Systematic Tuning of the Optical Properties of Discrete Complexes of Eu^II in Solution Using Counterions and Solvents

We describe a systematic study of the influence of halides and solvents on the optical properties of Eu^II-containing complexes in solution starting from well-defined crystalline precursors. Anionic halides, chloride and bromide, blue-shift the spectroscopic properties of Eu^II, whereas neutral ligands, methanol and acetonitrile, cause a red shift. This system provides evidence that Eu^II has a stronger affinity for chloride, and to some extent bromide, relative to acetonitrile but not methanol. We also describe a simple procedure using an ion-exchange resin for the exchange of iodide counterions to hexafluorophosphate. These findings are a step toward designing ligands that can tune the optical properties of Eu^II-containing complexes for solution-based applications.