Single-Crystal-Structure Directed Predesign of Cationic Covalent Organic Polymers for Rapidly Capturing 99TcO4–

99TcO4– with radioactive toxicity and high mobility is a potential radiation threat to the environment and organisms. Effective separation and fixation of 99TcO4– from nuclear waste are of paramount importance. Herein, we proposed a pre-evaluation strategy supported by single-crystal structures to prepare two kinds of cationic covalent organic polymers (COPs) with self-adaptive effects to remove 99TcO4–/ReO4– from aqueous solution. The as-prepared COPs showed rapid adsorption kinetics, large adsorption capacity, and good selectivity to ReO4–. Their application to radioactive 99TcO4– also exhibited high separation efficiency, with a removal rate of 97% within only 30 s. Studies on the adsorption mechanism based on single-crystal X-ray analysis, DFT calculation, and so forth confirmed that the host–guest structure–activity relationship derived from single crystals directly guides the structures and properties of the materials. This work provides a new approach for the controllable design of the source molecular crystal structure of complex structural materials to lead to excellent properties.