Fe(III) complexes with prolonged luminescence lifetimes via excited-state equilibration promoted by reversible intercomponent electron transfer.

Achieving long-lived luminescence in complexes of earth-abundant metals remains challenging because excited states in first-row transition-metal systems typically deactivate rapidly under ambient conditions. Strategies capable of prolonging emission lifetimes in such compounds are therefore of considerable interest. Here we show that iron(III) complexes incorporating pyrene-functionalized ligands display luminescence in fluid solution at room temperature with lifetimes up to 6.5 ns. Spectroscopic analysis indicates that excited-state equilibration occurs through reversible intramolecular electron transfer from the pyrene unit to the iron centre, generating a charge-separated state. Although the ligand-to-metal charge-transfer state can also undergo reversible energy transfer to nearby pyrene triplet states, intramolecular electron transfer dominates, leading to the formation of a pyrene + -iron(II) charge-separated state that acts as a long-lived excited-state reservoir. Equilibration involving this state produces biphasic emission from the iron centre. These findings identify reversible intercomponent electron transfer as a strategy for achieving prolonged luminescence lifetimes in complexes of earth-abundant metals.