridging O-atoms, and the other Pt-atom in the binuclear complex. A Pt–Pt distance of 2.876(28) Å and a Pt–P bond length of 2.32(4) Å are obtained. These values are in line with distances found in previous X-ray diffraction studies. The assignment of the EXAFS spectrum of the Pt2(P2O5H2)44– anion in its ground state is required for future time-resolved X-ray absorption measurements with the goal of determining the structure and dynamics of the complex in the 1,3A2u excited states