Abstract
Efficient near-infrared (NIR) to visible (VIS) light upconversion should combine large absorption coefficients ε(NIR) with very large quantum yields ϕ(UC) so that the overall brightness B(UC) = ε(NIR)·ϕ(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) ≤ ϕ(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 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((2)H(11/2),(4)S(3/2) → (4)I(15/2)) emission with ϕ(UC) = 3.6 × 10(-8) shows a record brightness of B(UC) = 2.8 × 10(-2) M(-1) cm(-1) (P(exc) = 25 W cm(-2)) for a molecular-based upconversion process.