Unveiling the Electron-Phonon Coupling Anisotropy in 2D Covalent Organic Frameworks

Abstract

Covalent organic frameworks (COFs) are emerging organic semiconductors with potential for energy conversion and storage. However, their electronic structure and electron-phonon interactions remain poorly understood. This issue is particularly important in 2D COFs, where covalent and van der Waals interactions along orthogonal directions manifest intrinsic anisotropy. In this work, seven 2D COFs with varied bond linkages, stacking patterns, and side-group conjugation were synthesized to investigate their electron-phonon interaction properties. The results show that electron-phonon coupling (EPC) exhibits obvious direction dependence. Strong EPC is observed along the interlayer direction, whereas negligible momentum exchange occurs along in-plane direction. A positive correlation between thermal conductivity and EPC strength was recognized, in contrast to the behavior of conventional semiconductors. Further analysis confirms that interlayer EPC hardens phonon frequencies and promotes in-plane acoustic phonon bunching, thereby enhancing thermal transport. A proof-of-concept study using a highly oriented COF film revealed extreme transport anisotropy. The thermal conductivity difference between perpendicular directions approaches two orders of magnitude, while the in-plane value reaches 20 W m^-1 K^-1 as determined by the laser flash method. These findings provide fundamental insight into the transport mechanism of 2D COFs and highlight their potential as efficient thermal conductors.