Using Ultrafast X-ray Spectroscopy To Address Questions in Ligand-Field Theory: The Excited State Spin and Structure...

We have employed a range of ultrafast X-ray spectroscopies in an effort to characterize the lowest energy excited state of [Fe(dcpp)₂]²⁺ (where dcpp is 2,6-(dicarboxypyridyl)pyridine). This compound exhibits an unusually short excited-state lifetime for a low-spin Fe(II) polypyridyl complex of 270 ps in a room-temperature fluid solution, raising questions as to whether the ligand-field strength of dcpp had pushed this system beyond the ⁵T₂/³T₁ crossing point and stabilizing the latter as the lowest energy excited state. Kα and Kβ X-ray emission spectroscopies have been used to unambiguously determine the quintet spin multiplicity of the long-lived excited state, thereby establishing the ⁵T₂ state as the lowest energy excited state of this compound. Geometric changes associated with the photoinduced ligand-field state conversion have also been monitored with extended X-ray absorption fine structure. The data show the typical average Fe-ligand bond length elongation of ∼0.18 Å for a ⁵T₂ state and suggest a high anisotropy of the primary coordination sphere around the metal center in the excited ⁵T₂ state, in stark contrast to the nearly perfect octahedral symmetry that characterizes the low-spin ¹A₁ ground state structure. This study illustrates how the application of time-resolved X-ray techniques can provide insights into the electronic structures of molecules-in particular, transition metal complexes-that are difficult if not impossible to obtain by other means.