Upconversion in Molecular Hetero-Nonanuclear Lanthanide Clusters in Solution

Up until now restricted to solid state chemistry and nanoparticles, upconversion (UC) phenomena by which low energy photons are piled up to engender higher energy emission, are progressively entering the field of molecular probes with a handful of molecular/supramolecular discrete complexes. Here we show that nonanuclear lanthanide complexes respresent a new class of solution state UC materials, and are straightforwardly crystalized from LnCl₃.6H₂O salts, triethylamine and acetylacetone, retaining their structural integrity in solution. For a composition of one Tb per eight Yb the nonanuclear clusters display a very efficient upconversion phenomenon with Tb luminescence in the visible region upon 980 nm NIR excitation of Yb. An unprecedented value of 1.0×10^-7 was obtained for the UC efficiency at only 2.86 W/cm², demonstrating these new molecular clusters to be up to 26 times more efficient than the best current molecular systems, the UC being observed down to a concentration of 10 nM.