Competing dynamics of intramolecular deactivation and bimolecular charge transfer processes in luminescent Fe() 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. Deactivation of an excited Fe III based dye has been investigated. Intramolecular deactivation is comparable to organic dyes, and ultrafast intermolecular deactivation via photoinduced disproportionation is observed over a range of low temperatures.