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

Abstract

Steady state and ultrafast spectroscopy on [Fe^III(phtmeimb)₂]PF₆ (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 (²LMCT) state was established based on Arrhenius analysis, indicating the direct deactivation of the ²LMCT 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]PF₆ to perform photocatalytic bimolecular reactions.