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 <i>ab</i>-plane, we observe that the room-temperature thermal conductivity along the interlayer <i>c</i>-axis (Λ_<i>c</i> = 0.29 W m^-1 K^-1) exceeds those along the <i>a</i>- and <i>b</i>-axes (Λ_<i>a</i> = 0.22 W m^-1 K^-1 and Λ_<i>b</i> = 0.24 W m^-1 K^-1), despite strong π-π stacking in the <i>ab</i>-plane. MD simulations reveal the origins of this counterintuitive observation, highlighting the pivotal role of phenyl side groups in facilitating phonon transport along the <i>c</i>-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 <i>c</i>-axis suggest more efficient thermal transport through the phenyl groups. Additionally, the 1/<i>T</i> 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.