A cadmium metal-organic framework, i.e., Cd-BBI, with ca. 43% solvent-accessible volume, was accessed starting from a rationally-designed tetraacid linker based on bisimidazole featuring concave shapes as the central core. The...

A fluorescent 4-connecting bisimidazole-tetraacid linker endowed with anthracene as the bulged core, that is, H 4 ANTBI, was rationally designed to access a metal–organic layered material, that is, Cd-ANTBI, with ca. 36% solvent-accesssible volume. Fluorescence is significantly suppressed in its crystalline state due to self quenching (ϕfl ca. 5%). However, 2D metal–organic framework nanosheets (MONs) generated by solvent-assisted exfoliation of the layered material in methanol were found to exhibit enhanced fluorescence (ϕfl ca. 20%). The formation of 2D MONs was unequivocally established by microscopic techniques such as atomic force microscopy, high-resolution transmission electron microscopy, and transmission electron microscopy–energy-dispersive spectroscopy analyses. Indeed, the extent of exfoliation in different solvents such as methanol, ethanol, and dimethyl sulfoxide was found to correlate with solvent properties (Gutmann’s donor numbers), attesting to the fact that solvents that can form hydrogen bonds with the imidazolium ions bring about exfoliation effectively. It is further shown for the first time that the suspension of 2D MONs serves as an efficient “turn-on” fluorescent material for sensing of the toxic nerve agent mimics; the observed detection limit for diethyl chlorophosphate is ca. 0.52 ppm.

A zinc metal–organic framework, i.e., Zn-MOF (Zn-DBC), with ca. 27% solvent-accessible void volume was synthesized from a rationally designed tetraacid based on sterically insulated dibenzo­[g,p]­chrysene core; the latter inherently features concave shapes. Due to rigidification of the fluorophore in the MOF, Zn-DBC exhibits a respectable fluorescence quantum yield of ca. 30% in the solid state. The fluorescent and water-stable Zn-DBC MOF was found to display intriguing temperature-dependent emission behavior with an activation barrier of 1.06 kcal/mol for radiationless deactivation from the singlet-excited state. It is shown that the Zn-MOF can be employed as an efficient sensory material for detection of hazardous “quat” dicationic herbicides in water by diffusion-limited “turn-off” fluorescence. Due to confinement of the cationic guest analytes within the pores of the MOF, the fluorescence quenching via excited-state charge transfer mechanism is shown to depend on the molecular size of the analyte in addition to the redox potentials. Remarkably, Zn-DBC permits sensing of DQ, a well-known toxic “quat” herbicide, with a detection limit as low as 2.8 ppm in water. The unique structural attributes of the Zn-MOF for highly efficient fluorescence sensing of toxic herbicides in water are thus exemplified for the first time.