Intramolecular sensitisation of europium(III) luminescence by 8-benzyloxyquinoline in aqueous solution

Abstract

Two new tetraazamacrocylic ligands have been prepared, in which one of the four nitrogens bears an 8-benzyloxyquinoline group, bound either via a simple methylene unit (CH 2) (ligand 1), or through a longer, amide linker (CH 2C(O)N(Me)CH 2) (ligand 2), in both cases at the 2-position of the chromophore. The synthesis of ligand 1 involved a reductive amination reaction of the free macrocycle with 8-benzyloxyquinoline-2-carboxaldehyde, whilst ligand 2 was prepared by a more conventional alkylation pathway. The other three nitrogens of the macrocycle are functionalised with acetate donors, leading to a DO3A-type ligand suitable for complexation of lanthanide ions. The europium(III) complexes of the two ligands have been prepared. Both are luminescent in aqueous solution, displaying europium-based emission upon excitation into the UV absorption bands of the chromophore. From the luminescence lifetimes measured in H 2O and D 2O, it is concluded that, whilst [Eu 2] has the one expected water molecule in the inner-sphere of the metal ion, [Eu 1] has no metal-bound water molecules. This is attributed to the coordination of the quinoline nitrogen to the metal ion, forcing the benzyloxy group into the space normally occupied by the axial water molecule. However, further analysis shows that the greatly superior quantum yield of [Eu 1] over [Eu 2] is due to the much higher efficiency of energy transfer in the former, and not to a reduction in non-radiative decay of the excited state; in fact, overall non-radiative deactivation is greater in [Eu 1] than in [Eu 2].