Controlling molecular assembly and charge transport of n-type organic semiconductors with sterically demanding substituents

Benzo[ de ]isoquinolino[1,8- gh ]quinolinetetracarboxylic diimide (BQQDI) n-type organic semiconductors demonstrate unique multi-fold intermolecular hydrogen-bonding interactions that lead to excellent aggregated structures, charge transports, and electron mobility. However, the robust intermolecular anchoring of BQQDI presents challenges for further fine-tuning molecular assemblies and organic semiconductor properties. Herein, we report the design and synthesis of two BQQDI derivatives with sterically demanding phenyl- and cyclohexyl-substituted BQQDI (Ph–BQQDI and Cy 6 –BQQDI), where the two organic semiconductors show distinct molecular assemblies and degrees of intermolecular orbital overlaps. In addition, the difference in their packing motifs led to strikingly different band structures that give rise to contrasting charge-transport capabilities. As a result, Cy 6 –BQQDI shows excellent transistor performances in both single-crystalline and polycrystalline thin-film organic field-effect transistors. residual d d 2 ), 1,1,1,3,3,3-hexafluoro-2-propanol- d 2 (HFIP- d 2 ) and 7.26 ppm for chloroform- d (CDCl 3 )). The data were presented in the following format: chemical shift, multiplicity (s = singlet, d = doublet, t = triplet, quint = quintet, m = multiplet), coupling constant in Hertz (Hz), signal area integration in natural numbers. Time-of-flight high-resolution mass (TOF-MS) spectrometry measurements were measured on a BRUKER compact-TKP2 mass spectrometer with the atmospheric pressure chemical ionization (APCI) method. Elemental analysis measurements were out on a JScience


Introduction
Charge transport that gives rise to electrical properties of organic semiconductors (OSCs) is typically governed by intermolecular orbital overlaps and controlling such intermolecular interactions to achieve effective charge-transport properties lies in the center of molecular design for high-performance OSCs 1,2 . In the past decades, intense investigations of highperformance OSCs in terms of molecular design and device engineering fueled the rapid development of applicable organic-based electronic devices such as organic field-effect transistors (OFETs) [3][4][5] , which offer mechanical flexibility and low-cost processing compared to traditional inorganic-based devices. In particular, the hole-transporting p-type OSCs have shown promising OFET performances with charge-carrier mobilities (µ) over 10 cm 2 V -1 s -1 , and not only do these materials lead to applicable devices, they also provided crucial information on charge transport and guidance for future molecular designs [6][7][8][9][10][11][12][13] . On the other hand, the electron-transporting n-type OSCs, which are an essential component for constructing organic-based logic circuits [14][15][16] , are generally inferior to state-of-the-art p-type OSCs in terms of µ. One of the challenges associated with the molecular design of n-type OSCs is that, owing to the energetics of charge injections and carrier transport, the lowest unoccupied molecular orbital (LUMO) level of n-type OSCs needs to be sufficiently low (< -4.0 eV) to protect charge carriers from oxidation by ambient oxygen and moisture 17,18 . While the airstability issue of n-type OSCs can be addressed by incorporations of electron-deficient moieties [19][20][21] and several studies have reported air-stable n-type OSCs with encouraging OFET performances [22][23][24][25] , design strategies that focus on effective intermolecular orbital overlaps (quantified by transfer integral t and effective mass m*) 26,27 and molecular assemblies for achieving favorable charge-transport properties and high electron mobility (µe) are still required. Recently, our group reported an air-stable and high-performance benzo [de]isoquinolino [1,8-gh]quinolinetetracarboxylic diimide (BQQDI) π-electron core 28 (π-core) (Fig. 1a). The BQQDI is structurally analogous to the widely studied perylenetetracarboxylic diimide (PDI) system ( Fig. 1a), though the electronegative nitrogen atoms in the BQQDI framework result in a DFT calculated deep-lying LUMO level of -4.17 eV (at the B3LYP/6-31G+(d) level) 29 for potential air-stable n-type charge transports, whereas the PDI π-core possesses a shallower LUMO level of -3.80 eV. Upon functionalization of the BQQDI π-core with phenethyl (PhC2-BQQDI) groups, multi-fold hydrogen-bonding interactions are formed between adjacent molecules in the transverse direction (Fig. 1b), and strong π-π interactions are also observed in the vertical direction. The resulting brickwork packing motifs show large t values which indicate twodimensional (2D)-like charge-transport properties, whereas simple PDI derivatives generally exhibit one-dimensional (1D) π-π stacking motif 30,31 that leads to anisotropic charge-transport capabilities. The PhC2-BQQDI derivative, forms favorable phenyl-to-phenyl edge-to-face interactions between each molecular layer (Fig. 1b), in addition to the aforementioned intermolecular features, which significantly reinforce the intermolecular orbital overlaps as well as suppression of molecular motions. As a result, PhC2-BQQDI exhibits an impressive µe of 3.0 cm 2 V -1 s -1 in solution-processed OFETs, and excellent robustness against thermal-and bias-stress, which are necessary features for practical organic electronic applications.
Despite the encouraging results of PhC2-BQQDI-based OSCs, the robust core-to-core and interlayer intermolecular interactions also pose challenges to further fine-tune molecular assemblies and charge-transport properties of BQQDI derivatives. By examining the packing structure of PhC2-BQQDI, we notice that the hydrogen-bonding interactions cause some degree of misalignment in the π-π stacking direction, which lead to an unbalanced chargetransport capability reflected by its t and m* values. Herein, we report the investigation of two BQQDI derivatives with phenyl (Ph-BQQDI) and cyclohexyl (Cy6-BQQDI) substituents on their molecular assemblies and charge-transport capabilities. From a chemical perspective, we envisage that the installment of these sterically demanding substituents close to the BQQDI πcore compared to PhC2-BQQDI may sufficiently weaken the hydrogen-bonding interactions in the transverse direction and reduce the misalignment in intermolecular orbital overlaps.
Owing to the different geometric and electronic properties, Ph-BQQDI and Cy6-BQQDI exhibit distinct intra and interlayer molecular assemblies that lead to contrasting chargetransport capabilities as well as OSC performances. The single crystals of Ph-BQQDI and Cy6-BQQDI were prepared using physical vapor transport (PVT) and solution-grown methods, respectively, and large plate-like crystals were obtained for both compounds (Fig. S5). Ph-BQQDI crystallizes in the monoclinic P21/c space group with a 2D brickwork packing motif. Each planar BQQ π-core forms a four-fold hydrogen-bonding interactions with O···H and N···H short contacts on each side with its adjacent molecules in the transverse direction, along with slipped π-π stacking interactions ( Fig. 3a). The substituents of Ph-BQQDI form multiple phenyl-to-phenyl interactions in the interlayer space (Fig. 3b), which has been shown to be a favorable feature for suppressing molecular fluctuations 28 . Within the brickwork assembly of Ph-BQQDI, distances of the π-stacks are found to be 3.36 Å and 3.37 Å (Fig. 3c), and the slight difference in distances is attributed to the misalignment between adjacent molecules in the transverse direction. The molecular assembly of Ph-BQQDI leads to a misalignment of LUMO of molecules in the π-π stacking direction (Fig. 3a), where only a small degree of LUMO overlaps is observed between the top molecule and the molecule in the bottom layer. By calculating the t values of Ph-BQQDI based on its crystal structure, it is evident that the misalignment in the assembly leads to different degrees of orbital overlaps with t1 and t3 equal to +77.23 meV and +48.57 meV, respectively (Fig. 3c) (Fig. 3a). However, transverse dimers of Cy6-BQQDI show a much smaller displacement in the long molecular axis direction than Ph-BQQDI dimers, and the reduced molecular misalignment of Cy6-BQQDI leads to a much more enhanced LUMO overlaps in the π-π stacking directions. In contrast to Ph-BQQDI, Cy6-BQQDI does not show any apparent interactions in the interlayer space ( Fig. 3b), and we speculate that this may lead to larger molecular fluctuations of Cy6-BQQDI than Ph-BQQDI. The 2D brickwork motif of Cy6-BQQDI shows a uniform π-π stacking distance of 3.33 Å, which corresponds to the same degree of intermolecular orbital overlap with t1 = t3 = +88.28 meV, which are larger than those of Ph-BQQDI. Even though the transverse dimer of Cy6-BQQDI demonstrates much weaker interaction energy than that of Ph-BQQDI dimer, the transverse intermolecular orbital overlap of Cy6-BQQDI that is quantified by t2 (+14.27 meV) is only slightly lower than that of Ph-BQQDI (+18.55 meV) (Fig. 3c). The uniform charge-transport capability exhibited by Cy6-BQQDI may indicate promising OSC performances 34 .  were directly laminated on a silicon substrate coated with a parylene insulating polymer, which has been used for laminated single-crystal OFETs 35 . The OSC single-crystalline thin films (7.79 nm thick) of the more soluble Cy6-BQQDI were prepared by the edge-casting method 36 on the AL-X601-coated silicon substrate, which is an excellent insulating layer for solutionprocessed n-type single-crystalline thin-film OFETs 28 . The maximum µe of Ph-BQQDI was measured to be 1.0 cm 2 V -1 s -1 (Fig. 5a). The highest µe of 2.3 cm 2 V -1 s -1 was achieved by the single-crystalline thin-film OFET using Cy6-BQQDI and an average µe of 1.8 ± 0.21 cm 2 V -1 s -1 was measured over 12 devices (Fig. S7), and the devices showed excellent air-stability over one month (Fig. S8). Thin-film X-ray diffractions of the OSC active layers of Ph-and Cy6-BQQDI reveal that their OFET channels directions correspond to the b-crystallographic axis and the [1 1 0] direction, respectively (Fig. S6). The molecular stacks of Cy6-BQQDI are roughly orthogonal to the OFET substrate with the π-π stackings that are parallel to the electron transport, whereas the molecular assembly of Ph-BQQDI creates more of an offset between the electron transport and the π-π stacking direction, which possibly leads to a less efficient electron transport. We evaluated the polycrystalline thin-film OFETs of Ph-and Cy6-BQQDI, and the highest µe of 0.16 cm 2 V -1 s -1 was obtained for Ph-BQQDI (Fig. S15), which is one-order lower than its single-crystalline device, likely due to large grain boundary of the polycrystalline thin films (Fig. S10). Cy6-BQQDI-based polycrystalline OFETs afforded the highest µe of 0.50 cm 2 V -1 s -1 on DTS (Fig. S16), and this promising result motivated us to explore other device conditions.

Results and Discussion
When changed the SAM from DTS to hexamethyldisilazane (HMDS), which decreases the ratio of face-on/edge-on assemblies (Fig. S12), the highest µe of 40 nm-thick polycrystalline devices of Cy6-BQQDI was further improved to 0.66 cm 2 V -1 s -1 (Fig. S14). We found that by reducing the Cy6-BQQDI OSC layer thickness from 40 nm to 20 nm on HMDS, an excellent highest µe of 1.0 cm 2 V -1 s -1 could be achieved, and the µe appeared to be independent of the channel length (100-500 µm) (Fig. S17-19). The device performances of polycrystalline Cy6-BQQDI on DTS and HMDS in air are also consistent over more than one month (Fig. S20).
The µe of polycrystalline Cy6-BQQDI is one of the highest among current BQQDI derivatives, though, we speculate that its overall polycrystalline device performance might be hampered by the orientational disordering of its thin-film molecular assembly, and further optimization of the deposition conditions is currently undergoing. Both single-and polycrystalline OFETs based on Cy6-BQQDI show significantly higher µe than those based on Ph-BQQDI, and the difference in their device performances are in agreement with their calculated t values, but more in-depth analysis of their charge-transport capabilities is required.   These data can be obtained free of charge at www.ccdc.cam.ac.uk/data_request/cif.