Shining light on the photoexcited states of first-row transition metal photocatalysts

Wallick, Rachel Frieda

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https://hdl.handle.net/2142/124643 Copy

Description

Title

Shining light on the photoexcited states of first-row transition metal photocatalysts

Author(s)

Wallick, Rachel Frieda

Issue Date

2024-04-16

Director of Research (if dissertation) or Advisor (if thesis)

• Vura-Weis, Josh
• Mirica, Liviu

Doctoral Committee Chair(s)

• Vura-Weis, Josh
• Mirica, Liviu

Committee Member(s)

• van der Veen, Renske
• Dlott, Dana

Department of Study

Chemistry

Discipline

Chemistry

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

• x-ray spectroscopy
• time-resolved spectroscopy
• photocatalysis
• ultrafast

Abstract

In this thesis, we have investigated the excited-states of several first-row transition metal photocatalytic systems through a suite of ultrafast optical and X-ray spectroscopies. Photocatalysis with first-row transition metal complexes is distinct from the well characterized photocatalysis with second- and third-row transition metal complexes due to the presence of low-lying ligand field states that deactivate the highly reactive metal-to-ligand charge transfer (MLCT) states within several picoseconds. We first characterize the excited-state of a Ni(II) photocatalyst, (dtbbpy)Ni(o-Tol)Cl, using time-resolved Ni L- and K-edge spectroscopy. We find that upon photoexcitation into the MLCT band, the complex forms a long-lived excited-state that we assign as a tetrahedral ligand-field state. While past work has presented indirect evidence for the formation of the tetrahedral ligand-field state, we present the first account of direct observation of this state. Next, we discuss the characterization of the excited-state of a bis-μ-oxo Fe(III) porphyrin system used for oxygen atom transfer via time-resolved Nitrogen K-edge spectroscopy. Upon photoexcitation, the Fe(III) porphyrin rings undergo disproportionation to form an Fe(II) porphyrin and a catalytically active Fe(IV)-oxo porphyrin. However, previous work has suggested that when tetra(phenyl)porphyrin is used, the disproportionated state forms in undetectable yields. We study a tetra(methylpyridnium)porphyrin system, and the Nitrogen K-edge spectroscopy suggests that the system primarily forms the disproportionated state upon photoexcitation. Unpublished work in our group suggests that larger Hammett parameters of the R-groups on the porphyrin lead to increased favorability of the formation of the disproportionated state. The Hammett parameter of methylpyridinium is ~2.5, which is quite large and would significantly favor the disproportionated state. Finally, we optically characterize a Ni pyridinophane photocatalyst that performs wavelength-dependent C-O cross-coupling. We aim to elucidate how the ultrafast dynamics change as a function of pump wavelength, providing an explanation for why the cross-coupling reactivity is wavelength dependent. The current study shows some differences in the ultrafast dynamics in the first ~2 ps after photoexcitation as a function of pump wavelength, but the long-lived states are the same regardless of excitation energy. This suggests that the photocatalysis may depend on an intramolecular process occurring within the first 2 ps after photoexcitation. We are in the process of performing a supplementary study to expand the wavelengths used. Theoretical calculations on different Ni systems suggest that photocatalysis depends on intramolecular processes happening on the ultrafast timescale, and this study may be indirect experimental evidence for the theory.

Graduation Semester

2024-05

Type of Resource

Thesis

Copyright and License Information

Copyright 2024 Rachel Wallick

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