POM-Based Viologen Compounds with Dual-Functional Color-Changing Modes for Integrated Smart Windows

At present, color-changing materials based on viologen have attracted extensive attention due to their outstanding color-changing properties. In this work, two viologen-based compounds containing polyoxometalate (POM) anions were successfully synthesized under solvothermal conditions, namely [H₂(4-pyby)(β-Mo₈O₂₆)]·2C₂H₇N (1) and {[Zn(4-pyby)Br](α-Mo₂^VMo₆^VIO₂₆)_0.5}·0.5H₂O (2) (4-pyby = 1-[4-(4-pyridinyl)butyl]-4-(4-pyridinyl) pyridinium, C₂H₇N = dimethylamine). Compound 1 exhibits a 0-dimensional structure, which can assemble into a two-dimensional supramolecular network by hydrogen bonds. Compound 2 forms a honeycomb-like two-dimensional layered network structure through metal covalent bonds. Furthermore, the stimuli-responsive chromic properties of compounds 1 and 2 were systematically investigated. Both compounds show reversible photochromic properties. At the same time, hydrogels based on compounds 1 and 2 were prepared, among which the photochromic hydrogel achieve a clear visualization of the color changes of these two compounds. The all-in-one devices prepared from the electrochromic hydrogels also show different color changes with the variation of voltage, especially compound 2, which shows a fast response time and excellent coloring efficiency. When the all-in-one devices are used to simulate smart windows, they show good temperature control ability and privacy protection. They have great potential for energy conservation and personal privacy protection.