Fluorine directed two-dimensional cruciform π π stacking in diketopyrrolopyrroles

Calvo-Castro, Jesus and Morris, Graeme and Kennedy, Alan R. and McHugh, Callum J. (2016) Fluorine directed two-dimensional cruciform π π stacking in diketopyrrolopyrroles. Crystal Growth and Design, 16 (9). pp. 5385-5393. ISSN 1528-7483

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    Enhanced bulk dimensionality in organic materials employed in optoelectronic devices is desirable and can overcome fabrication issues related to structural defects and grain boundaries. Herein, we report a novel fluorinated diketopyrrolopyrrole single crystal structure, which displays a unique, mutually orthogonal, 2 dimensional cruciform π π stacking arrangement. The crystal structure is characterised by an unusually large number of nearest neighbour dimer pairs which contribute to a greater thermal integrity than structurally analogous equivalents. Binding energies and charge transfer integrals were computed for all of the crystal extracted dimer pairs by means of M06-2X density functional at 6-311G(d) level. Although weak, a number of intermolecular interactions involving organic fluorine (C-F---H, πF---π and C-F---πF) were identified to influence the supramolecular assembly of these dimer pairs. Charge transfer integrals for the two π-π stacking crystal dimers were determined using the energy splitting in dimer method. Ambipolar charge transport favouring electron transfer approaching that of rubrene is predicted in both of these π-π stacks with a greater magnitude of coupling observed from those dimers perpetuating along the crystallographic a-axis. Charge transport behaviour in the single crystal is greatly influenced by selective fluorination of the N benzyl substituents and is consistent with the crystal extracted π-π stacking dimer geometries and their overall influence on wavefunction overlap. The reported structure is an interesting electron transport material that could be exploited, particularly in thin film based optoelectronic devices, where high bulk dimensionality is required.