Tracing the Full Bimolecular Photocycle of Iron(III)–Carbene Light Harvesters in Electron-Donating Solvents

Photoinduced bimolecular charge transfer processes involving the iron­(III) N-heterocyclic carbene (FeNHC) photosensitizer [Fe­(phtmeimb)₂]⁺ (phtmeimb = phenyltris­(3-methyl-imidazolin-2-ylidene)­borate) and triethylamine as well as N,N-dimethylaniline donors have been studied using optical spectroscopy. The full photocycle of charge separation and recombination down to ultrashort time scales was studied by investigating the excited-state dynamics up to high quencher concentrations. The unconventional doublet ligand-to-metal charge transfer (²LMCT) photoactive excited state exhibits donor-dependent charge separation rates of up to 1.25 ps^–1 that exceed the rates found for typical ruthenium-based systems and are instead more similar to results reported for organic sensitizers. The ultrafast charge transfer probed at high electron donor concentrations outpaces the solvent dynamics and goes beyond the classical Marcus electron transfer regime. Poor photoproduct yields are explained by donor-independent, fast charge recombination with rates of ∼0.2 ps^–1, thus inhibiting cage escape and photoproduct formation. This study thus shows that the ultimate bottlenecks for bimolecular photoredox processes involving these FeNHC photosensitizers can only be determined from the ultrafast dynamics of the full photocycle, which is of particular importance when the bimolecular charge transfer processes are not limited by the intrinsic excited-state lifetime of the photosensitizer.