CAREER: Time-Resolved X-ray Spectroscopy of Excited State Dynamics and Branching Underlying Photoinduced Metal-Halogen Bond Activation

With support from the Chemical Structure and Dynamics (CSD) program in the Division of Chemistry, and the Established Program to Stimulate Competitive Research (EPSCoR), Professor Aditi Bhattacherjee of the University of Iowa is investigating the early steps in the activation of single-site transition-metal catalysts when irradiated with visible light. Catalysts are molecules that facilitate chemical reactions but they themselves are not consumed during the reaction.

Understanding catalyst function is a challenge, as their chemical behavior oftentimes involves the motion of charge between the catalyst's subcomponents on very short time scales. Professor Bhattacherjee and her students will utilize femtosecond X-ray transient absorption spectroscopy to watch photoexcited charges as they move through a catalyst, determining both the time scales and pathways they take.

Their discoveries could lead to a better understanding of single-site catalysts used extensively in polymerization reactions. The education plans will weave professional skill development into chemistry curricula by developing a content-context balanced pedagogy that is mindful of diverse student experiences.

The central objective of this proposal is to identify the intersystem crossing rates and photochemical branching ratios in functionalized metallocene catalysts with atomic site specificity to inform a fundamental photophysical understanding of organometallic spin crossover concerning light and heavy atoms in multi-coordinating ligand fields. The charge transfer steps and spin-orbit coupling mechanisms in the photoactivation of these molecular catalysts will be resolved at the level of the frontier orbitals and the constituent metal and ligand spheres.

Atom-specific spectroscopies such as X-ray spectroscopy have the potential to resolve the charge transfer steps precisely. The energy gaps and orbital mixing coefficients will be measured directly in different charge transfer excited states, which can be compared with electronic structure calculations. The project will contribute to the advancement of tabletop ultrafast X-ray spectroscopy.

The results will also enable a mechanistic understanding of photocatalysis with earth-abundant metals and be of wide interest in fields encompassing non-adiabatic chemical dynamics, excited-state charge transfer, and photoredox chemistry.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.