Ultrafast Nonradiative Relaxation Limits the Efficiency of Photoinduced Bond Homolysis in Molecular LMCT Photocatalysts

Ligand-to-metal charge transfer photocatalysts (LMCT PCs) are being increasingly implemented toward construction and functionalization of organic molecules. Leveraging photoinduced metal-ligand bond homolysis, these PCs generate reactive intermediates ranging from halogen radicals to radical cross-coupling partners. Despite their growing synthetic utility, key mechanistic questions remain surrounding both the surprising inefficiency of this photodissociation chemistry and the puzzling success of nonprecious metal photocatalysts despite low energy ligand field states. Herein, femtosecond transient absorption (TA) spectroscopy is implemented to address these mysteries using FeCl₄^- and CeCl₆^2-, which photogenerate Cl• under mild conditions, as model systems. Ultrafast dynamics and complementary TA actinometry indicate that the homolysis efficiency is limited by relaxation to a lower energy excited state within the LMCT excited state manifold at a faster rate than relaxation to ligand field states. These results provide a mechanistic explanation for the low photochemical efficiencies observed in benchtop reactions that have encumbered otherwise proficient PCs.