Neutral Ru^II-based emitting materials: A prototypical study on factors governing radiationless transition in phosphorescent metal complexes

In addition to the metal-centered dd transition that is widely accepted as a dominant radiationless decay channel, other factors may also play important roles in governing the loss of phosphorescence efficiency for heavy-transition-metal complexes. To conduct our investigation, we synthesized two dicarbonylruthenium complexes with formulas [Ru(CO)₂(BQ) ₂] (1) and [Ru(CO)₂(DBQ)₂] (2), for which the cyclometalated ligands BQ and DBQ denote benzo-[h]quinoline and dibenzo[f,h]quinoxaline, respectively. Replacing one CO ligand with a P donor ligand such as PPh₂Me and PPhMe₂ caused one cyclometalated ligand to undergo a 180° rotation around the central metal atom, giving highly luminous metal complexes [Ru(CO)L(BQ)₂] and [Ru(CO)L(DBQ) ₂], where L = PPh₂Me and PPhMe₂ (3-6), with emission peaks λ_max in the range of 571-656 nm measured in the fluid state at room temperature. It is notable that the S₀-T ₁ energy gap for both 1 and 2 is much higher than that of 3-6, but the corresponding phosphorescent spectral intensity is much weaker. Using these cyclometalated Ru metal complexes as a prototype, our experimental results and theoretical analysis draw attention to the fact that, for complexes 1 and 2, the weaker spin-orbit coupling present within these molecules reduces the T ₁-S₀ interaction, from which the thermally activated radiationless deactivation may take place. This, in combination with the much smaller ³MLCT contribution than that observed in 3-6, rationalizes the lack of room-temperature emission for complexes 1 and 2.