Observation of Anisotropic Phonon-Electron Coupling in Two-Dimensional Covalent Organic Frameworks

Covalent organic frameworks (COFs) are promising organic semiconductors for energy conversion and storage, yet their electronic structure and electron-lattice interaction remain unclear. This is especially critical in 2D COFs, where covalent and van der Waals interactions along orthogonal dimensions create inherent anisotropy. Herein, six 2D COFs with diverse bond types, stacking modes, and side-group-controlled conjugation were synthesized to comparatively explore their electronic and thermal transport properties. It reveals that phonon-electron coupling (PEC) shows strong direction dependence. Along the c-axis, Fröhlich-like interactions generate quasi-large polarons, while negligible momentum exchange occurs in the a-b plane. Further testing showed that thermal conductivity and PEC strength are positively correlated, a phenomenon significantly differing from traditional semiconductors. We verify that the interlayer PEC hardens the phonon frequency and induces in-plane acoustic phonon bunching, which substantially benefits the heat transport efficiency. Proof-of-concept experiments with a highly oriented 2D COF film demonstrated extreme thermal transport anisotropy. The difference in λ values between the perpendicular directions is nearly two orders of magnitude, meanwhile, the in-plane λ value reaches up to 20 W·m⁻¹·K⁻¹. This work provides a basic understanding of the transport behaviors of 2D COFs and highlights their great potential as a highly thermal conductive material.