Research Progress of Carbon-carbon Bond Linked Two-dimensional Covalent-Organic Frameworks

Two-dimensional polymers (2DPs) are a type of planar polymer materials that possess regular porous structures.They fulfill the demand for thin, high-performing, and stable materials in flexible devices, making them highly potential candidates for applications in the field of flexible electronics.As a special class of covalent two-dimensional polymer materials, two-dimensional covalent organic frameworks (COFs) refer to crystalline porous materials with a two-dimensional topology formed by connecting π-conjugated building units through covalent bonds.The unique electronic structure of COFs gives them better electrical properties compared to other two-dimensional polymers.Furthermore, their unique periodic porous structure, high specific surface area, and excellent stability make them highly suitable for various applications such as ion transport, optoelectronic materials, and catalysis.Among these, carbon-carbon bond-linked COFs are regarded as one of the most promising types of two-dimensional polymers due to their excellent stability and good crystallinity.In recent years, many carbon-carbon bonded COFs with different structures and excellent properties have emerged based on different design principles and synthesis strategies.In this review, we summarize and introduce four common synthesis methods for preparing C＝C bonded COFs, namely solvent-thermal method, melt-polymerization method, interface polymerization method, and copper template method.Furthermore, we categorize C＝C bonded COFs into four classes: [C2＋C3], [C2＋C2], [C3＋C3], and [C4＋C2], according to the topological structure of the building units.We focus on analyzing the relationship between the composite structure of these COFs and their stability, electrical properties, catalytic performance, and other properties.Additionally, we compile and summarize the research progress of C＝C bonded COFs in terms of synthesis methods, structural innovation, performance improvement, and practical applications.This compilation will be beneficial for researchers in the subsequent studies of C＝C bonded COFs to select building units based on target structure and performance application and conduct pre-design.Furthermore, this review also includes previously overlooked C-C bonded COFs, providing a more comprehensive reference.In summary, this review aims to provide guidance for researchers in related fields to better design and synthesize multifunctional crystalline porous materials, thereby promoting the further development and application of carbon-carbon bond-linked COFs in various fields.