Structural characterization of excited state transition metal complexes by x-ray transient absorption spectroscopies

Excited state transition metal complexes (TMCs) undergo various steps of light-matter interactions to become highly energetic species capable of driving many photochemical reactions with implications on different applications. Characterizing transient electronic and nuclear structures and trajectories of TMC excited states are crucial for understanding and control of photochemical properties, such as excited state lifetime, reaction branching ratio, energy or electron transfer and catalytic kinetics. Over the past decades, methods in direct determination of excited state TMC structures have been developed using pulsed X-ray sources in X-ray spectroscopy, scattering and diffraction at synchrotrons, X-ray free electron lasers and table-top sources. The chapter focuses on X-ray absorption spectroscopy (XAS), a particularly suitable method for characterization of local electronic and nuclear structures of the metal centers as well as their interactions with the ligands with examples on those photoactive TMCs. The basic and fundamental aspects of X-ray transient absorption (XTA) spectroscopy will be described with the laser “pump,” X-ray “probe” scheme and consideration of sample environments. Soft X-ray spectroscopy will be described briefly at the end. Other variations of X-ray spectroscopies, such as X-ray emission spectroscopy (XES) and resonance inelastic X-ray scattering (RIXS), will not be covered in this chapter. Due many relevant existing literature over the past two decades, the examples given in this chapter are most familiar and understood by the author.