Photochromic Film Based on a Mixed-Heteroatom-Templated Er^III-Incorporated Polyoxometalate Used for Inkless Prints and Anti-Counterfeiting

Photochromic films have attracted increasing attention for their potential in information storage and photoswitchable devices. Developing novel photochromic materials is still a highly challenging topic. In this work, we successfully obtained an unprecedented mixed-heteroatom-templated Er^III-incorporated polyoxometalate [H₂N(CH₃)₂]₁₈Na₂K₂H₁₂{Er₂(H₂O)₁₀W₁₄O₄₃[SeTeW₁₅O₅₄][B-α-TeW₉O₃₃]₂}₂·117H₂O (1) by concurrently introducing the [TeO₃]^2- and [SeO₃]^2- heteroanion templates into the Er³⁺/WO₄^2- system. The most attractive feature is that this polyoxoanion of 1 consists of three kinds of polyoxometalate building units such as classical trivacant Keggin [B-α-TeW₉O₃₃]^8-, rarely seen trivacant Te^IV-Se^IV-oriented Dawson-like [SeTeW₁₅O₅₄]^10-, and extraordinary [W₁₄O₄₃]^2- with an infrequent [W^VIO₇(W^VI₅O₂₀)]^18- pentagon subunit. Based on the photochromism feature of polyoxometalates in the presence of organic electron donors under light irradiation, a photochromic composite film GEL/TEG/1 was fabricated by mixing 1 with gelatin (GEL) and triethylene glycol (TEG). The GEL/TEG/1 film shows good photochromic properties under 365 nm UV irradiation. The film changed color within 15 min under UV irradiation. The photochromic properties endow the film with fast and easy "'printing'" performance. The "printed" text or pattern in the film can be erased within 35 h in an atmospheric environment at room temperature, or colored film can be quickly erased within 3 h at 60 °C and the film can be reused for printing again. Moreover, the coloration and decoloration process can be repeated for at least 10 cycles, indicating that the GEL/TEG/1 film possesses favorable cycle performance. Further, the GEL/TEG/1 film can be used for ink-free printing and anti-counterfeiting encryption. This work not only offers a workable method for constructing mixed-heteroatom-templated polyoxometalates containing different kinds of uncommon building units but also offers some significant insights into the photochromic potential applications of polyoxometalate-based materials.