Ratiometric\nand Concomitant ‘Turn-Off/On’\nLuminescence Switch Based on Covalent–Organic Frameworks for\nNitrated Explosive Sensing in Aqueous Media

Luminescent\ncovalent–organic frameworks (LCOFs)\nhave great\npotential toward sensing explosive nitroaromatic compounds (NACs).\nHowever, intrinsically weak fluorescence properties of the imine-bonded\nCOFs limit their use as efficient explosive sensors. We report herein\ntetraphenylethylene-based LCOFs, <b>TPEPy</b> and <b>TPEB,</b> which show ‘turn-off’ fluorescence sensing upon the\naddition of different NACs through an intermolecular charge-transfer\n(ICT) process. Surprisingly, the <b>TPEPy</b> COF showed a fluorescence\nswitching from ‘turn-off’ to ‘turn-on’\nmode upon the addition of the highly acidic NAC, 2,4,6-trinitrophenol\n(TNP). The presence of a pyrene moiety presumably plays a crucial\nrole in the ‘turn-off/on’ property. The addition of\nTNP leads to the protonation of the imine-bonded N atom, and the columnar\nπ-stacking of the pyrene unit within the <b>TPEPy</b> COF\ngives rise to aggregate-induced enhanced luminescence, which dominates\nover the ICT process. This might be responsible for the fluorescence\n‘turn-on’ of the <b>TPEPy</b> COF, and there is\na large hypsochromic shift of the fluorescence emission maximum from\nλ<sub>max</sub> = 530 to λ<sub>max</sub> = 420 nm. This\nchange in the fluorescence emission passes through the near-white\nlight emission with CIE coordinates of (0.27, 0.35) (CIE coordinates\nof ideal white emission is 0.33, 0.33).