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

Alexander Britz*, Wojciech Gawelda, Tadesse A. Assefa, Lindsey L. Jamula, Jonathan T. Yarranton, Andreas Galler, Dmitry Khakhulin, Michael Diez, Manuel Harder, Gilles Doumy, Anne Marie March, Éva Bajnóczi, Zoltán Németh, Mátyás Imre Pápai, Emese Rozsályi, Dorottya Sárosiné Szemes, Hana Cho, Sriparna Mukherjee, Chang Liu, Tae Kyu KimRobert W. Schoenlein, Stephen H. Southworth, Linda Young, Elena Jakubikova, Nils Huse, György Vankó, Christian Bressler, James K. McCusker

*Corresponding author for this work

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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 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 5Tstate, 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.
Original languageEnglish
JournalInorganic Chemistry
Volume58
Issue number14
Pages (from-to)9341-9350
ISSN0020-1669
DOIs
Publication statusPublished - 2019

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