An intrinsically zwitter-ionic COF: a carboxylic acid and pseudo-tetrahedral sp3 nitrogen functionalized covalent organic framework with potential for humidity sensing

Abstract

Herein, we report a zwitter-ionic covalent organic framework (COF) simultaneously functionalized with carboxylic acid and protonatable sp³ nitrogen. The acidity and basicity of these functional groups are so well balanced that they can be easily protonated and deprotonated by using water/humidity. Color changes visibly display the dry and wet states. We believe that in the presence of moisture, it becomes zwitter-ionic, with the carboxylic acid getting deprotonated and concertedly protonating the sp³ nitrogen centers. This directly impacts the extent of conjugation in the carboxylate group carrying module of the COF, triggering the color change. This was corroborated by a lack of color change under humid conditions when a structurally equivalent COF devoid of carboxylic acid groups was employed. The pseudo-tetrahedral sp³ nitrogen centers and the freely rotatable bulky carboxylic acid group in the framework's backbone weaken the π-stacking between the COF layers; this allows the COF to exfoliate into nanosheets with ease. The organic dispersions of these nanosheets are highly luminescent under UV radiation. A trace quantity of water in the organic solvent completely quenches the luminescence, enabling the COF to act as a moisture sensor for organic solvents with a very impressive detection limit (LOD = 0.027% in acetonitrile; LOD = 0.128% in acetone). A Gaussian calculation reveals an alteration in the frontier orbitals of the zwitterionic vs. acid forms. DFT and MD modeling studies reveal that though the water's interaction with the COF is not strong, it is kinetically favored, which is important in reversible protonation–deprotonation chemistry. Thus, this designer COF represents an applicable solid-state humidity/water sensing material.