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

[Image: see text] A series of π-aromatic-rich cyclometalated ruthenium(II)-(2,2′-bipyridine) complexes ([Ru(bpy)(2)(π(Ar)-CM)](+)) in which π(Ar)-CM is diphenylpyrazine or 1-phenylisoquinoline were prepared. The [Ru(bpy)(2)(π(Ar)-CM)](+) complexes had remarkably high phosphorescence rate constants, k(RAD(p)), and the intrinsic phosphorescence efficiencies (ι(em(p)) = k(RAD(p))/(ν(em(p)))(3)) of these complexes were found to be twice the magnitudes of simply constructed cyclometalated ruthenium(II) complexes ([Ru(bpy)(2)(sc-CM)](+)), where ν(em(p)) is the phosphorescence frequency and sc-CM is 2-phenylpyridine, benzo[h]quinoline, or 2-phenylpyrimidine. Density functional theory (DFT) modeling of the [Ru(bpy)(2)(CM)](+) complexes indicated numerous singlet metal-to-ligand charge transfers for (1)MLCT-(Ru-bpy) and (1)MLCT-(Ru-CM), excited states in the low-energy absorption band and (1)ππ*-(aromatic ligand) ((1)ππ*-L(Ar)) excited states in the high-energy band. DFT modeling of these complexes also indicated phosphorescence-emitting state (T(e)) configurations with primary MLCT-(Ru-bpy) characteristics. The variation in ι(em(p)) for the spin-forbidden T(e) ((3)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 (1)MLCT states and second-order intensity perturbation from the significant configuration between the low-energy (1)MLCT and high-energy intense (1)ππ*-L(Ar) states. In addition, the observation of unusually high ι(em(p)) magnitudes for these [Ru(bpy)(2)(π(Ar)-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)(2)(sc-CM)](+) ions.