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)CoII(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.