Calix[4]resorcinarene-Based Porous Organic Cages: Synthesis and Applications

ConspectusPorous organic cages (POCs), characterized by well-defined cavity structures, have gradually emerged as crucial carriers in materials science, nanoscience, and bioscience over the past decade, owing to their unique recognition and encapsulation properties. However, there are still several key scientific issues in the current POCs system that require urgent attention: (1) The universal applicability of existing POC assemblies is fundamentally constrained by the limited tunability afforded to their constituent building blocks. (2) There is a need for new strategies to efficiently prepare robust POCs with good water and chemical stability, especially those with large cavities capable of encapsulating larger and/or more guests. (3) The exploration of POC applications is still in its initial stage, and the intrinsic relationship between the structure and properties of POCs is not yet clear. Therefore, it is imperative to explore new building blocks and synthetic methods for the efficient preparation of robust and large-sized POCs and to discover new functions related to their structural and cavity characteristics.Calix[4]resorcinarenes (C4RAs), a type of macrocyclic cavitand, possess adjustable intrinsic hydrophobic nanosized cavities and eight polar phenolic groups. They have been identified as excellent building blocks for constructing cage compounds. In 2020, we first reported the use of reticular chemistry in the preparation of three different types of novel β-ketoenamine-linked POCs, including [2 + 4] lanterns, unprecedented [3 + 6] triangular prisms, and extra-large [6 + 12] octahedrons, from the same tetraformylresorcin[4]arene (C4RACHO) cavitand, by simply varying the diamine ligands. Since then, significant progress in the rational design of C4RA-based POCs has enabled the development of diverse structures featuring [4 + 8], [6 + 8], [2 + 8 + 8], and [6 + 24 + 24] topologies, which have been tailored for many meaningful applications. In addition, a lot of robust C4RA-based POC crystals based on various efficient and stable covalent bond synthetic strategies, such as vinyl, imidazole, and dative iminoboronate linkages, have been synthesized based on different tetraformyl-functionalized C4RA cavitands and different organic linkers. These approaches effectively address the issues of efficient preparation, complex purification, and structural stability of POCs. Finally, taking advantage of the C4RA-based POC structural characteristics, such as high surface area, abundant interaction sites, and large intrinsic cavities, we have enhanced the capture ability of POCs for different types of molecules and ion guests. Moreover, we have made progress in the application fields of POCs, including separation and purification, energy storage, chiral recognition, catalysis, and bioscience applications.