[Misa]

What makes [Ru(bpy)3]2+ better than [Co(bpy)3]2+?

[Corribus]

Luminescence derives from an excited metal-to-ligand charge transfer (MLCT) state. The degree of luminescence in metal pyridyl complexes is usually related to the energetic relationship of the luminescent state (MLCT) and low-lying metal localized d-d transitions, which rapidly deactivate the excited state. In the ruthenium bipyridyl complex, these metal centered states are too far above the MLCT to be thermally activated, and the complex is luminescent. In the iron analog (and I assume the cobalt analog, which I know little about), the partially filled metal centered states rapidly quench the luminescence. A neater comparison is the associated Ru(tpy)₂²⁺ complex. In this one, the poor tpy bite angles weaken the ruthenium crystal field, which lowers the energy of the metal centered states. Population of these states is thermally possible at room temperature, leading to quenched luminescence - but at low temperature, thermal population is not possible, and so the luminescence becomes strong again.