So-called “dual emission” for 3MLCT luminescence in ruthenium complex ions: What is really happening?

Dual emission is a reality in many, but not all, ruthenium complex ions, even at room temperature and in fluid solution. It requires significantly different ligands and is more obvious in rigid media such as glasses at low temperature and crystalline powders. However, there are not just two unique lifetimes. Rather, there is a continuum of similar but slightly different lifetimes drawn from a bimodal distribution. In some of the cases that do not show dual emission, there still seems to be a continuum within a unimodal distribution. After reviewing a wide range of data, we describe possible interpretations. Most novel, but in our view most attractive, is a major role for ion pairing. If one admits that ions near charge transfer orbitals should have a stabilizing effect, and if one doubts that there would be any unique configuration for ion pairs, then it would seem that a continuous variation of excited state behavior is mandatory. This mechanism accounts for the fact that highly asymmetric substitutions at the 4-position of the phen ligand that most affects charge transfer always show dual emission and offer the most exaggerated differences between the two emissions. Other possibilities remain, given the data available at this time.