Structural Dynamics of Photochemical Reactions, Studied with Ultrafast X-ray Methods

The overarching scientific theme of the thesis regards energy transfer processes within the field of chemistry and biology with technical applications such as e.g. solar cells and photo catalysts. By developing existing methods and executing X-ray experiments aimed at investigating structural and electronic dynamics in photoactive molecules, the thesis aims to provide detailed understanding of such processes by imaging how atoms move as energy flows through a complex chemical system.

First, a systematic review will be given of the data analysis process of femtosecond Time-Resolved Wide Angle X-ray Scattering (TR-WAXS) solution data from modern X-ray Free Electron Laser (XFEL) facilities. In combination with structural modelling of complex dynamic photochemical systems, the experimental results allow for the creation of "MolecularMovies" and direct probing of the real-time atomic movements in such processes.

The first study presented in the thesis, focuses on developing a method to directly probe the sub-picosecond ground and excited state structural dynamics on the potential energy surfaces of photoexcited PtPOP/AgPtPOP molecules. These are model systems for a group of transition metal complexes of relevance to catalysis and light-harvesting purposes. A successful comparison with high-level quantum simulations allowed for an experimentally supported visualization of the structural dynamics.

The second study investigates the sub-picosecond structural changes in the solvation shell following an aqueous I−→I⁰ photoreaction. It represents a simple system for studying solvent reorganization upon a solute charge change as a model system for a variety of chemical and biological systems. Simultaneous measurements of the Xray absorption enabled direct coupling of the structural changes to changes in the electronic configuration to support the theory of increased electron back donation to I⁰ as a result of the structural changes in the solvent shell.
The final part of the thesis, introduces X-ray Cross Correlation Analysis (XCCA) as an additional and powerful tool to analyze the scattering signals from XFEL experiments. This method reveals additional information about the sample, as it takes advantage of the angular intensity correlations in the full 2-dimensional detector space, instead of reducing the data by azimuthal averaging. This approach revealed additional structural information about the PtPOP sample and a hidden timescale (∼ 50 ps) related to the rotational dephasing of the molecules.