Fabiana R. Gonçalves e Silva, Oscar L. Malta, Christine Reinhard, Hans-Ulrich Güdel, Claude Piguet, Jacques E. Moser and Jean-Claude G. Bünzli
Visible and Near-Infrared Luminescence of Lanthanide-Containing Dimetallic Triple-Stranded Helicates: Energy Transfer Mechanisms in the Sm^III and Yb^III Molecular Edifices
J. Phys. Chem. A 106, 1670-1677 (2002)                 

Abstract: The photophysical properties of the triple-stranded dimetallic helicates [Ln₂(L^C-2H)₃]·H₂O (Ln = Nd, Sm, Dy, Yb) are determined in water and D₂O 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 H₂O and D₂O, which compares favorably with experimental data. The main component of the ligand-to-metal energy transfer (97.5%) goes through a ³pp* ⁵G_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 ²F_5/2 (3E_1/2 + E_3/2) and ²F_7/2 (2E_1/2 + E_3/2) levels of Yb^III is consistent with D₃ symmetry.