Polyoxometalate-viologen photochromic hybrids for rapid solar ultraviolet light detection, photoluminescence-based UV probing and inkless and erasable printing.

In this work, a new crystalline polyoxometalate-viologen hybrid, (Pbpy)(Me2NH2)3[PW11ZnO40] (1: Pbpy = 1,1'-[1,4-phenylenebis-(methylene)]bis(4,4'-bipyridinium)), has been synthesized. It showed efficient ultraviolet light detection ability with an obvious colour change from pale yellow to blue and fast response with ultraviolet intensity as low as 0.006 mW cm-2 in narrow-band UV regions. The POM-viologen-based film of 1 has also been readily prepared on quartz substrates using a drop casting method and could exhibit different levels of colour changes under sunlight irradiation at different local times on a sunny day, indicating its potential to be used as a portable device for solar ultraviolet light detection. Ultraviolet light detection is not only reflected in colour changes, but also accompanies the photoluminescence phenomenon. The fluorescence intensity decreased with the increase of UV intensity, and when irradiated with an ultraviolet xenon lamp (8 mW cm-2) for about 5 min, the fluorescence intensity was almost completely quenched. The compound has the potential to achieve fluorescence based UV probing. The powdered sample of compound 1 could also be deposited in paper simply by coating it with a solution of ethanol. The paper can be used as an inkless and erasable print medium, which was found to remain clear for 11 d in the dark under an ambient atmosphere and was also reusable when erased.