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 pi-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 lambda(max) = 530 to lambda(max) = 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).