Upconversion in Molecular Hetero-Nonanuclear Lanthanide Clusters in Solution

Upuntil now restricted to solid state chemistry and nanoparticles, upconversion(UC) phenomena by which low energy photons are piled up to engender higherenergy emission, are progressively entering the field of molecular probes witha handful of molecular/supramolecular discrete complexes. Here we show thatnonanuclear lanthanide complexes respresent a new class of solution state UCmaterials, and are straightforwardly crystalized from LnCl3.6H2Osalts, triethylamine and acetylacetone, retaining their structural integrity insolution. For a composition of one Tb per eight Yb the nonanuclear clustersdisplay a very efficient upconversion phenomenon with Tb luminescence in thevisible region upon 980 nm NIR excitation of Yb. An unprecedented value of1.0×10-7 was obtained for the UC efficiency at only 2.86 W/cm2,demonstrating these new molecular clusters to be up to 26 times more efficientthan the best current molecular systems, the UC being observed down to aconcentration of 10 nM.