A Fluorescent Covalent Organic Cage for Ultrafast Detection of Picric Acid and HCl Vapor Sensing

Abstract Covalent organic cages (COCs) have recently gained massive attention owing to their solution processability and structural flexibility. Herein, we report an amine‐linked fluorescent COC (COC2) synthesized by adopting dynamic covalent imine chemistry followed by imine bond reduction and characterized with different spectroscopic techniques. The COC2 was utilized for highly sensitive, selective, and ultrafast detection of picric acid at the nanomolar level. The fluorescence quenching efficiency of PA towards the COC2 is 98.6 %, with a detection limit of 2.7 nM. PA sensing with the COC2, coated on a TLC plate and paper strip, exhibited an outstanding fluorescence quenching property. Furthermore, the COC2 unveiled solid‐state acidochromism upon exposure to HCl acid fumes and was transferred back to the original form on exposure to NH3 vapors.