Near-IR Charge-Transfer Emission at 77 K and Density Functional Theory Modeling of Ruthenium(II)-Dipyrrinato Chromophores: High Phosphorescence Efficiency of the Emitting State Related to Spin-Orbit Coupling Mediation of Intensity from Numerous Low-Energy Singlet Excited States.

Efficient charge-transfer (CT) phosphorescence in the near-IR (NIR) spectral region is reported for four substituted Ru-(R-dipyrrinato) complexes, [Ru(bpy)(R-dipy)](PF), where bpy is 2,2'-bipyridine and the substituent R is phenyl (ph), 2,4,6-trimethylphenyl, 4-carboxyphenyl (HOOC-ph), or 4-pyridinyl. The experimentally determined phosphorescence efficiency, ι = /(ν) (where and ν are the phosphorescence rate constant and the phosphorescence frequency, respectively), of the [Ru(bpy)(R-dipy)] complexes was approximately double that of [Ru(bpy)(Am)] complexes (Am = ammine ligand) in the NIR region. Density functional theory (DFT) modeling indicated two strikingly different electronic configurations of the triplet emitting state (T) in the two types of complexes.