Competing dynamics of intramolecular deactivation and bimolecular charge transfer processes in luminescent Fe( iii ) N-heterocyclic carbene complexes

Steady state and ultrafast spectroscopy on [Fe$^{III}$(phtmeimb)$_{2}$]PF$_{6}$ (phtmeimb = phenyl(tris(3-methylimidazol-2-ylidene))borate) was performed over a broad range of temperatures. The intramolecular deactivation dynamics of the luminescent doublet ligand-to-metal charge-transfer ($^{2}$LMCT) state was established based on Arrhenius analysis, indicating the direct deactivation of the $^{2}$LMCT state to the doublet ground state as a key limitation to the lifetime. In selected solvent environments photoinduced disproportionation generating short-lived Fe(IV) and Fe(II) complex pairs that subsequently undergo bimolecular recombination was observed. The forward charge separation process is found to be temperature-independent with a rate of ∼1 ps$^{–1}$. Subsequent charge recombination takes place in the inverted Marcus region with an effective barrier of 60 meV (483 cm$^{–1}$). Overall, the photoinduced intermolecular charge separation efficiently outcompetes the intramolecular deactivation over a broad range of temperatures, highlighting the potential of [Fe$^{III}$(phtmeimb)$_{2}$]PF$_{6}$ to perform photocatalytic bimolecular reactions.