Using Ultrafast X‑ray Spectroscopy To Address
Questions in Ligand-Field Theory: The Excited State Spin and Structure
of [Fe(dcpp)2]2+
Posted on 2019-06-26 - 14:47
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 moleculesin particular, transition metal complexesthat
are difficult if not impossible to obtain by other means.
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Britz, Alexander; Gawelda, Wojciech; Assefa, Tadesse A.; Jamula, Lindsey L.; Yarranton, Jonathan T.; Galler, Andreas; et al. (2019). Using Ultrafast X‑ray Spectroscopy To Address
Questions in Ligand-Field Theory: The Excited State Spin and Structure
of [Fe(dcpp)2]2+. ACS Publications. Collection. https://doi.org/10.1021/acs.inorgchem.9b01063