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 excited 3MLCT states of specifically deuterated Ru(bpy)3]2+ complexes doped into crystals of racemic Zn(bpy)3](ClO4)2. Results are consistent with and complementary to conclusions reached from excitation-line-narrowing experiments. Two sets of 3MLCT transitions are observed in conventional spectroscopy of Ru(bpy-d(n))(3-x)(bpy-d(m))x]2+ (x = 1, 2; n = 0, 2; m = 2, 8; n not = m) complexes doped into Zn(bpy)3](ClO4)2. The two sets coincide with the 3MLCT transitions observed for the homoleptic Ru(bpy-d(m))3]2+ and Ru(bpy-d(n))3]2+ complexes and can thus be assigned to localized 3MLCT transitions to the bpy-d(m) and bpy-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 3MLCT transitions associated with the bpy and bpy-d2 ligands in Ru(bpy)(bpy-d8)2]2+, Ru(bpy)2(bpy-d8)]2+, and Ru(bpy-d2)2(bpy-d8)]2+, side holes appear in the 3MLCT transitions associated with the bpy-d8 ligands approximately 40 and approximately 30 cm(-1) higher in energy. Since energy transfer to sites 40 or 30 cm(-1) higher in energy cannot occur at 1.8 K, the experiments unequivocally establish that the two sets of 3MLCT 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.