Deciphering and quantifying linear light upconversion in molecular erbium complexes

Single-center light upconversion corresponds to the pilling up of low-energy photons via successive
linear absorptions: a phenomenon commonly observed in lanthanide-doped low-phonon ionic solids
or nanoparticles. Its ultimate miniaturization in molecular complexes opens challenging perspectives
in term of improved reproducibility, chemical control and optical programming. However, highenergy
vibrations inherent to coordination complexes severely limit the efficiency of successive
excited-state absorptions (ESA) responsible for the gain in photon energy. By carefully wrapping
three polyaromatic ligand strands around trivalent erbium, we managed to induce low-power room
temperature near infrared (exc = 801 nm or 966 nm) to visible green (em = 522 nm and 545 nm) light
upconversion within mononuclear coordination complexes [Er(Lk)3]3+ operating either in the solid
state or in non-deuterated solution. The calculated upconversion quantum yields set the zero-level of
an elemental erbium-centered molecular ESA mechanism, a values which favorably compares with
cooperative upconversion (CU) previously implemented in sophisticated multisite Yb2Tb
supramolecular assemblies. The various dependences of the upconverted emission on the incident
excitation power imply different mechanisms, which can be tuned by molecular design.