Low-Temperature Spectra and Density Functional Theory Modeling of Ru(II)-Bipyridine Complexes with Cyclometalated Ancillary Ligands: The Excited State Spin-Orbit Coupling Origin of Variations in Emission Efficiencies

The 77 K emission spectra of cyclometalated ruthenium(II)-2,2'-bipyridine (CM-Ru-bpy) chromophores are very similar to those of related Ru-bpy complexes with am(m)ine or diimmine ancillary ligands, and density functional theory (DFT) modeling confirms that the lowest energy triplet metal to ligand charge transfer ((MLCT)-M-3) excited states of CM-Ru-bpy and related Ru-bpy complexes have very similar electronic configurations. However, the phosphorescence decay efficiencies of CM-Ru-bpy excited states are about twice those of the conventional Ru-bpy analogues. In contrast to the similar (MLCT)-M-3 excited state electronic configurations of the two classes of complexes, the CM-Ru-bpy chromophores have much broader visible region MLCT absorptions resulting from several overlapping transitions, even at 87 K. The emitting excited-state emission efficiencies depend on spin-orbit coupling (SOC) mediated intensity stealing from singlet excited states, and this work explores the relationship between the phosphorescence efficiency and visible region absorption spectra of Ru-bpy (MLCT)-M-3 excited states in the weak SOC limit. The intrinsic (MLCT)-M-3 emission efficiency, iota(em), depends on mixing with singlet excited states whose Ru-III-d pi-orbital angular momenta differ from that of the emitting state. DFT modeling of the (MLCT)-M-1 excited-state electronic configurations that contribute significantly to the lowest energy absorption bands have Ru-III-d pi orbitals that differ from those of their emitting (MLCT)-M-3 excited states. This leads to a very close relationship between iota(em) and the lowest energy MLCT band absorptivities in Ru-bpy chromophores. Thus, the larger number of (MLCT)-M-1 transitions that contribute to the lowest energy absorption bands accounts for the enhanced phosphorescence efficiency of Ru-bpy complexes with cyclometalated ancillary ligands.