New Donor–Acceptor–Donor Molecules with Pechmann Dye as the

Jan 7, 2013 - (1-3) This is simply due to their promising applications in low-cost, large-area, and flexible electronic devices, such as radio frequen...
1 downloads 11 Views 442KB Size
Download PDF
Article pubs.acs.org/cm

New Donor−Acceptor−Donor Molecules with Pechmann Dye as the Core Moiety for Solution-Processed Good-Performance Organic Field-Effect Transistors Zhengxu Cai, Yunlong Guo, Sifen Yang, Qian Peng, Hewei Luo, Zitong Liu,* Guanxin Zhang,* Yunqi Liu, and Deqing Zhang* Beijing National Laboratory for Molecular Sciences, Organic Solids Laboratory, Institute of Chemistry, Chinese Academy of Sciences (CAS), Beijing 100190, People’s Republic of China S Supporting Information *

ABSTRACT: In this paper, we report the synthesis and characterization of two new DA-D molecules (E)-5,5′-bis(5-(benzo[b]thiophen-2-yl)thiophen-2-yl)-1,1′-bis(2-ethylhexyl)-[3,3′-bipyrrolylidene]-2,2′(1H,1′H)-dione (BTBPD) and (E)-5,5′-bis- (5(benzo[b]furan-2-yl)thiophen-2-yl)-1,1′-bis(2-ethylhexyl)-[3,3′-bipyrrolylidene]2,2′(1H,1′H)-dione (BFBPD). They entail bipyrrolylidene-2,2′(1H,1′H)-dione (BPD, known as Pechmann dye) as the electron-accepting core that is flanked by two benzo[b]thiophene moieties and two benzo[b]furan moieties, respectively. Crystal structures of BTBPD and BFBPD provide solid evidence for the intermolecular donor−acceptor (D-A) interactions, which are favorable for improving charge transport performance. Organic field-effect transistors (OFETs) were prepared based on thin films of BTBPD and BFBPD through solution-processed technique. OFETs of BTBPD exhibit relatively high hole mobility up to 1.4 cm2 V−1 s−1 with high on/off ratio up to 106. In comparison, the hole mobility of OFETs with BFBPD (0.14 cm2 V−1 s−1) is relatively low, because of the poor thin-film morphology and low molecular ordering, even after annealing. Thin-film morphological and XRD studies were carried out to understand the variation of hole mobilities after annealing at different temperatures. The present studies clearly demonstrate the potentials of BPD that is planar and polar as the electron-acceptor moiety to build D-A molecules for organic semiconductors with good performance. KEYWORDS: donor−acceptor−donor molecules, Pechmann dye, intermolecular donor−acceptor interactions, organic field-effect transistors, hole-mobility

■

INTRODUCTION Solution-processed organic semiconductors and the resulting thin film organic field-effect transistors (OFETs) have received tremendous attention in recent years.1−3 This is simply due to their promising applications in low-cost, large-area, and flexible electronic devices, such as radio frequency identification (RFID) tags, smart cards, electronic papers, displayers, and sensors.4,5 So far, many organic semiconductors and resulting OFETs with high carrier mobility and good stability have been reported.6−11 However, new organic semiconductors of even better performances are highly desirable in order to realize their practical applications in flexible electronic devices. Recent studies demonstrate that appropriate connection of donor− acceptor (D-A) moieties is a viable approach to create new organic semiconductors of high performances by taking advantage of intermolecular donor and acceptor interactions.12−14 Both small molecules and macromolecules with DA frameworks are reported, and most of them exhibit impressive field-effect mobilities and even ambipolar semiconducting behaviors.15−17 Macromolecules with alternating D and A moieties usually outperform the respective small D−A molecules, in terms of carrier mobility.18,19 However, © 2013 American Chemical Society

conjugated macromolecules suffer batch-to-batch variation in terms of molecular weight and polydispersity that influence on the processability, microstructure, and, thus, semiconducting performances.20,21 In comparison, solution-processed small molecules offer several advantages over conjugated polymers, including the ease of purification, functionalization, and reproducibility.22−24 Therefore, small D-A molecules deserve further attention for new semiconductors and high-performance OFETs, as well as solar cells.25−27 Electron acceptors, which are utilized for construction of conjugated D-A molecules, include rylene diimides (e.g., naphthalene diimide and perylene diimide),28,29 benzothiadiazaole,9c,30 benzobisthiadiazole,31 isoindigo32,33 and diketopyrrolopyrrole.34−40 The outstanding semiconducting properties of these D-A molecules are highly associated with the unique electronic and structural features including at least (1) they entail planar electron-accepting moieties that are beneficial for intermolecular interactions, and (2) alkyl chains can be Received: November 23, 2012 Revised: January 2, 2013 Published: January 7, 2013 471

dx.doi.org/10.1021/cm303793g | Chem. Mater. 2013, 25, 471−478

Chemistry of Materials

Article

elemental analysis (see the Experimental Section). As expected, both BTBPD and BFBPD can be dissolved in common organic solvents such as CH2 Cl2 , CHCl3 , THF, and toluene. Thermogravimetric analysis (TGA) reveals that BTBPD and BFBPD are thermally stable below 250 °C (see Figure S1 in the Supporting Information). Figure 1a shows the cyclic voltammograms of BTBPD and BFBPD. One oxidation wave (with a shoulder peak) and two

introduced to improve the solubility and tuning intermolecular interactions.35 In this article, we report the successful application of bipyrrolylidene-2,2′(1H,1′H)-dione (BPD) as electron acceptor to construct D-A-D molecules for new organic semiconductors of good performances. BPD, which is known as Pechmann dye and was first synthesized by Hans von Pechmann in 1882,41,42 is a planar and polar electron acceptor moiety. Thus, incorporation of BPD into D-A molecules will be favorable for intermolecular interactions (donor−acceptor and π−π interactions). Alkyl chains can be introduced to amide groups to promote solubility and electron donors can be connected via various coupling reactions. Moreover, BPD possesses narrow band gap and absorbs strongly in the region of 450−720 nm.43 Therefore, it is anticipated that a platform of D-A molecules with BPD as the electron-accepting core can be established for creating new organic semiconductors for optoelectronic devices. Herein, we report two new D-A-D molecules (BTBPD and BFBPD; see Scheme 1) with BPD Scheme 1. Chemical Structures of BTBPD and BFBPD and Their Synthetic Routesa

a (i) 1: 2-ethylhexan-1-amine, CH2Cl2, room temperature (RT), 12 h; 2: HCl, 10%; (ii) lithium diisopropylamide, −78 °C, 30 min; (iii) 1,2dibromotetrachloroethane, −78 °C, 60 min 46%; (iv) 2tributylstannylbenzo[b]furan or 2-tributylstannylbenzo[b]thiophene, Pd2(dba)3, P(o-tol)3, THF, reflux, 1.5 h, 87% for BTBPD and 90% for BFBPD.

Figure 1. (a) Cyclic voltammograms of BTBPD (1.0 × 10−3 M) and BFBPD (1.0 × 10−3 M) in CH2Cl2 at a scan rate of 50 mV s−1, with Pt as the working and counter electrodes and an Ag/AgCl electrode (saturated KCl) as the reference electrode, and n-Bu4NPF6 (0.1 M) as supporting electrolyte; (b) normalized absorption spectra for solutions of BTBPD (1.0 × 10−5 M in CH2Cl2) and BFBPD (1.0 × 10−5 M in CH2Cl2) and their thin films.

core flanked by two electron-donating moieties (benzo[b]thiophen and benzo[b]furan, respectively). The results reveal that both BTBPD and BFBPD can be solution-processed and the thin-film OFETs exhibit relatively high mobilities up to 1.4 cm2 V−1 s−1 with an on/off ratio of 106. In addition, X-ray crystal structures of BTBPD and BFBPD provide solid evidence for intermolecular aromatic donor and acceptor interactions, which is beneficial for carrier transport.

reduction waves were observed for BTBPD and BFBPD. The redox potentials are listed in Table 1.45 Based on the respective onset oxidation and reduction potentials of BTBPD and BFBPD, their HOMO and LUMO energies as well as band gaps were estimated (see Table 1). Interestingly, both compounds possess the same HOMO energies (−5.0 eV), which are close to the work function of gold. This is indeed in agreement with the theoretical calculations (see the Supporting Information), which indicate that both HOMO and LUMO orbitals of BTBPD and BFBPD are mainly distributed on the central part of the molecules and electron-donating moieties of benzo[b]thiophen and benzo[b]furan make rather small contributions to the HOMO. This is probably due to the fact that the central BPD core is not coplanar with two benzo[b]thiophene moieties in BTBPD and two benzo[b]furan moieties in BFBPD, respectively; thus, overlaps among these moieties with BPD cores are weakened. BTBPD and BFBPD exhibit two absorption bands in the region of 330−900 nm, as depicted in Figure 1b. Compared to

■

RESULTS AND DISCUSSION Synthesis, Characterization, and HOMO/LUMO Energies. The synthesis of BTBPD and BFBPD was shown in Scheme 1. The synthesis started from compound 1, which was prepared according to the reported procedures.43 Reaction of 1 with 2-ethylhexan-1-amine yielded 2 in 15% yield.44 Treatment of 2 with lithium diisopropylamide (LDA) at −78 °C, followed by the addition of 1,2-dibromo-tetrachloroethane led to 3 in 46% yield. Finally, Stille-coupling of 3 with 2tributylstannylbenzo[b]thiophene and 2-tributylstannylbenzo[b]furan afforded dark metallic-lust solids BTBPD and BFBPD in 87% and 90% yields, respectively, after purification. The chemical structures of BTBPD and BFBPD were characterized with NMR and MS data, and their purities were checked with 472

dx.doi.org/10.1021/cm303793g | Chem. Mater. 2013, 25, 471−478

Chemistry of Materials

Article

Table 1. Absorption Maxima, Redox Potentials, HOMO/LUMO Energies, and Band Gaps of BTBPD and BFBPD λmax (nm) compound BTBPD BFBPD

HOMO (eV) film

solutiona b

304, 392, 666 (53000) 304, 396, 666 (40000)b

308, 404, 696 306, 400, 682

Eox11/2 (V) c

0.73 (0.75) 0.77 (0.75)c

LUMO (eV)

Ered1/2 (V)

exp.d

calc.e

exp.d

calc.e

band gap (eV)

−0.82 −1.26 −0.80−1.29

−5.0 −5.0

−4.7 −4.7

−3.7 −3.8

−2.9 −2.8

1.4f (1.3)g 1.4f (1.2)g

Measured in CH2Cl2 solutions of BTBPD (1.0 × 10−5 M) and BFBPD (1.0 × 10−5 M). bMolar extinction coefficient (εmax, M−1 cm−1). cShoulder oxidation peaks. dBased on the respective onset oxidation and reduction potentials of BTBPD and BFBPD with the following equations: LUMO = e f g ox −(Ered onset + 4.4) eV, HOMO = −(Eonset + 4.4) eV. Based on the DFT calculations. Based on the absorption spectral data. Based on redox potentials.

a

those in solutions, the absorption spectra of the thin films were red-shifted, albeit slightly. For instance, the absorption band at 666 nm in solution was red-shifted to 696 nm for the thin film of BTBPD. Such absorption spectral shifts should be due to the intermolecular interactions within thin films which may entail the electron donor and acceptor interactions. Based on the absorption spectra of their thin films, the band gaps of BTBPD and BFBPD were estimated to be 1.4 eV. These are in good agreement with those obtained with cyclic voltammetric data, as mentioned above (see Table 1). Crystal Structures. Crystal structures of BTBPD and BFBPD were determined and the crystallographic data are provided in the Supporting Information. Single crystals were grown by slow diffusion of MeOH into the concentrated solutions of both compounds in CHCl3. Both compounds belong to the triclinic system with different unit-cell parameters. BPD is symmetric around the central CC double bond, which exhibits trans-configuration (see Table 2).46 The BPD core is planar. This is probably due to the

Moreover, the electron-accepting BPD cores and the electron-donating benzo[b]thiophene moieties of neighboring layers are closely overlapped with a short distance of 3.336(5) Å (from S2 to BPD plane). The alkyl chains are interdigitated around the molecular columns formed through stacking of molecular layers via intermolecular aromatic D-A interactions. According to previous studies,6,24 such intermolecular donor and acceptor packing is beneficial for intermolecular ordering and close packing, thus improving carrier mobilities of organic semiconductors. Theoretical and solid-state MAS NMR studies were carried out to identify the intermolecular D-A interactions within certain D-A molecules.9,32 However, such intermolecular electron D-A interactions were not clearly confirmed by X-ray crystal structures previously for D-A conjugated molecules, to the best of our knowledge.38,47 BFBPD shows similar molecular structure as depicted in Table 3.48 Again, intramolecular weak hydrogen bonds exist Table 3. Molecular Structure and Intermolecular Arrangements for BFBPD (Alkyl Side Chains Were Omitted for the Sake of Clarity)a

Table 2. Molecular Structure and Intermolecular Arrangements for BTBPD (Alkyl Side Chains Were Omitted for the Sake of Clarity)a

a

Crystal data: C48H5N2O4S2, M = 783.02, orthorhombic, a = 9.5525(19) Å, b = 9.7452(19) Å, c = 12.660(3) Å, U = 1007.6 (3) Å3, T = 173(2) K, space group P1̅, Z = 1, 8240 reflections measured, 3944 unique (Rint = 0.0527), which were used in all calculations. The final wR(F2) was 0.2393 (all data). The deviation of interatomic distances was not indicated.

a

Crystal data: C48H50N2O2S4, M = 815.14, orthorhombic, a = 9.6290(19) Å, b = 9.900(2) Å, c = 12.559 (3) Å, U = 1026.2(4) Å3, T = 173(2) K, space group P1,̅ Z = 1, 7735 reflections measured, 3593 unique (Rint = 0.0676), which were used in all calculations. The final wR(F2) was 0.2348 (all data). The deviation of interatomic distances was not indicated.

between the O atoms of imide groups and the neighboring C− H groups. The central BPD core is planar, but conjugated moieties in BFBPD are not fully coplanar; the thiophene rings form dihedral angles of 18.96 o and 20.58 o with BPD core and the benzo[b]furan rings, respectively. BFBPD also shows similar intermolecular arrangements as depicted in Table 3. The interatomic contact (4.338(5) Å) of O atoms of neighboring benzo[b]furan rings is relatively long. Importantly, the BPD cores are interacted with benzo[b]furan moieties of adjacent layers with a separation of 3.321(5) Å (from O1 to the BPD plane).

intramolecular weak hydrogen bonds between the O atoms of imide groups and the neighboring C−H groups, as indicated in Table 2. However, conjugated moieties in BTBPD are not fully coplanar; the thiophene rings form dihedral angles of 23.56° and 27.88° with BPD core and the benzo[b]thiophene rings, respectively. Intermolecular arrangement of BTBPD is also depicted in Table 2. Molecules are assembled to form layers via intermolecular interactions (e.g., S···S short contacts, 3.522(5) Å). 473

dx.doi.org/10.1021/cm303793g | Chem. Mater. 2013, 25, 471−478

Chemistry of Materials

Article

Performances of Thin-Film OFETs. Bottom-gate/bottomcontact OFETs with thin films of BTBPD and BFBPD were fabricated with conventional techniques (see the Experimental Section). The device performances were measured under ambient condition and listed in Table 4. Both thin films of Table 4. Hole Mobilities (μh), Threshold Voltages (Vth), and Current On/Off Ratios (Ion/off) for Bottom Contact OFET Devices Based on Thin Films of BTBPD and BFBPD with OTS-Modified Si/SiO2 Substrate and Unmodified Gold as Electrodes at Different Annealing Temperaturesa temperature (°C) 25 80 100 120 140 25 80 100 120 140

μh (cm2 V−1 s−1) BTBPD 7.6−8.7 × 10−6 5.8−8.0 × 10−3 3.0−4.5 × 10−3 0.7−1.1 (1.4)b 6.0−8.4 × 10−3 BFBPD 3.7−7.9 × 10−3 0.03−0.09 0.05−0.14 0.05−0.08 7.8−9.6 × 10−3

Vth (V)

Ion/off

0−3 6−7 4−5 2−7 3−4

102 104 105 4 10 −106 105

0−2 5−10 8−15 5−9 3−4

105 10 −105 104 4 10 −105 104−105 4

a

The data were obtained based on more than 10 OFET devices (see the Supporting Information). bWith pentafluorobenzenethiol-modified gold as the source and drain electrodes.

BTBPD and BFBPD behave as p-type semiconductors. As an example, Figure 2 shows the output and transfer characteristics of the thin-film OFET for BTBPD after annealing at 120 °C. The as-prepared OFETs exhibited low mobility (8.7 × 10−6 cm2 V−1 s−1) and also low on/off ratio (102). Both the hole mobility and the on/off ratio increased after annealing thin films at different temperatures (see Figure S3 and Table S3 in the Supporting Information). Interestingly, OFETs of BTBPD exhibited relatively high hole mobility (up to 1.1 cm2 V−1 s−1) with high on/off ratio (up to 106) after just annealing at 120 °C for only 1.0 h.49 Moreover, the hole mobility could be further enhanced to 1.4 cm2 V−1 s−1 by employing pentafluorobenzenethiol-modified gold as the source and drain electrodes.50 OFETs of BFBPD were also investigated at different annealing temperatures, and the respective hole mobilities and on/off ratio are summarized in Table 4 and Figure 2. Similarly, hole mobilities increased after annealing thin films at different temperatures. The hole mobility increased from 7.9 × 10−3 cm2 V−1 s−1 to 0.09 cm2 V−1 s−1 after annealing at 80 °C for 1.0 h, and it reached 0.14 cm2 V−1 s−1 after further increasing the annealing temperature to 100 °C (see Figure S3 and Table S3 in the Supporting Information).51 However, the hole mobility decreased by further increasing the annealing temperatures.52 The chemical structures of two compounds are different just in two atoms; moreover, they show the same HOMO energies and similar crystal structures. However, OFETs of BFBPD exhibit relatively low hole mobilities, compared to those of BTBPD under the same conditions. This is probably related to the thin-film morphology of BTBPD, as discussed below. In addition, the stability of both OFET devices was examined in air with an average humidity of 20% for 20 days. As depicted in Figure S6 in the Supporting Information, OFETs of BTBPD and BFBPD exhibit good environmental stability.

Figure 2. (a) Output and (b) transfer characteristics of OFET with a thin film of BFBPD after annealing at 100 °C; (c) output and (d) transfer characteristics of OFET with a thin film of BTBPD after annealing at 120 °C (the transistor channel width and channel length were 1400 and 50 μm, respectively).

Thin-Film Morphology and XRD Studies. In order to understand the variation of the performances of OFETs of 474

dx.doi.org/10.1021/cm303793g | Chem. Mater. 2013, 25, 471−478

Chemistry of Materials

Article

BTBPD (see Figure S7 in the Supporting Information). Therefore, it may be concluded that intermolecular packing within the thin film of BTBPD after annealing at 120 °C is similar to that within the single crystal. Accordingly, intermolecular aromatic D-A interactions, as well as side-byside S···S short contacts, may also exist within the thin film of BTBPD after annealing at 120 °C. This may explain the good performance of OFETs after annealing at 120 °C, as discussed above. Figure 4 shows the AFM and optical microscopic images for the as-prepared thin film of BFBPD and the same film after

BTBPD and BFBPD after annealing, the respective thin-film morphology and XRD studies were performed. Figure 3 shows

Figure 3. AFM images, optical microscopic images, and XRD patterns for thin films of BTBPD after annealing at different temperatures.

the AFM images of thin films of BTBPD after annealing at different temperatures. Small domains with size of ∼100 nm were distributed within the as-prepared thin film of BTBPD. The domains were enlarged after annealing at 80 and 100 °C, but they were not interconnected. However, the thin film became more uniform after annealing at 120 °C. Alternatively, optical microscopic images (see Figure 3) also indicated the existence of small domains for the as-prepared thin film of BTBPD and the emergence of large domains after annealing at 120 °C. Correspondingly, the number and intensity of XRD peaks increased after annealing, as depicted in Figure 3. No diffraction peaks were detected for the as-prepared thin film, and weak diffraction peaks at 6.3°, 7.2°, 12.7°, 19.1°, and 21.6° emerged after annealing at 80 °C and the intensities for those at 7.2°, 14.6°, and 21.6° increased after the annealing temperature increased to 100 °C. Interestingly, the diffraction peaks at 7.2° and 21.6° were largely enhanced and that at 14.6° increased slightly. These XRD studies clearly manifest the enhancement of molecular ordering within thin films of BTBPD after annealing. The formation of continuous large domains and enhancement of molecular ordering are in agreement with the observation that hole mobilities of BTBPD increase with annealing (see Table 4). Close examination of the XRD pattern of a thin film of BTBPD after annealing at 120 °C may shed light on the intermolecular arrangements on the substrates (OTS-modified SiO2/Si). The appearance of diffraction peaks at 7.2°, 14.6°, and 21.6° manifests that a thin film of BTBPD adopts the typical lamellar structure after annealing. The first-order diffraction peak appears at 7.2°, corresponding to a d-spacing of 12.3 Å. The length of BTBPD along the short (ethyl) branched alkyl chains is 14.056 Å, which is close to the firstorder d-spacing. By considering the compatibility of the alkyl chains of BTBPD with OTS, the BTBPD molecules may be slightly inclined with alkyl chains onto the surface of the substrate. On the other hand, these three diffraction peaks correspond well to those of the XRD pattern of single-crystal

Figure 4. AFM images, optical microscopic images, and XRD patterns for thin films of BFBPD after annealing at different temperatures.

annealing at different temperatures. The as-prepared thin film entailed individual small domains of ∼100 nm, which were gradually enlarged after annealing at 80 and 100 °C. However, the domains were still not interconnected. The thin film of BFBPD became nonuniform with noncontinuous domains of different sizes after annealing at 120 °C. The optical microscopic studies yielded similar morphological changes for the thin film of BFBPD, as depicted in Figure 4. The asprepared thin film of BFBPD exhibited only a weak XRD peak at 5.7°. The same held true after annealing at 80 °C. The intensity of the peak at 5.7° increased and additional two weak diffraction peaks, at 11.7° and 17.4°, emerged after annealing at 100 °C. Further annealing at 120 °C led to the intensity reduction for the diffraction peak at 5.7°. These results reveal the poor thin-film morphology and weak molecular ordering within the thin film of BFBPD, even after annealing. This agrees well with the low hole mobility of the thin film of BFBPD, compared to that of BFBPD, under similar conditions. Although HOMO energies and crystal structures of both compounds are not largely affected by the respective two benzo[b]thiophene moieties in BTBPD and two benzo[b]furan moieties in BFBPD, they do influence the morphology and molecular ordering within their thin films, which were prepared with a spin-coating technique. The performance of OFETs of BFBPD may be improved by optimization of the interfaces between materials and substrates. 475

dx.doi.org/10.1021/cm303793g | Chem. Mater. 2013, 25, 471−478

Chemistry of Materials

■

Article

and dried over sodium sulfate. The residue was purified by column chromatography with the mixture of petroleum ether (60−90 °C) and CH2Cl2 (4:1, v/v) as the eluent. Compound 2 was obtained as a dark blue solid (102 mg, 15%). Mp: 93.3−94.5 °C. 1H NMR (300 MHz, CDCl3): δ 7.47 (d, 2H, J = 5 Hz), 7.43 (d, 2H, J = 3.5 Hz), 7.13 (dd, 2H, J = 3.9 Hz, J = 5.0 Hz), 7.09 (s, 2H), 3.79 (m, 4H), 1.60 (m, 2H), 1.20 (m,16H), 0.84−0.79 (m,12H); 13C NMR (75 MHz, CDCl3): δ 171.0, 145.2, 133.3, 128.3, 128.1, 128.0, 127.9, 103.5, 44.8, 38.9, 30.3, 28.4, 23.6, 23.0, 14.0, 10.5. MALDI-TOF: 551.3(M+H+), 573.3 (M +Na+). Elemental analysis: Calcd. for C32H42N2O2S2: C, 69.78; H, 7.69; N, 5.09; S, 11.64; Found: C, 69.80; H, 7.78; N, 4.96; S, 11.64. Synthesis of Compound 3. Lithium diisopropylamide (1.25 mL, 1.8 M solution in THF/n-heptanes/ethylbenzene) was added to a solution of compound 2 (490 mg, 0.89 mmol) in dry THF (50 mL) at −78 °C. The resulting solution was stirred for 30 min under nitrogen and then 1,2-dibromotetrachloroethane (900 mg, 2.76 mmol) in THF (5 mL) was added dropwise. After stirred for additional 1.0 h, 10 mL of water was added to the reaction mixture that was extracted with CH2Cl2 (3 × 50 mL). The organic layer was washed with saturated brine, dried over sodium sulfate, and evaporated to dryness. The residue was purified by column chromatography with petroleum ether (60−90 °C) and CH2Cl2 (4:1, v/v) as the eluent. Compound 2 was obtained as a dark blue solid (290 mg, 46%). Mp: 117.2−118.5 °C. 1H NMR (300 MHz, CDCl3): δ 7.17 (d, 2H, J = 3.9 Hz), 7.08 (d, 2H, J = 3.9 Hz), 7.02 (s, 2H), 3.80−3.66 (m, 4H), 1.60 (m, 2H), 1.28−1.20 (m, 16H), 0.85−0.80 (m, 12H); 13C NMR (75 MHz, CDCl3): δ 170.7, 144.3, 134.7, 131.1, 128.2, 127.9, 115.8, 103.7, 44.8, 38.9, 30.3, 28.4, 23.6, 23.0, 14.0, 10.5. MALDI-TOF: 709.0 (M+H+). Elemental analysis: Calcd. for C32H40Br2N2O2S2: C, 54.24; H, 5.96; N, 3.95; S, 9.05; Found: C, 54.28; H, 5.82; N, 3.91; S, 8.94. Synthesis of BTBPD. To the solution of compound 3 (100 mg, 0.14 mmol) and 2-tributylstannylbenzo[b]thiophene (278 mg, 0.66 mmol) in 50 mL of anhydrous tetrahydrofuran bubbled with nitrogen, tris(dibenzylideneacetone)dipalladium(0) (Pd2(dba)3) (3.0 mg) and tri(o-tolyl)phosphine (P(o-tol)3) (3.0 mg) were added in one portion. The mixture was stirred for 1.5 h at 60 °C under nitrogen. The mixture then was cooled to room temperature and poured into water. The organic phase was extracted with CHCl3 (3 × 50 mL). The organic layer then was washed with saturated brine, dried over sodium sulfate, and evaporated to dryness. The residue was purified by column chromatography with petroleum ether (60−90 °C) and CH2Cl2 (2:1, v/v) as the eluent. BTBPD was obtained as a dark blue solid (100 mg, 87%). Mp: 208.4−208.8 °C. 1H NMR (300 MHz, CDCl3): δ 7.82− 7.76 (m, 4H), 7.47 (m, 4H), 7.42−7.31 (m, 6H), 7.21 (s, 2H), 3.93− 3.80 (m, 4H), 1.73 (m, 2H), 1.31−1.27 (m, 16H), 0.90−0.85 (m, 12H); 13C NMR (150 MHz, CDCl3) δ 171.0, 144.4, 140.2, 139.5, 136.1, 132.9, 128.9, 127.5, 125.9, 125.1, 125.0, 123.8, 122.2, 121.0, 104.1, 45.0, 39.1, 30.4, 28.6, 23.7, 23.1, 14.1, 10.6. MALDI-TOF: 814.3 (M+), 837.4 (M+Na+). Elemental analysis: Calcd. for C48H50N2O2S4: C, 70.72; H, 6.18; N, 3.44, S, 15.73; Found: C, 70.45; H, 6.10; N,3.46, S, 15.98. Synthesis of BFBPD. To the solution of compound 3 (100 mg, 0.14 mmol) and 2-tributylstannylbenzo[b]furan (230 mg, 0.66 mmol) in 50 mL of anhydrous tetrahydrofuran bubbled with nitrogen, tris(dibenzylideneacetone)dipalladium(0) (Pd2(dba)3) (3.0 mg) and tri(o-tolyl)phosphine (P(o-tol)3) (3.0 mg) were added in one portion. The mixture was stirred for 1.5 h at 60 °C under nitrogen. Then, the mixture was cooled down to room temperature and poured into water. The organic phase was extracted by CHCl3 (3 × 50 mL); The organic layer then was washed with saturated brine, dried over sodium sulfate, and evaporated to dryness. The residue was purified by column chromatography with petroleum ether (60−90 °C) and CH2Cl2 (2:1, v/v) as the eluent. BFBPD was obtained as a dark blue solid (98 mg, 90%). Mp: 177.2−178.0 °C. 1H NMR (300 MHz, CDCl3): δ 7.59 (d, 2H, J = 7.5 Hz), 7.53−7.49 (m, 6H), 7.32−7.28 (m, 4H), 7.23 (m, 2H), 6.97 (s, 2H), 3.89−3.87 (m, 4H), 1.72 (m, 2H), 1.35−1.26 (m, 16H), 0.89−0.84 (m, 12H); 13C NMR (150 MHz, CDCl3): δ 171.0, 154.9, 150.1, 144.5, 135.7, 133.2, 128.9, 127.5, 125.4, 125.1, 123.5, 121.1, 111.2, 104.2, 103.0, 45.0, 39.1, 30.4, 28.5, 23.7, 23.0, 14.1, 10.6. MALDI-TOF: 783.4 (M+H). Elemental analysis: Calcd. for

CONCLUSIONS We reported the design and synthesis of two new D-A-D moleculesBTBPD and BFBPDwith BPD (Pechmann dye) as the electron-accepting core containing two benzo[b]thiophene moieties and two benzo[b]furan moieties, respectively. The single-crystal structures of both compounds were determined, and intermolecular aromatic donor−acceptor (DA) interactions were clearly confirmed. Organic field-effect transistors (OFETs) with thin films of BTBPD and BFBPD that were solution-processed were constructed with conventional techniques. OFETs of BTBPD exhibit relatively high hole mobility (up to 1.4 cm2 V−1 s−1) with high on/off ratios (up to 106). Because of the poor thin-film morphology and low molecular ordering, the hole mobility of OFETs of BFBPD (0.14 cm2 V−1 s−1) is relatively low. The present studies clearly demonstrate the potentials of the BPD moiety that is planar and polar as an electron-acceptor moiety to build D−A molecules for organic semiconductors of good performance. Various conjugated D-A frameworks (e.g., D-A-D) within either small molecules or macromolecules can be designed. Furthermore, as a dye, BPD shows strong absorptions up to 720 nm. The electronic structure and intermolecular packing of BPD may be further tuned by incorporation of appropriate electron donors. All these unique structural merits of BPD will enable us to establish a new molecular design platform for promising optoelectronic materials. Further studies along this vein are underway.

■

EXPERIMENTAL SECTION

Materials and Characterization Techniques. The reagents and starting materials were commercially available and used without any further purification, if not specified elsewhere. Compound 1 was synthesized according to the previous report.43 1 H NMR and 13C NMR spectra were recorded on Bruker ADVANCE III 300 MHz spectrometer, Bruker ADVANCE III 400 MHz spectrometer, and Bruker ADVANCE III 600 MHz spectrometer. Melting points were measured with Büchi B540 and uncorrected. MALDI-TOF MS spectra were recorded with BEFLEX III spectrometer. Elemental analysis was performed on a Carlo Erba model 1160 elemental analyzer. Solution and thin films absorption spectra were measured with a Jasco Model V-570 UV−vis spectrophotometer. Thermogravimetric analysis (TGA) (Shimadzu, Model DTG-60) measurements were performed under a nitrogen atmosphere at a heating rate of 10 °C/min. Cyclic voltammetric measurements were carried out in a conventional three-electrode cell using Pt wires 2 mm in diameter as working and counter electrodes, and an Ag/AgCl reference electrode on a computer-controlled Model CHI660C instrument at room temperature; n-Bu4NPF6 was used as the conducting electrolyte. X-ray crystallographic data were collected with a Bruker Smart CCD diffractometer through using graphitemonochromated Mo Kα radiation (λ = 0.71073 M). The computation was performed with the SHELXL-97 program. X-ray diffraction (XRD) measurements were carried out in the reflection mode at room temperature, using a 2-kW Rigaku XRD system. The thin films were imaged in air using a Digital Instruments Nanoscope V atomic force microscopy (AFM) system operated in tapping mode with a Nanoscope V instrument, and optical microscopic images were operated in tapping mode with an Olympus Model BX51 instrument. AFM samples and microscopic images were identical to those used in FET performance analysis. Synthesis of Compound 2. Compound 1 (0.42 g, 1.29 mmol) and 2-ethylhexan-1-amine (1.10 g, 8.52 mmol) were dissolved in CH2Cl2 (20 mL) at room temperature (RT) and the mixture was stirred for 12 h to form a clear solution. Then, 10 mL of HCl (10%, m/m) was added, and the reaction mixture turned blue immediately. After separation, the organic layer was washed with saturated NaHCO3 476

dx.doi.org/10.1021/cm303793g | Chem. Mater. 2013, 25, 471−478

Chemistry of Materials

Article

C48H50N2O4S2: C, 73.62; H, 6.44; N, 3.58, S, 8.19; Found: C, 73.44; H, 6.43, N, 3.56; S, 8.14. Fabrication of OFET Devices. A heavily doped n-type Si wafer and a layer of dry oxidized SiO2 (300 nm, with roughness lower than 0.1 nm and capacitance of 11 nF cm−2) were used as a gate electrode and gate dielectric layer, respectively. The drain-source (D-S) gold contacts were fabricated by photolithography. The substrates were cleaned in water, deionized water, alcohol, and rinsed in acetone. The surface then was modified with n-trichloro(octadecyl)silane (OTS). After that, the substrates were cleaned in n-hexane and CHCl3, followed by soaking in 20 mL of EtOH to which 40 μL of pentafluorobenzenethiol was added. The system was stewing for 60 min and the substrates were washed with EtOH. Compounds BTBPD and BFBPD were dissolved in CHCl3 (10 mg/mL) and spin-coated on above substrate (with or without modification of pentafluorobenzenethiol) at 2000 rpm for 30 s. The annealing process was carried out under vacuum for 1.0 h at each temperature. Field-effect characteristics of the devices were determined in air, using a Keithley Model 4200 SCS semiconductor parameter analyzer.

■

(9) (a) Li, L. Q.; Zhang, Y. J.; Li, H. X.; Tang, Q. X.; Jiang, L.; Chi, L. F.; Fuchs, H.; Hu, W. P. Adv. Funct. Mater. 2009, 19, 2987. (b) Li, L. Q.; Gao, P.; Schuermann, K. C.; Ostendorp, S.; Wang, W. C.; Du, C. A.; Lei, Y.; Fuchs, H.; De Cola, L.; Müllen, K.; Chi, L. F. J. Am. Chem. Soc. 2010, 132, 8807. (c) Tsao, H. N.; Cho, D. M.; Park, I.; Hansen, M. R.; Mavrinskiy, A.; Yoon, D. Y.; Graf, R.; Pisula, W.; Spiess, H. W.; Müllen, K. J. Am. Chem. Soc. 2011, 133, 2605. (10) Mitsui, C.; Soeda, J.; Miwa, K.; Tsuji, H.; Takeya, J.; Nakamura, E. J. Am. Chem. Soc. 2012, 134, 5448. (11) Chen, D. G.; Zhao, Y.; Zhong, C.; Gao, S. Q.; Yu, G.; Liu, Y. Q.; Qin, J. G. J. Mater. Chem. 2012, 22, 14639. (12) (a) Liang, Y. Y.; Wu, Y.; Feng, D. Q.; Tsai, S.-T.; Son, H.-J.; Li, G.; Yu, L. P. J. Am. Chem. Soc. 2009, 131, 56. (b) Lou, S. J.; Szarko, J. M.; Xu, T.; Yu, L. P.; Marks, T. J.; Chen, L. X. J. Am. Chem. Soc. 2011, 133, 20661. (c) Carsten, B.; Szarko, J. M.; Son, H. J.; Wang, W.; Lu, L. Y.; He, F.; Rolczynski, B. S.; Lou, S. J.; Chen, L. X.; Yu, L. P. J. Am. Chem. Soc. 2011, 133, 20468. (13) (a) Huang, F.; Chen, K.-S.; Yip, H.-L.; Hau, S. K.; Acton, O.; Zhang, Y.; Luo, J. D.; Jen, A. K.-Y. J. Am. Chem. Soc. 2009, 131, 13886. (b) Malley, K. M.; Li, C.-Z.; Yip, H.-L.; Jen, A. K.-Y. Adv. Energy Mater. 2012, 2, 82. (c) Yip, H.-L.; Jen, A. K.-Y. Energy Environ. Sci. 2012, 5, 5994. (14) Li, H.; Jiang, P.; Yi, C. Y.; Li, C.; Liu, S. X.; Tan, S. T.; Zhao, B.; Braun, J.; Meier, W.; Wandlowski, T.; Decurtins, S. Macromolecules 2010, 43, 8058. (15) Bijleveld, J. C.; Zoombelt, A. P.; Mathijssen, S. G. J.; Wienk, M. M.; Turbiez, M.; de Leeuw, D. M.; Janssen, R. A. J. J. Am. Chem. Soc. 2009, 131, 16616. (16) Bronstein, H.; Chen, Z. Y.; Ashraf, R. S.; Zhang, W. M.; Du, J. P.; Durrant, J. R.; Tuladhar, P. S.; Song, K.; Watkins, S. E.; Geerts, Y.; Wienk, M. M.; Janssen, R. A. J.; Anthopoulos, T.; Sirringhaus, H.; Heeney, M.; McCulloch, I. J. Am. Chem. Soc. 2011, 133, 3272. (17) Guo, X. G.; Ortiz, R. P.; Zheng, Y.; Kim, M. G.; Zhang, S. M.; Hu, Y.; Lu, G.; Facchetti, A.; Marks, T. J. J. Am. Chem. Soc. 2011, 133, 13685. (18) Osaka, I.; Shimawaki, M.; Mori, H.; Doi, I.; Miyazaki, E.; Koganezawa, T.; Takimiya, K. J. Am. Chem. Soc. 2012, 134, 5425. (19) Wu, J.-S.; Cheng, Y.-J.; Lin, T.-Y.; Chang, C.-Y.; Shih, P.-I.; Hsu, C.-S. Adv. Funct. Mater. 2012, 22, 1711. (20) Boudreault, P.-L. T.; Hennek, J. W.; Loser, S.; Ortiz, R. P.; Eckstein, B. J.; Facchetti, A.; Maris, T. J. Chem. Mater. 2012, 24, 2929. (21) Walker, B.; Tamayo, A. B.; Dang, X.-D.; Zalar, P.; Seo, J. H.; Garcia, A.; Tantiwiwat, M.; Nguyen, T.-Q. Adv. Funct. Mater. 2009, 19, 3063. (22) Lloyd, M. T.; Anthony, J. E.; Malliaras, G. G. Mater. Today 2007, 10, 34. (23) Allard, S.; Forster, M.; Souharce, B.; Thiem, H.; Scherf, U. Angew. Chem., Int. Ed. 2008, 47, 4070. (24) Coropceanu, V.; Cornil, J.; da Silva, D. A.; Olivier, Y.; Silbey, R.; Brédas, J.-L. Chem. Rev. 2007, 107, 926. (25) Loser, S.; Bruns, C. J.; Miyauchi, H.; Ortiz, R. P.; Facchetti, A.; Stupp, S. I.; Marks, T. J. J. Am. Chem. Soc. 2011, 133, 8142. (26) Ning, Z. J.; Fu, Y.; Tian, H. Energy Environ. Sci. 2010, 3, 1170. (27) Lee, O. P.; Yiu, A. T.; Beaujuge, P. M.; Woo, C. H.; Holcombe, T. W.; Millstone, J. E.; Douglas, J. D.; Chen, M. S.; Fréchet, J. M. J. Adv. Mater. 2011, 23, 5359. (28) (a) Zhan, X. W.; Facchetti, A.; Barlow, F.; Marks, T. J.; Ratner, M. A.; Wasielewski, M. R.; Marder, S. R. Adv. Mater. 2011, 23, 268. (b) Zhan, X. W.; Tan, Z. A.; Domercq, B.; An, Z. S.; Zhang, X.; Barlow, S.; Li, Y. F.; Zhu, D. B.; Kippelen, B.; Marder, S. R. J. Am. Chem. Soc. 2007, 129, 7246. (29) Chen, Z. H.; Zheng, Y.; Yan, H.; Facchetti, A. J. Am. Chem. Soc. 2009, 131, 8. (30) Ong, K.-H.; Lim, S.-L.; Tan, H.-S.; Wong, H.-K.; Li, J.; Ma, Z.; Moh, L. C. H.; Lim, S.-H.; De Mello, J. C.; Chen, Z.-K. Adv. Mater. 2011, 23, 1409. (31) (a) Yuen, J. D.; Fan, J.; Seifter, J.; Lim, B.; Hufschmid, R.; Heeger, A. J.; Wudl, F. J. Am. Chem. Soc. 2011, 133, 20799. (b) Yuen, J.

ASSOCIATED CONTENT

S Supporting Information *

TGA analysis, XRD patterns, theoretical calculations data, crystal data, 1H NMR, 13C NMR, and MALDI-TOF for new compounds. This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*E-mails: [email protected] (D.Z.), [email protected] (Z.L.), [email protected] (G.Z.). Notes

The authors declare no competing financial interest.

■ ■

ACKNOWLEDGMENTS The present research was financially supported by NSFC, the State Basic Program, and Chinese Academy of Sciences. REFERENCES

(1) (a) Murphy, A. R.; Fréchet, J. M. J. Chem. Rev. 2007, 107, 1066. (b) Beaujuge, P. M.; Fréchet, J. M. J. J. Am. Chem. Soc. 2011, 133, 20009. (2) Zaumseil, J.; Sirringhaus, H. Chem. Rev. 2007, 107, 1296. (3) Anthony, J. E.; Facchetti, A.; Heeney, M.; Marder, S. R.; Zhan, X. W. Adv. Mater. 2010, 22, 3876. (4) Di, C. A.; Liu, Y. Q.; Yu, G.; Zhu, D. B. Acc. Chem. Res. 2009, 42, 1573. (5) Wang, C. L.; Dong, H. L.; Hu, W. P.; Liu, Y. Q.; Zhu, D. B. Chem. Rev. 2012, 112, 2208. (6) (a) Tan, L. X.; Guo, Y. L.; Zhang, G. X.; Yang, Y.; Zhang, D. Q.; Yu, G.; Xu, W.; Liu, Y. Q. J. Mater. Chem. 2011, 21, 18042. (b) Tan, L. X.; Guo, Y. L.; Yang, Y.; Zhang, G. X.; Zhang, D. Q.; Yu, G.; Xu, W.; Liu, Y. Q. Chem. Sci. 2012, 3, 2530. (7) (a) Cao, Y.; Liu, S.; Shen, Q.; Yan, K.; Li, P. J.; Xu, J.; Yu, D. P.; Steigerwald, M. L.; Nuckolls, C.; Liu, Z. F.; Guo, X. F. Adv. Funct. Mater. 2009, 19, 2743. (b) Cao, Y.; Steigerwald, M. L.; Nuckolls, C.; Guo, X. F. Adv. Mater. 2010, 22, 20. (8) (a) Würthner, F.; Stolte, M. Chem. Commun. 2011, 47, 5109. (b) Steinmann, V.; Kronenberg, N. M.; Lenze, M. R.; Graf, S. M.; Hertel, D.; Meerholz, K.; Bürckstummer, H.; Tulyakova, E. V.; Würthner, F. Adv. Energy Mater. 2011, 1, 888. (c) Schmidt, R.; Oh, J. H.; Sun, Y.-S.; Deppisch, M.; Krause, A.-M.; Radacki, K.; Braunschweig, H.; Könemann, M.; Erk, P.; Bao, Z. N.; Würthner, F. J. Am. Chem. Soc. 2009, 131, 6215. (d) Li, H. Y.; Tee, B. C.-K; Cha, J. J.; Cui, Y.; Chung, J. W.; Lee, S. Y.; Bao, Z. N. J. Am. Chem. Soc. 2012, 134, 2760. 477

dx.doi.org/10.1021/cm303793g | Chem. Mater. 2013, 25, 471−478

Chemistry of Materials

Article

D.; Kumar, R.; Zakhidov, D.; Seifter, J.; Lim, B.; Heeger, A. J.; Wudl, F. Adv. Mater. 2011, 23, 3780. (32) (a) Lei, T.; Cao, Y.; Fan, Y. L.; Liu, C. J.; Yuan, S. C.; Pei, J. J. Am. Chem. Soc. 2011, 133, 6099. (b) Lei, T.; Cao, Y.; Zhou, X.; Peng, Y.; Bian, J.; Pei, J. Chem. Mater. 2012, 24, 1762. (33) Mei, J. G.; Kim, D. H.; Ayzner, A. L.; Toney, M. F.; Bao, Z. N. J. Am. Chem. Soc. 2011, 133, 20130. (34) Chen, Z. Y.; Lee, M. J.; Ashraf, R. S.; Gu, Y.; Albert-Seifried, S.; Nielsen, M. M.; Schroeder, B.; Anthopoulos, T. D.; Heeney, M.; McCulloch, I.; Sirringhaus, H. Adv. Mater. 2012, 24, 647. (35) Chen, H. J.; Guo, Y. L.; Yu, G.; Zhao, Y.; Zhang, J.; Gao, D.; Liu, H. T.; Liu, Y. Q. Adv. Mater. 2012, 24 (34), 4618−4622. (36) Loser, S.; Bruns, C. J.; Miyauchi, H.; Ortiz, R. P.; Facchetti, A.; Stupp, S. I.; Marks, T. J. J. Am. Chem. Soc. 2011, 133, 8142. (37) Qu, S. Y.; Tian, H. Chem. Commun. 2012, 48, 3039. (38) (a) Kim, C.; Liu, J. H.; Lin, J.; Tamayo, A. B.; Walker, B.; Wu, G.; Nguyen, T.-Q. Chem. Mater. 2012, 24, 1699. (b) Tantiwiwat, M.; Tamayo, A.; Luu, N.; Dang, X.-D.; Nguyen, T.-Q. J. Phys. Chem. C 2008, 112, 17402. (c) Liu, J. H.; Walker, B.; Tamayo, A.; Zhang, Y.; Nguyen T.-Q. Adv. Funct. Mater. 2013, 23 (1), 47−56. (39) Qiao, Y. L.; Guo, Y. L.; Yu, C. M.; Zhang, F. J.; Xu, W.; Liu, Y. Q.; Zhu, D. B. J. Am. Chem. Soc. 2012, 134, 4084. (40) Zhong, H. L.; Smith, J.; Rossbauer, S.; White, A. J. P.; Anthopoulos, T. D.; Heeney, M. Adv. Mater. 2012, 24, 3205. (41) Bogert, M. T.; Ritter, J. J. J. Am. Chem. Soc. 1924, 46, 2871. (42) von Pechmann, H. Chem. Ber. 1882, 15, 881. (43) (a) Norsten, T. B.; Kantchev, E. A. B.; Sullivan, M. B. Org. Lett. 2010, 12, 4816. (b) Kantchev, E. A. B.; Norsten, T. B.; Tan, M. L. Y.; Ng, J. J. Y.; Sullivan, M. B. Chem.Eur. J. 2012, 18, 695. (44) Hashimoto, H.; Shiratori, K.; Kawakita, K.; Tanaka, T.; Sekine, R.; Irikawa, H. Heterocycles 2005, 65, 1385. (45) The potentials were presented by reference to Ag/AgCl, and the potentials were calibrated against Fc/Fc+. The cyclic voltammogram of 2 was also measured. Compared to BTBPD and BFBPD, the oxidation waves of 2 was shifted to a more-positive region. The HOMO energy of 2 was estimated to be −5.2 eV, which was lower than those of BTBPD and BFBPD. This is due to the electrondonating properties of benzo[b]thiophene and benzo[b]furan groups in BTBPD and BFBPD. (46) Thermal disorders were found for the sulfur atoms of benzo[b] thiophene rings. In one conformation (with 46.4% probability), the benzo[b]thiophene and the neighboring thiophene rings are arranged in a “cis” manner, whereas they are arranged in “trans” manner in another conformation (with 53.6% probability). Thermal disorders were also detected for the carbon atoms of the alkyl chains. (47) Nguyen and her co-workers have just recently described the crystal structures of DPP-based electron donor−acceptor−donor molecules, but the respective intermolecular electron donor−acceptor interactions were not observed. (48) Thermal disorders were found for the carbon atoms of benzene ring of benzo[b]furan moieties and alkyl chains. (49) The hole mobility started to decrease by further increasing the annealing temperature to 140 °C. Figures S4 and S5 in the Supporting Information show the variation of the hole mobility of OFETs of BTBPD vs VGS and the hysteresis transfer characteristics. The results reveal that the hole mobility of OFETs of BTBPD is slightly affected by VGS and the measuring conditions. (50) Modification of the source-drain Au electrodes with pentafluorobenzenethiol is expected to modify the surface energy and importantly improve the work function of the Au electrodes, and thus the mobility is enhanced according to previous reports: (a) Gundlach, D. J.; Royer, J. E.; Park, S. K.; Subramanian, S.; Jurchescu, O. D.; Hamadani, B. H.; Moad, A. J.; Kline, R. J.; Teague, L. C.; Kirillov, O.; Richter, C. A.; Kushmerick, J. G.; Richter, L. J.; Parkin, S. R.; Jackson, T. N.; Anthony, J. E. Nat. Mater. 2008, 7, 216. (b) Ward, J. W.; Loth, M. A.; Kline, R. J.; Coll, M.; Ocal, C.; Anthony, J. E.; Jurchescu, O. D. J. Mater. Chem. 2012, 22, 19047. (51) Further enhancement of the hole mobility was not observed by employing the modified source−drain electrodes with pentafluor-

obenzenethiol. Similarly, the hole mobility of OFETs of BFBPD is slightly influenced by VGS and the measuring conditions, based on the data shown in Figures S4 and S5 in the Supporting Information. (52) OFETs of BFBPD exhibit positive threshold voltages (see Table 4). This also holds true for OFETs of BTBPD. Probably, this is due to the interactions of BFBPD and BTBPD with oxygen from air; as a result, their off-currents would be enhanced and positive threshold voltages (Vth) are required to induce the channel.

478

dx.doi.org/10.1021/cm303793g | Chem. Mater. 2013, 25, 471−478