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.