Density-functional theoretical study of fluorination effect on the electronic structure and electron drift mobilities of symmetric pentacene derivatives

A theoretical study using density functional theory is reported to gain insights how the side-specific symmetric fluorination of pentacene can change the geometry and the electronic structure of its neutral and mono-charged states. The commercial electric p- and n-type semiconductors, pentacene and perfluorinated pentacene, were selected as reference molecules. Depending on the substitution, the energies of the lowest unoccupied molecular orbitals (LUMOs) are changed from -2.92 eV to -3.92 eV. Comparing with the pentacene, the maximal hypsochromic shift of the optical bandgap can be reached by partial outermost substitution. On the other hand, the bathochromic shifts are predicted for the fluorination of inner rings. The potential role of intermolecular hydrogen H..F bonds stabilizing the crystal structure was estimated by mutual comparison of interaction energies for parallel-stacked and parallel-displaced dimer configurations. The drift mobilities were evaluated for the studied dimer configurations at the room temperature using the Marcus theory and the Einstein relation. The presented theoretical results indicate contradictory role of the fluorination in preparation of electric n-semiconductors. The increasing number of fluorine atoms improves the LUMO energy level but the dimer electron drift mobilities are decreased.