Tetrahydroxyhexanedioic Acid-Ornamented Phosphite-and-Bismuth-Bridging Eu ^III -Encapsulated Phospho(III)tungstate and Its Photochromic and Photoluminescence Switching Properties

Developing photoresponsive materials exhibiting both photochromic and photoluminescent switching properties presents a significant scientific challenge. Herein, a tetrahydroxyhexanedioic acid-ornamented phosphite-and-bismuth-bridging multi-Eu^III encapsulated phospho(III)tungstate [H₂N(CH₃)₂]₈Na₆[Eu₄W₆Bi₂O₁₂(HP^IIIO₃)₂(H₂THHA)₂(H₂O)₁₀][HP^III W₉O₃₃]₄ · 50H₂O (<b>1</b>, H₆THHA = tetrahydroxyhexanedioic acid) was obtained, whose polyanion encompasses four trivacant Keggin-type [B-α-HP^IIIW₉O₃₃]^8- units and a complex heterometallic {[Eu₄W₆Bi₂O₁₂(HP^IIIO₃)₂(H₂THHA)₂(H₂O)₁₀]}¹⁴⁺ cluster, wherein [HP^IIIO₃]^2- anions and Bi³⁺ ions serve as bridging linkers, while the long-chain multidentate H₂THHA^4- ligands coordinate with W and Bi atoms. Intriguingly, <b>1</b> exhibits remarkable photochromic behavior upon UV irradiation, rapidly changing from colorless to blue within 180 s and reverting to its original colorless state within 5 h in the absence of irradiation, suggesting its potential as a fast-responsive photochromic material. Moreover, the photoluminescence of <b>1</b> can be reversibly modulated through photochromic activation, indicating its applicability as a molecular switch. Importantly, the reversible color change can be cycled at least ten times without significant degradation. Spectroscopic analyses including IR, XRD, XPS, EPR and Raman confirm that the photochromic behavior arises from the structural reduction of W^VI centers to W^V species. Furthermore, the <b>1</b>-based photochromic film was fabricated and applied for erasable ink-free printing. This work highlights the effectiveness of integrating multicomponent systems in the design of polyoxometalates and broadens the scope of research on the photochromic properties of these materials.