Abstract
The tuning of molecular switches in solid state toward stimuli-responsive materials has attracted more and more attention in recent years. Herein, we report a switchable three-dimensional covalent organic framework (3D COF), which can undergo a reversible transformation through a hydroquinone/quinone redox reaction while retaining the crystallinity and porosity. Our results clearly show that the switching process gradually happened through the COF framework, with an almost quantitative conversion yield. In addition, the redox-triggered transformation will form different functional groups on the pore surface and modify the shape of pore channel, which can result in tunable gas separation property. This study strongly demonstrates 3D COFs can provide robust platforms for efficient tuning of molecular switches in solid state. More importantly, switching of these moieties in 3D COFs can remarkably modify the internal pore environment, which will thus enable the resulting materials with interesting stimuli-responsive properties.
Highlights
The tuning of molecular switches in solid state toward stimuli-responsive materials has attracted more and more attention in recent years
Based on our reported topology design to construct 3D COFs46,47, we designed and synthesized a linker TPB-HQ (Supplementary Fig. 1), which can react with tetra(p-aminophenyl)methane (TAPM) through [4 + 4] imine condensation reactions to form 3D-TPB-Covalent organic frameworks (COFs)-HQ (Fig. 1)
The formation of imine bonds was assessed by Fourier transform infrared (FT-IR) spectroscopy and solid-state NMR spectroscopy
Summary
The tuning of molecular switches in solid state toward stimuli-responsive materials has attracted more and more attention in recent years. Metal-organic frameworks (MOFs) have shown the ability in maintaining the property of molecular switches in solid state, by immobilizing them into the framework as organic components[12,13,14] It should be mentioned here, the moderate stability of MOFs will be a fatal obstacle to the development of this field[13], especially after considering their practical use as stimuli-responsive porous materials. Trabolsi et al reported an azobenzene-equipped 2D COF that can be used as light-operated reservoir[41] These systems have shown interesting stimuli-responsive properties, there still has some problems that need to be further improved, such as inefficient conversion. These drawbacks may probably be explained by their inherent 2D structures, as the strong π–π interaction in the adjacent layers can strongly restrict the transformation of molecular switches. Due to the modification of pore environment during the transformation process, 3D-TPB-COF-Q exhibits a much higher CO2/N2 selectivity compared to 3D-TPB-COF-HQ, indicating a remarkable stimuli-responsive separation effect
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