pH-Dependent Slipping and Exfoliation of Layered Covalent Organic Framework.

Layered/two-dimensional covalent organic frameworks (2D COF) are crystalline porous materials composed of light elements linked by strong covalent bonds. Interlayer force is one of the main factors directing the formation of a stacked-layer structure, which plays a vital role in the stability, crystallinity, and porosity of layered COF. The as-developed new way to modulate the interlayer force of imine-linked 2D TAPB-PDA-COF by only adjusting solution pH. In alkaline and neutral pH, pore size of COF decreases from 34 A due to the turbostratic effect. In highly acidic conditions (pH 1), the TAPB-PDA-COF shows faster and stronger turbostratic effect, thus causing the 2D structure to exfoliate. This yields bulk quantities of exfoliated few/single-layer 2D COF, which were well dispersed and displayed a clear Tyndall effect (TE). Furthermore, nanopipette-based electrochemical testing also confirms the slipping of layers with increase in acidic pH. A model of pH-dependent layer-slipping of TAPB-PDA-COF is then proposed. This controllable pH-dependent change in layer structure may open a new door for potential applications in controlled gas adsorption/desorption and drug loading/releasing.