Rational engineering of imine covalent organic frameworks with tunable emission for ratiometric fluorescence sensing...

The low emission of imine-based covalent organic frameworks (COFs) has limited their application in sensing fields. Exploring the guiding principle between framework structure and luminescence properties was crucial for rational design and fabrication of imine-linked COFs-based sensors. Herein, we report the solvothermal synthesis of three imine-linked COFs (COF, COF-OMe, COF-OH) using the monomers 2, 5-dimethoxyterephthalohydrazide (ETZ) with 1,3,5-triformylbenzene (TFB), 2,4,6-trimethoxybenzene-1,3,5-tricarbaldehyde (TBT) and 2-hydroxy-1,3,5-benzenetricarboxaldehyde (TFP), respectively. All the prepared COFs exhibited high crystallinity, robust chemical/thermal stability and a large surface area. More importantly, their emission red-shifted from blue light (475 nm) to red light (621 nm) with an increase in π-system density and strengthened intermolecular hydrogen bonding, which provided critical insights for the molecular engineering of luminescent properties in imine-based COFs. Notably, one of representative COF named COF-OH can selectively capture Au(III) and efficiently reduce them into blue-emitted gold nanocluster (Au NCs). Meanwhile, the fluorescence of COF-OH was quenched by fluorescence resonance energy transfer (FRET) and photoinduced electron transfer (PET). This unique feature endows COF-OH with significant potential for ratiometric fluorescence detection of Au(III).