Jay, R.; Norell, J.; Eckert, S.; Hantschmann, M.; Beye, M.; Kennedy, B.; Quevedo, W.; Schlotter, W.; Dakovski, G.; Minitti, M.; Hoffmann, M.; Mitra, A.; Moeller, S.; Nordlund, D.; Zhang, W.; Liang, H.; Kunnus, K.; Kubicek, K.; Techert, S.; Lundberg, M.; Wernet, P.; Gaffney, K.; Odelius, M.; Föhlisch, A.: Disentangling Transient Charge Density and Metal-Ligand Covalency in Photoexcited Ferricyanide with Femtosecond Resonant Inelastic Soft X-ray Scattering. The Journal of Physical Chemistry Letters 9 (2018), p. 3538-3543
10.1021/acs.jpclett.8b01429
Open Access Version
Abstract:
Soft X-ray spectroscopies are ideal probes of the local valence electronic structure of photocatalytically active metal sites. Here, we apply the selectivity of time- resolved resonant inelastic X-ray scattering at the iron L-edge to the transient charge distribution of an optically excited charge-transfer state in aqueous ferricyanide. Through comparison to steady-state spectra and quantum chemical calculations, the coupled effects of valence-shell closing and ligand-hole creation are experimentally and theoretically disentangled and described in terms of orbital occupancy, metal−ligand covalency, and ligand field splitting, thereby extending established steady-state concepts to the excited-state domain. π-Back-donation is found to be mainly determined by the metal site occupation, whereas the ligand hole instead influences σ-donation. Our results demonstrate how ultrafast resonant inelastic X-ray scattering can help characterize local charge distributions around catalytic metal centers in short-lived charge-transfer excited states, as a step toward future rationalization and tailoring of photocatalytic capabilities of transition-metal complexes.