Stimuli-Responsive Metal-Organic Framework (MOF) with Chemo-Switchable Properties for Colorimetric Detection of CHCl₃

The ligands 3,6-di(pyridin-4-yl)-1,4-dihydro-1,2,4,5-tetrazine (H₂ DPT), as a pillar spacer, and 4,4'-oxybis(benzoic acid) (H₂ OBA), as a dicarboxylate linker, have been used to develop a dihydrotetrazine-functionalized pillared metal-organic framework (MOF), [Zn(OBA)(H₂ DPT)_0.5 ]⋅DMF (TMU-34), as a functionally convertible stimuli-responsive MOF-based sensor. Sonochemically synthesized TMU-34 powder has been employed in the sensing of volatile organic compounds (VOCs). The results show that TMU-34 can act as a solid-state, naked-eye visual chemosensor for the detection of chloroform in the liquid and gas phases. TMU-34, as a chemo-switchable MOF with rapid response, undergoes a significant color change from yellow to pink through reversible dynamic conversion of dihydrotetrazine into tetrazine upon exposure to chloroform in the presence of other VOCs. Whereas other colorimetric MOF-based sensors show a wide range of color changes in the presence of different analytes, TMU-34 undergoes a color change only in the presence of chloroform. A mechanism for the observed color change is proposed on the basis of UV/Vis and ¹ H NMR spectroscopic studies. These observations reveal that TMU-34 represents one of the most promising candidates for the highly selective and sensitive detection of chloroform.