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Effects of Fluorine Substitution on the Intermolecular Interactions, Energetics, and Packing Behavior of N‑Benzyl Substituted Diketopyrrolopyrroles

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journal contribution
posted on 08.03.2016, 00:00 by Jesus Calvo-Castro, Graeme Morris, Alan R. Kennedy, Callum J. McHugh
Rationalizing the effects of molecular substitution in π-conjugated organic materials arising from well-defined intermolecular interactions, which can influence the formation of predefined packing motifs and control the emergence of π–π stacking represents a current challenge in supramolecular design. Significant effort is potentially required to manage the impact on solid state packing behavior in materials that have been molecularly tuned to carry out specific photophysical and electrochemical functions. In this regard, fluorine substitution in π-conjugated systems has seen a recent surge of interest, primarily aimed toward the development of materials with enhanced optical and optoelectronic behavior. In light of this interest, in the following study, we report the synthesis and single crystal structures from a series of four novel and structurally related, symmetric, fluorinated N-benzyl substituted diketopyrrolopyrroles (DPPs). Two of the investigated series exhibit slipped cofacial π–π dimer pairs, which are consistent with those reported by us previously in halogenated DPPs. Significantly, this characteristic stacking motif of N-benzyl substituted DPPs can be carefully modified via the replacement of hydrogen atoms with trifluoromethyl and isosteric fluorine–hydrogen substituents. In the case of trifluoromethyl substitution, we identify a previously unobserved packing motif exhibiting a framework of well-defined channels propagating along the length of the crystallographic c-axis. In each of the reported systems, all of the nearest neighbor dimer pairs have been identified and their intermolecular interaction energies computed by means of the M06-2X density functional at the 6-311G­(d) level. Through a detailed theoretical analysis involving the determination of cropped dimer energetics, organic fluorine is shown to play an active role in the stabilization of the crystal extracted dimer pairs through a number of additive and weak C–F---H, C–F---πF, and C–F---π intermolecular contacts. Contrary to recent reports, we demonstrate that substitution of hydrogen by fluorine can also lead to dramatic changes in solid state packing behavior as a consequence of these weak interactions. Given the importance of organic fluorine substitution in the construction of π-conjugated materials for optoelectronic materials, we feel that this work should be of interest to the wider community involved in supramolecular design of organic conjugated systems, and in particular to those investigating organic fluorine as well as diketopyrrolopyrrole containing architectures.