Light-emitting conjugated microporous polymers based on an excited-state intramolecular proton transfer strategy and selective switch-off sensing of anions

Conjugated microporous polymers (CMPs) are a novel class of porous materials that possess π-conjugated porous architectures, and are very popular to construct light-emitting materials. However, most CMPs emit weak luminescence. Here, highly emissive CMP-A and CMP-B were designed on the basis of an excited-state intramolecular proton transfer (ESIPT) strategy via intramolecular hydrogen bonds between hydroxyl groups and imine bonds, which endow the CMPs with strong orange-red luminescence with high absolute fluorescence quantum yield of 10% in the solid state. Interestingly, the ESIPT strategy restriction caused the fluorescence quenching pathway through the hydrogen-bond interaction between the hydroxyl groups and anions. The process could only be triggered by fluoride anions while other halogen anions (chloride and bromide) and acid anions (hex-phosphate, hydrogen sulfate, and nitrate) kept inactive, which offered the selective fluorescence switch-off sensing of the fluoride anions. Remarkably, the CMPs also exhibited high sensitivity and selectivity to fluoride anions. The detection limit was below two hundred nanomolar, which is ranked the best among fluoride sensor systems. This research opened a new structure design for chemical-sensors in porous materials.