Robust Thiazole-Linked Covalent Organic Frameworks with Post-Modified Azobenzene Groups: Photo-Regulated Dye Adsorption and Separation

Due to the intrinsic porosity and photo-regulated pore environment, azobenzene (Azo) functionalized covalent organic frameworks (COFs) show great potential for contaminant removal. However, the stability and degree of functionality of the COFs greatly affect the porosity and subsequent adsorption capacity and selectivity. Herein, a highly stable thiazole-linked COF containing phenolic hydroxyl groups (OH-COF) is constructed by using 4,4 ',4 '-(1,3,5-triazine-2,4,6-triyl)trianiline and 2,5-dihydroxyterephthalaldehyde in the presence of sulfur, which shows high crystallinity and specific surface. Azo-functionalized COFs ((Azo)x-COFs) are prepared by grafting different amounts azobenzene groups onto OH-COF through post-modification, whose aperture is conveniently tuned by the grafted azobenzene amount. The pore size of (Azo)x-COFs also can be reversibly adjusted under UV and visible light irradiation without affecting the crystallinity. Appropriate amount of Azo-grafted (Azo)0.1-COF possessed a high adsorption capacity (1216.93 mg g-1) to Congo red (CR). While the adsorption capacity of (Azo)1.0-COF to CR significantly increased 2.9 times to reach 1489.96 mg g-1 after UV irradiation. (Azo)1.0-COF also can selectively separate dye molecules with different sizes based on photo-regulated pore size, showing excellent reversibility and reusability. As such, the ability to photo-regulate the adsorption and interception of (Azo)x-COFs highlights their significance in functioning as smart porous nanomaterials for pollutant removal and water purification. Thiazole-linked azobenzene-functionalized COFs ((Azo)x-COFs) with high stability, crystallinity, and different azobenzene grafting amount are prepared through the post-modification method. (Azo)1.0-COF exhibits a 2.9 times improvement of adsorption capacity, and selectively separated different dyes based on the molecular size and adjusted the rejection rate of dyes by UV irradiation, which is ascribed to the intelligent closing/opening of molecular gates rendered by photo-isomerization of azobenzene groups. image