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

Abstract

The synthesis of the novel ligand L2, based on a 2,2′-bipyridine functionalized by a carboxylate function and a methyleneiminodiacetate group in positions 6 and 6′, is described. Its europium complex, [EuL2(H₂O)₃], has been prepared and characterized. The spectroscopic properties of [EuL2(H₂O)₃] were studied by means of absorption, and both steady-state and time-resolved luminescence spectroscopy. Although L2 displays a very good sensitization efficiency (η_sens. = 89%), the overall luminescence quantum yield of the complex is rather poor (2.6%). This results from strong non-radiative deactivations due to the presence of three water molecules in the first coordination sphere, as evidenced by luminescence lifetime measurements in H₂O and D₂O. The relaxation properties of the Gd³⁺ complexes obtained with ligands L2 and L1, the latter containing an additional pyridine ring in the aromatic core, were assessed by means of ¹⁷O NMR and nuclear magnetic relaxation dispersion (NMRD). The calculated water exchange rates for both complexes are faster than those of currently used contrast agents (k_ex²⁹⁸ = 14.0 ± 1.5 × 10⁶ and 11.1 ± 1.1 ×10⁶ s⁻¹ for [GdL1(H₂O)₂] and [GdL2(H₂O)₃], respectively). The rotational correlation time calculated for [GdL1(H₂O)₂] appeared to be long (110 ± 16 ps vs 65 ± 5ps for [GdL2(H₂O)₃]), pointing to a hindered rotation due to the larger aromatic frame. Finally, the interaction of the two Gd complexes with hydrogencarbonate and phosphate anions was studied by relaxivity measurements, showing that both anions are able to compete with water molecules as ligands for Gd³⁺, with HCO₃⁻ showing a better affinity.