Unraveling the microscopic mechanism of thermal conductivity in 2D COF–C60 assemblies via machine-learning potentials

Covalent organic framework–fullerene assemblies (C60@COF) are promising for energy storage and optoelectronics, but the microscopic mechanisms governing their thermal conductivity (κ) remain poorly understood and controversial. Here, we employ a neuroevolution potential (NEP) to investigate thermal transport in pristine COF-1 and C60@COF-1 through large-scale molecular dynamics simulations. Contrary to previous classical potential-based studies, our simulations reveal that C60 incorporation systematically reduces, rather than enhances, the in-plane κ of COF-1. Structural analyses show that the local bonding environment of the COF-1 framework remains largely intact, and confined C60 molecules are translationally localized, undergoing primarily librational and orientational motions. Phonon analysis indicates that C60 contributes negligibly to heat conduction, instead acting as localized molecular rattlers that strongly scatter low-frequency framework phonons, thereby shortening the phonon mean free path in C60@COF-1. Furthermore, stacking-dependent C60 libration and host-imposed confinement yield stronger host–guest coupling in AB-stacked C60@COF-1 compared to its AA-stacked counterpart, resulting in stronger phonon scattering and lower in-plane κ. This work elucidates the microscopic origin of thermal transport suppression in COF–C60 assemblies, establishes an NEP-based strategy for studying host–guest porous systems, and highlights confined molecular rattling as a tunable design parameter for regulating heat conduction in porous organic frameworks.