Syntheses and Color-Changing Properties of Three PW₁₁M (M = Ni, Cu, and Zn)-Based Viologen Compounds

Three POM–viologen compounds were synthesized at a temperature of 120 °C by a traditional hydrothermal technique, namely (1-Cby)·(1,1′-Bby)·[(α–PW₁₁O₃₉)Ni^II(1-Cby)]·7H₂O (PW₁₁Ni-BCby), (1-Cby)·(1,1′-Bby)·[(α-PW₁₁O₃₉)Cu^II(1-Cby)]·6H₂O (PW₁₁Cu-BCby), and (1-Cby)₂[H₅(α-PW₁₁O₃₉)₂Zn₂^II(1-Cby)₂]·27H₂O (PW₁₁Zn-Cby) (1-Cby·Br = 1-cyclopropylmethyl-4,4′-bipyridinyl-1-ium bromide; 1,1′-Bby·Br = 1,1′-bis(cyclopropylmethyl)-4,4′-bipyridine bromide). All three compounds contain PW₁₁M (M = Ni, Cu, and Zn), a transition metal M-substituted monosubstituted Keggin-type structure. These three compounds can exhibit a variety of excellent properties to stimulate discoloration in light, heat, and electricity. Moreover, PW₁₁Cu-BCby and PW₁₁Zn-Cby can be applied to inkless erasable printing and dual anticounterfeiting applications based on their photochromic behaviors.