《J. Am. Chem. Soc.》Published NJUST Researchers’ Achievements

Very recently, Prof. Zhang Gen’s group from School of Chemical Engineering of our university made a milestone to the field of dynamic covalent chemistry. The research result has been published in the top international chemistry journal,J. Am. Chem. Soc.titled as “Dynamic Transformation between Covalent Organic Frameworks and Discrete Organic Cages”. Zhen San and Wu Xiaowei are the co-first author, and professor Zhang Gen is the corresponding author. The link of the article is following as: https://pubs.acs.org/doi/10.1021/jacs.0c11073

Covalent organic frameworks (COFs) and covalent organic cages are two classes of crystalline porous organic materials with unprecedented properties. The formation of COFs and cages generally follows the principle of dynamic covalent chemistry (DCC), which allows dynamic conversion from cages to cages or COFs to COFs. However, the transformation between COFs and cages (cage-to-COF or COF-to-cage) have never been realized.

Figure 1. Synthesis of COF-NJST-1 and Cage-NJST-1

In this work, Zhang’s group proposed a DCC-induced linker exchange strategy to achieve such transformation at solid-liquid interface for the first time. Studies have shown that the transformation between COFs and cages mainly depends on the type of bond connecting these two organic porous materials, where the borate bond promotes the conversion of COF-to-cage, while imine bond leads to the conversion of cage-to-COF. Self-sorting experiments suggest that borate cages and imine COFs are thermodynamic minimum compounds. This research builds a bridge between discrete and polymeric organic scaffolds, and broadens the knowledge of chemistry and materials for porous materials science.

Figure 2. (a) PXRD of COF-NJST-1. (b-c) Top and side view of COF-NJST-1. (d) 1H NMR spectra for Cage-NJST-1. (e-f) Structure of discrete Cage-NJST-1

Furthermore, a DCC-induced linker exchange strategy for the structural transformation between infinite COFs and discrete cages. The transformation mainly dependents on the reversible covalent bonds linking COFs and cages. The studies showed that when boronate ester was used as linkage, COF-to-cage transformation can be achieved, and when imine bonding was applied, cage-to-COF conversion was accomplished. The transformation process was determined by time-dependent NMR and XRD spectra. Self-sorting experiments indicated that the boronate ester COFs and imine cages are thermodynamic minimum compounds which controlled the transformation. This work paves new view/way to study the crystalline nature between organic materials, further studies on the insightful experiments/mechanism that show how to control transformation in a more elaborate way to prepare more crystalline materials (MOF, COF, cycle, cage etc) are greatly needed and being proceeding in their lab.

This research was funded by the “Oversea High-level Talent”project, the Natural Science Foundation of Jiangsu Province, and the Key Laboratory for Soft Chemistry and Functional Materials of Ministry of Education.