Improving visible light sensitization of luminescent europium complexes

The synthesis and characterization of the new ligands L-1 , L-2 and L-4 are described with the series of four europium complexes of formula [EuLn (TTA)(3)] in which TTA refers to 2-thenoyltrifluoroacetonate and L-n to tridentate ligands with nitrogen containing heterocyclic structure, such as a 2,6-bis(3-methyl-pyrazolyl)-4-(p-toluyl-ethynyl)-triazine for L-1 , or terpyridines functionalized at the 4' position by a phenyl-vinylene for L-2 , a p-dimethylamino-phenylene for L-3 , or a p-aminophenyl-ethynylene for L-4 . The spectroscopic properties of the ligands and of the complexes are studied by means of UV-Vis absorption spectroscopy, as well as steady-state and time-resolved luminescence spectroscopy. All complexes display europium centred luminescence upon ligand excitation. Careful examination of the excitation spectra revealed differences in the ligand based sensitization efficiencies. For complexes of L (1) and L (2) , excitation of europium is mainly achieved through the TTA moieties and the photo-physical studies on [EuL1 (TTA)(3)] evidenced a weaker coordination of the bispyrazolyltriazine tridentate ligand, resulting from a partial decomplexation upon dilution. Complexes of L-3 and L-4 display intense excitation through the tridentate units, which extend down to 460 nm in the visible region. In the case of L-3 , selective excitation reveals the presence of a ligand-centred emission band at 520 nm which is likely ascribed to a L-3 centred charge transfer state.