Isotope and temperature dependence of dual emission in a mononuclear ruthenium(II) polypyridyl compound

Detailed photophysical measurements on a series of [Ru(bpy)₂(L)]ⁿ⁺ complexes, which are deuterated to varying degrees are reported, where bpy is 2,2′-bipyridyl and L is 3-(pyrazin-2-yl)-1,2,4- triazole (Hpztr) or 3-(pyridin-2-yl)-1,2,4-triazole (Hpytr). For the pyridinyltriazole complexes, deuteration of bpy leads to a doubling of the emission lifetime, while deuteration of the triazole ligand has no effect on the emission lifetime. For the complex containing the protonated pyrazinyltriazole ligand, deuteration of bpy does not affect the emission lifetime, but the lifetime is strongly affected by the deuteration of the triazole-containing ligand. These observations indicate that in the pyridinyltriazole complexes the emitting state is bpy based, while for the complex containing the protonated pyrazinyltriazole ligand the emission is pyrazine based. For the compound containing the deprotonated pyrazinyltriazole ligand a remarkable insensitivity of the emission lifetime to deuteration is observed. Time-resolved resonance Raman data clearly indicate that the emitting triplet state is bpy based at room temperature when the triazole is deprotonated and switches to being pyrazine based on protonation. Variable temperature emission studies on the deprotonated complex reveal a remarkable dual emission between 120 and 200 K. The two emitting states exhibit different lifetime dependences on temperature. It is proposed that the presence of these two, weakly coupled emitting states, one bpy and one pyrazine based, is responsible for the relative insensitivity of the emission lifetime of the deprotonated pyrazinyltriazole complex to deuteration.