Sub-100 fs Intersystem Crossing to a Metal-Centered Triplet in Ni(II)OEP Observed with M-Edge XANES

Nickel porphyrins have been extensively studied as photosensitizers due to their long-lived metal-centered excited states. The multiplicity of the (d,d) state, and/or the rate of intersystem crossing between singlet and triplet metal-centered states, has remained uncertain due to the spin-insensitivity of many spectral probes. In this work, we directly probe the metal 3d shell occupation of nickel(II) octaethylporphyrin (NiOEP) using femtosecond M_2,3-edge X-ray absorption near-edge structure (XANES). A tabletop high-harmonic source is used to perform 400 nm pump, extreme-ultraviolet probe transient absorption spectroscopy with ∼100 fs time resolution. Photoexcitation produces a (π,π*) state that evolves with a time constant of 48 fs to a vibrationally hot metal-centered triplet ³(d,d) excited state with a lifetime of 595 ps. The spin sensitivity of M-edge XANES allows the ³(d,d) state to be distinguished from a potential ¹(d,d) state, as shown by charge-transfer multiplet simulations and comparison to triplet nickel(II) oxide. Vibrational cooling of the hot triplet state occurs over tens of ps, with minimal change in the electronic structure of the nickel(II) center. No evidence of a ligand-to-metal charge-transfer or metal-to-ligand charge-transfer intermediate state is seen within the time resolution of the instrument, suggesting that if such a state exists in NiOEP it depopulates in <25 fs. Finally, this study demonstrates the ability of tabletop high-harmonic extreme ultraviolet sources to measure excited-state spin transitions in molecular transition-metal complexes.