Zinc tungstate encapsulated into a scarce Zn(ii)–viologen framework with photochromic, electrochromic and chemochromic properties

Abstract

Smart chromic materials reacting to physicochemical stimuli are widely applied in optical switches, smart windows, and chemical sensors. Currently, most materials only respond to a single stimulus, but those that respond to multiple external stimuli are still in the minority. Herein, we report a novel porous zinc tungstate@metaloxoviologen framework [Zn₃(Bcbpy)₆(H₂O)₂]-[ZnW₁₂O₄₀]·6H₂O (ZnW₁₂@MV, H₂BcbpyCl₂ = 1,1′-bis(3-carboxybenzyl)-4,4′-bipyridinium dichloride), which shows multiple stimulus-responsive properties due to a combination of different functional motifs, namely, viologen electron acceptors, luminescent zinc–oxygen-clusters, porous cationic frameworks, and ZnW₁₂O₄₀⁶⁻ electron donors. Generally, the large-sized polyoxometalate (POM) anions serving as structure-directing agents can easily direct the formation of the oligomeric metaloxoviologen cations, mainly because POMs may break down some linkages leaving larger spaces for themselves. The large ZnW₁₂O₄₀⁶⁻ anions in ZnW₁₂@MV are encapsulated into three-dimensional (3D) metaloxoviologen frameworks built up from the linkages of trinuclear zinc–oxygen clusters and Bcbpy viologens, which offer the first example of a 3D metaloxoviologen framework induced by large-sized POM anions. ZnW₁₂@MV shows a reversible chromic response to X-ray/UV and electricity via different stimulus-induced electron transfers between electron-rich POM anions and electron-deficient metaloxoviologen frameworks, whereas the coloration changes are ascribed to the formation of radical and mixed-valence colored state ZnW₁₂O₄₀⁶⁻ species. The photochromic behavior is accompanied by photoluminescence quenching. The discriminative response to different-sized amines is attributed to the formation of viologen radicals through host–guest electron transfer. These results indicate that the multi-stimulus response ZnW₁₂@MV can be applied in electrochromic devices, inkless erasable printing, and the detection of amines.