Annealed Covalent Organic Framework Thin Films for Exceptional Absorption of Ultrabroad Low‐Frequency Electromagnetic Waves

Abstract Different from harvesting of ultraviolet and visible lights via electronic transitions, absorption of low‐frequency electromagnetic waves is sophisticated in mechanism and poor in efficiency, imposing the structural design arduous and challenging. Here, the first example of exploring covalent organic frameworks for highly efficient absorption of low‐frequency electromagnetic waves is reported. Three pyrene frameworks are synthesized and annealed into porous networks, which upon mixture with paraffin are processed into thin films with tunable thickness. The films absorb ultrabroad low‐frequency electromagnetic waves covering S, C, X, and Ku bands and achieve exceptional efficiency of 99.999% with a thickness of only 2.5 mm and a loading content of only 20%. This result originates from a synergistic effect of conductivity, heteroatoms, and pores and outperforms the state‐of‐the‐art polymers, carbons, and metals. This approach opens a way to electromagnetic wave absorption.