Design and Control of Fe and Co Spin Crossover Dynamics Using Tabletop Femtosecond M-edge XANES

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With support from the Chemical Structure, Dynamics, and Mechanisms-A (CSDM-A) Program in the Division of Chemistry, Drs. Josh Vura-Weis and Greg Girolami at the University of Illinois at Urbana-Champaign are studying light-induced processes in iron- and cobalt-containing molecules. Light-absorbing molecules or chromophores are key components in certain solar cells and in the synthesis of some pharmaceuticals, but usually contain expensive ruthenium or iridium metal atoms. While the ruthenium and iridium-containing molecules store the energy they absorb from light for a long enough time to induce a chemical reaction, molecules with cheap and earth-abundant iron and cobalt atoms rapidly dissipate that energy as heat. This project will use insight gained from extreme ultraviolet absorption spectroscopy to better understand the processes leading to very fast energy loss in order to design new chromophores with long-lived excited states. Such molecules would reduce dependence on expensive precious metals in a wide range of solar energy conversion processes. This project will broaden participation in physical chemistry by hosting St. Elmo Brady Summer Scholars from historically black colleges and universities, as well as undergraduate research students from primarily undergraduate universities.The interplay between electron transfer and spin dynamics is a key factor in the photophysics and reactivity of complexes containing open-shell transition metals. Photoexcitation of such systems initiates a cascade of relaxation steps with competing rates of vibrational relaxation, intersystem crossing, and intramolecular electron transfer. In this tabletop work, femtosecond M-edge X-ray absorption spectroscopy is used to measure the relaxation dynamics in Fe(II) and Co(III) photosensitizers. The metal 3p-3d transitions in the extreme ultraviolet energy range are element, oxidation state, spin state, and ligand-field specific, and the laser-based source provides 30-femtosecond time resolution. Insights about the excited-state potential energy surfaces gained from this technique are used to design new Fe(II) chromophores with tailored vibrational modes that kinetically trap long-lived excited states. New ligands for Co(III) chromophores are being designed to provide metal-to-ligand charge transfer transitions in the visible region, and the excited-state dynamics of these chromophores are studied using transient absorption spectroscopy and quantum dynamics calculations. The new spectroscopic methods demonstrated in this proposal are expected to apply to a wide range of problems in physical and inorganic chemistry, from photocatalysis to solar cell design. The project also will provide advanced training in synthesis and spectroscopy for graduate and undergraduate students.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.