Merging benzotrithiophene covalent organic framework photocatalysis with TEMPO for selective oxidation of organic sulfides
image: TEMPO mediates hole transfer between BTT-sp2c-COF and organic sulfides
Credit: ©Science China Press
Covalent organic frameworks (COFs) are covalently linked crystalline reticular materials with highly ordered structures and permanent porosity. Due to the designable structures and controllable syntheses, various COFs have been constructed with rigid backbones linked by covalent bonds and widely applicated in photoactive materials. In particular, visible light photocatalysis of COFs has attracted extensive interest in recent years. Notably, the inherent sp2 carbon-conjugation enhances the chemical durability and in-plane π-electron delocalization, rendering COFs superior performance in various domains. As such, sp2 carbon-conjugated COFs also perform well in photocatalytic applications. Benzotrithiophene (BTT) as an electron-rich building block performs well in optoelectronic applications with excellent hole transport behavior. BTT, with coplanarity, extended π-conjugation, and facilitated π-electron delocalization, has emerged as the building block of choice and necessity in assembling COFs for photocatalysis.
It is of great significance to construct an efficient and stable photocatalytic organic conversion system. Therefore, a fully conjugated BTT COF, BTT-sp2c-COF is synthesized for the selective oxidation of organic sulfides. More importantly, TEMPO (2,2,6,6-tetramethylpiperidin-1-yl)oxyl, 1 mol%) could considerably accelerate the selective oxidation of organic sulfides with O2 over BTT-sp2c-COF. TEMPO mediates hole transfer between BTT-sp2c-COF and organic sulfides, and oxygen is incorporated into sulfoxides via an electron transfer pathway. Merging BTT-sp2c-COF photocatalysis with TEMPO generally applies to transforming organic sulfides into sulfoxides. This work implies the full π-conjugation of electron-rich building blocks into COFs is a viable strategy for selective visible light photocatalysis.
A plausible mechanism is proposed with O2•− as the dominant ROS of selective oxidation of organic sulfides by merging BTT-sp2c-COF photocatalysis with TEMPO. Upon green light irradiation, photo-induced excitation leads to the generation of e− and h+ on the surface of BTT-sp2c-COF. The separated charge carriers are injected into the reduction and oxidation procedure. TEMPO serves as a hole mediator between BTT-sp2c-COF and organic sulfides, and O-atoms are incorporated into sulfoxides via an electron transfer over BTT-sp2c-COF. The processes that photogenerated e− produces O2•− and photogenerated h+ promote the TEMPO cycle to activate sulfide substrate occur simultaneously. The activated sulfur cation radical is subjected to the attack of O2•− and transformed into peroxy sulfoxide product. Eventually, the final sulfoxide product is obtained under the function of CH3OH.
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Merging benzotrithiophene covalent organic framework photocatalysis with TEMPO for selective oxidation of organic sulfides