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

## TL;DR

This paper presents a new method to boost light upconversion using self-assembled molecules that convert near-infrared light to visible green light efficiently.

## Contribution

The study introduces a self-assembled single-molecule system with record brightness for molecular-based upconversion.

## Key findings

- A trinuclear complex [ZnErZn(L5)3]10+ was created with three IR-780 dyes around an Er(iii) cation.
- The system achieved a quantum yield of 3.6 × 10−8 and a brightness of 2.8 × 10−2 M−1 cm−1.
- Green upconverted emission was observed at room temperature in solution under 801 nm excitation.

## Abstract

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.

The virtues of self-assembly force the connection of three cationic cyanine dyes around a trivalent erbium emitter. Upconverted green emission is observed upon near-infrared excitation of a single molecule in solution at room temperature.