University of Illinois Urbana-Champaign

Probing the reaction coordinate of photoinduced spin-crossover with femtosecond M-edge XANES

Ash, Ryan

Content Files
ASH-DISSERTATION-2020.pdf

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https://hdl.handle.net/2142/109507

Description

Title
Probing the reaction coordinate of photoinduced spin-crossover with femtosecond M-edge XANES
Author(s)
Ash, Ryan
Issue Date
2020-11-30
Director of Research (if dissertation) or Advisor (if thesis)
Vura-Weis, Joshua
Doctoral Committee Chair(s)
Vura-Weis, Joshua
Committee Member(s)
  • Eden, J G
  • Girolami, Gregory S
  • van der Veen, Renske
Department of Study
Chemistry
Discipline
Chemistry
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Date of Ingest
2021-03-05T21:42:40Z
Keyword(s)
  • Physical Chemistry
  • M-edge XANES
  • Extreme Ultraviolet
  • Ultrafast Spectroscopy
  • Transition Metal Photophysics
  • Spin Crossover
Abstract
Photoinduced spin-crossover is studied using femtosecond M-edge XANES with a tabletop instrument based on a high-harmonic generation source. This technique probes the oxidation state, spin state and ligand field of the metal center with sub-50 fs time resolution, making it a powerful tool to study the ultrafast photophysics of transition metal complexes. Experimental spectra are interpreted with ligand field multiplet (LFM) simulations to extract information about the electronic structure of the metal center. General trends in the spectra of d0-d9 octahedral complexes with changing ligand field strengths are first presented to demonstrate the information contained in each M-edge spectrum. This framework is then used to study the ultrafast dynamics of a series of iron(II) complexes with varying ligand geometries. Key regions in the M-edge spectra are shown to indicate the presence of singlet, triplet and quintet metal-centered states, and a triplet intermediate state is identified in the relaxation cascade in Fe(phen)32+, Fe(tpy)22+, and Fe[(4-CF3)2bpca]2. Coherent oscillations are observed as a modification of the quintet spectrum and are shown to arise from modifications of the ligand field strength of the complex. NEVPT2 calculations are employed to show how different vibrational modes imprint a unique signature in the M-edge spectra and unravel a difference between the nuclear dynamics of Fe(tpy)22+ and Fe[(4-CF3)2bpca]2. Ongoing work is shown for the photophysics of a macrocyclic Fe(II) complex which has potential to afford long-lived metal-to-ligand charge transfer lifetimes with judicious choice of the axial ligand. Ultrafast valence tautomerism in a cobalt dioxolene complex is also studied. The transient M-edge XANES spectra show the involvement of a low-spin Co(II) before ultrafast intersystem crossing. Two relaxation pathways from the vibrationally hot and cold high-spin Co(II) state are also identified. Finally, the development of a thin-film cryostat suitable for transient M-edge XANES experiments is presented. The challenges of minimizing radiative heat transfer and cooling thin-film samples are shown and the cryostat’s operation is demonstrated for the complex Fe[(H2B(pz)2]2(bpy), which undergoes both thermal and photoinduced spin-crossover.
Graduation Semester
2020-12
Type of Resource
Thesis
Permalink
http://hdl.handle.net/2142/109507
Copyright and License Information
Copyright 2020 Ryan Ash

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