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 towards 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 non-precious metal photocatalysts despite low energy ligand field states. Herein, femtosecond transient absorption (TA) spectroscopy is implemented to address these mysteries using FeCl4− and CeCl62−, 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.