Solution-processable n-type organic field-effect transistor (OFET) materials based on carbonyl-bridged bithiazole and dioxocyclopentene-annelated thiophenes

Article abstract of DOI:10.1002/asia.201100098

A series of electronegative π-conjugated compounds composed of carbonyl-bridged bithiazole and alkyl-substituted dioxocyclopenta[b]thiophene were synthesized as a candidate for solution-processable n-type organic semiconductor materials and characterized on the basis of photophysical and electrochemical properties. Cyclic voltammetry measurements showed that the first half-wave reduction potentials of these compounds are between -0.97 and -1.14 V versus ferrocene/ferrocenium, which corresponds to lowest unoccupied molecular orbital energy levels between -3.83 and -3.66 eV. Thanks to hexyl or dodecyl groups in the molecules, the compounds are sufficiently soluble to realize the fabrication of their thin films through a spin-coating method. As a result, the prepared organic field-effect transistors based on these newly developed compounds exhibited n-channel characteristics not only under vacuum but also in air, and the best field-effect electron mobility observed under vacuum was 0.011 cm² V^-1 s^-1 with an on/off ratio of 10⁸ and a threshold voltage of 16 V. Copyright

Full text of DOI:10.1002/asia.201100098

FULL PAPERS
DOI: 10.1002/asia.201100098
Solution-Processable n-Type Organic Field-Effect Transistor (OFET)
Materials Based on Carbonyl-Bridged Bithiazole and Dioxocyclopentene-
Annelated Thiophenes
Masashi Nitani,[a] Yutaka Ie,*[a, b] Hirokazu Tada,[c] and Yoshio Aso*[a]
Abstract: A series of electronegative
p-conjugated compounds composed of
carbonyl-bridged bithiazole and alkyl-
and ꢀ1.14 V versus ferrocene/ferroce-
nium, which corresponds to lowest un-
occupied molecular orbital energy
levels between ꢀ3.83 and ꢀ3.66 eV.
Thanks to hexyl or dodecyl groups in
the molecules, the compounds are suffi-
ciently soluble to realize the fabrication
of their thin films through a spin-coat-
ing method. As a result, the prepared
organic field-effect transistors based on
these newly developed compounds ex-
hibited n-channel characteristics not
only under vacuum but also in air, and
the best field-effect electron mobility
substituted
dioxocyclopenta[b]thio-
phene were synthesized as a candidate
for solution-processable n-type organic
semiconductor materials and character-
ized on the basis of photophysical and
electrochemical properties. Cyclic vol-
tammetry measurements showed that
the first half-wave reduction potentials
of these compounds are between ꢀ0.97
Keywords: conjugation
·
organic
observed
under
vacuum
was
field-effect transistors · semiconduc-
tors · solution process · structure–
activity relationships
0.011 cm2 Vꢀ1 sꢀ1 with an on/off ratio of
108 and a threshold voltage of 16 V.
Introduction
Organic semiconducting materials based on p-conjugated
compounds are an essential component of organic electronic
devices, and considerable effort has been expended on de-
veloping organic field-effect transistor (OFET) materials.[2]
OFET materials can be generally classified into two types—
hole-transporting (p-type) and electron-transporting (n-
type) semiconducting materials. Although some p-type ma-
terials have acquired high hole mobility comparable to that
of amorphous silicon,[3] the development of n-type materials
suitable for practical use is still lacking.[4–23] Furthermore, so-
lution-processable n-type OFET materials are limited to
acene carboxylic diimide polymer,[24] thiophene dicarboxi-
mide polymer,[24] cyanomethylene-substituted conjugated
compounds,[25] and alkyl-substituted fullerenes.[26]
Recent studies on n-type OFET materials have identified
the following rational molecular design: 1) lowering the
lowest unoccupied molecular orbital (LUMO) energy level
to facilitate electron injection from metal electrodes and
2) invoking the intermolecular interactions to ease electron
transport in a solid state. Therefore, to meet these require-
ments, the introduction of electron-withdrawing groups into
planar p-conjugated systems has become a desirable molec-
ular design. We recently reported the synthesis of a new p-
conjugated system C-BTz composed of a carbonyl-bridged
bithiazole unit A as an electronegative central unit and tri-
fluoroacetylphenyl groups as an electronegative terminal
Organic electronics have become one of the most active re-
search fields because they can produce large-area, light-
weight, and flexible devices.[1] In addition, organic electronic
devices have another important characteristic: versatile solu-
tion processes such as roll-to-roll manufacturing and ink-jet
printing can be used in the fabrication of these devices. In
this context, it has been necessary to add solution processa-
bility to active materials to preserve their original potential
for organic electronics.
[a] M. Nitani, Prof. Y. Ie, Prof. Y. Aso
The Institute of Scientific and Industrial Research
Osaka University
8-1, Mihogaoka, Ibaraki, Osaka 567-0047 (Japan)
E-mail: yutakaie@sanken.osaka-u.ac.jp
aso@sanken.osaka-u.ac.jp
[b] Prof. Y. Ie
PRESTO-JST
4-1-8, Honcho, Kawaguchi, Saitama 333-0012 (Japan)
[c] Prof. H. Tada
The Graduate School of Engineering Science
Osaka University
1-3, Machikaneyama, Toyonaka, Osaka 560-8531 (Japan)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/asia.201100098.
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Chem. Asian J. 2011, 6, 2352 – 2361

Scheme 1. Chemical structures of A, B, 1, 2, 3, and C-BTz.
unit (Scheme 1).[23] Electrochemical measurements revealed
that unit A contributes largely to increasing the electronega-
tive character of C-BTz. In addition, X-ray crystallographic
B, and we have thus designed the new terminal units B6, B12,
and B66, which incorporate the alkyl groups to ensure solu-
bility. These alkyl groups are also expected to enhance inter-
molecular interactions owing to van der Waals interactions
between alkyl groups. In this study, we have also developed
electronegative p-conjugated systems 2-C6, 2-C12, and 3,
which are based on a combination of the central A unit and
modified terminal B units (Scheme 1). We report the prop-
erties and n-type OFET performances of these p-conjugated
systems.
analysis disclosed
a p-stacked dense-packing structure
caused by strong intermolecular interactions between the A
units. To incorporate these characteristics, C-BTz-based
OFETs were fabricated by a vacuum-deposition technique;
these OFETs showed good n-channel performance with an
electron mobility of 0.06 cm2 Vꢀ1 sꢀ1. However, the low solu-
bility of C-BTz in common organic solvents prevented us
from applying solution techniques in the fabrication of a
device. We have also reported the synthesis of a series of
oligothiophenes that contain difluorodioxocyclopenta[b]-
thiophene (B) as an electronegative terminal unit. These oli-
gothiophenes exhibited low-lying LUMO energy levels,[13]
and their OFETs fabricated by spin-coating exhibited n-type
performances with an electron mobility on the order of
10ꢀ4 cm2 Vꢀ1 sꢀ1,[27] thereby indicating that the B unit is a
highly potential terminal unit that brings n-type character to
conjugate systems. Based on these results, we have synthe-
sized compound 1 as a simple combination of A, B, and
soluble 3-dodecylthiophene (Scheme 1). Furthermore, these
results also motivated us to modify the chemical structure of
Results and Discussion
We first calculated the molecular orbitals of model com-
pounds by density functional theory (DFT) at the B3LYP/6-
31GACTHNUGRTENUNG(d,p) level. The alkyl groups of 1, 2, and 3 were replaced
with methyl groups (referred to as 1-C1, 2-C1, and 3-C1) to
ease the calculations.
As illustrated in Figure 1, the incorporation of electron-
rich thiophene rings between the central A unit and the ter-
minal B units for 1-C1 extends the effective conjugation and
increases the highest occupied molecular orbital (HOMO)
energy level. Compared with the partially localized LUMOs
on the carbonyl-bridged bithiazole unit for 1-C1 and 3-C1,
the LUMO of 2-C1 is delocalized over the p-conjugated
backbone. As a consequence, 2-C1 has a lower LUMO
energy level than the others. However, it is important to
mention that all compounds possess appropriate low-lying
LUMO energy levels for n-type OFET materials.
Abstract in Japanese:
The synthetic routes of 1, 2-C6, 2-C12, and 3 are outlined
in Scheme 2. Because the ketal-protected stannylated bithia-
zole (A-Sn) has been recently synthesized by our group,[23]
we concentrated our efforts on the synthesis of connecting
terminal units. Thus, compound 5 was synthesized by palla-
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Y. Aso and Y. Ie et al.
FULL PAPERS
Figure 1. HOMO and LUMO energies of model compounds calculated using DFT at the B3LYP/6-31GACTHNUGRTENUNG(d,p) level.
Scheme 2. Synthesis of 1, 2-C6 , 2-C12, and 3.
dium-catalyzed Stille coupling of 5-bromodifluorodioxocy-
clopenta[b]thiophene (B-Br) with 3-tri-n-butylstannyl-2-do-
decylthiophene, followed by bromination with N-bromosuc-
cinimide (NBS) in N,N-dimethylformamide (DMF). Dioxo-
cyclopenta[b]thiophene derivatives 7a–7c were prepared
following the same procedure reported previously.[12] Com-
pounds 7a and 7b were then fluorinated with Selectfluor in
the presence of tetrabutylammonium hydroxide (TBAH) to
yield the corresponding terminal units 8a and 8b, respec-
tively. In contrast, dihexyl-substituted compound 9 was ob-
tained by the reaction of 7c with 1-iodohexane in the pres-
ence of KF-Celite under microwave-irradiation condi-
tions.[28] Finally, the target compounds 1, 2-C6, 2-C12, and 3
were obtained through Stille coupling between stannylated
bithiazole A-Sn and the corresponding terminal units 8a,
8b, and 9, respectively, followed by deprotection of the diox-
olane group under acidic conditions. As we expected, all
compounds are soluble in common organic solvents such as
chloroform, toluene, and tetrahydrofuran (THF). Therefore,
these compounds can be purified by silica-gel column chro-
matography and preparative gel permeation chromatogra-
phy (GPC).
The UV-visible absorption spectra of the synthesized
target compounds were measured in a THF solution and in
films, and their data are summarized in Figure 2 and
Table 1. The 2-C12 spectrum was omitted from Figure 2 and
is represented by that of 2-C6 for clarity because these spec-
tra are almost completely coincident. As shown in Fig-
ure 2a, all the compounds exhibit an absorption maximum
between 420 and 470 nm and a broad shoulder between 500
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Solution-Processable n-Type Organic Field-Effect Transistor (OFET) Materials
Figure 2. a) UV-visible spectra of 1, 2-C6, and 3 in THF. b) UV-visible spectra of 1, 2-C6, and 3 films. c) CVs of 1, 2-C6, and 3 in fluorobenzene containing
0.1m nBu4NPF6.
Table 1. Absorption and electrochemical data of 1, 2-C6, 2-C12, and 3.
in n-type OFETs, this stability
is advantageous for the stable
operation of the devices. The
half-wave first reduction poten-
tials of 1, 2, and 3 are ꢀ1.14,
ꢀ0.97, and ꢀ1.09 V (versus fer-
rocene/ferrocenium (Fc/Fc+)),
respectively, which correspond
to the LUMO energy levels of
ꢀ3.66, ꢀ3.83, and ꢀ3.64 eV
lmax[a] [nm]
lonset[a] [nm]
Eogpt [eV]
E
[b] [V]
E
[V]
LUMO[c] [eV]
ox
1=2
red[b]
1=2
Compound
1
465 (489)
445 (453)
445 (453)
424 (421)
690 (745)
640 (702)
640 (700)
645 (710)
1.80 (1.66)
1.94 (1.77)
1.94 (1.77)
1.92 (1.75)
0.93, 1.13
n.d.
n.d.
ꢀ1.14, ꢀ1.32
ꢀ0.97, ꢀ1.15, ꢀ1.42
ꢀ0.97, ꢀ1.15, ꢀ1.42
ꢀ1.09, ꢀ1.52
ꢀ3.66
ꢀ3.83
ꢀ3.83
ꢀ3.71
2-C6
2-C12
3
n.d.
[a] In THF. [b] In C6H5F, 0.1m TBAPF6, V versus Fc/Fc+. [c] LUMO=ꢀ(Er1e=d2+4.8). The values of correspond-
ing films are in parentheses.
and 700 nm. By performing time-dependent DFT calcula-
tions at the B3LYP/6-31G(d,p) level, it has been estimated
under the assumption that the energy level of Fc/Fc+ is
A
ꢀ4.8 eV below the vacuum level by using Equation (1):[29]
that the absorption maximum and shoulder are mainly at-
tributed to the HOMO–LUMO+1 and HOMO–LUMO
transitions, respectively (see the Supporting Information);
this absorption behavior is quite similar to that of C-BTz.[23]
The optical HOMO–LUMO energy gaps (Eogpt) of 1, 2-C6,
and 3 are estimated to be 1.80, 1.94, and 1.92 eV, respective-
ly, from the onset of the absorption edges. Compound 1 has
a longer conjugation length than 2-C6 and 3 because it con-
tains additional thiophene rings between the carbonyl-
bridged bithiazole and the terminal units; therefore, it was
observed that the Eogpt of 1 is relatively small compared to
those of 2-C6 and 3. In the thin-film state, all the compounds
show the redshift of absorption (Figure 2b), which implies
the presence of intermolecular interactions of p-conjugated
backbones.
The electrochemical properties of compounds 1, 2-C6, 2-
C12, and 3 were investigated using cyclic voltammetry (CV)
measurements performed in fluorobenzene containing 0.1m
tetrabutylammonium hexafluorophosphate (TBAPF6) with a
scan rate of 100 mVsꢀ1 using a platinum disk as the working
electrode, platinum wire as the counter electrode, and Ag/
AgNO3 as the reference electrode. The cyclic voltammo-
grams of 1, 2-C6, and 3 are shown in Figure 2b, and their
electrochemical data are listed in Table 1. Oxidation waves
were observed only for compound 1 within the range of po-
tential window up to +1.5 V. However, all the compounds
exhibited multiple and reversible reduction waves, which in-
dicated kinetically stable formation of radical anionic spe-
cies. Considering the contribution of radical anionic species
Red
1=2
LUMO½eVꢁ ¼ ꢀðE þ 4:8Þ
ð1Þ
These estimated low-lying LUMO energy levels indicate
that these compounds can be a candidate for n-type OFET
materials. It should be noted that the trends for the LUMO
energy levels and HOMO–LUMO energy gaps estimated
from the experimental data are fairly consistent with those
of the molecular orbital calculations.
Charge carrier mobilities of 1, 2-C6, 2-C12, and 3 were
measured on OFETs with a bottom-contact configuration.
OFET devices were fabricated on the gate electrode of a p-
doped silicon substrate. Source and drain gold electrodes
with a channel width/channel length (W/L) of 38 mm/5 mm
or 294 mm/25 mm were prepatterned on a layer of SiO2 die-
lectric. The SiO2 dielectrics were subjected to various sur-
face treatments such as those using hexamethyldisilazane
and octadecyltrichlorosilane (ODTS), and we found that the
ODTS-treated substrates exhibited superior OFET charac-
teristics in this study. Thus, as an optimized condition, all
semiconductor films acting as active layers were spin-coated
from a solution in chloroform (1.0 wt%) on the ODTS-
treated SiO2 dielectric. The electrical characteristics of the
OFETs were measured under vacuum (10ꢀ3 Pa) at room
temperature. The field-effect electron mobility (me) was cal-
culated in the saturation regime at a source-drain voltage
(VSD) of 80 V by using Equation (2):
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Y. Aso and Y. Ie et al.
FULL PAPERS
tioned that the device performances were largely dependent
on post-annealing temperatures, and 1-, 2-C6-, and 3-based
devices exhibited the maximum electron mobility after an-
nealing at 2008C for 0.5 hours. In contrast, the annealing
temperature for 2-C12-based devices was limited to 1708C
because of the low melting point of 2-C12; further increase
in the annealing temperature resulted in the disappearance
of their OFET activity.
W
2L
2
ð2Þ
ISD
¼
CimðVGS ꢀ VthÞ
in which Ci denotes the capacitance of SiO2 and Vth denotes
the threshold gate voltage. As expected from their low-lying
LUMO energy levels and stable anionic species, all the com-
pounds exhibited n-type OFET characteristics, which are
summarized in Table 2. Among these characteristics, the
The OFETs were operated under ambient conditions to
investigate their stability in air. Transfer characteristics of
these devices are shown in Figure 4 and in the Supporting
Table 2. Field-effect characteristics of 1, 2-C6, 2-C12, and 3 under vacuum
and in air.
Compound Measurement conditions m [cm2 Vꢀ1 sꢀ1
]
Ion/Ioff Vth [V]
1
1
under vacuum
in air
under vacuum
in air
under vacuum
in air
under vacuum
in air
2.9ꢁ10ꢀ3
2.2ꢁ10ꢀ5
0.011
105
104
108
105
107
105
104
103
5
2
2-C6
2-C6
2-C12
2-C12
3
16
25
14
14
21
21
1.6ꢁ10ꢀ3
3.0ꢁ10ꢀ3
1.0ꢁ10ꢀ3
3.7ꢁ10ꢀ6
6.0ꢁ10ꢀ8
3
field-effect electron mobility of 2-C6 films attained was
0.011 cm2 Vꢀ1 sꢀ1. The 1- and 2-C12-based devices showed
moderate electron mobilities of the range of 10ꢀ3 cm2 Vꢀ1 sꢀ1,
and the 3-based film showed inferior n-type OFET charac-
teristics. Figure 3 represents the output and transfer charac-
teristics for OFETs fabricated using 1 and 2-C6, and the cor-
responding transfer characteristics of 2-C12 and 3 can be
found in the Supporting Information. It should be men-
Figure 4. Transfer (VSD =80 V) characteristics of a) 1- and b) 2-C6-based
OFET devices in air.
Information. As summarized in Table 2, although the elec-
tron mobilities were substantially decreased relative to
those measured under vacuum, all the compounds retained
n-type characteristics. Among them, 2-C6 showed an elec-
tron mobility of 1.6ꢁ10ꢀ3 cm2 Vꢀ1 sꢀ1 (Figure 4), which is one
order of magnitude larger than that of the spin-coated film
based on B-containing compounds.[13b] This improvement is
attributed to the introduction of the carbonyl-bridged bithia-
zole A unit. Additionally, it is worth noting that a drop in
the mobilities of the 2-C6- and 2-C12-based devices in air are
sufficiently smaller than that of 1. To further examine the
air stability of the present devices, the OFETs of 1 and 2-C6
were repeatedly biased in air by sweeping the gate voltage
(VGS) between ꢀ5 and 50 V under a fixed source-drain volt-
age (VSD) of 80 V, and their source-drain currents (ISD
)
versus VGS were monitored. As shown in Figure 5a, the 2-
C6-based device showed little hysteresis, even after 10
cycles, whereas the 1-based device showed large hysteresis
under the same biased conditions (Figure 5b). The current
decay of both films was small, thereby indicating that the
possibility of irreversible degradation of devices was exclud-
ed. Furthermore, the same measurements of the 1-based
device under vacuum showed small hysteresis compared
with that under air-exposed conditions (see the Supporting
Information). These results indicate that the 2-C6 film main-
tains sufficient stability against the penetration of electron-
trapping oxygen and/or moisture under the device-operation
conditions owing to the sufficiently deep LUMO energy
level of 2-C6.
Figure 3. a) Output and b) transfer (VSD =80 V) characteristics of a 1-
based OFET device. c) Output and d) transfer (VSD =80 V) characteris-
tics of a 2-C6-based OFET device. ISD, VSD, and VGS denote source-drain
current, source-drain voltage, and gate voltage, respectively.
To obtain information about the origins of the differences
in OFET performances, the structural order and morpholo-
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Solution-Processable n-Type Organic Field-Effect Transistor (OFET) Materials
film. The difference in grain size between 2-C6 and 2-C12
might be attributed to the different annealing temperature,
which can explain the lower electron mobility of 2-C12 than
that of 2-C6. Thus, the 2-C6-based OFET exhibited the high-
est n-type performance among the investigated compounds.
Conclusion
We have succeeded in the synthesis of a series of electroneg-
ative compounds containing carbonyl-bridged bithiazole as a
central unit and dioxocyclopenta[b]thiophene derivatives as
terminal units. The introduction of hexyl or dodecyl groups
into the p-conjugated systems increases their solubility
toward common organic solvents. In electrochemical meas-
urements, these compounds showed reversible first reduc-
tion waves between ꢀ0.97 and ꢀ1.14 V versus Fc/Fc+,
which indicated the kinetically stable formation of radical
anionic species and LUMO energy levels from ꢀ3.66 to
ꢀ3.83 eV. OFETs with the active layer of 1, 2-C61, 2-C12,
and 3 fabricated by spin coating exhibited n-type character-
istics. Among these OFETs, the 2-C6-based OFETs showed
the best electron mobility—as high as 0.011 cm2 Vꢀ1 sꢀ1 with
an on/off ratio of 108 and a threshold voltage of 16 V under
vacuum conditions. Furthermore, this device could operate
under air-exposed conditions with an electron mobility of
1.6ꢁ10ꢀ3 cm2 Vꢀ1 sꢀ1. XRD and AFM measurements showed
that the spin-coated film of 2-C6 had well-arranged ordering
with large grains. These results provide us with information
about the relationship between the molecular structures,
molecular properties, and OFET performances; in addition
to the low-lying LUMO energy level, the introduction of a
suitable alkyl group at appropriate positions to encourage
molecular high ordering in the solid state is essential to in-
crease the n-type characteristics in OFETs fabricated by a
solution process. Therefore, we conclude that we have ac-
complished the development of new solution-processable n-
type OFET materials based on carbonyl-bridged bithiazole
compounds through the structural modification of the elec-
tronegative terminal B unit; these materials not only con-
tribute to the solubilization of molecules but also improve
molecular ordering in the solid state. Our study has present-
ed a rational strategy of molecular design that can be used
to develop solution-processable n-type OFET materials.
Figure 5. Transfer (VSD =80 V) characteristics of a) 2-C6-based and b) 1-
based OFET device in air with repeated bias for up to 10 cycles.
gies of the thin films deposited on ODTS-modified SiO2/Si
substrates were studied by X-ray diffraction (XRD) and
atomic force microscopy (AFM). All spin-coated films have
about 70 nm thickness. As shown in Figure 6, all the as-spun
films exhibited no XRD reflection peaks. However, after
the annealing, the thin films of 1, 2-C6, and 2-C12 exhibited a
series of sharp reflection peaks corresponding to the d spac-
ing of 19, 17, and 24 ꢂ, respectively. These results indicate
that the amorphous as-spun films were transformed into
well-ordered crystalline films by thermal annealing, al-
though we were unable to analyze the origin of the observed
d spacing in the present study because of the lack of X-ray
crystallographic information. In contrast, the thin film of 3
remained amorphous regardless of annealing, which may be
the cause of the lower electron mobility for 3 relative to
those for 1 and 2. The XRD peak intensities of the film of 1
were weaker than those of the 2-C6 and 2-C12 films for simi-
lar film thicknesses. This indicates that the crystals and/or
molecules of 1 are randomly oriented between crystalline
grains compared to 2-C6 and 2-C12, which leads to the poor
air stability for 1-based OFETs.
Experimental Section
General Information
Column chromatography was performed on silica gel (KANTO Chemical
silica gel 60N, 40–50 mm). Thin-layer chromatography (TLC) plates were
visualized with UV light. Microwave irradiation was performed by using
a Biotage Initiator version 2.5. The microwave power output was set at
400 W. Preparative GPC was performed on a Japan Analytical Industry
LC-908 instrument equipped with JAI-GEL 1H/2H. Melting points are
uncorrected. 1H and 13C NMR spectra were recorded on JEOL LA-400
and LA-600 spectrometers in CDCl3 with tetramethylsilane as an internal
standard. Data are reported as follows: chemical shift in ppm (d), multi-
The AFM image of the annealed 2-C6 film displays appa-
rently large grains and a layered structure, whereas the films
of 1, 2-C12, and 3 exhibit similar morphologies with small
grains (Figure 7). This observation indicates that the 2-C6
film has an interconnected morphology suitable to both con-
structing efficient electron-transporting pathways and dis-
turbing the penetration of oxygen and/or moisture into the
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Y. Aso and Y. Ie et al.
FULL PAPERS
madzu UV-3100PC instrument. All
spectra were obtained using spectrog-
rade solvents. Cyclic voltammetry was
carried out on a BAS CV-620C vol-
tammetric analyzer. Elemental analy-
ses were performed on a PerkinElmer
LS-50B by the Elemental Analysis
Section of Comprehensive Analysis
Center (CAC) of The Institute of Sci-
entific and Industrial Research (ISIR),
Osaka University. The surface struc-
ture of the deposited organic films was
observed by using an atomic force mi-
croscope (Shimadzu, SPM9600), and
the film crystallinity was evaluated
with an X-ray diffractometer (Rigaku,
UltimaIV). X-ray diffraction patterns
were obtained using Bragg–Brentano
geometry with CuKa radiation as an X-
ray source with an acceleration voltage
of 40 kV and
40 mA. The scanning mode was q–2q
scans between 2.58 and 308 with
a beam current of
a
scanning step of 0.018. The thickness
of the active-layer organic films was
determined by a spectroscopic ellips-
ometer (HORIBA.UVISEL LT NIR-
NNG).
Materials
All reactions were carried out under a
nitrogen atmosphere. Solvents of the
highest purity grade were used as re-
ceived. Unless stated otherwise, all re-
agents were purchased from commer-
cial sources and used without purifica-
tion. 4-Hexyl-5-bromothiophene-2-car-
boxylic acid was prepared by using the
Figure 6. XRD data of a spin-coated film of a) 1, b) 2-C6, c) 2-C12, and d) 3 on SiO2 before and after annealing.
reported procedure.[30] 1H NMR spectroscopic data of 4-hexyl-5-bromo-
thiophene-2-carboxylic acid was in agreement with those previously re-
ported.
Synthesis of 4
5-Bromodifluorodioxocyclopenta[b]thiophene (B-Br; 230 mg, 0.86 mmol),
2-(tri-n-butylstannyl)-3-(dodecyl)thiophene (1.08 g, 2.00 mmol), and tetra-
kis(triphenylphosphine)palladium(0) (50 mg, 0.043 mmol) were placed in
a 200 mL round-bottomed flask and dissolved in toluene (5 mL). The re-
action mixture was stirred at 1208C for 17 h. After being cooled to room
temperature, the reaction mixture was filtered over Celite. After removal
of the solvent under reduced pressure, the residue was purified by
column chromatography on alumina (hexane/EtOAc 10:1) to give 4
(370 mg, 98%). Yellow solid; m.p. 74–758C; TLC: Rf =0.65 (hexane/ethyl
acetate 2:1); 1H NMR (400 MHz, CDCl3): d=7.48 (s, 1H), 7.47 (d, J=
5.1 Hz, 1H), 7.06 (d, J=5.1 Hz, 1H), 2.84 (s, 2H), 1.64 (m, 2H), 1.26 (m,
20H), 0.92 ppm (t, J=6.6 Hz, 3H); 13C NMR (100 MHz, CDCl3): d=
179.5 (t, J=24.9 Hz), 177.3 (t, J=25.9 Hz), 157.1, 154.1, 151.0, 144.9,
131.3, 128.9, 127.8, 118.1, 107.0 (t, J=267.3 Hz), 31.9, 30.3, 29.9, 29.7,
29.6, 29.54, 29.52, 29.4, 29.3, 22.7, 14.1 ppm; MS (EI): m/z: 438 [M+]; ele-
mental analysis calcd (%) for C23H28F2O2S2: C 62.98, H 6.43; found: C
62.85, H 6.51.
Synthesis of 5
Figure 7. AFM images of a spin-coated film of a) 1, b) 2-C6, c) 2-C12, and
d) 3 on SiO2 after annealing.
Compound 4 (133 mg, 0.30 mmol) was placed in a 50 mL round-bot-
tomed flask and dissolved in DMF (15 mL) and NBS (150 mg,
0.84 mmol) was added to the mixture. After stirring for 12 h at 908C, the
reaction was quenched by the addition of 2m aqueous Na2S2O3. The
aqueous layer was extracted with EtOAc, and the combined organic
layer was washed with water and dried over MgSO4. After removal of
plicity (s=singlet, d=doublet, t=triplet, m=multiplet), coupling con-
stant (Hz), and integration. Mass spectra were obtained on a Shimadzu
GCMS-QP-5050 instrument. UV-visible spectra were recorded on a Shi-
2358
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ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Asian J. 2011, 6, 2352 – 2361

Solution-Processable n-Type Organic Field-Effect Transistor (OFET) Materials
the solvent under reduced pressure, the residue was purified by column
chromatography on silica gel (hexane/EtOAc 10:1) to give 5 (120 mg,
77%). Yellow solid; m.p. 78–798C; 1H NMR (400 MHz, CDCl3): d=7.41
(s, 1H), 7.03 (s, 1H), 2.78 (s, 2H), 1.65 (m, 2H), 1.26 (m, 20H), 0.88 ppm
(t, J=6.6 Hz, 3H); 13C NMR (100 MHz, CDCl3): d=179.3 (t, J=
25.8 Hz), 177.2 (t, J=25.9 Hz), 155.2, 154.0, 151.2, 145.4, 134.0, 129.0,
118.4, 116.7, 106.9 (t, J=268.3 Hz), 31.9, 30.2, 29.8, 29.63, 29.61, 29.48,
29.42, 29.35, 29.33, 22.7, 14.1 ppm; MS (EI) m/z: 517 [M+]; elemental
analysis calcd (%) for C23H27Br1F2O2S2: C 53.38; H 5.26; found: C 53.12,
H 5.34.
48.2, 31.9, 29.70, 29.66, 29.64, 29.62, 29.58, 29.50, 29.35, 29.27, 28.41, 22.7,
14.1 ppm; MS (EI): m/z: 399 [M+]; elemental analysis calcd (%) for
C19H27BrO2S: C 57.14, H 6.81; found: C 57.32, H 7.09.
Synthesis of 7c
Compound 7c was synthesized by following the procedure for the prepa-
ration of 7a as a pale yellow solid with a yield of 42%. Pale brown solid;
m.p. 171–1728C; 1H NMR (400 MHz, CDCl3): d=7.42 (s, 1H), 3.38 ppm
(s, 2H); 13C NMR (100 MHz, CDCl3): d=188.8, 187.1, 157.2, 155.8,
131.0, 123.6, 47.7 ppm; MS (EI): m/z: 232 [M+]; elemental analysis calcd
(%) for C7H3Br1O2S: C 36.39, H 1.31; found: C 36.23, H 1.55.
Synthesis of 1
Compound 5 (130 mg, 0.25 mmol), A-Sn[23] (82 mg, 0.10 mmol), and tetra-
kis(triphenylphosphine)palladium(0) (6 mg, 0.005 mmol) were placed in a
test tube and dissolved in toluene (2.5 mL). The reaction mixture was
stirred at 1208C for 13 h. After removal of the solvent under reduced
pressure, the residue was washed with methanol and diethyl ether to give
ketal-protected compound 1’ (64 mg, 58%). Dark purple solid; 1H NMR
(400 MHz, CDCl3): d=7.54 (s, 2H), 7.44 (s, 2H), 4.58 (s, 4H), 2.85 (t, J=
7.6 Hz, 4H), 1.74 (m, 4H), 1.27 (m, 40H), 0.88 ppm (t, J=6.6 Hz, 6H);
MALDI TOF-MS: m/z: calcd: 1111.44; found: 1111.34 [M+].
Synthesis of 8a
Compound 7a (300 mg, 0.952 mmol) and Selectfluor (809 mg, 2.28 mmol)
were placed in a 50 mL round-bottomed flask and dissolved in THF
(10 mL). Tetrabutylammonium hydroxide (37% MeOH solution, 1.47 g,
2.09 mmol) was added to the mixture at 08C. After stirring for 12 h at
room temperature, the reaction was quenched by addition of water, and
the organic layer was separated. The aqueous layer was extracted with
ethyl acetate, and the combined organic layer was washed with water and
dried over MgSO4. After removal of the solvent under reduced pressure,
the residue was purified by column chromatography on silica gel
(hexane/ethyl acetate 20:1) to give 7 (260 mg, 78%). Colorless solid; m.p.
Synthesis of 1’
1
Compound 1’ (15 mg, 0.013 mmol) was placed in a 100 mL round-bot-
tomed flask and dissolved in AcOH (1 mL). Aqueous HCl (12m, 0.1 mL)
was added to the mixture at 1008C. The reaction mixture was stirred at
1008C for 2 h. After being cooled at room temperature, the reaction was
quenched by the addition of H2O. The solid was collected and washed
with H2O, methanol, and diethyl ether followed by purification by
column chromatography on silica gel (chloroform) and GPC to give pure
1 (8 mg, 56%). Dark brown solid; m.p. >3008C; 1H NMR (400 MHz,
CDCl3): d=7.52 (s, 2H), 7.44 (s, 2H), 2.84 (t, J=7.6 Hz, 4H), 1.74 (m,
4H), 1.27 (m, 40H), 0.88 ppm (t, J=6.6 Hz, 6H); 13C NMR (150 MHz,
CDCl3): d=179.2 (t, J=26.0 Hz), 177.1 (t, J=26.0 Hz), 176.7, 162.2,
154.8, 154.3, 154.0, 151.5, 145.8, 142.1, 137.7, 130.6, 130.1, 118.9, 106.8 (t,
J=267.2 Hz), 31.9, 30.1, 30.0, 29.7, 29.64, 29.57, 29.41, 29.37, 22.7,
14.1 ppm; MALDI TOF-MS: m/z: calcd: 1067.39; found: 1068.6 [M+];
elemental analysis calcd (%) for C53H54F4N2O5S6: C 59.64, H 5.10, N
2.62; found: C 59.48, H 5.13, N 2.37.
46–478C; TLC: Rf =0.29 (hexane/ethyl acetate 2:1); H NMR (400 MHz,
CDCl3): d=2.89 (t, J=7.6 Hz, 2H), 1.61 (m, 2H), 1.33 (m, 8H),
0.90 ppm (t, J=6.6 Hz, 3H); 13C NMR (100 MHz, CDCl3): d=178.8 (t,
J=24.9 Hz), 176.7 (t, J=24.9 Hz), 152.2, 151.1, 141.1, 132.5, 106.1 (t, J=
268.3 Hz), 31.4, 29.0, 28.8, 27.7, 22.5, 14.0 ppm; MS (EI): m/z: 352 [M+];
elemental analysis calcd (%) for C13H13Br1F2O2S: C 44.46, H 3.73; found:
C 44.44, H 4.02.
Synthesis of 8b
Compound 8b was synthesized following the procedure for the prepara-
tion of 8a as a brown solid with a yield of 90%. Colorless solid; m.p. 58–
598C; 1H NMR (400 MHz, CDCl3): d=2.88 (t, J=7.6 Hz, 2H), 1.61 (m,
2H), 1.26 (m, 20H), 0.88 ppm (t, J=6.6 Hz, 3H); 13C NMR (100 MHz,
CDCl3): d=178.8 (t, J=24.9 Hz), 176.8 (t, J=24.9 Hz), 152.2, 151.1,
141.1, 132.5, 106.2 (t, J=268.3 Hz), 31.9, 29.70, 29.66, 29.64, 29.62, 29.58,
29.50, 29.35, 29.27, 28.41, 22.7, 14.1 ppm; MS (EI): m/z: 435 [M+].
Synthesis of 7a
Synthesis of 2-C6
4-Hexyl-5-bromothiophene-2-carboxylic acid[30] (6a; 1.16 g, 4.00 mmol)
and thionyl chloride (8 mL) were placed in a three-necked round-bot-
tomed flask. The reaction mixture was stirred at 608C for 1 h. After re-
moval of thionyl chloride under reduced pressure, the residue was dis-
solved in nitrobenzene (30 mL). Next, anhydrous aluminum trichloride
(AlCl3; 1.75 g, 13.1 mmol) was added to it, and degassed with nitrogen. A
solution of malonyl chloride (600 mg, 4.26 mmol) in nitrobenzene
(10 mL) was then added dropwise to the stirred mixture. After stirring at
808C for 3 h, the reaction was quenched by the addition of 10% aqueous
oxalic acid (40 mL). The aqueous layer was extracted with CH2Cl2, and
the combined organic layer was washed with water and dried over
MgSO4. After removal of the solvent under reduced pressure, the residue
was purified by column chromatography on silica gel (hexane/ethyl ace-
tate 20:1) to yield 7a (705 mg, 56%). Colorless solid; m.p. 87.0–87.58C;
TLC: Rf =0.37 (hexane/ethyl acetate 20:1); 1H NMR (400 MHz, CDCl3):
d=3.38 (s, 2H), 2.83 (t, J=7.6 Hz, 2H), 1.56 (m, 2H), 1.32 (m, 8H),
0.89 ppm (t, J=6.6 Hz, 3H); 13C NMR (100 MHz, CDCl3): d=189.6,
187.3, 154.6, 154.1, 139.1, 127.6, 48.2, 31.4, 29.0, 28.8, 27.5, 22.5, 14.0 ppm;
MS (EI): m/z: 315 [M+]; elemental analysis calcd (%) for C13H15BrO2S:
C 49.53, H 4.80; found: C 49.25, H 4.92.
Compound 8a (180 mg, 0.513 mmol), A-Sn[23] (178 mg, 0.218 mmol), and
tetrakis(triphenylphosphine)palladium(0) (15 mg, 0.013 mmol) were
placed in a test tube and dissolved in toluene (2 mL). The reaction mix-
ture was stirred at 1208C for 13 h. After removal of the solvent under re-
duced pressure, the residue was washed with methanol and diethyl ether
to give ketal-protected compound 2-C6’ (100 mg, 59%). Red solid;
1H NMR (400 MHz, CDCl3): d=4.59 (m, 4H), 3.25 (t, J=8.3 Hz, 4H),
1.74 (m, 4H), 1.53 (m, 4H), 1.26 (m, 12H), 0.87 ppm (t, J=6.6 Hz, 6H);
MS (EI): m/z: 778 [M+].
Synthesis of 2-C6’
Compound 2-C6’ (100 mg, 0.128 mmol) was placed in a 50 mL round-bot-
tomed flask and dissolved in AcOH (10 mL). Aqueous HCl (12m, 1 mL)
was added to the mixture at 1008C. The reaction mixture was stirred at
1008C for 2 h. After being cooled at room temperature, the reaction was
quenched by the addition of H2O. The solid was collected and washed
with H2O, methanol, and diethyl ether followed by purification by
column chromatography on silica gel (chloroform) and GPC to give pure
2-C6 (49 mg, 52%). Dark brown solid; m.p. 211–2128C; 1H NMR
(400 MHz, CDCl3): d=3.28 (t, J=8.3 Hz, 4H), 1.75 (m, 4H), 1.53 (m,
4H), 1.26 (m, 12H), 0.87 ppm (t, J=6.6 Hz, 6H); 13C NMR (150 MHz,
CDCl3): d=179.8 (t, J=26.0 Hz), 177.6 (t, J=26.0 Hz), 175.5, 159.9,
154.7, 152.9, 152.6, 148.3, 144.3, 139.3, 106.5 (t, J=268.7 Hz), 31.4, 29.5,
29.4, 28.4, 22.5, 14.0 ppm; MS (EI): m/z: 734 [M+]; elemental analysis
calcd (%) for C33H26F4N2O5S4: C 53.94, H 3.57, N 3.81; found: C 53.87, H
3.80, N 3.80.
Synthesis of 7b
Compound 7b was synthesized by following the procedure for the prepa-
ration of 7a as a brown solid with a yield of 27%. Colorless solid;
m.p. 71–728C; 1H NMR (400 MHz, CDCl3): d=3.37 (s, 2H), 2.82 (t, J=
7.6 Hz, 2H), 1.55 (m, 2H), 1.26 (m, 20H), 0.88 ppm (t, J=6.6 Hz, 3H);
13C NMR (100 MHz, CDCl3): d=189.7, 187.4, 154.7, 154.2, 139.3, 127.7,
Chem. Asian J. 2011, 6, 2352 – 2361
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemasianj.org
2359

Y. Aso and Y. Ie et al.
FULL PAPERS
Synthesis of 2-C12
solution in chloroform (1.0 wt%) on the ODTS-treated SiO2 dielectric.
The characteristics of the OFET devices were measured at room temper-
ature under a pressure of 10ꢀ3 Pa without exposure to air after fabricating
the active layer. The field-effect mobility (m) was calculated in the satu-
rated region at VSD =80 V. The current on/off ratio was determined from
Compound 2-C12 was synthesized following the procedure for the prepa-
ration of 2-C6 as a dark brown solid with a yield of 79%. Dark brown
solid; m.p. 163–1648C; 1H NMR (400 MHz, CDCl3): d=3.27 (t, J=
8.3 Hz, 4H), 1.74 (m, 4H), 1.26 (m, 40H), 0.87 ppm (t, J=6.6 Hz, 6H);
13C NMR (150 MHz, CDCl3): d=179.9 (t, J=26.0 Hz), 177.7 (t, J=
26.0 Hz), 175.5, 156.0, 154.8, 152.9, 152.7, 148.3, 144.2, 139.4, 106.5 (t, J=
268.7 Hz), 31.9, 29.70, 29.66, 29.64, 29.62, 29.58, 29.50, 29.35, 29.27, 28.41,
22.7, 14.1 ppm; MALDI TOF-MS: m/z: calcd: 903.14; found: 903.36 [M+
]; elemental analysis calcd (%) for C19H27BrO2S: C 57.14, H 6.81; found:
C 57.20, H 6.87.
the ISD at VGS =0 V (Ioff) and VGS =80 V (Ion
)
Acknowledgements
Synthesis of 9
This study was supported by the Industrial Technology Research Grant
Program in 2008 from the New Energy and Industrial Technology Devel-
opment Organization (NEDO) of Japan and a Grant-in-Aid for Scientific
Research from the Ministry of Education, Culture, Sports, Science and
Technology, Japan, and by cooperative research with Sumitomo Chemical
Co., Ltd. Thanks are extended to the Elemental Analysis Section of
Comprehensive Analysis Center (CAC) of the Institute of Scientific and
Industrial Research (ISIR), Osaka University, for assistance in obtaining
elemental analyses.
Compound 7c (540 mg, 2.34 mmol), 1-iodohexane (3.30 g, 15.6 mmol),
and potassium fluoride on Celite (50 wt%, 2.4 g) were placed in a test
tube and dissolved in acetonitrile (20 mL). After stirring for 40 min at
1508C under microwave irradiation, the reaction was quenched by addi-
tion of 10% aqueous HCl and the organic layer was separated. The aque-
ous layer was extracted with CH2Cl2, and the combined organic layer was
washed with water and dried over MgSO4. After removal of the solvent
under reduced pressure, the residue was purified by column chromatog-
raphy on silica gel (hexane/ethyl acetate 3:1) to give 8 (469 mg, 50%).
Colorless oil; TLC: Rf =0.7 (hexane/ethyl acetate 3:1); 1H NMR
(400 MHz, CDCl3): d=7.41 (s, 1H), 1.75 (m, 4H), 1.10 (m, 16H),
0.82 ppm (t, J=7.3 Hz, 6H); 13C NMR (100 MHz, CDCl3): d=196.9,
194.9, 156.3, 155.2, 130.8, 123.4, 62.8, 35.3, 31.3, 29.5, 24.8, 22.4, 13.9 ppm;
MS (EI): m/z: 400 [M+]; elemental analysis calcd (%) for C19H27BrO2S:
C 57.14, H 6.81; found: C 57.20, H 6.87.
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Synthesis of 3
Compound 9 (67 mg, 0.17 mmol), A-Sn[23] (67 mg, 0.082 mmol), and tetra-
kis(triphenylphosphine)palladium(0) (8 mg, 0.007 mmol) were placed in a
test tube and dissolved in toluene (1 mL). The reaction mixture was
stirred at 1208C for 13 h. After removal of the solvent under reduced
pressure, the residue was washed with methanol and diethyl ether to give
ketal-protected compound 3’ (37 mg, 56%). Red solid; 1H NMR
(400 MHz, CDCl3): d=7.69 (s, 2H), 4.59 (s, 4H), 1.80 (q, J=5.8 Hz,
8H), 1.79 (q, J=5.8 Hz, 8H), 1.10 (m, 32H), 0.82 ppm (t, J=6.8 Hz,
12H); MS (EI): m/z: 875 [M+].
Synthesis of 3’
Compound 3’ (37 mg, 0.042 mmol) was placed in a 100 mL round-bot-
tomed flask and dissolved in AcOH (5 mL). Aqueous HCl (12m, 0.5 mL)
was added to the mixture at 1008C. The reaction mixture was stirred at
1008C for 2 h. After being cooled at room temperature, the reaction was
quenched by the addition of H2O. The solid was collected and washed
with H2O, methanol, and diethyl ether followed by purification by
column chromatography on silica gel (chloroform) and GPC to give pure
1
3 (31 mg, 88%). Dark brown solid; m.p. 222–2238C; H NMR (400 MHz,
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0.82 ppm (t, J=6.8 Hz, 12H); 13C NMR (150 MHz, CDCl3): d=197.6,
195.7, 176.2, 162.2, 157.0, 155.8, 154.8, 150.2, 143.5, 118.4, 64.4, 35.5, 31.3,
29.6, 24.9, 22.4, 14.0 ppm; MS (EI) m/z: 831 [M+]; elemental analysis
calcd (%) for C45H54N2O5S4: C 65.03, H 6.55, N 3.37; found: C 64.74, H
6.58, N 3.08.
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Bottom-Contact Device
The field-effect mobility of 1, 2-C6, 2-C12, and 3 was measured using
bottom-contact thin-film field-effect transistor (FET) geometry. The p-
doped silicon substrate functions as the gate electrode. The thermally
grown silicon oxide dielectric layer on the gate substrate was 300 nm
thick and had a capacitance of 10.0 nFcmꢀ2. Interdigital source and drain
electrodes were constructed with gold (30 nm) that were formed on the
SiO2 layer. The channel width (W) and channel length (L) were 38 mm
and 5 mm, or 294 mm and 25 mm, respectively. The silicon oxide surface
was first washed with acetone and 2-propanol. The silicon oxide surface
was then activated by ozone treatment and pretreated with octadecyltri-
chlorosilane (ODTS). The semiconductor layer was spin-coated from a
2360
www.chemasianj.org
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Asian J. 2011, 6, 2352 – 2361

Solution-Processable n-Type Organic Field-Effect Transistor (OFET) Materials
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