Transformation of Porous Organic Cages and Covalent Organic Frameworks with Efficient Iodine Vapor Capture Performance

The reaction of 5,5 '-([2,2 '-bipyridine]-5,5 '-diyl)diisophthalaldehyde (BPDDP) with cyclohexanediamine and tively, affords a nitrogen-rich porous organic cage BPPOC and two USTB-1 and USTB-2 (USTB = University of Science and Technology Beijing), under suitable conditions. Interestingly, BPPOC with a single-crystal X-ray diffraction structure is able to successfully transform into USTB-1 and USTB-2 (newly converted COFs denoted as USTB-1c and USTB-2c, respectively) upon exchange of the imine unit of cyclohexanediamine in the cage by BZ and BPDA. Such a transformation also enables the isolation of analogous COFs (USTB-3c and USTB-4c) on the basis of an isostructural organic cage, BTPOC, which is derived from 5,5 '-([2,2 '-bithiophene]-4,4 '-diyl)diisophthalaldehyde (BTDDP) and cyclohexanediamine. However, the conventional solvothermal reaction between BTDDP and BPDA leads to an impure phase of USTB-4 containing incompletely converted aldehyde groups due to the limited solubility of the building block. The newly prepared COFs have been characterized by nuclear magnetic resonance spectroscopy, Fourier transform infrared spectroscopy, scanning electron microscopy, and transmission electron microscopy. In particular, BPPOC is able to absorb the iodine vapor with an uptake of 5.64 g g-1, breaking the porous organic cage's (POC's) record value of 3.78 g g-1. Nevertheless, the cage-derived COFs exhibit improved iodine vapor adsorption capability in comparison with the directly synthesized counterparts, with the highest uptake of 5.80 g g-1 for USTB-1c. The mechanism investigation unveils the superiority of nitrogen atoms to sulfur atoms for POCs in iodine vapor capture with the assistance of definite crystal structures. This, in combination with porosity, synergistically influences the iodine vapor capture capacity of COFs.

Automated summary

The reaction of BPDDP with cyclohexanediamine results in the synthesis of nitrogen-rich porous organic cage BPPOC and two USTB-1 and USTB-2 COFs. The newly prepared COFs show enhanced iodine vapor adsorption capability and can be transformed between different structures by exchanging the imine unit. This transformation is influenced by the crystal structures and the porosity of the materials, with nitrogen atoms proving superior to sulfur atoms for POCs in iodine vapor capture.