Electrophosphorescent divalent osmium and ruthenium complexes: A density functional theory investigation of their electronic and spectroscopic properties

Structures of divalent osmium and ruthenium complexes of [M(bpy)3]2+, [M(bpy)2P2]2+ and [M(dpb)2P2]2+ (M=Ru, Os, bpy=2,2′-bipyridine, dpb=4,4′-diphenyl-2,2′-bipyridine, P2=cis-1,2-bis(diphenylphosphino)ethylene) in the ground and the lowest triplet states were fully optimized with density functional theory. The features of the absorption bands and phosphorescent emission from the lowest triplet state for the complexes have been described by time-dependent density functional theory. The [Os(bpy)3]2+ complex in methanol solution exhibits two strong absorption bands at ∼495 and ∼445 nm, which are well reproduced by the present calculations. There is one absorption peak observed at 440-460 nm for the divalent Ru polypyridyl complexes in methanol solution, which is close to the TDDFT calculated values of 415-440 nm for the complex of [Ru(bpy)3]2+. All evidences from the optimized structures, the calculated atomic charges and spin densities for the triplet complexes support the conclusion that phosphorescent emission in the divalent Os and Ru complexes originate from the triplet metal-to-ligand-charge-transfer state. This has been discussed in detail.