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MOF/COF materials can detect TNT (2,4,6-trinitrobenzene) through fluorescence quenching (turn-off) or fluorescence activation (turn-on) mechanisms. However, direct fluorescence activation is relatively rare and requires a carefully designed mechanism.
Electron transfer inhibition strategy:
The fluorescence of electron-rich MOF/COF materials (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 binding of TNT may restore some fluorescence (J. Am. Chem. Soc. 2016, 138, 3031).
The TNT triggers structural changes:
Design dynamic covalent COFs, where the reaction between TNT and amines leads to bond reorganization, releasing the locked fluorescent groups (such as aggregation-induced emission agents AIEgens) (Angew. Chem. Int. Ed. 2020, 59, 8446).
Competitive Coordination Illumination:
The luminescence of fluorescent ligands (such as europium complexes) in metal MOFs is inhibited by the nitro group of TNT. The addition of competing ligands (such as phosphate) can displace TNT and restore the 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 a signal reversal mechanism.
