Luminescent materials under IR radiation?

is there a database somewhere of materials that are luminescent when exposed to Infra Red radiation?




There are two types of materials I'm aware of that produce light in the way I think you want: two-photon fluorescence and photon upconversion. Two-photon fluorescence is basically the same as normal fluorescence but two photons are absorbed simultaneously, emitting a single photon when returning to the ground state. [Photon upconversion](http://en.wikipedia.org/wiki/Photon_upconversion) similarly involves the absorption of multiple photons, but sequentially (by various mechanisms), followed by emission of a single photon.


Two-photon fluorescence is mostly seen used for [two-photon excitation microscopy](http://en.wikipedia.org/wiki/Two-photon_excitation_microscopy) and useful dyes tend to be big conjugated organic systems. Materials used for photon upconversion are generally lanthanide-doped nanoparticles and they've found many uses in applications traditionally served by quantum dots and organic fluorophores. They're both particularly useful in medical imaging as IR can penetrate tissue deeply, but [upconverting nanoparticles](http://onlinelibrary.wiley.com/doi/10.1002/anie.201005159/full) have a big advantage in that two-photon fluorescence requires very high excitation intensity (pretty much just lasers) as both photons must be absorbed simultaneously, but upconversion processes are sequential and can thus occur at lower intensity. Though, for some applications, the intensity requirements for two-photon absorption are useful as one can play tricks with focusing such that only a small region receives enough excitation light to emit, making imaging small areas possible.


As for a database of compounds, Thermo-Fisher has a [catalogue of two-photon dyes](https://www.thermofisher.com/ca/en/home/references/molecular-probes-the-handbook/technical-notes-and-product-highlights/fluorescent-probes-for-two-photon-microscopy.html), but I don't know that there are a lot of commercial manufacturers of upconverting nanoparticles, though Sigma-Aldrich formerly carried up converting nanocrystal under the Sunstone brand.


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I am not aware of any materials that have this property. There are certainly molecules that emit fluorescence in the IR, when irradiated with shorter wavelength light, but I can't think of any that would emit in the IR after being irradiated in the IR in a manner that can be called "luminescence". Luminescence describes the specific phenomenon where a photon of a particular wavelength had excited a molecule to a higher-energy electronic state, the molecule then relaxes to a lower energy state, with emission of a second photon, which is always of longer wavelength than the original photon. Because molecular electronic transitions are typically in the UV-visible region of the spectrum, the phenomenon of luminescence is generally restricted to those spectral regions.


One particular issue with IR luminescence is that it would be pretty difficult to tell apart from normal thermal IR emission ... luminescence refers specifically to light that is emitted outside of the thermal spectrum of an object. That means that an object would need to be pretty cold initially, in order to distinguish luminescence in the IR region. And again, since the emitted wavelength is necessarily longer than that of the absorbed wavelength, a molecule that was luminescent under IR irradiation would emit deeper in the IR than where it absorbed, making all of this harder.


Having said that, there are certainly molecules with electronic transitions that absorb in the IR, and if you got them cold enough and used an appropriate spectrometer, you might be able to observe IR luminescence from them. It's something that would be restricted to pretty highly specified research labs though.


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*In addition to Michael D. M. Dryden's excellent answer:*


Freshly prepared **graphene oxide** seemingly shows two-photon absorption in the IR and deactivation by fluorescence, as described in an 2014 article in 
*Nature Science Reports*. ([DOI](http://dx.doi.org/10.1038/srep06090))


Fig. 2D in that article shows a linear correlation of the emission intensity at $\lambda\_{\textrm{em}} = \textrm{680 nm}$ with the square of the intensity of the excitation at $\lambda\_{\textrm{exc}} = \textrm{1140 nm}$.


Fig. 2C shows that the emission wavelength can be tuned by variation of the excitation wavelength!


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