Recent advances in fluorescent 3D covalent organic frameworks

Covalent organic frameworks (COFs) represent an emerging class of crystalline porous organic materials constructed by covalently linking organic molecules into two-dimensional (2D) or three-dimensional (3D) structure, which is the cutting-edge research in chemistry and material science. Due to their large surface areas, low density, high stability and ordered frameworks, COFs are promising materials for adsorption and separation, catalysis, energy storage, sensing and so on. Among them, the application of fluorescent COFs in the field of sensing has attracted a lot of attention with the following advantages: (1) Specific molecular recognition can be achieved based on the tailor-made structure of fluorescent COFs; (2) the inherent porous structure of COFs can achieve efficient sensing by facilitating the interaction between the guest molecule and the functional unit; (3) their excellent stability can maintain the structure intact during the sensing process, which is important in recycling; and (4) the structure-property-function relationships can guide the design of fluorescent COFs with superior performance. Up to now, most of the reported fluorescent COFs were fluorescent 2D COFs and few fluorescent 3D COFs have been reported. In fact, 3D COFs, in which the molecular building blocks are extended in three dimensions by covalent self-assembly connections, can efficiently avoid the fluorescence quenching caused by the stacking of fluorescent units. Therefore, 3D COFs are considered as an ideal candidate for the construction of luminescent materials. In this review, we summarized the recent research progress in fluorescent 3D COF, including the methods used to construct fluorescent 3D COFs and their applications in chemical sensing, light-emitting materials and bioimaging. In addition, the challenges and prospects of fluorescent 3D COF are also discussed. In the first part, we discussed the synthetic strategies used to construct fluorescent 3D COFs. In general, fluorescent 3D COFs can be obtained by direct synthesis of the corresponding fluorescent building blocks or by post-synthetic modification of the pre-prepared frameworks. Many fluorescent units, including pyrene, tetraphenylene, and anthracene have been successfully incorporated into 3D COFs to construct fluorescent 3D COFs. Besides, fluorescent 3D COFs can also be synthesized directly via the formation of conjugated C=C bond. In addition, luminescent ions could also be successfully immobilized into the framework via post-synthetic modification method to construct fluorescent 3D COFs. Then we summarized the applications of these fluorescent 3D COFs in detection (e.g., explosive detection, volatile organic compounds sensing and metal ions detection), fabricating optoelectronics, and cell imaging. In the last part, we give the conclusion about the review, including a summary of the construction methods of fluorescent 3D COFs and their applications in sensing, optoelectronics, and imaging. However, there are still few studies on fluorescent 3D COFs due to the problems that hinder the development. First, the fluorophores suitable for the construction of fluorescent 3D COFs are quite limited. Second, the synthesis and structure determination of fluorescent 3D COFs are very challenging. Third, there are rather few studies on structure-property-function relationships, which makes their target design and synthesis very challenging. Since the excitation energy dissipation caused by Schiff base and the aggregation caused fluorescence quenching are the main reasons for the weak fluorescence of 3D COFs, constructing 3D COFs with C=C bonds and suppressing the aggregation quenching between fluorescent units or constructing 3D COFs with AIE molecules are potential strategies for the synthesis of fluorescent 3D COFs with strong fluorescence. We believe with the efforts from all the researchers worldwide, fluorescent 3D COFs will develop rapidly as the design, synthesis and structure determination of 3D COFs continue to evolve.