Covalent organic frameworks (COFs) are an emerging class of crystalline porous polymers that have attracted significant attention due to their tunable properties and structural robustness. As a result, COFs with luminescent properties are of great interest for fields such as chemical sensing, solid-state light emitters, photocatalysis, and optoelectronics. However, the bottom-up synthesis of luminescent COF systems remains a challenge in the field due to an abundance of competing non-radiative pathways, including phenomena such as aggregate caused quenching (ACQ). To overcome these obstacles, there has been a burgeoning investigation into the luminescent and photophysical properties of COFs. This review will highlight methods used to fabricate luminescent COFs and discuss the factors that are critical for their production. A collection of known luminescent COF systems will be featured. In addition, the ability to utilize the photophysical properties of COFs for applications related to photocatalysis, solid-state light emitters, and chemical sensing will be addressed. An outlook will address the current progress and remaining challenges facing the field to ultimately expand the scope of their applications.

The luminescent and photophysical properties of covalent organic frameworks.

Atomically 2D thin-layered structures, such as graphene nanosheets, graphitic carbon nitride nanosheets (g-C3N4), hexagonal boron nitride, and transition metal dichalcogenides are emerging as fascinating materials for a good array of domains owing to their rare physicochemical characteristics. In particular, graphitic carbon nitride has turned into a hot subject in the scientific community due to numerous qualities such as simple preparation, electrochemical properties, high adsorption capacity, good photochemical properties, thermal stability, and acid-alkali chemical resistance, etc. Basically, g-C3N4 is considered as a polymeric material consisting of N and C atoms forming a tri-s-triazine network connected by planar amino groups. In comparison with most C-based materials, g-C3N4 possesses electron-rich characteristics, basic moieties, and hydrogen-bonding groups owing to the presence of hydrogen and nitrogen atoms; therefore, it is taken into account as an interesting nominee to further complement carbon in applications of functional materials. Nevertheless, g-C3N4 has some intrinsic limitations and drawbacks mainly related to a relatively poor specific surface area, rapid charge recombination, a limited light absorption range, and a poor dispersibility in both aqueous and organic mediums. To overcome these shortcomings, numerous chemical modification approaches have been conducted with the aim of expanding the range of application of g-C3N4 and enhancing its properties. In the current review, the comprehensive survey is conducted on g-C3N4 chemical functionalization strategies including covalent and noncovalent approaches. Covalent approaches consist of establishing covalent linkage between the g-C3N4 structure and the chemical modifier such as oxidation/carboxylation, amidation, polymer grafting, etc., whereas the noncovalent approaches mainly consist of physical bonding and intermolecular interaction such as van der Waals interactions, electrostatic interactions, π-π interactions, and so on. Furthermore, the preparation, characterization, and diverse applications of functionalized g-C3N4 in various domains are described and recapped. We believe that this work will inspire scientists and readers to conduct research with the aim of exploring other functionalization strategies for this material in numerous applications.

Emerging Chemical Functionalization of g-C3N4: Covalent/Noncovalent Modifications and Applications.

Covalent organic frameworks (COFs), as a new emerging class of highly crystalline advanced porous materials with fascinating structural tunability and diversity as well as the desired semiconductor properties, have gained significant attention as highly promising and efficient photocatalysts or designer platforms for a variety of photocatalytic applications in recent years; thus a comprehensive review is timely to summarize the advances of this field. In this review, a background and brief timeline concerning the developments and key achievements of COFs are provided. Afterwards, a systematic overview of the potential photocatalytic applications realized to date in the fast growing field of COFs is provided with the aim of presenting a full blueprint of COFs for possible photochemical energy conversion and reactions. Finally, the challenges remaining and personal perspectives on further development of this type of material for photocatalysis are presented.

/pdf/covalent-organic-frameworks-emerging-high-performance-1bs4av2oh6.pdf

Covalent organic frameworks: emerging high-performance platforms for efficient photocatalytic applications

Recent developments in the photocatalytic applications of covalent organic frameworks: A review

Covalent-Organic-Framework-Based Composite Materials

Designing photocatalysts with heterostructures is an effective way to promote visible-light-driven photocatalytic degradation. Herein, a series of 2D/2D heterojunction photocatalysts, denoted as CuPor-Ph-COF/g-C3N4 composites, were prepared through in situ synthesis on the surface of g-C3N4 by a facile liquid-assisted grinding method. The photocatalytic performance of the as-prepared CuPor-Ph-COF/g-C3N4 composites was evaluated by the degradation of a model pollutant rhodamine B. The CuPor-Ph-COF/g-C3N4 composites displayed superior photocatalytic performance to pure g-C3N4 or pure CuPor-Ph-COF because of the faster separation of photogenerated charges. This represents the first composite fabricated between a 2D porphyrin-based covalent organic framework (COF) and g-C3N4, demonstrating not only the possibility but also more importantly the affordability of the application of costly porphyrin-based COFs in catalysis.

A hybrid of g-C 3 N 4 and porphyrin-based covalent organic frameworks via liquid-assisted grinding for enhanced visible-light-driven photoactivity.

Two nitrogen-rich azo-bridged porphyrin porous organic polymers (Azo-Por-Bpy-POP and Azo-Por-Dadp-POP) with high surface areas were prepared by coupling 5,10,15,20-tetra(p-nitrophenyl)-porphyrin with the aromatic amines of 2,2′-bipyridine-5,5′-diamine (Bpy) and diaminodiphenyl (Dadp). Azo-Por-Bpy-POP and Azo-Por-Dadp-POP display high photocatalytic reduction activity for CO2 to CO under visible-light irradiation without a sacrificial reagent or metal co-catalyst. Azo-Por-Bpy-POP exhibits the highest photoreduction for CO2 with CO as the only carbonaceous reduction product with a production rate of 38.75 μmol g−1 h−1. Theoretical investigations indicate a stronger electrostatic interaction between CO2 and Azo-Por-Bpy-POP than Azo-Por-Dadp-POP, which favors CO2 photoreduction.

Enhui Zhang

Papers

A hybrid of g-C 3 N 4 and porphyrin-based covalent organic frameworks via liquid-assisted grinding for enhanced visible-light-driven photoactivity.

Metal-free azo-bridged porphyrin porous organic polymers for visible-light-driven CO2 reduction to CO with high selectivity.