Stack bonding in pentacene and its derivatives

Understanding the nature of π-stacking interactions is important to molecular recognition, self-assembly, and organic semiconductors. The stack bond order (SBO) model of π-stacking has shown that the conformations of dimers are found at orientations where the combinations of monomer MOs are overall bonding within the stack. DFT calculations show that parallel displaced minima found on the potential energy surface for the π-stacked dimers of pentacene and perfluoropentacene occur when the dimer MOs are constructed from combinations of monomer MOs with an allowed SBO. An examination of the MOs of π-stacked dimers extracted from X-ray structures of alkynyl derivatives like TIPS-pentacene pack at one or more of the minima expected to show similar MO patterns. The π-stacking variability within these materials can be attributed to a balance between the minima allowed by SBO theory and steric effects within the lattice. The offset orientation of the pentacene cores observed in packing of these materials is attributed to the increased overlap of monomer lobes in the dimer and a reduction in two-orbital-four-electron repulsions. Charge mobility estimated from the frontier MOs of the dimer is related to the MO structure that favors PD conformations.