The design of new photochromic polymers incorporating covalently or ionically linked spiropyran/polyoxometalate hybrids

Abstract

The incorporation of highly photochromic polyoxometalates (POMs) with spiropyran (SP) appended groups into polymeric matrices has been investigated via two synthetic approaches. First of all, a new unsymmetrical Anderson-type POM bearing both a photochromic SP entity and a methacrylate (MA) unit has been synthesized (SP-POM-MA). This compound has been copolymerized with methylmethacrylate (MMA) as a monomer leading to a covalently linked SP-POM polymer material (poly(SP-POM-MA-co-MA)). In a second part, a symmetrical Anderson-type POM functionalized by two SP units (SP-POM-SP) has been ionically associated with the cationic polymer poly(2-(dimethylamino)ethyl methacrylate) (poly(SP-POM-SP-DMAEMA⁺)). The different hybrid materials elaborated have been fully characterized via¹H NMR spectroscopy, IR spectroscopy, thermogravimetric analysis, X-ray photoelectron spectroscopy and size exclusion chromatography, assessing unambiguously the integrity of the SP-POM units once introduced into the polymer. The solid-state photophysical properties of these materials have been fully investigated under ambient conditions. The ionic poly(SP-POM-SP-DMAEMA⁺) polymer with 7.7% weight in SP, exhibits highly efficient photochromic performances including a fast and intense coloration response under low power UV irradiation, a strong photocoloration contrast, and a very slow thermal fading process. This can be explained by the great stabilization of the photogenerated zwitterionic merocyanine form in the polar cavities of the cationic polymer. In addition, this material exhibits a relatively low photofatigue. For the covalent poly(SP-POM-MA-co-MA) polymer containing 1.1% weight in SP, as expected the photocoloration contrast is lower, but noticeably, showing that even low loading in SP-POM can afford photoresponsive materials.