Time-resolved X-ray absorption spectroscopy for the study of molecular systems relevant for artificial photosynthesis

• Recent advances of time-resolved X-ray absorption spectroscopy are reviewed.
• Excited state structure of photosensitizer is determined from XAS spectra.
• Charge transfer in supramolecular systems is probed using X-ray spectroscopy.
• Oxidation state and structure of intermediates of hydrogen-evolving catalyst is determined.

Transition metal coordination compounds have a rich photochemistry and are interesting candidates as both light harvesters (photosensitizers) and catalysts in photocatalytic systems. Knowledge of electronic and molecular structure of excited states of photosensitizers and intermediates of catalysts is a key topic for rational design of systems for artificial photosynthesis. We describe recent advances in the field of time-resolved X-ray absorption spectroscopy that provide information on local structure around metal centers, their orbital structure and oxidation state, and thereby insights into the mechanisms of their photochemical reactions. Photosensitizers with metal centers, multicomponent molecular catalytic systems, and supramolecular model sensitizer-catalysts with two metal centers are used as examples to demonstrate the possibilities of the technique. We overview different experimental methods that can be used to investigate intermediates with lifetimes in the range from hundreds of picoseconds to hundreds of microseconds. Theoretical methods to extract the structural and electronic information from X-ray absorption near edge structure spectroscopy (XANES) are also discussed.