Coupling between local vibrations and charge transport in discotic materials

Charge transport between molecular units is sensitive to coupling between thermal motion and the electronic structure of these units. The thermal motion of a short column of discotic molecules is simulated by a molecular dynamics simulation in order to provide the correct distribution of distorted molecular units, including the effects of the alkoxy tails. Ab initio calculations are then used to determine the electronic structure of the distorted triphenylene cores and the time dependence of the interaction between these. This goes beyond previous works that have calculated changes in charge transfer by finite displacement of rigid molecular units along translational and rotational coordinates. Two timescales and types of motion are distinguished: slow whole body motions of the disks, and internal vibrational modes of the aromatic cores. The effects of these motions are separated via their different timescales, and are found to be of equal importance in the interaction between the HOMOs of neighbouring molecules. Electron/phonon coupling from these small molecular distortions may be important not only for charge transport, but also for loss of carriers via relaxation of electronically excited states.