Enhanced Layered-Herringbone Packing due to ... - ACS Publications

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Enhanced Layered-Herringbone Packing due to Long Alkyl Chain Substitution in Solution-Processable Organic Semiconductors Hiromi Minemawari,*,† Mutsuo Tanaka,† Seiji Tsuzuki,† Satoru Inoue,†,∥ Toshikazu Yamada,† Reiji Kumai,‡ Yukihiro Shimoi,† and Tatsuo Hasegawa*,†,§ †

National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba 305-8565, Japan Condensed Matter Research Center (CMRC) and Photon Factory, Institute of Materials Structure Science, High Energy Accelerator Research Organization (KEK), Tsukuba 305-0801, Japan § Department of Applied Physics, The University of Tokyo, Tokyo 113-8656, Japan ‡

S Supporting Information *

ABSTRACT: Herein, we report the stabilization and modulation of layered-herringbone (LHB) packing, which is known to afford high-performance organic thin-film transistors, based on crystal structure analyses and calculations of intermolecular interaction energies for alkyl-substituted organic semiconductor (OSC) crystals. We systematically investigated the alkyl chain-length dependence of the crystal structures, solvent solubilities, and thermal characteristics for three series of symmetrically and asymmetrically alkyl-substituted benzothieno[3,2-b][1]benzothiophenes (BTBTs). All the series exhibit LHB packing when the BTBTs are substituted with relatively long alkyl chains (−CnH2n+1), i.e., n ≥ 4 for monoalkylated, n ≥ 6 for dialkylated, and n ≥ 5 for phenyl-alkylated BTBTs. LHB packing is also evident in the nonsubstituted and diethyl-substituted BTBTs, although those substituted with short alkyl chains generally did not feature LHB packing because of their lack of interchain ordering. The density functional theory calculations of the intermolecular interactions revealed that the BTBT cores inherently generate LHB packing, and the stability is increasingly enhanced by the alignment of longer alkyl chains. It was also found that the LHB packing is stabilized by keeping the size ratios of the total intermolecular attractive forces between the T-shaped and slipped parallel contacts at about 3:2 for all the LHB compounds, despite the slight structural modifications generated by the substituents. We discuss the effects of alkyl substitutions to modulate the LHB packing of the BTBT cores and thus the two-dimensional carrier transport in layered OSC crystals.

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layers with flat semiconductor−insulator interfaces; thus, it facilitates efficient two-dimensional carrier transport to generate high-performance organic TFTs. In fact, most of the representative OSC cores, such as pentacene,10,11 rubrene,12,13 dinaphtho[2,3-b:2′,3′-f ]thieno[3,2-b]thiophene (DNTT),14,15 and benzothieno[3,2-b][1]benzothiophene (BTBT),16−19 or others20,21 are known to generate LHB packing. As for the development of OSCs, recent attention has primarily focused on a class of solution-processable OSCs with the aim of the inexpensive production of electronic products using print production (i.e., printed electronics) technologies.22,23 In fact, it has been reported that some solutionprocessed OSCs exhibit LHB packing and thus generate the highest-performance organic TFTs.24,25 As a subsequent step toward applications in printed electronics, it is necessary to develop OSCs with both sufficient solvent solubility and

INTRODUCTION

The design and control of molecular packing motifs in organic semiconductor (OSC) crystals is critical to realizing efficient carrier transport and, consequently, high device performance in organic electronic devices.1−3 This is related to electronic band formation in semiconducting organic molecular solids, which is predominantly determined by close intermolecular π−π interactions as well as nearly perfect alignment of the component molecules to form crystals.4,5 Among the variety of packing motifs, layered-herringbone (LHB) packing is unique and is known to be the most suitable for obtaining high-performance organic thin-film transistors (TFTs).6,7 LHB packing is characterized by (or defined as) the formation of isolable molecular layers composed of two types of intermolecular contacts within the respective layers, i.e., Tshaped (core-to-edge) and slipped parallel (core-to-core) contacts between the neighboring planar π-electron skeletons that are arranged in glide and translational symmetry, respectively.8,9 The highly layered crystallinity characteristic of LHB packing is optimal for the production of uniform channel © XXXX American Chemical Society

Received: October 29, 2016 Revised: January 4, 2017

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DOI: 10.1021/acs.chemmater.6b04628 Chem. Mater. XXXX, XXX, XXX−XXX

Chemistry of Materials

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thermal stability (although these features are often mutually exclusive) in addition to excellent carrier transport characteristics. Accordingly, the application of a wide variety of chemical substitutions to the π-electron skeletons is intriguing in order to optimize the characteristics of the materials. To date, the substitution effects have been studied for a variety of OSC cores, but individually.16−19,26−43 Recently, the alkyl chain-length dependence of the packing motif was systematically studied for a series of phenyl-alkylated BTBT compounds with asymmetric phenyl and normal alkyl substitutions.41,44,45 The compounds exhibit an isomorphous LHB packing motif with long alkyl chain substituents (n ≥ 5). The respective layers are composed of unipolar orientations of the component asymmetric molecules, and the obtained unipolar layers form an alternating antiparallel alignment such that the alkyl chains (and phenyl rings) are in tail-to-tail (headto-head) contact. This type of packing motif is referred to as bilayer-type LHB and is unique to asymmetric OSC molecules. In contrast, the Ph-BTBT-Cn compounds with short alkyl chains (n ≤ 4) do not exhibit bilayer-type LHB, but instead feature quasiherringbone packing of molecules with alternating long-axis orientations within the respective layers. However, the effects of the substituents on the appearance, stability, and modulation of the LHB packing remain unclear. Further systematic investigation of the chemical substitution effects is necessary to tune and optimize the solution-processable OSCs to generate both excellent carrier transport and material characteristics. In this study, we investigated the stabilization and modulation of LHB packing in three series of alkylsubstituted BTBTs: asymmetric monoalkylated BTBT (monoCn-BTBT), symmetric dialkylated BTBT (di-Cn-BTBT), and asymmetric phenyl-alkylated BTBT (Ph-BTBT-Cn), as shown in Figure 1. The dependence of the crystal structures, solvent

Article

EXPERIMENTAL SECTION

Materials. The preparation of the materials used in this study is summarized in the Supporting Information. Structural Characterizations. Single crystals for structural analysis were obtained via recrystallization from toluene or 1,2dichlorobenzene at room temperature and carefully collected from the solution using mesh-type LithoLoops (400 μm diameter).46 The X-ray diffraction data were collected using a Rigaku VariMax with RAPID using Mo Kα radiation for mono-Cn-BTBT (n = 3, 4) and di-Cn-BTBT (n = 2−5) and Cu Kα radiation for mono-C9-BTBT from a graphite monochromator equipped with a Rigaku cylindrical imaging plate diffractometer at 300 K. For mono-C2-BTBT, the measurement was performed using synchrotron radiation (λ = 0.6893 Å)47 at 300 K on the BL-8B beamline at the Photon Factory (PF), High Energy Accelerator Research Organization (KEK). All of the calculations were performed using the Rapid-AUTO program and CrystalStructure crystallographic software packages.48 The structures were solved by direct methods.49,50 The final refinements of the non-hydrogen atoms were performed with anisotropic thermal factors. The final cycle of full-matrix least-squares refinements on F2 were performed using SHELXL97.51 The crystallographic data are summarized in Tables S1 and S2 (Supporting Information). For mono-Cn-BTBT (n = 4−16), the powder X-ray diffraction measurements were also conducted using synchrotron radiation (λ = 1.553 Å)52 on the BL-8B beamline at PFKEK. The data were collected using a Rigaku DSC imaging plate system at 300 K (Supporting Information). The data were deposited at Cambridge Crystallographic Data Center as supplementary publications CCDC 1525673−1525680. Solubility. The solubilities of BTBT, mono-Cn-BTBT (n = 1−16), and di-Cn-BTBT (n = 1−10) were evaluated by adding chloroform or 1,2-dichlorobenzene to a powdered sample (∼10 mg) in increments of 10 μL and stirring the mixture to dissolve the solid sample completely at 25 °C. The solubility (mmol/L) was calculated from the total amount of solvent and the sample weight. Thermal Properties. The samples were thermally analyzed by thermogravimetric-differential thermal analysis (TG-DTA; TG/DTA 220, Seiko instruments Inc.) and differential scanning calorimetry (DSC; DSC7000X, Hitachi High-Tech Science Corporation). For the DSC measurements, the powdered sample was heated and subsequently cooled at rates of 5 K/min (first and second scan) and 10 K/min (third scan). The obtained DSC curves are shown in Figures S3 and S5, and the determined phase transition temperatures are summarized in Tables S4 and S5. Details of some broad endothermic/ exothermic peaks observed at around 50−90 °C in the DSC curves for mono-Cn-BTBT (n = 8, 9, 10, and 11) were also provided in Figure S3. Theoretical Calculations. Ab initio molecular orbital and DFT calculations of the intermolecular interactions were performed with the Gaussian 09 software package using the crystal geometries.53 The total intermolecular interaction energy (Eint) was calculated at the B97D/6311G** level. The Hartree−Fock level interaction energy (EHF) was obtained using the 6-311G** basis set. The basis set superposition error (BSSE)54 was corrected using the counterpoise procedure.55 The BTBT−BTBT, BTBT−alkyl chain, and alkyl chain−alkyl chain interactions in the mono-Cn-BTBT, di-Cn-BTBT, and Ph-BTBT-Cn crystals were calculated using the structures of the fragments in the crystals. The dangling bonds of fragments were capped by hydrogen atoms in the calculations. We also decomposed the intermolecular interaction energy from the viewpoint of its origin as follows: Electrostatic energy (Ees) was calculated as the interactions between distributed multipoles56,57 of interacting molecules using the ORIENT version 3.2.58 Distributed multipoles up to hexadecapole on all atoms were obtained from B97D/6-311G** level wave functions of isolated molecules using the GDMA program.59 Induction energy (Eind) was calculated as the interactions of polarizable sites with the electric field produced by the distributed multipoles of monomers.60 The atomic polarizabilities of carbon (α = 10 au, 1 au =1.648777 × 10−41 J−1 C2 m2) and sulfur (α = 20 au) were used for the calculations.61 The EHF is approximately the sum of the electrostatic, induction, and short-range (orbital−orbital)

Figure 1. Chemical structures of three series of alkylated BTBTs.

solubilities, and thermal characteristics on the alkyl chain length was investigated for these compounds. On the basis of the results, we discuss how LHB (or bilayer-type LHB) packing is affected by the substituted alkyl chain lengths. We also reveal the close relationship between the structural variation and solubility and thermal characteristics in these materials. Then, we investigate the structural characteristics that stabilize the LHB (or bilayer-type LHB) packing via ab initio molecular orbital and DFT calculations of the intermolecular interactions. On the basis of analyses of the intermolecular interactions between the neighboring T-shaped and slipped parallel contacts, we discuss the origins of slight structural modifications caused by the substitutions and their effects on the two-dimensional carrier transport in layered OSC crystals. B


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Figure 2. Crystal packing structures of BTBT and mono-Cn-BTBTs. interaction energy (Eshort). Accordingly, the Eshort was calculated according to the equation Eshort = EHF − Ees − Eind. The electron correlation contribution (Ecorr) was calculated according to the equation Ecorr = Eint − EHF. The contribution of the dispersion interaction is discussed on the basis of the size of Ecorr, since the electron correlation contribution to the interaction energy (Ecorr) is mainly dispersion energy. The electrostatic potential on the surface of molecule and atomic charges were calculated using the B97D/6311G** wave functions. The atomic charges were calculated by electrostatic potential fitting using the Merz−Singh−Kollman scheme.62,63 The intermolecular transfer integrals between the HOMOs of adjacent molecules in the crystal were calculated using the Amsterdam Density Functional (ADF) program64 with the PW91/TZP level65 based on the crystal geometries. The calculations for BTBT, di-CnBTBT, and Ph-BTBT-Cn were performed using the structural data provided in CCDC-975935;18 CCDC-677772, -679293, -679294;17 CCDC-1017499;44 and CCDC-1400438−140044245 deposited at the Cambridge Crystallographic Data Center.

packing of mono-C2-BTBT, the orientations of the molecular long axis are alternately aligned between all the pairs of neighboring T-shaped contacts. Thus, independent alkyl chain layers do not form. A one-dimensional ladder-like packing motif is formed along the crystallographic b axis by intermolecular S···S and S···C contacts that are shorter than the sum of the van der Waals radii.66 A change in the alkyl chain length from n = 2 to n = 3 also causes a drastic change in the packing motif. Mono-C3-BTBT crystallizes into a noncentrosymmetric structure with Cc symmetry, and thus all the molecules are oriented in a unipolar mode to generate polar crystals. In the crystal, molecules do not form layers but are shifted along the molecular long axis by half of the molecular length. The neighboring molecules at different depths along the molecular long axis are strongly connected to each other in a face-to-edge fashion to form one-dimensional molecular chains along the a axis. These results indicate that substitutions with short alkyl chains disrupt the layer-by-layer or LHB packing; this is most likely because interchain ordering becomes unfavorable. It should also be pointed out that although the crystal structures are not layer-by-layer in mono-C2-BTBT and mono-C3-BTBT, the molecular packing motifs are composed of T-shaped (coreto-edge) and slipped parallel (core-to-core) contacts between the neighboring planar π-electron skeletons. This implies that herringbone packing is a universal property of organic materials with π-electron skeletons. Mono-C4-BTBT and mono-C9-BTBT crystallize into monoclinic (P21/a) and triclinic (P1̅) structures, respectively;

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RESULTS AND DISCUSSION Crystal Structure Analyses. Full structural analyses were successfully conducted for the mono-Cn-BTBT compounds with n = 2, 3, 4, and 9 and di-Cn-BTBT compounds with n = 2, 3, 4, and 5. All the crystallographic data for these compounds are provided in Tables S1 and S2. The unit cell molecular packing structures for the mono-CnBTBTs and unsubstituted BTBT (n = 0) are presented in Figure 2. The unsubstituted BTBT exhibits LHB packing, while substitution with short alkyl chains (n = 2 and 3) affords crystals that do not show LHB packing. In the molecular C


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Chemistry of Materials however, both compounds exhibit very similar bilayer-type LHB packing.41,44,45 The formation of bilayer-type LHB was also revealed in mono-C8-BTBT using powder XRD analyses.39,67 Independent alkyl chain layers form in these crystals. The intermolecular short S···S and S···C contacts were densely formed between adjacent molecules within the herringbone layers for both mono-C4-BTBT and mono-C9-BTBT. Full structural analyses of single crystals were not successful for the other compounds because of the very highly layered crystals and resultant thin flake-like nature of the obtained crystals. In order to establish the alkyl chain-length dependence of the crystal structures, powder X-ray diffraction experiments were conducted for all the mono-Cn-BTBTs with long alkyl chains (n = 4−16). The d-spacing values were estimated from the reflections that correspond to the (00l) reflections (details are provided in the Supporting Information). The results are shown in Figure 3. The obtained d-spacing values systematically

Figure 4. Photographs of typical crystals of mono-Cn-BTBT obtained by recrystallization from solution from 1,2-dichlorobenzene. Needleshaped crystals were obtained for mono-C2- and mono-C3-BTBT, isolated rhombic crystals for mono-C4-BTBT, and two-dimensional films composed of hexagonal thin layers for longer n.

lengths larger than n, at which the long-axis length of BTBT (or Ph-BTBT) and substituted alkyl chain lengths become comparable: The propyl (n = 4) and pentyl (n = 5) groups are almost the same length as BTBT and Ph-BTBT, respectively. In Figure 5, we show the crystal structures of the di-CnBTBTs with n = 2, 3, 4, 5, and 8. The LHB packing is evident when n = 2; however, the respective ethyl groups do not sit adjacent to each other, but rather contact the neighboring benzene rings in the crystal, which differs from the LHB packing seen in mono-Cn-BTBTs with long alkyl chains. Another isomorphous packing motif is formed in the di-CnBTBT crystals with n = 3, 4, and 5: The π-electron skeletons stack on each other to generate a π-stacking motif in which the slipped parallel contacts dominate and the T-shaped contacts do not form. Materials Characteristics by Alkyl Substitutions. Figure 6 presents the solubility and thermal properties of the mono-CnBTBT, di-Cn-BTBT, and Ph-BTBT-Cn compounds plotted as a function of the alkyl chain lengths. The trends in the solubilities of all three series of compounds with increasing chain length are similar: The solubility rapidly increases with increasing n from 0 to 2, and then gradually decreases with further increasing alkyl chain length. These characteristics can be understood in terms of the structural variation and stabilization by the substituted alkyl chains. The compounds with n ≤ 3 do not form isolated alkyl chain layers resulting in the increased solubility with increasing n up to 2. It is most likely that the solubility in this range is enhanced by the introduction of alkyl chains that have different affinities to solvents than the πelectron skeletons, as is usually seen for organic compounds.43 In contrast, the solubility decreases with increasing n in the compounds that show isomorphous LHB packing. As we already discussed for the Ph-BTBT-Cn compounds,45 these results imply that the solubility is fundamentally determined by the total cohesive energy of the crystals, where the increase of the alkyl chain length leads to an increase of the cohesive energy, as is associated with the formation of independent alkyl chain layers. However, it is not clear why the solubility of di-Cn-

Figure 3. Alkyl chain-length dependences of the d-spacings calculated for the (00l) refractions in the powder XRD patterns of mono-CnBTBT (red ●). The corresponding values derived from the unit cell parameters of single crystals are also plotted (black ◆, mono-CnBTBT; blue ▲, di-Cn-BTBT; yellow ■, Ph-BTBT-Cn; light blue ◆, BTBT).

increase with increasing alkyl chain length. The values also roughly correspond to the nominal length of two mono-CnBTBT molecules. While the diffractions in the low-angle region were observed as clear single peaks for n = 4−15, those for n = 16 appeared as double peaks, indicating the coexistence of different polymorphic forms (Figure S2). According to these results, it is most likely that all the compounds with long alkyl chains (n ≥ 4) form bilayer-type LHB. We have investigated several conditions of recrystallization, including the use of various organic solvents, for the single crystal growth of mono-Cn-BTBTs and found that the compounds with n ≥ 4 exhibit a common trend of crystal habit to afford fairly thin flake crystals as presented in Figure 4. In contrast, needle-shaped crystals were obtained for mono-C2BTBT and mono-C3-BTBT, which reflects the anisotropic intermolecular interactions in the crystals (the crystals grew along the direction of the largest interactions that occurred in the crystal; along the b axis in mono-C2-BTBT and along the a axis in mono-C3-BTBT). These observations suggest that the molecular packing motif significantly influences the crystal habits in the case of mono-Cn-BTBT. These results are very similar to those obtained for PhBTBT-Cn (n ≥ 5).45 We suggest that the formation of the bilayer-type LHB is common for asymmetric OSC molecules with long alkyl chains.68 We also point out that the formation of the bilayer-type LHB seems to take place with alkyl chain D


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Figure 5. Crystal packing structures of di-Cn-BTBTs. Hydrogen atoms are omitted for clarity for n = 3, 4, and 5.

contrast, the mono-Cn-BTBTs do not exhibit liquid crystal phases (the DSC curves are shown in Figure S3).38,39 The observed gradual decrease in the melting point with increasing alkyl chain length of the substituents was attributed to the contribution of the thermally induced molecular dynamic motions of the alkyl chains within the layer-by-layer crystal packing, which became more pronounced for longer alkyl chains at high temperatures. All the results demonstrate that the structural features govern the solubilities and thermal characteristics of the three series of compounds. This is associated with the cohesive forces between the alkyl chains, which increase with increasing n.45 The results also indicate that the alkyl chains play crucial roles in the formation of layer-by-layer and LHB packing motifs for the formation of isolated alkyl chain layers in all the alkylated BTBTs. Intermolecular Interactions in the LHB Packing. To theoretically investigate the cohesive energies of the crystals, we evaluated the intermolecular interaction energies between neighboring molecules in the three series of crystals based on DFT calculations. We used the B97D functional, which is known to afford reasonable estimations for interaction energies of aromatic hydrocarbons and heterocycles.71 Figure 7 summarizes the results for the compounds showing LHB packing including the intermolecular interaction energies at the six nearest neighbor contacts of the T-shaped (e2, e3, e5, and e6) and slipped parallel (e1 and e4) configurations within the herringbone layers. We found that the interaction energy systematically increases with increasing n, while maintaining the size ratios between the T-shaped and slipped parallel contacts at about 3:2 for all the LHB compounds despite the slight structural variations. The enhancement of the intermolecular interaction energies with increasing n was ascribed to contributions from the attractive forces between the alkyl chains as shown in Figure 7c. This means that the size ratio of interchain interaction energies between the T-shaped and slipped parallel contacts should remain at 3:2, irrespective of the alkyl chain length. Note that the alignment between the

Figure 6. Solubilities at 25 °C (left) and thermal characteristics (right) of mono-Cn-BTBTs (top), di-Cn-BTBTs (middle), and Ph-BTBT-Cn (bottom) compounds: yellow ■, point of 5% weight loss for TGDTA; red ●, melting or clearing point; green ▲, crystal to liquid crystal phase transition; and blue ▲, liquid crystal to liquid crystal phase transition. Open circles and triangles in the plots for di-CnBTBTs represent the literature data.16

BTBT sharply decreases from n = 9 to n = 10. These results afford clear evidence of the fastener (or zipper) effect.69,70 The effects of n on the thermal characteristics of the monoCn-BTBTs and di-Cn-BTBTs are similar: The melting temperature sharply decreases from n = 0 to n = 3 and then gradually decreases with further increasing alkyl chain length. In contrast, the melting temperature of Ph-BTBT-Cn reaches a maximum at n = 2. Note that the di-Cn-BTBTs exhibit liquid crystal phases when n ≥ 6 at around 100 °C,16 and the Ph-BTBT-Cn compounds exhibit liquid crystal phases when n ≥ 5 at around 150 °C41,45 at which the compounds form LHB packing. In E


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Figure 7. (a) Schematic view of LHB packing motif; (b) calculated intermolecular interaction energies at the T-shaped (e2, e3, e5, and e6) and slipped parallel (e1 and e4) contacts in the LHB packing; (c) contributions from BTBT−BTBT, BTBT−alkyl chain, and alkyl chain−alkyl chain interactions in mono-Cn-BTBT; and (d) alignment between the alkyl chains within the layer.

interactions in determining the molecular arrangement of BTBT and substituted BTBT molecules in the crystals. The electrostatic potentials calculated for BTBT and Ph-BTBT-C3 are shown in Figure 8. The blue region has positive charge, and

alkyl chains is similar to that between the BTBT cores; the molecular planes formed by the trans-alkyl chains are also arranged according to the T-shaped and slipped parallel contacts, as presented in Figure 7d. It is most likely that the size ratio should remain the same to stabilize the LHB packing structures. It is most likely that the size ratio should remain the same to stabilize the LHB packing structures. This size ratio is reasonable considering the number ratio between the component T-shaped and slipped parallel contacts (i.e., 2:1) in the LHB packing motifs. The LHB packing of the BTBT cores was modified by the effect of alkyl substitution. For example, the herringbone angle at the T-shaped contacts becomes narrower with increasing n, as presented in Figure S10. In addition, both the interplanar distance at the slipped parallel contacts and the intermolecular distance between the second nearest neighbor molecules also decrease with increasing n. These structural variations lead to the slight decrease in the volume of LHB packing of the BTBT cores with increasing n, as shown in Figure S11; this means that the BTBT cores are packed more tightly within the herringbone layers because of the long alkyl chain substitutions, which clearly demonstrates the fastener effect.69,70 We point out that the intermolecular interaction energy between the BTBT cores remains almost constant in the size and size ratios between the T-shaped and slipped parallel contacts, despite the modulation of molecular arrangement within the layer, as shown in Figure 7c. The results indicate that the LHB packing of the BTBT cores remains stable despite slight structural modifications that are caused by the alkyl-chain substitutions. Origin of the Intermolecular Attractive Forces. The charge distributions and electrostatic potentials of BTBT and Ph-BTBT-C3 were calculated to discuss the role of electrostatic

Figure 8. Electrostatic potentials calculated for BTBT and phenylpropylated BTBT (Ph-BTBT-C3) from B97D/6-311G** wave functions.

the red region has negative charge. The electrostatic potentials calculated for the two molecules show that the hydrogen atoms of benzene and thiophene rings (BTBT cores) have positive charge and the inner part of rings have negative charge. The charges on sulfur atoms and propyl group of Ph-BTBT-C3 are small. The positive charges on hydrogen atoms of BTBT and the BTBT core of Ph-BTBT-C3 are 0.09−0.24 e (e = 1.60 × 10−19 C). The charges on sulfur atoms are −0.05 to −0.06 e, respectively. Owing to the substantial positive charges, the hydrogen atoms of BTBT cores do not prefer to have contact. No hydrogen atoms of BTBT cores have close contact in the LHB packing crystals. Apparently, the repulsive electrostatic F


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The hole transport properties of BTBT and substituted BTBT crystals suggest the existence of the orbital−orbital interactions between neighboring molecules in the crystals. The orbital−orbital interactions were sometimes claimed as the dominant source of the attraction in the crystals owing to the existence of the orbital−orbital interactions. However, the analysis of contribution of each energy term to the interaction energy clearly shows that the dispersion interactions are the primary source of the attraction and the total orbital−orbital interactions are repulsive. The existence of the orbital−orbital interactions does not necessarily indicate that the orbital− orbital interactions are the main cause of the attraction. The analysis of the intermolecular interactions shows that the orbital−orbital interactions in the crystal are the result of the short contact of the neighboring molecules due to strong dispersion interactions. Vairation of the Intermolecular Transfer Integral. To investigate the hole transport properties within the LHB crystals, the transfer integrals between the HOMOs of adjacent molecules were calculated using the Amsterdam density functional (ADF) program package.64 Figure 9 summarizes

interactions between the hydrogen atoms should play certain (but not dominant as described below) roles in the formation of the LHB packing. The contribution of each intermolecular force to the interaction energy between slipped parallel (e1) and T-shaped (e2) neighboring BTBT molecules in the crystal was calculated to investigate the origin of intermolecular attractive forces. The results are summarized in Table 1. Table 1. Electrostatic, Induction, Short-Range (Orbital− Orbital), and Dispersion Interactions between Neighboring BTBT Moleculesa

slipped parallel (e1) T-shaped (e2)

Eintb

Eesc

Eindd

Eshorte

Ecorrf

−5.14 −7.40

−0.80 −0.32

−0.17 −0.26

3.65 4.42

−7.82 −11.24

a

Energy in kcal/mol. The geometries of BTBT dimers are shown in Figures 2 and 7. bTotal interaction energy calculated at the B97D/6311G** level. cElectrostatic energy. dInduction energy. eShort-range (orbital−orbital) interaction energy (=EHF − Ees − Eind). HF/6311G** level interaction energy was used as the EHF. Eshort is mainly the sum of the exchange-repulsion and charge-transfer energy. f Contribution of electron correlation on the interaction energy (=Eint − EHF). Ecorr is mainly dispersion energy. See text.

The Ecorr calculated for e1 and e2 are −7.82 and −11.24 kcal/ mol, respectively. The large negative Ecorr values imply that the dispersion interactions are the dominant attractive force between the neighboring BTBT molecules in the crystal. The electrostatic energies (Ees) for e1 and e2 are −0.80 and −0.32 kcal/mol, respectively. They are as small as 10% and 3% of the Ecorr. The contributions of the induction interactions to the attraction are negligible. The induction energies (Eind) for e1 and e2 are −0.17 and −0.26 kcal/mol, respectively. The shortrange (orbital−orbital) interaction (Eshort) is mainly the exchange repulsion and charge-transfer interactions. The total short-range (orbital−orbital) interaction energies (Eshort) calculated for e1 and e2 (3.65 and 4.42 kcal/mol, respectively) are repulsive. It would be useful to mention that similar results were reported for the intermolecular interactions of other aromatic molecules. The interaction energy calculations for the benzene, naphthalene, and thiophene dimers8,72,73 and the neighboring oligothienoacenes in the crystals74 show that the dispersion interactions are the major source of the attraction. The analysis of intermolecular interactions as presented above affords clear evidence that the dispersion interactions are the primary source of the attraction between neighboring BTBT derivatives in the crystals, as the dispersion interactions are also the major source of the attractive interactions of substituents (alkyl and phenyl groups). The electrostatic interactions between neutral molecules are highly orientation dependent in general. Nonetheless, the dispersion interactions, which are the dominant attractive interactions in the BTBT crystals, will be the primary intermolecular force in determining the molecular arrangement in the crystals.

Figure 9. Calculated transfer integrals at the T-shaped (t2, t3, t5, and t6) and slipped parallel (t1 and t4) contacts in the LHB packing motifs of mono-Cn-BTBT, di-Cn-BTBT, and Ph-BTBT-Cn.

the results for the six nearest neighbor contacts with the Tshaped (t2, t3, t5, and t6) and slipped parallel (t1 and t4) configurations. The estimated transfer integrals exhibit large anisotropy between the T-shaped and slipped parallel contacts. This feature is naturally significantly different from that of the anisotropy of the intermolecular interaction energies: The transfer integral at the slipped parallel contacts is about three times larger than that at the T-shaped contacts in unsubstituted BTBT. This anisotropy increases upon substitution with relatively short alkyl chains (n = 4 for mono-Cn-BTBT and n = 2 and 8 for di-Cn-BTBT). In contrast, the transfer integral at the T-shaped contacts increases upon substitutions with longer alkyl chains, which results in the decreasing anisotropy in the transfer integral with increasing n. This variation should be due to the tighter packing of the BTBT cores with longer alkyl chain substitution, as discussed above. The results indicate that the molecular fastener effect should facilitate two-dimensional charge transport in the alkylated BTBTs. We conclude that the substituted alkyl chain lengths for the BTBT cores are critical to achieving highperformance organic TFTs, as well as improving the solubility and thermal characteristics. G



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CONCLUSIONS We investigated the stabilization and modulation of LHB packing in alkyl-substituted BTBTs. We found that substitutions with long alkyl chains effectively stabilize LHB packing for three series of monoalkylated, dialkylated, and phenyl-alkylated BTBTs, whereas substitutions with short alkyl chains impede the LHB packing. The solubilities, thermal characteristics, and charge transport properties of the alkylated BTBTs correlate with the variation of the molecular packing motifs. These results are supported by the DFT calculations for intermolecular interactions. These findings contribute to the development of high-performance solution-processable OSCs that show sufficient solvent solubility and thermal stability in addition to excellent carrier transport characteristics for future applications in flexible and printed electronics.

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ASSOCIATED CONTENT

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.6b04628. Synthesis of materials, summary of single crystal and powder XRD analyses, DSC profiles, calculated intermolecular interaction energies and transfer integrals, and modulation of herringbone packing structures (PDF) Crystal data for mono-C2-BTBT (CIF) Crystal data for mono-C3-BTBT (CIF) Crystal data for mono-C4-BTBT (CIF) Crystal data for mono-C9-BTBT (CIF) Crystal data for di-C2-BTBT (CIF) Crystal data for di-C3-BTBT (CIF) Crystal data for di-C4-BTBT (CIF) Crystal data for di-C5-BTBT (CIF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Hiromi Minemawari: 0000-0003-3157-7905 Present Address ∥

REFERENCES

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S Supporting Information *

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Article

Nippon Kayaku Co. Ltd., Tokyo 115-8588, Japan

Notes

The authors declare no competing financial interest. The corresponding Crystallographic Information Files (CIFs) were deposited at the Cambridge Crystallographic Data Center.

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ACKNOWLEDGMENTS We are grateful to Nippon Kayaku Co., Ltd., for providing BTBT. We thank Prof. Jun-ichi Hanna and Dr. Hiroaki Iino (Tokyo Inst. Tech.) for valuable discussions; Prof. Tomoyuki Akutagawa, Dr. Norihisa Hoshino (Tohoku University), and Dr. Kensuke Kobayashi (KEK) for their help in X-ray measurements; and Ms. Yuko Hanawa (AIST) for assistance for solubility and thermal property measurements. The synchrotron X-ray experiment was performed with the approval of the Photon Factory Program Advisory Committee (2014S2001). This work was partly supported by JSPS KAKENHI Grant 26246014. H


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