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MOF/COF materials can detect TNT (2,4,6-trinitrobenzene) through fluorescence quenching (turn-off) or excitation (turn-on) mechanisms. However, direct fluorescence excitation is relatively rare and requires a clever design of the mechanism:
Electron transfer inhibition strategy:
The fluorescence of electron-rich MOF/COF (such as those containing pyrene or triphenylamine units) is usually quenched by TNT (electron transfer). However, if a strong quenching agent (such as nitrobenzene) is introduced beforehand, the competition for binding by TNT may restore some fluorescence (J. Am. Chem. Soc. 2016, 138, 3031).
TNT-triggered structural changes:
Design dynamic covalent COF, where TNT reacts with amino groups to cause bond reformation, releasing the locked fluorescent group (such as an aggregation-induced emission molecule AIEgen) (Angew. Chem. Int. Ed. 2020, 59, 8446).
Competitive coordination excitation:
The fluorescence of the fluorescent ligand in metal MOF (such as europium complexes) is inhibited by the nitro group of TNT. Adding a competitive ligand (such as phosphate) can replace TNT and restore fluorescence (ACS Sens. 2018, 3, 2434). Challenge: The high electron affinity of TNT makes it more prone to quenching fluorescence, thus requiring the reverse design of the signal reversal mechanism.
