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.