Abstract A series of seven emissive europium (III) and terbium (III) complexes was prepared, incorporating a 3-pyridyl-4-azaxanthone or 3-pyrazolyl-4-azaxanthone sensitising moiety within a polydentate macrocyclic ligand. High overall emission quantum yields in aqueous media are attenuated in the presence of protein or certain oxy anions due to displacement of the N, N′-chelated sensitiser. Nevertheless, these complexes are taken into cells and ...