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 (3)dd state as intermediate quencher state of the (3)MLCT 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 (3)dd state from the (3)MLCT 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 (3)dd state by ~4000 cm(-1) each, compared to [Ru(bpy)3](2+).