Synthesis and properties of semiconducting bispyrrolothiophenes for organic field-effect transistors

Article abstract of DOI:10.1002/chem.201304914

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 field-effect-transistor (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 unit with a λ_max 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 cm ²V^-1s^-1 and the I_on/I_off value is greater than 10⁶. Highly soluble bispyrrolothiophenes have been synthesized from vinyl azides by using transition-metal-catalyzed C-H bond functionalization (see scheme; TFT=thin-film transistor). The solution-state properties and field-effect-transistor (FET) electrical behavior of these compounds were investigated.

Full text of DOI:10.1002/chem.201304914

DOI: 10.1002/chem.201304914
Full Paper
&
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]
Abstract: A series of new highly soluble bispyrrolothio-
phenes were synthesized from vinyl azides by using transi-
tion-metal-catalyzed CÀH-bond functionalization. In addition
to modifying the substituents present on the end-pyrrolo-
thiophene moieties, the arene linker in between the two
units was also varied. The solution-state properties and field-
effect-transistor (FET) electrical behavior of these bispyrrolo-
thiophenes was compared. Our investigations identified that
the optical properties and oxidation potential of our com-
pounds were dominated by the pyrrolothiophene unit with
a lmax value of approximately 400 nm and oxidation at ap-
proximately 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 VÀ1 sÀ1 and the Ion/Ioff
value is greater than 106.
Introduction
Interest in semiconducting polymers and fused-oligomeric aro-
matic molecules for charge transport and the storage/conver-
sion 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 VÀ1 sÀ1 and are as high as approximately
15 cm2 VÀ1 sÀ1. Despite this significant progress, the develop-
ment of new, easily modifiable organic materials continues to
inspire synthetic groups to further develop our understanding
of charge-transport phenomena in organic solids and imple-
ment them into solution-processed OFETs.[13]
Scheme 1. Heteroaromatic electronic materials. PEDOT-PSS=poly(2,3-dihy-
drothieno-1,4-dioxin)–poly(styrenesulfonate), PXDOP=poly[(3,4-alkylene-
dioxy)pyrrole-2,5-diyl].
cles have promising intrinsic electronic properties and can ach-
ieve good environmental stability. Although unsubstituted
polythiophenes 1a 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., 1b),[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 pro-
cesses have been developed that use the polythiophene
PEDOT-PSS (2; scheme 1) for antistatic treatments and electro-
chromatic applications.[18] Although the implementation of
Arguably, the most success has been achieved for semicon-
ductors containing thiophenes (Scheme 1).[2,6] These heterocy-
[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: tgd@uic.edu
[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.
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polythiophenes in FET devices has been successful,[18] the in-
herent nonuniform nature of the polymers makes understand-
ing 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 con-
tinues to motivate the synthesis of new materials. Similar to
polythiophenes, polypyrroles are well established as (semi)con-
ducting 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 alk-
ylenedioxy 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 pro-
cesses have been established that take advantage of the im-
proved properties in PXDOP derivatives.[24] In contrast, the de-
velopment of electronic materials that contain both pyrroles
and thiophenes is still emerging (such as 5),[25] and we antici-
pated that compounds containing the thieno-[3,2-b]pyrrole
structural motif, which is readily accessible from vinyl azide 7
Table 1. MO computations of pyrrolothiophene electronic structures.[a]
Compound
EH [eV]
À4.60
EL [eV]
À2.39
EG [eV]
2.21
À4.47
À5.08
À0.93
À1.85
3.54
3.23
À4.49
À4.87
À5.15
À1.69
À2.20
À1.83
2.80
2.67
3.32
À5.08
À2.77
2.31
[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.
II
by using the Rh2 -catalyzed CÀH-bond amination reaction de-
veloped by us,[26] would build on the successes of these com-
pounds 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 solution-
processed OFETs.
electronic-structure calculations for the gas-phase neutral
states of these molecules were carried out by using DFT calcu-
lations at the B3LYP/6-31G** level of theory with Spartan PC
(Spartan ‘08, Version 1.0.0; Wavefunction Inc.) Here, the molec-
ular-orbital energies of pentacene (EH =À4.60, EL =À2.39 eV)
are taken as references. The new computed structures include
two bispyrrolothiophene unsubstituted cores with the corre-
sponding end-functionalized methyl ester group and two addi-
tional bispyrrolothiophene units bridged by phenylene (elec-
tron neutral) or benzothiadiazole (electron-poor) spacers. From
the results of the molecular-orbital (MO) calculations, the
EH values range from approximately À4.5 eV (unsubstituted
systems) to approximately À4.9/À5.1 eV for the functionalized
derivatives. It is clear that to achieve a sufficiently low HOMO
energy, lower than À4.60 eV of the reference value, it is essen-
tial to functionalize the pyrrolothiophene units with the CO2R
substituents. The fact that the relatively weak electron-with-
drawing 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).
Results and Discussion
Molecular-orbital computations
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, be-
cause 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 accu-
racy of density-functional methods, which now provide accu-
rate estimates of molecular geometries,[28] including dihedral
angles and rotational barriers,[29] dipole moments,[30] and elec-
tronic-structure properties, such as electron affinity,[31,32] ioniza-
tion potential,[32] band gaps, and ground-state vibrational fre-
quencies.[33,34]
Synthesis of pyrrolothiophene semiconductors
The synthetic route to the potential N-heterocyclic electronic
materials for our study was designed to be modular and facili-
tate the synthesis of a focused library of symmetrical com-
pounds that contain two pyrrolothiophene moieties
(Scheme 2). We envisioned that this library could be created
most efficiently if the penultimate step of the synthesis intro-
duced an arene linker between the pyrrolothiophenes. We an-
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
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Scheme 2. Synthetic route to bispyrrolothiophenes. cod=1,5-cycloocta-
diene, dba=dibenzylideneacetone, dibpy=bipyridine, DME=1,2-dimeth-
oxyethane, dppf=1,1’-bis(diphenylphosphino)ferrocene, esp=a,a,a’,a’-tet-
ramethyl-1,3-benzenedipropionate, HBpin= pinacolborane, xphos=2-dicy-
clohexylphosphino-2’,4’,6’-triisopropylbiphenyl.
Figure 1. Variation of the linker identity in bispyrrolothiophenes.
ticipated that the identity of the arene would allow us to mod-
ulate 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 condensa-
tion of 8 and 2-thiophenecarboxaldehyde provided vinyl azide
7. We anticipated that the pyrrole moiety might be created
bispyrrolothiophenes 6. In addition to modifying the MO posi-
tions, we anticipated that the identity of the linker might also
influence how these molecules stack in the thin film. In partic-
ular, 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 brick-
layer 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 re-
lationship, dibromides of the proposed arene linkers were
used in the Suzuki reaction to produce bispyrrolothiophenes
6a–f. Irrespective of the identity of the arene linker, these com-
pounds 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.
II
from 7 by using the Rh2 -catalyzed CÀH-bond amination reac-
tion developed by us[20a] and were delighted to see that expo-
sure of this azide to [Rh2(esp)2] (1 mol%) triggered a CÀH-bond
amination reaction to produce pyrrolothiophene 9. Important-
ly, catalysis of this CÀH-bond amination step by using the
II
Rh2 –carboxylate complex enabled reduction of the reaction
temperature from 145 to 758C for the analogous thermolysis
II
reaction.[35] We found that our Rh2 -catalyzed CÀH-bond amina-
tion 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 n-
dodecyl group on the pyrrole nitrogen atom of 9 to ensure
the solubility of our bispyrrolothiophenes in a wide range of
solvents. Iridium-catalyzed CÀH borylation installed the pinoco-
late ester group without disturbing the methyl ester substitu-
ent in 11.[36] Iterative Suzuki reactions of 11 installed an arene
linker. The yield of the linker installation was significantly di-
minished when both cross-coupling reactions were attempted
in one step.
In addition to modifying the identity of the arene linker, bis-
pyrrolothiophenes 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 bispyrro-
lothiophenes, several other groups were appended to the pyr-
role 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 prop-
erties 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 bis-
pyrrolothiophene 6g following steps (d)–(f) in Scheme 2. In
To investigate the effect of the linker identity on the proper-
ties of the bispyrrolothiophene, we chose electron-rich, elec-
tron-neutral, and electron-poor arenes and heteroarenes as
linkers (Figure 1). The incorporation of these moieties was ex-
pected to influence the electronic parameters of the resulting
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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-vol-
tammetry 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 refer-
ence electrode and nBu4NPF6 as the electrolyte. Most of the N-
dodecyl bispyrrolothiophenes 6a–f exhibited well-defined irre-
versible multiple oxidation potentials with the fluorenyl-substi-
tuted 6e being the only exception. The oxidative potentials of
our bispyrrolothiophenes compare favorably to those reported
for poly- and oligomeric thiophenes. Typically substituted tet-
rathiophenes 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 sys-
tems (1.1–1.5 V).[44] Comparison of the cyclic voltammetry data
of bispyrrolothiophenes 6a–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 À0.9 V
(i.e., 6a–f). We believe that the homogeneity of our data re-
veals 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 thio-
phene unit.
Scheme 3. Variation of the N substituent on the bispyrrolothiophene.
line with our expectations, 6g was soluble in a range of organ-
ic solvents, including toluene and hexanes.
In addition to alkyl groups, arenes were also appended to
the pyrrole nitrogen atom (Scheme 3). Electron-rich, electron-
neutral, 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 com-
pounds were readily accessed: a Cu-mediated N-arylation reac-
tion[41] of pyrrolothiophene 9 with aryl boronic acids afforded
15. The resulting N-arylpyrrolothiophenes were smoothly ela-
borated to 6—6j following steps (d)–(f) in Scheme 2. The solu-
bility of the resulting bispyrrolothiophenes was significantly di-
minished relative to those bearing an n-dodecyl group.
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, 6g–j). In every case, the oxidative potential
remained approximately À0.9 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-bispyr-
rolothiophenes exhibited a broad response that consisted of
multiple peaks. This response type has been previously ob-
served in polythiophenes and polydithienopyrroles and was at-
tributed to polymerization of the oligomeric material.[19b,45]
Solution-state properties of bispyrrolothiophenes
The electronic properties of the bispyrrolothiophenes were ex-
amined by means of optical spectroscopy and cyclic voltam-
metry (Table 2). All of the bispyrrolothiophenes exhibit two pri-
mary 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 rela-
tive 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-trans-
fer character.[19c,42] Although the lmax values of bispyrrolothio-
phenes 6 do not vary considerably, the onset of absorption de-
pended on the identity of the linker molecule with the elec-
tron-deficient benzodithiazole absorbing at a longer wave-
length than the electron-rich dithiophene (Table 2, entries 3
Field-effect transistor and film microstructure
The charge-transport properties of the new semiconductors
were investigated by fabricating top-gate/bottom-contact thin-
film 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 mgmLÀ1). After mild-film annealing (1108C) in air for 5 min,
the polymeric dielectric layer (i.e., CYTOP) was spin-coated.
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The devices were annealed for an additional 10 min
at 1108C in air before depositing the Au-gate elec-
trode by thermal evaporation. All the device process-
ing and electrical measurements were performed in
an ambient atmosphere, except for the Au contact
vapor deposition. Field-effect mobilities m were calcu-
lated in the saturation regime by using m=
(2ISDL)/[WCi(VSGÀVth)2]; in which ISD =saturation cur-
rent, L=length in cm, W=width in cm, VSG =fixed
gate voltage, and Vth =threshold voltage.
Table 2. UV/Vis spectroscopic data for bispyrrolothiophenes 6.
Bispyrrolothiophene 6
lmax
lonset
EG
[eV]
[nm]
[nm]
396
440
435
2.8
276
396
413
2.85
The best-performing OFETs were obtained when
thin films of our bispyrrolothiophenes were pro-
cessed with dichlorobenzene as the solvent (Table 4).
Less organized films—and as a consequence, no ac-
tivity—were obtained when lower-boiling-point sol-
vents were used to deposit the semiconductors. Sir-
ringhaus and co-workers established that the use of
high-boiling-point solvents enhances device per-
formance because the slower evaporation facilitates
slow growth of highly crystalline films.[46] The activity
of our FETs depended on the structure of the bispyr-
rolothiophene. When an ester moiety and the n-do-
decyl groups were present on pyrrolothiophenes 6a–
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.,
6g–j).
279
394
453
450
442
2.73
2.75
2.80
302
398
275
399
289
396
449
446
2.76
2.78
301
390
278
394
448
450
455
2.76
2.75
2.72
Although poor hole mobilities (<10À3 cm2 VÀ1 sÀ1
)
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 VÀ1 sÀ1). On the
other hand, a very good FET performance was
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 VÀ1 sÀ1 and Ion/Ioff values of greater than 106.
These results are quite exciting because these films
were annealed at only 1108C for 5 minutes in air
after spin-coating. A possible explanation for the mo-
bility increase of semiconductor 6 f could be due to
enhanced crystallinity of this film (see below) pro-
moted by intramolecular interactions.[47] Alternatively,
the enhanced device performance of semiconductor
305
394
293
393
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 electron-
rich pyrrolothiophene moiety.
The microstructure of the thin films of the bispyrrolothio-
phenes was investigated with wide-angle X-ray diffraction
(WAXRD). Interestingly, with the exception of 6 f (Figure 4a), all
the films are amorphous. Analysis of the XRD plot of the 6 f
thin film clearly reveals a broad reflection centered at approxi-
mately 3.98, thus indicating a poorly crystalline film. From this
reflection, a molecular d spacing of approximately 11 ꢁ is
Figure 2. Structure of the top-gate/bottom-contact TFT transistor used in
this study.
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Table 3. Cyclic-voltammetry data for bispyrrolothiophenes.
onset
pa
onset
[V][b]
pc
Bispyrrolothiophene[a]
E
[V][b]
E
6a
6b
6c
6d
6e
6 f
6g
6h
6i
0.94
1.33
0.92
0.94
0.97
1.03
0.89
1.56
1.07
1.38
À0.86
À0.98
À0.87
À0.92
À0.87
À0.98
À0.92
À0.99
À0.89
À0.87
6j
[a] Concentration of bispyrrolothiophene=10À3 m. [b] Conditions: 0.1m
nBu4NPF6 in CH2Cl2, scan rate=50 mVsÀ1, 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 poly-
crystalline nature of 6 f was further confirmed by the AFM
image shown in Figure 4b, which indicates the formation of
rectangular-shaped crystallites. However, the overall XRD/AFM/
FET data indicate that despite the amorphous/very poor crys-
talline 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-mole-
cule semiconductors. Thus, we feel that FET optimization of
these systems by finding more compatible dielectric/electrical-
contact treatments or by additional structural modifications to
enhance thin-film crystallinity is worth additional efforts.
Figure 4. a) WAXRD of a representative polycrystaline film 6j and amor-
phous films 6a,b of bispyrrolothiophenes. b) AFM image of 6 f.
Conclusion
Table 4. Performance of bispyrrolothiophenes in thin-film FETs.[a]
The synthesis, solution-state properties, and OFET
performance of a new series of bispyrrolothiophenes
[c]
Bispyrrolothiophene 6[b]
msat
[cm2 VÀ1 sÀ1
VT
[V]
Ion/Ioff
has been reported. The synthesis of these semicon-
ductors was facilitated by using transition-metal-cata-
]
lyzed CÀH-bond functionalization. The pyrrolothio-
0.03
À60
À63
À39
À74
À58
4ꢂ105
3ꢂ105
2ꢂ105
3ꢂ103
7ꢂ102
phene structural motif was established from a vinyl
II
azide by using the Rh2 -catalyzed CÀH-bond amina-
tion reaction developed by us. To enable a modular
synthesis, a subsequent IrI-catalyzed CÀH-bond bor-
ylation provided the substrate for a Suzuki–Miyaura
cross-coupling reaction, which allowed ready varia-
tion of the identity of the arene linker between the
two pyrrolothiophene moieties. The solution-state
properties and FET electrical behavior of these bispyr-
rolothiophenes 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 approx-
imately 400 nm and an electrochemical oxidation at
approximately 1 V, independent of the arene central
linker. In contrast, when FET active (i.e., 6a–f), the
performance of the FET devices fabricated from spin-
coated films of these bispyrrolothiophenes depended
on the identity of the arene linker. The bispyrrolothio-
0.004
0.1
0.04
3ꢂ10À5
0.28
À29
4ꢂ106
[a] Thin films created for 10 mgmLÀ1 solutions in dichlorobenzene. [b] Com-
pounds 6g–j were inactive. [c] Computed from the slope of ID1/2 =f(VD); capacitance=
2.3 nFcmÀ2
.
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phene linked with benzothiodiazole 6 f exhibited a respectable
hole mobility that approached 0.3 cm2 VÀ1 sÀ1 and a Ion/Ioff value
of greater than 106. Lower performances (i.e., mobilities up to
0.04–0.06 cm2 VÀ1 sÀ1) were obtained for compounds with more
electron-rich arene linkers, whereas poorer mobilities were ob-
tained for the systems with no linkers or the bulky fluorine
unit (0.03 to ~10À4 cm2 VÀ1 sÀ1). 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 mo-
bilities and that incorporation of an electron-deficient zone be-
tween 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-alk-
ylation substituents and carboxyalkyl end-functionalization.
Acknowledgements
We are grateful to the Petroleum Research Fund administered
by the American Chemical Society (51853-ND7 to T.G.D.), The
National Science Foundation (CHE-1265630 to T.G.D.), and the
University of Illinois at Chicago for their generous financial sup-
port.
Keywords: azides · charge transfer · field-effect transistors ·
heterocycles · semiconductors · thin films
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Received: December 16, 2013
Published online on && &&, 0000
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Chem. Eur. J. 2014, 20, 1 – 9
www.chemeurj.org
8
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!

Full Paper
FULL PAPER
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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 CÀH
bond functionalization (see scheme;
TFT=thin-film transistor). The solution-
state properties and field-effect-transis-
tor (FET) electrical behavior of these
compounds were investigated.
Synthesis and Properties of
Semiconducting Bispyrrolothiophenes
for Organic Field-Effect Transistors
Chem. Eur. J. 2014, 20, 1 – 9
www.chemeurj.org
9
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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These are not the final page numbers! ÞÞ