Investigation of Ln 3+ complexation by a DOTA derivative substituted by an imidazothiadiazole: synthesis, solution structure, luminescence and relaxation properties.

We investigated the coordination properties of original macrocyclic Ln 3+ complexes comprising an imidazothiadiazole heterocycle. The thermodynamic stability of the Gd 3+ complex was determined by a combination of potentiometric and photophysical measurements. The kinetic inertness was assessed in highly acidic media. The solution structure of the Ln 3+ complex was unambiguously determined by a set of photophysical measurements and 1 H, 13 C, 89 Y NMR data in combination with DFT calculations, which proved coordination of the heterocycle to Ln 3+ . The ability of the imidazothiadiazole moiety to sensitize Tb 3+ luminescence was investigated. Finally, the relaxation properties were investigated by recording 1 H nuclear magnetic relaxation dispersion (NMRD) profiles and 17 O measurements. The water exchange rate is similar to that of GdDOTA as the less negative charge of the ligand is compensated for by the presence of a bulky heterocycle. Relaxivity is constant over a large range of pH values, demonstrating the favorable properties of the complex for imaging purposes.