Three-Motif Molecular Junction Type Covalent Organic Frameworks for Efficient Photocatalytic Aerobic Oxidation

Abstract

Covalent organic frameworks (COFs), with the features of flexible structure regulation and easy introduction of functional groups, have aroused broad interest in the field of photocatalysis. However, due to the low light absorption intensity, low photoelectron conversion efficiency, and lack of suitable active sites, it remains a great challenge to achieve efficient photocatalytic aerobic oxidation reactions. Herein, based on reticular chemistry, we rationally designed a series of three-motif molecular junction type COFs, which formed dual photosensitizer coupled redox molecular junctions containing multifunctional COF photocatalysts. Significantly, due to the strong light adsorption ability of dual photosensitizer units and integrated oxidation and reduction features, the PY-BT COF exhibited the highest activity for photocatalytic aerobic oxidation. Especially, it achieved a photocatalytic benzylamine conversion efficiency of 99.9% in 2.5 h, which is much higher than that of the two-motif molecular junctions with only one photosensitizer or redox unit lacking COFs. The mechanism of selective aerobic oxidation was studied through comprehensive experiments and density functional theory calculations. The results showed that the photoinduced electron transfer occurred from PY and then through triphenylamine to BT. Furthermore, the thermodynamics energy for benzylamine oxidation on PY-BT COF was much lower than that for others, which confirmed the synergistic effect of dual photosensitizer coupled redox molecular junction COFs. This work provided a new strategy for the design of functional COFs with three-motif molecular junctions and also represented a new insight into the multifunctional COFs for organic catalytic reactions.

Two-Dimensional Covalent Organic Frameworks with Pentagonal Pores

“…Covalent organic frameworks (COFs) have emerged as a new kind of porous polymers, which allow the assembly of various organic building blocks through strong covalent bonds at atomic precision. − Owing to their specific construction modes, most COFs have exhibited multiple advantages, such as high crystallinity, flexible structures, and strong robustness, which endow these materials with tremendous application foreground in gas adsorption, − heterogeneous catalysis, − optoelectronics, − sensing, − and energy storage. − Although a large number of two-dimensional (2D) COFs with triangular, tetragonal, and hexagonal pores have been developed through the rational combination of building blocks with different symmetries, − the construction of COFs with pentagonal pores remains a great challenge as it conflicts with the principle of plane geometry. − …”