Identifying Driving and Spectator Phonon Modes in Pentacene Exciton Transport

In organic semiconductors, the crystal packing motif is known to modify electronic properties, such as exciton transport dynamics. Phonon vibrations can drive or hinder exciton transport, and understanding the role of these intermolecular vibrations can aid in the rational design of materials for improved solar cell efficiency. In this article, we use double pulse spatially offset femtosecond stimulated Raman spectroscopy (SOFSRS) to identify the functional role of phonon modes in pentacene exciton transport. In SOFSRS, we photoexcite our sample at a spatially offset position relative to the Raman pump and probe, which allows us to track changes in the excited state structure over micrometer length scales and femtosecond time scales during exciton transport. We first measure the phonon modes in a single crystal and then use optical pulse shaping to selectively amplify each mode and measure the resulting exciton transport dynamics along the fast and slow transport axes using SOFSRS. We compare the resulting dynamics with a single pulse excitation SOFSRS to assign driving and spectator phonon modes. We find that a 91 cm^-1 phonon mode drives exciton transport preferentially along the slow transport axis. We also find two modes at 161 and 176 cm^-1 that drive an increase in the overall excited state population. By comparing these to first-principles density functional theory calculations, we assign a plausible mechanism for exciton-phonon coupling. This study presents a new experimental method that can determine the functional role of phonon vibrations in mediating exciton transport.