Highly luminescent, visible-emitting lanthanide macrocyclic chelates stable in water and derived from the cyclen framework

Two new tetraazamacrocyclic ligands are designed with the aim of sensitizing the luminescence of Tb(III) and Eu(III) ions in water: L5 {1,4,7,10-tetrakis[N-(phenacyl)carbamoylmethyl]- 1,4,7,10-tetraazacyclododecane} and L6 {1,4,7,10-tetrakis[N-(4-phenylphenacyl)carbamoylmethyl]- 1,4,7,10-tetraazacyclododecane}. These ligands react with lanthanide trifluoromethanesulfonates to yield stable 1:1 complexes in water (log K = 12.89 ± 0.15 for EuL5). X-ray diffraction on [Tb(L5)(H₂O)](CF₃SO₃)₃ (P1̄, a = 13.308(3) Å, b = 14.338(3) Å, c = 16.130(3) Å α = 101.37(3)°, β = 96.16(3)°, γ = 98.60(3)°) shows the Tb(III) ion lying on a C₄ axis and being 9-coordinate, with one water molecule bound in its inner coordination sphere. The absolute quantum yields are determined in aerated water for the complexes formed with ions used in fluoroimmunoassays (Ln = Sm, Eu, Tb, and Dy). Large values are found for [Tb(H₂O)(L5)]³⁺ and [Eu(H₂O)(L6)]³⁺, in line with the molecular design of the receptors: 23.1% and 24.7%, respectively. The intense luminescence of these ions results from efficient intersystem crossing and L → Ln energy transfer processes, as well as from a suitable shielding of the emitting ions from radiationless deactivation.