Self-assembled multidyes-sensitized erbium single molecules for boosting energy transfer light-upconversion in solution.

Efficient near-infrared (NIR) to visible (VIS) light-upconversion should combine large absorption coefficients NIR with massive quantum yields UC so that the overall brightness BUC = NIRUC is maximum. Relying on linear optics, several photons are collected by strongly absorbing dyes, stored on long-lived intermediate excited states and finally piled up using mechanisms of simple or double operator natures. The miniaturization to implement detectable linear light-upconversion in a single molecule is challenging because of the existence of the thermal vibrational bath which increases non-radiative relaxation and limits quantum yields to 10−9 ≤ UC ≤ 10−6. An acceptable brightness thus requires the connection of a maximum of cationic cyanine dyes around trivalent lanthanide luminophores. Taking advantage of the thermodynamic benefit brought by strict self-assembly processes, three cationic IR-780 dye could be arranged around a single Er(III) cation in the trinuclear [ZnErZn(L5)3]10+ triple-stranded helicate. NIR excitation at 801 nm in acetonitrile at room temperature induces light-upconversion via the energy transfer upconversion (ETU) mechanism. The final green Er(2H11/2,4S3/24I15/2) emission with UC = 3.610−8 shows a record brightness of BUC = 2.810−2 M−1cm−1 (Pexc = 25 Wcm−2) for a molecular-based upconversion process.