Air-stable n-type organic field-effect transistors based on 4,9-dihydro-s-indaceno[1,2-b:5,6-b']dithiazole-4,9-dione unit

Article abstract of DOI:10.1021/cm301985q

4,9-Dihydro-s-indaceno[1,2-b:5,6-b′]dithiazole-4,9-dione (IDD) was designed as a novel electronegative unit, and the π-conjugated compound (2C-TzPhTz) containing it was synthesized as a candidate for air-stable n-type organic field-effect transistor (OFET

Full text of DOI:10.1021/cm301985q

Article
pubs.acs.org/cm
Air-Stable n‑Type Organic Field-Effect Transistors Based on 4,9-
Dihydro‑s‑indaceno[1,2‑b:5,6‑b′]dithiazole-4,9-dione Unit
Yutaka Ie,*,†,‡ Masashi Ueta,† Masashi Nitani,† Norimitsu Tohnai,‡,§ Mikiji Miyata,§ Hirokazu Tada,∥
and Yoshio Aso*,†
†The Institute of Scientific and Industrial Research (ISIR), Osaka University, 8-1, Mihogaoka, Ibaraki, Osaka 567-0047, Japan
‡JST-PRESTO, 4-1-8, Honcho, Kawaguchi, Saitama 333-0012, Japan
§Department of Materials and Life Science, Graduate School of Engineering, Osaka University, 2-1 Yamadaoka, Suita, Osaka
565-0871, Japan
∥Department of Materials Engineering Science, Division of Materials Physics, Graduate School of Engineering Science, Osaka
University, 1-3, Machikaneyama, Toyonaka, Osaka 560-8531, Japan
S
* Supporting Information
ABSTRACT: 4,9-Dihydro-s-indaceno[1,2-b:5,6-b′]dithiazole-
4,9-dione (IDD) was designed as a novel electronegative unit,
and the π-conjugated compound (2C-TzPhTz) containing it
was synthesized as a candidate for air-stable n-type organic field-
effect transistor (OFET) materials. Cyclic voltammetry
measurements revealed that the IDD unit contributes to
lowering the lowest unoccupied molecular orbital (LUMO)
energy level. X-ray crystallographic analysis of 2C-TzPhTz
showed an almost planar molecular geometry and dense
molecular packing, which is advantageous to electron transport.
OFETs based on 2C-TzPhTz showed high electron mobility of
up to 0.39 cm2 V−1 s−1, which is one of the highest electron
mobilities observed among pentacyclic dione-based materials.
Top-contact OFET devices showed operating stability and long-term stability under ambient conditions, attributed to the low-
lying LUMO energy level and dense packing in the solid state. Furthermore, bottom-contact OFETs also maintained good
electron mobility beyond 0.1 cm2 V−1 s−1 under air-exposed conditions. We demonstrated that n-type OFETs are more sensitive
to H2O than O2 and found that the acquirement of air stability for the 2C-TzPhTz-based OFET is due to the increased stability
against not only O2 but also H2O. All of these results indicate that IDD is a potentially useful building unit for high-performance
air-stable n-type semiconducting materials.
KEYWORDS: organic field-effect transistor, electron-transporting material, organic electronics, structure−property relationships
Moreover, stable device operation under ambient conditions
has been pursued for practical applications of n-type OFETs,
which has consequently brought a more difficult hurdle to the
development of suitable new materials. Several research works
have proposed that the acquirement of both the low-lying
LUMO energy level to overcome the oxidation of anionic
carriers with ambient oxidants (O2 and H2O) and the dense
packing in the solid state to reduce the permeation of ambient
oxidants are highly desirable.5a,6a In this context, the design of
new electron-accepting units is still of significance for the
development of n-type semiconducting materials since thin-film
OFET materials producing both air-stable device operation and
high electron mobility beyond 0.1 cm2 V−1 s−1 are limited to
only a few π-conjugated systems such as acene carboxylic
INTRODUCTION
■
π-Conjugated systems have tremendous potential as semi-
conducting materials for organic electronics such as organic
light-emitting diodes, organic field-effect transistors (OFETs),
and organic photovoltaic cells, since they can be applicable to
low-cost, large-area, and flexible electronic devices.1 OFETs are
one of the most actively investigated applications within this
field. Recent significant progress has shown that hole-
transporting (p-type) OFET materials have achieved high
hole mobilities beyond the value of amorphous silicon.2,3 In
contrast, there remain major challenges concerning the
development of electron-transporting (n-type) OFET materi-
als.4 This lag in development is predominately due to the
difficulty in determining a molecular design suitable for existing
guidelines: (1) lowering the lowest unoccupied molecular
orbital (LUMO) energy level for efficient electron injection
from metal electrodes and (2) invoking strong intermolecular
interactions for efficient electron transport in the solid state.
Received: June 25, 2012
Revised: July 26, 2012
Published: July 26, 2012
© 2012 American Chemical Society
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Figure 1. Chemical structures and HOMO and LUMO energies calculated with DFT at the B3LYP/6-31G(d,p) level.
Scheme 1. Synthesis of 2C-TzPhTz
diimide compounds,5−10 dicyanomethylene-substituted conju-
gated compounds,10−14 metal phthalocyanine,15 azapentacene
compounds,16 and benzobis(thiadiazole) compounds.17
dithiazole-4,9-dione (IDD) as a new electronegative unit after
first calculating the molecular orbitals of IDD together with
those of CBB by density functional theory (DFT) at the
B3LYP/6-31G(d,p) level. As shown in Figure 1, in both cases,
the HOMOs and LUMOs are spread over the entire π-
frameworks and the carbonyl groups strongly partcipate in the
LUMOs. In reflection of the different number of carbonyl
groups, IDD possesses a notably lower LUMO energy level
than does CBB. These results encouraged us to develop the π-
conjugated compound 2C-TzPhTz that consists of IDD and
trifluoroacetylphenyl groups. In this study, we report on the
synthesis, properties, and structures of 2C-TzPhTz and its
performance as an active layer of n-type OFETs. We
successfully obtained high field-effect performances with
electron mobility of up to 0.39 cm2 V−1 s−1, and the OFET
devices showed good stability under ambient conditions
because of the concurrent features of a low-lying LUMO
energy level and dense packing in the solid state. We will also
report on the investigation of the influence of O2 and H2O,
Recently, we reported on carbonyl-bridged bithiazole (CBB)
as a new electronegative unit (Figure 1),18 in which the
combination of an electron-withdrawing thiazole ring with
carbonyl bridging contributes to enhancing both the electron-
accepting characteristics and the intermolecular interactions.18a
OFETs made from C-BTz, which is composed of the central
CBB unit and trifluoroacetylphenyl groups19 as an electro-
negative terminal unit, have achieved good electron mobility,
up to 0.06 cm2 V−1s−1, and moderate air stability.18a With this
result in mind, we anticipated that expansion of the π-
conjugation from the tricyclic mono-one system to a
pentacyclic dione system could further enhance both the
electron-accepting characteristics and the intermolecular over-
lapping of the π-systems, resulting in higher electron mobility
as well as air stability for the OFETs.20−22 Therefore, in this
study, we designed 4,9-dihydro-s-indaceno[1,2-b:5,6-b′]-
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independently, on n-type OFET performance in order to clarify
the origin of such air stability.
RESULTS AND DISCUSSION
■
Synthesis, Properties, and Structures. In general, an
intramolecular Friedel−Crafts acylation reaction is utilized for
the synthesis of pentacyclic dione compounds such as
indenofluore-4,9-dione23 and 4,9-dihydro-s-indaceno[1,2-b:5,6-
b′]dithiophene-4,9-dione24 derivatives. However, this method is
not applicable to the synthesis of the title dione compound due
to the considerable electron-deficient character of thiazole
compared to benzene and thiophene. Furthermore, although
we have previously established the synthesis of a carbonyl-
bridged bithiazole derivative through double lithiation of 2,2′-
bis(triisopropylsilyl)-5,5′-bithiazole and subsequent treatment
with ethyl 1-piperidine carboxylate,18a a similar one-pot
reaction failed to afford key intermediate 4. Thus, we
synthesized bisamide compound 3 and then applied an
intramolecular acylation reaction via double lithiation of 3 to
give 4 in a good yield (Scheme 1). After acetal protection of the
carbonyl groups and concomitant removal of the triisopro-
pylsilyl groups under acidic conditions, we converted 5 into a
stannylated derivative 6. Finally, the 2C-TzPhTz target
compound was prepared by the Stille coupling reaction of 6
with 4′-bromo-2,2,2-trifluoroacetophenone followed by acidic
acetal deprotection. The solubility of 2C-TzPhTz toward
common organic solvents such as CHCl3, chlorobenzene, and
o-dichlorobenzene is very low. Thus, the 2C-TzPhTz
compound was purified by gradient sublimation in a high
vacuum and unambiguously characterized by MS, solid-state
NMR spectroscopy, and elemental analysis (see the Supporting
Information). As shown in Figure S1(a) (Supporting
Information), thermogravimetric analysis (TGA) of 2C-
TzPhTz showed a high thermal stability with 5% weight loss
at 438 °C in a nitrogen atmosphere. As shown in Figure S1(b)
(Supporting Information), the UV−vis absorption spectrum of
2C-TzPhTz in THF exhibits an intense absorption band at 408
nm, accompanied with a shoulder between 450 and 650 nm.
According to time-dependent DFT calculations at the B3LYP/
6-31G(d,p) level, this absorption band and shoulder can be
mainly ascribed to the HOMO−LUMO+1 (433 nm) and
HOMO−LUMO (642 nm) transitions with oscillator strengths
(f) of 1.48 and 0.16, respectively.
Figure 2. (a) Cyclic voltammograms of 4 (blue) and 7 (black) in
fluorobenzene at room temperature containing 0.1 M TBAPF6. (b)
Cyclic voltammogram of 2C-TzPhTz in benzonitrile at 180 °C
containing 0.1 M TBAPF6. (c) Differential pulse voltammogram of
2C-TzPhTz in benzonitrile at 180 °C containing 0.1 M TBAPF6.
good agreement with the above-mentioned theoretical results.
The cyclic voltammogram of 2C-TzPhTz in benzonitrile at the
high temperature of 180 °C shows one reversible reduction
wave within a potential window of benzonitrile (Figure 2b),
and the Ered of −1.01 V was determined by a differential
1/2
pulse voltammetry measurement (Figure 2c). On the basis of
this Ered1/2 value and on the premise that the energy level of Fc/
Fc+ is −4.8 eV below the vacuum level,26 the LUMO energy
level of 2C-TzPhTz is estimated to be −3.79 eV, which is lower
than that of C-BTz (−3.64 eV).18a
In order to investigate the molecular structure and packing in
the solid state, X-ray crystallographic analysis of 2C-TzPhTz
was carried out. Suitable single crystals were obtained by
recrystallization from an iodobenzene solution. The 2C-
TzPhTz molecules form a monoclinic primitive lattice with a
space group of P21/a and unit cell parameters of a =
13.4677(11) Å, b = 4.7775(3) Å, c = 18.764(2) Å, and β =
94.002(5)°. The molecular structure of 2C-TzPhTz reveals an
almost planar geometry with inter-ring dihedral angles less than
2° (Figure 3a). As shown in Figure 3b, in contrast to the dense
side-by-side packing of C-BTz,18a the C2-symmetric molecular
structure and two carbonyl groups on the pentacyclic unit in
2C-TzPhTz induce several short contacts between neighboring
units, resulting in the formation of a cross-oriented layer-by-
layer packing. In each layer, intermolecular π−π interactions are
observed with an interplanar distance of 3.38 Å, revealing a
more dense π-stacking configuration than that of C-BTz (3.40
and 3.52 Å).18a On the basis of this packing diagram, the
intermolecular transfer integrals (t) between LUMOs of
neighboring molecules were calculated using PW91/TZP, as
implemented in the Amsterdam Density Functional (ADF)
program.27 As shown in Figure 4, large overlaps are observed
To investigate the electrochemical properties of 2C-TzPhTz,
cyclic voltammetry (CV) measurements were attempted.
However, CV measurement of 2C-TzPhTz was restricted due
to the lack of sufficient solubility. Thus, to directly compare the
electrochemical properties of IDD and CBB units, CV
measurements of pentacyclic unit 4 and carbonyl-bridged
bithiazole derivative 7 (structure shown in Figure 2) were
performed in fluorobenzene containing 0.1 M tetrabutylammo-
nium hexafluorophosphate (TBAPF6) as the supporting
electrolyte at a scan rate of 100 mV s−1. A platinum disk, a
platinum wire, and Ag/AgNO3 were used as the working
electrode, counter electrode, and reference electrode, respec-
tively. All potentials were calibrated against a ferrocene/
ferrocenium (Fc/Fc+) couple as the internal standard. As
shown in Figure 2a, 4 gave two reversible reduction waves with
half-wave potentials Ered1/2 = −1.17 and −1.58 V, whose values
are positively shifted compared to those of 7 (Ered = −1.47
1/2
and −2.14 V).25 This result clearly indicates that the
pentacyclic dione unit contributes to an increase in the
electron-accepting nature compared to the tricyclic system, in
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Figure 5. (a) AFM image and (b) XRD data of 2C-TzPhTz film on
ODTS-modified SiO2.
XRD measurement of the film exhibits regular-interval peaks up
to the fifth order (Figure 5b), indicating that 2C-TzPhTz forms
a crystalline structure in the thin films. The first peak at 2θ =
4.7° corresponds to a d-spacing of 18.8 Å, which is almost equal
to the length of the crystallographic c-axis. Thus, 2C-TzPhTz
molecules are positioned on a stacking orientation parallel to
the SiO2 surface with their long axes inclined at an angle of
46.2°. Notably, this orientation is advantageous for charge
transport in the OFET geometry.
Figure 3. (a) Molecular structure and (b) packing diagram of 2C-
TzPhTz.
To evaluate the transistor characteristics of 2C-TzPhTz,
OFET devices with bottom-gate top-contact geometry were
fabricated by patterning gold electrodes over the organic thin
films using a shadow mask to define a channel width (W) of 5
mm and a channel length (L) 20 μm. FET parameters obtained
from the measurements are summarized in Table 1. Under a
Table 1. Field-Effect Characteristics of 2C-TzPhTz and C-
BTz in a Vacuum and Air
measurement
conditions
μe,
Ion/
run compound geometry
cm2 V−1 s−1 Ioff
Vth, V
−51
1
2
3
4
5
2C-
TzPhTz
top-
contact
under vacuum
0.13
0.11
0.39
0.14
0.03
102
102
106
107
103
2C-
TzPhTz
top-
contact
in air
−39
23
2C-
TzPhTz
bottom-
contact
under vacuum
in air
2C-
TzPhTz
bottom-
contact
24
a
C-BTz
top-
contact
under vacuum
33
a
Figure 4. Estimated transfer integrals of LUMOs for (a) 2C-TzPhTz
and (b) C-BTz between neighboring molecules.
Reference 18a.
along the π-stacked direction, and the transfer integral for 2C-
TzPhTz is larger than that for the π-stacking of C-BTz (71.7
meV) under the same calculation method. This result supports
that 2C-TzPhTz displays an efficient electron-transport
character along the π-stacking direction.
Morphologies of the Thin Films and Field-Effect
Properties. The morphology and structural order of 2C-
TzPhTz in the solid state were investigated by atomic force
microscopy (AFM) and X-ray diffraction (XRD). The 2C-
TzPhTz thin films were prepared on octadecyltrichlorosilane
(ODTS)-modified SiO2/Si substrates by vacuum deposition at
a substrate temperature of 170 °C and subsequent annealing at
300 °C for 1 h in a vacuum. As shown in Figure 5a, the AFM
image of the 2C-TzPhTz film shows crystal-shaped submi-
crometer-sized grains with no observable grain boundaries. The
vacuum, the OFET device using 2C-TzPhTz showed typical n-
channel responses with an electron mobility (μe) of 0.13 cm2
V−1 s−1 (Figure S2, Supporting Information), which is almost
four times higher than that of C-BTz under the same device
conditions (runs 1 and 5 in Table 1). This higher electron
mobility is consistent with the above-described estimation that
the stacking of 2C-TzPhTz has larger transfer integrals
between the LUMOs than does C-BTz. Moreover, on the
basis of calculated energies for neutral and radical-anionic
states,28 the reorganization energy of 2C-TzPhTz is estimated
to be 311 meV, which is smaller than that of C-BTz (356
meV). This result also supports that the introduction of the
IDD unit gives rise to the potential for increased electron
transport characteristics. As expected from the low-lying
LUMO energy level and dense packing in the solid state,
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these 2C-TzPhTz-based OFET devices are found to be stable
under air-exposed conditions, maintaining a μe of 0.11 cm2 V−1
s−1 (run 2). To examine their operational stability in air, the
device was repeatedly biased by a gate-voltage (VGS) sweep
from −20 to 80 V under a fixed source−drain voltage (VDS) at
80 V, and its source−drain currents (IDS) were collected. As can
be seen in Figure 6a, the observed transfer characteristics show
Figure 7. (a) Transfer characteristics of a bottom-contact OFET using
2C-TzPhTz. (b) Output characteristics of the same device at source−
drain voltage of 120 V. VDS denotes source−drain voltage.
Figure 6. (a) Transfer characteristics of a top-contact OFET using 2C-
TzPhTz with repeated biases up to 10 cycles under air-exposed
conditions at source−drain voltage of 80 V. IDS and VGS denote
source−drain current and gate voltage, respectively. (b) Electron
mobility of a top-contact OFET using 2C-TzPhTz versus storage
period under air-exposed conditions.
attributed to the complete exclusion of atmospheric contam-
inants, and the improved on/off current ratio and positive shift
of the threshold voltage are probably due to the small area of
gold electrode−organic layer contacts that restrains the surplus
currents.30
Surprisingly, after exposure to air, the bottom-contact
OFETs still maintained a high electron mobility of 0.14 cm2
V−1 s−1 (run 4 in Table 1). Although the electron mobility
deteriorates slightly compared to that in a vacuum, the value is
an identical level with those observed in the top-contact
devices. At present, bottom-gate bottom-contact OFETs with
both air-stable device operation and high electron mobility
exceeding 0.1 cm2 V−1 s−1 have been limited to only a few
precedents.8a,10b Since we could not observe significant
differences in the morphology and crystallinity between the
thin-films of the top-contact and bottom-contact OFETs, we
can speculate that, for the bottom-contact OFETs, the organic
thin-films are not well-organized at the interface with the gold
electrodes,31 resulting in them being susceptible to the
intrusion of atmospheric contaminants.32 It is well-known
that the performance deterioration of n-type OFETs in air is
caused by O2 and H2O as the main ambient oxidants. On the
basis of the reduction potentials of O2 (+0.57 V vs SCE,
corresponding to +0.19 V vs Fc/Fc+) and H2O (−0.66 V vs
SCE, corresponding to −1.04 V vs Fc/Fc+) in aqueous
media,33,34 it has been proposed that the O2-oxidation process
against anionic carriers has more serious influence on the
thermodynamic stability of n-type OFETs than the H2O-
oxidation process.6c,33 In fact, several researches revealed the
influence of O2 on n-type OFET performances.5a,35,36 In terms
of a kinetic barrier of closely stacked conjugated polymers
against the penetration of O2, the OFET performances in a
wide range of O2 pressures were also investigated.37 In contrast,
little hysteresis under the repeatedly biased conditions, tracing
almost overlapped curves even after 10 cycles. Furthermore, on
monitoring the long-term stability under air-exposed con-
ditions, the 2C-TzPhTz-based OFET only showed slight
changes in the mobility even after 3 months, as shown in
Figure 6b. These results demonstrate that 2C-TzPhTz is a
promising electron-transporting semiconducting material for
practical OFET application.
To obtain further insight into intrinsic device performances,
OFETs with bottom-gate bottom-contact geometry were
fabricated. This device configuration is also desirable for
practical transistor applications.8a,29 Since, on our fabrication−
measurement equipment for bottom-gate bottom-contact
OFETs, the performances can be evaluated in a high vacuum
without exposing the device to air after preparing the thin-films,
the influence of atmospheric contaminants is minimized during
device processing. For this measurement, source and drain gold
electrodes with a W/L of 294 mm/25 μm were prepatterned on
a 300 nm layer of SiO2 dielectric and thin-films of 2C-TzPhTz
were vacuum-deposited by the above-mentioned conditions
onto the ODTS-modified SiO2/Si substrates. Figure 7 shows
the transfer and output characteristics of the devices that exhibit
a high electron mobility of 0.39 cm2 V−1 s−1, an on/off current
ratio (Ion/Ioff) of 106, and a threshold voltage (Vth) of 23 V (run
4 in Table 1). Compared to the top-contact OFETs with a
similar channel length, the improved electron mobility is mainly
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the influence of H2O on n-type OFET performances has not
been fully clarified.8a,38 Under these circumstances, to reveal the
charge-trapping ability of O2 and H2O individually, we
compared the characteristics of the bottom-contact OFETs
under dry air (O2: 20−22%, H2O < 0.5 ppm) and under humid
N2 (O2 < 0.1 ppm, around 80% of humidity). For the
investigation of the relationship between molecular structures
and the device stability, we measured the OFET performances
of 2C-TzPhTz as an air-stable material, C-BTz as a moderate
air-stable material, and our previously reported 819b as an air-
unstable reference material (structure shown in Figure 8). Note
levels after re-evacuating the devices, indicating that the air-
instability is not due to irreversible chemical instability. These
results clearly indicate that H2O has more influence on the
charge trapping of n-type OFETs than does O2. By assuming
the reduction potential of O2 (−1.25 V vs Fc/Fc+) in aprotic
media,39 the electron affinities of O2 and H2O and the LUMO
energy levels of the molecules are diagrammed in Figure 9. In
Figure 9. Energy diagram. LUMO energy levels of 2C-TzPhTz, C-
BTz, and 8 were estimated from CV.
spite of the energetically mismatched relationship, as described
above, 8 and C-BTz barely showed the n-type OFET
characteristics under dry air and humid N2, respectively,
which might be brought by compensation with the kinetic
barrier of the stacked hydrophobic molecules. On the other
hand, 2C-TzPhTz is superior to C-BTz in terms of the
energetically matched LUMO energy level. In addition, the
kinetic barrier of 2C-TzPhTz derived from the dense packing is
expected. Therefore, these combined effects contribute to
accomplishing the high device stability toward not only O2 but
also H2O. In other words, the expansion of π-conjugation from
BCC to IDD is advantageous to an increase in both the
thermodynamic and the kinetic stabilities of n-type OFETs.
Figure 8. Transfer characteristics of bottom-contact OFETs using (a)
2C-TzPhTz, (b) C-BTz, and (c) 8 at a source−drain voltage of 80 V.
that 8 possesses an electrochemically estimated LUMO energy
level of −3.20 eV. To equalize the kinetic penetration of O2 or
H2O into organic thin films, the thicknesses of the films were
defined to be 10 nm for all the devices. It should be mentioned
that 2C-TzPhTz, C-BTz, and 8 take a π-stacked crystalline
structure in the solid state, and their interplanar distances of
3.38, 3.40, 3.52, and 3.45 Å, respectively, are comparable to the
kinetic diameter of O2 (3.46 Å) and longer than that of H2O
(2.65 Å).36 We first measured the OFET characteristics in a
vacuum chamber. Then, the measurements were performed
after introducing the dry air or humid N2 to the chamber and
keeping the conditions for 1 h at room temperature. All the
devices were biased by a VGS sweep from −20 to 60 V under a
fixed VDS of 80 V, and their IDS values were collected. As shown
in Figure 8a,b, both 2C-TzPhTz and C-BTz almost maintained
their OFET characteristics in dry air. On the other hand, in
humid N2, 2C-TzPhTz showed one order of decrease in
electron mobility against that in a vacuum, and, furthermore, a
decrease in two orders was observed for C-BTz. The
pronounced decrease of the electron mobility for 2C-TzPhTz
compared to that above-mentioned under air-exposed con-
ditions comes from the increased humidity from around 50%
for the ambient air to 80% for the humid N2. In contrast to 2C-
TzPhTz and C-BTz, 8 showed an apparent deterioration even
under dry air, and the n-type characteristics were completely
quenched under humid N2 (Figure 8c). Note that, in all the
cases, the degraded performances can be recovered to the initial
CONCLUSION
■
In summary, we have successfully demonstrated that 4,9-
dihydro-s-indaceno[1,2-b:5,6-b′]dithiazole-4,9-dione (IDD) is a
promising electronegative unit for constructing π-conjugated
systems for n-type OFET materials. The derivative of IDD
combined with trifluoroacetylphenyl groups, 2C-TzPhTz,
shows high electron mobility of up to 0.39 cm2 V−1 s−1 on
OFETs, which is one of the highest values reported for
pentacyclic dione-based materials. The high OFET perform-
ance is considered to originate from its low-lying LUMO
energy level and an appropriate molecular arrangement in the
solid state for charge-carrier transport, which were unambig-
uously investigated by electrochemistry and X-ray analysis.
Besides the good OFET performances, it is notable that top-
contact OFET devices show operating stability and long-term
stability under air-exposed conditions. Furthermore, bottom-
contact OFETs also maintained high n-type characteristics
under air-exposed conditions, and electron mobility beyond 0.1
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236−242 °C; 1H NMR (400 MHz, CDCl3, δ) 7.44 (s, 2H), 3.76−3.70
(m, 4H), 3.26−3.13 (m, 4H), 1.77−1.39 (m, 12H); 13C NMR (150
MHz, CDCl3, δ) 165.9, 140.6, 131.9, 118.9, 48.3, 43.1, 26.6, 25.8, 24.7;
MS (EI) m/z 457 [M+]. Anal. Calcd for C18H22Br2N2O2: C, 47.18; H,
4.84; N, 6.11. Found: C, 47.18; H, 4.97; N, 5.96.
cm2 V−1 s−1 was observed. We investigated the influence of O2
and H2O, independently, on n-type OFET performance in
order to clarify the origin of such air stability. We successfully
demonstrated that n-type OFETs are more sensitive to H2O
than O2 for our investigated compounds and found that the
acquirement of air stability for 2C-TzPhTz-based OFET is due
to the increased stability against not only O2 but also H2O. This
feature is accomplished by the introduction of the IDD unit,
which is advantageous to increase both thermodynamic and
kinetic stabilities for the n-type OFET operation. Further
improvements in device performance for 2C-TzPhTz through
optimization of the device structure are anticipated. The
present results indicate that, for the novel electron-transporting
materials based on IDD, further improvement in transport
properties and/or solution processability can be achieved by
fine-modification of the constituent molecules.
Synthesis of 3. 1 (11.4 g, 24.8 mmol), 2 (31.6 g, 59.6 mmol), and
tetrakis(triphenylphosphine)palladium(0) (2.90 g, 2.48 mmol) were
placed in a pressure vessel and dissolved in toluene (100 mL). The
reaction mixture was stirred at 120 °C for 10 h. After removal of the
solvent under reduced pressure, the residue was washed with methanol
and ethyl acetate to give compound 3 (16.7 g, 86%). White solid; mp
1
255−259 °C; H NMR (400 MHz, CDCl3, δ) 8.28 (s, 2H), 7.52 (s,
2H), 3.95−3.84 (m, 2H), 3.41−3.28 (m, 2H), 3.00−2.88 (m, 4H),
1.53−1.34 (m, 10H), 1.29−1.19 (m, 2H), 1.18−1.12 (m, 36H), 0.70−
0.58 (m, 2H); 13C NMR (150 MHz, CDCl3, δ) 172.4, 168.1, 144.9,
137.3, 136.9, 128.9, 48.1, 42.9, 26.1, 25.3, 24.5, 18.8, 12.0; MS (EI) m/
z 779 [M+]. Anal. Calcd for C42H66N4O2S2Si2: C, 64.73; H, 8.54; N,
7.19. Found: C, 64.79; H, 8.57; N, 7.28.
Synthesis of 4. 3 (500 mg, 0.64 mmol) in THF (45 mL) was added
to lithium diisopropylamide (1.0 M, 14 mL) in THF at −40 °C. After
stirring for 1 h, the reaction was quenched by the addition of H2O.
The aqueous layer was extracted with CHCl3, 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/CHCl3 =
2.5/1) to yield 4 (343 mg, 89%). Purple solid; mp 255−260 °C; TLC
Rf = 0.29 (2.5:1 hexane/CHCl3); 1H NMR (400 MHz, CDCl3, δ) 7.37
(s, 2H), 1.53 (hept, 6H, J = 7.6 Hz), 1.18 (d, 36H, J = 7.6 Hz); 13C
NMR (150 MHz, CDCl3, δ) 183.6, 178.0, 159.8, 157.3, 140.5, 138.4,
117.0, 18.8, 12.0; MS (EI) m/z 609 [M+]. Anal. Calcd for
C32H44N2O2S2Si2: C, 63.11; H, 7.28; N, 4.60. Found: C, 62.82; H,
7.33; N, 4.32.
EXPERIMENTAL SECTION
■
General Information. Column chromatography was performed
on silica gel (KANTO Chemical silica gel 60 N, 40−50 μm) or neutral
alumina (Merck Aluminum oxide 90 standardized). Thin-layer
chromatography (TLC) plates were visualized with UV. Preparative
gel-permeation chromatography (GPC) was performed using a Japan
Analytical Industry LC-908 equipped with a JAI-GEL 1H/2H column.
1
Melting points were uncorrected. H NMR and 13C NMR spectra
were recorded using a JEOL JMM-ECS400 and JCM-ECA600 in
CDCl3 with tetramethylsilane as an internal standard. Solid-state H
1
NMR and 13C NMR spectra were recorded using a Bruker AVANCE
III600WB. The NMR data are reported as follows: chemical shift in
ppm (δ), multiplicity (s = singlet, d = doublet, t = triplet, hept =
heptet, m = multiplet), coupling constant (Hz), and integration. Mass
spectra were obtained on a Shimadzu GCMS-QP-5050. UV−visible
spectra were recorded using a Shimadzu UV-3100PC. All spectra were
obtained in spectrograde solvents. Cyclic voltammetry and differential
pulse voltammetry were carried out using a BAS CV-620C
voltammetric analyzer. Elemental analyses were performed on
PerkinElmer LS-50B by the Elemental Analysis Section of the
Comprehensive Analysis Center (CAC) of ISIR, Osaka University.
The surface structures of the deposited organic film were observed by
atomic force microscopy (Shimadzu, SPM9600), and the film
crystallinity was evaluated by an X-ray diffractometer (Rigaku,
RINT2500). X-ray diffraction patterns were obtained using Bragg−
Brentano geometry with Cu Kα radiation as an X-ray source with an
acceleration voltage of 40 kV and a beam current of 40 mA. The
scanning mode was set to θ−2θ scans between 2.5 and 30° with
scanning steps of 0.01°.
Synthesis of 5. 4 (47 mg, 0.077 mmol), p-toluenesulfonic acid (133
mg, 0.77 mmol), and 2,2-dibutyl-1,3-propanediol (58 mg, 0.031
mmol) were placed in a three-necked round-bottomed flask and
dissolved with benzene (30 mL). The reaction mixture was stirred at
110 °C for 8 h. After removal of the solvent under reduced pressure,
the residue was purified by column chromatography on silica gel
(hexane/ethyl acetate =20/1) to yield 5 (42 mg, 85%). White solid;
1
mp 268−275 °C; TLC Rf = 0.27 (20:1 hexane/ethyl acetate); H
NMR (400 MHz, CDCl3, δ) 8.68 (s, 2H), 7.46 (s, 2H), 4.60 (s, 2H),
4.57 (s, 2H), 3.82 (s, 2H), 3.79 (s, 2H); 13C NMR (150 MHz, CDCl3,
δ) 163.8, 153.6, 149.2, 137.4, 132.9, 116.5, 100.1, 71.8, 35.7, 32.9, 31.1,
26.0, 24.7, 24.0, 23.9, 14.6, 14.4; MS (EI) m/z 636 [M+]. Anal. Calcd
for C36H48N2O4S2: C, 67.89; H, 7.60; N, 4.40. Found: C, 67.66; H,
7.58; N, 4.35.
Synthesis of 6. 5 (237 mg, 0.37 mmol) was placed in a three-
necked round-bottomed flask and dissolved with THF (5 mL). n-BuLi
(1.57 M, 0.7 mL) was added dropwise to the mixture at −40 °C. After
stirring for 1 h, SnBu3Cl (0.33 mL, 1.23 mmol) was added. After
stirring for 0.5 h at room temperature, the reaction was quenched by
the addition of H2O (2 mL). The aqueous layer was extracted with
ethyl acetate, and the combined organic layers were washed with water
and dried over MgSO4. After removal of the solvent under reduced
pressure, the residue was purified by column chromatography on
alumina (hexane/CHCl3 = 10/1) followed by further purification with
Materials. All reactions were carried out under a nitrogen
atmosphere. Especially, the reactions using lithium diisopropylamide
or n-BuLi were carried out under complete exclusion of water and
oxygen. The reaction using thionyl chloride was performed under a
fume food. The removed thionyl chloride was disposed after
neutralization with sodium carbonate. Solvents of the highest purity
grade were used as received. Unless stated otherwise, all reagents were
purchased from commercial sources and used without purification. 1,4-
Dibromo-2,5-benzenedicarboxylic acid was prepared by the previously
reported procedure.40
1
GPC (CHCl3) to give 6 (396 mg, 88%) as a yellow solid: H NMR
(400 MHz, CDCl3, δ) 7.41 (s, 2H), 4.76 (s, 2H), 4.73 (s, 2H), 3.77 (s,
2H), 3.74 (s, 2H).
Synthesis of 1. 1,4-Dibromo-2,5-benzenedicarboxylic acid (10 g,
30.9 mmol), dimethylformamide (a few drops), and thionyl chloride
(50 mL) were placed in a 100 mL round-bottomed flask. The reaction
mixture was stirred at 80 °C for 1 h. After removal of thionyl chloride
under reduced pressure, the residue was dissolved in dichloromethane
(300 mL) at 0 °C. Triethylamine (25.9 mL, 185 mmol) and piperidine
(18.4 mL, 185 mmol) were added to the mixture, and the resulting
mixture was allowed to warm to room temperature. After stirring for 2
h, the solution was poured into water and extracted with dichloro-
methane. The extract was dried over MgSO4. After removal of the
solvent under reduced pressure, the residue was washed with methanol
and diethyl ether to give compound 1 (12 g, 83%). White solid; mp
Synthesis of 2C-TzPhTz. 6 (322 mg, 0.27 mmol), 4′-bromo-2,2,2-
trifluoroacetophenone (201 mg, 0.80 mmol), and tetrakis-
(triphenylphosphine)palladium(0) (31 mg, 0.027 mmol) were placed
in a pressure vessel and dissolved with toluene (3 mL). The reaction
mixture was stirred at 120 °C for 13 h. After removal of the solvent
under reduced pressure, the residue was washed with methanol and
diethyl ether. This compound was placed in a 100 mL round-
bottomed flask and dissolved with AcOH (50 mL). Aq. HCl (12 M, 3
mL) was added to the mixture at 100 °C. The reaction mixture was
stirred for 2 h. After being cooled to room temperature, the reaction
was quenched by the addition of H2O. The solid was collected and
3291
dx.doi.org/10.1021/cm301985q | Chem. Mater. 2012, 24, 3285−3293

Chemistry of Materials
Article
washed with H2O, methanol, and diethyl ether followed by purification
ASSOCIATED CONTENT
■
under gradient sublimation under a high vacuum (10−6 Pa) to give
S
* Supporting Information
1
pure 2C-TzPhTz (77 mg, 45%) as a green solid; mp >300 °C; H
NMR spectra, TGA curve, UV−vis absorption spectrum,
computational details, and OFET characteristics. This material
is available free of charge via the Internet at http://pubs.acs.org.
NMR (600 MHz, solid, δ) 5.72 (br); 13C NMR (150 MHz, solid, δ)
183.2, 175.4, 171.4, 154.5, 137.8, 135.7, 130.5, 128.2, 125.9, 124.0,
115.6; MS (EI) m/z 640 [M+]. Anal. Calcd for C30H10F6N2O4S2: C,
56.25; H, 1.57; N, 4.37. Found: C, 56.17; H, 1.65; N, 4.37.
AUTHOR INFORMATION
X-ray Information. Single crystal diffraction: Diffraction data for
2C-TzPhTz were collected using synchrotron radiation (λ = 0.8000
Å) at the BL38B1 in the SPring-8, with approval of JASRI
(2010A1266). The cell refinements were performed with the
HKL2000 software.41 Direct method (SIR-2004) was used for the
structure solution.42 Calculation was performed with the observed
reflections [I > 2σ(I)] using the CrystalStructure crystallographic
software packages,43 except for refinement, which was performed using
SHELXL-97.44 All non-hydrogen atoms were refined with anisotropic
displacement parameters, and hydrogen atoms were placed in idealized
positions and refined as rigid atoms with the relative isotropic
displacement parameters.
C15H5F3NO2S: Mw = 320.27, a = 13.4677(11) Å, b = 4.7775(3) Å, c
= 18.764(2) Å, α = 90°, β = 94.002(5)°, γ = 90°, V = 1204.4(2) Å3, T
= 93 K, monoclinic, space group P21/a (No. 14), Z = 4, ρcalcd = 1.766 g
cm−3, 2455 unique reflections, final R1 and wR2 values 0.0784 (I >
2.0σ(I)) and 0.2401 (all data), respectively. Crystallographic data
(excluding structure factors) for the structure reported in this paper
have been deposited with the Cambridge Crystallographic Data Centre
as supplementary publication no. CCDC-866114.
■
Corresponding Author
*E-mail: yutakaie@sanken.osaka-u.ac.jp (Y.I.); aso@sanken.
osaka-u.ac.jp (Y.A.).
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by 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. The synchrotron radiation
experiments were performed at the BL38B1 in the SPring-8
facility with the approval of the Japan Synchrotron Radiation
Research Institute (JASRI) (Proposal 2010A1266). Thanks are
extended to the CAC, ISIR, for assistance in obtaining
elemental analyses.
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