Tabletop Femtosecond M-edge X-ray Absorption Near-Edge Structure of FeTPPCl: Metalloporphyrin Photophysics from the Perspective of the Metal

Abstract

Iron porphyrins are the active sites of many natural and artificial catalysts, and their photoinduced dynamics have been described as either relaxation into a vibrationally hot ground state or as a cascade through metal-centered states. In this work, we directly probe the metal center of iron(III) tetraphenyl porphyrin chloride (FeTPPCl) using femtosecond M-edge X-ray absorption near-edge structure (XANES) spectroscopy. Photoexcitation at 400 nm produces a (π,π*) state that evolves in 70 fs to an iron(II) ligand-to-metal charge transfer (LMCT) state. The LMCT state relaxes to a vibrationally hot ground state in 1.13 ps, without involvement of (d,d) intermediates. The tabletop extreme-ultraviolet probe, combined with semiempirical ligand field multiplet calculations, clearly distinguishes between metal-centered and ligand-centered excited states and resolves competing accounts of Fe(III) porphyrin relaxation. This work introduces tabletop M-edge XANES as a valuable tool for measuring femtosecond dynamics of molecular transition metal complexes in the condensed phase.

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

“…Previous visible-light transient absorption spectra of NiOEP in toluene produced similar spectra and time constants, additionally noting that the solvent environment modifies the timescale of each relaxation process but not the spectral shape. 7 The ~1-ps and few-ps time constants have been assigned as intramolecular vibrational relaxation through a cascade of vibrational modes, with a possible influence from intermolecular vibrational relaxation at long times.7,36 The major difference in the thin-film results is the long-lived spectrum, which we attribute to local heating in the film, in agreement with previous studies 19,…”

“…The triplet 3 atoms and is likely due to thermal heating within the thin film. 19,35 Information about the thermal baseline, whose rise is a good match for the 26.3 ps component of the global fit, is shown in the SI.…”

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

“…The 3p → 3d dipole-allowed transitions lying between 30-100 eV contain information about the oxidation state, spin state, and coordination geometry of the metal center, and can be predictably reproduced using ligand-field multiplet (LFM) simulations. [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.…”

Competition Between Intersystem Crossing and Vibrational Cooling in Photoinduced Valence Tautomerism of a Cobalt-Dioxolene Complex Revealed with Femtosecond M-Edge XANES

“…The 3p → 3d dipole-allowed transitions lying between 30-100 eV contain information about the oxidation state, spin state, and coordination geometry of the metal center, and can be predictably reproduced using ligand-field multiplet (LFM) simulations. [28][29][30][31] The development of ultrafast XUV light sources based on high-harmonic generation (HHG) has extended the applicability of M 2,3 -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 30,32,33 and in transition metal oxide semiconductors, [34][35][36][37][38] but its applicability to molecular cobalt complexes has not yet been demonstrated.…”

“…[28][29][30][31] The development of ultrafast XUV light sources based on high-harmonic generation (HHG) has extended the applicability of M 2,3 -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 30,32,33 and in transition metal oxide semiconductors, [34][35][36][37][38] 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.…”