Molecular Mimicry and Assemblies of Asymmetric Organic Semiconductors


 Molecular assembly is a crucial factor for charge transports in organic semiconductors (OSCs), and molecularly flexible alkyl chain substitution is a key design feature for achieving desired molecular assemblies. However, the high degree of freedom of alkyl chains leads to molecular fluctuations that are detrimental to OSC performances. Stabilization of alkyl chains via intermolecular interactions in packing structures exists in biological and materials systems, and such a strategy can be harnessed in OSCs to suppress molecular fluctuations. Here, we present a robust synthetic strategy for a series of asymmetric n-type benzo[de]isoquinolino[1,8-gh]quinolinetetracarboxylic diimide (BQQDI) OSCs with various alkyl chain lengths, and certain alkyl chains exhibit an unusual molecular mimicry with energetically favorable gauche conformer that shows isomorphic structures and small molecular fluctuations. Asymmetric n-type OSC with the optimum chain length exhibits satisfactory solubility, excellent electron mobility, and large-area single-crystalline thin films are fabricated for practical organic electronics.

*Corresponding Author: 23 Toshihiro Okamoto, tokamoto@k.u-tokyo.ac.jp 24 25 26 27 28 29 30 31 32 33 Introduction 15 Molecularly flexible alkyl chains are of vital importance for controlling molecular 16 assemblies of functional materials and biomolecules from liquid crystals 1 to lipid bilayers 2 . In 17 the area of printable and flexible small-molecule organic semiconductors (OSCs), which self-18 aggregate via intermolecular interactions, effective molecular assemblies by rational alkyl 19 chain engineering can lead to strong intermolecular orbital overlaps 3 , high charge-carrier 20 mobilities (µ), and band-like charge transports 4-7 . Another critical role of long alkyl chains is 21 to control solubility and crystallinity of OSCs for solution-processability with common organic 22 solvents and achieve large-area printable electronics 8 . However, one common phenomenon 23 with alkyl chains in functional materials and biological systems is the anti-gauche 24 isomerization at different temperatures due to the relatively small energy difference between 25 these rotamers, 9 and such a thermal disordering causes severe molecular fluctuations in 26 molecular systems. Although these properties may be harnessed for materials applications such 27 as stimuli sensors 10 , molecular fluctuations have been shown to be a detrimental factor for 28 charge-transport properties as they disrupt intermolecular orbital overlaps [11][12][13][14][15][16] . Currently, our 29 knowledge on suppressing molecular fluctuations of OSCs from a molecular design point of 30 view has been limited, but in biological systems, certain membrane proteins show stabilization 31 of amino acid sidechains via intermolecular hydrogen bonding interactions 17,18 , and similar 32 stabilization effect of alkyl chain conformations can also be observed in host-guest materials 33 3 systems 18-20 . On the basis of these findings, inspirations can be drawn from nature for 1 molecular design of OSCs, where motions of alkyl chains are stabilized by appropriate 2 intermolecular interactions to achieve suppression of molecular fluctuations and such a feature 3 is especially crucial for the molecular design of future high-performance OSCs. PhC2-BQQDI-Cn (Cn: linear alkyl chains, n-CnH2n+1). 8 Herein, we present an effective stabilization of alkyl chains with suppressed molecular 9 fluctuations on electron-transporting n-type OSCs, which are an urgently demanded 10 component for all-organic logic circuits [21][22][23][24][25] along with high-performance p-type 11 counterparts [26][27][28][29][30][31][32] . Recently, our group developed a benzo [de]isoquinolino [1,[8][9][10][11][12] gh]quinolinetetracarboxylic diimide (BQQDI) π-electron core (π-core) for high-performance 13 n-type OSCs (Fig. 1a) [33][34][35] . Though the BQQDI molecular structure bares similarity with the 14 vastly studied perylene diimide (PDI) π-core [36][37][38] , the incorporated electronegative nitrogen 15 atoms in BQQDI leads to a deep-lying lowest unoccupied molecular orbital (LUMO) level 16 (Fig. 1a), which offers air-stability in n-type OSC operations without further chemical 17 modifications. Symmetric phenethyl-substituted-BQQDI (PhC2-BQQDI) shows four-fold 18 intermolecular hydrogen-bonding interactions and phenyl-to-phenyl interactions in the 19 interlayers of the crystal structure (Fig. 1b). Molecular dynamic (MD) simulations suggest both 20 intra and interlayer interactions of PhC2-BQQDI suppress molecular fluctuations in the solid-1 state and ensure consistent molecular orbital overlaps for undisrupted charge transport (Fig.   2 1b). As a result, PhC2-BQQDI exhibits an outstanding and reliable µe of 3.0 cm 2 V -1 s -1 . 3 However, the high-performance PhC2-BQQDI exhibits issues of low solubility and difficulties 4 with large-area thin-film fabrications due to the lack of alkyl chains. To further explore the 5 molecular design for high-performance solution-processed OSCs with suppressed molecular 6 fluctuations, we envisage an asymmetric PhC2-BQQDI-Cn (n = 5, 6, and 7) approach ( Fig.   7 1c), where the favorable phenethyl sidechain is preserved on one side, and the substitution of 8 flexible alkyl chains on the other side of BQQDI may lead to fine-tuning of molecular 9 assemblies. The lack of flexible alkyl chains in PhC2-BQQDI results in low solubility and high 10 temperature is required for solution-processed device fabrication, but the introduction of alkyl 11 chains in current asymmetric PhC2-BQQDI-Cn derivatives show orders of magnitude higher 12 solubility that improves the solution processability of high-performance n-type OSCs. We 13 discover that PhC2-BQQDI-Cn alkyl chains mimic the overall shape of the surrounding phenyl 14 groups by adopting the gauche conformation that is stabilized by multiple CH···π interactions 15 that demonstrate the molecular mimicry assembly, and its alkyl chains surprisingly exhibit a 16 similar degree of molecular fluctuations as the high-performance PhC2-BQQDI with rigid 17 phenyl groups. In particular, the PhC2-BQQDI-C5 derivative exhibits high n-type OSC 18 performances in solution-processed OFETs, and inch-scale single-crystalline thin films are 19 obtained for the fabrication of large-area electronics using the continuous edge-casting 20 method 39,40 due to its high solution processability. 21

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Synthesis. Although nature synthesizes asymmetric biomolecules with marvelous 23 efficiencies 41 , it is a challenge for synthetic chemists to prepare asymmetric compounds due to 24 poor selectivity and low yields 42 . In the case of n-type OSCs, several studies on the synthesis 25 of asymmetric PDI have been reported 43-45 . The commonly employed strategy involves 26 sequential imidizations that results in the desired monoimidized product, along with unreacted 27 and difunctionalized species, which are attributed to the high reactivity of primary R-NH2. 28 Owing to the relatively low solubility of PDI, purification of the desired asymmetric compound 29 can be difficult, which leads to poor yields and low purity. Thus, we speculate that similar 30 synthetic strategies for asymmetric PDI may not be applicable to the less-soluble BQQDI, and 31 a more rational and selective method is developed to prepare asymmetric BQQDI. We lower 32 the reactivity of the phenethylamine by functionalizing it with the p-methoxybenzyl (PMB) 33 5 group (Fig. 2), which is a heat-stable protecting group that can endure imidization reactions at 1 elevated temperatures and eventually be removed by acid 46 . synthesis of PhC2-BQQDI-Cn (n= 5, 6, and 7). 5 Here, we began our synthesis from the previously reported trichlorophenyl formate- 6 containing compound 1 (Fig. 2) 47 , as the versatile electron-deficient formate can be readily 7 displaced by alkylamines and purified by column chromatography. The PMB-protected amine 8 was then reacted with compound 1 in refluxed o-dichlorobenzene (o-DCB) for 40 min to give 9 intermediate 2 in 45% yield. Although the first reaction generated the desired and 10 difunctionalized products, as well as unreacted compound 1 indicated by high-performance 11 liquid chromatography (HPLC), compound 2 was readily isolated by column chromatography. 12 From the key precursor 2, we carried out a highly selective one-pot synthesis to furnish a series 13 of PhC2-BQQDI-Cn (n = 5, 6, and 7) from intermediate 2 (Fig. 2), as PMB removal and ring-14 closing steps can be simultaneously facilitated by TfOH, and the one-pot synthesis resulted in 15 excellent yields of 90-92%. One drawback for the high-performance PhC2-BQQDI is the low 16 solubility in common organic solvents due to the lack of flexible alkyl chains, and high 17 temperature (>150 ˚C) is required for large-area device fabrications. Current asymmetric PhC2-18 BQQDI-Cn show more than one order of magnitude higher solubility in o-DCB than PhC2-19 BQQDI (Supplementary Table 1), which indicates high solution-processability. 20 Fundamental properties. The thermal stability of PhC2-BQQDI-Cn compounds was 21 evaluated by thermalgravimetric-differential thermal analysis (TG-DTA), and the crystal phase 22 stability/transition was measured by differential scanning calorimetry (DSC). All PhC2- 23 6 BQQDI-Cn derivatives showed excellent thermal stability with 5% weight loss temperatures 1 (T95) and decomposition temperatures above 370 ˚C and 380 ˚C, respectively (Supplementary 2 Fig. 8). DSC measurements indicated no apparent phase transitions of PhC2-BQQDI-Cn up to 3 250˚C ( Supplementary Fig. 9). All PhC2-BQQDI-Cn derivatives exhibited completely 4 reversible reduction waves in cyclic voltammetry (CV) measurements ( Supplementary Fig.   5 10). The length of alkyl chains did not impose noticeable effects in electrochemical properties, 6 as all derivatives showed first half-width reduction waves at -0.68 V that corresponded to the 7 lowest unoccupied molecular orbital (LUMO) ELUMO = -4.12 eV, and the second reduction 8 waves at -1.0 V appeared to be reversible and electrochemically stable. The electrochemical 9 properties of PhC2-BQQDI-Cn indicate a deep-lying LUMO level that is suitable for air-stable  Table 2), and 13 all derivatives crystallized in the monoclinic system. Within each crystallographic layer, PhC2-14 BQQDI-Cn derivatives form the 2D brickwork packing motif with vertical π-π stacking and 15 lateral hydrogen-bonding interactions (Fig. 3a). To our surprise, the asymmetric molecules did 16 not form the expected phenyl-to-phenyl interlayer interactions shown in PhC2-BQQDI, 17 instead, PhC2-BQQDI-Cn derivatives demonstrate the layer-by-layer molecular assembly 18 where the alkyl chains interact with phenyl groups in the adjacent layer (Fig. 3a). An intriguing 19 finding of PhC2-BQQDI-Cn derivatives is their molecular mimicry assemblies by the alkyl 20 chain conformations. Instead of the expected linear anti conformation, PhC2-BQQDI-C5 21 shows a gauche conformation at the C2-C3 bond with a torsion angle of -71.4˚, and PhC2- 22 BQQDI-C6 exhibits gauche conformations at C2-C3, and C4-C5 bonds with torsion angles of 23 69.3˚ and -4.8˚, respectively. By further extending the alkyl chain to n = 7, such a molecular 24 mimicry is still retained, with torsion angles of -64.1˚ and 101.1˚ (Fig. 3a). Though, the long 25 C7 alkyl chain with the gauche conformation likely causes severe thermal disordering 26 compared to other two derivatives. We then performed a torsion angle energy scan at C2-C3 27 of PhC2-BQQDI-C5 (Fig. 3b), and the potential energy profile of the single molecule showed 28 a textbook-like profile, where the most energetically favorable rotamer is the anti form, though 29 the gauche conformation is merely 0.86 kcal mol -1 higher than that of the anti conformation. 30 However, in the crystal structure of PhC2-BQQDI-C5, each alkyl chain is surrounded by four 31 phenyl groups, and when this "aromatic pocket" is taken into consideration in the DFT 32 calculation, the gauche rotamer becomes the most stable form and the anti rotamer is now 25 33 7 kcal mol -1 higher in energy than the gauche due to steric hinderance (Fig. 3c). The stabilization 1 of the molecular mimicry is arguably attributed to the multiple CH···π interactions within the 2 aromatic pocket, and the large energy barrier between the anti and gauche rotamers would 3 make conformational isomerization unlikely at room temperature, and usual alkyl chain 4 molecular fluctuations may be suppressed. The powders of PhC2-BQQDI-C5 are further 5 subjected to temperature-variant powder X-ray diffractions (PXRD) at SPring-8 RIKEN interlayer phenyl-to-phenyl interactions. Surprisingly, despite having the molecularly flexible 1 alkyl group, the interlayer chains of asymmetric PhC2-BQQDI-C5 also show similarly small 2 degree of molecular fluctuations (small B-factors) as PhC2-BQQDI (Fig. 5a), which is possibly 3 due to the stabilization effect on the molecular mimicry by the "aromatic pocket" (Fig. 3d), and 4 the π-core of PhC2-BQQDI-C5 also shows small degree of fluctuations. Based on this result, 5 we argue that the alkyl chain PhC2-BQQDI-C5 does not behave as an ordinary flexible alkyl 6 chain, but it rather mimics a structurally rigid phenyl group, which leads to suppressed 7 molecular fluctuations. Similarly, PhC2-BQQDI-C6 also exhibits small amplitude of molecular 8 fluctuations in the π-cores, but the alkyl chains show noticeably large B-factors and 9 destabilization of the molecular mimicry conformation (Fig. 5b). The thermally disordered 10 PhC2-BQQDI-C7 expectedly demonstrates large degree of molecular fluctuations in the alkyl 11 chains and the molecular mimicry in the single-crystal structure is no longer retained during 12 the MD simulations (Fig. 5c). In addition, the π-cores of PhC2-BQQDI-C7 show larger 13 amplitudes of molecular fluctuations which could potentially affect the charge-transport 14 capability.  20 revealing the magnitude of the dynamic fluctuations. 21 11 We calculated the variant t values to understand the effect of molecular fluctuations on 1 charge transport in the π-π stacking directions of PhC2-BQQDI-Cn based on the MD 2 simulations. PhC2-BQQDI-C5 exhibits the smallest standard deviations (σ) of 23.0 meV and 3 13.2 meV in the t1 and t3 directions, respectively (Fig. 5d), which suggests that the charge-4 transport capability of PhC2-BQQDI-C5 does not appear to be affected by molecular 5 fluctuations. The σ values of PhC2-BQQDI-C5 are in fact lower than the high-performance 6 PhC2-BQQDI (13.9 meV and 24.2 meV) 33 in their respective directions, which further 7 demonstrates the effectiveness of molecular mimicry in suppressing molecular fluctuations. 8 On the other hand, PhC2-BQQDI-Cn (n = 6 and 7) result in large σ of t values compared to 9 PhC2-BQQDI-C5 due to their molecular fluctuations (Fig. 5e-f), which suggest potentially 10 compromised charge-transport capabilities. In addition, it has been reported that the ratio of 11 σ and averaged t values (σ/tAvg.) quantifies the effect of molecular fluctuations on charge 12 transport 55 , and PhC2-BQQDI-C5 shows the smallest σ/tAvg. in both t1 and t3 directions (0.30 13 and 0.40) (Fig. 5d) among current derivatives, indicating its promising OSC performances.  Supplementary Fig. 12-14). PhC2-BQQDI-C5 with the molecular mimicry demonstrates 20 excellent single-crystalline thin films (Fig. 6a) and the best OFET behavior demonstrating 21 textbook-like transfer and output characteristics with a high µe of 1.4 cm 2 V -1 s -1 and a 22 reliability factor (r) 54 of 0.86, which leads to an effective µe of 1.0 cm 2 V -1 s -1 (Fig. 6b-c). The 23 highest µe of 1.4 cm 2 V -1 s -1 can be obtained for PhC2-BQQDI-C5, and it shows an averaged 24 µe of 1.2 cm 2 V -1 s -1 over seven devices using the same fabrication method (Supplementary 25 Fig. 15), which further demonstrates the reliability of its OFET performance. PhC2-BQQDI- 26 C6 with the similar charge-transport capability and exhibits a similar µe of 1.2 cm 2 V -1 s -1 , 27 though it displays a non-ideal transfer curve with a low r of 0.28 and an effective µe of 0.33 28 cm 2 V -1 s -1 (Supplementary Fig. 16a). Similarly, PhC2-BQQDI-C7 demonstrates nonlinearity 29 in its transfer characteristic, its highest µe of 1.0 cm 2 V -1 s -1 is accompanied by a low r of 0.36 30 and an effective µe of 0.36 cm 2 V -1 s -1 (Supplementary Fig. 16b). The device performances of  (Supplementary Fig 17). crystalline thin film with channels in every 30˚ relative to the printing direction (L = ~40 µm, 12 W = ~90 µm), and the resulting azimuthal µe. 13 In light of the excellent OFET performance of PhC2-BQQDI-C5, we successfully fabricated 14 its centimeter-scale single-crystalline thin films using our recently reported continuous edge- 15 casting solution-processed method 39,40 ( Supplementary Fig. 18), the excellent processability is 16 attributed to its high crystallinity and suitable solubility, which also shows potentials for 17 applicable electronics. We examined the device performance as well as anisotropic µe of the 18 large-area single-crystalline thin film of PhC2-BQQDI-C5 (Fig. 6d). For the preliminary 19 devices of PhC2-BQQDI-C5, we constructed the OFET channel along its crystal growth 20 direction (b-axis). However, both our effective mass and anisotropic µe of the large-area single- 21 crystalline device suggest that the b-axis/printing direction is not the best charge-transport 22 direction. From the plotted azimuthal µe (Fig. 6e), it is apparent that the a-axis direction gives 1 the best charge transport, and the experimental result herein is approximately consistent with 2 our angular-dependent inversed effective mass calculations. We anticipate that with optimized 3 device engineering condition, PhC2-BQQDI-C5 has the potential to show further improved µe 4 with the appropriate OFET channel direction. 5

6
In summary, we have developed an effective and efficient synthetic strategy for asymmetric 7 PhC2-BQQDI-Cn (n = 5, 6, and 7) compounds. The intriguing molecular mimicry of alkyl 8 chains in the single-crystal structure of PhC2-BQQDI-C5 with the gauche conformations have 9 shown to be energetically favorable and persistent at elevated temperatures attributed to the 10 CH···π stabilization from the neighboring phenyl groups. Although varying the alkyl chain 11 length did not appear to impose a pronounce effect on the intralayer charge transport of PhC2-12 BQQDI-Cn, we noticed a dramatic difference in molecular fluctuations from these derivatives 13 that may have distinct consequences in their charge-transport capabilities. The alkyl chains of OSCs with the highest µe of 1.4 cm 2 V -1 s -1 , and an excellent averaged µe of 1.2 cm 2 V -1 s -1 is 20 obtained over seven devices. In addition, we demonstrate that PhC2-BQQDI-C5 possesses 21 suitable solubility and high crystalline thin-film quality for large-area device fabrication that is 22 promising for the development of future printable organic electronics. 23 25 purchased from Tokyo Chemical Industry Co., Ltd and Kanto Chemicals, respectively, and o-     and refined by applying riding model, all other atoms were refined anisotropically. 10 Crystallographic Preparation of OFET substrates. A highly n ++ -doped silicon wafer was used as the substrate, 19 which the surface was treated by a fluorinated insulating polymer, AL-X601 (40 nm). The 20 highly n ++ -doped silicon wafer with thermally grown SiO2 layer (200 nm) was ultrasonicated 21 in acetone and isopropanol, and then dried on a hotplate in air. Following UV−O3 treatment, 22 AL-X601 diluted with propylene glycol monomethyl ether acetate (PGMEA) was spin-coated 23 onto the wafer and cured at 150 ˚C for 30 min in an air. 24 Fabrication of solution-processed single-crystalline thin films. PhC2-BQQDI-Cn were 25 investigated in the bottom-gate, top-contact OFET structure. Preparations of single-crystalline 26 thin films were carried out by the solution-processed edge-casting method 53 . Thin-film crystals 27 of PhC2-BQQDI-Cn were grown from 0.02-0.03 wt% 1-methylnaphthalene solutions at 90- 28 115 ˚C. After the completion of crystallization, thin films were thoroughly dried in a vacuum 29 oven at 100 °C for 10 hours. Then, 40 nm-thick gold layers were vacuum deposited through a 30 metal shadow mask, acting as source and drain electrodes. Objective channel regions were 31 edged by the conventional Nd:YAG laser etching technique or manually by using cotton swabs. 32 Before measurements, thermal annealing at 100 ˚C for 10 hours were conducted to remove 1 residual water and improve gold electrode−semiconductor contacts.

2
Fabrication of large-area single-crystalline thin films. The single-crystalline film of PhC2- 3 BQQDI-C5 (0.02 wt% in 1-methylnaphthalene) was prepared on a glass substrate encapsulated 4 by a 55 nm-thick AL-X601 insulating layer by means of the continuous edge-casting method. 5 The stage temperature was maintained at 140 ˚C and the velocity of the moving stage was set 6 to 24 µm s -1 . dynamics, an all-atom model was employed in accordance with generalized Amber force field 20 parameters 60 . The partial atomic charges of the simulated molecules were calculated using the 21 restrained electrostatic potential (RESP) 61 methodology, based on DFT calculations with the 22 6-31G(d) basis set using the GAUSSIAN 09 program 59 . 23 For each system, the pre-equilibration run was initially performed at the given temperature for 24 5 ns after the steepest descent energy minimization. All systems were subjected to pre- 25 equilibration runs in the NTV ensemble before their equilibration runs. During the pre- 26 equilibration runs for the NTV ensemble, the Berendsen thermostat 62 was used to maintain the 27 temperature of the system with relaxation time of 0.2 ps and the volume of the MD cell was 28 kept constant. Subsequently, for the NTP ensemble the equilibration run was performed using 29 the Nosé-Hoover thermostat 63-65 and Parrinello-Rahman barostat 66 with relaxation times of 1.0 30 and 5.0 ps, respectively. For all MD simulations in the NTP ensemble, the pressure of the 31 system was kept at 1.0 bar. The smooth particle-mesh Ewald (PME) 67 method was employed 1 to treat the long-rang electrostatic interactions and the real space cutoff and the grid spacing 2 are 1.2 and 0.30 nm, respectively. The time step was set to 1 fs. 3 To compare temperature dependence of thermal atomic fluctuations between different 4 molecules, we calculated the B-factors related to the thermal stability as expressed below: where ∆ is the root mean square fluctuations (RMSF) of atom . The RMSF values can be 7 estimated by using following equation: where is the time step, � � is the position coordinate of atom , and � is the average of 10 � �. The RMSF values were analyzed from MD trajectories during the last 10 ns in the 11 equilibrium.
12 Data availability 13 The data reported in this study are available from the corresponding author (Toshihiro