are considered as suitable candidates for various applications such as gas storage, separation, and catalysis because of their highly ordered structure and tailored porosity (microporosity (<2 nm), mesoporosity (2-50 nm), and macroporosity (>50 nm)). [15,16] However, the development of a kind of porous organic materials with a predesignable structure remains a great challenge, which is crucial to achieve desirable and tunable functionalities. The emergence of "reticular chemistry" enlightened the discovery of a kind of novel porous polymeric material, [17][18][19] namely covalent organic frameworks (COFs). COFs are crystalline, porous, organic polymers composed of organic building blocks and interconnected by strong covalent bonds, which are receiving substantial attention due to their unique features of light weight, good thermal stability, permanent and periodic porosity, adjustable pore sizes, and large surface area. [20][21][22][23][24][25] Inspired by these merits, COFs have emerged as ideal candidates for a wide range of applications, such as gas storage and separation, [26][27][28][29][30] adsorption, [31][32][33] catalysis, [34][35][36][37] sensors, [38,39] optoelectronics devices, [40,41] drug delivery, [42][43][44] energy storage and conversion. [45][46][47][48][49] Unlike the traditional amorphous organic polymers which are formed by strong but irreversible covalent bonds, COFs allow reversible condensation between building blocks, [50,51] leading to the construction of highly ordered crystalline structures due to the "error correction" during the crystallization process. [52,53] Depending on the topological establishment of building blocks, COFs are classified into 2D COFs and 3D COFs. [54,55] Notably, 2D COFs are commonly regarded as a new class of layer-stacked materials and have attracted considerable interests because they exhibit not only 2D extended layered structures but also periodic columnar π-arrays. [56][57][58] The first two 2D COFs linked by boroxine and boronate ester were reported in 2005, [59] which were highly sensitive to acid, base, and even moisture, demonstrating reasonably poor chemical stability. Since then, a large number of robust 2D COFs have been reported to be constructed from numerous chemically stable linkages such as triazine, [60] imine, [61][62][63][64] hydrazone, [65] imide, [66,67] ether, [68] olefin, [69] and oxazole [70,71] (Figure 1a). In addition, based on the symmetry of the monomers, 2D COFs can be designed in various topologies (tetragonal, hexagonal, rhombic, and trigonal), exhibiting different pore sizes and shapes (Figure 1b). By Covalent organic frameworks (COFs), an emerging class of organic crystalline polymers with highly oriented structures and permanent porosity, can adopt 2D or 3D architectures depending on the different topological diagrams of the monomers. Notably, 2D COFs have particularly gained much attention due to the extraordinary merits of their extended in-plane π-conjugation and topologically ordered columnar π-arrays. These properties tog...
