# Yb2‐Tb Upconversion in a Hetero‐Trimetallic Molecular Lanthanide Complex

## TL;DR

Researchers developed a lanthanide complex that efficiently converts photons in water, even with long distances between metal ions.

## Contribution

A new hetero-trimetallic lanthanide complex enables efficient photon upconversion with site-specific control and long intermetallic distances.

## Key findings

- The Yb2Tb complex shows efficient photon upconversion in both D2O and H2O.
- Yb2→ Tb energy transfer is not affected by OH quenching from the solvent.
- Upconversion remains effective despite long intermetallic distances (11.5–25 Å).

## Abstract

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).

Lanthanides are at the forefront of photon upconversion in molecular systems; however, the chemical nature of the lanthanides makes site‐specific coordination chemistry difficult to achieve. Here, we employ kinetically stabile building blocks to achieve hetero‐trimetallic 4f complexes with complete site‐specific chemical control. We observe efficient Yb2
→ Tb upconversion in aqueous solution despite relatively long intermetallic distances.