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 doddering lanthanide ions. The hetero-trimetallic complex Yb2Tb shows efficient Yb2 → Tb photon upconversion via cooperative sensitisation 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 is 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 sensitisation upconversion energy transfer remains effective despite relatively long intermetallic distances between donor pairs (13.5-25 Å) and between the Yb2 donor and the Tb acceptor (11.5-13.5 Å).