Self-assembled multidye-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
εNIR with very large quantum yields ϕUC so that the overall brightness BUC = εNIR·ϕUC 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 ≤ ϕUC ≤ 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 dyes 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 ϕUC = 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.