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.