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

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.

Name thesis:degree_name

Ph.D.

Level thesis:degree_level

Dissertation

Discipline thesis:degree_discipline

Chemistry

Grantor

University of Illinois at Urbana-Champaign

Year dc:date

2021

Author dc:creator

• Ash, Ryan

Contributors dc:contributor

• Vura-Weis, Joshua
• Eden, J G
• Girolami, Gregory S
• van der Veen, Renske

• Physical Chemistry
• M-edge XANES
• Extreme Ultraviolet
• Ultrafast Spectroscopy
• Transition Metal Photophysics
• Spin Crossover

Statement dc:rights

• Copyright 2020 Ryan Ash

Language dc:language

en

Handle dc:identifier

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

OAI identifier oai:identifier

oai:www.ideals.illinois.edu:2142/109507