Experimental Evidence of Ultrafast Quenching of the ³MLCT Luminescence in Ruthenium(II) Tris-bipyridyl Complexes via a ³dd State

Ultrafast transient absorption spectroscopy serves to identify the 3dd state as intermediate quencher state of the 3MLCT luminescence in the non-luminescent ruthenium complexes [Ru(m-bpy)3]2+ (m-bpy = 6-methyl-2,2′-bipyridine) and [Ru(tm-bpy)3]2+ (tm-bpy = 4,4′,6,6′-tetramethyl-2′,2′-bipyridine). For [Ru(m-bpy)3]2+, the population of the 3dd state from the 3MLCT state occurs within 1.6 ps, while the return to the ground state takes 450 ps. For [Ru(tm-bpy)3]2+, the corresponding values are 0.16 and 7.5 ps, respectively. According to DFT calculations, methyl groups added in the 6 and 6′ positions of bipyridine stabilize the 3dd state by ∼4000 cm–1 each, compared to [Ru(bpy)3]2+