Polymetallic Lanthanide Complexes with PAMAM-Naphthalimide Dendritic Ligands:  Luminescent Lanthanide Complexes Formed in Solution

Generation 3 PAMAM dendrimers functionalized with 2,3-naphthalimide chromophoric groups on the end branches were synthesized, and the formation of Eu³⁺ polymetallic complexes was investigated. The luminescence properties of these complexes upon binding were fully characterized. On addition of Eu³⁺ to the dendrimer solution, lanthanide luminescence appears. The formation of a luminescent species corresponding to a dendrimer:lanthanide ratio of 1:8 was determined by luminescence batch titration and indicated by the maximum of Eu³⁺ emission. This indicates an overall average coordination number of 7.5 around each lanthanide metal cation. This is the first report of such characterization in the literature. Luminescence lifetimes indicate that the metal cation is well protected from nonradiative deactivation by the dendritic structure. Despite the limited efficiency of the sensitization of Eu³⁺, the absolute quantum yield being 0.0006, the good protection of the eight lanthanide cations bound in the dendrimer structure and the high absorptivity leads to the red emission from Eu³⁺ that is easily observed in solution under irradiation with 354 nm UV light.