Upconversion of light with molecular and supramolecular lanthanide complexes

Upconversion (UC) is the process by which the energy of multiple photons is absorbed by a compound and restored in the form of a photon of higher energy than the incident light, resulting in an anti-Stokes process. Although studied theoretically since the middle of the last century and experimentally observed in the 1960's, the process was up to recently mainly restricted to solid state devices and ultimately to nanoparticles at the end of the century. At the same period, different researches were directed towards the possibility to observe UC at the molecular level and it is only recently that the phenomenon could be observed in discrete molecular entities in solution with still very few examples. This review aims at explaining the difficulties encountered at the molecular level compared to the solid state and summarizes the results reported to date on UC at the molecular scale. (C) 2020 Elsevier B.V. All rights reserved.