Polyazamacrocycles based on a tetraaminoacetate moiety and a (poly)pyridine intracyclic unit: direct synthesis and application to the photosensitization of Eu(III) and Tb(III) ions in aqueous solutions

A series of five new 15-, 18- or 21-membered polyazamacrocycles (L₁-L₅) based on a pyridine, bipyridine or terpyridine unit and a triethylenetetraminetetraacetic acid (TTTA) skeleton is described. In ligands L₄ and L₅ the azaheterocycle contains an additional extracyclic functionality (ester group) suitable for covalently attachment to bioactive molecules. The synthetic procedure is based on the use of a linear tetra-N-alkylated tetramine synthon incorporating masked acetate arms and an efficient metal template ion effect, which controls the crucial macrocyclization step. In the case of L₁-L₃, the formation of lanthanide complexes with europium(III) and terbium(III) was investigated and the fluorescence characteristics of the complexes were established. In this series, the terbium(III) complex derived from the bipyridine ligand exhibits the highest lifetime and quantum yield values (τ=2.18 ms, Φ=26%).