Inkless printing and anti-counterfeiting enabled by a photochromic film with a Dawson-type Zr-substituted phosphotungstate

Photoresponsive functional materials, especially photochromic materials, have recently emerged as critical players in advancing cutting-edge optical applications. Herein, a Dawson-type Zr-substituted phosphotungstate, [NH 2 (CH 3 ) 2 ] 6 H 3 [( α 1 -P 2 W 17 O 61 )Zr(HCIT)]·9H 2 O ( 1 ; H 4 CIT = citric acid), was synthesized from a hexavacant precursor [H 2 P 2 W 12 O 48 ] 12− via a conventional solution method. A notable feature of compound 1 is that the substitution site of the polyanion [( α 1 -P 2 W 17 O 61 )Zr(HCIT)] 9− is positioned at the equator of the Dawson structure. This sharply contrasts with the previously reported polar substitution sites. Compound 1 was systematically characterized by IR, PXRD, XPS, UV, 31 P NMR and TGA. This compound possesses conspicuous photochromism with the half-life (t 1/2 ) achieved 0.140 min. The in vitro cytotoxicity assessment of compound 1 revealed minimal toxic effects on the normal cell lines NIH-3 T3 (mouse embryonic fibroblasts) and AML-12 (mouse normal hepatocytes). Even at a high concentration of 80 μg/mL, cell viability remained greater than 90 %. Additionally, a photochromic composite film GEL/TEG/ 1 was prepared by incorporating 1 into a triethylene glycol (TEG) and gelatin (GEL) matrix. The GEL/TEG/ 1 composite film exhibited a pronounced color change within 30 s of UV irradiation (λ = 365 nm) and reached saturation in 6 min. Photochromic films demonstrate rapid, light-induced patterning capabilities that enable facile image formation without conventional printing processes. The photopatterned images on the film demonstrated reversible behavior, with complete erasure occurring within 3 h under ambient conditions. The GEL/TEG/ 1 film demonstrated excellent cycling stability, maintaining reproducible coloration–decoloration behavior over at least 10 consecutive cycles without significant performance degradation. The GEL/TEG/ 1 film demonstrates potential for applications in ink-free patterning and advanced anti-counterfeiting technologies, leveraging its reversible photochromic properties.