High Intrinsic Phosphorescence Efficiency and Density Functional Theory Modeling of Ru(II)-Bipyridine Complexes with π-Aromatic-Rich Cyclometalated Ligands: Attributions of Spin-Orbit Coupling Perturbation and Efficient Configurational Mixing of Singlet Excited States.

A series of π-aromatic-rich cyclometalated ruthenium(II)-(2,2'-bipyridine) complexes ([Ru(bpy)(π-CM)]) in which π-CM is diphenylpyrazine or 1-phenylisoquinoline were prepared. The [Ru(bpy)(π-CM)] complexes had remarkably high phosphorescence rate constants, , and the intrinsic phosphorescence efficiencies (ι = /(ν)) of these complexes were found to be twice the magnitudes of simply constructed cyclometalated ruthenium(II) complexes ([Ru(bpy)(sc-CM)]), where ν is the phosphorescence frequency and sc-CM is 2-phenylpyridine, benzo[]quinoline, or 2-phenylpyrimidine. Density functional theory (DFT) modeling of the [Ru(bpy)(CM)] complexes indicated numerous singlet metal-to-ligand charge transfers for MLCT-(Ru-bpy) and MLCT-(Ru-CM), excited states in the low-energy absorption band and ππ*-(aromatic ligand) (ππ*-L) excited states in the high-energy band. DFT modeling of these complexes also indicated phosphorescence-emitting state (T) configurations with primary MLCT-(Ru-bpy) characteristics. The variation in ι for the spin-forbidden T (MLCT-(Ru-bpy)) excited state of the complex system that was examined in this study can be understood through the spin-orbit coupling (SOC)-mediated sum of intensity stealing (∑SOCM-IS) contribution from the primary intensity of the low-energy MLCT states and second-order intensity perturbation from the significant configuration between the low-energy MLCT and high-energy intense ππ*-L states. In addition, the observation of unusually high ι magnitudes for these [Ru(bpy)(π-CM)] complexes can be attributed to the values for both intensity factors in the ∑SOCM-IS formalism being individually greater than those for [Ru(bpy)(sc-CM)] ions.