Triptycene Based 3D Covalent Organic Frameworks (COFs)—An Emerging Class of 3D Structures

Abstract

Covalent Organic Frameworks (COFs) are a newly emerged class of porous materials consisting of organic building blocks linked by strong covalent bonds. The physical and chemical properties of COFs, i.e., modularity, porosity, well-developed specific surface area, crystallinity, and chemical-thermal stability, make them a good application material, especially in the aspects of adsorption and gas separation. The organic compositions of their building blocks also render them with biocompatible properties; therefore, they also have potential in biomedical applications. Depending on the symmetry of the building blocks, COF materials form two-dimensional (2D COF) or three-dimensional (3D COF) crystal structures. 3D COF structures have a higher specific surface area, they are much lighter due to their low density, and they have a larger volume than 2D COF crystals, but, unlike the latter, 3D COF crystals are less frequently obtained and studied. Selecting and obtaining suitable building blocks to form a stable 3D COF crystal structure is challenging and therefore of interest to the chemical community. Triptycene, due to its 3D structure, is a versatile building block for the synthesis of 3D COFs. Polymeric materials containing triptycene fragments show good thermal stability parameters and have a very well-developed surface area. They often tend to be characterized by more than one type of porosity and exhibit impressive gas adsorption properties. The introduction of a triptycene backbone into the structure of 3D COFs is a relatively new procedure, the results of which only began to be published in 2020. Triptycene-based 3D COFs show interesting physicochemical properties, i.e., high physical stability and high specific surface area. In addition, they have variable porosities with different pore diameters, capable of adsorbing both gases and large biological molecules. These promising parameters, guaranteed by the addition of a triptycene backbone to the 3D structure of COFs, may create new opportunities for the application of such materials in many industrial and biomedical areas. This review aims to draw attention to the symmetry of the building blocks used for COF synthesis. In particular, we discussed triptycene as a building block for the synthesis of 3D COFs and we present the latest results in this area.