Phenyl Side Groups Enhance Phonon Transport in Rubrene Crystals

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 (Λ c = 0.29 W m –1 K –1 ) exceeds those along the a - and b -axes (Λ a = 0.22 W m –1 K –1 and Λ b = 0.24 W m –1 K –1 ), despite strong π–π 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.