Fluorescent benzothiazole-based covalent-organic frameworks with intramolecular dual “lock” for ratiometric detection of fenthion

Fluorescent covalent-organic frameworks (COFs) are attracting extensive interest owing to their long-range ordered structure, well-ordered pores, and larger specific surface area. However, it is very difficult to obtain COFs with strong fluorescence because of intramolecular rotation. Here, a benzothiazole-based COFTFP-DBD with a strong intramolecular dual "lock" was prepared. Schiff base and sulfhydryl form benzothiazole ring as the first "lock," and hydroxyl and nitrogen atoms of benzothiazole ring form hydrogen bonds as the second "lock," thus providing strong intramolecular dual "lock." The intramolecular dual "lock" can reduce the intramolecular rotation effectively, and the hydrogen bonds achieve the excited-state intramolecular proton transfer, which results in a large Stokes shift. Accordingly, the COFTFP-DBD showed strong dual fluorescence emission at both 340 nm and 510 nm. The COFTFP-DBD can be used to detect fenthion selectively and visually based on two fluorescence changes due to fluorescence resonance energy transfer. The linear range is 9.31 ng/mL to 90 μg/mL, and the detection limit is 3.10 ng/mL. This work provides an effective strategy for rational selection and design of functional monomers to improve the fluorescence performance of COFs.