Effects of fluorine substitution on the intermolecular interactions, energetics and packing behaviour of N-benzyl substituted diketopyrrolopyrroles

Calvo-Castro, Jesus and Morris, Graeme and Kennedy, Alan R. and McHugh, Callum J. (2016) Effects of fluorine substitution on the intermolecular interactions, energetics and packing behaviour of N-benzyl substituted diketopyrrolopyrroles. Crystal Growth and Design, 16 (4). pp. 2371-2384. ISSN 1528-7483 (https://doi.org/10.1021/acs.cgd.6b00157)

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Abstract

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.

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