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]·[Ni₄(H₂O)₂Ge₂Mo₁₈O₆₈(MoO₂)₂]_0.5}·10H₂O (1), {[Ni^II(nibipy)₂(4,4′-bipy)(H₂O)₂]·(SiMo₁₂O₄₀)}·2H₂O (2), and [(Hnibipy)₂·(SiMo₁₂O₄₀)]·2.5H₂O (3) (dibipy·Cl = 1-(3,5-dimethyl-isoxazol-4-ylmethyl)-[4,4’]bipyridinyl-1-ium chloride, nibipy·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 [Ni₄(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.