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.

上转换（UC）是这样的过程，通过该过程，多个光子的能量被化合物吸收并以能量比入射光高的光子的形式恢复，从而导致了反斯托克斯过程。尽管从上世纪中叶开始就进行了理论研究，并在1960年代进行了实验观察，但直到最近，该过程主要限于固态器件，最终在本世纪末才局限于纳米颗粒。同时，针对在分子水平观察UC的可能性进行了不同的研究，直到最近，在溶液中的离散分子实体中仍可以观察到这种现象，仍然只有很少的例子。