Using Ultrafast X-ray Spectroscopy To Address Questions in Ligand-Field Theory: The Excited State Spin and Structure of [Fe(dcpp)2]2+

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 T-5(2)/T-3(1) crossing point and stabilizing the latter as the lowest energy excited state. K alpha and K beta X-ray emission spectroscopies have been used to unambiguously determine the quintet spin multiplicity of the long-lived excited state, thereby establishing the T-5(2) 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 similar to 0.18 angstrom for a T-5(2) state and suggest a high anisotropy of the primary coordination sphere around the metal center in the excited T-5(2) state, in stark contrast to the nearly perfect octahedral symmetry that characterizes the low-spin (1)A(1) 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.