Ultrafast Excited State Dynamics for FeCl3 as an Efficient Photo-Oxidant

In this study, we investigate the excited state dynamics of FeCl3 in various solvents using ultrafast transient absorption technique to unravel the effective performance of FeCl3 as a photocatalyst in oxidizing various aromatic substrates under visible and sunlight irradiation. We demonstrate that the key step leading to the formation of [Cl•Fe(II)Ln]* species is the charge transfer from the chloride ligand to the metal Fe atom. These species can engage in geminate and non-geminate pathways; however, the latter fashion is the critical step in determining the electron transfer from the substrate to the catalyst. Such primary oxidation step of the substrate can occur at the fastest of 110 ps time scale. Remarkably, utilizing methanol as an alternative solvent to acetonitrile resulted in the absence of transient signals as MeOH molecules, after replacing the inner sphere molecules surrounding FeCl3, exhibit a higher efficiency in quenching the [Cl•Fe(II)Ln]* species compared to MeCN. Our ultrafast studies pave the way for controlling these reaction rates for earth abundant materials through fine tuning of ligands and organic substrates.