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

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 Tb3+ complexes have been analyzed and the sensitization of the Tb3+ by the heterocyclic moiety demonstrated with modest quantum yields. The corresponding Gd3+ 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. Efficient routes to the synthesis of DO3 A-imidazo[2,1-b][1,3,4]thiadiazole complexes from readily accessible starting materials were investigated. The photophysical properties of the Tb3+ complexes have been determined. The corresponding Gd3+ complexes were also investigated and show a higher relaxivity than GdDOTA, without any aggregation process. image