Ultrafast X-ray Spectroscopies of Transition Metal Complexes Relevant for Catalysis

Transition metal (TM) complexes are ubiquitous in both technological and biological catalytic systems. For a detailed understanding of their reactivity, a knowledge of the fundamental processes during chemical reactions is crucial. These primary processes involve correlated changes in spin state, molecular orbitals as well as the geometric structure of the reactant and occur on femtosecond (fs) time- and Angström (A) length scales. Using optical laser pump - X-ray spectroscopic probe techniques the aforementioned dynamics of solvated TM complexes can be tracked with 100-picosecond(ps)-resolution at synchrotrons and 100-fs-resolution at X-ray Free Electron Lasers (XFELs). Time-resolved (TR) hard X-ray absorption (XAS) and emission (XES) spectroscopy is exploited to site-selectively probe the optically induced changes in structure and molecular orbitals of TM complexes with high relevance for catalysis. A novel setup for TR XAS at PETRA III has been implemented, consisting of a repetitionrate- tunable synchronized MHz fiber amplifier laser and a data acquisition (DAQ) strategy which is capable of measuring multi-photon events with single photon resolution at MHz repetition rates. This system permits measuring TR XAS with signal-to-noise ratios comparable to those of static measurements. We have probed the optically induced changes in the unoccupied orbitals of an iridium photosensitizer (IrPS) by measuring its TR X-ray absorption near edge structure (XANES). Monitoring the IrPS in a fully functioning system for photocatalytic hydrogen generation allowed following the electron transfer from an electron donor to the optically excited IrPS*. Furthermore, we have characterized the excited state spin and structure of a possible candidate towards low-spin Fe(II) photosensitizers via TR XES and the TR extended X-ray absorption fine structure (EXAFS). Finally we have investigated an octahedral high-valent Fe(V) complex, which is formed via UV flash photolysis from its Fe(III) precursor. The ultrafast high-valent Fe(V) formation was monitored using TR XANES at the SACLA XFEL with ~300 fs resolution and the formation was found to occur on the same timescale or faster than the temporal resolution. TR XES was employed to identify a ~40-ps-lived pentacoordinated intermediate state in a competing ligand exchange reaction channel