Design and optical investigations of a spironaphthoxazine/polyoxometalate/spiropyran triad

Abstract

The assembly of two distinct organic photochromic groups and polyoxometalate (POM) within a single molecule has been realized. In the hetero-functionalized Anderson polyanion MnMo₆O₁₈(SP)(SN), one spiropyran and one spironaphthoxazine cap the two faces of a planar {Mn^IIIMo₆^VIO₁₈} fragment. A related symmetrical POM where the {Mn^IIIMo₆^VIO₁₈} inorganic moiety is connected to two spironaphthoxazines has also been obtained, and isolated as an organoammonium (MnMo₆O₁₈(SN)₂) or a cationic spiropyran ((SP)₃MnMo₆O₁₈(SN)₂) salt. The structures of MnMo₆O₁₈(SN)₂ and MnMo₆O₁₈(SP)(SN) have been solved by single-crystal X-ray diffraction analysis. In solution, both compounds exhibit electrochromic properties with multiple redox coloured states. The solid-state photochromism of all the compounds has also been extensively studied at room temperature. Contrary to the organic spironaphtoxazine derivative SNTris which is very poorly photochromic, the pale yellow colour of MnMo₆O₁₈(SN)₂ shifts to deep blue with high coloration contrast which underlines the positive effect of the grafting onto the inorganic platform. In addition, MnMo₆O₁₈(SN)₂ also shows good cyclability and exhibits a fast bleaching process. Finally, the temperature dependence of the solid-state photochromic properties of MnMo₆O₁₈(SN)₂ is also reported and compared to that of the related MnMo₆O₁₈(SP)₂. The photo-coloration process which is attenuated at low temperatures in both cases is clearly more affected for MnMo₆O₁₈(SN)₂. Moreover, the thermal fading process becomes significant at 340 K for MnMo₆O₁₈(SP)₂ and 260 K for MnMo₆O₁₈(SN)₂ underlining strong different optical behaviours between both materials.