Four POM-Viologen Color-Changing Materials with Fast Color Response under Various External Stimuli

Four kinds of polyoxometalate (POM)-viologen compounds were synthesized by hydrothermal method, namely (1–cby) 2 ·[H 2 (SiMo 12 O 40 )]·2H 2 O ( 1 ), (1–cby) 2 ·[H 2 (SiW 12 O 40 )]·2H 2 O ( 2 ), (1–cby) 2 ·(1,1′–bcby) 2 ·{H 4 [Co 4 (H 2 O) 2 (PW 9 O 34 ) 2 ]}·12H 2 O ( 3 ), (1–cby)·(1,1′–bcby)·[H(α–PW 11 O 39 )Co II (1–cby)]·8H 2 O ( 4 ) (1–cby·Br = 1–Cyclopropylmethyl–[4,4′]bipyridinyl–1–ium bromide, 1,1′–bcby·Br = 1,1′–Bis-cyclopropylmethyl–[4,4′]bipyridinyl–1–ium bromide). These four POM-viologen compounds exhibit one-dimensional supramolecular network structures. Especially, compound 3 contains a rare sandwich POM subunit {Co 4 (H 2 O) 2 (PW 9 O 34 ) 2 } 10– . These four compounds can be used as color-changing materials, and they all exhibit noticeable color changes upon exposure to light, heat, and electricity. The discoloration mechanism involves viologen derivatives with electron-deficient properties accepting electrons from POM with electron-rich properties under external stimulation, leading to the formation of viologen free radicals. Among them, compounds 1 and 2 also have good properties for ink-free erasable printing, double anticounterfeiting, and ultraviolet detector because of their rapid color response to ultraviolet (UV) light. In addition, compounds 1 – 4 also show different color changes in the detection of volatile amines.