Fluorine directed two-dimensional cruciform π π stacking in diketopyrrolopyrroles

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 characterized by an unusually large number of nearest neighbor 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 extracted dimer pairs by means of the M06-2X density functional at the 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 ...

*Corresponding authors: j.calvo-castro@herts.ac.uk, callum.mchugh@uws.ac.uk 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, π F ---π and C-F---π F ) 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.

INTRODUCTION
The concept of dimensionality is widely employed to describe both molecular and bulk structural properties in organic semiconductors. 1,2 High bulk dimensionality is proposed to be advantageous, overcoming significant fabrication issues relating to structural defects and grain boundaries in charge mediating devices. It is accepted that linear conjugated systems often display the most efficient bulk assemblies for charge transport based on 1-dimensional π-π stacking with high electronic coupling and transfer integrals. These systems are often surpassed in device performance however, by molecular scaffolds displaying more effective solid state aggregation and a greater number of electronic interactions that consist of 2-dimensional or 3-dimensional π-π stacking in mutually orthogonal directions. 1 Many examples of conjugated π-systems that exhibit higher orders of molecular and bulk dimensionality have been reported, including the archetypal swivel and spiro-centred oligothiophene cruciforms, perylenediimides, TIPS pentacene cruciforms, dinapththothiophenes, benzobisthiazoles and tetrathiafulvalene analogues . First reported in 1974, 5 diketopyrrolopyrroles (DPPs) are ubiquitously employed in industry as high performance organic pigments, owing to their highly desirable colouristic and fastness properties. [6][7][8][9][10][11] More recently, materials based upon DPPs have attracted interest as promising charge transfer mediating materials in optoelectronic devices such as organic light emitting diodes, organic field effect transistors and organic photovoltaics for solar energy conversion. [12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28] The concept of enhanced dimensionality in DPP containing architectures has been reported, although this has focused mainly on an increase in molecular dimensionality based around linear conjugated and non-conjugated systems . 29 In star-shaped molecules such as those described, strong quadrupole based aggregation in the solid state limits charge carrier performance in field effect transistor applications. One potential strategy to improve charge transport behaviour in such star-shaped environments is through the correct choice and position of the solubilising N-substituents, aimed to direct more efficient solid state interactions . 1 We are engaged in the rational design and systematic engineering of crystalline and thin film architectures based upon N-substituted DPPs [30][31][32][33][34][35] and have recently demonstrated in a number of systems that small structural variations can systematically influence and control the packing motifs via manipulation of their single crystal intermolecular interactions. [30][31][32][33] We are specifically interested in the dramatic changes observed to intermonomer displacements in crystal derived π-π stacking dimer pairs in these systems, which are reported to play a crucial role in determining the charge transfer properties of organic semiconductors. [30][31][32][33][36][37][38] We have shown that N-benzylation is an effective strategy in the molecular design of DPPs, with N-benzyl substituted single crystal examples characterised by close alignment along their short molecular axis with the emergence of slipped cofacial 1-dimensional π-π stacks 30-33 unlike their pigment and alkylated counterparts. [39][40][41][42] This close intermonomer alignment along the short molecular axis in N-benzyl DPP architectures is associated to larger computed intermolecular interactions which are desirable for the enhancement of π-π stacking thermal integrity and to furthermore maximise wavefunction overlap, critical in optimising hole and electron transfer behaviour.
In addition, halogen substitutions have been widely explored in π-conjugated organic semiconductors and we have shown that they play an active role in determining the solid state behaviour in N-benzyl DPPs. Fluorine substitution of hydrogen is often employed in the life sciences and solid state chemistry [43][44][45][46][47][48][49][50][51][52][53][54] in light of their similar polarizabilities, with substantial change in their occupied volume. Physical and chemical properties in comparative systems are influenced by the significant variation in electronegativity, such as the reversal of the electron density when comparing perfluorinated rings and their non-fluorinated analogues and is often associated to the appearance of an array of intermolecular interactions such as C-F---H, C-F---F-C, C-F---π F , π F ---π and π F ---π F contacts. 45,46,[50][51][52][53] Hydrogen bonding interactions involving organic fluorine are rarely encountered despite its high electronegativity and is related to the low polarisability of this atom. 45,46,55 We have previously reported that isosteric fluorine substitution in N-benzyl substituted DPPs plays an important role in determining solid state packing behaviour and can be tuned to access unique structural motifs. 30 The π-π stacking regime of crystal structures bearing direct fluorination of the DPP core phenyl rings and benzyl groups display the now characteristic 1-dimensional slipped π-π stacking motif arising from N-benzyl substitution, with the degree of long and short molecular axes slip controlled by the position of fluorine substitution. Surprisingly, this type of stacking behaviour is diminished completely by trifluoromethyl substitution at the para position of the core phenyl rings, leading to novel packing motifs underpinned by an orthogonal orientation of the π-π stacking dimer pairs and formation of a non-covalent organic framework characterised by well-defined channels perpetuating along the length of a single crystallographic axis. 30 6 In all of the DPP crystal structures reported to date by our group, π-π stacking behaviour has been 1-dimensional in the bulk state, despite the star-shaped structure of the molecular building blocks involved. In the following study, we extend our earlier work on fluorine substitution in DPPs and report the synthesis and characterisation of a novel symmetrical fluorine substituted N-benzyl DPP single crystal structure, which surprisingly, exhibits 2-dimensional π-π stacking (Scheme 1). This new structure, HHFBDPP, was given a name in the form of XYZBDPP, in line with our previously reported series 30 for ease of comparison throughout the present study, where X and Y denote the substitution on the para and meta positions of DPP core phenyl rings respectively. In turn, Z denotes isosteric substitution of the phenylic atoms for fluorine atoms within the benzyl groups. The crystal structure of HHFBDPP exhibits a greater number of nearest neighbour dimer pairs (vide infra) when compared to all previous DPP based systems examined by us [30][31][32] and more importantly all of its dimer pairs are centrosymmetric, facilitating a theoretical determination of their charge transfer properties via the energy splitting in dimer method. Two separate dimer pairs were observed to form one-dimensional π-π stacking motifs and their respective π-π dimer stacking arrangements are illustrated in Figure 1. The first of these two π-π dimer pairs (dimer pair II) exhibits the characteristic 30-32 slipped-cofacial intermonomer arrangement associated with N-benzyl substitution, with a large displacement along the long molecular axis (Δx = 9.13 Å) and a shift along the short molecular axis (Δy = 1.64 Å), consistent with that reported by us previously for the meta fluorinated analogue, HFFBDPP (Δx and Δy = 9.12 and 2.31 Å respectively). 30 The latter reinforced our conclusion that isosteric fluorine substitution of the benzyl phenylic hydrogen atoms leads to larger displacements along the long and short molecular axes than those exhibited by non-fluorinated equivalents, such as HHHBDPP (where Δx and Δy = 4.52 and 0.05 Å respectively). 31 The second π-π stacking interaction based on dimer pair (V) of HHFBDPP is characterised by unusual intermonomer displacements (Δx = 0.58 and Δy = 4.43 Å) which are much more in line with those observed for non N-substituted DPP pigments. To the best of our knowledge, the crystal structure of HHFBDPP represents the first N-substituted DPP derivative to exhibit such a large displacement along the short molecular axis in a π-π stacking interaction. In addition, π-π stacking dimer pairs II and V extend into mutually orthogonal one-dimensional π-π slipped cofacial stacking motifs running parallel to the b and a crystallographic axes respectively. This cruciform arrangement of π-π stacking interactions is unique and HHFBDPP represents the first example of a DPP where π-π induced high order bulk dimensionality is observed in the single crystal structure. In the following report, the nature of 8 the computed binding energies and charge transfer properties of these distinctive π-π stacking assemblies will be discussed.

Reagents and instrumentation. Unless otherwise specified, all starting materials and reagents
were purchased from Fisher Scientific, Sigma-Aldrich or VWR and used as received without further purification. 1 H NMR and 13 C NMR proton decoupled spectra were determined using a Bruker AV3 400 MHz spectrometer (in CDCl 3 ). Elemental analyses were carried out using the service provided at the University of Strathclyde in Glasgow, UK. FTIR analyses were carried out on the neat samples by attenuated total reflectance using a Thermo Scientific Nicolet iS5 FTIR Spectrometer, with an iD5 ATR (Diamond) sampling accessory.
Synthesis.  (4) 57 Transfer integrals for holes (t h ) and electrons (t e ) were computed within the framework of the energy splitting in dimer method where t h and t e are given by half the splitting between the dimer HOMO/HOMO(-1) and

2,5-bis(pentafluorobenzyl)-3,6-diphenylpyrrolo[3,4-c]pyrrole-1,4-dione (HHFBDPP).
LUMO/LUMO(+1) orbitals respectively. 36 All molecular modelling studies in this work were carried out using Truhlar's density functional M06-2X 58 at the 6-311G(d) level as implemented in Spartan10 59 software unless otherwise stated. This density functional has been shown to give a good account of the dimer interactions of π-conjugated systems. 38,60 RESULTS AND DISCUSSION Structural description. As is common for related structures, 30,31,33 the molecular structure of HHFBDPP has crystallographically imposed i symmetry (Z' = 0.5) with the centre of symmetry lying in the middle of the C1-C1' bond. This necessitates an anti-conformation with the C 6 F 5 groups lying above and below the plane of the DPP ring. Previous structures in this family have been described as π-π stacks with a continuum of structures with different degrees of slip along the long molecular axis. [30][31][32][33] Small degrees of slip correspond to close contacts between the DPP rings and the (non-benzyl) C 6 X 5 rings whilst larger degrees of slip correspond to contacts between pairs of C 6 X 5 rings. HHFBDPP has a structure that fits into this continuum albeit with a large long axis slip that is most similar to the series' previously described end member HFFBDPP. The centroid to centroid distance between the phenyl rings is 3.792 Å and so like HFFBDPP this is a relatively long contact that lies outside the contact distance of van-der-Waals interactions. With respect to van-der-Waals distances, the shortest contacts are π-π interactions between the fluorinated rings (shortest C…C distance 3.293 (2)  In light of the crucial role played by π-π stacking motifs in determining the charge transport properties of organic conjugated materials, [36][37][38] it is significant that two of the identified dimer pairs of HHFBDPP (dimer pairs II and V) exhibit such a stacking assembly (Figure 2). Dimer pair II is characterised by the slipped cofacial π-π stacking motif observed previously in N-benzyl substituted DPPs, where the displacement along the long and short molecular axes (see to display such a displaced cofacial π-π stacking interaction along the short molecular axis.  Figure 2 illustrates the unique supramolecular arrangement of π-π stacking observed in HHFBDPP, where dimer pairs II and V pairs form mutually orthogonal one-dimensional π-π slipped cofacial stacking motifs that perpetuate the length of the b and a crystallographic axes respectively. The supramolecular assembly of both π-π stacking domains affords a cruciform arrangement in the bulk which is highly advantageous in organic semiconductors. In addition, the π-π stacking dimer pairs II and V were observed to exhibit the largest computed intermolecular interaction energies from all of those identified in the single crystal structure of HHFBDPP (see Table 2). For dimer pair II, which exhibits a closer alignment along the short molecular axis than dimer pair V (Δy = 1.64 and 4.43 Å for dimer pairs II and V respectively), an intermolecular interaction energy of -41.08 kJ mol -1 was computed. This is lower than that computed for the π-π stacking dimer pair of the non-fluorinated equivalent, HHHBDPP for which we reported ΔE CP of -70.20 kJ mol -1 , 31 but greater than that computed for the analogue bearing fluorine substituents on the benzyl phenyl rings and at the meta positions of the DPP core phenyl rings (HFFBDPP, where ΔE CP = -22.46 kJ mol -1 ). 30 We associate the differences in interaction energies of the respective π-π stacks to the interactions facilitated by their different displacements along the long and short molecular axes, as illustrated in Figure 1 (Δx/Δy = 4.52/0.05, 9.13/1.64 and 9.12/2.31 Å for HHHBDPP, HHFBDPP and HFFBDPP respectively). In turn, a larger computed binding energy of -56.17 kJ mol -1 was determined for the π-π stacking dimer pair V of HHFBDPP which we similarly ascribe to the closer intermonomer alignment observed for this dimer pair along the long molecular axis (Δx = 9.13 and 0.58 Å for dimer pairs II and V respectively) and therefore more favourable intermolecular interactions. Both π-π stacking dimer pairs of HHFBDPP were observed to exhibit greater binding energies than those computed for the cofacial π-π dimer pair identified in the single crystal structure of rubrene (ΔE CP = -35.60 kJ mol -1 using M06-2X/6-311G*), which is an archetypal material widely employed as a charge transfer mediator in optoelectronic devices. 37,65,66 Therefore, we anticipate a greater thermal integrity of the two π-π stacking dimer assemblies reported for HHFBDPP; a property which is highly desirable in light of the sensitivity (vide infra) of computed charge transfer integrals to small intermonomeric displacements. 30  The identified nearest neighbour dimer pairs extracted from the single crystal structure of HHFBDPP are presented in Figure 3. The role of pentafluorobenzyl (FB) substitution in these environments was investigated by producing a series of systematically cropped dimer pairs (DPP in Table 2) and computing their interaction energies, in line with our previous studies, [30][31][32] where the fluorine substituted benzyl groups were cropped and substituted with hydrogen atoms to generate their pigment-type equivalents. These results are summarised in Table 2. induced stabilisation of the dimer pair II. The latter is associated to a weak intermolecular interaction between the electropositive para phenylic hydrogen atoms and the electron deficient pentafluorophenyl rings. In turn, most of the computed binding energy can be associated to the quasi-eclipsed π-π interaction between the DPP core phenyl rings and an electrostatic interaction between the electronegative carbonyl oxygen and the electropositive meta phenylic hydrogen atoms located 2.542 Å apart (Figure 4). This is consistent with the computed pentafluorobenzyl induced stabilisation of the meta-fluorinated analogue, HFFBDPP (ΔE CP = -22.46 and -16.75 kJ mol -1 for XYZBDPP and DPP of the π-π dimer pair of HFFBDPP respectively), 30 which we attribute to their similar intermonomer displacements along the short and long molecular axes as illustrated in Figure 1. Unlike the fluorinated species HHFBDPP and HFFBDPP, the non-fluorinated analogue, HHHBDPP is characterised by a substantial destabilisation on removal of the benzyl groups (ΔE CP = -70.10 and -41.90 kJ mol -1 for XYZBDPP and DPP of π-π dimer pair of HHHBDPP respectively). This greater benzyl induced stabilisation for the π-π dimer pair of HHHBDPP is attributed to a close electrostatic intermolecular interaction between the electropositive methylene hydrogens and electronegative carbonyl oxygen atoms, which is precluded in the fluorinated systems given the larger shift along the long molecular axis (Δx = 4.52, 9.13 and 9.12 Å for π-π dimer pairs of HHHBDPP, HHFBDPP (II) and HFFBDPP respectively) 30,31 as illustrated in Figure 5. In contrast to π-π dimer pair II in HHFBDPP, the π-π stacking dimer pair V, which is characterised by significant displacement along the short molecular axis was observed to exhibit 20 a larger pentafluorobenzyl induced stabilisation (ΔE CP =-56.17 and -25.25 kJ mol -1 for XYZBDPP and DPP of dimer pair V respectively). We associate this two-fold stabilisation to intermolecular interactions involving a slipped-cofacial [68][69][70] contact between the electron deficient pentafluorinated phenyl rings and two different electrostatic intermolecular interactions between the electronegative carbonyl oxygen of one monomer and the electropositive methylene and ortho phenylic hydrogen atoms of the other monomer, which are separated by 3.106 and 2.423 Å respectively ( Figure 6). respectively. In light of the very small charge transfer integrals computed for dimer pairs I, III, IV, VI, VII and VIII (SI 2) we focus in the following on those computed for the cruciform π-π stacking dimer assemblies II and V ( Figure 2).
Theoretical hole and electron transfer integrals for the π-π stacking dimer pairs II and V were determined to be t h /t e = 0.27/0.79 kJ mol -1 and t h /t e = 3.47/5.12 kJ mol -1 respectively. Based upon these crystal extracted dimer geometries we would therefore predict ambipolar charge transport behaviour from HHFBDPP, with this single crystal structure favouring electron transport, via either π-π stacking domain, with more efficient electron transport occurring along the overlap, which for dimer pair V in HHFBDPP, is controlled by the unusual short molecular axis slip observed in the π-π stack. Whilst the predicted electron transport behaviour of HHFBDPP does not supersede that determined for HHHBDPP 31 it does approach that computed by us for rubrene (t e = 7.46 kJ mol -1 ) 26 . Given the additional favourable impact that may be expected on the overall electron affinity of HHFBDPP as a result of fluorination, we anticipate that this DPP system could be an interesting electron transport material that should be investigated further. In addition, unlike DPP single crystal structures reported by us previously, orthogonal π-π charge transport may be expected for HHFBDPP, albeit to a lesser extent along the crystallographic b-axis associated with the direction of π-π stacking in dimer pair II. Therefore, despite lower hole and electron integrals compared with HHHBDPP, this structure might be expected to demonstrate more effective overall charge transport in thin film based devices, owing to the enhanced dimensionality of charge transport pathways available; particularly when considering the presence of grain boundaries and other environmental influences that may arise during device fabrication and which can adversely impact on overall device performance.
To investigate the role of the pentafluorobenzyl substitution in controlling the electronic coupling and charge carrier type in this system, we computed transfer integrals for artificially generated dimer geometries as extracted from the single crystal structure of HHFBDPP, where the pentafluorobenzyl groups were removed and replaced with hydrogen atoms. Negligible anticipated that this effect should be limited to structurally related π-π dimer pairs characterised by large shifts along their short molecular axis, and accordingly has not been previously observed to this extent. We report that, although weak, a number of intermolecular interactions involving organic fluorine (C-F---H, π F ---π and C-F---π F ) are involved in the supramolecular assembly of these dimers. In common with structural analogues reported previously, two of the dimer pairs in HHFBDPP exhibit slipped cofacial π-π stacking interactions that are characteristic of N-benzyl substitution and which are known to be crucial in the development of effective charge transfer mediating materials. One of these π-π stacking dimer pairs exhibits intermonomer displacements in line with previously reported N-benzyl substituted systems, with shifts along the long and short molecular axes that are consistent with pentafluorobenzyl substitution. To our surprise, the other π-π stacking dimer pair was significantly displaced along its short molecular axis in a cofacial dimer arrangement, which is more characteristic of non N-substituted DPP pigment analogues. The π-π stacking dimer pairs in the single crystal structure extend into mutually orthogonal 1-dimensional π-π slipped cofacial stacking motifs running the length of the a and b crystallographic axes, forming a completely unique 2-dimensional supramolecular cruciform arrangement. To the best of our knowledge, this behaviour of π-π stacking interactions has not been reported previously and HHFBDPP represents the first example of an N-substituted DPP where π-π controlled high order bulk dimensionality is observed in the solid state. Charge transfer integrals for the two π-π stacking dimer pairs in HHFBDPP were determined using the energy splitting in dimer method. Ambipolar charge transport favouring electron transfer approaching that of rubrene is predicted in both of the π-π stacks with a greater magnitude of coupling observed from those dimers perpetuating along the crystallographic a-axis. We propose that charge transport behaviour in HHFBDPP 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. Given the distinctive electronic structure of this system, heavily influenced by its unique solid state packing behaviour, we anticipate that HHFBDPP is an interesting electron transport material that should be investigated further, particularly in crystalline thin film based devices where high bulk dimensionality is desirable.

SUPPORTING INFORMATION
Position and computed hole and electron transfer integrals for each of the crystal derived dimer pairs. X-ray crystallographic information files (CIF) are available free of charge via the Internet