Visible Light Photocatalytic Systems Based On π-conjugated Molecules For Selective Oxidation Reactions

Visible light photocatalysis can carry out chemical reactions under mild conditions,which is one of the strategies to alleviate the problems of environmental degradation and energy shortage.Wherein visible light-induced activation of O₂ is a crucial step in photocatalytic selective aerobic oxidation reactions.Furthermore,the construction of efficient photocatalysts is the key to improving visible light utilization.In this context,a series of photoredox active semiconductors were constructed based onπ-conjugated molecules and applied to the research of visible light-induced activation of O₂,thereby realizing the selective oxidation of amines and sulfides.Firstly,based on the strategy of modifying Ti O₂ withπ-conjugated molecules,1,2-dihydroxyanthraquinone-Ti O₂（1,2-DHA-Ti O₂）and N-hydroxyphthalimide-Ti O₂（NHPI-Ti O₂）were constructed as two complex photocatalysts,which achieved the activation of O₂ and selective oxidation of sulfides or amines with the cooperative effect of（2,2,6,6-tetramethylpiperidin-1-yl）oxyl（TEMPO）under visible light irradiation.In this study,the twoπ-conjugated molecules were anchored on the surface of Ti O₂ by double hydroxyl bonding or N-hydroxyl bonding,respectively.The introduction of TEMPO promoted the charge transfer and ensured the stability ofπ-conjugated molecules,which finally enhanced the activity of the photocatalytic system.Secondly,theπ-conjugated molecules were directly connected by covalent bonds to construct covalent organic framework photocatalysts,which is beneficial to the fast transport of charges in the skeleton structure.Therefore,based on theπ-conjugated triazine with excellent optoelectronic activity,an sp² carbon-conjugated triazine-based COF was built,namely TP-COF.Using O₂ as the terminal oxidant,TP-COF accomplished blue light-induced selective oxidation of sulfides and amines by virtue of the large specific surface area,extendedπ-conjugated skeleton,good stability,and fast charge transfer.This work reveals the critical role ofπ-conjugated molecules as building blocks for the construction of photoactive COFs,emphasizing the designability of COFs.Lastly,π-conjugated molecule pyrene was selected as the building block to synthesize two pyrene-based COFs,namely Py-sp²c-COF and Py-COF.The results indicated that Py-sp²c-COF had a more stable chemical structure and exhibited better photocatalytic performance in the selective aerobic oxidation of sulfides and amines.More importantly,TEMPO can establish a cooperative system with Py-sp²c-COF,which significantly increases the reaction efficiency of the photocatalytic system and maintains the stability of COFs during the reaction.This study evidences that 2D COFs can establish a cooperative photocatalytic system with TEMPO.In conclusion,four types of photocatalysts based onπ-conjugated molecules have been assembled successfully,which realized the visible light-induced activation of O₂for the selective aerobic oxidation of sulfides and amines.The effectiveness of the molecular design and cooperative strategy has been established,offering insights for constructing photocatalytic selective aerobic oxidation reactions.