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.