A solar ultraviolet sensor based on fluorescent polyoxometalate and viologen

A novel solar ultraviolet sensitive composite film consisting of fluorescent polyoxometalate and viologen has been readily prepared on solid substrates using a drop casting method, and can be colored and bleached by photoreduction and air oxidation processes. A photoswitchable emission has also been observed with the fluorescence intensity returning to its original value after bleaching of the blue-colored film, which is due to the fluorescence resonance energy transfer (FRET) from the excited states of the Eu(III) ions to colored viologen. The decoloration process takes about 2.5 hours under ambient conditions but can be accelerated by humid air. The absorbance change of the composite film is less than 15% even after 8 cycles of variation between the colorless and blue color states. This is also the reason for the emission intensity change of the composite film. Thus, quick-responsive photochromism and switchable fluorescence are realized by a POM-viologen-based film which shows great potential to realize portable devices for solar ultraviolet detection.