Polymorphic Covalent Organic Frameworks: Molecularly Defined Pore Structures and Iodine Adsorption Property

Radioactive iodine-capturing materials are urgently needed for the emerging challenges in nuclear waste disposal. The various pore structures of covalent organic frameworks (COFs) render them promising candidates for efficient iodine adsorption. However, the detailed structure-property relationship of COFs in iodine adsorption remains elusive. Herein, two polymorphic COFs with significantly different crystalline structures are obtained based on the same building blocks with varied molecular ratios. The two COFs both have high crystallinity, high specific surface area, and excellent chemical and thermal stability. Compared with the [C₄+C₄] topology (PyT-2) with an AA stacking form, the [C₄+C₂] topology (PyT-1) with an AB stacking form has more twisted pore channels and complex ink-bottle pores. At ambient conditions, PyT-1 and PyT-2 both exhibit good adsorption properties for iodine capture either in a gaseous or liquid medium. Remarkably, PyT-1 presents an excellent maximum adsorption capacity (0.635 g g^-1), and the adsorption limit of PyT-2 is 0.445 g g^-1 in an n-hexane solution with an iodine concentration of 400 mg L^-1, which is highly comparable to the state-of-the-art iodine absorption performance. This study provides a guide for the future molecular design strategy toward novel iodine adsorbents.