Design and synthesis of covalent organic frameworks

Covalent organic frameworks (COFs) materials are crystalline polymer networks constructed by organic building blocks. Their tunable structure, low density, well-defined pores, high surface areas and high thermal stability render them have promising applications in gas storage and separations, energy storage, catalysis, sensing and optoelectronic. This review focuses on the design principle of COF based on the structure and functionality, and summarizes the recent synthesize method of COF bulk powder, single layer COFs and COF films. The design principle of COFs is classified into the structural-oriented and functional-oriented protocols. For structural design, the basic principle is reticular chemistry. The geometry of the building units and the symmetry of the reaction group determine the dimension, topology, structure, and the pore size and shape of COFs. The reaction types of COF are usually based on thermodynamic controlled reversible reaction. Moreover, the functionality of COFs can be easily modulated via three strategies, including directly using functionalized building blocks to construct COFs, post-synthetic modification of the pre-prepared framework, and encapsulation of functional molecules into the channel of COFs. Along with the structural and functional design, various COF materials have been reported and greatly boosted the developments of this field. Then we summarized the synthetic methodologies of COFs. In general, the developed synthetic methodologies follow three aspects: (1) fast and efficient; (2) for large-scale industrial production; (3) for processing shaped-body of COF. Recently, there are several available methods to fabricate COF into free-standing films including the liquid-liquid interfacial polymerization, vapor-assisted conversion and baking the molecular precursors. These approaches indicate a way forward for accessing COF into films and enable applying to real industrial applications. Although there has been a significant expansion of COF chemistries over the past 15 years, yet this field is far from mature, the article ends with a perspective on the future developments in this growing field and discusses the challenges remain for these materials. To date, it is still hard to obtain single crystals, thus advanced characterization techniques and theoretical calculation methods are highly demanded to be developed. The mechanisms for the nucleus formation and growth are still needed to be deeply investigated. In addition, exploring novel topologies and linkages of COF are necessary from the chemistry perspective. Moreover, in order to precisely construct and tune the functions of COF materials, further studies on the structure-property correlation via the directly design functional building blocks and encapsulation of functional molecules into the pores are still required. In terms of the processing COF from the particle to shaped bodies and to devices would be critically important in many practical applications, like gas separation, catalysis and organic light-emitting diodes, thus further development towards the efficient synthesis of COF films or membranes with large area, high orientation, no defect, and uniform thickness would be of great interest. For COF materials, advances in the fast, efficient and economic fabrication methods and the exploration of new functionalization would make them become the commercialized material for industrial application in the fields of energy, health, environment and so on.