DOI: 10.1002/chem.201304914

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& Organic Semiconductors

Synthesis and Properties of Semiconducting Bispyrrolothiophenes for Organic Field-Effect Transistors Crystalann Jones,[a] Damien Boudinet,[b] Yu Xia,[b] Mitch Denti,[b] Adita Das,[a] Antonio Facchetti,*[b] and Tom G. Driver*[a]

pounds were dominated by the pyrrolothiophene unit with a lmax value of approximately 400 nm and oxidation at approximately 1 V. FET devices constructed with thin films of these bispyrrolothiophenes were also fabricated by means of thin-film solution processing. One of these compounds, a bispyrrolothiophene linked with benzothiodiazole, exhibits a mobility of approximately 0.3 cm2 V 1 s 1 and the Ion/Ioff value is greater than 106.

Abstract: A series of new highly soluble bispyrrolothiophenes were synthesized from vinyl azides by using transition-metal-catalyzed C H-bond functionalization. In addition to modifying the substituents present on the end-pyrrolothiophene moieties, the arene linker in between the two units was also varied. The solution-state properties and fieldeffect-transistor (FET) electrical behavior of these bispyrrolothiophenes was compared. Our investigations identified that the optical properties and oxidation potential of our com-

Introduction Interest in semiconducting polymers and fused-oligomeric aromatic molecules for charge transport and the storage/conversion of energy has exploded because of their potential to be low-cost alternatives to inorganic materials, easy processibility, and tunable synthesis.[1] Although initial polythiophenes and acenes for solution-processed organic field-effect transistors (OFETs) suffered from poor solubility, easy oxidation, or lengthy syntheses for functionalization,[2–5] tremendous progress has been made.[6] Recently, impressive performances have been achieved, once demonstrated only using pentacene and single-crystal rubrene.[7] For example, field-effect transistors (FETs) based on soluble (benzo-)fused heretoaromatic small molecules,[8] perylenediimides,[9] and diketopyrrole (DPP)-based polymers[10–12] now achieve hole and electron mobilities that well surpass 5 cm2 V 1 s 1 and are as high as approximately 15 cm2 V 1 s 1. Despite this significant progress, the development of new, easily modifiable organic materials continues to inspire synthetic groups to further develop our understanding of charge-transport phenomena in organic solids and implement them into solution-processed OFETs.[13] Arguably, the most success has been achieved for semiconductors containing thiophenes (Scheme 1).[2, 6] These heterocy-

Scheme 1. Heteroaromatic electronic materials. PEDOT-PSS = poly(2,3-dihydrothieno-1,4-dioxin)–poly(styrenesulfonate), PXDOP = poly[(3,4-alkylenedioxy)pyrrole-2,5-diyl].

cles have promising intrinsic electronic properties and can achieve good environmental stability. Although unsubstituted polythiophenes 1 a are semiconducting, their poor solubility severely restricts their processibility.[14] Core functionalization with alkyl chains at the 3 position of the thiophene addresses the poor solubility of the material (e.g., 1 b),[15] but varying the identity of this substituent can be synthetically challenging. Moreover, these monomers require a stereoregular, efficient polymerization to optimize the polymer molecular weight and performance.[16, 17] Despite these challenges, commercial processes have been developed that use the polythiophene PEDOT-PSS (2; scheme 1) for antistatic treatments and electrochromatic applications.[18] Although the implementation of

[a] C. Jones, A. Das, Prof. Dr. T. G. Driver Department of Chemistry University of Illinois at Chicago 845 West Taylor Street, Chicago, IL, 60607 (USA) E-mail: [email protected] [b] Dr. D. Boudinet, Dr. Y. Xia, M. Denti, Dr. A. Facchetti Polyera Corporation 8045 Lamon Avenue, Skokie, IL, 60077 (USA) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201304914. Chem. Eur. J. 2014, 20, 1 – 9

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Full Paper polythiophenes in FET devices has been successful,[18] the inherent nonuniform nature of the polymers makes understanding and control of the relationship between the molecular structure and electronic properties challenging. Consequently, the identification of small molecules that exhibit promising electronic structures and transport properties in devices continues to motivate the synthesis of new materials. Similar to polythiophenes, polypyrroles are well established as (semi)conducting polymers.[19, 20] Polypyrrole, however, has a band gap of approximately 2.7 eV and is easily oxidized.[21, 22] Structural modification of the pyrrole unit has been shown to improve the properties of the resulting material; that is, fusing an alkylenedioxy bridge onto the pyrrole repeating unit (i.e., PXDOP (4); scheme 1) lowers the band gap (1.6 eV) and maintains the low-oxidation potential relative to polypyrrole.[23] Although not as extensively developed as polythiophenes, commercial processes have been established that take advantage of the improved properties in PXDOP derivatives.[24] In contrast, the development of electronic materials that contain both pyrroles and thiophenes is still emerging (such as 5),[25] and we anticipated that compounds containing the thieno-[3,2-b]pyrrole structural motif, which is readily accessible from vinyl azide 7 by using the Rh2II-catalyzed C H-bond amination reaction developed by us,[26] would build on the successes of these compounds by maintaining the promising electronic properties of thiophenes and provide an easily modifiable pyrrole N atom to append substituents to modify the solubility and packing of the resulting compounds in the thin film. Although the thieno[3,2-b]pyrrole-motif has been used in boron dipyrromethene difluoride (BODIPY) dyes,[27] to the best of our knowledge, this structural motif has never been reported for OFET organic semiconductors. Herein, we describe the synthesis of a series of new low-molecular-weight bispyrrolothiophene compounds, their characterization, and their implementation into solutionprocessed OFETs.

Table 1. MO computations of pyrrolothiophene electronic structures.[a] Compound

Molecular-orbital computations of pyrrolothiophene electronic structures were performed prior to synthetic efforts to guide the core-substitution pattern. We anticipated that annulation/ linkage of this core with thiophene or benzenoid rings, because of the highly electron-rich nature of the pyrrole ring, would result in cores with very high HOMO energies. Thus, the corresponding FET devices will be difficult to switch off. The utility of such modeling has advanced with the increased accuracy of density-functional methods, which now provide accurate estimates of molecular geometries,[28] including dihedral angles and rotational barriers,[29] dipole moments,[30] and electronic-structure properties, such as electron affinity,[31, 32] ionization potential,[32] band gaps, and ground-state vibrational frequencies.[33, 34] Table 1 collects the density-functional theory (DFT) structures computed in this study and the corresponding HOMO and LUMO energies and energy gaps. Geometry optimizations and Chem. Eur. J. 2014, 20, 1 – 9

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EG [eV]

4.60

2.39

2.21

4.47

0.93

3.54

5.08

1.85

3.23

4.49

1.69

2.80

4.87

2.20

2.67

5.15

1.83

3.32

5.08

2.77

2.31

electronic-structure calculations for the gas-phase neutral states of these molecules were carried out by using DFT calculations at the B3LYP/6-31G** level of theory with Spartan PC (Spartan ‘08, Version 1.0.0; Wavefunction Inc.) Here, the molecular-orbital energies of pentacene (EH = 4.60, EL = 2.39 eV) are taken as references. The new computed structures include two bispyrrolothiophene unsubstituted cores with the corresponding end-functionalized methyl ester group and two additional bispyrrolothiophene units bridged by phenylene (electron neutral) or benzothiadiazole (electron-poor) spacers. From the results of the molecular-orbital (MO) calculations, the EH values range from approximately 4.5 eV (unsubstituted systems) to approximately 4.9/ 5.1 eV for the functionalized derivatives. It is clear that to achieve a sufficiently low HOMO energy, lower than 4.60 eV of the reference value, it is essential to functionalize the pyrrolothiophene units with the CO2R substituents. The fact that the relatively weak electron-withdrawing carboxylate substituent is sufficient to lower the MO energy to an acceptable level is very important because this group is present at the final step of the pyrrolythiophene-unit synthesis (see below).

Molecular-orbital computations

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EL [eV]

[a] DFT calculations carried out at the B3LYP/6-31G** level using Spartan ‘08 PC; EH = energy of the HOMO, EL = energy of the LUMO, EG = energy of the HOMO/LUMO gap.

Results and Discussion

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EH [eV]

Synthesis of pyrrolothiophene semiconductors The synthetic route to the potential N-heterocyclic electronic materials for our study was designed to be modular and facilitate the synthesis of a focused library of symmetrical compounds that contain two pyrrolothiophene moieties (Scheme 2). We envisioned that this library could be created most efficiently if the penultimate step of the synthesis introduced an arene linker between the pyrrolothiophenes. We an2

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Scheme 2. Synthetic route to bispyrrolothiophenes. cod = 1,5-cyclooctadiene, dba = dibenzylideneacetone, dibpy = bipyridine, DME = 1,2-dimethoxyethane, dppf = 1,1’-bis(diphenylphosphino)ferrocene, esp = a,a,a’,a’-tetramethyl-1,3-benzenedipropionate, HBpin = pinacolborane, xphos = 2-dicyclohexylphosphino-2’,4’,6’-triisopropylbiphenyl.

Figure 1. Variation of the linker identity in bispyrrolothiophenes.

ticipated that the identity of the arene would allow us to modulate the electronic properties of our compounds. Toward this end, a series of bispyrrolothiophenes were synthesized in six steps from commercially available 2-thiophenecarboxaldehyde and methyl azidoacetate 8 (Scheme 2). Knoevenagel condensation of 8 and 2-thiophenecarboxaldehyde provided vinyl azide 7. We anticipated that the pyrrole moiety might be created from 7 by using the Rh2II-catalyzed C H-bond amination reaction developed by us[20a] and were delighted to see that exposure of this azide to [Rh2(esp)2] (1 mol %) triggered a C H-bond amination reaction to produce pyrrolothiophene 9. Importantly, catalysis of this C H-bond amination step by using the Rh2II–carboxylate complex enabled reduction of the reaction temperature from 145 to 75 8C for the analogous thermolysis reaction.[35] We found that our Rh2II-catalyzed C H-bond amination reaction could be performed on a multigram scale with only 1 mol % of [Rh2(esp)2] without attenuation of the reaction yield. For our initial series of compounds, we appended an ndodecyl group on the pyrrole nitrogen atom of 9 to ensure the solubility of our bispyrrolothiophenes in a wide range of solvents. Iridium-catalyzed C H borylation installed the pinocolate ester group without disturbing the methyl ester substituent in 11.[36] Iterative Suzuki reactions of 11 installed an arene linker. The yield of the linker installation was significantly diminished when both cross-coupling reactions were attempted in one step. To investigate the effect of the linker identity on the properties of the bispyrrolothiophene, we chose electron-rich, electron-neutral, and electron-poor arenes and heteroarenes as linkers (Figure 1). The incorporation of these moieties was expected to influence the electronic parameters of the resulting

bispyrrolothiophenes 6. In addition to modifying the MO positions, we anticipated that the identity of the linker might also influence how these molecules stack in the thin film. In particular, introduction of the electron-deficient benzodithioazole was anticipated to promote the formation of a “bricklayer” structure in the thin film used to produce a FET.[37] The bricklayer structure would be favored because the composition of a mixture of electron-rich and electron-deficient p systems should favor close cofacial p stacking.[38] To investigate this relationship, dibromides of the proposed arene linkers were used in the Suzuki reaction to produce bispyrrolothiophenes 6 a–f. Irrespective of the identity of the arene linker, these compounds were soluble in a range of organic solvents including toluene, and even hexanes, to enable their use in printable OFETs if they exhibited promising electronic properties. In addition to modifying the identity of the arene linker, bispyrrolothiophenes were synthesized that varied the identity of the pyrrole N-substituent (Scheme 3). Although the n-dodecyl group was initially chosen to ensure solubility of the bispyrrolothiophenes, several other groups were appended to the pyrrole nitrogen atom. A methyl group was chosen to investigate the effect of shortening the length of the N-alkyl substituent on the properties of the bispyrrolothiophene because the length of the alkyl substituent plays a critical role in the properties of 3-substituted thiophene oligomers.[39] To maintain the solubility of the bispyrrolothiophene in toluene, the methyl ester groups were replaced with n-hexyl ester moieties. This compound was synthesized from pyrrolothiophene 13 through a Fe(OAc)3-catalyzed transesterification reaction that produced 14.[40] The resulting pyrrolothiophene was transformed into bispyrrolothiophene 6 g following steps (d)–(f) in Scheme 2. In

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Full Paper and 6). Changing the identity of the substituent on the pyrrole nitrogen atom, however, did not affect either the lmax or lonset values. These data suggest that the optical activity of these molecules is dominated by the pyrrolothiophene moiety presumably because the N-aryl substituent and the aryl linker rotate out of planarity in solution to minimize destabilizing steric interactions. Our bispyrrolothiophenes were analyzed by using cyclic-voltammetry data to determine the effect of the arene linker on the electronic properties (Table 3). Cyclic voltammograms of bispyrrolothiophenes were obtained by using a Ag/AgCl reference electrode and nBu4NPF6 as the electrolyte. Most of the Ndodecyl bispyrrolothiophenes 6 a–f exhibited well-defined irreversible multiple oxidation potentials with the fluorenyl-substituted 6 e being the only exception. The oxidative potentials of our bispyrrolothiophenes compare favorably to those reported for poly- and oligomeric thiophenes. Typically substituted tetrathiophenes undergo oxidation at approximately 2 V.[43] In contrast, our bispyrrolothiophenes oxidize at significantly lower potentials and our observed values agree well with those of other oligomers containing fused-ring thiophene systems (1.1–1.5 V).[44] Comparison of the cyclic voltammetry data of bispyrrolothiophenes 6 a–f reveals that oxidative potentials of our compounds are dominated by the pyrrolothiophene moiety (Table 3); therefore, irrespective of the electronic nature of the linker arene, the anodic potentials occur around 0.9 V (i.e., 6 a–f). We believe that the homogeneity of our data reveals that little overlap of the p system of the arene linker and pyrrolothiophene occurs in solution presumably to minimize the steric interactions between the ortho-hydrogen atom of the arene linker and the hydrogen atom at C3 in the thiophene unit. The effect of structural modification of the pyrrolothiophene motif was also investigated. Neither decreasing the length of the N-alkyl substituent nor swapping the N-alkyl substituent for an N-aryl group affected the magnitude of the oxidation potential (Table 3, 6 g–j). In every case, the oxidative potential remained approximately 0.9 V. As before, we attribute the lack of change in these results to little overlap of the p system of the N-aryl substituent with that of the pyrrolothiophene. The N-aryl substituent, however, did exert a deleterious effect on the appearance of the voltammogram; in contrast to the sharp peaks observed with the N-dodecyl group, N-aryl-bispyrrolothiophenes exhibited a broad response that consisted of multiple peaks. This response type has been previously observed in polythiophenes and polydithienopyrroles and was attributed to polymerization of the oligomeric material.[19b, 45]

Scheme 3. Variation of the N substituent on the bispyrrolothiophene.

line with our expectations, 6 g was soluble in a range of organic solvents, including toluene and hexanes. In addition to alkyl groups, arenes were also appended to the pyrrole nitrogen atom (Scheme 3). Electron-rich, electronneutral, and electron-poor arenes were chosen to further modify the electronic nature of the bispyrrolothiophene and to affect thin-film packing of these compounds. These compounds were readily accessed: a Cu-mediated N-arylation reaction[41] of pyrrolothiophene 9 with aryl boronic acids afforded 15. The resulting N-arylpyrrolothiophenes were smoothly elaborated to 6—6 j following steps (d)–(f) in Scheme 2. The solubility of the resulting bispyrrolothiophenes was significantly diminished relative to those bearing an n-dodecyl group. Solution-state properties of bispyrrolothiophenes The electronic properties of the bispyrrolothiophenes were examined by means of optical spectroscopy and cyclic voltammetry (Table 2). All of the bispyrrolothiophenes exhibit two primary optical transitions: a sharp absorption at approximately l = 280 nm and a broad peak at approximately l = 395 nm. This latter peak exhibits a prominent, broad shoulder at l = 420 nm. Based on the similar appearance of these spectra relative to dithieno[3,2-b:2’,3’-d]pyrroles, the high energy peak is tentatively assigned to a p–p* transition and the broad peak to a lower energy transition that exhibited some charge-transfer character.[19c, 42] Although the lmax values of bispyrrolothiophenes 6 do not vary considerably, the onset of absorption depended on the identity of the linker molecule with the electron-deficient benzodithiazole absorbing at a longer wavelength than the electron-rich dithiophene (Table 2, entries 3 &

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Field-effect transistor and film microstructure The charge-transport properties of the new semiconductors were investigated by fabricating top-gate/bottom-contact thinfilm transistors (TFTs; Figure 2). The semiconductor thin films (ca. 40–50 nm) were deposited on untreated Au (S-D contacts)/ glass substrates by spin-coating the solutions (ca. 5– 10 mg mL 1). After mild-film annealing (110 8C) in air for 5 min, the polymeric dielectric layer (i.e., CYTOP) was spin-coated. 4

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Full Paper The devices were annealed for an additional 10 min at 110 8C in air before depositing the Au-gate elecEG trode by thermal evaporation. All the device process[eV] ing and electrical measurements were performed in an ambient atmosphere, except for the Au contact vapor deposition. Field-effect mobilities m were calcu2.8 lated in the saturation regime by using m = (2ISDL)/[WCi(VSG Vth)2]; in which ISD = saturation current, L = length in cm, W = width in cm, VSG = fixed 2.85 gate voltage, and Vth = threshold voltage. The best-performing OFETs were obtained when 2.73 thin films of our bispyrrolothiophenes were processed with dichlorobenzene as the solvent (Table 4). Less organized films—and as a consequence, no ac2.75 tivity—were obtained when lower-boiling-point solvents were used to deposit the semiconductors. Sirringhaus and co-workers established that the use of 2.80 high-boiling-point solvents enhances device performance because the slower evaporation facilitates slow growth of highly crystalline films.[46] The activity 2.76 of our FETs depended on the structure of the bispyrrolothiophene. When an ester moiety and the n-dodecyl groups were present on pyrrolothiophenes 6 a– 2.78 f, the devices were active. No field-effect transport was observed, however, when the n-dodecyl group was replaced with an aryl group probably because the larger ring prevents efficient p–p stacking (i.e., 6 g–j). 2.76 Although poor hole mobilities (< 10 3 cm2 V 1 s 1) were measured with the dithiophene and fluorine bridges, better performances were achieved without the bridge and with benzene and naphthalene (hole mobilities were ca. 0.02–0.06 cm2 V 1 s 1). On the other hand, a very good FET performance was 2.75 observed with bispyrrolothiophene 6 f containing a benzothiodiazole moiety (Figure 3). Spin-coated and only mildly annealed films of this semiconductor exhibited charge-carrier mobilities that approached 0.3 cm2 V 1 s 1 and Ion/Ioff values of greater than 106. These results are quite exciting because these films 2.72 were annealed at only 110 8C for 5 minutes in air after spin-coating. A possible explanation for the mobility increase of semiconductor 6 f could be due to enhanced crystallinity of this film (see below) promoted by intramolecular interactions.[47] Alternatively, the enhanced device performance of semiconductor 6 f could be due to a bricklayer crystalline thin film.[25] This structure should be favored because the electron-deficient benzothiodiazole would prefer to p stack with the electronrich pyrrolothiophene moiety. The microstructure of the thin films of the bispyrrolothiophenes was investigated with wide-angle X-ray diffraction (WAXRD). Interestingly, with the exception of 6 f (Figure 4 a), all the films are amorphous. Analysis of the XRD plot of the 6 f thin film clearly reveals a broad reflection centered at approximately 3.98, thus indicating a poorly crystalline film. From this reflection, a molecular d spacing of approximately 11  is

Table 2. UV/Vis spectroscopic data for bispyrrolothiophenes 6. Bispyrrolothiophene 6

lmax [nm]

lonset [nm]

396

440

276 396 413

435

279 394

453

302 398

450

275 399

442

289 396

449

301 390

446

278 394

448

305 394

450

293 393

455

Figure 2. Structure of the top-gate/bottom-contact TFT transistor used in this study. Chem. Eur. J. 2014, 20, 1 – 9

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Full Paper Table 3. Cyclic-voltammetry data for bispyrrolothiophenes. Bispyrrolothiophene[a]

E onset [V][b] pa

6a 6b 6c 6d 6e 6f 6g 6h 6i 6j

0.94 1.33 0.92 0.94 0.97 1.03 0.89 1.56 1.07 1.38

E onset [V][b] pc 0.86 0.98 0.87 0.92 0.87 0.98 0.92 0.99 0.89 0.87

[a] Concentration of bispyrrolothiophene = 10 3 m. [b] Conditions: 0.1 m nBu4NPF6 in CH2Cl2, scan rate = 50 mV s 1, Ag/AgCl reference electrode. Figure 3. Representative FET a) transfer and b) output for 6 f. The curves shown are for the indicated bispyrrolothiophenes.

found, which is consistent with MO computations. The polycrystalline nature of 6 f was further confirmed by the AFM image shown in Figure 4 b, which indicates the formation of rectangular-shaped crystallites. However, the overall XRD/AFM/ FET data indicate that despite the amorphous/very poor crystalline nature of these films, respectable FET mobilities can be achieved for few of these compounds. Although some classes of conjugated polymer do exhibit good performances in the amorphous state,[1e] this result is not common for small-molecule semiconductors. Thus, we feel that FET optimization of these systems by finding more compatible dielectric/electricalcontact treatments or by additional structural modifications to enhance thin-film crystallinity is worth additional efforts.

Figure 4. a) WAXRD of a representative polycrystaline film 6 j and amorphous films 6 a, b of bispyrrolothiophenes. b) AFM image of 6 f.

Conclusion Table 4. Performance of bispyrrolothiophenes in thin-film FETs.[a] [b]

Bispyrrolothiophene 6

[c] sat 2

m [cm V 1 s 1]

Ion/Ioff

VT [V]

0.03

60

4  105

0.004

63

3  105

0.1

39

2  105

0.04

74

3  103

58

7  102

29

4  106

3  10

0.28

5

[a] Thin films created for 10 mg mL 1 solutions in dichlorobenzene. [b] Compounds 6 g–j were inactive. [c] Computed from the slope of ID1/2 = f(VD); capacitance = 2.3 nF cm 2.

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The synthesis, solution-state properties, and OFET performance of a new series of bispyrrolothiophenes has been reported. The synthesis of these semiconductors was facilitated by using transition-metal-catalyzed C H-bond functionalization. The pyrrolothiophene structural motif was established from a vinyl azide by using the Rh2II-catalyzed C H-bond amination reaction developed by us. To enable a modular synthesis, a subsequent IrI-catalyzed C H-bond borylation provided the substrate for a Suzuki–Miyaura cross-coupling reaction, which allowed ready variation of the identity of the arene linker between the two pyrrolothiophene moieties. The solution-state properties and FET electrical behavior of these bispyrrolothiophenes was compared. Our investigations identified that the optical properties and oxidation potential of our compounds were dominated by the pyrrolothiophene moiety with a lmax value at approximately 400 nm and an electrochemical oxidation at approximately 1 V, independent of the arene central linker. In contrast, when FET active (i.e., 6 a–f), the performance of the FET devices fabricated from spincoated films of these bispyrrolothiophenes depended on the identity of the arene linker. The bispyrrolothio 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

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Full Paper phene linked with benzothiodiazole 6 f exhibited a respectable hole mobility that approached 0.3 cm2 V 1 s 1 and a Ion/Ioff value of greater than 106. Lower performances (i.e., mobilities up to 0.04–0.06 cm2 V 1 s 1) were obtained for compounds with more electron-rich arene linkers, whereas poorer mobilities were obtained for the systems with no linkers or the bulky fluorine unit (0.03 to ~ 10 4 cm2 V 1 s 1). No field effect was observed for bispyrrolothiophenes bearing N-aryl substituents or those lacking ester groups at C3. These results suggest that fairly amorphous small molecules can exhibit decent field-effect mobilities and that incorporation of an electron-deficient zone between two electron-rich pyrrolothiophenes promotes minimal crystallinity, which further enhances FET performance. Thus, further performance optimization in this family will employ the use of more p-extended electron-poor units with short N-alkylation substituents and carboxyalkyl end-functionalization.

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FULL PAPER & Organic Semiconductors C. Jones, D. Boudinet, Y. Xia, M. Denti, A. Das, A. Facchetti,* T. G. Driver* && – && Highly soluble bispyrrolothiophenes have been synthesized from vinyl azides by using transition-metal-catalyzed C H bond functionalization (see scheme;

Chem. Eur. J. 2014, 20, 1 – 9

TFT = thin-film transistor). The solutionstate properties and field-effect-transistor (FET) electrical behavior of these compounds were investigated.

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Synthesis and Properties of Semiconducting Bispyrrolothiophenes for Organic Field-Effect Transistors

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