Solid-state fluorescent pH sensors based on pyrimidine/purine-cored covalent organic framework

Fluorescent covalent organic frameworks have been increasingly used as chemical sensors due to their inherent outstanding nature. Here we report two cases of β-ketoenamine typed fluorescent COFs, pyrimidine-cored covalent organic framework (TpApy-COF) and purine-cored covalent organic framework (TpApu-COF). The emission intensities of TpApy-COF and TpApu-COF in aqueous dispersion descend gradually with the ascending of pH value in the range of from 1 to 13. The reversible fluorescence transformation along with the pH change indicates the favorable reusabilities of TpApy-COF and TpApu-COF. The successful assembly of fluorescent liquid glue and fluorescent crayon, as well as their fluorescent turn on and turn off sensing of HCl and NH3, suggest positive practicability and processability. The formation and disintegration of extra intramolecular H-bonding during protonation and deprotonation play crucial roles in the reversible fluorescence switching.