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 3 d transition metal complexes.