Intermolecular charge transfer between heterocyclic oligomers. Effects of heteroatom and molecular packing on hopping transport in organic semiconductors

For electron or hole transfer between neighboring conducting polymer strands or oligomers, the intrinsic charge-transfer rate is dictated by the charge-resonance integral and by the reorganization energy due to geometric relaxation. To explain conduction anisotropy and other solid-state effects, a multivariate, systematic analysis of bandwidth as a function of intermolecular orientations is undertaken for a series of oligoheterocycles, using first-principles methods. While cofacial oligomers show the greatest bandwidths at a given intermolecular C-C contact distance, for a fixed center-to-center intermolecular distance, tilted π-stacking increases π-overlap (particularly for LUMO orbitals) and decreases electrostatic repulsion, yielding optimum tilt angles for packing of ∼40-60° at small intermolecular separations. The calculations also reveal that bandwidths and intrinsic mobilities of holes and electrons in conjugated oligoheterocycles can be quite comparable.