Zwitterionic manganese(II) MOF based on bisimidazole tetracarboxylic acid ligand exhibiting reversible photochromism, erasable inkless printing and amine-selective sensing

Abstract

As chromic materials, viologen-based metal-organic frameworks (MOFs) with the electron-deficient bipyridinium have received enormous attentions. The imidazole nitrogen atoms possess the high basicity, which tends to be protonated, leading to the electron-deficient imidazolium that also may induce color changes. Along this line, by using the bisimidazole tetracarboxylic acid, a three-dimensional (3D) zwitterionic Mn-MOF was synthesized under solvothermal conditions, namely [Mn(H₄L)(H₂O)₂]·2H₂O (Mn-MOF-1) (H₆L = 4,4′,4″,4‴- (1,4-phenylenebis (1H-imidazole-2,4,5-triyl))tetrabenzoic acid). Mn-MOF-1 shows reversible color changes with high contrast color from light yellow to green under visible light irradiation, which can be attributed to the generations of radicals through photoinduced intermolecular electron transfer (ET) between carboxylate and imidazolium, which can be confirmed by the electron paramagnetic resonance (EPR) and the density function theory (DFT) calculations. To advance the practical application, we have fabricated [Mn-MOF-1]-filter paper and [Mn-MOF-1]-PMMA films, which can be used for inkless and erasable printing. Besides, Mn-MOF-1 can also show color changes for the small primary amines such as ethylamine (EA), n-propylamine (PA) and n-butylamine (BA). This size-selective chromic behaviors can be interpreted with host-guest electron transfer generating radicals in the specific confined space. This work provides a promising strategy to promote the development of stimulus-responsive multifunctional MOF-based materials.