Probes of the Metal-to-Ligand Charge-Transfer Excited States in Ruthenium-Am(m)ine-Bipyridine Complexes: The Effects of NH/ND and CH/CD Isotopic Substitution on the 77 K Luminescence

Abstract

The effects of ligand perdeuteration on the metal-to-ligand charge-transfer (MLCT) excited-state emission properties at 77 K are described for several [Ru(L)4bpy]2+ complexes in which the emission process is nominally {RuIII,bpy−} → {RuII,bpy}. The perdeuteration of the 2,2‘-bipyridine (bpy) ligand is found to increase the zero-point energy differences between the ground states and MLCT excited states by amounts that vary from 0 ± 10 to 70 ± 10 cm-1 depending on the ligands L. This indicates that there are some vibrational modes with smaller force constants in the excited states than in the ground states for most of these complexes. These blue shifts increase approximately as the energy difference between the excited and ground states decreases, but they are otherwise not strongly correlated with the number of bipyridine ligands in the complex. Careful comparisons of the [Ru(L)4(d8-bpy)]2+ and [Ru(L)4(h8-bpy]2+ emission spectra are used to resolve the very weak vibronic contributions of the C−H stretching modes as the composite contributions of the corresponding vibrational reorganizational energies. The largest of these, 25 ± 10 cm-1, is found for the complexes with L = py or bpy/2 and smaller when L = NH3. Perdeuteration of the am(m)ine ligands (NH3, en, or [14]aneN4) has no significant effect on the zero-point energy difference, and the contributions of the NH stretching vibrational modes to the emission band shape are too weak to resolve. Ligand perdeuteration does increase the excited-state lifetimes by a factor that is roughly proportional to the excited-state−ground-state energy difference, even though the CH and NH vibrational reorganizational energies are too small for nuclear tunneling involving these modes to dominate the relaxation process. It is proposed that metal−ligand skeletal vibrational modes and configurational mixing between metal-centered, bpy-ligand-centered, and MLCT excited states are important in determining the zero-point energy differences, while a large number of different combinations of relatively low-frequency vibrational modes must contribute to the nonradiative relaxation of the MLCT excited states.