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

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.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...

Nature of halogen‐centered intermolecular interactions in crystal growth and design: Fluorine‐centered interactions in dimers in crystalline hexafluoropropylene as a prototype

“…These results, together with those reported by others, also give an indication that fully fluorinated aromatic compounds can sustain directional interactions to form ordered supramolecular architectures on various metal surfaces . Clearly, the replacement of hydrogen by fluorine in molecules can lead to dramatic changes in the solid‐state packing behavior as a consequence of its weak attractive interaction with other sites …”

Twist and shout: a surprising synergy between aryl and N-substituents defines the computed charge transport properties in a series of crystalline diketopyrrolopyrroles

“…28,29,31,32 We have T M06-2X density functional in the theoretical analysis of charge transfer integrals, reorganisation energies and interaction energetics in DPPs, 27-33 many of which are comparable or supersede those computed for the single crystal structures of state of the art organic semiconductors such as rubrene. 29 The M06-2X semilocal density functional has been shown to quantitatively agree with state-of-theart quantum mechanical calculations describing noncovalent interactions through its inclusion of important medium distant London dispersion forces.…”

“…In all cases, N-benzylated systems were observed to surpass the computed intermolecular interactions compared with their alkylated counterparts, thus reinforcing our previous assertion regarding the positive role of benzyl substitution in enhancing the thermal stability opairs in DPP single crystals. 28,29,31,32 We report for the phenyl substituted systems, PADPP and PBDPP, that removal of either the alkyl or benzyl groups affords ca. 40% destabilisation of the total binding -T attributed to an electrostatic intermolecular interaction between electropositive methylene protons and electronegative carboxylic oxygen atoms situated 2.85 Å apart in PBDPP and to a stabilising dispersive interaction between the aliphatic C 6 chains and DPP core phenyl rings in PADPP (Figure 4).…”

“…In all cases, transfer integrals were determined using the energy splitting in dimer methodology (ESI), as reported by us previously. 28,29,31,32,44 Via analysis of the computed t h/e -stacking dimer pair of PBDPP exhibits the largest hole transfer integral for all of the investigated systems, with electronic coupling approaching that of rubrene (t h = 10.69 and 12.40 kJ mol-1 -PBDPP and rubrene respectively). Of considerable interest to us was the significant deterioration in the magnitude of the transfer integrals for both holes and electrons upon alkyl substitution on the lactam nitrogen atoms with phenyl substitution (t h /t e = 10.69/6.13 and 1.71/1.52 kJ mol -1 -PBDPP and PADPP respectively), which can be readily accounted for as a consequence of the dimer stacking displacements exhibited by these systems (vide supra) and subsequent lower wavefunction overlap as illustrated in Figure 6.…”

“…In the thiophene structures, N-benzylation also exerts a positive influence on inter-Å in TBDPP Å in TADPP. It is clear, as discussed by us previously, 28,29,31,32 when -stacking dimer interactions and their spatial overlap in DPP single crystals to maximise wavefunction overlap and electronic coupling between monomers, that careful consideration of both the core aryl and N-substituents is required (vide infra).The dihedral angle between core aryl rings and the DPP central core was determined for each structure. It was observed in line with previous reports, 36 that regardless of substitution on the lactam nitrogen atoms, phenyl based systems always exhibit greater dihedral angles than their near-planar thiophene and furan equivalents (32.7/22.5(2), 0.6/1.0(5) and 9.1/3.7(4) ° for PADPP/PBDPP, FADPP/FBDPP and TADPP/TBDPP respectively).…”