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

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(²H_11/2,⁴S_3/2 → ⁴I_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.