A stable electron-deficient metal-organic framework for colorimetric and luminescence sensing of phenols and anilines

A 3D metal-organic framework (LVMOF-1) with unique electron-deficient channels was synthesized and its sensing properties for electron-rich benzene derivatives were demonstrated. The MOF is built of robust [Eu(OH)(COO)₂]_n columns and tetratopic viologen-based crosslinkers and shows excellent chemical stability. The structure integrates Eu(iii) centers to luminesce and viologen moieties to accept electrons, and most notably, the electron-deficient viologen moieties, like those in box-like diviologen cyclophanes, are ideally spaced for sandwiching electron-rich aromatic rings. The MOF shows a bimodal response (color and luminescence) to phenols, anilines, benzenediols and aminophenols, with excellent selectivity against a wide range of other organic molecules. The chromogenic phenomena allow facile, quick and naked-eye test-paper detection of these priority contaminants in water, while the luminescence response affords very fast and sensitive quantitative detection. In particular, the detection limits for anilines and benzenediols are as low as 1-9 ppb. The charge transfer and energy transfer mechanisms for the sensing properties were elucidated on the basis of X-ray crystallography after single-crystal-to-single-crystal adsorption and orbital energy analyses according to electrochemical and spectroscopic data and also DFT calculations. The MOF bridges the gap between discrete cyclophanes functioning in solution and extended porous lattices in the solid state and can provide a blueprint for further development of sensory MOFs.