Highly crystalline benzothiadiazole covalent organic framework for enhanced Cr(VI) photocatalytic reduction by constructing donor-acceptor structure

Covalent organic frameworks (COFs) have gained significant attention in environmental remediation. In this paper, we synthesized two sp2 carbon-conjugated COFs (HDU-107 and HDU-108) using [1,1 ':4 ',1"-terphenyl]4,4"-dicarbonitrile and 2,2 '-(benzo[c][1,2,5]thiadiazole-4,7-diylbis(4,1-phenylene))diacetonitrile as the core, and benzo[1,2-b:3,4-b ':5,6-b '']trithiophene-2,5,8-tricarbaldehyde as the linker, for efficient photocatalytic reduction of Chromium (VI) (Cr(VI)). HDU-108 successfully constructed an electron donor-acceptor (D-A) structure using electron-rich thiophene as the electron donor and benzothiadiazole as the acceptor, exhibiting excellent photocatalytic ability due to its structure, which facilitated the spatial separation of charge carriers and reduced charge complexation. This ability was far superior to that of HDU-107. For 10 mg of HDU-108, the efficiency of Cr(VI) reduction in 120 minutes was more than 99.9 %. And the cycle experiments confirmed its good reusability and stability. e- played a dominant role in the photoreduction of Cr(VI). Moreover, densityfunctional theory (DFT) simulations indicated that constructing D-A structures was an effective strategy for modulating photocatalytic activity. The charge separation was triggered by the polarization of the electron-rich BTT nucleus to the local charge density, while the cyanine conjugation provided the active center to concentrate the electronegativity. In conclusion, HDU-108, is expected to be a highly efficient photocatalytic material for environmental remediation.