Sub-100 fs Intersystem Crossing to a Metal-CenteredTriplet 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 M2,3-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 tri...