Driving Fluorescence by Forming AIEgens in a Hollow Azine-Linked Covalent Organic Framework

Highly conjugated two-dimensional covalent organic frameworks (2D-COFs) exhibit strong aggregation-caused quenching due to insurmountable π–π stacking. Nevertheless, their viscous microenvironment is conducive to the luminescence from aggregation-induced emission luminogens (AIEgens). Herein, differing from monomer-based AIE COFs, salicylaldehyde azine (SA), as an AIEgen, formed in the 2D azine COF (ACOF) for the first time through the condensation between two non-AIE monomers, 2-hydroxy-1,3,5-triformylbenzene and hydrazine (N2H4). To improve the dispersibility and accessibility, a sacrificial template was adopted to form a hollow ACOF (HACOF) by growing the ACOF in situ on ZIF-90 that was then digested under mild acidic conditions. Thus, a well-designed AIE structure, excellent dispersibility, small size, and low density were integrated. Strong red emission at 635 nm with a large Stokes shift of 260 nm was observed from the HACOF in different solvents and even the solid state, with quantum yield up to 13.2%. Intramolecular hydrogen bonding is the key to form the AIE segments for HACOF luminescence, proved by a non-emissive hydroxyl-free COF prepared with 1,3,5-triformylbenzene. The emission is completely quenched by ammonia (NH3), which breaks the hydrogen bond and unlocks the rotors of SA segments to disable AIE modules. The special emission mechanism endowed the HACOF with a low background, a low limit of detection (157 nM), and a broad response range (0.1–300 μM) for sensing NH3. Therefore, this novel HACOF sheds light on functionalized AIE COFs with a structure-dependent emission and quenching mechanism.