Fluorine Directed Two-Dimensional Cruciform π–π Stacking in Diketopyrrolopyrroles

Abstract

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 2 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.

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).…”

Exploring structure based charge transport relationships in phenyl diketopyrrolopyrrole single crystals using a 2D π–π dimer model system

“…2 In this regard, significant efforts have been devoted to develop an in-depth understanding of the role that systematic substitutions on common core motifs can exert on charge carrier properties and subsequent device performance. 2,[6][7][8][9][10][11] In-silico design based upon an evaluation of intrinsic material properties which dictate charge transport behavior and subsequent performance in organic semiconductors represents a highly prized asset in materials development, providing a tool that can facilitate the identification of superior materials by molecular argument. Among the numerous types of small π-conjugated systems investigated, diketopyrrolopyrrole (DPP) based materials, largely employed in industry as high performance pigments, 12,13 have more recently attracted an increasing surge of interest in optoelectronics.…”