Playing with Polyoxometalates to Build Molecular Materials Exhibiting Negative Solid-State Photochromism

Four novel ionic assemblies combining polyoxometalates (POMs) and the cationic photochromic spiropyran SPPy⁺ (1',3',3'-trimethyl-6-nitro-8-pyridiniomethylspiro[2H-[1]benzopyran-2,2'-indoline]), namely, (SPPy)₂[W₆O₁₉]·2CH₃CN, (SPPy)_2.6(H)_0.4[PW₁₂O₄₀]·CH₃CN, (SPPy)_2.1(H)_1.9[β-Mo₈O₂₆]·CH₃CN, and (SPPy)_3.7(H)_0.3[β-Mo₈O₂₆], were synthesized and extensively characterized using complementary techniques, including structural characterization of (SPPy)₂[W₆O₁₉]·2CH₃CN by X-ray diffraction. In particular, solid-state ¹³C NMR spectroscopy was used for the first time on POM/Spiro compounds to assess the proportion of the merocyanine form in these supramolecular materials. Strikingly, in contrast to the deeply colored (SPPy)_2.6(H)_0.4[PW₁₂O₄₀]·CH₃CN and (SPPy)_2.1(H)_1.9[β-Mo₈O₂₆]·CH₃CN assemblies, which are nonphotochromic in the solid state at room temperature, (SPPy)₂[W₆O₁₉]·2CH₃CN and (SPPy)_3.7(H)_0.3[β-Mo₈O₂₆] exhibit strong photocontrast under soft-light irradiation. These observations highlight the crucial role of both the nature of the POM and the synthesis conditions on the optical properties of the final materials. The solid-state photochromic properties of these species have been fully investigated, and the coloration and fading kinetic parameters have been determined. Importantly, once discolored, (SPPy)₂[W₆O₁₉]·2CH₃CN and (SPPy)_3.7(H)_0.3[β-Mo₈O₂₆] turn back to a colored state in the dark, making them the first POM/Spiro hybrid materials to exhibit solid-state negative photochromism.