Ligand manipulation of charge transfer excited state relaxation and spin crossover in [Fe(2,2′-bipyridine)(2)(CN)(2)]

We have used femtosecond resolution UV-visible and Kβ x-ray emission spectroscopy to characterize the electronic excited state dynamics of [Fe(bpy)(2)(CN)(2)], where bpy=2,2′-bipyridine, initiated by metal-to-ligand charge transfer (MLCT) excitation. The excited-state absorption in the transient UV-visible spectra, associated with the 2,2′-bipyridine radical anion, provides a robust marker for the MLCT excited state, while the transient Kβ x-ray emission spectra provide a clear measure of intermediate and high spin metal-centered excited states. From these measurements, we conclude that the MLCT state of [Fe(bpy)(2)(CN)(2)] undergoes ultrafast spin crossover to a metal-centered quintet excited state through a short lived metal-centered triplet transient species. These measurements of [Fe(bpy)(2)(CN)(2)] complement prior measurement performed on [Fe(bpy)(3)](2+) and [Fe(bpy)(CN)(4)](2−) in dimethylsulfoxide solution and help complete the chemical series [Fe(bpy)(N)(CN)(6–2N)](2N-4), where N = 1–3. The measurements confirm that simple ligand modifications can significantly change the relaxation pathways and excited state lifetimes and support the further investigation of light harvesting and photocatalytic applications of 3d transition metal complexes.