Understanding Anomalous Cage-Escape Dynamics in Photoredox Processes Driven by a Fe(III) N‑Heterocyclic Carbene Complex

Solvent cage-escape dynamics of bimolecular photoredox products in solution has been investigated computationally through a combination of molecular dynamics simulations and quantum chemical calculations. The present work focuses on the photoinduced oxidation of the organic electron donor dimethylaniline (DMA) by a Fe(III) N-heterocyclic carbene photosensitizer (Fe(III)NHC+) in two different solvents, serving as an example of current interest due to their relevance for the development of earth-abundant photocatalytic systems. Calculated solvent cage-escape yields of radical-cation and neutral photoproducts (DMA•+ and Fe(II)NHC, respectively) by molecular dynamics simulations reveal more favorable solvation in acetonitrile than in dichloromethane following the initial photoinduced charge-separation. These results agree with basic expectations from solvent polarity considerations but give an opposite trend compared to experimentally reported cage-escape yields. Alternative cage-escape mechanisms were therefore considered computationally to account for the anomalous experimental cage-escape yields. Both quantum chemical calculations and molecular dynamics simulations support the formation of radical-cation dimers ([(DMA)2]•+), allowing for more efficient charge migration involving the radical-cations as the polarity of the solvent is decreased. The results further demonstrate the ability of the counterion (PF6–) to stabilize the photoproducts through radical-cation–anion pairing, suggesting that these bimolecular interactions can also play an important role to preferentially promote photoproduct formation in less polar solvents. Both radical-cation dimer formation and radical-cation–counterion interactions are therefore proposed to provide additional pathways that help to explain the experimental observations of anomalous solvation dependence of the cage-escape dynamics in the investigated system. The broader implications of the bimolecular cage-escape processes on photocatalytic reaction dynamics are also considered based on our findings about light-induced intermolecular interactions.