# IR fluorescent material for outdoor use

I am experimenting with IR video cameras. They have a broad sensitivity peak at 940 nm. There is a filter blocking 750 nm and shorter in front of the sensor.

I am looking for an IR fluorescent substance to use for markers in the scene the camera is observing. Something that can be turned on with ultraviolet illumination bursts so I can differentiate between my markers and other IR sources.

My hope is for some affordable and harmless substance. Something that could be mixed with polyester, acrylic or epoxy potting compound wold be nice. An already solid watherproof material is even better.

My searches so far have found some IR fluorescent stains for microscopy but I am afraid they are expensive.

This might be a physics question also but I think chemists works a lot with IR absorption and IR fluorescence.

## 1 Answer

A commercial solution would be a dye like IRF940A (from QCR Solutions) with an excitation maximum at $$\lambda_{\mathrm{exc}} = 405\,\mathrm{nm}$$ and an emission maximum at $$\lambda_{\mathrm{em}} = 940\,\mathrm{nm}$$. The supplier does not provide any information on the structure of the dye.

### What compounds are promising candidates?

Ytterbium(III) complexes!
In a recent article,published in J. Photochem. Photobiol. A, 2015, *309, 65-71 (DOI), J. Legendziewicz and coworkers prepared a $$\ce{Yb(III)}$$ phthalocyanine with acetate as a chelating axial ligand and examined the optical properties in solution and in PMMA polymer. In PMMA, the compound shows a Soret band with a maximum at $$\lambda_{\mathrm{abs}} = 350\,\mathrm{nm}$$. Excitation in the Soret band result in emission with a maximum around around $$\lambda_{\mathrm{em}} = 700\,\mathrm{nm}$$.

Other examples for even larger wavelength differences, resulting from ligand-to-metal energy transfer are known for $$\ce{Yb(III)}$$ complexes bound to eosines or fluoresceines as the primary chromophor.