Re and Br X‑ray Absorption Near-Edge Structure Study of the Ground and Excited States of [ReBr(CO)₃(bpy)] Interpreted by DFT and TD-DFT Calculations

X-ray absorption spectra of fac-[ReBr­(CO)₃(bpy)] near the Re L₃- and Br K-edges were measured in a steady-state mode as well as time-resolved at 630 ps after 355 nm laser pulse excitation. Relativistic spin–orbit time-dependent density functional theory (TD-DFT) calculations account well for the shape of the near-edge absorption (the ″white line″) of the ground-state Re spectrum, assigning the lowest-lying transitions as core-to-ligand metal-to-ligand charge transfer from Re 2p_3/2 into predominantly π*­(bpy) molecular orbitals (MOs) containing small 5d contributions, followed in energy by transitions into π* Re­(CO)₃ and delocalized σ*/π* MOs. Transitions gain their intensities from Re 5d and 6s participation in the target orbitals. The 5d character is distributed over many unoccupied MOs; the 5d contribution to any single empty MO does not exceed 29%. The Br K-edge spectrum is dominated by the ionization edge and multiple scattering features, the pre-edge electronic transitions being very weak. Time-resolved spectra measured upon formation of the lowest electronic excited state show changes characteristic of simultaneous Re and Br electronic depopulation: shifts of the Re and Br edges and the Re white line to higher energies and emergence of new intense pre-edge features that are attributed by TD-DFT to transitions from Re 2p_3/2 and Br 1s orbitals into a vacancy in the HOMO-1 created by electronic excitation. Experimental spectra together with quantum chemical calculations provide a direct evidence for a ReBr­(CO)₃ → bpy delocalized charge transfer character of the lowest excited state. Steady-state as well as time-resolved Re L₃ spectra of [ReCl­(CO)₃(bpy)] and [Re­(Etpy)­(CO)₃(bpy)]⁺ are very similar to those of the Br complex, in agreement with similar (TD) DFT calculated transition energies as well as delocalized excited-state spin densities and charge changes upon excitation.