Ultrafast electron transfer and intersystem crossing in bimetallic molecules

Increasingly, the role of electron spin is an important topic in electron transfer (ET). Coupling between the electronic, spin, and nuclear degrees of freedom play an important role in driving ET and intersystem crossing (ISC) on the ultrafast timescale. In this thesis, ultrafast optical and X-ray techniques are used to characterize the ET and ISC dynamics of a prototypical bimetallic FeIICoIII molecule following metal-to-metal charge transfer (MMCT) excitation. Femtosecond optical transient absorption (OTA) spectroscopy supported by time-dependent density functional theory (TD-DFT) find evidence of long-lived FeII ligand-field excited states, suggesting that the MMCT state decays by subpicosecond ISC and back ET (BET). Using synchrotron-based K-edge X-ray transient absorption (XTA), the longest-lived excited state is characterized as an FeII high-spin based on the metal-ligand bond elongation evidenced by extended X-ray absorption fine structure (EXAFS). Then, synchrotron-based L-edge XTA is used to show that the shorter-lived Fe-centered excited state is a triplet state that exhibits charge delocalization between the FeII and CoIII centers. This triplet state is further characterized by extreme ultraviolet (XUV) TA with a high-harmonic-generation source. Next, femtosecond hard X-ray experiments are performed with an X-ray free electron laser source to characterize the ultrafast ISC+BET reaction. Kβ X-ray emission spectroscopy provides the oxidation-state and spin-state specificity required to monitor the electronic and spin degrees of freedom during the reaction. In conjunction, K-edge XTA linear anisotropy tracks the geometry changes during the reaction, allowing a view of the vibrational motion in the molecular frame. Then, with this (achiral) FeIICoIII complex thoroughly characterized, we developed a novel chiral analogue to examine effects of chirality on the ISC+BET reaction. Initial UV-vis absorption and OTA experiments show that the chiral substituent minimally perturbs the electronic structure and dynamics of the complex. Furthermore, no changes in the ISC+BET rate are detected within the time resolution of the OTA experiment. Overall, this thesis sheds new light on the coupling between electronic, nuclear, and spin degrees of freedom following MMCT in a bimetallic molecule.

Name thesis:degree_name

Ph.D.

Level thesis:degree_level

Dissertation

Discipline thesis:degree_discipline

Chemistry

Grantor

University of Illinois Urbana-Champaign

Year dc:date

2025

Author dc:creator

• Burke, John Henry

Contributors dc:contributor

• Vura-Weis, Josh
• van der Veen, Renske
• Mirica, Liviu
• Makri, Nancy

• Mixed valence
• dimer
• bimetallic
• inorganic
• organometallic
• ferrocene
• metallocene
• MMCT
• IVCT
• charge transfer
• femtosecond
• picosecond
• laser
• photophysics
• transient absorption
• X-ray
• XUV
• absorption
• XAS
• XES
• XANES
• EXAFS
• spectroscopy
• synchrotron
• X-ray free electron laser
• XFEL
• high harmonic generation
• HHG
• chiral induced spin selectivity effect
• CISS
• chirality
• high spin
• quintet
• excited state

Statement dc:rights

• Copyright 2025 John Henry Burke

Language dc:language

en

Handle dc:identifier

https://hdl.handle.net/2142/132786

OAI identifier oai:identifier

oai:www.ideals.illinois.edu:2142/132786