Relaxation and luminescence studies on hydrated bipyridyl- andterpyridyl-based lanthanide complexes

摘要

The synthesis of the novel ligand L2, based on a 2,2'-bipyridine functionalized by a carboxylatefunction and a methyleneiminodiacetate group in positions 6 and 6', is described. Its europiumcomplex, [EuL2(H_2O)_3], has been prepared and characterized. The spectroscopic properties of[EuL2(H_2O)_3] were studied by means of absorption, and both steady-state and time-resolvedluminescence spectroscopy. Although L2 displays a very good sensitization efficiency (η_(sens)=89%), theoverall luminescence quantum yield of the complex is rather poor (2.6%). This results from strongnon-radiative deactivations due to the presence of three water molecules in the first coordinationsphere, as evidenced by luminescence lifetime measurements in H_2Oand D_2O. The relaxation propertiesof the Gd" complexes obtained with ligands L2 and L1, the latter containing an additional pyridinering in the aromatic core, were assessed by means of ~(17)ONMR and nuclear magnetic relaxationdispersion (NMRD). The calculated water exchange rates for both complexes are faster than those ofcurrently used contrast agents (k_(ex)~(298)= 14.0 ± 1.5 × 10~6and 11.1 ± 1.1 x10~6s~(-1) for [GdL1(H_2O)_2] and[GdL2(H_2O)_3], respectively). The rotational correlation time calculated for [GdL1 (H_2O)_2] appeared tobe long (110 ± 16 ps vs 65 ± 5ps for [GdL2(H_2O)_3]), pointing to a hindered rotation due to the largeraromatic frame. Finally, the interaction of the two Gd complexes with hydrogencarbonate andphosphate anions was studied by relaxivity measurements, showing that both anions are able tocompete with water molecules as ligands for Gd~(3+), with HCO_3~- showing a better affinity.