# What fluorescent materials absorb visible light and emit UVC light?

What fluorescent materials, if any, exhibit anti-stokes shift such that they absorb visible light and emit ultraviolet light in the range of 250-285 nm? I'm interested in particular in materials that are efficient in their conversion and not difficult to acquire.

• Almost by definition, upconversion is not efficient... – Jon Custer Jan 21 '19 at 22:47
• @JonCuster I think I know what you are referring to, that it takes multiple lower-energy photons to produce one higher-energy photon. I would like to use "efficient" in the sense that the input energy primarily gets converted to ultraviolet light in the desired range. – Calvin Jan 21 '19 at 23:47
• Yeah, and that is the problem. For two photon processes you need lots of intensity and even then the efficiency is lousy. Or you are asking for multiple sequential absorption through states with really long lifetimes. Also really unlikely. It just isn’t happening... – Jon Custer Jan 22 '19 at 0:25

It is not possible in a single molecule as energy conservation will forbid it. There is not enough thermal energy, $$k_BT \approx 210$$ wavenumbers, visible to uv thousands of wavenumbers. It is sort of possible using triplet-triplet annihilation ($$\ce{T + T \to S^* + S}$$ ) but this needs significant initial excitation and long lived triplets, which means de-oxygenated solution, and produces the same singlets as were excited initially. There are also inorganic up-conversion materials, for example some lanthanides, such as ytterbium based ones will convert near ir light to visible light.