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.