Impact of Systematic Structural Variation on the Energetics of π–π 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 π-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, π–π stacks has been identified
within the single crystal structure series and intermolecular interaction
energies and charge transfer integrals for the π–π
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 π–π
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 π–π 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.