Four POM-Viologen Color-Changing Materials with Fast Color Response under Various External Stimuli

Four kinds of polyoxometalate (POM)-viologen compounds were synthesized by hydrothermal method, namely (1-cby)₂·[H₂(SiMo₁₂O₄₀)]·2H₂O (1), (1-cby)₂·[H₂(SiW₁₂O₄₀)]·2H₂O (2), (1-cby)₂·(1,1'-bcby)₂·{H₄[Co₄(H₂O)₂(PW₉O₃₄)₂]}·12H₂O (3), (1-cby)·(1,1'-bcby)·[H(α-PW₁₁O₃₉)Co^II(1-cby)]·8H₂O (4) (1-cby·Br = 1-Cyclopropylmethyl-[4,4']bipyridinyl-1-ium bromide, 1,1'-bcby·Br = 1,1'-Bis-cyclopropylmethyl-[4,4']bipyridinyl-1-ium bromide). These four POM-viologen compounds exhibit one-dimensional supramolecular network structures. Especially, compound 3 contains a rare sandwich POM subunit {Co₄(H₂O)₂(PW₉O₃₄)₂}^10-. These four compounds can be used as color-changing materials, and they all exhibit noticeable color changes upon exposure to light, heat, and electricity. The discoloration mechanism involves viologen derivatives with electron-deficient properties accepting electrons from POM with electron-rich properties under external stimulation, leading to the formation of viologen free radicals. Among them, compounds 1 and 2 also have good properties for ink-free erasable printing, double anticounterfeiting, and ultraviolet detector because of their rapid color response to ultraviolet (UV) light. In addition, compounds 1-4 also show different color changes in the detection of volatile amines.