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

Abstract

Nickel porphyrins have been extenstively 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 an LMCT or MLCT 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 table highharmonic XUV sources to measure excited-state spin transitions in molecular transition metal complexes. 3

Ultrafast photoinduced dynamics in Prussian blue analogues

“…[6][7][8][9][10] This leads to a fast depopulation of the MLCT state into the ligand manifold (or metal-centred states) with different spin quantum numbers. A lot of the recent understanding of the very fast spin changes in inorganic complexes comes from the development of new techniques, such as X-ray free-electron lasers (XFELs) 8,9,[11][12][13][14][15][16] (see a recent review by Chergui and Collet 17 ), table-top X-ray sources, 18,19 and ultrafast electron diffraction. 20,21 Furthermore, developments in computational methods have been essential.…”

Elucidation of Complex Triplet Excited State Dynamics in Pd(II) Biladiene Tetrapyrroles

“…Additionally, porphyrins containing first row transition metals are known to undergo rapid, vibrationally excited d-d transitions between singlet and triplet states that show moderate excitation-wavelength dependent dynamics. [30][31] Certain nickel porphyrins, such as Ni(OEP) (OEP = octaethylporphyrin) can exhibit wavelength-dependent photophysics involving metal-centered, vibrationally-excited singlet states rapidly (sub-ps) converting to triplet d-centered states. These metal-centered triplet states, which are dominated by d-d transitions, undergo nonradiative deactivation via the lowest-lying triplet state in several hundred ps.…”

Photoinduced Valence Tautomerism of a Cobalt-Dioxolene Complex Revealed with Femtosecond M-Edge XANES

“…[29][30][31][32] The development of ultrafast XUV light sources based on high-harmonic generation (HHG) has extended the applicability of M-edge spectroscopy to study the dynamics of 3d transition metal complexes. Femtosecond M-edge XANES has been used to measure excited-state relaxation dynamics in Fe and Ni complexes 31,33,34 and in transition metal oxide semiconductors, [35][36][37][38][39] but its applicability to molecular cobalt complexes has not yet been demonstrated. In the present work, we use this emerging technique to measure the mechanism and timescale of ultrafast valence tautomerism in Co(DQ) 2 from the perspective of the metal center, revealing an initial LS Co II state that precedes competitive vibrational cooling and intersystem crossing processes.…”