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Intermolecular interactions and energetics in the crystalline π-π stacks and associated model dimer systems of asymmetric halogenated diketopyrrolopyrroles

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Four novel systematically fluorinated DPPs and their single crystal structures are reported.Structures involving direct fluorination of the DPP core phenyl rings and N-benzyl groups, display 1-dimentional π-π stacking motifs; a characteristic of N-benzyl substitution, where long and short molecular axis displacement is induced by isosteric substitution of phenylic hydrogen atoms for fluorine atoms. This characteristic stacking behaviour is destroyed upon trifluoromethyl substitution at the para position of the core phenyl rings, in one case affording a novel molecular conformation and π-π dimer pair exhibiting a higher intermolecular interaction energy than any other structurally analogous DPP based system reported previously. This crystal structure also exhibits a unique orthogonal association of the π-π dimer pairs along the crystallographic a and b axes, resulting in the formation of a framework that is characterised by well-defined channels perpetuating along the length of the crystallographic c axis. The role of fluorine induced stabilisation and its impact on optoelectronic properties in these systems is identified via analysis of computed intermolecular interactions for all the crystal extracted nearest neighbour dimer pairs and their associated cropped equivalents. Our results clearly reinforce the positive role of benzyl substitution in DPP crystal structures to enhance optoelectronic behaviour. More importantly they demonstrate the significant impact small changes in molecular structure can have on the solid state properties of this molecular motif, particularly when fluorination is involved.ABSTRACT: Rationalising 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 behaviour 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 towards 3 the development of materials with enhanced optical and optoelectronic behaviour. 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 fluorinehydrogen substituents. In the case of trifluoromethyl substitution, we identify a previously unobserved packing motif exhibiting a framework...

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