Polyanionic Polydentate Europium Complexes as Ultrabright One- or Two-photon Bioprobes

A family of europium (III) complexes based on a polydentate ligand functionalized by charge-transfer antennae presents remarkable one- and two-photon photophysical proper-ties in water or buffer. A detailed analysis of their emission properties suggests that the wrapping of the ligand around the central rare-earth ion results in an overall Cs symmetry in agreement with the theoretical simulation and that about 65-70 % of the emission intensity is concentrated in the hypersensitive ⁵ D₀ →⁷ F₂ transition at 615 nm. Their brightness is excellent, in the range of the best lanthanide bioprobes making them very attractive for bio-imaging experiments.