X‑ray Absorption Spectroscopy of Ground and Excited Rhenium–Carbonyl–Diimine Complexes: Evidence for a Two-Center Electron Transfer

Steady-state and picosecond time-resolved X-ray absorption spectroscopy is used to study the ground and lowest triplet states of [ReX(CO)3(bpy)] n+, X = Etpy (n = 1), Cl, or Br (n = 0). We demonstrate that the transient spectra at both the Re L3- and Br K-edges show the emergence of a pre-edge feature, absent in the ground-state spectrum, which is associated with the electron hole created in the highest occupied molecular orbital following photoexcitation. Importantly, these features have the same dynamics, confirming previous predictions that the low-lying excited states of these complexes involve a two-center charge transfer from both the Re and the ligand, X. We also demonstrate that the DFT optimized ground and excited structures allow us to reproduce the experimental XANES and EXAFS spectra. The ground-state structural refinement shows that the Br atom contributes very little to the latter, whereas the Re–C–O scattering paths are dominant due to the so-called focusing effect. For the excited-state spectrum, the Re–X bond undergoes one of the largest changes but still remains a weak contribution to the photoinduced changes of the EXAFS spectrum.