Phenyl Side Groups Enhance Phonon Transport in Rubrene Crystals

XT Xu and JZ Xu and YQ Cai and N Chowdhury and C Zhang and DG Cahill and L Chen and CD Frisbie and XJ Wang, JOURNAL OF THE AMERICAN CHEMICAL SOCIETY, 147, 42803-42814 (2025).

DOI: 10.1021/jacs.5c15105

Rubrene single crystals, renowned for their record-high hole mobility among organic semiconductors, are widely used in organic electronics where thermal management is essential. This study reports a comprehensive investigation of thermal transport in rubrene along its three primary crystallographic directions, integrating crystal growth, structural characterization, thermal measurements, and molecular dynamics (MD) simulations. Contrary to the well-established 2D nature of charge transport favored in the ab-plane, we observe that the room- temperature thermal conductivity along the interlayer c-axis (Lambda c = 0.29 W m-1 K-1) exceeds those along the a- and b-axes (Lambda a = 0.22 W m-1 K-1 and Lambda b = 0.24 W m-1 K-1), despite strong pi-pi stacking in the ab-plane. MD simulations reveal the origins of this counterintuitive observation, highlighting the pivotal role of phenyl side groups in facilitating phonon transport along the c-axis. Phonon dispersion and density of states analyses indicate enhanced low-frequency vibrational modes (0-3.7 THz) associated with the side groups. Participation ratio analyses confirm that vibrational modes in both the side groups and backbone have comparable degrees of spatial localization for carrying heat; however, higher phonon group velocities along the c-axis suggest more efficient thermal transport through the phenyl groups. Additionally, the 1/T temperature dependence of thermal conductivities along all three axes suggests crystalline behavior in rubrene as might be expected despite its complex molecular structure. These findings uncover a previously underappreciated role of side group dynamics in phonon transport in molecular crystals and provide new insights into developing thermal management strategies for organic electronic devices.

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