Intermolecular Interactions and Energetics in the Crystalline π-π Stacks and Associated Model Dimer Systems of Asymmetric Halogenated Diketopyrrolopyrroles

Four novel structurally analogous asymmetric, halogenated N-benzyl substituted diketopyrrolopyrroles (DPP) have been synthesized, and their crystal structures obtained. All four crystal structures exhibit pi-pi stacks with very small displacements along their short molecular axes, which based upon our previous studies involving symmetrical DPPs is a characteristic of N-benzyl substitution. Intermolecular interaction energies were computed for extracted crystal pi-pi dimer pairs by means of M06-2X density functional at the 6-311G(d) level to investigate the most energetically favored position of the halogen atoms in FBDPP and CIBDPP structures. In addition, effective stabilization energies arising from both benzyl and halogen substitution in these derivatives and in BrBDPP and IBDPP pi-pi dirtier pairs were determined in order to probe the impact of these groups on the resulting dimer stability. Effects of the intermonomer displacements along the long molecular axis, which have been shown by us previously to significantly influence wavefunction overlap and effective electronic coupling, were investigated in detail using aligned and anti-aligned model systems of CIDPP and BrDPP. The predictions of these model systems are remarkably consistent with the observed displacements in their crystal derived pi-pi dimer pair equivalents, offering insight into the effective role of intermolecular contacts in crystal structures involving this molecular motif, particularly with a view toward crystal engineering in these systems. As a result, we believe that this study should be of significant interest to the growing DPP based materials community and in general to those investigating the detailed manner by which substituents can be employed in the supramolecular design of crystalline molecular architectures.