Halochromic Benzazolo-oxazolidine/Polyoxometalate Supramolecular Materials with Reversible High-Contrast Color-Switching

The first halochromic supramolecular ionic assemblies of a stimulus-responsive benzazolo-oxazolidine (BOX) derivative with polyoxometalates (POMs) have been elaborated. First, the conversion of the 7,9,9-trimethyl-10-(4-dimethylamino)styryl-indolino[2,1-b]oxazolidine (3) neutral ring-closed form to its monoprotonated (3o) or diprotonated (3op) ring-open cations has been triggered by controlled acidification, in the presence of the Lindqvist-type [M₆O₁₉]²⁻ (M = Mo and W) POMs. The three resulting hybrid materials (3o)2[M6O19] (M = Mo and W) and (3op)2[W6O19]Cl2 were structurally characterized by single-crystal X-ray diffraction. The solid-state halochromic properties of films of (3o)2[Mo6O19] (M = Mo and W) have been fully investigated. Upon exposure to HCl vapor, the films exhibit an immediate high-contrast color-switching due to the conversion of 3o to 3op following the incorporation of protons and chloride ions into the POM-BOX frameworks. The initial color of the films is progressively recovered by releasing HCl vapor. The kinetics of the releasing processes have been quantified and compared with those of a film of the organic reference (3o)Cl·CH3CN, revealing that the 3op molecule is better stabilized in the crystalline state when associated with the POM units via noncovalent interactions. (3o)2[Mo6O19] (M = Mo and W) also exhibit a good cyclability over successive HCl vapor trapping-releasing cycles.