  Molecular vibrations play a critical role in the charge transport properties
of weakly van der Waals bonded organic semiconductors. To understand which
specific phonon modes contribute most strongly to the electron-phonon coupling
and ensuing thermal energetic disorder in some of the most widely studied high
mobility molecular semiconductors, state-of-the-art quantum mechanical
simulations of the vibrational modes and the ensuing electron phonon coupling
constants are combined with experimental measurements of the low-frequency
vibrations using inelastic neutron scattering and terahertz time-domain
spectroscopy. In this way, the long-axis sliding motion is identified as a
killer phonon mode, which in some molecules contributes more than 80% to the
total thermal disorder. Based on this insight, a way to rationalize mobility
trends between different materials and derive important molecular design
guidelines for new high mobility molecular semiconductors is suggested.
