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Abstract

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 $^{5}T_{2}/^{3}T_{1}$ 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 $^{5}T_{2}$ 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 $∼0.18 Å$ for a $^{5}T_{2}$ state and suggest a high anisotropy of the primary coordination sphere around the metal center in the excited $^{5}T_{2}$ 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.

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