Substantial luminescence enhancement in ternary europium complexes by coordination of different ionic ligands

We demonstrate in a general and comprehensive manner that a substantial enhancement of luminescence in europium complexes can be achieved by increasing ionic ligand diversity. For the complexes studied, the measured boosts in the quantum efficiency of luminescence obtained using this strategy ranged from a minimum of 100% to a maximum of 543%. We formalize this concept by means of mathematical inequalities which describe the fact that either the quantum efficiency η, or the radiative decay rate Arad, of a mixed ligand complex will be larger than the average of the same property for the respective same-ligand complexes. We further introduce the concept of structure hardening by ligands, by linking it quantitatively for the first time with the value of the non-radiative decay rate Anrad, and interpret it in terms of the lowest vibrational frequency of the complex. In light of all these concepts and results, we conclude that luminescence can be boosted in europium complexes by increasing ligand diversity, which, in turn, should be ideally obtained with the help of hardener ligands. This general and comprehensive strategy aims at increasing Arad while simultaneously decreasing Anrad.