Structure of clusters of pentacene molecules and their polarizabilities

We calculate and compare the ground state structures of one, two and three pentacene molecule clusters, using B3LYP and B97-D density functionals, and polarizabilities in the ground state structure to examine the extent of anisotropy for applications in electronic device fabrication. B3LYP and B97-D functionals perform sufficiently well for a single pentacene molecule, but for two and three molecule clusters, the inclusion of dispersive interactions through B97-D becomes necessary. For two and three molecule clusters the energy landscape near ground state is flat in molecular coordinates, implying multiple related structures may be accessible at room temperature. Further, for the ground state structures, the polarizability along the long axis of the molecule is larger than the other two directions. This may make it possible to explain the beneficial effect of applying an electric field during processing of pentacene films in electronic devices, as observed experimentally; specifically, in this context, the structural and polarizability data presented could allow for development of models for nucleation and growth of pentacene films.