Ultrafast spectroscopy uncovers the mechanistic underpinnings of next-generation photocatalysts

Rachel Weiss Clark, Bryan Kudisch

Florida State University

A rapidly developing sect of photoredox catalysis utilizes extremely redox active excited states accessed with visible light to promote desirable photochemical transformations. High-valent metal complexes have shown great promise to this end, with recently demonstrated ability to selectively oxidize various substrates outside of even the solvent oxidation window by accessing photochemically active ligand-to-metal charge transfer (LMCT) states. Despite impressive and diverse reported photoreactivity, low per-photon efficiency (typically below 10%) remains a limitation of this class of photoredox reactions, and this low efficiency seems to be a general feature of even structurally and chemically dissimilar metal-ligand combinations in the LMCT photocatalysis literature. In order to address the mechanistic underpinnings of these photochemical efficiencies, we surveyed a variety of LMCT active photocatalysts derived from multiple high-valent metals (Ce⁴⁺, Fe³⁺, Ti⁴⁺) in acetonitrile for the formation of chlorine radicals (Cl•) by employing steady-state actinometric methods, ultrafast transient absorption spectroscopy, and ultrafast fluorescence upconversion spectroscopy. Our preliminary results are consistent with a common deactivation mechanism among the investigated complexes, despite their disparate electronic structures, redox activities, and coordination geometries. We tentatively assign the origin of low photochemical quantum yields to an ultrafast kinetic competition between this deactivation mechanism and the productive bond homolysis pathway from a common, excited state intermediate, and we can quantitatively connect the ultrafast competition to observed steady-state photoredox reaction rates. This investigation paves the way for further ultrafast mechanistic investigation of nascent and high-potential photoredox catalysts towards both fundamental, comprehensive understanding of connections between ultrafast photophysics and benchtop photochemistry as well as the discovery of other impactful and efficient photoredox platforms.

Oral presentation