Synthesis, Luminescence and Relaxometric Properties of Macrocyclic Lanthanide Complexes Appended with an Imidazothiadiazole, a Heterocyclic Bioactive Scaffold

Abstract The syntheses of two macrocycles containing original imidazothiadiazole heterocycles are presented. The protonation constants of the ligands have been determined by potentiometry and photophysical measurements. The photophysical properties of the corresponding Tb 3+ complexes have been analyzed and the sensitization of the Tb 3+ by the heterocyclic moiety demonstrated with modest quantum yields. The corresponding Gd 3+ complexes are monohydrated and display higher relaxivities than GdDOTA, which could be explained by the larger size of these complexes. The different substitutions on the heterocycle do not affect strongly the protonation constants of the ligand, the quantum yield, water exchange rate or relaxation properties of the system.