Synthesis of meta and para-substituted aromatic sulfonate derivatives of polydentate phenylazaphosphinate ligands: enhancement of the water solubility of emissive europium(III) EuroTracker® dyes

The synthesis is described of a series of meta and para-substituted phenylsulfonate derivatives of very bright Eu(III) complexes with arylphosphinate groups and strongly absorbing arylalkynylpyridine moieties. The synthetic route involved the early introduction of trifluoroethyl esters to protect the sulfonic acid group, withstanding the use of reagents including acetyl bromide and mCPBA, and tolerating acid-catalysed esterification and Sonogashira reaction conditions. The Eu(III) complexes exhibit enhanced water solubility; their photophysical properties are not perturbed significantly by introduction of the anionic sulfonate groups.