Anthracene-Bisimidazole Tetraacid Linker-Based Metal–Organic Nanosheets for Turn-On Fluorescence Sensing of Nerve Agent Mimics

A fluorescent 4-connecting bisimidazole-tetraacid linker endowed with anthracene as the bulged core, that is, H4ANTBI, 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.