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)(H(2)O)](CF(3)SO(3))(3) (P1 macro, a = 13.308(3) A, b = 14.338(3) A, c = 16.130(3) A, alpha = 101.37(3) degrees, beta = 96.16(3) degrees, gamma = 98.60(3) degrees ) shows the Tb(III) ion lying on a C(4) 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(H(2)O)(L5)](3+) and [Eu(H(2)O)(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.