Elucidating Cooperative Luminescence and Cooperative Sensitisation Upconversion of Nonanuclear Lanthanide Complexes in Solution

Upconversion nanoparticles 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 we explore, for the first time, molecular solution-state cooperative luminescence (CL) upconversion arising from a Yb excited state and investigate the mechanistic origin behind cooperative sensitisation (CS) upconversion in Yb/ Tb systems. Counterintuitively, the best UC performances were obtained for Yb/Tb ratios close to parity, resulting in the brightest molecular upconversion complexes with an unprecedented quantum yield of 2.8 × 10-6 at a low laser power density of 2.86 W/cm2.