Π-Skeleton Tailoring of Olefin-Linked Covalent Organic Frameworks Achieving Low Exciton Binding Energy for Photo-Enhanced Uranium Extraction from Seawater

Adsorption-photocatalysis technology based on covalent organic frameworks (COFs) offers an alternative method for advancing the field of uranium extraction from seawater. When determining the photocatalytic activity of COFs, the binding energy of excitons (E-b) functions is the decisive factor. Nevertheless, the majority of reported COFs have a large E-b, which seriously restricts their application in the field of photocatalysis. Using a practical pi-skeleton engineering strategy, the current study synthesizes three donor-acceptor olefin-linked COFs containing amidoxime units in an effort to minimize E-b. Theoretical and experimental results reveal that the construction of planar and continuous pi-electron delocalization channels can significantly reduce E-b and promote the separation of electron-hole pairs, thereby enhancing the photocatalytic activities. Moreover, the E-b of the TTh-COF-AO with a planar pi-skeleton donor is significantly reduced, and exhibits a substantially smaller E-b (38.4 meV). Under visible light irradiation, a high photo-enhanced uranium extraction capacity of 10.24 mg g(-1) is achieved from natural seawater without the addition of sacrificial reagents, which is superior to the majority of olefin-linked COFs that have been reported to date. This study, therefore, paves the way for the development of tailored, efficient COFs photocatalysts for the extraction of uranium from seawater.