Three Viologen Compounds Containing Keggin and Mo-Capped Weakley Polyoxometalates: Photo-/Thermo-/Electro-Chromic Studies

Multiresponsive color-changing materials have important applications in daily life. In this work, three photochromic, thermochromic, and electrochromic compounds were synthesized by introducing two kinds of polyoxometalates into the viologen system, namely, {[Ni(dibipy)(4,4 '-bipyH)(H2O)(3)]<middle dot>[Ni-4(H2O)(2)Ge2Mo18O68(MoO2)(2)](0.5)}<middle dot>10H(2)O (1), {[Ni-II(nibipy)(2)(4,4 '-bipy)(H2O)(2)]<middle dot>(SiMo12O40)}<middle dot>2H(2)O (2), and [(Hnibipy)(2)<middle dot>(SiMo12O40)]<middle dot>2.5H(2)O (3) (dibipy<middle dot>Cl = 1-(3,5-dimethyl-isoxazol-4-ylmethyl)-[4,4']bipyridinyl-1-ium chloride, nibipy<middle dot>Cl = 1-(4-Nitro-benzyl)-[4,4']bipyridinyl-1-ium chloride and 4,4 '-bipy = 4,4 '-bipyridine). Compound 1 contains the Mo-capped weakley anion [Ni-4(H2O)(2)(Ge2Mo18O68(MoO2)(2)](8-), which is very rare in polyoxometalates. Both 1 and 2 own mixed organic ligands to modify anions, such as viologen and 4,4 '-bipy. Compounds 1 and 3 exhibit fast responsive photochromic effects, while compound 2 has rapid recovery of thermochromic properties. Sandwich-type rigid devices based on compounds 1-3 also exhibit electrochromic properties. In addition, hydrogels and poly(methyl methacrylate)-based mixed matrix membrane systems were introduced to achieve portable visual UV detectors. The applications of these compounds in metal ion detection and anticounterfeiting were also studied.