Polyoxometalates–viologen photochromic hybrids for rapid solar ultraviolet light detection, photoluminescence and inkless and erasable prints
In this work, a new crystalline polyoxometalates–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 obvious colour change from pale yellow to blue and fast response with ultraviolet intensity as low as 0.006 mW/cm2 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 change by sunlight irradiation at different local times in one day realizing portable devices for solar ultraviolet light detection. Ultraviolet light detection is not only reflected in colour changes, but also accompanies photoluminescence phenomenon. The fluorescence intensity decreased with the increase of UV intensity, and when irradiated upon ultraviolet xenon lamp (8 mW/cm2) for about 5 min, fluorescence intensity was almost completely quenched. The compound is potential to become fluorescence based UV probing. The powdered sample of compound 1 could also be deposited in paper simply by coating them with a solution of ethanol. The papers can be used as inkless and erasable print media, which was found to remain clear for 11 d in the dark under an ambient atmosphere and was also reusable when erasured.