An Oxalate- and Phosphite-Bridging Multi-HoIII-Encapsulated Tellurotungstate and Its Photochromic Performance.

Photochromic functional materials have attracted increasing interest for advanced smart devices, reversible optical switching, and rewritable information storage. Herein, a multi-HoIII-encapsulated tellurotungstate [H2N(CH3)2]18Na2H2[Ho4(OX)2(HPO3)2W6O10][B-α-TeW8O31]4(NO3)2·56H2O (H2OX = H2C2O4, 1) was synthesized. Its polyanion is constructed from four tetravacant Keggin [B-α-TeW8O31]10- subunits and a HPO32--bridged decanuclear heterometallic [Ho4(HPO3)2(OX)2W6O10]20+ cluster. Interestingly, two oxalate ligands link a pair of {Ho2(HPO3)W3O5[B-α-TeW8O31]2}8- moieties to consolidate the overall polyanion. Enlightened by the inherent photochromic feature of polyoxometalates induced by photoinduced electron transfer in organic donor environments, a flexible transparent photochromic PVA/GL/1 film was fabricated by embedding 1 into a poly(vinyl alcohol) (PVA) matrix with glycerol (GL). The resulting PVA/GL/1 film exhibits photochromic behavior upon 365 nm UV irradiation. Notably, this film achieves fast coloration within 2 s and reaches the color saturation state at 30 s. Moreover, the preprinted texts and patterns can be completely faded within 5 h at room temperature, and the erasure time can be shortened to 2 h under 50 °C. This coloration-decoloration process can be steadily repeated for over 10 cycles, manifesting superior cycle performance. Benefiting from its fast response, tunable photochromic behavior, and cyclic stability, the PVA/GL/1 film may hold great application prospects in erasable inkless printing, anticounterfeiting encryption, and information storage.