While the low-absorption cross section of lanthanide-based upconversion systems, in which the trivalent lanthanides (Ln3+) are responsible for converting low- to high-energy photons, has restricted their application to intense incident light, the emergence of a cascade sensitization through an organic dye antenna capable of broadly harvesting near-infrared (NIR) light in upconversion nanoparticles opened new horizons in the field. With the aim of pushing molecular upconversion within the range of practical applications, we show herein how the incorporation of an NIR organic dye antenna into the ligand scaffold of a mononuclear erbium coordination complex boosts the upconversion brightness of the molecule to such an extent that a low-power (0.7 W·cm–2) NIR laser excitation of [L6Er­(hfa)3]+ (hfa = hexafluoroacetylacetonate) at 801 nm results in a measurable visible upconverted signal in a dilute solution (5 × 10–4 M) at room temperature. Connecting the NIR dye antenna to the Er3+ activator in a single discrete molecule cures the inherent low-efficient metal-based excited-state absorption mechanism with a powerful indirect sensitization via an energy transfer upconversion, which drastically improves the molecular-based upconverted Er3+-centered visible emission.

Whereas dye-sensitized lanthanide-doped nanoparticles represent an unquestionable advance for pushing linear near-infrared (NIR) to visible-light upconversion within the frame of applications, analogous improvements are difficult to mimic for related but intramolecular processes induced at the molecular level in coordination complexes. Major difficulties arise from the cationic nature of the target cyanine-containing sensitizers (S), which drastically limits their thermodynamic affinities for catching the lanthanide activators (A) required for performing linear light upconversion. In this context, the rare previous design of stable dye-containing molecular SA light-upconverters required large S···A distances at the cost of the operation of only poorly efficient intramolecular S → A energy transfers and global sensitization. With the synthesis of the compact ligand [L2]+, we exploit here the benefit of using a single sulfur connector between the dye and the binding unit for counterbalancing the drastic electrostatic penalty which is expected to prevent metal complexation. Quantitative amounts of nine-coordinate [L2Er­(hfac)3]+ molecular adducts could be finally prepared in solution at millimolar concentrations, while the S···A distance has been reduced by 40% to reach circa 0.7 nm. Detailed photophysical studies demonstrate the operation of a three times improved energy transfer upconversion (ETU) mechanism for molecular [L2Er­(hfac)3]+ in acetonitrile at room temperature, thanks to the boosted heavy atom effect operating in the close cyanine/Er pair. NIR excitation at 801 nm can thus be upconverted into visible light (525–545 nm) with an unprecedented brightness of B up(801 nm) = 2.0(1) × 10–3 M–1·cm–1 for a molecular lanthanide complex.