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

Intersystem Crossing ◽

Edge Structure ◽

State Spin ◽

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 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 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 high-harmonic XUV sources to measure excited-state spin transitions in molecular transition metal complexes.

Tracking the Metal-Centered Triplet in Photoinduced Spin Crossover of Fe(phen)32+ with Tabletop Femtosecond M-edge XANES

10.26434/chemrxiv.8259011.v1 ◽

2019 ◽

Author(s):

Kaili Zhang ◽

Ryan Ash ◽

Gregory S Girolami ◽

Josh Vura-Weis

Keyword(s):

Transient Absorption ◽

Extreme Ultraviolet ◽

Xanes Spectrum ◽

Fast Time ◽

Edge Structure ◽

X Ray ◽

Ligand Charge Transfer ◽

X Ray Absorption ◽

Mlct State

Fe(II) coordination complexes are promising alternatives to Ru(II) and Ir(III) chromophores for photoredox chemistry and solar energy conversion, but rapid deactivation of the initial metal-to-ligand charge transfer (MLCT) state to low-lying (d,d) states limits their performance. Relaxation to a 5T2g state is postulated to occur via a metal-centered triplet state, but this mechanism remains controversial. We use femtosecond extreme ultraviolet (XUV) transient absorption spectroscopy to measure the excited-state relaxation of Fe(phen)32+ and conclusively identify a 3T intermediate that forms in 170 fs and decays to a vibrationally hot 5T2g state in 40 fs. The shape of this M2,3-edge X-ray absorption near edge structure (XANES) spectrum is sensitive to the electronic structure of the metal center, and the high spin sensitivity, fast time resolution, and tabletop convenience of XUV transient absorption make it a powerful new tool for measuring the complex photophysics of transition metal complexes.

Ballistic ΔS=2 Intersystem Crossing in a Cobalt Cubane Following Ligand-Field Excitation Probed by Extreme Ultraviolet Spectroscopy

Physical Chemistry Chemical Physics ◽

10.1039/d1cp04136c ◽

2021 ◽

Author(s):

Yusef Shari'ati ◽

Josh Vura-Weis

Keyword(s):

Excited State ◽

Extreme Ultraviolet ◽

Xanes Spectroscopy ◽

Ligand Field ◽

Ultraviolet Spectroscopy ◽

Intersystem Crossing ◽

Edge Structure ◽

X Ray ◽

Field Excitation ◽

Femtosecond M2,3-edge X-ray absorption near-edge structure (XANES) spectroscopy is used to probe the excited-state dynamics of the cobalt cubane [CoIII4O4](OAc)4(py)4 (OAc = acetate, py = pyridine), a model for water...

Ballistic ΔS=2 Intersystem Crossing in a Cobalt Cubane Following Ligand-Field Excitation Probed by Extreme Ultraviolet Spectroscopy

10.33774/chemrxiv-2021-fh9gb ◽

2021 ◽

Author(s):

Yusef Shari'ati ◽

Josh Vura-Weis

Keyword(s):

Excited State ◽

Water Oxidation ◽

Extreme Ultraviolet ◽

Ligand Field ◽

Ultraviolet Spectroscopy ◽

Intersystem Crossing ◽

Edge Structure ◽

X Ray ◽

Field Excitation ◽

Femtosecond M2,3-edge X-ray absorption near-edge structure (XANES) spectroscopy is used to probe the excited-state dynamics of the cobalt cubane [CoIII4O4](OAc)4(py)4 (OAc = acetate, py = pyridine), a model for water oxidation catalysts. After ligand-field excitation, intersystem crossing to a metal-centered quintet occurs in 38 fs. 30% of the hot quintet undergoes ballistic back-ISC directly to the singlet ground stat, with the remainder relaxing to a long-lived triplet.

Tracking the Metal-Centered Triplet in Photoinduced Spin Crossover of Fe(phen)32+ with Tabletop Femtosecond M-edge XANES

10.26434/chemrxiv.8259011.v2 ◽

2019 ◽

Author(s):

Kaili Zhang ◽

Ryan Ash ◽

Gregory S Girolami ◽

Josh Vura-Weis

Keyword(s):

Transient Absorption ◽

Extreme Ultraviolet ◽

Xanes Spectrum ◽

Fast Time ◽

Edge Structure ◽

X Ray ◽

Ligand Charge Transfer ◽

X Ray Absorption ◽

Mlct State

Fe(II) coordination complexes are promising alternatives to Ru(II) and Ir(III) chromophores for photoredox chemistry and solar energy conversion, but rapid deactivation of the initial metal-to-ligand charge transfer (MLCT) state to low-lying (d,d) states limits their performance. Relaxation to a 5T2g state is postulated to occur via a metal-centered triplet state, but this mechanism remains controversial. We use femtosecond extreme ultraviolet (XUV) transient absorption spectroscopy to measure the excited-state relaxation of Fe(phen)32+ and conclusively identify a 3T intermediate that forms in 170 fs and decays to a vibrationally hot 5T2g state in 40 fs. A coherent vibrational wavepacket with a period of 250 fs and damping time of 0.66 ps is observed on the 5T2g surface, and the spectrum of this oscillation serves as a fingerprint for the Fe-N symmetric stretch. The results show that the shape of the M2,3-edge X-ray absorption near edge structure (XANES) spectrum is sensitive to the electronic structure of the metal center, and the high spin sensitivity, fast time resolution, and tabletop convenience of XUV transient absorption make it a powerful tool for studying the complex photophysics of transition metal complexes.

Tracking the Metal-Centered Triplet in Photoinduced Spin Crossover of Fe(phen)32+ with Tabletop Femtosecond M-edge XANES

10.26434/chemrxiv.8259011 ◽

2019 ◽

Author(s):

Kaili Zhang ◽

Ryan Ash ◽

Gregory S Girolami ◽

Josh Vura-Weis

Keyword(s):

Transient Absorption ◽

Extreme Ultraviolet ◽

Xanes Spectrum ◽

Fast Time ◽

Edge Structure ◽

X Ray ◽

Ligand Charge Transfer ◽

X Ray Absorption ◽

Mlct State

Fe(II) coordination complexes are promising alternatives to Ru(II) and Ir(III) chromophores for photoredox chemistry and solar energy conversion, but rapid deactivation of the initial metal-to-ligand charge transfer (MLCT) state to low-lying (d,d) states limits their performance. Relaxation to a 5T2g state is postulated to occur via a metal-centered triplet state, but this mechanism remains controversial. We use femtosecond extreme ultraviolet (XUV) transient absorption spectroscopy to measure the excited-state relaxation of Fe(phen)32+ and conclusively identify a 3T intermediate that forms in 170 fs and decays to a vibrationally hot 5T2g state in 40 fs. A coherent vibrational wavepacket with a period of 250 fs and damping time of 0.66 ps is observed on the 5T2g surface, and the spectrum of this oscillation serves as a fingerprint for the Fe-N symmetric stretch. The results show that the shape of the M2,3-edge X-ray absorption near edge structure (XANES) spectrum is sensitive to the electronic structure of the metal center, and the high spin sensitivity, fast time resolution, and tabletop convenience of XUV transient absorption make it a powerful tool for studying the complex photophysics of transition metal complexes.

Line-shape broadening of an autoionizing state in helium at high XUV intensity

New Journal of Physics ◽

10.1088/1367-2630/ac3b2e ◽

2021 ◽

Author(s):

Lennart Aufleger ◽

Patrick Friebel ◽

Patrick Rupprecht ◽

Alexander Magunia ◽

Thomas Ding ◽

...

Keyword(s):

Excited State ◽

Strong Coupling ◽

Line Shape ◽

Transient Absorption ◽

Extreme Ultraviolet ◽

Model Simulation ◽

Autoionizing State ◽

Doubly Excited ◽

Level Model

Abstract We study the interaction of intense extreme ultraviolet (XUV) light with the 2s2p doubly excited state in helium. In addition to previously understood energy-level and phase shifts, high XUV intensities may lead to other absorption line shape distortions. Here, we report on experimental transient-absorption spectroscopy results on the 2s2p line width modification in helium in intense stochastic XUV fields. A few-level model simulation is realized to investigate the origins of this effect. We find that the line shape broadening is connected to the strong coupling of the ground state to the 2s2p doubly excited state which is embedded in the ionization continuum. As the broadening takes place for intensities lower than for other strong-coupling processes, e.g. observing asymmetry changes of the absorption profile, this signature can be identified already in an intermediate intensity regime. These findings are in general relevant for resonant inner shell transitions in nonlinear experiments with XUV and x-ray photon energies at high intensity.

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

10.26434/chemrxiv.8283647.v2 ◽

2019 ◽

Author(s):

Ryan Ash ◽

Kaili Zhang ◽

Josh Vura-Weis

Keyword(s):

Excited State ◽

Charge Transfer ◽

High Spin ◽

Ground State ◽

Extreme Ultraviolet ◽

High Harmonic ◽

Ligand Field ◽

Intersystem Crossing ◽

Valence Tautomerism ◽

Reduced Forms

Cobalt complexes that undergo charge-transfer induced spin-transitions (CTIST) or valence tautomerism (VT) from low spin (LS) Co(III) to high spin (HS) Co(II) are potential candidates for magneto-optical switches. We use M-edge XANES spectroscopy with 40 fs time resolution to measure the excited-state dynamics of Co(III)(Cat-N-SQ)(Cat-N-BQ), where Cat-N-BQ and Cat-N-SQ are the singly and doubly reduced forms of the 2-(2-hydroxy-3,5-di-tert-butylphenyl-imino)-4,6-di-tert-butylcyclohexa-3,5-dienone ligand. The extreme ultraviolet probe pulses, produced using a tabletop high-harmonic generation light source, measure 3p3d transitions and are sensitive to the spin and oxidation state of the Co center. Photoexcitation at 525 nm produces a low-spin Co(II) ligand-to-metal charge transfer state which undergoes intersystem crossing to high-spin Co(II) in 67 fs. Vibrational cooling from this hot HS Co(II) state competes on the hundreds-of-fs timescale with back-intersystem crossing to the ground state, with 60% of the population trapped in a cold HS Co(II) state for 24 ps. Ligand field multiplet simulations accurately reproduce the ground-state spectra and support the excited-state assignments. This work demonstrates the ability of M-edge XANES to measure ultrafast photophysics of molecular Co complexes.

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

10.26434/chemrxiv.8283647 ◽

2019 ◽

Author(s):

Ryan Ash ◽

Kaili Zhang ◽

Josh Vura-Weis

Keyword(s):

Excited State ◽

Charge Transfer ◽

High Spin ◽

Ground State ◽

Extreme Ultraviolet ◽

High Harmonic ◽

Ligand Field ◽

Intersystem Crossing ◽

Valence Tautomerism ◽

Reduced Forms

Cobalt complexes that undergo charge-transfer induced spin-transitions (CTIST) or valence tautomerism (VT) from low spin (LS) Co(III) to high spin (HS) Co(II) are potential candidates for magneto-optical switches. We use M-edge XANES spectroscopy with 40 fs time resolution to measure the excited-state dynamics of Co(III)(Cat-N-SQ)(Cat-N-BQ), where Cat-N-BQ and Cat-N-SQ are the singly and doubly reduced forms of the 2-(2-hydroxy-3,5-di-tert-butylphenyl-imino)-4,6-di-tert-butylcyclohexa-3,5-dienone ligand. The extreme ultraviolet probe pulses, produced using a tabletop high-harmonic generation light source, measure 3p3d transitions and are sensitive to the spin and oxidation state of the Co center. Photoexcitation at 525 nm produces a low-spin Co(II) ligand-to-metal charge transfer state which undergoes intersystem crossing to high-spin Co(II) in 67 fs. Vibrational cooling from this hot HS Co(II) state competes on the hundreds-of-fs timescale with back-intersystem crossing to the ground state, with 60% of the population trapped in a cold HS Co(II) state for 24 ps. Ligand field multiplet simulations accurately reproduce the ground-state spectra and support the excited-state assignments. This work demonstrates the ability of M-edge XANES to measure ultrafast photophysics of molecular Co complexes.

Alternative difference analysis scheme combiningR-space EXAFS fit with global optimization XANES fit for X-ray transient absorption spectroscopy

Journal of Synchrotron Radiation ◽

10.1107/s1600577517005719 ◽

2017 ◽

Vol 24 (4) ◽

pp. 818-824 ◽

Cited By ~ 3

Author(s):

Fei Zhan ◽

Ye Tao ◽

Haifeng Zhao

Keyword(s):

Absorption Spectroscopy ◽

Transient Absorption ◽

Difference Spectrum ◽

Structure Change ◽

Edge Structure ◽

Difference Analysis ◽

X Ray ◽

Analysis Scheme ◽

Time-resolved X-ray absorption spectroscopy (TR-XAS), based on the laser-pump/X-ray-probe method, is powerful in capturing the change of the geometrical and electronic structure of the absorbing atom upon excitation. TR-XAS data analysis is generally performed on the laser-on minus laser-off difference spectrum. Here, a new analysis scheme is presented for the TR-XAS difference fitting in both the extended X-ray absorption fine-structure (EXAFS) and the X-ray absorption near-edge structure (XANES) regions.R-space EXAFS difference fitting could quickly provide the main quantitative structure change of the first shell. The XANES fitting part introduces a global non-derivative optimization algorithm and optimizes the local structure change in a flexible way where both the core XAS calculation package and the search method in the fitting shell are changeable. The scheme was applied to the TR-XAS difference analysis of Fe(phen)3spin crossover complex and yielded reliable distance change and excitation population.