Multifaceted Deactivation Dynamics of Fe(II) -Heterocyclic Carbene Photosensitizers.

Excited state dynamics of three iron(II) carbene complexes that serve as prototype Earth-abundant photosensitizers were investigated by ultrafast optical spectroscopy. Significant differences in the dynamics between the investigated complexes down to femtosecond time scales are used to characterize fundamental differences in the depopulation of triplet metal-to-ligand charge-transfer (MLCT) excited states in the presence of energetically accessible triplet metal-centered (MC) states. Novel insights into the full deactivation cascades of the investigated complexes include evidence of the need to revise the deactivation model for a prominent iron carbene prototype complex, a refined understanding of complex MC dynamics, and a quantitative discrimination between activated and barrierless deactivation steps along the MLCT → MC → GS path. Overall, the study provides an improved understanding of photophysical limitations and opportunities for the use of iron(II)-based photosensitizers in photochemical applications.