Visible and near-infrared luminescence of lanthanide-containing dimetallic triple-stranded helicates:: Energy transfer mechanisms in the SmIII and YbIII molecular edifices

The photophysical properties of the triple-stranded dimetallic helicates [Ln(2)(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, SMIII, 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 WIT 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 3pipi* --> (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 F-2(5/2) (3E(1/2) + E-3/2) and F-2(7/2) (2E(1/2) + E-3/2) levels of Yb-III is consistent with D-3 symmetry.