Yb₂-Tb Upconversion in a Hetero-Trimetallic Molecular Lanthanide Complex

Photon Upconversion in molecular hetero-metallic lanthanide systems is challenged by the lack of chemical diversity displayed by the lanthanide ions. Here, we report the multi-photon photophysical properties of a series of molecular hetero-trimetallic lanthanide complexes Yb₂Ln (Ln = Eu³⁺, Gd³⁺, Tb³⁺) assembled from kinetically inert building blocks providing site-specific chemical control regarding introduction of differing lanthanide ions. The hetero-trimetallic complex Yb₂Tb shows efficient Yb₂ → Tb photon upconversion via cooperative sensitization in both D₂O and H₂O. By contrast, Yb₂Eu does not show Yb₂ → Eu upconversion, while Yb₂Gd has been used as a spectroscopic blank. We find that the Yb₂ → Tb energy transfer appears to be independent of OH quenching from the solvent. Additionally, we report the intermetallic distances in the complex using density functional theory and molecular dynamics simulations. We find that the Yb₂ → Tb cooperative sensitization upconversion energy transfer remains effective despite relatively long intermetallic distances between donor pairs (13.5-25 Å) and between the Yb donors and the Tb acceptor (11.5-13.5 Å).