Photochromic\nFilm Based on a Mixed-Heteroatom-Templated\nEr<sup>III</sup>-Incorporated Polyoxometalate Used for Inkless Prints\nand Anti-Counterfeiting

Photochromic\nfilms have attracted increasing attention for their\npotential in information storage and photoswitchable devices. Developing\nnovel photochromic materials is still a highly challenging topic.\nIn this work, we successfully obtained an unprecedented mixed-heteroatom-templated\nEr<sup>III</sup>-incorporated polyoxometalate [H<sub>2</sub>N(CH<sub>3</sub>)<sub>2</sub>]<sub>18</sub>Na<sub>2</sub>K<sub>2</sub>H<sub>12</sub>­{Er<sub>2</sub>(H<sub>2</sub>O)<sub>10</sub>W<sub>14</sub>O<sub>43</sub>[SeTeW<sub>15</sub>O<sub>54</sub>]­[B-α-TeW<sub>9</sub>O<sub>33</sub>]<sub>2</sub>}<sub>2</sub>·117H<sub>2</sub>O (<b>1</b>) by concurrently introducing the [TeO<sub>3</sub>]<sup>2–</sup> and [SeO<sub>3</sub>]<sup>2–</sup> heteroanion\ntemplates into the Er<sup>3+</sup>/WO<sub>4</sub><sup>2–</sup> system. The most attractive feature is that this polyoxoanion of <b>1</b> consists of three kinds of polyoxometalate building units\nsuch as classical trivacant Keggin [B-α-TeW<sub>9</sub>O<sub>33</sub>]<sup>8–</sup>, rarely seen trivacant Te<sup>IV</sup>–Se<sup>IV</sup>-oriented Dawson-like [SeTeW<sub>15</sub>O<sub>54</sub>]<sup>10–</sup>, and extraordinary [W<sub>14</sub>O<sub>43</sub>]<sup>2–</sup> with an infrequent [W<sup>VI</sup>O<sub>7</sub>(W<sup>VI</sup><sub>5</sub>O<sub>20</sub>)]<sup>18–</sup> pentagon subunit. Based on the photochromism feature of polyoxometalates\nin the presence of organic electron donors under light irradiation,\na photochromic composite film GEL/TEG/<b>1</b> was fabricated\nby mixing <b>1</b> with gelatin (GEL) and triethylene glycol\n(TEG). The GEL/TEG/<b>1</b> film shows good photochromic properties\nunder 365 nm UV irradiation. The film changed color within 15 min\nunder UV irradiation. The photochromic properties endow the film with\nfast and easy “‘printing’” performance.\nThe “printed” text or pattern in the film can be erased\nwithin 35 h in an atmospheric environment at room temperature, or\ncolored film can be quickly erased within 3 h at 60 °C and the\nfilm can be reused for printing again. Moreover, the coloration and\ndecoloration process can be repeated for at least 10 cycles, indicating\nthat the GEL/TEG/<b>1</b> film possesses favorable cycle performance.\nFurther, the GEL/TEG/<b>1</b> film can be used for ink-free\nprinting and anti-counterfeiting encryption. This work not only offers\na workable method for constructing mixed-heteroatom-templated polyoxometalates\ncontaining different kinds of uncommon building units but also offers\nsome significant insights into the photochromic potential applications\nof polyoxometalate-based materials.