The influence of alkyl group regiochemistry and backbone fluorination on the packing and transistor performance of N-cyanoimine functionalised indacenodithiophenes

The influence of backbone fluorination and sidechain regiochemistry on an indacendithiophene (IDT) core containing electron withdrawing N-cyanoimine groups is investigated.

distances (~ 3.40 Å), significant differences in electron transfer integrals for the two regioisomers were observed, relating to differences in relative molecular displacement along the π-stacking direction. Organic thin-film transistors fabricated via blade-coating displayed electron mobility up to 0.13 cm 2 V -1 s -1 for the isomer with the larger transfer integral.

Introduction
Conjugated organic semiconductors (OSCs) are one of the key enabling materials for the predicted next generation of printed electronic devices. 1, 2 Extensive research effort has resulted in the development of a number of promising solution processable materials. However, progress in the development of p-type (or hole transporting) materials has far outstripped that of n-type (or electron transporting) materials in terms of charge carrier mobility, scalability and stability. [3][4][5][6] New n-type materials are required for use in a number of applications, from the development of CMOS-type circuits with low power consumption to electron-transporting interlayers in hybrid photovoltaic devices. [7][8][9][10] In designing such n-type materials, it is important that the OSC has a high electron affinity to facilitate electron injection and charge transport. This is usually expressed in terms of the energy level of the lowest unoccupied molecular orbital (LUMO), which should be sufficiently low-lying to enable charge injection from common electrode materials. This is typically achieved by the functionalization of an aromatic core with a number of strongly electron withdrawing groups, like nitriles, carbonyl, (di)imide and halogens. 9,[11][12][13][14] Low lying LUMO levels can also help to improve the ambient stability of the resulting reduced species. [15][16][17][18] In addition to energetic considerations, it is also important for charge transport that the material has a high degree of solid-state ordering with a strong electronic coupling between adjacent units. [19][20][21][22] A number of fused aromatic cores have been investigated as potential n-type materials. One particularly interesting class is that based on fused aromatics held rigidly co-planar by bridging (hetero)atoms, often termed ladder-type materials. 23,24 The high rigidity and coplanarity helps to reduce reorganization energy during charge transfer and facilitates delocalization of the conjugated system. Within the extensive class of ladder-type materials, we and others, have been particularly interested in 4,9-dihydro-s-indaceno[1,2-b:5,6-b′]dithiophene (IDT), in which a central benzene ring is flanked by two thienyl units held co-planar by bridging carbon atoms. [25][26][27][28] The high symmetry of IDT, coupled with its terminal thienyl groups, tends to afford linear materials with low torsional disorder, which have good p-type transistor performance.
The promising performance of the IDT core has prompted interest in its utilization as an n-type material in transistor devices. 27,29,30 Since IDT is inherently electron rich, as a consequence of the two fused thiophene rings, efforts have focussed on the functionalization of IDT with various electron withdrawing groups, either in the terminal or bridgehead positions. 29,[31][32][33][34] We recently reported that the functionalization of the bridgehead position of IDT with the strongly electron withdrawing dicyanomethylene group together with simultaneous fluorination of the central benzene ring resulted in a material with a low lying LUMO and promising n-type performance. 35 Unusually, we found that this material demonstrated improved solubility compared to the analogous non-fluorinated material, which appeared to result from the steric interactions between the fluorine and the dicyanomethylene group leading to a bowing of the structure.
In this report, we aimed to reduce the molecular distortion whilst retaining the deep LUMO level by combining central core fluorination with the addition of a less sterically demanding electron withdrawing group in the bridgehead positions of the IDT. One such group is the N-cyanoimine group (=N-CN), which has been relatively unexplored in the context of n-type materials. [36][37][38] Recently, Rault-Berthelot and co-workers reported that IDT functionalised with N-cyanoimine exhibited a very similar LUMO level to IDT functionalized with dicyanomethylene, thereby demonstrating its strongly electron-withdrawing nature. 39 Building upon these results, we hereby report the synthesis and characterization of two novel fluorinated IDT derivatives (diFIDT-di(N(CN))) containing the N-cyanoimine group. In addition, we also investigate how changing the regiochemistry of the required solubilizing hexyl group from the alpha (2,7-) to the beta (3,8-) positions of the IDT core influences solid-state packing and device performance.

Design and Synthesis
As a starting point we examined the predicted effect on backbone planarity of the N-cyanoimine group by performing molecular modelling using DFT with a B3LYP level of theory and 6-311+G(d,p) basis set. The N-cyanoimine group is asymmetric and therefore three stereoisomers could possibly be formed by its attachment to the IDT bridging positions. DFT calculations were performed to establish the ground state conformation and HOMO/LUMO energy levels of each isomer ( Figure   S1). The results indicated that the structure is planar in all three isomers, suggesting that unlike dicyanomethylene, N-cyanoimine would not introduce any undesired steric interactions with the fluorinated core. The Z,Z-isomer was predicted to be the most stable, as indicated by a lower HOMO energy level. These calculations also predicted a low-lying LUMO energy level (-4.25 eV), indicating suitability for n-type applications. 2,7-Dihexyl-diFIDT-di(N(CN)) and 3,8-dihexyl-diFIDT-di(N(CN)) were synthesised following a modified literature procedure (Scheme 1). 36,40 The starting 2,7-dihexyl-diFIDT-di(O) was synthesised following our recently reported route, 35  A planar slipped π-π stacking arrangement is formed along the a axis, however adjacent stacks (along the molecular short axis) are arranged almost perpendicular to each other (85.3 °) with poor interstack registry. Alkyl chain interdigitation is observed between stacks along the long axis (Figure 1d & e). This is in contrast to the non-fluorinated analogue 2,7-dihexyl-IDT-di(N(CN)), which displays a planar slipped π-π stacking motif without such tilting (Figure 1f & g). This change in packing motif upon core fluorination is similar to that observed for the dicyanomethylene-containing analogue, 2,7dihexyl-diFIDT-di(C(CN) 2 ), reported in our previous study. 35 There is co-facial packing of the conjugated core within a stack, along the a axis, with an interplanar stacking distance of 3.41 Å and a slipping distance of 3.24 Å (angle 43.6 °) ( Figure S22). These values are similar to 2,7-dihexyl-IDT-di(N(CN)) (interplanar stacking distance of 3.41 Å, slipping distance of 3.51 Å).
One key factor affecting charge transport through organic crystals is the electronic coupling between frontier molecular orbitals on neighbouring units, known as the transfer integral. A larger value indicates better overlap between frontier molecular orbitals on neighbouring units (LUMO -LUMO in the case of electron transport) and therefore the potential for more efficient charge transfer. 2,7-Dihexyl-diFIDT-di(N(CN)) displays a one-dimensional (1D) charge transfer pathway, with an electron transfer integral of 106 meV for the main π-stack. This value is high, and larger than that calculated for 2,7-dihexyl-IDT-di(N(CN)) (87 meV), indicating that core fluorination is expected to be beneficial for charge transfer.
To gain insight into the intermolecular interaction energies, symmetry adapted perturbation theory (SAPT0/jun-cc-pvdz) calculations were performed. SAPT0 analysis allows the total interaction energies between molecular pairs extracted from the crystal structures to be decomposed into the A planar slipped π-π stacking arrangement is formed along the a axis and alkyl chain interdigitation is observed between stacks (Figure 3d& e). Stacks are arranged co-planar along the short axis. This is contrasting to that observed for 2,7-dihexyl-diFIDT-di(N(CN)), where a perpendicular arrangement is present. Hence, the position of the alkyl chain is shown to have a significant impact on the crystal packing. There is co-facial packing of the conjugated core within a stack, along the a axis, with an interplanar stacking distance of 3.39 Å and a slipping distance of 3.25 Å (angle 43.8 °) ( Figure S27). These values are almost identical to those present in 2,7-dihexyl-diFIDT-di (N(CN)). (N(CN)) displays a one-dimensional (1D) charge transfer pathway, with an electron transfer integral of 29 meV for the main π-stack. This is much lower than for 2,7-dihexyl-diFIDT-di(N(CN)) (106 meV), which indicates that there is a poorer LUMO -LUMO interaction. Such a large difference is surprising, considering that the interplanar stacking distance and slipping distance are so similar, but highlights the sensitivity of charge transfer to the relative positioning of the frontier molecular orbitals on neighbouring molecules. This phenomenon has been widely studied,

Electrochemical Properties
Electrochemical studies of both dialkyl-diFIDT-di(N(CN)) analogues were performed by cyclic voltammetry (CV) using a 0.1 M solution of Bu 4 NPF 6 in DCM solutions and a ferrocene (Fc/Fc + ) internal standard. Potentials were swept from 0 V to a positive maximum, then swept back to a negative maximum before returning to 0 V. In line with best practice, energy levels were estimated from E 1/2 (when reversibility was observed) or E i (when irreversibility was observed), where E 1/2 is the half-wave potential and E i is the inflection-point potential. 43 N(CN)) demonstrated a small depression (0.16 eV) due to the introduction of fluorine groups onto the central phenyl moiety.

UV-Vis Absorption Spectra
The absorption spectra for both dialkyl-diFIDT-di (N(CN)

Thermal Properties
The thermal behaviour of both dialkyl-diFIDT-di(N(CN)) analogues was investigated by a combination of thermal gravimetric analysis (TGA, Figure S30) and differential scanning calorimetry (DSC, Figure S31). The key data is summarised in Error! Reference source not found.. (N(CN)) analogues exhibited high thermal stabilities (temperature at which 5% mass loss is reached, T d > 320 °C), though a distinct drop in mass (~ 1% loss) can be seen at around 250 -275 °C. This feature was also present for the non-fluorinated analogue (2,7-dihexyl-IDT-di (N(CN))), as previously reported by Rault-Berthelot et al., though this feature was not explicitly discussed. [39] The T d of 364 °C reported for 2,7-dihexyl-IDT-di (N(CN)) was > 40 °C higher than for 2,7-dihexyl-diFIDT-di(N(CN)) (T d = 321 °C), indicating that core fluorination was detrimental for the thermal

Organic Field-Effect Transistors
The charge transport behaviour of both materials was investigated in OFET devices. Previously reported transistors utilizing the non-fluorinated 2,7-dihexyl-IDT-di(N(CN)) were fabricated via vacuum deposition, achieving relatively modest device performance with a charge carrier mobility up to 1.4 x 10 -3 cm 2 V -1 s -1 . 39 In our case we found that the solubility of both dialkyl-diFIDTdi (N(CN)) OSCs was sufficient in chlorobenzene to fabricate transistor devices by blade-coating, a potentially scalable technique for solution based devices. 35,44 Devices were fabricated in a bottom- contact top-gate (BC -TG) configuration, with the active layer deposited onto the substrate at 100 °C, followed by spin coating of the gate dielectric (Cytop TM ) and thermal evaporation of the aluminium gate electrode. The key results are summarised in Table 1. All devices exhibited unipolar electron transport, reflecting the role of the deep LUMO level in facilitating electron transport. In addition, the deep HOMO energy level (~ -6.05 eV) imparted a large energy level offset with the Au S/D electrodes, helping to suppress hole injection and possible ambipolar behaviour.
The saturated mobility was extracted from the linear part of the square root of the drain current ( Figure S32a & S33a). The absence of any double slope in the plot endorses the reliability of the data, 45 although a weak gate voltage-dependence of the saturated mobility above V T suggests some device non-idealities were present (Figure S32c & S33c). Furthermore, the large variations in the threshold voltages suggest the presence of interfacial traps and a strong relation to the local processing conditions during coating.
All devices displayed performance at least an order of magnitude higher than previously reported for the vacuum-processed, non-fluorinated 2,7-dihexyl-IDT-di (N(CN)), highlighting the beneficial role of core fluorination.
Single crystal analysis of 2,7-dihexyl-diFIDT-di (N(CN)) and 3,8-dihexyl-diFIDT-di(N(CN)) demonstrated that they arrange into highly ordered planar slipped stacking arrangements with very close π-π stacking distances (~ 3.40 Å) and minimal π-π slippage. Despite exhibiting similar stacking distances, a significant difference in electron transfer integrals for the two regioisomers was observed, relating to differences in relative molecular displacement along the π-stacking direction. OFET show that the inclusion of the N-cyanoimine groups cause less backbone distortion than the previously investigated dicyanomethylene group. The combination of N-cyanoimine with core fluorination leads to very low lying LUMO levels and appears a useful strategy in the design of n-type materials.

Conflicts of interest
There are no conflicts to declare.     N(CN)) analogues, compared to the non-fluorinated analogue. a) extracted from CV measurements, b) data for this compound taken from Rault-Berthelot et al. [25] , c) reanalysed as E 1/2 , d) in THF, e) in toluene, f) in chloroform, g) no transitions observed in DSC.