Cooperative Luminescence and Cooperative Sensitisation Upconversion of Lanthanide Complexes in Solution

Upconversion materials have led to various breakthrough applications in solar energy conversion, imaging, and biomedicine. One key impediment is the facilitation of such processes at the molecular scale in solution where quenching effects are much more pronounced. In this work, molecular solution‐state cooperative luminescence (CL) upconversion arising from a Yb excited state is explored and the mechanistic origin behind cooperative sensitisation (CS) upconversion in Yb/Tb systems is investigated. Counterintuitively, the best UC performances were obtained for Yb/Tb ratios close to parity, resulting in the brightest molecular upconversion complexes with a quantum yield of 2.8×10−6 at a low laser power density of 2.86 W cm−2. Cooperative luminescence and cooperative sensitization are observed for the first time in solution at the molecular scale with nona‐nuclear clusters containing nine Yb atoms or a mixture of Yb and Tb ones. The cooperative sensitization upconversion processes in the mixed clusters were systematically investigated, showing the best performances for equimolar amounts of Yb and Tb in the clusters and using deuterated solvents and ligands.