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) 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.