Engineering Covalent Organic Frameworks as Heterogeneous Photocatalysts for Organic Transformations

Abstract

Covalent organic frameworks (COFs) are an emerging category of organic polymers with highly porous crystalline structures. In the last decade, reports on the use of COFs as heterogeneous photocatalysts for organic transformations have shown significant progress. Still, comprehensive reviews on the mechanisms of the photocatalytic organic transformations using COFs are lacking. This Review provides a comprehensive and systematic overview of COF-based photocatalysts for organic transformations. Firstly, we discuss the photophysical properties and the characterization methods of COF-based photocatalysts. Then, the general photocatalytic mechanism, the advantages, and the strategies to improve the photocatalytic efficiency of COF-based photocatalysts are summarized. After that, advanced examples of COF-based photocatalysts for organic transformations are analyzed with regard to the underlying mechanisms. The Review ends with a critical perspective on the challenges and prospects.

Low band gap semiconducting covalent organic framework films with enhanced photocatalytic hydrogen evolution

“…[11][12][13][14] COFs are covalently connected two-(2D) or three-(3D) dimensional crystalline porous polymers that can be potentially applied in gas adsorption, separation, catalysis, energy storage, sensing and biomedical applications. [15][16][17][18] Particularly, 2D-COFs are promising as photocatalysts due to their extended pconjugated networks, which can serve as light harvesters, and their high surface areas, which provide a large number of accessible catalytically active sites for the required redox reactions. 19,20 The bottom-up synthesis of COFs is advantageous to introduce different types of p-conjugated building blocks, enabling tuneable band gaps, optical and electronic properties.…”

Extending the π-conjugation system of covalent organic frameworks for more efficient photocatalytic H2O2 production

“…In photocatalytic processes, there are in general three essential steps to convert light energy into chemical energy, namely the light-harvesting process, the generation and migration of photogenerated electron–hole pairs, and surface redox reactions. 17 COFs are good semiconductors to ensure sufficient mobility for the separation and migration of electron–hole pairs. 18 The advantage of COFs compared to other heterogeneous photocatalysts is their capacity to ensure efficient separation of electron–hole pairs through columnar π arrays, which also significantly narrows their bandgaps.…”

“…18 The advantage of COFs compared to other heterogeneous photocatalysts is their capacity to ensure efficient separation of electron-hole pairs through columnar π arrays, which also significantly narrows their bandgaps. 17 Additionally, the π-π stacking interaction, the long-range order, and the regularly integrated building blocks enable the transport and transformation of photogenerated charges. 19 Recently, our group reported for the first time the use of two imine-linked COFs for the photocatalytic metal-free production of H 2 O 2 through the ORR.…”