Converting the covalent organic framework linkage from hydrazone to thiadiazole toward blue light-powered selective conversion of organic sulfides

Abstract

Covalent organic frameworks (COFs) are composed of various organic linkers with dynamic covalent bonds as linkages, which, in part, establish functionality. Nevertheless, these covalent bonds are usually not robust enough under reaction conditions and, therefore, can be converted into irreversible ones because the application of COFs hinges on the characteristics of these linkages. In this work, TFPT-COF, a triazine-based COF with a hydrazone linkage, is constructed. Subsequently, the hydrazone linkage is converted to a thiadiazole linkage through thionation and oxidative cyclization at 115 °C, converting TFPT-COF to TDA-COF with a robust and irreversible thiadiazole linkage. The band gap is narrowed, and the electronic structure is altered from TFPT-COF to TDA-COF based on density functional theory calculations. Besides, experimental results suggest that TDA-COF possesses broadened light absorption and improved optoelectronic properties compared to TFPT-COF. Converting TFPT-COF to TDA-COF significantly shifts the photocatalytic activity for the selective conversion of thioanisoles to sulfoxides with oxygen (O₂). TDA-COF drives the blue light-powered conversion of thioanisoles to sulfoxides with O₂, whereas TFPT-COF shows almost no activity. TDA-COF is especially accessible for converting various sulfides to sulfoxides with high selectivities, while maintaining high photocatalytic activity and stability over six cycles. This work demonstrates that converting the dynamic linkages of COFs to irreversible ones contributes positively to photocatalytic activities.