Taming Lanthanide-Centered Upconversion at the Molecular Level

Considered at the
beginning of the 21th century as being incompatible with the presence
of closely bound high-energy oscillators, lanthanide-centered superexcitation,
which is the raising of an already excited electron to an even higher
level by excited-state energy absorption, is therefore a very active
topic strictly limited to the statistical doping of low-phonon bulk
solids and nanoparticles. We show here that molecular lanthanide-containing
coordination complexes may be judiciously tuned to overcome these
limitations and to induce near-infrared (NIR)-to-visible (VIS)-light
upconversion via the successive absorption of two low-energy photons
using linear-optical responses. Whereas single-ion-centered excited-state
absorption mechanisms remain difficult to implement in lanthanide
complexes, the skillful design of intramolecular intermetallic energy-transfer
processes operating in multimetallic architectures is at the origin
of the recent programming of erbium-centered molecular upconversion.