Crystal Engineering of Biphenylene-Containing Acenes for High

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Crystal Engineering of Biphenylene-Containing Acenes for High-Mobility Organic Semiconductors Jinlian Wang, Ming Chu, Jian-Xun Fan, Tsz-Ki Lau, Ai-Min Ren, Xinhui Lu, and Qian Miao J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b12671 • Publication Date (Web): 30 Jan 2019 Downloaded from http://pubs.acs.org on January 31, 2019

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Journal of the American Chemical Society

Crystal Engineering of Biphenylene-Containing Acenes for HighMobility Organic Semiconductors Jinlian Wang 1, Ming Chu 1, Jian-Xun Fan 2, 4, Tsz-Ki Lau 3, Ai-Min Ren 2, Xinhui Lu 3, Qian Miao 1, 5 * Department of Chemistry, The Chinese University of Hong Kong, Shatin, New Territories, Hong Kong, China of Theoretical Chemistry, Laboratory of Theoretical and Computational Chemistry, Jilin University, Changchun, 130023, China 3 Department of Physics, The Chinese University of Hong Kong, Shatin, New Territories, Hong Kong, China 4 College of Chemistry and Material, Weinan Normal University, Weinan, China 1

2 Institute

Institute of Molecular Functional Materials (Areas of Excellence Scheme, University Grants Committee), Hong Kong, China 5

ABSTRACT: Herein we report synthesis, crystal structures and semiconductor properties of new derivatives of bisnaphtho[2',3':3, 4]cyclobut[1,2-b:1',2'-i]anthracene (BNCBA). It is found that the π-π stacking of BNCBA in single crystals can be largely modified by alkyl substituting groups of different lengths. In particular, the tetrahexyl derivative exhibits π-π stacking with an unusual zigzag arrangement. The variation of molecular packing also leads to change of charge transport characteristics as found from the theoretical calculation of mobility on the basis of single crystal structures. All of these BNCBA derivatives function as p-type semiconductors in solution-processed thin film transistors, and the tetrahexyl derivative exhibits field effect mobility as high as 2.9 cm2/Vs.

INTRODUCTION Acenes and their derivatives are one of the most important groups of organic semiconductors, particularly applied in organic thin film transistors (OTFTs),1 which are elemental units in light-weight, flexible and low-cost organic electronic devices. The best known example of acenes is pentacene, which has led p-type small-molecule organic semiconductors as a benchmark for applications in OTFTs. As a result of consisting of linearly annelated benzene rings, acenes have only one aromatic sextet 2 regardless of their length. Therefore acenes become less stable in accompany with reduction of aromaticity when the length of the acene increases.3 Such instability is an obstacle to the development of larger acenes for organic semiconductors with enhanced spatial electronic delocalization. One strategy to solve this problem is to extend acenes through four-membered rings, which increase the length of acenes without compromising their stability or changing their linear shape.4 Using this strategy, Swager 5 and Xia 6, 7 designed and synthesized a series of biphenylenecontaining acenens, such as 1a and 1b, which are derivatives of bisnaphtho[2',3':3,4]cyclobut[1,2-b:1',2'-i]anthracene (BNCBA) as shown in Figure 1. BNCBA has the same number of six-membered rings as heptacene, but is much more stable than heptacene because insertion of four-membered rings into the linear π-backbone increases the number of aromatic sextets. In contrast to acenes, biphenylene-containing acenes in the solid state have been rarely explored as organic semiconductors, and no biphenylene-containing acenes were documented as p-type organic semiconductors in OTFTs to the best of our knowledge 8, 9 likely due to lack of capability to optimize the molecular packing of biphenylene-containing acenes in the solid state.

Figure 1 Structures of substituted bisnaphtho[2',3':3,4]cyclobut[1, 2-b:1',2'-i]anthracene (1a–c and 2a–c) with the biphenylene moieties highlighted in light blue.

Molecular packing is of key importance to the performance of organic semiconductors in OTFTs10 because charge transport at the microscopic level is governed by electronic coupling between neighboring semiconductor molecules, in a way that intimately depends on both the relative positions of interacting molecules and the phase and nodal properties of πorbitals.11, 12 As inspired by earlier success in tuning molecular packing of acenes with different substituents,10, 13 we studied crystal engineering of biphenylene-containing acenes by attaching alkyl groups of different lengths to the linear πbackbone. Herein we report synthesis, crystal structures and semiconductor properties of new derivatives of BNCBA, 1c and 2a–c, which were studied in comparison with the earlier reported derivative 1b (Figure 1). It is found that the π-π stacking of BNCBA in single crystals can be largely modified

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by different substituting groups, and, in particular, 2b exhibits π-π stacking with an unusual zigzag arrangement. In solutionprocessed thin film transistors, all of these compounds function as p-type organic semiconductors and 2b exhibits field effect mobility as high as 2.9 cm2/ Vs.

RESULTS AND DISCUSSION Scheme 1 shows the two-step synthesis of 1c and 2a–c 6 using Xia's strategy from 1,4-dialkylbenzooxanorbornadienes (3a–c), which were prepared by the Diels-Alder reaction between in situ generated benzyne and the corresponding 2,5-dialkyl furans.14 The palladiumcatalyzed C−H activated annulation between 3a–c and dibromoanthracene 4a15 or 4b16 formed four-membered rings and gave 5 and 6a–c as a mixture of two diastereomers. The yields of 5 and 6a–c are lower than that in the synthesis of 1b using the same method presumably because the longer alkyl chains in 3a–c bring extra steric hindrance to the C−H activated annulation reaction. The subsequent aromatization under acidic conditions afforded 1c and 2a–c in yields of 60– 80%. Compounds 1c and 2a–c are all yellow powders, which are soluble in CH2Cl2 or CHCl3. They are stable both in the solid state and in solution under ambient air and light. For example, the 1H NMR of 2b in CDCl3 did not change after the solution was kept under ambient air and light for three weeks as shown in Figure S1 in the Supporting Information. Scheme 1 Synthesis of BNCBA derivatives 1c and 2a–c.

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to those of 1b–c by about 25 nm as a result of extra conjugation effect of the ethynyl groups in 1b–c. Upon irradiation with UV light, the yellow solutions of 1b–c exhibited green fluorescence, while those of 2a–c exhibited blue fluorescence. As shown in Figure S2b in the Supporting Information, the fluorescence spectra of 1b–c and 2a–c exhibit small Stokes shifts (2 to 8 nm) in agreement with the rigid πbackbone of 1b–c and 2a–c. In the test window of CV, 1b–c and 2a–c all exhibited one reversible oxidation wave but did not exhibit any reduction waves as shown in Figure S3 (Supporting Information). The half-wave oxidiation potentials of 1b–c and 2a–c versus ferrocenium/ferrocene are shown in Table 1, and on the basis of these potentials, the highest occupied molecular orbital (HOMO) energy levels are estimated17 and shown in Table 1. The slightly lower HOMO energy levels of 1b–c are in agreement with the electronwithdrawing effect of ethynyl groups. The reversible oxidation of 1b–c and 2a–c suggests that they may function as p-type semiconductors in the solid state. Table 1 Oxidation potentials, absorption, emission, and HOMO energy levels of 1b–c and 2a–c. Eoxa (V)

HOMOb (eV)

Egc (eV)

λAbs maxd (nm)

λEm maxe (nm)

1b

0.77

-5.87

2.48

491

499

1c

0.80

-5.90

2.48

493

501

2a

0.74

-5.84

2.63

466

472

2b

0.73

-5.83

2.64

466

468

2c

0.74

-5.84

2.63

466

468

a Half-wave potential versus ferrocenium/ferrocene. b Estimated from HOMO = –5.10 – Eox (eV). c Estimated from the absorption edge in the UV-vis absorption spectrum. d The longest wavelength absorption maximum as measured from a 1×10-5 M solution in CHCl3. e The shortest wavelength emission maximum as measured from a 1×10-5 M solution in CHCl3.

Reagents and conditions: (a) Pd(OAc)2, JohnPhos, Cs2CO3, THF, 130°C in a sealed tube. (b) concd HCl (aq), i-PrOH/CHCl3, 80 °C.

The optical properties of 1b–c and 2a–c were studied with UV-vis absorption spectroscopy and fluorescence spectroscopy, and their electrochemical properties were studied with cyclic voltammetry (CV). As shown in Figure S2a in the Supporting Information, 2a–c exhibited essentially the same absorption spectra with the longest-wavelength absorption maximum at 466 nm, which is blue shifted relative

Slow evaporation of solvents from solutions of 1b–c and 2a–c afforded yellow or orange crystals suitable for X-ray crystallography. The crystal structure of 1b obtained in this study is almost the same as that reported earlier.6 In the crystals, the nonacyclic backbones in 1b–c and 2a–c are all flat and essentially the same as those in the reported crystal structure of 1a.5 In particular, the four-membered rings in 1b– c and 2a–c exhibit two long C–C bonds of 1.49–1.51 Å and two slightly shorter C–C bonds of 1.44–1.45 Å. The fourmembered rings are bonded to four C atoms with C–C bond lengths (1.34–1.36 Å) very close to the typical bond lengths of C–C double bonds in alkenes (1.31–1.34 Å), presenting a radialene-like structure similar to that in [3]phenylene.18 With varied substituting groups, 1b–c and 2a–c in the crystals exhibit very different packing modes for their linearly shaped π-backbones as detailed below. In order to better understand the relationship between molecular packing and charge transport, we also calculated the transfer integral for holes (Vh) between adjacent molecules in the crystals of 1b–c and 2a–c. The transfer integral is a key factor that determines the charge

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Journal of the American Chemical Society transfer rate between neighboring molecules according to the Marcus theory of charge transfer,12, 19 and a larger transfer integral leads to higher drift mobility of charge carriers. Figure 2 shows the molecular packing of 1b in the crystals, which exhibits herringbone packing with π-π overlap between adjacent molecules.20 As shown in Figure 2a, the nonacyclic backbones of 1b stack along two directions with displacement along the long axis of π-backbone. The space between π-stacks of 1b is occupied by the triisopropyl groups. Within each stack, two adjacent π-planes overlap with about five rings (Figure 2b) and are separated by a distance of 3.47 Å (Figure 2a). For calculation of charge transfer integrals, three different dimers of 1b in the crystral structure are identified as D1, D2 and D3, which are shown with blue, red and green doubleheaded arrows in Figure 2a, respectively. The two stacked molecules in Figure 2b are dimer D1. The transfer integral for holes is calculated to be 50.04 meV for D1, 0.28 meV for D2 and 0.93 meV for D3. This indicates that the charge transport along the π-stack of 1b is dominant, while the charge transport between adjacent π-stacks is negligible.

stack, two adjacent π-planes are separated by a distance of 3.47 Å (Figure 3a) and overlap with about three and half rings (Figure 3b). In comparison to 1b, adjacent molecules of 1c exhibits larger displacement along the long axis of the πbackbone as a result of replacing methyl groups in 1b with hexyl groups. For calculation of charge transfer integrals, three different dimers of 1c in the crystral structure are identified as D1, D2 and D3, which are shown with blue, red and green double-headed arrows in Figure 3a, respectively. The two stacked molecules in Figure 3b are dimer D2. The transfer integral for holes is calculated to be 3.65 meV for D1, 21.02 meV for D2 and 0.39 meV for D3. This indicates that major charge transport in the crystal of 1c occurs along the direction of π-stacking, while minor charge transport occurs between adjacent π-stacks.

Figure 3 Molecular packing of 1c in the crystals: (a) 1D πstacking as viewed along the long axis of the polycyclic backbone; (b) two π-stacked molecules of 1c with the bottom molecule shown in light blue. (H atoms are removed for clarification, triisopropylsilylethynyl and hexyl groups in Figure 3a are shown as wires, and other atoms are shown as ellipsoids set at 50% probability.)

Figure 2 Molecular packing of 1b in the crystals: (a) herringbone packing with π-π overlap; (b) two π-stacked molecules of 1b with the bottom molecule shown in light blue. (H atoms are removed for clarification, triisopropylsilylethynyl and methyl groups in Figure a are shown as wires, and other atoms are shown as ellipsoids set at 50% probability.)

Figure 3a shows the molecular packing of 1c in the crystals, which exhibits one-dimensional (1D) π-stacking. Within each

As shown in Figure 4a, tetrabutyl derivative 2a exhibits 1D π-stacking with a π-to-π distance of 3.35 Å between two adjacent molecules. Figure 4b shows two π-stacked molecules of 2a, which exhibit apparent displacement along the short axis of the π-backbone. As a result, the π-π overlap mainly occurs between the edges of the two π-backbones. For calculation of charge transfer integrals, three different dimers of 2a in the crystral structure are identified as D1, D2 and D3, which are shown with blue, red and green double-headed arrows in Figure 4a, respectively. The two stacked molecules in Figure 4b are dimer D1. The transfer integral for holes is calculated to be 38.87 meV for D1, 0.20 meV for D2 and 0.01 meV for D3, respectively. This indicates a 1D nature of charge transport in the crystal of 2a with dominant charge transport



along the π-stack and negligible charge transport between adjacent π-stacks.

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are removed for clarification, hexyl groups in Figure 5a are shown as wires, and other atoms are shown as ellipsoids set at 50% probability.)

The crystal structure of 2b contains two crystallographically independent molecules, which are shown in grey and light blue in Figure 5. In contrast to 2a, tetrahexyl derivative 2b in the crystals exhibits an unusal type of two-dimensional (2D) π-stacking with a zigzag arrangement of the stacked πbackbones.21 Figure 5a shows 2D π-stacking of 2b with π-to-π distances of 3.54 Å and 3.67 Å between adjacent molecules. Figure 5b shows the zigzag arrangement of 2b, which results in overlap of about only one ring between two adjacent πplanes. For calculation of charge transfer integrals, three different dimers of 2b in the crystral structure are identified as D1, D2 and D3, which are shown with blue, red and green double-headed arrows in Figure 5a, respectively. The transfer integral for holes is calculated to be 21.92 meV for D1, 31.15 meV for D2 and 0.00 meV for D3. This indicates a 2D nature of charge transport in the crystal of 2b. Each molecule of 2b can effectively transport charge through face-to-face π-overlap to four adjacent molecules that are above or below it in the zigzag arrangement.

Figure 4 Molecular packing of 2a in the crystals: (a) 1D πstacking; (b) two π-stacked molecules of 2a with the bottom molecule shown in light blue. (H atoms are removed for clarification, butyl groups in Figure 4a are shown as wires, and other atoms are shown as ellipsoids set at 50% probability.)

Figure 5 Molecular packing of 2b in the crystals: (a) 2D πstacking; (b) zigzag arrangement of π-backbones as viewed along the a-axis of the unit cell. (The crystallographically independent molecules are shown in grey and light blue, respectively. H atoms

Figure 6 Molecular packing of 2c in the crystals: (a) 1D πstacking; (b) two π-stacked molecules of 2c with the bottom molecule shown in light blue. (H atoms are removed for


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Journal of the American Chemical Society clarification, octyl groups in Figure 6a are shown as wires, and other atoms are shown as ellipsoids set at 50% probability.)

Similar to 1b, tetraoctyl derivative 2c exhibits herringbone packing with π-π overlap between adjacent molecules as shown in Figure 6a. The π-backbones of 2c stack along two directions with a π-to-π distance of 3.50 Å between two adjacent molecules. Two adjacent π-stacks interact with each other through edge-to-face interactions, which involve carbonto-carbon contacts as short as 3.72 Å. Figure 6b shows two πstacked molecules of 2c, which are arranged in a way similar to that of 2a exhibiting apparent displacement along the short axis of the π-backbone. As a result, the π-π overlap mainly occurs between the edges of the two π-backbones. For calculation of charge transfer integrals, two different dimers of 2c in the crystral structure are identified as D1 and D2, which are shown with blue and red double-headed arrows in Figure 6a, respectively. The two stacked molecules in Figure 6b are dimer D1. The transfer integral for holes is calculated to be 49.33 meV for D1 and 6.94 meV for D2. This indicates that 2c is similar to 1c with the major charge transport along the direction of π-stacking and the minor charge transport between adjacent π-stacks. On the basis of the transfer integrals, hole mobilities of 1b– c and 2a–c were calculated with the tunneling effect 22 taken into account. Table 2 shows the calculated mobility along each channel as identified in the crystal structure of each compound. These mobilities indicate a 1D characteristic for hole transport in the crystals of 1b, 1c and 2a, where the transport pathway of holes is dominated by the direction along the 1D π-π stacking but is negligible in other channels. In a πstack, each molecule can only transfer holes to one molecule above itself and one molecule below itself. In contrast, the hole transport in the crystal of 2b has a 2D characteristic with high mobilities along two channels. In other words, each molecule in the 2D π-π stacking of 2b can transfer holes efficiently to four neighboring molecules (two above itself and two below itself). The hole transport in the crystal of 2c is considered as quasi-1D with the major pathway along the direction of π-π stacking and the minor pathway between the neighboring π-stacks through the edge-to-face interactions. To fabricate OTFTs, thin films of 1b-c and 2a–c were deposited by drop-casting their solutions in CHCl3 onto a highly doped silicon wafer substrate, which was inclined to induce alignment of crystallites in the films as shown in Figure 7a.23, 24 Before deposition of the organic films, the silicon wafer was coated with a layer of alumina to form the dielectric,25 and the surface of alumina was pre-treated with a self-assembled monolayer of 12cyclohexyldodecylphosphonic acid (CDPA), which provided an ordered dielectric surface wettable by common organic solvents.26 As observed from the polarized light micrographs (Figure S8 in the Supporting Information), the film of 1b consisted of small crystalline needles that lacked alignment, while the film of 1c consisted of well aligned stripes of hundreds of micrometer long. The different film morphologies of 1b and 1c are attributable to the fact that 1b is much less soluble than 1c. The drop-cast films of 2a–c had similar appearance under the polarized light microscope and all consisted of roughly aligned fibers. Fabrication of top-contact OTFTs was completed by vacuum deposition of gold on the

films of 1b–c and 2a–c through a shadow mask to form topcontact source and drain electrodes. In particular, the shadow mask was placed on the films of 1c and 2a–c in a way that resulted in conduction channels roughly along the preferential growth direction of the ribbons or fibers as shown in Figure S8 (Supporting Information).

Figure 7 (a) Schematic drawing for the process of drop casting organic thin films; (b) reflection polarized light micrograph for a drop-casted film of 2b; (c) drain current (ID) versus gate-source voltage (VGS) with drain-source voltage (VDS) at −5 V for an OTFT of 2b with an active channel of W = 1 mm and L = 180 μm as measured in air.

Table 2. Transfer integrals for holes, calculated mobilities and measured mobilities of 1b–c and 2a–c.

1b

1c

2a

2b

2c

Channel

Transfer Integral (meV) a

Calculated Mobility b (cm2/Vs)

D1

50.04

29.24

D2

0.28

0.003

D3

0.93

0.028

D1

3.65

0.29

D2

21.02

14.58

D3

0.39

0.012

D1

38.87

2.90

D2

0.20

0.0005

D3

0.01

0.00

D1

21.92

7.88

D2

31.15

20.41

D3

0.00

0.00

D1

49.33

13.99

D2

6.94

2.85

Measured Mobility c (cm2/Vs) (2.4 ± 1.5)×10−4 highest: 5.3×10−4 (6.0 ± 2.4)×10−3 highest: 1.4×10−2 (2.8 ± 1.8)×10−2 highest: 8.7×10−2 1.2 ± 0.5 highest: 2.9 (8.6 ± 3.9)×10−3 highest: 1.9×10−2

a

Transfer integrals for hole were calculated at the PW91/TZP level. b The computational methods are detailed in the Supporting



Information. c Data collected from at least 30 channels on 7 independent substrates for each compound in air.

As measured in ambient air, 1b–c and 2a–c all behaved as p-type semiconductors in the transistors. Figure 7c shows the typical transfer I-V curves measured from the best performing transistors of 2b, and Figure S15 (Supporting Information) shows the typical transfer I-V curves measured from the best performing transistors of the other four compounds. From the transfer I-V curve in the saturation regime at a drain voltage of −5 V, the field effect mobility was extracted using the equation: ID = (μWCi/2L)(VGS–VT)2, where ID is the drain current, μ is field effect mobility, Ci is the capacitance per unit area for the CDPA-modified alumina measured as 134 ± 14 nF/cm2,27 W is the channel width, L is the channel length, and VGS and VT are the gate-source and threshold voltage, respectively. The field effect mobilities of 1b–c and 2a–c are summarized in Table 2. The field effect mobility of 2b extracted from the transfer I-V curve shown in Figure 7c is as high as 2.9 cm2/ Vs, while the field effect mobilities of other four compounds are lower than 2b by one to three orders of magnitude. As shown in Figure 7c, the transistor of 2b exhibited a poor on/off ratio of 103, which is a result of relatively low on-current and high off-current. The low oncurrent can be attributed to the high threshold voltage (−3.85 V), which is in agreement with the low HOMO energy level of 2b (−5.83 eV). The high off-current can be attributed to the dielectric layer, which was tens of nanometer thick to achieve low voltage operation but suffered from a leakage current at the level of nA as shown in Figure S16 in the Supporting Information. In order to understand the measured field effect mobilities of 1b–c and 2a–c in relation to the solid state structures, the films were studied with X-ray diffraction and atomic force microscopy (AFM). The out-of-plane X-ray diffractions (Figure S11 in the Supporting Information) from the films of 1b and 2a exhibit one very small peak, while those from the films of 1c, 2b and 2c exhibit one strong peak in accompany with one or two higher order peaks. The strong diffractions from the films of 1c, 2b and 2c can be indexed as the (001), (010) and (101) diffractions according to the corresponding single crystal structures, respectively. This indicates that the crystallites in these films have the corresponding crystallographic plane parallel to the substrate surface. On the other hand, the very weak diffractions from the films of 1b and 2a indicate poor ordering along the direction perpendicular to the substrate surface as a result of randomly oriented crystallites or amorphous domains in these films. The AFM images (Figure S9 in the Supporting Information) show that the film of 1b consists of sub-micrometer scaled rods, while the film of 1c consists of very smooth stripes of more than 2 μm wide. The AFM image from the film of 2a shows micrometer-sized domains shaped like seaweed, while the AFM images from the films of 2b and 2c reveal irregular domains of smaller size (hundreds of nanometer). This indicates that the fibers of 2a–c in their polarized light micrographs in fact consist of smaller domains lacking good alignment. The films of 1c and 2b were further studied with grazing incidence wide-angle X-ray scattering (GIWAXS). Figure S12a in the Supporting Information shows the GIWAXS patterns of 1c, which was obtained when the

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incident beam was along the growth direction of the crystal stripes. These peaks, which can be grouped into two sets, indicate that the film of 1c has high in-plane ordering but the molecules of 1c in different crystal stripes are not aligned along the same direction. Careful analysis of these diffraction peaks reveals three directions of growth in the crystal stripes of 1c as shown in Figure S13 in the Supporting Information. In contrast, the GIWAXS pattern of 2b (Figure S12b in the Supporting Information) exhibits (011) and (01−1) diffraction rings, which indicate a random in-plane orientation. The (210) diffraction of 2b occurs near the x axis indicating that the (210) crystallographic plane is almost perpendicular to the substrate surface. This is in agreement with the fact that the (010) plane of 2b is parallel to the substrate surface since the (010) and (210) planes form an angle of 85.5°. On the basis of the above observations from the X-ray diffraction and AFM, it can be concluded that the measured mobilities are averaged results from charge transport along different directions within individual domains and crossing grain boundaries. As a result, the measured mobilities are lower than the maximum calculated mobilities. The low mobilities of 1b and 2a can mainly be attributed to the randomly oriented domains in their films, and the low mobility of 2c can mainly be attributed to the large amount of deep grain boundaries as found from the AFM (Figure S9 in the Supporting Information). When the (010) plane of 2b is parallel with the substrate surface, the π-plane of 2b forms an angle of 80.8° with the substrate surface (Figure S14 in the Supporting Information) and the channels D1 and D2 are both parallel with the substrate. Therefore, the high mobility of 2b benefits from the favorable edge-on orientation of molecules on the dielectric surface and the 2D nature of charge transport. On the other hand, the low mobility of 1c can be explained with the fact that molecules of 1c in the crystal stripes are oriented along three different directions and only one direction is along the direction of π-stacking (Channel D2 in Figure 3) with high calculated mobility.

CONCLUSION In conclusion, we demonstrate for the first time that biphenylene-containing analogues of acene function as highmobility p-type organic semiconductors in solution-processed OTFTs. The molecular packing of BNCBA in the solid state is largely modified by attaching alkyl groups of different lengths to the linear π-backbone. The variation of molecular packing also leads to change of charge transport characteristics as found from the theoretical calculation of mobility on the basis of single crystal structures. Having 2D π-π stacking with an unusual zigzag arrangement, tetrahexyl derivative 2b exhibits field effect mobility as high as 2.9 cm2/Vs. This study suggests that biphenylene-containing analogues of acene are promising candidates for high-mobility semiconductors.

ASSOCIATED CONTENT Supporting Information including synthesis and characterization of 1c and 2a–c, preparation and characterization of thin films, transistors, details of computational study, CIF files for 1b–c and 2a–c. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION


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Journal of the American Chemical Society Corresponding Author *E-mail: [email protected]

Notes The authors declare no competing financial interest

ACKNOWLEDGMENT

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We thank Ms. Hoi Shan Chan (the Chinese University of Hong Kong) for the single crystal crystallography. This work was supported by the Research Grants Council of Hong Kong (GRF 14300217), the University Grants Committee of Hong Kong (project number: AoE/P-03/08) and the Natural Science Foundation of China (No. 21473071).

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Crystal Engineering of Biphenylene-Containing Acenes for High

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