Importance of Intermolecular Interactions in Assessing Hopping Mobilities in Organic Field Effect Transistors:  Pentacene versus Dithiophene-tetrathiafulvalene

We report on a computational study to understand the high mobility found in dithiophene-tetrathiafulvalene (DT-TTF) transistors, with respect to that known for pentacene, using an extended measure of the reorganization energy (λ_reorg). We demonstrate the high importance of considering intermolecular interactions to assess hole-hopping mobilities. We find that due to the steric, polarizable environment and the structural changes induced by local intermolecular charge delocalization, the calculated λ_reorg for DT-TTF decreases from 0.574 eV in the isolated molecule to 0.042 eV in the crystal embedded molecule, which is of the same order of the previously reported value found for the isolated pentacene molecule, 0.098 eV.