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), {[NiII(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

A Series of POM–Viologen Photo-/Electrochromic Hybrids and Hydrogels Acting as Multifunctional Sensors for Detecting UV, Hg²⁺, and Organic Amines

In this work, POM anions were introduced into the viologen system in order to synthesize POM–viologen hybrid compounds with excellent properties. Three new POM–viologen compounds, {CdII(tybipy)(DMF)2[β-Mo8O26]0.5Cl} (1), {CoII(tybipy)2(DMF)2[H2(β-Mo8O26)]2}·4C2H7N (2), and (tybipy)4·(β-Mo8O26) (3) (tybipy·Br = 1-thiophen-3-ylmethyl-[4,4′]bipyridinyl-1-ium bromide), have been prepared by a solvothermal method, and their structures were characterized. POM anions are modified by mixing organic ligands with transition metals in compounds 1 and 2. However, compound 3 is a supramolecular structure constructed by hydrogen bonding interactions between the dissociative viologen and POM anions. These three compounds have rapid photoresponse and photochromic ability, which can be made into mixed matrix membranes and hydrogels for UV detection. The rigid sandwich devices prepared by compounds 1–3 have achieved ultrafast electrochromism and recovery. In addition, photochromic hydrogels based on compounds 1–3 can achieve ultrafast photochromic recovery. Compounds 1–3 can be used in ink-free printing and Hg2+ fluorescence detecting. Compounds 1 and 2 can also be used as organic amine detectors. Combined with photochromism and fluorescence detection of Hg2+, visual test papers for Hg2+, Cu2+, and Co2+ were successfully realized, which can improve the portability and detection speed of heavy metal ions in the actual environment