Side-chain torsional dynamics strongly influence charge transport in organic semiconductors

The role of low-frequency (terahertz) vibrational motions on charge carrier dynamics in organic semiconductors (OSCs) is becoming well-known, and efforts are underway to rationally design new materials to mitigate these detrimental effects. However, most efforts have focused on stabilizing the fused-ring semiconducting `core', often by functionalizing with various side-groups, yet questions regarding the role of such modifications on electron-phonon couplings are still outstanding. In this work, the influence of thiophene rings sigma-bonded directly to the pi-conjugated cores is explored. The manner in which these groups alter low-frequency vibrational, and resulting electronic, dynamics is quantified using a theoretical approach employing fully-periodic density functional theory (DFT) simulations. Ultimately, these results showcase how the equilibrium geometry and corresponding electronic structure are directly related to detrimental electron-phonon coupling, which have important implications for the design of improved organic optoelectronic materials.