Influence of molecular thermal motion on electronic excitation transport in anthracene crystals

Anisotropies in both the diffusion coefficient of triplet excitons and the mobilities of electrons and holes were numerically calculated in each direction of anthracene crystal axes at 95 and 290 K. Dynamic fluctuations of the molecular position, obtained by a molecular-dynamics simulation method, were used in ab initio calculations which determined the electronic excitation transport based on a hopping model. Compared with results from investigations considering only static perfect lattices, improvements were obtained in particular for the triplet exciton in the ab crystal plane and for the hole in all directions. However, as previously found in several studies, a discrepancy still remains between the theoretical and experimental anisotropies of the electron and triplet-exciton transport in the c' direction as compared with those in the ab plane. It is suggested this discrepancy is caused by static structural fluctuations, such as lattice defects in real crystals.