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Soluble Dinaphtho[2,3-b:2',3'-f]thieno[3,2-b]thiophene Derivatives for Solution-Processed Organic Field-Effect Transistors Masanori Sawamoto, Myeong Jin Kang, Eigo Miyazaki, Hiroyoshi Sugino, Itaru Osaka, and Kazuo Takimiya ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b10477 • Publication Date (Web): 19 Jan 2016 Downloaded from http://pubs.acs.org on January 21, 2016

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ACS Applied Materials & Interfaces

Soluble b]thiophene

Dinaphtho[2,3-b:2',3'-f]thieno[3,2Derivatives

for

Solution-Processed

Organic Field-Effect Transistors Masanori Sawamoto§,¶, Myeong Jin Kang†, Eigo Miyazaki†, Hiroyoshi Sugino§, Itaru Osaka§, Kazuo Takimiya§,†*

§

Emergent Molecular Function Research Group, RIKEN Center for Emergent Matter Science

(CEMS), Wako, Saitama 351-0198, Japan

¶

Program in Physics and Functional Materials Science, Graduate School of Science and

Engineering, Saitama University, Saitama 338-8570, Japan

†

Department of Applied Chemistry, Graduate School of Engineering, Hiroshima University,

Higashi-Hiroshima 739-8527, Japan

KEYWORDS: Organic semiconductor, Thienoacene, Solution process, Branched alkyl group, Organic field-effect transistor, High mobility

1 ACS Paragon Plus Environment


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ABSTRACT:

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We demonstrate a new approach to solution-processable dinaphtho[2,3-b:2',3'-

f]thieno[3,2-b]thiophene (DNTT) derivatives that can afford good thin-film transistors having mobilities higher than 0.1 cm2 V–1 s–1. The key molecular design strategy is the introduction of one branched alkyl group at the edge of the DNTT core, which improves solubility while retaining semiconducting characteristics in the thin-film state. Dialkylation, i.e., the introduction of two branched alkyl groups on the DNTT core, had a detrimental effect on the semiconducting properties. Although the physicochemical properties of the mono- and dialkylated derivatives at the molecular level were almost the same, the thin-film absorption spectra and the ionization potentials (IPs) were markedly different, indicating that the intermolecular interaction in the thin-film state was affected by the number of alkyl groups. Indeed, the packing structures of the monoalkylated DNTTs in the thin-film state, which were estimated from the XRD patterns, were similar to that of parent DNTT, indicating the existence of the lamella structure with the herringbone packing motif. In sharp contrast, the XRD patterns of the dialkylated DNTT thin films showed poor crystallinity and the packing structures were significantly different from that of parent DNTT. All the results of structural characterization in the thin-film state and evaluation of device characteristics of the DNTT derivatives with branched alkyl groups indicate that the introduction of a branched alkyl group in the molecular long-axis direction is an effective way to solubilize the rigid, largely πextended organic semiconducting core without interfering with the semiconducting characteristics in the thin-film state.


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Introduction High expectations are placed on printed electronics as an emerging technology that can differentiate future electronics from conventional ones and attain low-cost production and large-area and flexible device applications.1 Solution-processable organic semiconductors, one of the key materials for realizing printed electronics, are presumed to have several noteworthy properties, including high mobility and good environmental stability,2–5 and therefore, many researchers have risen to the challenge of elaborating such superior soluble organic semiconductors. Dinaphtho[2,3-b:2',3'f]thieno[3,2-b]thiophene (DNTT, Figure 1)6 and its derivatives are promising organic semiconductors that afford high-mobility organic field-effect transistors (OFETs) with good environmental stability and operational stability. Such excellent properties as a semiconducting material have been exploited in the fabrication of state-of-the-art, ultrathin, flexible electronic circuits and sensors on plastic substrates.7–9 Although these prime examples of DNTT-based transistors testify to the superiority of DNTT as an organic semiconductor, the low solubility of DNTT in common organic solvents is a drawback for use in the field of printed electronics. In order to exploit the superiority of DNTT as a promising core structure for organic semiconducting materials, it is important to make DNTT derivatives soluble in organic solvents. To this end, the primary question is how to solubilize the DNTT core while retaining the semiconducting properties. Several solubilizing modifications that preserve the semiconducting characteristics of rigid, insoluble organic semiconducting frameworks have been reported. One example is the introduction of long alkyl groups10,11 or bulky substituents12–16 in such a way that the substituents do not violate, rather enhance the intermolecular interaction between the semiconducting cores in the condensed phase. Another example is the so-called “precursor” approach17–19 where a precursor with a non-planar, deformed structure, being different from original semiconducting π-conjugated systems, for example, a Diels-Alder cycloaddition adduct, can react



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thermally or photochemically after solution deposition at the precursor stage on the substrate to reproduce active semiconducting materials. The development of soluble DNTT derivatives via the introduction of long alkyl groups has been attempted (Figure 1),20,21 but the resulting alkylated DNTTs (Cn-DNTTs) are not very soluble in ordinary organic solvents at room temperature. Only one solution processing technique carried out under specific conditions is available for the fabrication of solution-processed OFETs,22 in sharp contrast to the case of highly soluble dialkylated [1]benzothiono[3,2-b][1]benzothiophenes (BTBTs),10 which are lower homologs of Cn-DNTTs and afford various solution-processed OFETs23–26 that show very high mobilities of up to 43 cm2 V–1 s–1 and good stability.27 On the other hand, Kimura and coworkers have developed a soluble DNTT precursor (5,14-N-phenylmaleimidedinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophene, Figure 1) for the fabrication of solution-processed high-performance OFETs with good environmental stability.28,29 Although the precursor approach to soluble DNTTs is quite successful, the synthesis of the precursor is carried out with a less effective Diels-Alder reaction, in which the yields is low (approximately 15%). In the present work, we re-examined the alkylation approach to soluble DNTTs by employing branched alkyl groups, and found that the introduction of branched alkyl group(s) on DNTT enhanced solubility. Furthermore, moderately soluble monoalkylated DNTTs afforded solutionprocessed OFETs having mobilities higher than 0.1 cm2 V–1 s–1. We report the synthesis, electronic properties, thin-film structures, and FET characteristics of solution-processed OFETs.

Figure 1. Molecular structures of DNTT, alkylated DNTTs (Cn-DNTTs), and DNTT precursor.


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Results and discussion Molecular design

It has been widely demonstrated and commonly recognized that

branched alkyl groups, such as a 2-ethylhexyl (EH) group, can effectively enhance solubility when introduced on π-extended semiconducting oligomers and polymers. On the other hand, the presence of branched alkyl groups would influence the intermolecular interaction between the π-conjugated cores and hence the electronic properties of the resulting thin films. For this reason, the number, shape, and size of branched alkyl groups should be tuned carefully.30,31 With these as background, we designed several DNTT derivatives (Figure 2) in order to investigate the effects of branched alkyl groups, and the first branched alkyl group we examined was the EH group. We tried to design an effective synthetic method for both symmetrical 2,9-bis(2-ethylhexyl)-DNTT (1a) and unsymmetrical 2-(2-ethyhexyl)-DNTT (2a). Then, we modified the structure of the branched alkyl group by changing the branching position in the trunk part of the group from the second (EH) to the third (3-ethylheptyl, EHep) or fourth (4-ethyloctyl, EO) carbon atom. As a result, we synthesized three each of 2,9-dialkyl- (1) and 2-alkyl DNTT derivatives (2) as follows.

Figure 2. Molecular structures of branched alkyl DNTTs.

Synthesis

As a result of intensive work aimed at designing a synthetic procedure for

DNTT and its derivatives,6,32,33 we established a general method for the synthesis, which started with the conversion of 6-substituted 2-methoxynaphthalene derivatives into symmetrical 2,9disubstituted DNTT derivatives.32 For the synthesis of branched-alkylated DNTTs (1 and 2), we first synthesized corresponding 6-alkyl-2-methoxynaphthalenes (3a–c, Scheme 1). Depending on the branching position in the alkyl groups, three approaches were employed; for the synthesis of 25 ACS Paragon Plus Environment


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ethylhexyl derivative (3a), conventional Kumada-Tamao coupling34 using the corresponding Grignard reagent and 6-bromo-2-methoxynaphthalene afforded the desired 6-(2-ethylhexyl) derivative (Scheme 1a). On the other hand, the synthesis of 3-ethylheptyl derivative (3b) was initiated from commercial 6-methoxy-2-naphthaldehyde, which was reacted with 2-ethylhexyl lithium that was in situ generated from 1-bromo-2-ethylhexane to give the corresponding benzylic alcohol intermediate. The alcoholic functionality was efficiently removed by ionic deoxygenation with triethylsilane in the presence of trifluoroacetic acid to afford 6-(3-ethylheptyl)-2methoxynaphthalene (3b, Scheme 1b).35,36 Further increase of the number of carbon atoms between the naphthalene core and the branching carbon atom in the alkyl group was achieved by Sonogashira coupling37 between 6-bromo-2-methoxynaphthalene and 4-ethyloct-1-yne that was in situ generated from the corresponding 1-trimethylsilyl (TMS) derivative,38,39 followed by hydrogenation to give 6-(4-ethyloctyl)-2-methoxynaphthalene (3c, Scheme 1c).

Scheme 1. Synthesis of 6-alkyl-2-methoxynaphthalenes (3a–c).

(a) OMe

OMe

R-MgBr

3a: R = 2-ethylhexyl Br

NiCl2(dppp)

R

69%

(b) Li

OMe

OMe

OMe

Et3SiH

R OHC

46%

CF3CO2H

OH

82%

3b: R = 3-ethylheptyl

(c) TMS OMe Br

Pd(dppf)Cl2 CuI TBAF, Et3N

OMe

OMe

Pd/C, H2 quant.

quant

R 3c: R = 4-ethyloctyl


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Following the reported procedure,32 these 6-alkyl-2-methoxynaphthalenes (3) were readily converted into 2,9-dialkyl-DNTTs in good yields via a five-step reaction that consisted of methylthiolation at the 3-position to give 4, functional group transformation of the 2-methoxy group into corresponding trifluoromethanesulfonyloxy derivative (6), coupling reaction with 1,2bis(tributylstannyl)ethene to give precursor (7), and conversion of 7 into DNTT derivatives (1) via the iodine-promoted thieno[3,2-b]thiophene formation reaction (Scheme 2). Scheme 2. Synthesis of 2,9-dialkyl DNTTs (1a–c) from 6-alkyl-2-methoxynaphthalenes (3).

For the synthesis of unsymmetrical 2-alkylated DNTTs, we examined two approaches targeting 2-(2-ethylhexyl)-DNTT (2a) as a model case from 6a (Scheme 3). The first approach was the consecutive thiophene annulation reaction that had been demonstrated to be quite effective for the synthesis of unsymmetrical BTBT derivatives and related compounds.40 Starting from 6a, a TMSprotected acetylene moiety was introduced via the Sonogashira or Stille coupling reaction, and resulting acetylene derivative (8) was reacted with 2-naphthylsulfenylchloride, which was in situ generated from 2-naphthalenethiol and N-chlorosuccinimide, followed by treatment with tetrabutylammonium fluoride to give 7-(2-ethylhexyl)-3-(2-naphthylthio)naphtho[2,3-b]thiophene (9).41 After its conversion into bromide (10), intramolecular aryl-aryl coupling catalyzed by bis(triphenylphosphine)palladium(II) dichloride afforded desired 2a in ca. 12% total yield via a four-step synthesis from 6a. 7 ACS Paragon Plus Environment


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The second approach was the use of a non-selective coupling reaction between two different 3methylthio-2-(trifluoromethanesulfonyloxy)naphthalenes (6) and 1,2-bis(tributylstannyl)ethene, followed by the iodine-promoted thieno[3,2-b]thiophene formation reaction (Scheme 3).32 The first coupling reaction of 6a and 6d afforded unsymmetrical intermediate (11a) together with two symmetrical byproducts. Chromatographic purification effectively isolated desired 11a in 44% yield, which was readily converted into corresponding DNTT derivative (2a) in an almost quantitative yield, resulting in a total yield higher than 40% via only two steps from 6a. In order to effectively use 6a, the most important intermediate in the syntheses of 2a, the first selective approach was initially thought to be more desirable than the second one because the second approach required non-selective random coupling where the maximum yield of the unsymmetrical intermediate was 50%, provided that the reactivities of substrates 6a and 6d were identical. In the actual synthesis, however, it turned out that the first selective approach had several drawbacks, including low isolated yield in each reaction, particularly the final step (42% isolated yield) and the second step (44% isolated yield), where an unexpected byproduct, naphthalene disulfide, was very difficult to remove from the reaction mixture. In contrast, although the yield of the unsymmetrical product in the initial non-selective coupling reaction of the second approach was 44%, the well-established iodine-promoted thienothiophene formation proceeded almost quantitatively, resulting in a quite efficient synthesis of desired 2a. From these comparisons, we conclude that the second non-selective approach is more practical than the first one, and carried out the synthesis of other derivatives with different alkyl groups using this approach (Scheme 4). Purification of the DNTT derivatives was done by recrystallization and gradient sublimation under reduced pressure to give analytical and device-grade samples.

Scheme 3. Two approaches to the synthesis of 2-(2-ethylhexyl)-DNTT (2a).


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Scheme 4. Synthesis of 2-(3-ethylheptyl)-DNTT (2b) and 2-(4-ethyloctyl)-DNTT (2c).

Molecular properties

Molecular properties, such as solubilities, melting points,

oxidation potentials (Figure S1), absorption spectra (Figure 3), and expected HOMO energy levels (EHOMOs) and HOMO-LUMO energy gaps (Egs), of the new DNTT derivatives together with parent DNTT and C10-DNTT as references are summarized in Table 1. The solubilities of the new DNTT derivatives are, as expected, enhanced significantly, particularly for dialkylated derivatives (1a–c); for example, compared with the solubility of C10-DNTT (0.070 g L–1 at 50 °C in toluene), the solubilities of the new DNTT derivatives are ca. 10–100 times higher under identical conditions. The EHOMOs and Egs of the derivatives as evaluated by solution electrochemistry and absorption spectrum measurements in solution are virtually the same as those of parent DNTT and C10-DNTT, indicating no significant electronic perturbation by the branched alkyl groups.



1x10

1a

5

1b 5x10

ε / M–1 cm –1

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1c

4

0 1x10

5

5x10

4

2a 2b 2c

0 250

300

350

400

450

500

Wavelength / nm

Figure 3. Absorption spectra in chloroform of 1 and 2.

Thin-film deposition and electronic properties in the thin-film state

Thin films of the new

DNTT derivatives could be deposited by spin-coating from their hot chloroform solutions (ca. 0.2 to 0.8 wt%) or vacuum evaporation. Both the spin-coated and vapor-deposited thin films consisted of polycrystalline grains, similar to conventional thin films of small-molecule organic semiconductors (Figure S3). The optical properties of the thin films were significantly affected by not the deposition method but the number and shape of branched alkyl groups. Figure 4 shows the photoelectron spectra and the absorption spectra of the spin-coated thin films. It is apparent that the ionization potentials (IPs) and the absorption spectra of the thin films are largely dependent on the number and branching position of alkyl groups (Table 2). In particular, IP of 1a is 5.7 eV, which is significantly larger by 0.3 eV than that of DNTT. This can be explained by the reduced intermolecular interaction in the thin-film state due to the two 2-ethylhexyl groups at close proximity to the DNTT core.42 Such a drastic effect can be mitigated by positioning the branching position in the alkyl groups far from the DNTT core; IPs of 1b and 1c are 5.3 and 5.5 eV, respectively (Figure 4a). It is 10 ACS Paragon Plus Environment

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reasonable to consider that the effect of the branched alkyl groups is dependent on the distance between the branching carbon atoms and the DNTT core. In addition, the branching carbon atoms may exert an odd-even effect that alters the direction of the branched alkyl groups such that they point toward the DNTT core (1a and 1c) or away from it (1b). In sharp contrast, all the 2-alkylated (monoalkylated) derivatives (2a–c) showed IPs of around 5.1 eV, similar to that of C10-DNTT (4.9 eV).20 The absorption spectra (Figure 4b) also reflected the intermolecular interaction in the thin-film state. Among the alkylated derivatives, 1a having the largest IP produced the most blue-shifted peak (λmax: 420 nm), whereas the absorption peaks of 1b and 1c thin films were slightly red-shifted (around 424–429 nm). In sharp contrast, very pronounced red shifts were observed for monoalkylated derivatives (2a–c), which showed absorption peaks at approximately 445 nm regardless of the branching position of the alkyl groups. By comparing the thin-film absorption spectra with those of DNTT and C10-DNTT (Table 2), we conclude that, as discussed on the basis of IP data, 2a–c possessing one alkyl group exhibit larger intermolecular interaction in the thin-film state than 1a–c having two alkyl groups.

(a)

30

1/2

/ cps

1/2

40

20

(b) 1a 1b

1a 1b 1c

Absorbance / arb.unit

50

Yield

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1c 2a 2b 2c

2a 2b 2c

10

0 4.5

5

5.5 Energy / eV

6

400

420

440

460

480

500

Wavelength / nm

Figure 4. Photoelectron spectra (a) and absorption spectra (b) of spin-coated thin films of 1 and 2.

Thin-film transistor devices based on dialkylated derivatives (1a–c)

Thin-film transistors with a

top-contact bottom-gate configuration were fabricated by using the spin-coated thin films. Table 3 11 ACS Paragon Plus Environment


summarizes the device metrics of 1b and 1c. Although the performances of TFTs are dependent on the alkyl groups, the device performances are generally poor compared with those of parent DNTT or C10-DNTT. In particular, no clear FET response was observed for the 1a-based OFETs. On the other hand, 1b yielded the best device characteristics: hole mobility was 0.011 cm2 V–1 s–1 and Ion/off was 104 (Figure 5). The best mobility value is lower than those of parent DNTT6 or C10-DNTT20 by more than two orders of magnitude. 0.002

-5

V = -60 V

(a)

-6

10

-2x10

(b)

d

V = - 60 V g

-6

10

1/2

0.0015

-6

-1.5x10

-7

-8

10

-6

-1x10

d

d

0.001

-I / A

-I / A

d

(-I )

1/2

/A

10

0.0005

V = - 50 V g

-7

-5x10

-9

10

V = - 40 V g

0 -60

-10

-50

-40

-30

-20 -10 V /V

0

10

10 20

0

0

-10

-20

g

0.0002

-7

V = -60 V

(c)

-30 V /V

-40

-50

-60

d

10

-2x10

-8

(d)

d

V = - 60 V g

-8

10

-1.5x10

-8

-10

10

d

d

-I / A

10

-I / A

1/2

/A

1/2

-9

0.0001

d

(-I )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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-1x10

-8

-5x10

-9

V = - 50 V g

V = - 40 V g

0 -60

-11

-50

-40

-30

-20 -10 V /V

0

10

10 20

0

V = - 30~0 V g

0

-10

-20

g

-30 -40 V /V

-50

-60

d

Figure 5. Transfer (a, c) and output (b, d) characteristics of top-contact, bottom-gate OFET devices (L = 40 µm, W = 3000 µm) with spin-coated thin films of 1b (a, b) and 1c (c, d) fabricated on HMDS-treated substrates. 12 ACS Paragon Plus Environment

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These poor device characteristics can be rationalized by the ordering nature in the thin-film state as elucidated by thin-film XRD measurements (Figure 6). In the out-of-plane XRDs (Figure 6a), although the thin films showed peaks up to the second (1a), third (1b), or third (1c) order, respectively, the extracted d-spacings, ca. 16 Å (1a), ca. 23 Å (1b), and 24 Å (1c), were obviously shorter than the molecular lengths of the structural model with the stretched trunk part of alkyl groups, ca. 28–29, 31–32, and 33–34 Å for 1a, 1b, and 1c, respectively (Figure S4). These out-ofplane XRDs clearly demonstrate that the molecules in the thin film do not stand perpendicular to the substrate surface, i.e., the molecules do not show the edge-on molecular orientation, which is a common feature of high-performance organic semiconductors, such as pentacene and DNTT.6 Besides the different molecular orientation from the edge-on one, their in-plane XRDs (Figure 6b) showed no clear peaks, indicating that the molecular ordering in the transverse direction of the substrate surface was also poor. The poor ordering in the thin-film state is qualitatively consistent with the optical properties of their thin films (Figure 4). From these experimental results, we conclude that the introduction of two branched alkyl groups at both ends of the DNTT core is not a promising strategy for the development of high-performance soluble organic semiconductors.

(a)

(b)

001

001

0

5

002

002

1a

003

1b

003

10 15 20 2θ / degree

Intensity / arb.unit

001 002

Intensity / arb. unit

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1b

1c

1c

25

1a

30

15

20 25 2θχ / degree

30

Figure 6. Out-of-plane (a) and in-plane (b) XRDs of 1a–c thin films on Si/SiO2 substrate.



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Thin-film transistors based on monoalkylated derivatives (2a–c)

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In sharp contrast to the

poor semiconducting characteristics of the devices based on 1a–c, monoalkylated derivatives (2a–c) afforded devices with decent transistor characteristics. Using spin-coated thin films of 2a–c, devices featuring mobilities as high as 0.5 cm2 V–1 s–1 and Ion/off of 104–107 were fabricated (Table 4, Figure 7). Interestingly, 2b-based devices obtained after several runs of device fabrication showed mobilities of up to 1.6 cm2 V–1 s–1 and Ion/off of 105 (Figure S5), although the reproducibility was not very good. This could be related to the relatively low solubilities of the monoalkylated derivatives compared with the dialkylated ones (Table 1), which resulted in the low uniformity of the spincoated thin films and eventually the large variation of device characteristics. These promising performances of the monoalkylated derivatives impelled us to further confirm their potential as high-performance organic semiconductors. We fabricated OFET devices by using the vapordeposited thin films of 2a–c (Figure 8, Table S2). Vapor deposition afforded thin films having much better uniformity in their appearance, and the device characteristics were rather reproducible. By optimizing the substrate temperature during thin-film deposition and surface treatment of the Si/SiO2 substrates, the device performances were markedly improved; the extracted mobilities from the saturation regime were higher than those recorded for the solution-processed devices; the mobilities of 2b- and 2c-based devices reached 2.5 cm2 V–1 s–1 and Ion/off was approximately 107, which were comparable to those of parent DNTT-based devices.6 These results indicate that the present monosubstituted DNTT derivatives have high potential as solution-processable organic semiconductors.


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-3

0.02

10

V = -60 V

(a)

d

-4

-4x10

d

d

-I / A

10

-I / A

1/2

/A

1/2 d

-7

g

-3x10

-6

0.01

V = - 60 V

-4

-5

10 10

(-I )

(b)

-4

10

0.015

V = - 50 V g

-4

-2x10

-8

10

0.005

V = - 40 V

-4

g

-1x10

-9

10

V = - 30 V g

-10

0 -60

-50

-40

-30

-20 -10 V /V

0

10

20

10

0

0

-10

-20

g

-3

d

-6x10

-4

10

0.015

-60

V = - 60 V g

-4

-4x10

d

-I / A

d

-7

10

d

1/2

/A

1/2

0.01

g

-3x10

-9

-1x10

-30

-20 -10 V /V

0

10

g

V = - 30 V g

-4

V = - 20 V g

-10

-40

V = - 40 V

-4

-2x10

10 0 -60 -50

V = - 50 V

-4

-8

10

0.005

10 20

0

0

-10

-20

10

-4

10

-5

10

-6

10

-7

10

-8

10

-9

10

-10

-50

-60

d

0.005

-2x10

V = - 60 V g

-4

V = - 50 V g

d

0.01

(f)

-I / A

/A

1/2

0.015

-3

d

d

10

-I / A

V = -60 V

(e)

-30 -40 V /V d

g

1/2

-50

-5

10 10

(-I )

(d)

-4

-5x10

-6

0.02

-40

-4

10

V = -60 V

(c)

-30

V /V d

-I / A

0.02

(-I )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


-1x10

-4

V = - 40 V g

V = - 30 V g

V = - 20 V g

0 -60

-50

-40

-30

-20

-10

V /V

0

10

20

0

0

-10

-20

-30 -40 V /V

-50

-60

d

g

Figure 7. Transfer (a, c, e) and output (b, d, f) characteristics of solution-processed FET devices fabricated on Si/SiO2 substrate with 2a (a, b), 2b (c, d), and 2c (e, f) as the active layer. 15 ACS Paragon Plus Environment


0.03

-3

10

-4

10

-5

10

-6

0.02

10

-7

10

-8

0.01

10

-9

0 -60

-50

-40

-30

-20 -10 V /V

0

10

10 20

(b) V = - 60 V g

d

(-I )

1/2

/A

1/2

0.04

10

-0.002

-0.0015 V = - 50 V g

d

0.05

-2

-I / A

d

10

d

V = -60 V

(a)

-I / A

0.06

-0.001 V = - 40 V g

-0.0005 V = - 30 V g

-10

0

0

-10

-20

0.06

-2

V = -60 V

(c)

d

0.05

10

-0.002

10

-4

0.04

d

d

d

-I / A

-6

10

-I / A

1/2

/A

V = - 60 V g

10

(-I )

1/2

-0.0015

-0.001

V = - 50 V g

-7

10

0.02

-8

10 0.01

-0.0005

V = - 40 V g

-9

10

V = - 30 V

-10

-50

-40

-30

-20 -10 V /V

0

10

10 20

0

g

0

-10

-20

g

0.03

-2

10

-3

10

-4

10

-5

10

-6

10

-7

10

-8

10

-9

10

-10

d

0.02 0.01

-50

-40

-30

-20 -10 V /V

-60

0

10

20

-0.0025

(f)

V = - 60 V g

-0.002 V = - 50 V

-0.0015

g

d

/A

1/2

0.04

10

-I / A

d

0.05

0 -60

-50

d

V = -60 V

(e)

-30 -40 V /V d

-I / A

0.06

-60

-5

0.03

0 -60

-50

(d)

-3

10

1/2

-30 -40 V /V d

g

(-I )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 47

-0.001 V = - 40 V g

-0.0005 V = - 30 V g

0

0

-10

-20

-30 -40 V /V

-50

-60

d

g


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Figure 8. Transfer (a, c, e) and output (b, d, f) characteristics of vapor-processed FET devices fabricated on octyltrichlorosilane (OTS)-treated Si/SiO2 substrate with 2a (a, b), 2b (c, d), and 2c (e, f) as the active layer.

Packing structures of monoalkylated DNTTs in the thin-film state

We suspected that the

promising device characteristics should be related to the packing structure in the thin-film state. Then, we attempted to prepare single crystals of 2a–c, but were unable to obtain crystals with sufficient quality for X-ray single-crystal analysis. Alternatively, we measured thin-film XRDs of the spin-coated thin films (Figure 9). In sharp contrast to those of 1a–c (Figure 6a), the out-of-plane XRDs of all the thin films of 2a–c showed a series of peaks assignable to lamella structures (Figure 9a), which are typical of thin films of parent DNTT or C10-DNTT as superior semiconducting layers. The calculated interlayer spacings (d-spacings) are 25.8, 27.1, and 28.3 Å for 2a, 2b, and 2c, respectively. These d-spacings are longer than the estimated molecular lengths of the derivatives with the stretched trunk part of the branched alkyl group (22.6, 23.5, and 24.9 Å, respectively, Figure S6). From these findings, we speculate that there exist molecules pointing in opposite directions, i.e., “anti-parallel” or “staggered” orientation, in the edge-on manner in each lamella layer (Figure S7). In fact, similar “anti-parallel” packing structures were observed in extended πsystems unsymmetrically substituted with a bulky isopropyl,16 t-butyl,16 or TMS group.43 On the other hand, for related unsymmetrical BTBT derivatives with a phenyl group and an alkyl group longer than pentyl (C5), the "two-layer anti-parallel" structure exists, in which the orientation of molecules in one molecular layer is the same, but the molecular orientation in the next layer is the opposite.44 In this case, an intermolecular cohesive force (van der Waals interaction) induced by the long alkyl groups, often called the “fastener effect” or the “zipper effect,”42,45 may play a critical role in the determination of the packing structure. Although the present DNTT derivatives have 17 ACS Paragon Plus Environment


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alkyl groups longer than C5, branching can suppress the intermolecular van der Waals interaction between the alkyl groups, causing repulsion in the packing structure and thereby resulting in “antiparallel” packing. On the other hand, the in-plane XRDs of the three compounds show three distinct peaks in the 2θχ =15 to 30º regime (Figure 9b), which are typical of the in-plane XRD patterns of thin films of organic semiconductors with the herringbone arrangement in each lamella layer. This feature in the in-plane XRD is quite similar to those of parent DNTT and other related materials.46 Provided that the crystallites in the thin film belong to monoclinic space groups or ones with higher symmetry and have the preferred orientation along the normal to the substrate as the crystallographic c-axis as in the case of DNTT, the three peaks can be indexed as 110, 020, and 120, respectively.47,48 This assignment then provides the ab unit cell of each compound. The speculated sizes of the ab cells, a = ca. 6.2, b = 8.0 Å (Table 5), are almost identical for the three compounds within experimental error and slightly larger in the b-axis direction than those of DNTT (a = 6.187, b = 7.662 Å), implying that the large branched alkyl groups may slightly push the DNTT cores away from the adjacent molecules. Nevertheless, it is interesting to point out that even with such large alkyl groups, the molecules crystallize into the herringbone packing, indicating the fairly strong cohesive nature of the DNTT core. As a result, the high-performance thin-film transistors based on 2a–c can be explained from a structural point of view.


Page 19 of 47

(a)

001

(b) 110

002 005

120

020

2a

2a


003


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2b

110

120

020

2b

110

120

020

2c

0

5

10

15

2θ / degree

20

25

2c

30

15

20

25

30

2θχ / degree

Figure 9. Out-of-plane (a) and in-plane (b) XRDs of 2a–c thin films on Si/SiO2 substrate.

Conclusion Aiming to develop DNTT-based solution-processable organic semiconductors, we examined the effects of introducing branched alkyl groups, such as 2-ethylhexyl, 3-ethylheptyl, and 4-ethyloctyl groups, on the DNTT core. As expected, the solubilities of the newly developed DNTT derivatives were markedly improved; in particular, dialkylated DNTT derivatives (1a–c) showed solubilities that were as high as 16 g L–1 in hot toluene. However, judging from the very poor transistor characteristics and the mobilities that were more than two orders of magnitude lower than that of parent DNTT, 1a–c were proved useless as an active semiconducting material in solution-processed OFETs. Their thin-film XRDs revealed that they afforded thin films with poor crystallinity compared with parent DNTT. This meant that the intermolecular ordering in the thin-film state was weakened by the two branched alkyl groups, resulting in poor intermolecular orbital overlap in the thin-film state. In sharp contrast, the introduction of one branched alkyl group on the DNTT core did not lower solubility and yielded good semiconducting characteristics: OFET devices fabricated 19 ACS Paragon Plus Environment


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with the spin-coated thin films worked properly, showing mobilities of up to 0.5 cm2 V–1 s–1, and ones fabricated with vapor-deposited thin films afforded mobilities that were as high as 2.5 cm2 V–1 s–1, indicating their potential as solution-processable high-performance organic semiconductors. It is intriguing that the monoalkylation retains the structural characteristics of the parent DNTT and other related high-performance DNTT derivatives in the thin-film state, namely, the lamella structure with the herringbone packing motif on the substrate as confirmed by thin-film XRD measurement, consistent with the high mobility of their thin-film OFETs. As a result, the introduction of one branched alkyl group at the 2-position of DNTT enhanced solubility while retaining decent semiconducting characteristics. Among the derivatives with different branching positions of the alkyl groups, 2-ethylhexyl derivative (2a) having branching position closest to the DNTT core afforded slightly inferior device characteristics compared with 3-ethylheptyl (2b) and 4ethyloctyl derivatives (2c) in the vapor-processed OFETs, indicating that the close proximity of branching position can decrease the intermolecular orbital overlap in the solid state. Although there may be slight differences in the device characteristics depending on the branched alkyl groups, the most important knowledge we acquired from the present study is that such a simple modification as the introduction of one branched alkyl group on a largely π-extended thienoacene core, which can be accomplished by synthesis, is a promising strategy for the development of solution-processable organic semiconductors. The strategy would be applicable to other organic semiconductors that are based on naked thienoacenes and related compounds.

Experimental Synthesis General: All chemicals and solvents are of reagent grade unless otherwise indicated. Tetrahydrofuran (THF), N,N-dimethylformamide (DMF), dichloromethane, and toluene were purified with standard procedures prior to use. 4-Ethyl-1-trimethylsilyloct-1-yne36,37 and 320 ACS Paragon Plus Environment

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Page 21 of 47

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methylthio-2-(trifluoromethanesulfonyloxy)naphthalene (6d)32 were prepared as reported. All reactions were carried out under nitrogen atmosphere unless otherwise mentioned. Melting points were uncorrected. 1H and

13

C NMR spectra operated at 400 and 100 MHz, respectively, were

obtained in deuterated chloroform (CDCl3) with TMS as the internal reference; chemical shifts (δ) are reported in parts per million (ppm). EI-MS spectra were obtained using an electron impact ionization procedure (70 eV). 6-(2-Ethylhexyl)-2-methoxynaphthalene (3a)

To a solution of 6-bromo-2-methoxynaphthalene

(10 g, 42 mmol), Ni(dppp)Cl2 (2.3 g, 4.2 mmol) in THF (100 mL) was added at 0 °C 2ethylhexylmagnesium bromide solution in diethyl ether, prepared from 2-ethylhexyl bromide (36 g, 168 mmol) and Mg (4.1 g, 172 mmol) in diethyl ether (170 mL), and the resulting mixture was stirring at rt for 6 h. After cooling, the mixture was diluted with water (ca. 100 mL) and filtered to remove unreacted Mg and other solid precipitates. The filtrate was extracted with diethyl ether (100 mL × 2), and the combined extracts were washed with brine, dried (MgSO4), and evaporated in vacuo to give 6-(2-ethylhexyl)-2-methoxynaphthalene (3a) as pale yellow oil (7.6 g, 69%). 1H NMR δ 0.85–0.91 (m, 6H), 1.23–1.36 (m, 8H), 1.64 (spt, J = 6.0 Hz, 1H), 2.59–2.71 (m, 2H), 3.91 (s, 3H), 7.09–7.14 (m, 2H), 7.27 (dd, J = 8.5, 1.8 Hz, 1H), 7.50 (br. s, 1H), 7.65 (d, J = 7.1 Hz, 1H), 7.67 (d, J = 8.2 Hz, 1H);

13

C NMR δ 11.0, 14.3, 23.2, 25.6, 29.0, 32.5, 40.3, 41.2, 55.5, 105.8,

118.7, 126.6, 127.3, 128.7, 129.0, 129.2, 133.0, 137.3, 157.2; EI-MS (70 eV) m/z = 270 [M+]; HRMS (EI) Calcd for C19H26O [M+]: 270.1984, found: 270.1978. 6-(3-Ethylheptyl)-2-methoxynaphthalene (3b)

Lithium (360 mg, 52 mmol) was placed in a

100-mL round-bottomed flask, and dispersed in THF (30 mL). After the solution was cooled to 0 °C, THF solution (25 mL) of 6-methoxyl-2-naphthaldehyde (3.0 g, 16 mmol) and 2-ethylhexyl bromide (9.3 mL, 52 mmol) was added dropwise over 1.5 h. The reaction mixture was stirred at rt for 1.5 h. Saturated aqueous ammonium chloride solution (30 mL), chloroform, (50 mL) and water were added successively. The aqueous layer was separated, and extracted with chloroform (50 mL 21 ACS Paragon Plus Environment


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Page 22 of 47

× 2). The combined organic layers were washed with brine, dried (MgSO4), and concentrated in vacuo. The residue was purified by column chromatography on silica gel eluted with chloroform to give 6-(3-ethyl-2-hydroxyheptyl)-2-methoxynaphthalene (2.2 g, 46%) as pale yellow oil, which was used to the next reaction without further purification. 1H NMR δ 0.80–0.93 (m, 6H), 1.15–1.50 (m, 9H), 1.61–1.88 (m, 2H), 1.82 (d, J = 3.2 Hz, 1H), 3.92 (s, 3H), 4.83–4.95 (m, 1H), 7.10–7.18 (m, 2H), 7.46 (dd, J = 8.5, 1.6 Hz, 1H), 7.71 (s, 1H), 7.72 (d, J = 3.7 Hz, 1H), 7.74 (d, J = 3.2 Hz, 1H); 13

C NMR δ (10.5, 10.7), 14.3, 23.3, (25.6, 26.3), (28.7, 28.8), (32.6, 33.1), (35.5, 35.6), 43.3, 55.5,

(73.0, 73.0), 105.8, 119.1, (124.6, 124.7), 124.8, 127.3, 128.9, 129.5, 134.2, (140.5, 140.6), 157.8; EI-MS (70 eV) m/z = 300 [M+]; HR-MS (APCI) Calcd for C20H28O2 [M+]: 300.20838, found: 300.20816. Trifluoroacetic acid (11 g, 100 mmol) was added into a solution of 6-(3-ethyl-1-hydroxyheptyl)-2methoxynaphthalene (3.0 g, 10 mmol) in dichloromethane (50 mL). After the mixture was cooled to 0 °C, triethylsilane (1.6 mL, 10 mmol) was added, and the reaction mixture was stirred at the same temperature for 30 min. The resulting mixture was the diluted with water (50 mL) and extracted with dichloromethane (50 mL × 2). The combined organic layer was washed with brine, dried (MgSO4) and concentrated in vacuo. The residue was purified by column chromatography on silica gel eluted with hexane to give 3b (2.3 g, 82%) as pale yellow oil. 1H NMR δ 0.84–0.95 (m, 6H), 1.21–1.43 (m, 9H), 1.58–1.68 (m, 2H), 2.66–2.74 (m, 2H), 3.91 (s, 3H), 7.09–7.14 (m, 2H), 7.30 (dd, J = 8.5, 1.6 Hz, 1H), 7.54 (s, 1H), 7.65 (d, J = 3.2 Hz, 1H), 7.67 (d, J = 4.6 Hz, 1H); 13C NMR δ 11.0, 14.3, 23.3, 25.9, 29.1, 32.9, 33.3, 35.4, 38.7, 55.4, 105.8, 118.7, 126.2, 126.8, 128.1, 129.0, 129.3, 133.0, 138.7, 157.2; EI-MS (70 eV) m/z = 284 [M+]; HR-MS (APCI) Calcd for C20H29O [M+H]+: 285.22159, found: 285.22129. 6-(4-Ethyloctyl)-2-methoxynaphthalene (3c)

Tetrabutylammonium fluoride (1M in THF, 84

mL, 84 mmol) was added to a solution of 2-bromo-6-methoxynaphthalene (5.0 g, 21 mmol), 4ethyl-1-trimethylsilyloct-1-yne36,37 (13.3 g, 64 mmol), Pd(dppf)Cl2·CH2Cl2 (3.4 g, 4.2 mmol) and 22 ACS Paragon Plus Environment

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CuI (800 mg, 4.2 mmol) in toluene (100 mL) at rt. After the mixture was stirred at rt for 1 h, triethylamine (100 mL) was added, and the resulting mixture was stirred at 70 °C for 18h. After cooling to rt, the mixture was diluted with water (100 mL) and extracted with hexane (100 mL × 2). The combined extracts were washed with brine, dried (MgSO4), and concentrated in vacuo to give 6-(4-ethyloctyn-1-ly)-2-methoxynaphthalene (15.3 g, quantitative) as colorless oil, which was directly utilized in the next hydrogenation reaction without further purification. 1H NMR δ 0.86– 1.00 (m, 6H), 1.24–1.39 (m, 4H), 1.42–1.55 (m, 1H), 2.43 (d, J = 5.5 Hz, 2H), 3.91 (s, 3H), 7.08 (d, J = 2.3 Hz, 1H), 7.13 (dd, J = 8.9, 2.5 Hz, 1H), 7.42 (dd, J = 8.2, 1.4 Hz, 1H), 7.63 (d, J = 8.7 Hz, 1H), 7.66 (d, J = 9.2 Hz, 1H), 7.82 (s, 1H); 13C NMR δ 11.4, 14.3, 23.1, 23.5, 26.3, 29.2, 33.0, 39.1, 55.5, 81.9, 88.9, 105.9, 119.3, 126.7, 128.7, 129.2, 129.5, 130.9, 133.8, 158.1; EI-MS (70 eV) m/z = 294 [M+]; HR-MS (EI) Calcd for C21H26O [M+]: 294.1984, found: 294.1985. A solution of 6-(4-ethyloctyn-1-ly)-2-methoxynaphthalene (15.3 g, crude product) and 10 wt% Pd/C (2.8 g, 2.6 mmol) in THF (150 mL) was purged with hydrogen gas, and the mixture was stirred at rt, and the progress of hydrogenation was traced by GC-MS analysis. The catalyst was then filtered off, and the filtrate was concentrated in vacuo. Column chromatography of the residue on silica gel eluted with hexane gave 3c as colorless oil (12.5 g, quantitative). 1H NMR δ 0.77–0.93 (m, 6H), 1.12–1.41 (m, 11H), 1.60–1.73 (m, 2H), 2.66–2.77 (m, 2H), 3.91 (s, 3H), 7.08–7.14 (m, 2H), 7.30 (dd, J = 8.5, 1.6 Hz, 1H), 7.54 (s, 1H), 7.65 (d, J = 4.1 Hz, 1H), 7.68 (d, J = 4.6 Hz, 1H); 13

C NMR δ 11.0, 14.3, 23.3, 26.0, 28.8, 29.1, 33.0, 33.1, 36.5, 38.9, 55.4, 105.8, 118.7, 126.3,

126.7, 128.0, 129.0, 129.3, 133.0, 138.3, 157.2; EI-MS (70 eV) m/z = 298 [M+]; HR-MS (EI) Calcd for C21H30O [M+]: 298.2297, found: 298.2303. 6-(2-Ethylhexyl)-3-methylthio-2-methoxynaphthalene (4a)

To a solution of 6-(2-

ethylhexyl)-2-methoxynaphthalene (3a, 7.0 g, 26 mmol) in THF (40 mL) was added 1.6 M hexane solution of n-BuLi (33 mL, 53 mmol) at 0 °C. After the mixture was stirred for 1 h at rt, dimethyldisulfide (4.6 mL, 51 mmol) was added to the solution at 0 °C, and the resulting mixture 23 ACS Paragon Plus Environment


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Page 24 of 47

was stirred for 1 h at rt. The mixture was poured into a saturated aqueous ammonium chloride solution (100 mL) and was extracted with hexane (100 mL × 2). The combined extracts were washed with brine, dried (MgSO4), and concentrated in vacuo to give crude 6-(2-ethylhexyl)-3methylthio-2-methoxynaphthalene (4a) contaminated with the 1-methylthio isomer (15/>15/>15

210

200

1.12/1.32

–5.3

402/417

3.0

2a

1.0/2.4

0.90/1.4/5.3

334

328

1.14/1.34

–5.3

402/417

3.0

2b

0.72/1.3

0.62/1.0/3.1

368

356

1.13/1.34

–5.3

402/419

3.0

2c

1.1/1.5

0.73/1.1/2.7

358

355

1.14/1.35

–5.3

402/417

3.0

DNTTg

-/< 0.06

-/-/< 0.06

>400

>350

–i

–5.4

402/416

2.9

C10-DNTTh

-/< 0.06

-/0.070/0.86

310

118, 224, –k 298, 310j

–5.4

402/420

2.9

a

Solubilities were determined as concentration of saturated solution. Data were obtained at rt and 50 °C in chloroform, and at rt, 50, and 100 °C in

toluene. b See Figure S2. c V vs Ag/AgCl. Pt as working and counter electrodes, PhCN as solvent, Bu4NPF6 (0.1 M) as supporting electrolyte, scan rate = 0.1 V s–1. All the potentials were calibrated with Fc/Fc+ (E1/2 = +0.60 V measured under identical conditions). d Estimated with the following equation: EHOMO = –(4.20 + Eonset). e In chloroform solution. f Calculated from λedge. g Data from reference 6. h Data from reference 20. i See reference 6. j Peak data from reference 20. k See reference 20.

37

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Table 2. Thin-film properties of DNTT derivatives.

Compound

IP / eV a

λ peak/edge / nm b

Eg / eV c

1a

5.7

420/435

2.8

1b

5.3

424/447

2.8

1c

5.5

429/446

2.8

2a

5.1

443/471

2.6

2b

5.1

446/474

2.6

2c

5.1

445/470

2.6

DNTT d

5.4

447/473

2.6

C10-DNTT e

4.9

470/483

2.6

a

Determined by photoelectron spectroscopic measurement of spin-coated thin films on ITO substrates b Measurements of spin-coated thin films on quartz substrates were conducted. c Calculated from λedge. d Data from reference 6. e Data from reference 20.


38

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Table 3. Characteristics of OFETs based on 1b and 1c.

Compound 1b

1c

a

Spin-coating SAM

µ / cm2 V–1 s–1 a

Vth / V

Ion/off

untreated

1.4 (0.59) × 10–3

–25

~ 105

HMDS

1.1 (0.57)× 10–2

–38

~ 104

OTS

9.8 (5.4) × 10–3

–28

~ 107

ODTS

5.5 (2.6) × 10–3

–26

~ 106

untreated

5.1 (4.0) × 10–5

–25

~ 105

HMDS

5.0 (4.4) × 10–5

–44

~ 104

OTS

1.6 (0.85) × 10–5

–21

~ 104

ODTS

5.7 (2.7) × 10–5

–21

~ 106

Typical values obtained from more than 15 devices. Values in parentheses are average values.


39


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Table 4. Characteristics of solution-processed OFETs based on 2a–c. Compound

SAM

µ / cm2 V–1 s–1 a

Vth / V

Ion/off

2a

untreated

0.36 (0.27)

–11

~ 107

HMDS

0.11 (0.066)

–6.4

~ 105

OTS

0.12 (0.032)

–11

~ 105

ODTS

0.021 (0.015)

–5.9

~ 105

untreated

0.11 (0.064)

–8.3

~ 105

HMDS

0.28 (0.11)

–4.4

~ 106

OTS

0.086 (0.046)

–7.8

~ 106

ODTS

0.19 (0.091)

–9.2

~ 105

untreated

0.55 (0.16)

–8.6

~ 105

HMDS

0.28 (0.13)

–15

~ 105

OTS

0.075 (0.029)

0.30

~ 104

ODTS

0.16 (0.056)

–9.0

~ 104

2b

2c


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Table 5. Structural parameters of ab-cell extracted from XRD data.

Compound

d-spacinga / Å

Molecular length (l) / Åb

a / Åc

b/ Åd

2a

25.8

21.4

6.2

8.0

2b

27.3

22.4

6.2

8.0

2c

28.3

23.6

6.2

8.0

DNTT e

16.3

15.3

6.1

7.7

(16.21(1)) d

(15.35)d

(6.187(4)) d

(7.662(6)) d

a

Calculated interlayer spacing from 001 reflection. b Obtained from molecular geometries optimized by DFT calculations (B3LYP/6-31G*). Value in parentheses for DNTT was obtained by single-crystal X-ray analysis. c Length of crystallographic axis in the stacking direction was estimated from in-plane XRD data. Value in parentheses for DNTT was obtained by singlecrystal X-ray analysis. d Length of crystallographic b-axis (side-by-side direction) was estimated from in-plane XRD data. Value in parentheses for DNTT was obtained by single-crystal X-ray analysis. e See reference 6.


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