The combination of time-resolved X-ray and optical spectroscopic techniques provides a powerful means of probing the behavior of homogeneous transition metal complexes following optical excitation. In this talk I will present our recent investigation of the photoinduced dynamics of the aqueous ferrate(VI) ion, a remarkable example of a hexavalent iron complex that is stable under ambient conditions and can serve as an excellent source of oxidizing potential in both catalysis and energy storage applications. Despite its broad utility, the competing photochemical and photophysical relaxation pathways of the excited state ferrate(VI) ion remain largely uncharacterized. We have successfully identified the timescales, intermediates, and branching ratios for both pathways, providing a clear picture of the excited state dynamics that will help guide environmental engineering efforts toward novel, non-toxic methods of oxidative water treatment. I will also discuss our work on the Cu(I)-catalyzed [2 + 2] photocycloaddition reaction, which provides a simple, single-step route to cyclobutane moieties in organic syntheses that would otherwise be disfavored or impossible due to ring strain and steric interactions. By observing the intermolecular dimerizations of two model cyclic olefins from femtosecond to microsecond timescales, we have found that the reaction of norbornene proceeds via a metal-to-ligand charge transfer state, while the reaction of cyclohexene proceeds via an initial ligand photoisomerization step. Our investigation reveals how this photocatalytic reaction may be directed along strikingly disparate trajectories with only very minor changes to the structure of the substrate. Finally, I will conclude with a brief summary of our current work to apply these strategies to intermolecular photochemistry, using the photocatalytic epoxidation of cycloolefins by [FeTPP]<sub>2</sub>O as an illustrative example.

Disentangling photophysical and photochemical relaxation pathways in transition metal complexes using X-ray and optical transient absorption spectroscopies

ACS Spring 2022

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