Visible and Near-Infrared Luminescence of Lanthanide-Containing Dimetallic Triple-Stranded Helicates:  Energy Transfer Mechanisms in the Sm

The photophysical properties of the triple-stranded dimetallic helicates [Ln2(LC−2H)3]·H2O (Ln = Nd, Sm, Dy, Yb) are determined in water and D2O solutions, and energy transfer processes are modeled for SmIII. The luminescence of NdIII, SmIII, and YbIII is sensitized by (LC−2H)2-, but the energy transfer from the ligand to the LnIII ions is not complete, resulting in residual ligand emission. The luminescence of the NdIII helicate is very weak due to nonradiative de-excitation processes. On the other hand, the YbIII and SmIII helicates exhibit fair quantum yields, 1.8% and 1.1% in deuterated water, respectively. The energy transfer rates between (LC−2H)2- and SmIII 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 3ππ* → 5G5/2(1) path, and the operative mechanism is of the exchange type. For the YbIII helicate, minor effects of oxygen on the sensitization of YbIII and nanosecond time-resolved spectroscopy point to the energy transfer mechanism being consistent with a recently proposed pathway involving fast electron transfer and YbII. No up-conversion process could be identified. Ligand-field splitting of the 2F5/2 (3E1/2 + E3/2) and 2F7/2 (2E1/2 + E3/2) levels of YbIII is consistent with D3 symmetry.