The production of renewable solar fuels on a global scale requires the development of water oxidation catalysts from earth-abundant elements. The tetrametallic oxo-cluster, [CoIII4O4](OAc)4(py)4 (OAc = acetate, py = pyridine), is a promising molecular compound that has attracted significant interest due to its provocative “cubane” structure which bears a strong resemblance to the active core of the oxygen-evolving complex of photosystem II in plants. In this talk is discussed the ultrafast spectroscopic results of Co4O4 ligand-field excitation with green visible light. Femtosecond M2,3-edge X-ray absorption near-edge structure (XANES) spectroscopy is used as a metal-sensitive probe in the extreme ultraviolet (XUV) region of the electromagnetic spectrum near 50-90 eV. Transient spectra are assigned by comparison with ligand-field multiplet simulations, and reveal a rapid singlet→quintet intersystem crossing (ISC) in 38 fs. The data show a 30% drop in quintet population after a further 94 fs, suggesting a surprisingly efficient ballistic back-ISC directly to the singlet ground state, with the remainder of the excited population relaxing to a long-lived triplet state. The merits of exponential vs. non-exponential kinetics to describe these dynamics are discussed, as well as the greater implications for ultrafast ISC in transition metal complexes.

Ballistic ΔS=2 intersystem crossing in a cobalt cubane following ligand-field excitation probed by extreme ultraviolet spectroscopy

ACS Spring 2022

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