Four Anderson-type POM-based viologen compounds for information encryption and food freshness detection

In this paper, Anderson-type polyoxometalates (POMs) were combined with a viologen ligand to synthesize four compounds under solvothermal conditions, namely (1,3–bcbby)₃·(TeMo₆O₂₄)⋅5H₂O (1),(1,3–bcbby)·[M₂(H₂O)₆TeMo₆O₂₄] (2: M = Zn; 3: M = Ni; 4: M = Cu) (1,3-bcbby·2Cl = 1,1′-bis(3-carboxybenzyl)-4,4′-bipyridine dichloride). Compound 1 forms one-dimensional supramolecular chains and two-dimensional supramolecular layers through hydrogen bondings. Isomorphic compounds 2−4 form one-dimensional inorganic chains through covalent bonds between metal ions and POMs, which were then organized into two-dimensional layered structures through hydrogen bondings. All four compounds can be used as color-changing materials. Among them, compounds 1 and 2 exhibit pronounced reversible photochromic properties under a xenon lamp irradiation, while compounds 3 and 4 show good thermochromic properties. Capitalizing on the prompt response of compounds 1 and 2 to ultraviolet light, they are utilized on filter paper for application in ink-free erasable printing. Additionally, when these four compounds are exposed to atmospheres containing ammonia, ethylenediamine or diethylenetriamine, they exhibit distinct color changes. Based on this character, compound 2 was formulated into a portable amine detection strip, which can detect the freshness of pork in real-time through color changes of the detection strip.