Impact of Systematic Structural Variation on the Energetics of pi-pi Stacking Interactions and Associated Computed Charge Transfer Integrals of Crystalline Diketopyrrolopyrroles

Control over solid state structure is critical for effective performance in optoelectronic devices bearing if-conjugated charge mediating organic materials. A series of five structurally related N-benzyl-substituted diketopyrrolopyrroles (DPPs) differing solely in 2 out their 60 atoms were synthesized and crystal structures obtained. Systematic variation of the long axis aligned, pi-pi stacks has been identified within the single crystal structure series and intermolecular interaction energies and charge transfer integrals for the pi-pi stacks have been computed by means of density functional theory (M06-2X/6-311G(d)). The computed intermolecular interaction energies as well as charge transfer integrals were further investigated utilizing a series of systematically cropped dimer pairs, highlighting the crucial role of the benzyl/halo substitution on stabilization of these pi-pi dimers. Two of the DPPs, including a new polymorph of a previously reported structure exhibit twice the intermolecular interaction energy and comparable hole transfer integrals to Rubrene, one of the most efficient hole conducting materials known. The computed properties for all of the pi-pi dimer systems reported herein are consistent with trends predicted by a model system. As such these materials show great promise as charge mediators in organic electronic applications and may be exploited in systematic structure activity based investigations of charge transfer theory.