Can intramolecular rotors govern the thermal conductivity of molecular materials?

Controlling thermal transport at the nanoscale through phonon engineering remains a significant challenge in thermal management and nanodevice design. Molecular materials present a promising pathway for thermal control owing to their structural flexibility and dynamic behavior encompassing both intra- and intermolecular motions. However, the influence of these dynamics on thermal transport remains poorly understood. This study focuses on intramolecular rotation as a representative motion within molecular materials. These rotations are characterized by low-frequency spectra but fall outside the classification of standard normal modes. Through theoretical calculations, we assessed the thermal conductivity of self-assembled molecular multilayers, investigating the role of hindered and free rotations within these layers in influencing thermal transport. Our findings reveal that intramolecular rotation has minimal impact on thermal transport, even as rotational modes shift from cooperative hindered rotation to nearly free rotation through thermal activation. In contrast, intermolecular dynamics-particularly interlayer motions-play a dominant role in determining thermal conductivity. These motions drive the transition from ballistic to diffusive transport through thermal activation. These findings highlight intermolecular dynamics as the primary modes for thermal control, while interactions between intramolecular dynamics and thermal conduction pathways remain weak.