Chemical interactions and dynamics with femtosecond X-ray spectroscopy and the role of X-ray free-electron lasers

Abstract

X-ray free-electron lasers with intense, tuneable and short-pulse X-ray radiation are transformative tools for the investigation of transition-metal complexes and metalloproteins. This becomes apparent in particular when combining the experimental observables from X-ray spectroscopy with modern theoretical tools for calculations of electronic structures and X-ray spectra from first principles. The combination gives new insights into how charge and spin densities change in chemical reactions and how they determine reactivity. This is demonstrated for the investigations of structural dynamics with metal K-edge absorption spectroscopy, spin states in excited-state dynamics with metal 3p-3d exchange interactions, the frontier-orbital interactions in dissociation and substitution reactions with metal-specific X-ray spectroscopy, and studies of metal oxidation states with femtosecond pulses for ‘probe-before-destroy’ spectroscopy. The role of X-ray free-electron lasers is addressed with thoughts about how they enable ‘bringing back together’ different aspects of the same problem and this is thought to go beyond a conventional review paper where these aspects are formulated in italic font type in a prequel, an interlude and in a sequel. This article is part of the theme issue ‘Measurement of ultrafast electronic and structural dynamics with X-rays'.

Hot Branching Dynamics in a Light‐Harvesting Iron Carbene Complex Revealed by Ultrafast X‐ray Emission Spectroscopy

“…[8] Forinstance,inthe extensively studied prototype Fe II molecule Fe II (bpy) 3 (bpy = 2,2'-bipyridine), the charge-separated MLCT state deactivates on an ultrafast timescale into high-spin MC states. [9,10] Similar ultrafast transitions into 3 MC states have been observed and described for ruthenium sensitizer complexes. [11][12][13] By destabilizing the MC scavenger states we,a nd other groups,h ave recently developed iron complexes with significantly slower deactivation of the MLCT states.…”

Hot Branching Dynamics in a Light‐Harvesting Iron Carbene Complex Revealed by Ultrafast X‐ray Emission Spectroscopy

“…[8] Forinstance,inthe extensively studied prototype Fe II molecule Fe II (bpy) 3 (bpy = 2,2'-bipyridine), the charge-separated MLCT state deactivates on an ultrafast timescale into high-spin MC states. [9,10] Similar ultrafast transitions into 3 MC states have been observed and described for ruthenium sensitizer complexes. [11][12][13] By destabilizing the MC scavenger states we,a nd other groups,h ave recently developed iron complexes with significantly slower deactivation of the MLCT states.…”

“…Traditional low‐spin Fe II polypyridyl complexes generally show ultrafast <100 fs decay of metal‐to‐ligand charge transfer states (MLCT) into metal centered (MC) states . For instance, in the extensively studied prototype Fe II molecule Fe II (bpy) 3 (bpy=2,2′‐bipyridine), the charge‐separated MLCT state deactivates on an ultrafast timescale into high‐spin MC states . Similar ultrafast transitions into 3 MC states have been observed and described for ruthenium sensitizer complexes .…”

“…As an element‐specific technique, X‐ray spectroscopy is a powerful tool to probe the electronic structure of the metal site and surrounding ligands. With the recent advent of X‐ray free electron laser (XFEL) facilities with high flux and a temporal resolution better than 30 fs, it has become possible to identify the nature of the excited states directly . In these studies, the oxidation state, spin, and charge‐transfer character of the molecular state were identified by modeling the measured time‐resolved spectra with a linear combination of measured or calculated static reference spectra.…”

“…[23,24] As an element-specific technique,X -ray spectroscopy is apowerful tool to probe the electronic structure of the metal site and surrounding ligands.With the recent advent of X-ray free electron laser (XFEL) facilities with high flux and at emporal resolution better than 30 fs, [25] it has become possible to identify the nature of the excited states directly. [10,[26][27][28][29][30][31][32] In these studies,t he oxidation state,s pin, and charge-transfer character of the molecular state were identified by modeling the measured time-resolved spectra with al inear combination of measured or calculated static reference spectra. Here,w eu se ultrafast K a and K b X-ray emission spectroscopy (XES) to investigate the excited-state dynamics of one of the recently introduced Fe II carbene complexes,F e II (bpy)(btz) 2 (PF 6 ) 2 [btz = 4,4'-bis(1,2,3-triazol-5-ylidene)] with significantly altered photophysics compared to prototype polypyridyl Fe II complexes such as Fe II (bpy) 3 (Figure 1).…”