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The title compounds were synthesized and studied by solution and single-crystal absorption, luminescence, and excitation spectroscopy. The f-f luminescence is induced in the Tm(3+) and Yb(3+) complexes in solution by exciting into the (1)Pi-(1)Pi absorptions of the ligand in the UV. A single-configurational coordinate model is proposed to rationalize the nonradiative relaxation step from ligand-centered to metal-centered excited states in [Yb(dpa)(3)](3-) (dpa = 2,6-pyridinedicarboxylate). Direct f-f excitation is used in crystals of Na(3)[Tm(dpa)(3)].13H(2)O and Na(3)[Yb(dpa)(3)].13H(2)O to induce f-f luminescence. From low-temperature, high-resolution absorption, luminescence, and excitation spectra, the ligand-field splittings in the relevant states can be determined. It was impossible to induce NIR to VIS upconversion in any of the complexes. This is mainly due to the fact that nonradiative relaxation among the f-f excited states is highly competitive, even in [Yb(dpa)(3)](3-) with an energy gap between (2)F(5/2) and (2)F(7/2) of about 10000 cm(-1). It can be rationalized on the basis of an adapted energy gap law. No luminescence at all could be detected in Na(3)[Er(dpa)(3)].13H(2)O.

The photophysical properties of the triple-stranded dimetallic helicates [Ln 2 (L C -2H) 3 ]‚H 2 O (Ln ) Nd, Sm, Dy, Yb) are determined in water and D 2 O solutions, and energy transfer processes are modeled for Sm III . The luminescence of Nd III , Sm III , and Yb III is sensitized by (L C -2H) 2-, but the energy transfer from the ligand to the Ln III ions is not complete, resulting in residual ligand emission. The luminescence of the Nd III helicate is very weak due to nonradiative de-excitation processes. On the other hand, the Yb III and Sm III helicates exhibit fair quantum yields, 1.8% and 1.1% in deuterated water, respectively. The energy transfer rates between (L C -2H) 2-and Sm III levels are calculated by direct and exchange Coulomb interaction models. This theoretical modeling coupled to numerical solutions of the rate equations leads to an estimate of the emission quantum yields in H 2 O and D 2 O, which compares favorably with experimental data. The main component of the ligandto-metal energy transfer (97.5%) goes through a 3 ππ* f 5 G 5/2 (1) path, and the operative mechanism is of the exchange type. For the Yb III helicate, minor effects of oxygen on the sensitization of Yb III and nanosecond time-resolved spectroscopy point to the energy transfer mechanism being consistent with a recently proposed pathway involving fast electron transfer and Yb II . No up-conversion process could be identified. Ligand-field splitting of the 2 F 5/2 (3E 1/2 + E 3/2 ) and 2 F 7/2 (2E 1/2 + E 3/2 ) levels of Yb III is consistent with D 3 symmetry.