Abstract
A series of europium(III) and terbium(III) complexes of three 1,4,7‐triazacyclononane‐based pyridine containing ligands were synthesized. The three ligands differ from each other in the substitution of the pyridine pendant arm, namely they have a carboxylic acid, an ethylamide, or an ethyl ester substituent, i.e., these ligands are 6,6′,6″‐[1,4,7‐triazacyclononane‐1,4,7‐triyltris(methylene)]tris[pyridine‐2‐carboxylic acid] (H~3~tpatcn), ‐tris[pyridine‐2‐carboxamide] (tpatcnam), and ‐tris[pyridine‐2‐carboxylic acid] triethyl ester (tpatcnes) respectively. The quantum yields of both the europium(III) and terbium(III) emission, upon ligand excitation, were highly dependent upon ligand substitution, with a ca. 50‐fold decrease for the carboxamide derivative in comparison to the picolinic acid (=pyridine‐2‐carboxylic acid) based ligand. Detailed analysis of the radiative rate constants and the energy of the triplet states for the three ligand systems revealed a less efficient energy transfer for the carboxamide‐based systems. The stability of the three ligand systems in H~2~O was investigated. Although hydrolysis of the ethyl ester occurred in H~2~O for the Ln(tpatcnes)~3~ complexes, the tripositive Ln(tpatcnam)~3~ complexes and the neutral [Ln(tpatcn)] complexes showed high stability in H~2~O which makes them suitable for application in biological media. The [Tb(tpatcn)] complex formed easily in H~2~O and was thermodynamically stable at physiological pH (pTb 14.9), whereas the Ln(tpatcnam)~3~ complexes showed a very high kinetic stability in H~2~O, and once prepared in organic solvents, remained undissociated in H~2~O.