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)[H-2(SiMo12O40)]2H(2)O (1), (1-cby)(2)[H-2(SiW12O40)]2H(2)O (2), (1-cby)(2)(1,1 '-bcby)(2){H-4[Co-4(H2O)(2)(PW9O34)(2)]}12H(2)O (3), (1-cby)(1,1 '-bcby)[H(alpha-PW11O39)Co-II(1-cby)]8H(2)O (4) (1-cbyBr = 1-Cyclopropylmethyl-[4,4 ']bipyridinyl-1-ium bromide, 1,1 '-bcbyBr = 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 {Co-4(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.