Effect of size and light power on the fluorescence yield of rubrene nanocrystals

Rubrene nanocrystals were prepared from a toluene emulsion. For the bigger one (>500nm), the absorption spectrum is deformed at wavelength where the absobance of a single nanocristal is higher that 0.1. The nanocristals exhibit a long lived fluorescence in air. The crystal structure protects singlet states from oxygen. But the fluorescence yield spans from 0.7 down to 0.08 decreasing as their size increases. Surface of rubrene nanocrystals does not create quenching defects. The low fluorescence yield is due to the presence of impurities that induce a static quenching of the fluorescence. Nanocrystals smaller than 50 nm have a high average fluorescence yield because of the low probability of presence of an impurity. We have shown that the decrease of the fluorescence yield with the power of the excitation light and the amplification of the effect by removing oxygen are due to the Triplet-Singlet annihilation that inhibits fluorescence when more than one excited state is present in the nanocrystal.