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Toshiki Higashino, Satoru Inoue, [Shunto Arai](https://orcid.org/0000-0002-0055-3006), Seiji Tsuzuki, Hiroyuki Matsui, Reiji Kumai, Kiyofumi Takaba, Saori Maki-Yonekura, Hirofumi Kurokawa, Ichiro Inoue, Kensuke Tono, Koji Yonekura, Tatsuo Hasegawa

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This document is the Accepted Manuscript version of a Published Work that appeared in final form in Chemistry of Materials, copyright © 2024 American Chemical Society after peer review and technical editing by the publisher. To access the final edited and published work see https://doi.org/10.1021/acs.chemmater.3c02500[In Copyright](http://rightsstatements.org/vocab/InC/1.0/)

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[Effects of Thiophene-Fused Isomer on High-Layered Crystallinity in π-Extended and Alkylated Organic Semiconductors](https://mdr.nims.go.jp/datasets/4dd9d6a2-4cee-4a63-831a-b2ba7a6e5726)

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Effects of thiophene-fused isomer on high layered crystallinity in π-extended and alkylated organic semiconductors Toshiki Higashino,*a Satoru Inoue,b Shunto Arai,b,c Seiji Tsuzuki,b Hiroyuki Matsui,d Reiji Kumai,e Kiyofumi Takaba,f Saori Maki-Yonekura,f Hirofumi Kurokawa,g Ichiro Inoue,f Kensuke Tono,f,h Koji Yonekura,f,g and Tatsuo Hasegawab a Research Institute for Advanced Electronics and Photonics, National Institute of Advanced Industrial Science and Tech-nology (AIST), Tsukuba, Ibaraki 305-8565, Japan b Department of Applied Physics, The University of Tokyo, Hongo, Tokyo 113-8656, Japan c Research Center for Macromolecules and Biomaterials, National Institute for Materials Science (NIMS), Tsukuba, Ibaraki 305-0044, Japan d Research Center for Organic Electronics, Yamagata University, Yonezawa, Yamagata 992-8510, Japan e Condensed Matter Research Center (CMRC) and Photon Factory, Institute of Materials Structure Science, High Energy Accelerator Research Organization (KEK), Tsukuba, Ibaraki 305-0801, Japan f RIKEN SPring-8 Center, Kouto, Sayo, Hyogo 679-5148, Japan g Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Katahira, Aoba-ku, Sendai 980-8577, Japan h Japan Synchrotron Radiation Research Institute, 1-1-1 Kouto, Sayo, Hyogo, 679-5198, Japan  ABSTRACT: Here we systematically investigated the effects of thiophene-fused isomer and the end-cap substitution on high lay-ered crystallinity, film formability, and field-effect transistor characteristics in π-extended and alkylated organic semiconductors (OSCs). We developed four kinds of unsymmetric rod-like OSCs based on syn-/anti-isomers of ben-zothieno[6,5-b]-/benzothieno[5,6-b]-benzothieno[3,2-b]thiophene (BTBTT) with phenyl/alkyl substitutions with different alkyl chain lengths; synCn and antiCn (n = 6, 10). The layered molecular packing motifs of the compounds are distinct by the thiophene orientation of isomeric π-cores but are unaffected by the alkyl chain length. The synCn forms a bilayer-type layered herringbone (b-LHB) packing composed of head-to-head arrangement of unidirectionally-aligned molecular layers with showing high layered crystallinity and high carrier mobility over 10 cm2 V−1 s−1. By contrast, the antiCn forms antiparallel alkyl-interdigitated herring-bone (aai-HB) structure in which the respective π-core layer is composed of alternating antiparallel alignment of π-cores and the alkyl chains are interdigitated with each other between the adjacent π-core layers. The latter shows relatively poor crystalline-film formability and moderate carrier mobility. Dispersion-corrected density functional theory calculations of intermolecular interaction energy reveal that the overall shape of the rigid π-core components is crucial for achieving unidirectionally-aligned and close-ly-packed 2D π-core layers, and that the flexible end-cap substituents strengthen and balance the layered crystallinity. The findings will be crucial for designing and developing the highly layered crystalline and high-performance OSCs.INTRODUCTION Fused-ring π-electron skeletons (or π-cores) based on thien-oacenes constitute the fundamental material bases of organic semiconductors (OSCs) towards versatile electronics applica-tions.1-4 Among them, linearly extended π-cores, fused in acene-/phenacene-like manners, frequently form self-organized two-dimensional (2D) layered molecular pack-ing that is optimal for efficient 2D carrier transport in organic field-effect transistors (OFETs).5-7 Some OSCs based on linear π-cores exhibit superior film-forming ability (denoted as lay-ered crystallinity) in solution and thus afford high-performance printed OFETs.8-21 However, many similar molecules do not show the high layered crystallinity, or do not afford excellent device characteristics even if the layered as-sembly is achieved.22-29 It is quite important to establish how to design and develop the OSC molecules showing efficient carrier transport as well as high layered crystallinity in the OSC molecules.  The carrier transport characteristics are primarily deter-mined by the packing motifs of π-core components, which could be tuned by the modulation of π-cores and/or side-chain components in the OSC molecules. Recently, it was shown   that the layered crystallinity is considerably enhanced by end-cap substitutions with long alkyl chains and/or phenyl groups, as is demonstrated for some linear π-cores, such as benzothieno[3,2-b][1]benzothiophene (BTBT).30-33 The un-symmetrically substituted molecules frequently form bi-layer-type layered herringbone (b-LHB) packing, composed of head-to-head arrangement of unidirectionally-aligned mono-molecular layers, eventually affording high layered crystallin-ity.34-39 In the crystals, substituted long alkyl chains form alkyl chain layers which synergetically enhance the LHB packing of unidirectionally-aligned π-core layers.40,41 The phenyl groups also assist the formation of the LHB packing motif. In partic-ular, these end-cap effects allow to form b-LHB packing even for the unsymmetric π-cores whose unsubstituted moieties crystallize into antiparallel side-by-side arrangements.42 In contrast to the end-cap effects, little is known on the modulation effect of π-cores on the layered crystallinity and carrier transport characteristics. This is partly related to the difficulty in isolating thiophene-fused isomers of the π-cores in chemical syntheses.43-57 It is also anticipated that the iso-meric thiophene part easily presents positional disorder in the crystals.58-60 Nonetheless, there exists a vast variety of struc-tural isomers in candidate π-cores depending on the connect-ing pattern of the benzene and thiophene rings, even if they are limited to linear π-cores.61-66 It is still unclear how such struc-tural isomerism affects the intermolecular contact and transport characteristics, though it holds great promise in mo-lecular design towards realizing practical high-performance OSCs. In this study, we focus on the structural isomeric effect of BTBTT (BTBTT: ben-zothieno[6,5-b]benzothieno[3,2-b]thiophene, Scheme 1(a)); the orientation of terminal thiophene moiety can be changed to syn- and anti-forms.67,68 We developed pure syn-/anti-thiophene-fused isomers of BTBTT-based OSCs with end-capped substitutions with phenyl and alkyl groups. The syn/anti isomerization in the BTBTT skeleton leads to the variation of reorganization energy, one of the critical factors defining carrier transport properties (Figure S1). We found that the synCn forms the b-LHB packing with showing high layered crystallinity and high mobility, whereas the antiCn presents a unique antiparallel alkyl-interdigitated herringbone (aai-HB) packing with showing poor film-forming ability and moderate mobility. Based on the results and on the quantum chemical intermolecular interaction calculations, we discuss the origin of the isomeric effect on the layered crystallinity of the OSC molecules in terms of the molecular geometry and the intermolecular forces. RESULTS AND DISCUSSION Material synthesis. Synthetic strategy for the target Ph-BTBTT-Cn derivatives is illustrated in Scheme 1(b). It shows two alternative synthetic routes, as classified in terms of final thiophene-ring formation reaction.69,70 The first one, which was reported previously, has a demerit, as it inevitably affords undesirable product associated with the two possible    Scheme 1 (a) Chemical structures of syn-/anti-isomers of Ph-BTBTT-Cn. (b) Synthetic strategy for Ph-BTBTT-Cn derivatives employing a thiophene ring formation reaction. Previously reported route (green) and newly proposed route (purple) using the benzothio-phene-/benzodithiophene-involved precursor, respectively. Note that the former precursor has two possible reaction sites including an un-desirable position marked with gray asterisk affording isomeric byproducts. (c) Synthetic scheme of syn-/anti-isomers of Ph-BTBTT-Cn.    reaction sites on the phenyl-substituted benzothiophene unit. To avoid it, we employed another precursor comprising the benzodithiophene unit, which allows to afford syn-/anti-BTBTT skeletons selectively and efficiently. According to the improved route as depicted in Scheme 1(c), we carried out the synthesis of a series of the BTBTT-isomers (synCn and antiCn). Using syn-/anti-isomers of benzodithio-phene (BDT) as starting materials, we formed unsymmetric skeletons (4) by introducing an alkyl chain unit and a phenyl group in a step-by-step fashion though the lithiation/borylation reactions and the Suzuki−Miyaura cross-coupling reactions. After the oxidation of 4, the intramolecular cyclization of the precursors (5) formed the BTBTT backbone. The crude prod-ucts were finally purified by vacuum sublimation to afford a white powder for synCn and a pale yellow powder for antiCn. The present synthetic route remarkably improves the yields at the final step from 20% to 50-60% because the unnecessary cyclization at the carbon, denoted by gray star, in Scheme 1(b) is avoided.42 The total six steps were conducted in good over-all yields of around 20%.  Solubility and thermal properties. All the BTBTT-isomers are sufficiently soluble for solution-based thin-film processing in some aromatic or chlorinated solvents at room temperature (Figure 1). The solubility almost doubles for the same isomer as the alkyl chain shortens from n = 10 to n = 6, and this trend well matches the results for other thieno-acene OSCs.8,31,32 In a comparison of the same alkyl chain lengths, antiCn shows approximately 60% less solubility than synCn. All the compounds show a high thermal stability of the solid state up to at least 200 °C and have liquid crystalline (LC) phases, as shown by the thermogravimetry-differential thermal analysis (TG-DTA) in Figures 1 and S2. The temper-atures of the crystal-LC and LC-isotropic phase transitions increase slightly with shortening the alkyl chain, and are much higher in antiCn than in synCn. The higher (lower) thermal stability of antiCn (synCn) is well consistent with the trend of the lower (higher) solubility, when we consider that these characteristics are determined by the crystal packing stability. The results also imply that the packing motif is distinct be-tween synCn and antiCn.  Single-crystal growth and molecular packing. Figures 2(a)-(d) present micrographs of single crystals or recrystal-lized products from solutions for all the compounds. The crystal habits are clearly different between synCn and antiCn,  Figure 2 Optical images of the single crystals of (a) synC10 and (b) synC6 fixed on a mesh-type Litholoop and the crystalline solids of (c) antiC10 and (d) antiC6. Experimental crystal structures projected along the intralayer direction for (e) synC10, (f) synC6, (g) antiC10, and (h) antiC6. Herringbone packings of (i) synC6 and (j) antiC6 (The hexyl chains are omitted for clarity.). Top and front views of one molecule of (k) synC6 and (l) antiC6 in the crystals. Green broken frame: approximate molecular shapes of the rigid π-conjugated skeleton composed of the phenyl ring and the BTBTT π-core.  Figure 1 Solubility in chlorobenzene at 25 °C (red solid circle) and phase-transition behaviors (bluish bar) of the BTBTT-isomers.   but are independent of the alkyl chain length, as seen in the micrographs. The synCn affords plate-like single crystals suitable for X-ray diffraction analysis, whereas it is difficult to grow large and thick single crystals for antiCn; ultrathin flake-like crystals stacked to each other are mostly obtained, even under an optimized growth condition. Although suffi-cient x-ray diffraction intensity for structure analysis was not obtained from the antiCn crystal, the crystal structure of an-tiCn was successfully achieved by serial X-ray diffraction using free electron lasers (XFELs) and electron diffraction analyses. The resulting molecular packings are shown in Fig-ures 2(e)-(j), and the crystallographic data are listed in Table S1. As expected, the molecular packing motifs are distinct between synCn and antiCn. The synC6 forms a b-LHB packing being isomorphous to synC10,42 while both antiC6 and antiC10 form unique aai-HB packing, in which the re-spective π-core layers are formed by a herringbone-type over-lap arrangement between π-core moieties (i.e., composed of BTBTT core and phenyl ring) of adjacent molecules aligned parallel and antiparallel to each other, whereas the alkyl-chain moieties are interdigitated with each other between the π-core layers. Despite these different layered structures, the herring-bone angles (θHB) between the adjacent π-cores are almost the same for synCn and antiCn (Table S2). The notable variation of the molecular packing between synCn and antiCn should be clearly ascribed to the effect of isomeric change in BTBTT π-cores, the origin of which will be discussed in later section.  As seen from the crystal structures shown in Figures 2(k) and (l)), synCn and antiCn present particular difference in term of the whole molecular shape in the crystals. As for the rigid π-core moieties, synCn is composed of a slightly bent rod-like π-core, while antiCn has more linearly extended π-core. The feature is associated with the thiophene-fused isomeric effect on the shape of rigid π-core, and is responsible for the change of the molecular packing between synCn and antiCn, as discussed later. In contrast, the alkyl-chain orienta-tion as to the π-core is also much different between synCn and antiCn; the alkyl chain is directed close to the molecular long axis in synCn with the bent angle of about 10°, while that is considerably bent of about 45° from the long axis of π-core in antiCn (Table S2). Both the alkyl conformations of synCn and antiCn are different from those of the geometry of isolat-ed molecules optimized by DFT calculations (Table S2). Here, we briefly describe the key results of the crystallo-graphic analysis of antiCn. The general molecular arrange-ment is identical for antiC6 and antiC10, but antiC6 is more symmetric (space group P21/c) with two independent mole-cules in the unit cell (Z' = 2), while antiC10 is less symmetric (P1) with Z' = 8. The neighboring molecules of antiC6 are related by symmetry operations, but in the analysis of antiC10, these relationships are only pseudo-symmetric. By assuming that all molecules are independent with the space group P1, a model structure that better matches the diffraction data was finally achieved. The obtained structure shows slight differ-ences in the molecular conformation among the eight inde-pendent molecules, in terms of the dihedral angles between the π-core and phenyl group/alkyl chain and the herringbone an-gles formed by the π-cores. Various aspects and origin of these features will be reported shortly. Solution-processed crystalline thin films and their char-acteristics. Figure 3 summarizes results of drop-casting and blade-coating of the solutions of the compounds onto sub-strates; crossed-Nicols polarized micrographs are shown for the obtained thin films and products. The results of simple drop casting show a clear difference in the thin-film formabil-ity of the compounds; synCn produces uniform millime-ter-scale thin films composed of single-crystalline domains that show clear brightness contrast in the images, while an-tiCn affords needle-like recrystallized products but not thin-film products. In contrast, the blade-coating technique allows to form millimeter-scale crystalline thin films for all compounds. However, the difference in the film-forming abil-ity between synCn and antiCn is easily observed from the images; Fairly large single-crystalline domains are obtained for synCn, while antiCn affords elongated films composed of multiple domains that do not have clear boundaries to each other, where the extinction ratio in the crossed-Nicols image is not as clear as the synCn crystalline films, even after the film process conditions were fully optimized.   Figure 3 (a) A schematic of a drop-casting method. (b) Crossed-Nicols polarized micrographs of the drop-casted crystalline solids of the BTBTT-isomers. (c) A schematic of a blade-coating method. (d) Crossed-Nicols polarized micrographs of the blade-coated thin films of the BTBTT-isomers. The white dashed line presents a single-crystal domain.    The crystalline states of both drop-casted and blade-coated films were examined by X-ray diffraction measurements (Fig-ure S3). Clear Bragg reflections were observed in the synCn films; out-of-plane (00l) and single in-plane peak at 2θ of 18.09° for synC6 and 18.18° for synC10, corresponding to the (020) diffractions (Figures S3(a) and (c)). The result indicates the crystal packing of the films with bulk crystals (Figure S3(e)). In contrast, a single weak out-of-plane diffraction is only observed for the antiCn films (Figures S3(b) and (d)), although no in-plane Bragg reflections were observed in the measurements. The d-spacing is estimated as about 25.2 Å for antiC6 and 29.6 Å for antiC10, which are strictly different from the (002) reflections of bulk crystals (23.4 Å for antiC6 and 27.7 Å for antiC10) but roughly corresponds to the length of linearly-extended molecules (23.8 Å for antiC6 and 28.8 Å for antiC10). Atomic force microscopy (AFM) measurements reveal that the step height in the antiC6 thin film is estimated to be about 2.4 nm that approximately corresponds to the sin-gle-molecular length of antiC6 (Figure S4). We consider that the antiCn films should form smectic LC-like states, as sche-matically presented in Figure S3(f)),71-76 and that the films do not have a long-range order suitable for XRD measurements. However, the details of the molecular arrangements in the films are not clear. TFT characteristics. We fabricated bottom-gate, top-contact (BGTC)-type TFTs composed of an SiO2 gate dielectric layer passivated by a parylene C layer, gold source-drain electrodes, and drop-casted films of synCn or blade-coated films of antiCn as channel layer (Figure 4(a)). Typical p-type characteristics are observed for all the com-pounds, as shown in Figures 4(b) and S5. All the TFTs show negligible hysteresis with almost the same threshold voltages of around –20 V. This indicates that the presence of carrier traps at the semiconductor/dielectric interface does not depend largely on the crystalline states of the OSC thin films.77,78 In contrast, the carrier transport properties strongly depend on the  Figure 4 (a) A schematic of the BGTC-TFT device. (b) Transfer curves and the mobility plots in the saturation regions for the TFT devices based on the drop-casted thin films for syn-isomers and based on the blade-coated thin films for anti-isomers. Table 1 Intermolecular interaction energies (Eint) for (a) synC6 and (b) antiC6, between the central target molecule, shown exclusively by a space-filling model, and the neighboring molecules, shown by a ball and stick model, with interatomic distance shorter than 4 Å. The in-termolecular contacts mostly between π-core moieties that form intralayer herringbone arrangements are defined as “Intralayer”. In con-trast, interlayer intermolecular contacts at the phenyl group and at the alkyl chain of the target molecule are defined as “Interlayer phenyl” and “Interlayer alkyl”, respectively, where the latter involves interdigitated contacts between alkyl chains. The “Intralayer” contacts were decomposed into dispersion (Edisp), electrostatic (Ees), induction (Eind), and short-range (Eshort) interactions, by energy decomposition analy-sis. All energies were averaged for the crystallographically independent molecules.    isomeric BTBTT π-cores; synCn exhibit high saturation mo-bilities over 10 cm2 V−1 s−1 (average: 9.0 ± 1.5 cm2 V−1 s−1 for synC10 and 8.7 ± 1.6 cm2 V−1 s−1 for synC6), while antiCn show relatively low mobilities (average: 0.041 ± 0.0036 cm2 V−1 s−1 for antiC10 and 0.036 ± 0.0055 cm2 V−1 s−1 for an-tiC6). A similar tendency is observed in current on/off ratios: over 107 for synCn and around 105 for antiCn, respectively. These differences in carrier transport properties are closely related to the crystalline states of the OSC thin films. Intermolecular interaction energies. To understand why the b-LHB and aai-HB lattices are formed with synC6 and antiC6, respectively and selectively, we conducted disper-sion-corrected density functional theory (DFT) calculations for intermolecular interactions using the optimized crystal struc-tures of synC6 and antiC6. The crystal structures were opti-mized starting from the structures obtained by the X-ray and electron diffraction measurements. The interaction energies were calculated for molecular pairs consisting of a molecule and its surrounding neighbors with the shortest interatomic distance of 4 Å or less. Table 1 shows sum of interaction energy between molecules adjacent along inter- and intralayer directions, respectively. The total interaction energies are almost same between the different crystal lattices of synC6 and antiC6. The intralayer interaction is dominant for both crystals, as they are composed of the rod-like OSC molecules. In contrast, the interlayer in-teraction becomes more important in aai-HB lattices of an-tiC6 (−1.71 + −12.18 = −13.89 kcal/mol) than in b-LHB lat-tices of synC6 (−5.04 + −2.92 = −7.96 kcal/mol), which can be clearly ascribed to the contribution of interdigitated con-tacts between alkyl chains. These features demonstrate the higher layered crystallinity in b-LHB lattices of synC6 than in aai-HB lattices of antiC6. Interestingly, the interaction energy calculated only between the adjacent π-core moieties are al-   Figure 5 Molecular packing of the theoretically optimized crystal structures for synC6 and antiC6. (a) Molecular arrangement in a slipped-parallel relationship. Green broken frame: approximate molecular shapes of the π-conjugated rigid skeleton composed of the phe-nyl ring and the BTBTT π-core. Gray solid line: intermolecular distances between the centroids of the adjacent benzene rings of the π-conjugated rigid skeleton, showing the uniformity of the molecular arrangements. (b) Lattice energy per one molecule. (c) Side and (d) front views of the herringbone packing of the π-skeleton. Red and blue broken lines: arrangement pattern of the adjacent sulfur atoms in the benzodithiophene unit highlighted by a space-filling model with yellow and brown color.    most same between the different crystal lattices (−77.99 kcal/mol in b-LHB lattices of synC6 vs. −77.73 kcal/mol in aai-HB lattices of antiC6). The results imply that the b-LHB and aai-HB lattices are competitive in the syn- and an-ti-isomers, though it is not clear whether the distinct π-cores are suitable or not for the formation of parallel/antiparallel intralayer arrangements, respectively. Origin of isomeric effect on distinct molecular packing. In the crystals formed by neutral molecules such as the OSCs used in the present study, attractive intermolecular forces are mainly derived from dispersion forces,79,80 as is confirmed in Table 1. Repulsive force owing to the short-range orbital in-teraction should be balanced with the attractive forces, result-ing in the formation of stable crystal packings.81 Thus, larger lattice energy can be gained by closer intermolecular distance with keeping atom-atom distance larger than certain distances to avoid short-range repulsion. Conversely, crystal structures that involve larger void space between adjacent molecules becomes more unstable. The rule should be the basic origin why the different isomeric molecules afford distinct molecular packing. Keeping the facts in mind, we investigated why the thio-phene-fused isomers of synC6 and antiC6 form the b-LHB and aai-HB packings, respectively. For this purpose, we hy-pothetically generated the opposite types of packings (aai-HB lattice for synC6 and the b-LHB lattice for antiC6), and com-pared their stability with that of actual crystals. Each hypo-thetical lattice is modeled by substituting synC6 (antiC6) with antiC6 (synC6) in the b-LHB (aai-HB) lattices, and then sub-jected the initial models to geometry optimizations (See the Experimental for details). The obtained molecular packing is presented in Figure 5(a), with the lattice parameters listed in Table S3 and the calculated powder patterns shown in Figure S6 (See Figure S7 for the transfer integrals and band structures calculated from these actual and hypothetical lattices). Based on the method which we call as “lattice-swapping approach”, we found that the hypothetical packings for both isomers form slightly larger unit cell volume and are less stable than the actual packings, showing larger lattice energies of approxi-mately 1.5 kcal/mol, as depicted in Figure 5(b). The calculated results are quite consistent with the actual packings for synC6 and antiC6.  Based on the findings, we discuss the stabilities of actual and hypothetical packings in terms of the slightly different geometry of rigid π-conjugated skeletons that should affect the formation of close intralayer intermolecular packing or the opening between adjacent molecules. We first focus on the relatively bulky sulfur atoms and compare the arrangement patterns in the HB packing, as presented in Figures 5(c) and (d). The sulfur atoms are arranged in a slightly distorted hex-agonal manner in the actual lattices, whereas they are uneven-ly arranged in the hypothetical aai-HB lattice of synC6. The latter feature should be unfavorable for close intermolecular packing. We also examined slipped-parallel arrangement in the HB packing as shown in Figure 5(a). It is found that the π-core of synC6 is slightly bent and thus not suitable for close and uniform antiparallel molecular arrangement in the hypo-thetical aai-HB lattice. Finally, we investigated the intermo-lecular C−H···π distance in the T-shaped arrangements (Figure 6 and Table S4), as the fundamental unit for the HB pack-ing.82,83 We found that the H···π distances between the C-H terminals and the contacting π-electron plane are mostly shorter in the actual lattice than in the hypothetical lattice for both the isomers. These results corroborate that the actual lat-tices should be selectively formed by the tighter HB packing.  CONCLUSION We successfully synthesized two pairs of syn-/anti-isomers of synCn and antiCn with different alkyl chain lengths (n = 6, 10), and investigated the effect of unsymmetric thienoacenes involving different thiophene-fused isomers on the crystal packings and solid-state characteristics. All the compounds show sufficient solvent solubility for solution processing at room temperature and high thermal stability of the solid state up to 200 °C. These structural isomers afford distinct types of packing motifs, irrespective of the alkyl chain lengths: The synCn crystallizes into the b-LHB packing composed of head-to-head arrangement of unidirectionally-aligned mono-molecular layers, while the antiCn forms aai-HB packing composed of alternating antiparallel alignment of π-cores and the alkyl chains are interdigitated with each other between the adjacent π-core layers. The observed packing difference is clearly associated with the film-forming ability and the TFT properties: The synCn easily forms a single-crystalline films that exhibit high carrier mobility over 10 cm2 V−1 s−1, while the antiCn shows poor film-forming ability and moderate mobility. By comparing with hypothetical optimized structure models, we found that the isomeric effect on the distinct na-ture of molecular packing originates from the slightly different geometry of rigid π-conjugated skeleton that affects the for-mation of close intralayer intermolecular packing or the open-ing between adjacent molecules. We believe that these find-  Figure 6 (a) Electrostatic potentials calculated for synC6 and an-tiC6 from B3LYP/6-311G** wave functions. (b) Schematics of intermolecular C−H···π interactions in the T-shaped arrangement. (c) Averaged distances of the H···π contacts.   ings will open further potential for crystal engineering in func-tional OSCs based on unsymmetric π-cores.  EXPERIMENTAL SECTION Materials. Details of syntheses and characterizations for the title compounds, synCn and antiCn, is summarized in the Supporting Information.  Thermal Properties. Thermal properties were investigated by thermogravimetry-differential thermal analysis (TG-DTA; STA7200RV, Hitachi High-Tech Science Co.). TG-DTA measurements were performed in the range between room temperature and 500 °C at a heating rate of 5 K min−1 under N2 atmosphere. Crystal Growth. Plate-like single crystals of synCn suita-ble for X-ray diffraction were obtained by recrystallization from anisole solutions under ambient conditions. One crystal was collected from the solution using mesh-type LithoLoops (Molecular Dimensions Ltd.). Flake-like ultrathin single crys-tals of antiCn suitable for electron diffraction were obtained by recrystallization from mixed solutions of chlorobenzene and anisole (1:2 v/v).  X-ray Crystal Structure Analysis. X-ray crystallography was carried out for synC6. The single-crystal X-ray diffraction data were collected using a Rigaku AFC10 four-circle diffrac-tometer equipped with a Pilatus 200 K hybrid pixel detector and a graphite monochromated Mo Kα radiation source (λ = 0.71073 Å). Data reduction with empirical absorption correc-tion was performed using the CrysAlisPro software package.84 The structure was solved by direct methods using the SHELXT program85 and was refined by the full-matrix least-squares method using SHELXL86 by applying aniso-tropic temperature factors for all nonhydrogen atoms. The hydrogen atoms were placed at geometrically calculated posi-tions. All calculations were carried out by using the crystallo-graphic software package Olex2-1.2.87 Serial XFEL Crystal Structure Analysis. XFEL crystal-lography88 was carried out for serial crystal structure analyses from ultrathin flake-like crystals of the compound antiC6. The flakes were suspended in low-viscosity liquid paraffin and spread over a polyimide plate with an area of 4 x 4 mm2. The XFEL measurement was performed at Beamline 3, SACLA XFEL facility89. The photon energy of the XFEL and the beam size were adjusted to 15.0 keV and ~1 μm, respectively. The sample plate was scanned over the entire plate with XZ-translation and phi-rotation movements, being exposed every 10 μm by XFEL pulses. The diffraction patterns were recorded on an MX300-HS detector (Rayonix) placed 90 mm downward from the sample plane. All data collection was performed at room temperature. Total 182,878 images were collected. The reference cell parameters, monoclinic, a = 13 Å, b = 7.8 Å, c = 47 Å, β = 97°, were obtained from rotational electron diffraction (see the next Electron Crystal Structure Analysis section) and provided for processing the serial dif-fraction images with CrystFEL suite.90 6,308 images (3.4% of the total collected images) were successfully indexed from extracted 24,253 images (13.3%) showing diffraction spots. The integrated and merged diffraction intensities were then used for ab initio phasing with SHELXT85, and the initial structure was refined with SHELXL86. The data were tested for twinning using PLATON91 and revealed the twin operator (-1 0 0/0 -1 0/0.8 0 1), which was included in the refinement. The bond geometry was restrained using the values from the structure of synC10. All non-hydrogen atoms were modeled with anisotropic displacement parameters and hydrogen atoms were generated using a riding model. The crystallographic parameters are summarized in Table S1. Electron Crystal Structure Analysis. Electron crystallog-raphy was carried out92 for crystal structure analyses from ultrathin flake-like crystals of the compounds antiCn at n = 6 and n = 10. The crystals obtained by drying up the anisole solution of antiCn were spread with Novec7100 (3M) or ani-sole onto a holey carbon film-coated TEM grid. The crystals were examined with a CRYO ARM 300 microscope (JEOL Ltd., Japan) operated at an accelerating voltage of 300 kV. Diffraction patterns were recorded on a DE-64 detector (Direct Electron, USA). Sequential frames were collected per crystal under parallel illumination by continuously rotating the sam-ple stage from -68° to 68° at a rotation speed of 1 °/s. Total 96 and 67 rotational series, respectively for antiC6 and 10, were collected at cryogenic temperature (~93 K) using SerialEM and ParallEM93,94. Diffraction data were then processed with DIALS95. For antiC6, the common lattice parameters found in the rotation series were referred in Serial XFEL Crystal Structure Analysis. For antiC10, the initial phase was ob-tained by molecular replacement96 using a part of the structure, as the direct method failed to yield reasonable solutions. The atomic model was manually adjusted and refined. The crystal-lographic parameters are included in Table S1. Thin-Film Processing. To obtain thin films of synCn and antiCn, a drop-casting and blade-coating technique were em-ployed. A heavily p-doped Si wafer covered with a 100 nm thermally grown SiO2 dielectric layer was used as a substrate after being cleaned by sequential sonication in deionized water, acetone, 2-propyl alcohol, and deionized water. On the Si/SiO2 (100 nm) wafer, a parylene C layer was deposited by chemical vapor deposition using a PDS 2010 LABCOTER (Specialty Coating Systems, USA) and DPX-C (di-chloro-di-p-xylylene) as a starting material. The thickness of the parylene C layer was evaluated to be around 40 nm. Solution-processes were done on the parylene C layer. Drop-casting was performed under a simple sealed atmosphere at room temperature using 0.02 wt% anisole (FUJIFILM Wako Pure Chemical Industries Ltd, 99.0%) solutions of synCn and 0.02 wt% chlorobenzene (Sigma-Aldrich, 99.9%) solutions of antiCn. A thin glass plate coated with Cytop (CTL-809 M; AGC Inc., Japan) was used as the coating blade, and its motion was controlled using a stepping motor (SHOT-302GS; Sigma Koki Co., Ltd., Ja-pan). Blade-coating was performed using 0.02 wt% chloro-benzene solutions of synCn and antiCn at a sweep rate of 0.5 μm s–1 under ambient conditions for synCn and 7.5 μm s–1 under 60 °C for antiCn. Thin-Film Properties. XRD measurements of the solu-tion-processed thin films were carried out using a thin-film diffractometer (SOR-SmartLab; Rigaku Co., Ltd.), combined with monochromatized synchrotron X-ray radiation of an en-ergy of 10 keV (1.238 Å) at the beamlines BL-7C of Photon Factory (PF), High-Energy Accelerator Research Organization (KEK). Atomic force microscopy (AFM) images of the thin films were recorded on a commercial AFM system (Dimen-  sion Edge, Bruker Co., Ltd.) in a tapping mode under ambient conditions. Fabrication of TFTs. Gold source/drain electrodes were patterned on the synCn and antiCn thin films by thermal deposition using a shadow mask; the channel length and width of the devices, used for measurements of TFT characteristics, were 50 and 340 μm for synC10, 100 and 765 μm for synC6, and 50 and 500 μm for antiCn, respectively. The total capaci-tance of the gate dielectric composed of the SiO2 and parylene C layers was estimated at approximately 26 nF cm–2. For proper mobility evaluation, parts of the thin films outside (and between) the channels were trimmed using a homemade mi-cromanipulator. The two-probe characteristics of the bot-tom-gate top-contact TFTs were measured under a N2 atmos-phere using a High-Power Source/Monitor Unit Module (E5280A; Keysight Technologies). The saturation mobility (µsat) was calculated from the following equation: ID = (W/2L)Ciµsat(VG−Vth)2, where ID is the drain current, W is the channel width, L is the channel length, Ci is the capacitance per unit area of the gate dielectric layer, and VG and Vth are the gate and threshold voltage, respectively. Computational Methods. The Quantum ESSPRESSO pro-gram97,98 was used to optimize the crystal structures and cal-culations of lattice energies. The PBE functional99 and Grim-me’s D3 dispersion correction100 were used for the DFT cal-culations. The cutoff energies of plane wave basis set and charge density were 49 Ry and 441 Ry, respectively. The atomic positions and cell parameters were optimized in the optimizations of crystal structures. The lattice energies (Elattice) were obtained according to the following equation, Elattice = Ecryst – N Emono, where Ecryst is the energy of the unit cell of crystal after geometry optimization, which contains N mole-cules, and Emono is the energy of the optimized isolated mole-cule. Each molecule was isolated by the cubic unit cell with the edge of 35 Å in the geometry optimization. The hypothet-ical b-LHB lattice of synC6 and aai-HB lattice of antiC6 were generated through the following procedure: The atomic coordinates obtained from the diffraction experiments were partially modified using GaussView software,101 converting the molecules from synC6 (antiC6) to antiC6 (synC6), whose modified coordinates were used as initial models for the optimizations of crystal structures. Intermolecular interaction energies (Eint) between neighbor-ing molecules were calculated at the B3LYP/6–311G** lev-el102 with Grimme’s D3 dispersion correction using the Gaussian16 program,103 based on the optimized crystal geom-etries. The basis set superposition error (BSSE)104 was cor-rected by the counterpoise method.105 The interaction energies between the π-conjugated skeletons composed of the phenyl group and the BTBTT core were calculated using the fragment structures in the crystals. The dangling bonds of the fragments were capped by hydrogen atoms in the calculations. The intermolecular interaction energy was categorized into the contributions of dispersion force (Edisp), the electrostatic force (Ees), the induction force (Eind), and short-range or-bital-orbital interation (Eshort). The Ees and Eind were calculated using the ORIENT version 3.2.106 Ees was calculated as the interactions between distributed multipoles107 of interacting molecules using the ORIENT version 3.2. Distributed multi-poles up to hexadecapole on all atoms were obtained from B3LYP/6-311G** level wave functions of isolated molecules using the GDMA program.108 The Eind was calculated as inter-actions of polarizable sites with the electric field produced by the distributed multipoles of monomers.109 The atomic polar-izabilities of carbon (a = 10 au) and sulfur (a = 20 au) were used for the calculations.110 Distributed multipoles were used only to estimate the Ees and Eind. The interaction energy calcu-lated without the dispersion correction (Eb3lyp) is approximate-ly the sum of the Ees, Eind, and Eshort. Thus, the Eshort was calcu-lated according to the equation; Eshort = Eb3lyp – Ees – Eind. The Edisp was calculated as the difference between the calculated interaction energies with and without dispersion correction according to the equation; Edisp = Eint – Eb3lyp. Reorganization energies of the neutral molecules under pos-itive charging were calculated with the four points meth-od,111,112 using the Gaussian16 program at the B3LYP/6-311G** level. Transfer integrals between neighboring molecules were calculated for synC6 and antiC6 using the Gaussian16 pro-gram at the PW91/TZVP level using the HOMO generated from the theoretically optimized crystal geometries. The transfer integral tAB between the molecular orbitals |A⟩ and |B⟩ was calculated by using the following equation: 𝑡AB =〈𝐴|𝐹|𝐵〉 −12(〈𝐴|𝐹|𝐴〉 + 〈𝐵|𝐹|𝐵〉)〈𝐴|𝑆|𝐵〉1 − 〈𝐴|S|𝐵〉2 where F is the Fock matrix and S is the overlap matrix. Electronic structures of synC6 and antiC6 were calculated at the PBE level of theory99 using the plane wave basis set and pseudopotentials as implemented in the Quantum Espresso program.97,98 The self-consistent calculations are computed with the cutoff energy of 25 Ry for plane wave and 225 Ry for charge density, respectively, and the Brillouin-zone integra-tions of a 3 × 3 × 1 k-points mesh.  ASSOCIATED CONTENT The Supporting Information is available free of charge on the ACS Publications website at DOI: Synthetic procedures and characterization data, thermogra-vimetry-differential thermal analysis (TG-DTA), crystallo-graphic data obtained from the diffraction measurements and obtained through the theoretical optimization, molecular con-formation analysis, and thin-film characterizations. Crystallographic data for synC6 (CIF) Crystallographic data for antiC6 (CIF) Crystallographic data for antiC10 (CIF) AUTHOR INFORMATION Corresponding Author * E-mail: t-higashino@aist.go.jp  ORCID Toshiki Higashino: 0000-0002-9227-8207 Satoru Inoue: 0000-0002-1943-1594 Shunto Arai: 0000-0002-0055-3006 Seiji Tsuzuki: 0000-0001-8518-0300 mailto:t-higashino@aist.go.jp  Hiroyuki Matsui: 0000-0003-1956-9603 Reiji Kumai: 0000-0002-5320-0028 Kiyofumi Takaba: 0000-0002-2927-0608 Saori Maki-Yonekura: 0000-0002-8295-8794 Hirofumi Kurokawa: 0000-0002-1144-3400 Kensuke Tono: 0000-0003-1218-3759 Koji Yonekura: 0000-0001-5520-4391 Tatsuo Hasegawa:0000-0001-5187-7483 Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This study was partly supported by JSPS KAKENHI Grant Numbers 20H05867, 21K14699, 21H04651, and 18K19161, JST CREST Grant Number JPMJCR18J2, and JST-Mirai Program (Grant Number JPMJMI20G5). 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