Syntheses and Color-Changing\nProperties of Three PW<sub>11</sub>M (M = Ni, Cu, and Zn)-Based Viologen\nCompounds

Three POM–viologen compounds were synthesized\nat a temperature\nof 120 °C by a traditional hydrothermal technique, namely (1-Cby)·(1,1′-Bby)·[(α–PW<sub>11</sub>O<sub>39</sub>)Ni<sup>II</sup>(1-Cby)]·7H<sub>2</sub>O (<b>PW</b><sub><b>11</b></sub><b>Ni-BCby</b>),\n(1-Cby)·(1,1′-Bby)·[(α-PW<sub>11</sub>O<sub>39</sub>)Cu<sup>II</sup>(1-Cby)]·6H<sub>2</sub>O (<b>PW</b><sub><b>11</b></sub><b>Cu-BCby</b>), and (1-Cby)<sub>2</sub>[H<sub>5</sub>(α-PW<sub>11</sub>O<sub>39</sub>)<sub>2</sub>Zn<sub>2</sub><sup>II</sup>(1-Cby)<sub>2</sub>]·27H<sub>2</sub>O (<b>PW</b><sub><b>11</b></sub><b>Zn-Cby</b>) (1-Cby·Br = 1-cyclopropylmethyl-4,4′-bipyridinyl-1-ium\nbromide; 1,1′-Bby·Br = 1,1′-bis(cyclopropylmethyl)-4,4′-bipyridine\nbromide). All three compounds contain PW<sub>11</sub>M (M = Ni, Cu,\nand Zn), a transition metal M-substituted monosubstituted Keggin-type\nstructure. These three compounds can exhibit a variety of excellent\nproperties to stimulate discoloration in light, heat, and electricity.\nMoreover, <b>PW</b><sub><b>11</b></sub><b>Cu-BCby</b> and <b>PW</b><sub><b>11</b></sub><b>Zn-Cby</b> can be applied to inkless erasable printing and dual anticounterfeiting\napplications based on their photochromic behaviors.