Synthesis and characterization of N-Acyl-substituted PyrroloTTF derivatives and improved air-stability of PyrroloTTF-based OFETs

Article abstract of DOI:10.1246/cl.2008.1088

As novel bis(pyrrolo[3,4-d])tetrathiafulvalene (PyTTF)-based organic semiconductors, N-acyl-substituted PyTTFs 4 were synthesized and evaluated. Their oxidation potentials determined by cyclic voltammetry were shifted cathodically by more than 0.35 V comp

Full text of DOI:10.1246/cl.2008.1088

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1088
Chemistry Letters Vol.37, No.10 (2008)
Synthesis and Characterization of N-Acyl-substituted PyrroloTTF Derivatives
and Improved Air-stability of PyrroloTTF-based OFETs
Iori Doi,1 Eigo Miyazaki,1 and Kazuo Takimiyaꢀ1;2
1Department of Applied Chemistry, Graduate School of Engineering, Hiroshima University, Higashi-Hiroshima 739-8527
2Institute for Advanced Materials Research, Hiroshima University, Higashi-Hiroshima 739-8530
(Received August 8, 2008; CL-080771; E-mail: ktakimi@hiroshima-u.ac.jp)
As novel bis(pyrrolo[3,4-d])tetrathiafulvalene (PyTTF)-
O
O
O
R
based organic semiconductors, N-acyl-substituted PyTTFs 4
were synthesized and evaluated. Their oxidation potentials de-
termined by cyclic voltammetry were shifted cathodically by
more than 0.35 V compared with the N-alkyl-PyTTFs 3. In ac-
cordance with elevated oxidation potentials, their vapor-process-
ed field-effect transistors showed improved air-stability with
mobilities up to 0.08 cm2 Vꢁ1 sꢁ1 under ambient conditions.
R
O
S
S
S
S
R
O R
N
N
2
NaH
74 ~ 88%
R = CH3 (4a), C5H11 (4b), C11H23 (4c)
Scheme 1. Synthesis of N-acyl-substituted PyTTFs 4.
paper, we describe the synthesis of N-acyl-PyTTFs and the char-
acteristics of FET devices using their vapor deposited thin films.
N-alkylation of the parent PyTTF (2) was easily done by a
reaction of 2 with alkyl halide in the presence of sodium
hydride.5–8 Thus, introduction of acyl substituents on 2 was first
attempted under a similar condition using acyl chlorides. The
reaction however, gave a complex mixture, and 4 could not be
isolated. Instead, the use of acid anhydrides turned out to give
4 successfully in reasonable isolated yields (Scheme 1). Thus,
N-acetyl- (4a), n-hexanoyl- (4b), and n-dodecanoyl-PyTTF
(4c) were obtained. All new derivatives were fully characterized
by spectroscopic and combustion analyses.
Unambiguous structural determination of N-n-hexanoyl
derivative 4b was achieved by single-crystal X-ray analysis.
In the crystal structure, two crystallographically independent
molecules exist (Figure S1). Although the conformations of
n-C5H11 chains in two molecules are different, the central core
parts of the molecules are similar to each other: the carbonyl
moieties are in the same plane that the PyTTF part defined,
indicating that the conjugation effectively extends to the N-acyl
moieties (Figure 2).
Organic field-effect transistors (OFETs) have attracted cur-
rent attention for potential applications to various electronic de-
vices, such as flexible displays, inexpensive electronic papers,
and radio-frequency-identification tags.1 Recent development
of new organic semiconductors and processing techniques have
achieved high-performance OFETs with field-effect mobility
(ꢀFET) higher than 1.0 cm2 Vꢁ1 sꢁ1 in thin-film-transistor set-
tings, making OFETs a viable alternative to the current amor-
phous-silicon-based FETs.2 Among a wide variety of p-channel
organic semiconductors so far developed, tetrathiafulvalene
(TTF) derivatives have occupied an important position:3 one
of the highest FET mobilities so far attained (1.4 cm2 Vꢁ1 sꢁ1
)
was achieved by dithieno[3,4-d]tetrathiafulvalene (DT-TTF, 1,
Figure 1).4 We recently reported that N-alkyl-substituted bis-
(pyrrolo[3,4-d])tetrathiafulvalenes (R2-PyTTFs, 3, Figure 1)),
highly soluble heteroaromatic-annulated TTF derivatives isoe-
lectronic with DT-TTF, are useful as solution-processible
organic semiconductors.5 Thus fabricated FET devices with
their spin-coated thin films showed FET mobility as high as
0.017 cm2 Vꢁ1 sꢁ1 in air. Although their FET performances were
relatively good for solution-processed OFETs without intensive
optimizations in device fabrication, their sensitivity to ambient
air was a severe drawback: shortly after exposure to air, off-cur-
rent of the OFETs rose significantly. The poor stability in air is
likely due to their low oxidation potentials (E1=2 ¼ ꢁ0:11 V vs.
Fc/Fcþ), in other words, the higher-lying energy level of the
highest occupied molecular orbital (HOMO), which makes 3
susceptible to air oxidation.
Cyclic voltammograms of 4 showed two reversible oxida-
tion couples typical for the TTF derivatives. (Figure S3) As ex-
pected, the oxidation peaks of 4 (E1 ¼ þ0:25 V vs. Fc/Fcþ)
1=2
were shifted cathodically by more than 0.35 V compared with
those of 3, indicating a significant decrease of the energy level
of the HOMO. The HOMO level of 4 estimated from the oxida-
tion onset is ca 5.0 eV below the vacuum level, which is almost
comparable with that of pentacene, known as a representative
organic semiconductor.
In contrast to the highly soluble N-alkyl-PyTTFs 3, the
solubilities of 4 even with the long alkyl chains were very poor:
In order to stabilize 2-based organic semiconductors, we in-
tended to introduce electron-withdrawing groups on the nitrogen
atoms instead of rather electron-donating N-alkyl groups and
synthesized N-acyl-substituted PyTTFs (4, Scheme 1). In this
S
S
S
S
S
S
S
S
H2n+1Cn
N
N CnH2n+1
X
X
n = 4, 8, 12, 16, 20
X = S: DT-TTF (1)
X = NH: PyTTF (2)
R2-PyTTF (3)
Figure 1. Structures of DT-TTF and Py-TTFs.
Figure 2. Molecular structure of 4b.
Copyright ꢀ 2008 The Chemical Society of Japan

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Chemistry Letters Vol.37, No.10 (2008)
1089
In summery, we successfully synthesized N-acyl-PyTTFs 4
and evaluated the FET devices based on their vapor-deposited
thin films. The electron-withdrawing acyl groups introduced
brought about lower HOMO levels than those of N-alkyl-
PyTTFs 3, which contributes to stabilization of 4-based FET
devices under ambient conditions without decreasing the
FET characteristics. These results indicate that the tuning of
the energy level of HOMO is important in developing air-stable
p-channel organic semiconductors and air-stable OFETs.
(a)
(b)
-5 10-5
-4 10-5
-3 10-5
-2 10-5
-1 10-5
0 100
10-4
10-5
10-6
10-7
10-8
10-9
V
d = -60 V
Vg = -60 V
-50 V
fresh
7h
20h
30h
-40 V
-30 V
-20 V
-10, 0 V
-60
-40
-20
0
20
0
-10
-20
-30
-40
-50
-60
This work was supported by a Grants-in-Aid for Scientific
Research (No. 20350088) from the Ministry of Education,
Culture, Sports, Science and Technology, Japan.
Vd / V
Vg / V
Figure 3. FET characteristics of 4b-based OFET. (a) output
characteristics and (b) transfer characteristics.
References and Notes
the solubility in toluene at rt was 0.01 g Lꢁ1 for 4b and
0.002 g Lꢁ1 for 4c, which were lower than those of 3 by
more than two orders of magnitude. Although the reasons for
the extremely low solubility of 4 are not clear, we speculate
that incorporation of the carbonyl groups in the conjugation of
PyTTF core and thus induced planer and rigid molecular struc-
ture confirmed by X-ray structural analysis account for it.
Poor solubility of 4 in organic solvents prevented the depo-
sition of thin films by solution processes. Instead, physical vapor
deposition of 4 gave homogeneous thin films with metallic luster
on Si/SiO2 substrates except for 4c, which thermally decom-
posed during vapor deposition. The thin films consist of well-
ordered polycrystalline grains as confirmed by the atomic force
microscopy (AFM) images (Figure S4). Such morphology is a
prerequisite for high-performance OFETs.
1
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The XRD measurements of the thin films of 4a and 4b indi-
cated that the films have crystalline order in the direction of the
substrate normal (Figure S5). The calculated interlayer distances
˚
˚
(d-spacings) are 17.0 A for 4a and 22.6 A for 4b. The d-spacing
of 4b is almost the same as the length of the crystallographic c
axis in the bulk single crystal, which suggests that molecular
arrangements in the thin film and in the bulk single crystal are
similar to each other.
3
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On top of the evaporated thin films, gold source and drain
electrodes were thermally deposited through a shadow mask to
define the channel of the transistors with a length (L) of 50 or
100 mm and width (W) of 1.5 mm, respectively. The OFET
devices were evaluated under ambient conditions, and the 4a-
and 4b-based devices showed normally-off FET characteristics
(Figure 3). Extracted FET mobilities from the saturation regime
are up to 0.05 cm2 Vꢁ1 sꢁ1 for 4a-based devices and 0.08
cm2 Vꢁ1 sꢁ1 for 4b-based devices, respectively (Table S1).
As already mentioned, the 3-based OFETs suffered from
severe air-oxidation: the fresh devices showed typical normal-
ly-off FET characteristics. However, exposure to air for 2 h
increased the off-current significantly, higher than that of the
fresh device by a factor of more than two orders of magnitude
(Figure S6). In contrast, the present 4-based devices showed
improved air-stability. As shown in Figure 3b, the devices
showed typical normally-off FET responses even after exposure
to ambient lab conditions for more than 9 h. This stabilization
can be accounted for by the lowered HOMO energy level of 4
by introduction of the electron-withdrawing acyl groups on the
PyTTF core.
4
5
6
7
8
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Efficient synthetic route for the parent PyTTF (2) was
established. See supporting Information.
Supporting Information is available electronically on the
CSJ-Journal Web site, http://www.csj.jp/journals/chem-lett/
index.html.
Published on the web (Advance View) September 25, 2008; doi:10.1246/cl.2008.1088