Ratiometric and Concomitant ‘Turn-Off/On’ Luminescence Switch Based on Covalent–Organic Frameworks for Nitrated Explosive Sensing in Aqueous Media

Luminescent covalent–organic frameworks (LCOFs) have great potential toward sensing explosive nitroaromatic compounds (NACs). However, intrinsically weak fluorescence properties of the imine-bonded COFs limit their use as efficient explosive sensors. We report herein tetraphenylethylene-based LCOFs, TPEPy and TPEB, which show ‘turn-off’ fluorescence sensing upon the addition of different NACs through an intermolecular charge-transfer (ICT) process. Surprisingly, the TPEPy COF showed a fluorescence switching from ‘turn-off’ to ‘turn-on’ mode upon the addition of the highly acidic NAC, 2,4,6-trinitrophenol (TNP). The presence of a pyrene moiety presumably plays a crucial role in the ‘turn-off/on’ property. The addition of TNP leads to the protonation of the imine-bonded N atom, and the columnar π-stacking of the pyrene unit within the TPEPy COF gives rise to aggregate-induced enhanced luminescence, which dominates over the ICT process. This might be responsible for the fluorescence ‘turn-on’ of the TPEPy COF, and there is a large hypsochromic shift of the fluorescence emission maximum from λmax = 530 to λmax = 420 nm. This change in the fluorescence emission passes through the near-white light emission with CIE coordinates of (0.27, 0.35) (CIE coordinates of ideal white emission is 0.33, 0.33).