Acetonitrile Ligand Photosubstitution in Ru(II) Complexes Directly from the 3MLCT State

The excited states of the series [Ru(tpy)(L)(CH3CN)]n+ (1–11) (n = 1, 2) containing bidentate ligands L with varying electron-donating ability were characterized through Arrhenius analysis of the temperature dependence of their excited-state lifetimes. Complexes 1–11 undergo photoinduced CH3CN dissociation upon 450 nm irradiation with ligand exchange quantum yields that increase with the energy barrier to populating a dissociative triplet ligand field (3LF) state from the lowest-energy triplet metal-to-ligand charge transfer (3MLCT) excited state. Combined with DFT calculations, the data indicate that ligand photodissociation in 1–11 occurs directly from the 3MLCT state instead of a 3LF state. This finding is in contrast to the generally accepted mechanism for ligand photodissociation in Ru(II) complexes and indicates that alternative pathways for photoinduced ligand dissociation are available. These results can widely impact design principles for applications that require ligand photodissociation, such as photochemotherapy and photocatalysis, as well as for those where photosubstitution is undesirable, such as solar energy conversion.