Probing the electronic structures of coordination compounds by transient spectral hole-burning. Applications to specifically deuterated [Ru(bpy)(3)](2+) complexes

Transient spectral hole-burning (THB), a powerful technique for probing the electronic structures of coordination compounds,is applied to the lowest e xcited (MLCT)-M-3 states of specifically deuterated [Ru(bpy)(3)](2+) comple xes doped into crystals of racemic [Zn(bpy)(3)](ClO4)(2). Results are consi stent with and complementary to conclusions reached from excitation-line-na rrowing experiments. Two sets of (MLCT)-M-3 transitions are observed in con ventional spectroscopy of [Ru(bpy-d(n))(3-x)(bpy-d(m))(x)](2+) (x = 1, 2; n = 0, 2; m = 2, 8; n not equal m) complexes doped into [Zn(bpy)(3)](ClO4)(2 ). The two sets coincide with the (MLCT)-M-3 transitions observed for the h omoleptic [Ru(bpy-d(m))(3)](2+) and [Ru(bpy-d(n))(3)](2+) complexes and can thus be assigned to localized (MLCT)-M-3 transitions to the bpy-d(m) and b py-d(n) ligands. The THB experiments presented in this paper exclude a two- site hypothesis. When spectral holes are burnt at 1.8 K into (MLCT)-M-3 tra nsitions associated with the bpy and bpy-d(2) ligands in [Ru(bpy)(bpy-d(8)) (2)](2+), [Ru(bpy)(2)(bpy-d(8))](2+), and [Ru(bpy-d(2))(2)(bpy-d(8))](2+), side holes appear in the (MLCT)-M-3 transitions associated with the bpy-d(8 ) ligands approximate to 40 and approximate to 30 cm(-1) higher in energy. Since energy transfer to sites 40 or 30 cm(-1) higher in energy cannot occu r at 1.8 K, the experiments unequivocally establish that the two sets of (M LCT)-M-3 transitions observed for [Ru(bpy-d(n))(3-x)(bpy-d(m))(x)](2+) (x = 1, 2) complexes in [Zn(bpy)(3)](ClO4)(2) occur on one molecular cation.