Enhancing Spin-Orbit coupling in covalent organic polymers to facilitate intersystem crossing for uranium (VI) photoreduction.

Photocatalytic removal and recovery of uranium from wastewater by heightening reactive oxygen species, especially superoxide radical (O2•-), is of great significance for radioactive contamination remediation. However, limited by the sluggish intersystem crossing (ISC) rates, the generation of triplet excited states is inefficient in polymeric semiconductors, thereby lower the level of O2•-. Herein, we first furnish a heteroatomic nitrogen-embedded strategy to enhance spin-orbit coupling (SOC) capacity of covalent organic polymers (COPs) by decorating benzo[c][1,2,5]thiadiazole-based COPs with the benzene, triazine, and tris([1,2,4]triazolo)[4,3-a:4',3'-c:4'',3''-e][1,3,5]triazine (TTT) cores, resulting in SOC value of 0.03, 0.10 and 0.41 cm-1, respectively. Thus, the TTT-functionalized COPs exhibited the highest ISC rates and the longest triplet excited state with a lifetime of 30.5 μs, dramatically facilitating the photoreduction of U(VI) (95.0 %, 1.5 h). This work validates that enhancing SOC ability is an effective strategy to improve ISC rate, which contributed to the rational design of polymeric semiconductors with intersystem crossing behaviors.