A series of isopolymolybdate-viologen hybrids with photo-, thermo- and electro-chromic properties.

The combination of electron-deficient viologen ligands with electron-rich POMs is a typical acceptor-donor system that has recently received much attention. Under solvothermal and hydrothermal conditions, by introducing three symmetric viologen ligands into POM-based hybrid materials, we successfully constructed four POMs-viologen inorganic-organic hybrid compounds, namely (1,3-bcbpy) 2 ·(δ-Mo 8 O 26 ) (1) (1,3-bcbpy·2Cl = 1,1'-bis(3-carboxybenzyl)-4,4'-bipyridine dichloride), {Co II (1,4-bcbpy) 2 (H 2 O) 2 [H 2 (β-Mo 8 O 26 )]}·2H 2 O·2CH 2 O (2), (1,4-bcbpy) 2 ·(δ-Mo 8 O 26 )·2H 2 O (3) (1,4-bcbpy·2Cl = 1,1'-bis(4-carboxybenzyl)-4,4'-bipyridine dichloride, CH 2 O = formaldehyde), and {Cu II (1,1-pmbby) 2 (H 2 O)[H 2 (β-Mo 8 O 26 ) 2 ]}·5H 2 O·C 2 H 7 N (4) (1,1-pmbby·2Cl = 1,1'-[1,4-phenylbis(methylene)]bis-(4,4'-bipyridine)dichloride, C 2 H 7 N = dimethylamine). These four compounds exhibit different fascinating structures, especially compound 4 is a typical metal-organic framework. Compounds 1-4 exhibit good discoloration behaviors under various external stimuli. For example, compounds 1-4 showed a positive response to the irradiation from a 300 W Xe lamp. When a positive voltage was applied to the ECD based on compounds 1-4, 1/2/3/4-ECD underwent a significant color conversion. What's more, compound 4 also showed obvious discoloration results after heating. In a word, 1-4 are multifunctional discoloration materials under different external stimuli. In addition, the coated filter paper prepared based on compound 3 can be used as a new printing material medium and can be successfully applied in erasable inkless printing and dual anti-counterfeiting.