Syntheses and performance study of three POM-viologen compounds with photo- and electric-stimulation response

In recent years, stimulus responsive materials have received widespread attention. Under solvothermal conditions, three polyoxometalates-viologen organic-inorganic hybrid compounds were successfully constructed by combining a viologen ligand 1-(3-Nitro-benzyl)-[4,4']bipyridinyl-1-ium bromide (1,3-nibipy·Br) with octamolybdate, namely [Cu₂(1,3-nibipy)₄(H₂O)₂(β-Mo₈O₂₆)₂]·2H₂O (1), [Cu₂(1,3-nibipy)₄(H₂O)₄(β-Mo₈O₂₆)]·(β-Mo₈O₂₆) (2) and (1,3-Hnibipy)₂·(β-Mo₈O₂₆) (3). These three compounds can exhibit color changing properties under both light and electrical stimulation. Through characterizations of PXRD, FT-IR, UV-vis spectra, XPS, EPR, CV, and other tests, the photochromic and electrochromic properties of these three compounds are caused by the generation of viologen radicals. Compounds 1-3 have a rapid photoresponse efficiency and can be made into mixed matrix films for use as ultraviolet detectors. In addition, coated filter paper synthesized from acetonitrile and compounds can serve as an innovative erasable ink-free printing material medium, which is suitable for various erasable ink-free printing and anti-counterfeiting fields. We further investigated the electrochromic devices prepared from compounds 1-3, which achieved color change at a voltage of around -0.2 V and exhibited good stability after 500 cycles.