New photoresponsive charge-transfer spiropyran/polyoxometalate assemblies with highly tunable optical properties

Seven new photochromic hybrid organic–inorganic supramolecular assemblies have been synthesized by combining a photoswitchable cationic spiropyran (SP) with different polyoxometalate (POM) complexes, and the structures of five of them have been solved by single-crystal X-ray diffraction analysis. These materials differ by the nature of the POM (e.g. [Mo8O26]4−, [M(OH)6Mo6O18]3− (M = Al, Fe), and [Mo6O19]2−), the SP/POM ratio and the crystallized solvent molecules. Their optical properties before and under low-power UV irradiation have been thoroughly investigated by diffuse reflectance spectroscopy, and compared with those of the previously reported compound (SP)3[PMo12O40]. All seven SP/POM assemblies exhibit improved solid-state photochromic performances. The latter vary with the composition and the design of the hybrid frameworks, and structure–property relationships have been evidenced. Especially, the coloration of the materials before UV exposure is governed by a low-energy intermolecular charge-transfer (CT) transition between SP donor and POM acceptor. The CT transition energy can be tailored by tuning the intrinsic ligand-to-metal charge-transfer (LMCT) of the POM unit, which allows drastic improvement of the photocoloration contrasts. Besides, the coloration kinetics has been systematically quantified revealing that the SP photoisomerization strongly varies in the series. These results have been interpreted taking into account several physical parameters (SP structural characteristics, SP/POM and SP/solvent solid-state interactions, molar volume). Finally, based on studies involving two of the reported compounds, it has been evidenced that, under ambient conditions, such materials are remarkable quasi-bistable systems which exhibit fast and comparable coloration and fading rates, and show very good cyclabilities.