The Photochromism of UiO-66 Encapsulated Polyoxometalate Composites

We present here the synthesis of H5[(PV2Mo10O40)@Zr-UiO-66] (1) composite and H5[(PV2Mo10O40)@Ce-UiO-66] (2) composite via postsynthetic impregnation of H5[PV2Mo10O40] polyoxometalate into the pores of the UiO-66 metal-organic framework (MOF). The composites 1 and 2 are characterized by IR, PXRD, ICP, BET, CV, TGA, SEM-EDX, and HR-TEM techniques. The DMF-solvated H5[(PV2Mo10O40)@Zr-UiO-66-DMF] (1-DMF) and H5[(PV2Mo10O40)@Ce-UiO-66-DMF] (2-DMF) are prepared by soaking composites 1 and 2 in DMF solvent for 12 h. The yellow composites H5[(PV2Mo10O40)@Zr-UiO-66-DMF] and H5[(PV2Mo10O40)@Ce-UiO-66-DMF] on exposure to sunlight, are converted to the blue-colored H4[(PVIVVMo10O40)@Zr-UiO-66] 1red and H4[(PVIVVMo10O40)@Ce-UiO-66] 2red. The photoreduction of the Keggin anion by DMF results in the formation of the blue-colored composites 1red and 2red. The reduced composites 1red and 2red are characterized by diffuse reflectance, EPR, and X-ray photoelectron spectroscopy (XPS). The blue-colored 1red, upon heating at 70 ̊C, and 2red, upon immersion in H2O, are converted back to the yellow-colored H5[(PV2Mo10O40)@Zr-UiO-66] 1reg and H5[(PV2Mo10O40)@Ce-UiO-66] 2reg, respectively. The three reversible photochromic cycles of composites 1 and 2 are studied by solid-state electronic spectroscopy. The photochromism of composites 1 and 2 is mediated by DMF solvent, and the composites do not exhibit photochromism without DMF solvation.