Stable Hydrazone-Linked Covalent Organic Frameworks Containing O,N,O′-Chelating Sites for Fe(III) Detection in Water

Two stable crystalline
hydrazone-linked covalent organic frameworks (COFs) (Bth-Dha and Bth-Dma) containing functional O,N,O′-chelating
sites have been designed and successfully synthesized by the Schiff-base
condensation reactions between benzene-1,3,5-tricarbohydrazide (Bth) and 2,5-dihydroxyterephthalaldehyde (Dha) or 2,5-dimethoxyterephthal-aldehyde (Dma), respectively. Bth-Dma exhibits strong fluorescence in the solid
state and in an aqueous dispersion, while no fluorescence can be observed
for Bth-Dha. Interestingly, the as-synthesized Bth-Dma can be used as a turn-off fluorescence sensor for
the Fe­(III) ion in aqueous solution with outstanding selectivity and
sensitivity. The recognition process can be attributed to the coordination
interaction between Fe­(III) ion and the O,N,O′-chelating sites
in the pore wall of Bth-Dma COF, as verified
by X-ray photoelectron spectroscopy and 1H NMR spectroscopy.
To the best of our knowledge, this is the first report on the rational
design of luminescent COF with predesigned O,N,O′-chelating
sites as a fluorescence sensor for highly selective and sensitive
metal ion detection. This work may pave the way for designing luminescent
COF sensors with functional binding sites for detecting specific metal
ions.