Rare earth inorganic-organic hybrid compounds based on Keggin-type polyoxometalate {SiW12} with fast-responsive photochromism and switchable luminescence properties

Four rare earth inorganic-organic hybrid compounds based on Keggin-type polyoxometalate {SiW12} with general formula [{RE(DMF)4(H2O)4RE(DMF)6}2{SiW12O40}3] (RE = La(1), Pr(2), Sm(3), Eu(4), DMF = N,N-dimethylformamide) were synthesized by the conventional solution method. Structure analyses indicate that four compounds are isostructural and are all built from three [SiW12O40]4– polyoxoanions and DMF ligands linked by RE3+. Compounds 1–4 are extremely sensitive to UV light and present excellent photochromic properties, in which the coloring and fading time of samples are both no more than 2 min, and the reversible coloring-fading process can be repeated for at least 10 cycles with little fatigue, suggesting that compounds 1–4 can be a series of fast-responsive and durable photochromic materials. Moreover, the solid-state photoluminescence spectra of compounds 3(Sm) and 4(Eu) display strong characteristic emissions of rare earth ions based on f–f transitions. Meanwhile, compound 4(Eu) has a longer fluorescence lifetime up to 566.74 μs. Notably, compounds 3(Sm) and 4(Eu) exhibit the switchable luminescence behavior induced by photochromism, which allows them to be used as potential molecular switches.