Comparative study of the luminescence properties and relative stability of a series of europium(III) complexes bearing one to four coordinated azaxanthone groups

The synthesis and spectral properties are reported of a set of four europium(III) complexes containing between one and four 3-azaxanthone sensitising chromophores. Each complex is based on a macrocyclic cyclen core. The emission and aqueous solution ¹H NMR spectral properties are compared, revealing information about the local complex symmetry on the experimental timescale. A C₂-symmetric mono-cationic complex shows an overall emission quantum yield of 10% in HEPES buffer and exhibits the best relative stability profile in the presence of a million-fold excess of added metal salts or EDTA, as indicated by the time-dependence of overall europium emission, following indirect excitation of the 3-azaxanthone chromophore.