Europium Complexes of Tris(dipicolinato) Derivatives Coupled to Methylumbelliferone: A Double Sensitization

A para-poly(oxyethylene) dipicolinic acid derivative coupled to methylumbelliferone was synthesized and used as a ligand for coordination with europium ions. Characterization of the complex by mass spectrometry and spectrophotometry was performed in aqueous solution and points to the formation of the 1:3 Eu/L complex. Physicochemical properties are close to those of already reported dipicolinato complexes, confirming that the presence of a coumarin chromophore does not interfere with complex formation. Photophysical measurements result in observed lifetimes of 1.4 ms and intrinsic quantum yields, Phi(Eu)(Eu), ranging from 33 to 50% (depending on the solvent used). Selective excitation can be performed either mostly on the dipicolinate (DPA) backbone (at 280 nm) or exclusively on the coupled coumarin chromophore in the near visible range (320 nm). Despite the spatial distance between the antenna and the lanthanide ion, europium emission was observed upon 320 nm excitation together with the coumarin emission. The different sensitization pathways have been explored and rationalized. Finally, the complex has been used to probe the ratio of a binary mixture of solvent, establishing the potential interest of this new class of ligands in many fields of research, from photophysics to applied chemistry.