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.
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5$\begingroup$ Almost by definition, upconversion is not efficient... $\endgroup$– Jon CusterCommented Jan 21, 2019 at 22:47
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$\begingroup$ @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. $\endgroup$– CalvinCommented Jan 21, 2019 at 23:47
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1$\begingroup$ 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... $\endgroup$– Jon CusterCommented Jan 22, 2019 at 0:25
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2 Answers
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If you want to convert visible light into UV, you have two options.
Two-photon excited fluorescence. The process is inherently wasteful, but the field is studied due to applications in microscopy. See example paper on fluorophores https://pubs.acs.org/doi/abs/10.1021/ol017150e . Also, there are commercially available dyes.
Optical frequency multiplier. The process is fairly efficient, but I think it requires laser as the light source. Also, AFAIK it doesn't involve excitation of a molecule, but is an arcane optical device exploiting non-linear optical properties of some materials. Most laser diodes in violet and UV range use frequency multipliers to up-convert light from more convenient red and green diodes.
Naturally I think that the second option is closer to what you want, but in this case you should look for commercial products, that might be available as "laser frequency doubler/tripler". Good luck.
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1$\begingroup$ Two photon absorption is a nice way of doing this but you will need a short pulse laser to make it work well. Laser frequency doublers etc are widely used in many laser systems and use non-centrosymetric crystals such as potassium dihydrogen phosphate (KDP). You can also generate continuum spectra with short (sub picosecond) laser pulses focussed into, say, water or sapphire, but these are not what as asked about. $\endgroup$ Commented Jan 22, 2019 at 15:54
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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.
