Synthesis of methoxy-substituted picenes: Substitution position effect on their electronic and single-crystal structures

Article abstract of DOI:10.1021/jo500543h

A series of picenes having methoxy groups was synthesized through Pd-catalyzed Suzuki-Miyaura couplings or Wittig reaction/intramolecular cyclization sequences, and their physicochemical properties and single-crystal structures were evaluated. The substit

Full text of DOI:10.1021/jo500543h

Article
pubs.acs.org/joc
Synthesis of Methoxy-Substituted Picenes: Substitution Position
Effect on Their Electronic and Single-Crystal Structures
Hiroki Mori,† Xi-chao Chen,† Ning-hui Chang,† Shino Hamao,‡ Yoshihiro Kubozono,‡,§
Kiyohiko Nakajima,∥ and Yasushi Nishihara*,†,§
†Division of Earth, Life, and Molecular Sciences, Graduate School of Natural Science and Technology, Okayama University, 3-1-1
Tsushimanaka, Kita-ku, Okayama 700-8530, Japan
‡Research Laboratory for Surface Science, Okayama University, 3-1-1 Tsushimanaka, Kita-ku, Okayama 700-8530, Japan
§ACT-C, Japan Science and Technology Agency, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan
∥Department of Chemistry, Aichi University of Education, 1 Hirosawa, Igaya-cho, Kariya, Aichi 448-8542, Japan
S
* Supporting Information
ABSTRACT: A series of picenes having methoxy groups was synthesized through Pd-catalyzed Suzuki−Miyaura couplings or
Wittig reaction/intramolecular cyclization sequences, and their physicochemical properties and single-crystal structures were
evaluated. The substitution position effects between the outer 1,12-, 2,11-, and 4,9-position and the inner 3,10-position are quite
different; the former showed the same electronic structure as that of picene, but the latter results in a HOMO geometry different
from those of picene and other methoxy picenes. In addition, crystal structures of four types of methoxy-substituted picenes 4a−
c,e strongly depend on their substitution position and number of methoxy groups, which dramatically changes the structures
from the fully anisotropic 1D π-stacked structure to a unique 3D herringbone structure due to steric hindrance of methoxy
groups. The calculations of transfer integrals based on their single-crystal structures reveal that the methoxy picenes have
intermolecular overlaps less effective than that of the parent nonsubstituted picene. These results are attributed not only to the
packing structure but also to electronic structures such as the HOMO distribution. The preliminary OFET of the representative
4c,e showed hole mobilities significantly lower than that of picene due to their less effective intermolecular overlaps, as predicted
by the calculated transfer integrals.
rings are fused in an armchair structure. It represents a novel
and promising class of materials for organic electronics. An
FET device with a thin film of picene showed a high mobility
(μ) value of more than 5 cm2 V−1 s−1, and its FET properties
are known to be improved with the μ value as well as the on−
off ratio under air or O2.4 Additionally, picene can be used in
superconductors: a picene doped with an alkali metal such as
potassium or rubidium exhibited superconductivity below 18
K.5
INTRODUCTION
■
Aromatic compounds with extended π conjugation have
attracted significant interest because of their high potential
for use in organic electronic devices such as organic field-effect
transistors (OFETs), light-emitting diodes (OLEDs), and
photovoltaic cells (OPVs).1 Among them, a linearly fused
system, such as pentacene and its derivatives, has been
intensively applied to organic materials for electronic devices.2
However, pentacene is quite unstable on exposure to light and
air due to its relatively high HOMO energy level (−5.0 eV)
originating from its highly π-conjugated nature, which limits its
practical application in electronic devices.3 On the other hand,
picene, an isostructural analogue of pentacene, is known to be a
stable compound (EHOMO = −5.8 eV), in which the benzene
To further utilize picene for organic semiconducting
materials, an efficient method of synthesizing functionalized
Received: March 7, 2014
Published: May 2, 2014
© 2014 American Chemical Society
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Figure 1. Structures of methoxy picenes 4a−e in this study.
Scheme 1. Synthesis of 4a−d by Suzuki−Miyaura Coupling/Intramolecular Cyclization Sequences
Scheme 2. Synthesis of 4e via Wittig Reaction
picene is highly desirable for the following reasons. First, the
introduction of solubilizing groups on the picene core would
allow the development of soluble picene-based materials
suitable for the solution process, while nonsubstituted picene
generally has poor solubility. Second, the introduction of
substituents can alter a perturbation on the basis of the
electronic structures of the core, resulting in different HOMO
and LUMO energy levels, which are very important for
electronic devices. Third, the kind, the position, and the
number of substituents significantly affect the packing structure
in the solid state, which is critical for efficient carrier transport.
Thus, many researchers have succeeded in developing efficient
methods of synthesizing parent and functionalized picenes.6
However, there are still several drawbacks to these methods,
such as a tedious series of multiple steps needed for their
preparation, the use of expensive reagents and catalysts, and the
limitations of the substrates. Furthermore, there is no example
of highly selective synthesis of various isomeric picenes, and
hence a systematic comparison of electronic structures and
solid-state properties in a series of substituted picenes has not
been investigated. Very recently, we have reported a new
method for the synthesis of substituted picenes through the
palladium-catalyzed Suzuki−Miyaura cross-coupling of stereo-
defined (Z)-alkenylboronates with polyhalobenzene and
sequential intramolecular double cyclization via direct C−H
bond activation.7 In order to analyze the systematic trends of
physical properties of the isomeric picenes, we herein report the
synthesis of five kinds of methoxy-substituted picenes in
different substitution positions (Figure 1), their physicochem-
ical properties and intermolecular transfer integrals (ts) based
on their single crystal X-ray analyses, and FET mobilities of
several representative methoxy picenes.
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abs
Figure 2. Absorption spectra (1 × 10−5 M) and fluorescence spectra with excitation wavelength at λmax (1 × 10−6 M) of 4a−e in CH2Cl2.
a
b
RESULTS AND DISCUSSION
Table 1. UV−Vis and Fluorescence Data of 4a−e
■
c
ε, M−1
Egopt, eV /
λ
,
max d
Stokes shift,
cm−1
abs
em
Synthesis of 4a−d by Suzuki−Miyaura Coupling/
λmax
nm
,
compd
cm−1
λedge, nm
nm
Intramolecular Cyclization Sequences. With our previ-
ously reported method of synthesizing substituted picenes for
reference,7 we investigated the Suzuki−Miyaura coupling of
stereodefined (Z)-alkenylboronates 1a−d bearing methoxy
groups with 1,4-dichloro-2,3-diiodobenzene (2) to generate
3a−d, which were further subjected to Pd-catalyzed intra-
molecular double cyclization via C−H activation,8,9 affording
the corresponding di- and tetramethoxy picenes 4a−d in 24−
42% yields (Scheme 1). The origin of relatively low yields of
the desired products can be attributed to the occurrence of
isomerizarion that leads to the formation of E isomers of 3a−d
during Suzuki−Miyaura coupling and/or protodechlorination
over the course of the subsequent double cyclization.
Synthesis of 4e by Wittig Reaction/Intramolecular
Cyclization Sequences. Since the aforementioned method of
synthesis selectively gave rise to the formation of 4a through
intramolecular double cyclization, we synthesized 3,10-
dimethoxypicene (4e) using a different strategy. In this new
method shown in Scheme 2, the precursor 9 has only one
reaction site, which enabled the selective synthesis of 4e after
the Pd-catalyzed intramolecular double cyclization. First, (2-
bromo-5-methoxybenzyl)triphenylphosphonium bromide (7),
derived from commercially available 5, reacted with phthalic
dialdehyde (8) to give the precursor (Z,Z)-9 in 52% yield.10
After the optimization of the conditions shown in Table S1
(Supporting Information), the subsequent Pd-catalyzed intra-
molecular cyclization afforded the desired product 4e in 42%
yield.
e
picene
285
290
290
289
94300
49700
55100
42500
3.23/384
3.13/396
3.08/402
3.13/396
3.14/395
378
388
391
389
382
212
201
331
267
242
e
4a
e
4b
4c
4d
262,
288
50200,
43100
4e
291
41500
b
3.15/393
385
273
a
c
1 × 10−5 M in CH2Cl2. 1 × 10−6 M in CH2Cl2. Estimated with the
absorption edge (λedge). Wavelength of fluorescence maximum. Our
d
e
previous work.7
maxima (λmaxabs) of 4a−e were slightly red shifted in
comparison to that of picene. Among them, 4a,b,d have a
relatively intense absorption at around 270 nm in comparison
to that of picene. In particular, this transition of 4d is the most
intense and has a λmax value of 262 nm, which indicates that the
introduction of methoxy groups to the 1,12- and 4,9-positions
may enhance their transition. Methoxy picenes 4a−d exhibited
red-shifted absorption bands at around 360−380 nm,
corresponding to an S0 → S1 transition.11 In addition, new
and more intense absorption peaks around 340−380 nm
derived from an S0 → S1 transition may reflect the difference in
their electronic structures. HOMO−LUMO energy gaps (Eg)
estimated from their absorption edges are 3.13 eV for 4a,c, 3.08
eV for 4b, and 3.14 eV for 4d, which are smaller than that of
picene. These results suggest that the strong electron-donating
nature of methoxy groups directly affects the electronic
structures of the picene core, resulting in red-shifted absorption
bands, intense absorption peaks, and smaller energy gaps. In
sharp contrast, the absorption spectrum of 4e also showed a
red-shifted absorption band around 360−380 nm, but the
UV−Vis Absorption and Fluorescence Spectra of 4a−
e. UV−vis absorption spectra of methoxy picenes 4a−e are
shown in Figure 2, and their optical properties are summarized
in Table 1 and compared with those of picene. The absorption
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spectral shape did not change dramatically in comparison to
that of picene, though 4e also has two strong electron-donating
methoxy groups. In particular, no absorption peak was observed
around 340−360 nm in 4e, suggesting that the substitution
effect of 3,10-positions is small, as evidenced from our previous
work,7 or that 4e may have a different electronic structure in
comparison to that of picene and the four other methoxy
picenes 4a−d.
The fluorescence spectra of 4a−e have nearly identical
features and exhibited a vibronic structure that is similar to that
of picene. The emission maxima (λmaxem) were bathochromi-
cally shifted from that of the parent picene, but the substitution
position effect on their fluorescence spectra is rather small.
Stokes shifts of methoxy picenes, except for 4a, increased with
an increasing number of methoxy groups, indicating that the
introduction of methoxy groups onto the 2,3,4,9,10,11-
positions leads to large structural changes between the ground
and excited states. However, 4a has a smaller Stokes shift than
that of picene, implying that 4a has a rigid and coplanar
structure due to the introduction of methoxy groups onto the
1,12-positions in the excited state.
elevated significantly as the number of methoxy groups
increased. In these cases, methoxy groups introduced onto
the picene framework in any position simply act as the strong
electron-donating group, even in 4e. Actually, as UV−vis results
suggested, the electronic structure of 4e may be different from
those of picene and the four other methoxy picenes 4a−d,
although the HOMO energy level of 4e also increases in the
same manner as for 4a−d by methoxy substituents. The
elevation of HOMO energy levels found in the five methoxy
picenes 4a−e may contribute to a lowering of the contact
resistance between the gold source electrodes and active layers
(4a−e) in OFET devices.
Theoretical Calculations of HOMO and LUMO in 4a−e.
To investigate the electronic structures of methoxy picenes 4a−
e in detail, theoretical calculations of their HOMO and LUMO
were carried out with density functional theory (DFT) at the
B3LYP/6-31G(d) level using Gaussian 09, Revision A.02.12
Figure 4 shows the molecular orbitals and calculated energy
diagrams of 4a−e. In 4a,b,d, the geometries of both the
HOMO and LUMO are quite similar to those of picene: the
coefficient of the HOMO in these methoxy picenes was
delocalized not only over the entire molecular π framework but
also onto the oxygen atoms of the terminal methoxy groups.
Reflecting this feature, HOMO levels of 4a,b,d were elevated
significantly as the number of methoxy substituents increased.
In addition, their HOMO−LUMO energy gaps become small,
which is consistent with the results of their absorption spectra.
These results can be understood if it is considered that the
introduction of methoxy groups onto the 1,12-, 4,9-, and
2,4,9,11-positions directly affects the frontier orbitals of the
picene core. The geometry of HOMO in 4c is also similar to
those of picene and 4a,b,d, but a small difference was observed.
The geometry of the LUMO and LUMO+1 in 4c is reversed
with respect to picene (Figure S1, Supporting Information); the
energy difference between the LUMO (−1.08 eV) and LUMO
+1 (−1.05 eV) in 4c is quite small. This result suggests that the
substitution in the 2,11-positions has the effect of stabilizing the
LUMO. In fact, although 4b with methoxy groups in the 2,11-
positions does not have an inverted LUMO and LUMO+1, the
energy difference between the LUMO and LUMO+1 in 4b is
also quite small (LUMO = −0.83 eV, LUMO+1 = −0.84 eV).
For this reason, a small difference in the absorption spectra
among 4a,c,d bearing two methoxy groups could be observed.
However, the obtained actual energy gaps of 4a,c,d were almost
identical, which suggests that the electronic structure of 4c is
similar to that of 4a,d.
In contrast, the HOMO of 4e is quite different from that of
picene, though the geometry of the LUMO is the same. This
feature is different from what we found in our previous work in
the case of 3,10-trimethylsilyl picene.7 Interestingly, the
HOMO and HOMO-1 in 4e are reversed with respect to
that of picene (Figure S1, Supporting Information). The
HOMO and HOMO-1 energy levels of 4e are −5.24 and −5.38
eV, respectively, which suggests that methoxy groups in the
3,10-positions stabilize the HOMO energy level, whereas the
HOMO-1 energy level becomes very unstable. Thus, the small
substitution effect on absorption spectra between picene and 4e
had a significant effect on their intrinsically different electronic
structures. This change in HOMO distribution may affect their
intermolecular overlaps: i.e., intermolecular transfer integrals of
HOMO in the solid state.13
Electrochemical Properties of 4a−e. Next, the electro-
chemical properties of methoxy picenes 4a−e were investigated
by cyclic voltammetry. Figure 3 shows the cyclic voltammo-
Figure 3. Cyclic voltammograms of 4a−e in CH2Cl2 solution
containing 0.1 M Bu4NPF6 as supporting electrolyte at a scan rate
of 100 mV s−1.
grams of 4a−e, and their electrochemical data are summarized
in Table 2. Cyclic voltammograms of the five methoxy picenes
4a−e showed one oxidation wave with a clear reduction
process, and they showed oxidation onsets at +0.49−0.73 V (vs
Fc/Fc+). In comparison with the parent picene, their HOMO
energy levels, determined to be 5.29−5.56 eV below the
vacuum level from the onset of the oxidation peaks, were
Table 2. Electrochemical Data of 4a−e
a
b
c
compd
Eonset, V
EHOMO, eV
ELUMO, eV
d
picene
+1.00
+0.71
+0.49
+0.73
+0.76
+0.71
−5.80
−5.51
−5.29
−5.53
−5.56
−5.51
−2.57
−2.38
−2.21
−2.40
−2.42
−2.36
d
4a
d
4b
4c
4d
4e
a
b
Oxidation onset vs Fc/Fc+. Estimated with the equation EHOMO
c
From LUMO orbitals of all methoxy picenes 4a−e,
coefficients of the LUMO were also slightly delocalized onto
(eV) = −4.8 − E1/2 − Eonset. Calculated according to the formula
ELUMO = EHOMO + Egopt Our previous work.7
.
d
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Figure 4. Calculated HOMOs and LUMOs of picene and 4a−e at the DFT B3LYP/6-31(d) level.
the oxygen atoms, but their contributions were rather smaller
than those of HOMO. This result suggests that the substitution
effect of methoxy groups on LUMO levels (destabilization of
the LUMO by the methoxy groups) is small, resulting in a
HOMO−LUMO energy gap smaller than that of picene.
To further investigate the application of these methoxy
picenes to semiconducting materials, we calculated the
molecular reorganization energies of holes (λhs), which may
potentially affect the hole-transporting ability.14 The calculated
λhs are 185 meV for picene, 168 meV for 4a, 277 meV for 4b,
244 meV for 4c, 220 meV for 4d, and 260 meV for 4e. In
general, alkoxy substituents in the core tend to show λ values
larger than that of the parent core because large geometrical
changes in the alkoxy-substituted materials occur upon
oxidation.15 However, to our surprise, 4a has a λ value smaller
than that of picene. In order to understand these differences, we
investigated the geometrical changes upon oxidation in the
parent picene and methoxy picenes 4a−e. Their calculated
bond length changes between the optimized neutral and
cationic states are summarized in Table S2 (Supporting
Information). Among picenes 4a,c−e having two methoxy
groups, the largest C−C bond length changes are almost the
same (0.020−0.023 Å), but significant geometrical changes in
the C−O bonds are observed in 4c−e; the C−O bond lengths
decreased by 0.026 Å for 4c, 0.024 Å for 4d, and 0.030 Å for 4e
upon oxidation. These large geometrical changes may be
attributed to their quinoidal species in the cationic state, as
evidenced from their optimized structure. In particular, the p-
anisole-like structure of 4e may readily form a quinoidal
structure upon oxidation, resulting in the largest geometrical
change in the C−O bonds and thus the largest λ value among
dimethoxy picenes.16 Whereas geometrical changes in the C−O
bonds in 4a are also observed, the change is relatively small
(−0.016 Å). From the optimized structure of 4a in both the
neutral and cationic states, oxygen atoms of 4a could possibly
form hydrogen bonds with the neighboring hydrogen atoms
(i.e., the hydrogen atoms on a central benzene ring of picene;
calculated distance 2.01 Å), leading to a more planar and rigid
backbone. This is the reason 4a has a λ value smaller than that
of picene. These calculations have led us to conclude that only
4a can be a good candidate as an effective hole-transporting
material, but 4a was found to show a small transfer integral (t)
in its crystal structure, as described below.
Single-Crystal Structure Analyses and Transfer Inte-
grals of HOMO in 4a−c,e. As shown in Figure 5, X-ray
diffraction analyses unambiguously clarified the structures of
dimethoxypicenes 4a,7 4c, and 4e and tetramethoxypicene 4b.17
The calculations of intermolecular transfer integrals (ts)
between the HOMOs in these single crystals were also carried
out with the PW91 functional and Slater-type triple-ζ
Figure 5. ORTEP drawings of 4b,c,e determined by X-ray
crystallography with 30% probability thermal ellipsoids. Hydrogen
atoms are omitted for simplicity.
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polarization (TZP) basis sets using the Amsterdam Density
Functional (ADF) package (Figure 6).18 These calculation
results enabled us to predict their hole-transporting character-
istics.19
anisotropic 1D π-stacked structure typically limits effective
carrier transport due to its discontinuous structure.21 This
suggests that the crystal structure of 4b is probably unsuitable
for a high-performance OFET.
In contrast, dimethoxypicenes 4a,c crystallize in the
orthorhombic space groups Pna21 and P212121, respectively.
Both packing structures are similar, and they partially form the
herringbone packing motif. In 4a, one dimer arranged in a
herringbone manner is oriented perpendicularly with respect to
another dimer plane. The transfer integrals in the c axis
direction (t1) and ab plane (t2, face-to-edge interaction) are 6
and 28 meV, respectively, which are rather small; in particular,
t1 is significantly lower than that of nonsubstituted picene (68
meV; Figure S2, Supporting Information). This is probably
because of the distorted molecular structure of 4a, which
prevents effective intermolecular interaction. Thus, the crystals
of 4a might be less effective for carrier transport in OFET
devices. In comparison with 4a, the packing structure of 4c is a
layer by layer structure with a herringbone arrangement, but it
is a different structure in comparison to a typical layer-by-layer
structure, as seen in that of a representative thienoacene.21 The
molecules of each layer are tilted in the opposite direction and
have mutually orthogonally oriented structures. Compound 4c
exhibited relatively large transfer integrals only in the
crystallographic a axis direction (t1 = 36 meV), whereas the
values for the other segments (t2 = 4 meV and t3 = 5 meV) are
significantly smaller than those of picene (56 and 63 meV,
respectively; Figure S2). Compound 4c has a structure with
slippage in the molecular long axis direction by about half of the
length of a molecule to avoid steric hindrance of two methoxy
groups. Therefore, intermolecular orbital overlaps in the
crystallographic bc plane in 4c are small, resulting in small
transfer integrals.
Finally, 4e crystallizes in the orthorhombic space group
P212121, which is the same as that of 4c. 4e has the smallest
changes from picene among the four methoxy picenes. From
the molecular arrangements projected in the c axis direction,
the crystal structure of 4e appears to have a typical layer-by-
layer structure with a herringbone packing motif. However,
molecule 4e in the crystal slipped along the molecular long axis
direction by about half the length of a molecule. This unique
structure is similar to that of 3,10-dimethyldinaphtho[2,3-
b:2′,3′-f ]thieno[3,2-b]thiophene (3,10-DMDNTT) recently
reported by Takimiya and co-workers.22 The crystal structure
of 3,10-DMDNTT can be described as a “3D-herringbone”
packing arrangement that retains large intermolecular overlaps
in the a axis direction (67 meV) and has relatively large
overlaps (9 meV) in the bc plane. Its thin film acted as a
superior semiconductor channel in FETs with a mobility of up
to 0.8 cm2 V−1 s−1. However, the transfer integral along the a
axis direction (ta) in the crystal of 4e was found to be 11 meV,
whereas the values for the other segments tp1 and tp2 were 7 and
2 meV, respectively. According to the picene crystal structure,
there is slight displacement in the molecular long axis. Thus,
the structure of 4e with large slippage reduces its electronic
coupling between adjacent molecules (i.e., tp1 and tp2). In
addition, one possible reason for these low transfer integrals
along the a axis direction may be attributable to their HOMO
density distribution; the HOMO coefficients in 4e reside
densely on the CC bonds perpendicular to the molecular
long axis direction. In fact, as can be visualized from models
shown in Figure S3 (Supporting Information), the HOMO
distribution of 4e offers less effective electronic coupling in the
Figure 6. Packing structures and calculated intermolecular transfer
integrals (ts) between the HOMOs of methoxy picenes using the ADF
package (PW91/TZP): (a) 4a; (b) 4b; (c) 4c; (d) 4e.
The single-crystal structures of methoxy picenes 4a−c,e are
quite different from that of nonsubstituted picene.20 These
crystal structures strongly depend on the substitution position
and the number of methoxy groups. From MO calculations, an
intermolecular electronic coupling of 4a−c depends on their
packing structures because they have almost the same HOMOs
as that of picene.
Compound 4b was found to crystallize in the monoclinic
space group P21/n, which is similar to that of picene. In the
crystal lattice, four methoxy groups are in a conformation that
extends in the same aromatic plane, but the directions of
methoxy groups between the 2,11- and 4,9-positions are
different. The packing structure of 4b can thus be described as a
one-dimensional (1D) π−π stacking motif. Each molecule
slipped along the bc plane and stacked in the opposite direction.
In addition, each layer exists orthogonal to its neighbor. This
might be explained by the steric effect of four bulky methoxy
groups and their unsymmetrical structure. The transfer integrals
in the stacks (i.e., along the crystallographic a axis direction),
designated as t1, have a large value (74 meV). However, this
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Figure 7. Transfer (left) and output (right) curves of OFETs using (a) 4c (L = 200 μm) and (b) 4e (L = 100 μm).
Table 3. FET Characteristics of 4c,e-Based OFET Devices Fabricated on SiO2 Gate Dielectric (W = 500 μm)
compd
sample
L, μm
μh, cm2 V−1 s−1
Vth, V
−3
−8
−14
−22
−7
−21
−25
−6
−13(9)
−69
Ion/off
S, V decade−1
4c
1
50
50
5.8 × 10−5
4.5 × 10−5
4.4 × 10−5
4.4 × 10−5
1.0 × 10−4
4.2 × 10−5
9.5 × 10−5
3.4 × 10−4
1(1) × 10−4
2.5 × 10−3
1.1 × 10−2
1.7 × 10−2
3.0 × 10−2
6.2 × 10−3
1.4 × 10−2
1.4 × 10−2
2.1 × 10−2
1.2 × 10−2
1.2 × 10−2
1.4(8) × 10−2
1.1 × 103
9.7 × 102
2.8 × 102
3.1 × 102
6.2 × 102
1.0 × 102
1.5 × 102
1.5 × 102
5(4) × 102
9.3 × 104
7.5 × 104
4.9 × 104
5.3 × 104
1.9 × 104
2.0 × 104
6.7 × 103
3.0 × 104
1.3 × 104
1.4 × 104
4(3) × 104
6.6
7.1
7.9
5.7
5.2
8.6
6.7
6.3
2
3
100
200
200
300
450
450
4
5
6
7
8
av
1
7(1)
4e
50
100
200
200
200
285
350
450
450
600
4.5
5.0
3.8
5.6
6.1
4.2
10.5
3.2
2.4
2.6
2
−31
−43
−58
−28
−52
−40
−36
−48
3
4
5
6
7
8
9
10
av
−48
−45(12)
5(2)
stacked direction than picene because 4e has a more
discontinuous HOMO distribution. We concluded that it was
for this reason that low transfer integrals along the a axis
direction were observed in 4e.
OFET Characteristics and Structure−Property Rela-
tionship of 4c,e. From a structural and theoretical point of
view, from among the four methoxy picenes whose structures
were analyzed, 4c,e have the potential to be OFET materials.
Thus, we fabricated and characterized OFET devices using
thermal evaporated thin films of 4c,e with a device having a
bottom-gate, top-contact configuration. The obtained transfer
and output curves are shown in Figure 7, and extracted
parameters as a function of channel length (L) from the
saturation regime including the field-effect hole mobilities (μh),
threshold voltages (Vth), on−off ratios (Ion/off), and subthres-
hold slopes (S) are summarized in Table 3. 4c- and 4e-based
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Figure 8. XRD and simulated powder patterns of (a) 4c and (b) 4e on an Si/SiO2 substrate.
OFET devices both showed typical p-channel FET character-
istics, and remarkable channel-length dependence was not
observed. Owing to the HOMO energy levels of 4c,e being
higher than that of picene, smaller threshold voltages (Vth) for
both 4c- and 4e-based devices were observed. However, the
extracted average hole mobilities of 4c- and 4e-based devices
were 9.6 × 10−5 and 1.4 × 10−2 cm2 V−1 s−1, respectively, which
are 2 and 4 orders of magnitude lower, respectively, than that of
picene-based OFET.4 Considering the small t values of 4c,e, the
present low OFET characteristics are consistent. However, the
field-effect mobility of 4c is significantly lower than that of 4e,
though 4c has t values larger than those of 4e, a finding which
we find rather puzzling.
of methoxy groups onto the 3,10-positions (i.e., substitution in
the long molecular axis direction) is the most effective approach
for the development of high-performance semiconducting
materials.
CONCLUSION
■
Five picenes bearing two or four methoxy groups at different
positions have been selectively synthesized via the palladium-
catalyzed Suzuki−Miyaura coupling or Wittig reaction and
subsequent intramolecular double cyclization via C−H bond
functionalizations. From the physicochemical properties and
MO calculations of methoxy picenes, the positions and the
numbers of methoxy groups significantly affected their
electronic properties. Methoxy groups introduced onto the
picene framework in the 1,12-, 2,11-, and 4,9-positions had a
direct effect on their electronic structures, but the HOMO
changes to another geometrical arrangement as a result of
introducing the methoxy group onto the 3,10-positions. The
single-crystal structure analyses of four methoxy picenes
revealed that their structures are quite different from that of
picene and are also influenced by the positions and the number
of methoxy groups. The crystal structures were dramatically
changed from a fully anisotropic 1D π-stacked structure to a
unique 3D herringbone structure. From the calculations of
transfer integrals on the basis of their single-crystal structures, it
is evident that their intermolecular electronic coupling depends
on their packing structures and/or HOMO distribution. The
preliminary results of OFET with 4c,e showed average hole
mobilities of 9.6 × 10−5 and 1.4 × 10−2 cm2 V−1 s−1,
respectively, which are 2 and 4 orders of magnitude lower,
respectively, than that of picene due to less effective
intermolecular overlaps. However, 4e showed the smallest
structural changes in the packing structure and carrier mobility
higher than that of 4c, indicating that the introduction of
methoxy groups onto the 3,10-positions may be an effective
approach for the future development of high-performance
semiconducting materials. Such a structure−property relation-
ship will provide us with an important clue to the design of
picene-based high-performance materials in organic electronic
devices. Furthermore, an understanding of the crystal structures
of the four methoxy picenes examined in this study serves as
motivation to design new types of picene derivatives having
crystal structures that allow high-performance OFETs to be
realized.
In order to answer this question, we first investigated the
molecular orientation in thin film by out-of-plane and in-plane
XRD measurements (Figure 8). In a thin film of 4c, no
diffraction pattern was observed, indicating that it is almost
amorphous on an Si/SiO2 substrate. One possible explanation
of this low crystalline nature may be its weak intermolecular
interaction owing to its unsymmetrical structure. This
amorphous nature of a thin film of 4c makes it unsuitable as
an efficient carrier transport in OFETs: i.e., it would result in
low hole mobility. From these results, we found that the
introduction of methoxy groups onto the 2,11-positions in the
picene core led to large structural changes and less effective
intermolecular overlaps. In contrast, defined diffraction peaks in
both out-of-plane and in-plane XRD were observed in a thin
film of 4e on an Si/SiO2 substrate. A comparison of the XRD
patterns with the simulated powder patterns from the bulk
single crystal clearly shows that the crystal structures of a thin
film and solid state of 4e are quite different. We found that the
3D herringbone structure of 4e is a metastable state. Two
identical diffraction peaks at 2θ = 5.28 and 10.62° were
observed in the out-of-plane XRD. The calculated d spacing is
16.7 Å, which is almost the same as the molecular length (17.4
Å) of 4e as evidenced from its single-crystal structure. This
result suggests that 4e forms a typical layer-by-layer structure in
its thin film similar to the parent picene, which resembles the
case of 2,9-dimethyldinaphtho[2,3-b:2′,3′-f ]thieno[3,2-b]-
thiophene (2,9-DMDNTT) reported by Takimiya and co-
workers.22 Although 4e has a crystal structure similar to that of
picene in thin films, it seems to have less effective
intermolecular overlaps due to the more discontinuous
HOMO distribution as seen from theoretical calculations,
resulting in lower carrier mobility. However, 4e showed the
smallest structural changes in the packing structure and higher
carrier mobility than that of 4c, indicating that the introduction
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quenched with 1 M HCl (3 mL), and extracted with chloroform (3 ×
10 mL). The combined organic extracts were washed thoroughly with
water and dried over anhydrous magnesium sulfate, filtered, and
concentrated in vacuo. The crude residue was then purified by
chromatography and HPLC to afford the product 4d (28 mg, 42%
yield) as a white solid. Mp: >300 °C. FT-IR (neat, cm−1): 3003 (m),
2959 (m), 2936 (m), 2907 (m), 1607 (s), 1589 (s), 1508 (m), 1456
(s), 1416 (s), 1261 (s), 1254 (s), 1043 (s), 775 (s), 731 (m), 527 (s).
1H NMR (CDCl3, 600 MHz, room temperature): δ 4.08 (s, 6H), 7.04
(d, J = 7.8 Hz, 2H), 7.64 (t, 2H), 8.42 (d, J = 8.4 Hz, 2H), 8.48 (d, J =
9.6 Hz, 2H), 8.79 (d, J = 9.6 Hz, 2H), 8.89 (s, 2 H). 13C{1H} NMR
was not observed due to the poor solubility. MS could not be obtained
due to poor solubility. HRMS (EI): calcd for C24H18O2 338.1307,
found 338.1290.
EXPERIMENTAL SECTION
■
General Experimental Methods. All of the reactions were
carried out under an Ar atmosphere using standard Schlenk
techniques. Glassware was dried in an oven (130 °C) and heated
under reduced pressure prior to use. The 1H and 13C{1H} NMR
spectra were recorded on 300 MHz spectrometers. High-resolution
mass spectra (HRMS) were obtained by fast atom bombardment
(FAB) using a double-focusing magnetic sector mass spectrometer.
Solvents employed as eluents and for all other routine operations, as
well as the anhydrous solvents and all reagents used, were purchased
from commercial suppliers and employed without any further
purification. The stereodefined (Z)-alkenylboronates 1a−d23 and
1,4-dichloro-2,3-diiodobenzene (2)24 were prepared in accordance
with the references.
1,12-Dimethoxypicene (4a). White solid. Isolated yield: 24%. Mp:
>300 °C. FT-IR (KBr, cm−1): 2930 (w), 1522 (m), 1450 (s), 1427 (s),
1269 (s), 1238 (s), 1140 (s), 1059 (s), 841 (s), 820 (s), 748 (s), 704
(w). 1H NMR (CDCl3, 600 MHz, room temperature): δ 4.21 (s, 6H),
7.21 (q, J = 12 Hz, 2H), 7.58−7.64 (m, 4H), 7.96 (d, J = 9.6 Hz, 2H),
8.83 (d, J = 9.0 Hz, 2H), 9.92 (s, 2H). 13C{1H} NMR (CDCl3, 150
MHz, room temperature): δ 55.9, 108.2, 121.1, 121.4, 122.5, 126.5,
126.6, 127.1, 128.5, 128.8, 134.3, 158.9. MS could not be detected
because of the high boiling point. Anal. Calcd for C24H18O2: C, 85.18;
H, 5.36. Found: C, 85.52; H, 5.16.
2,4,9,11-Tetramethoxypicene (4b). White solid. Isolated yield:
31%. Mp: >300 °C. FT-IR (KBr, cm−1): 2999 (w), 1618 (s), 1454
(m), 1416 (m), 1383 (m), 1261 (s), 1148 (s), 1047 (s), 808 (m), 644
(w). 1H NMR (CDCl3, 600 MHz, room temperature): δ 4.06 (s, 6H),
4.08 (s, 6H), 6.70 (d, J = 2.4 Hz, 2H), 7.73 (d, J = 1.8 Hz, 2H), 8.37
(d, J = 9.6 Hz, 2H), 8.64 (d, J = 9.0 Hz, 2H), 8.74 (s, 2H). 13C{1H}
NMR (CDCl3, 150 MHz, room temperature): δ 55.7, 56.0, 95.5, 97.9,
118.9, 119.2, 121.1, 121.6, 127.9, 129.8, 132.6, 157.3, 159.3. HRMS
(EI): calcd for C26H22O4 398.1518, found 398.1502.
Synthesis of 1,4-Dichloro-2,3-bis[(1Z)-2-(2-methoxyphenyl)-
ethenyl]benzene (3d). To a stirred solution of 1d (780 mg, 3
mmol) were added PEPPIS-IPr (67.9 mg 0.1 mmol, 10 mol %) and
1,4-dichloro-2,3-diiodobenzene (2; 398.8 mg, 1 mmol) in toluene (10
mL) and an aqueous solution of KOH (3 M) at room temperature.
The reaction mixture was stirred at 110 °C for 12 h and then
quenched with 1 M HCl. The organic layers extracted with diethyl
ether (3 × 10 mL) were washed with brine and dried over MgSO4.
The subsequent filtration, evaporation of the solvent, and purification
by silica gel column chromatography using ethyl acetate/hexane (1/5)
as the eluents yielded 3d (210 mg, 0.51 mmol, 51%) as a yellow oil.
FT-IR (neat, cm−1): 3007 (m), 2957 (m), 2836 (m), 1607 (s), 1512
(s), 1439 (m), 1304 (s), 1254 (s), 1177 (s), 1128 (m), 1034 (s), 839
(s), 754 (m), 507 (m). 1H NMR showed a complex mixture of various
stereoisomers. MS (EI, m/z (relative intensity)): 410 (M+, 5), 304 (5),
302 (6), 289 (4), 227 (11), 189 (3), 131 (3), 122 (10), 121 (100), 77
(3). HRMS (EI): calcd for C24H20Cl2O2 410.0840, found 410.0806.
1,4-Dichloro-2,3-bis[(1Z)-2-(3-methoxyphenyl)ethenyl]benzene
(3a). Yellow oil. Yield: 67%. FT-IR (neat, cm−1): 2938 (w), 1597 (s),
1578 (s), 1489 (s), 1435 (s), 1260 (s), 1153 (m), 1042 (s), 860 (w),
795 (s), 689 (m). 1H NMR (CDCl3, 300 MHz, room temperature): δ
3.60 (s, 6H), 6.10 (d, J = 12 Hz, 2H), 6.49 (d, J = 3.9 Hz, 3H), 6.55 (t,
J = 6.6 Hz, 3H), 6.68−6.75 (m, 2H), 7.05−7.10 (m, 2H), 7.31 (s, 2H).
13C{1H} NMR (CDCl3, 75 MHz, room temperature): δ 54.9, 112.9,
2,11-Dimethoxypicene (4c). White solid. Isolated yield: 10%. Mp:
>300 °C. FT-IR (neat, cm−1): 3005 (w), 2930 (w), 2832 (w), 1607
(m), 1452 (m), 1433 (m), 1211 (s), 1172 (m), 1047 (m), 1030 (m),
1
842 (m), 824 (s), 787 (s), 525 (m). H NMR (CDCl3, 600 MHz,
room temperature): δ 4.08 (s, 6H), 7.32 (d, J = 1.6 Hz, 1H), 7.33 (d, J
= 1.6 Hz, 1H), 7.92 (d, J = 5.6 Hz, 2H), 7.95 (d, J = 6.0 Hz, 2H), 8.18
(d, J = 1.6 Hz, 2H), 8.63 (d, J = 6.0 Hz, 2H), 8.81 (s, 2H). 13C{1H}
NMR (CDCl3, 150 MHz, room temperature): δ 55.5, 103.9, 117.2,
119.4, 121.4, 126.8, 127.0, 128.0, 129.1, 129.9, 131.8, 158.5. MS (EI,
m/z (relative intensity)): 338 (M+, 92), 323 (31), 296 (18), 295 (70),
281 (23), 280 (100), 252 (21), 250 (19), 140 (21), 126 (45), 125
(24), 113 (13). Anal. Calcd for C24H18O2: C, 85.18, H, 5.36. Found:
C, 84.92, H, 5.12.
113.6, 119.3, 124.0, 125.6, 128.7, 129.1, 129.2, 132.2, 133.2, 136.0,
137.4, 138.0. MS (EI, m/z (relative intensity)): 411 (M+, 5), 410 (19),
302 (16), 289 (27), 227 (79), 121 (100), 91 (14). Anal. Calcd for
C24H20Cl2O2: C, 70.08; H, 4.90. Found: C, 69.93; H, 4.78.
1,4-Dichloro-2,3-bis[(1Z)-2-(2,4-dimethoxyphenyl)ethenyl]-
benzene (3b). Yellow oil. Yield: 38%. FT-IR (neat, cm−1): 2938 (w),
1608 (s), 1503 (s), 1464 (w), 1292 (s), 1290 (s), 1159, (s), 823 (w).
1H NMR (CDCl3, 300 MHz, room temperature): δ 3.76 (s, 6H), 3.80
Synthesis of 1-Bromo-2-(bromomethyl)-4-methoxybenzene
(6).25 2-Bromo-5-methoxytoluene (5; 5.0 g, 24.0 mmol), N-
bromosuccinimide (5.1 g, 28.8 mmol), and AIBN (0.1 g, 0.5 mmol,
2 mol %) were suspended in benzene (100 mL). The reaction mixture
was heated to reflux for 1 h and then cooled to room temperature. The
resulting mixture was filtered, and the filtrate was diluted with diethyl
ether (75 mL) and washed with water (2 × 100 mL) and brine (1 ×
50 mL). The organic layers were dried over MgSO4 and concentrated.
The crude solid was purified by silica gel column chromatography
using ethyl acetate/hexane (1/10) to afford 6 (5.5 g, 21 mmol, 86%)
as a white solid.
Synthesis of (2-Bromo-5-methoxybenzyl)triphenylphos-
phonium Bromide (7).26 To a solution of triphenylphosphine
(1.18 g, 4.5 mmol) in toluene was added 2-bromo-5-methoxybenzyl
bromide (6; 1.25 g, 4.5 mmol). The solution was heated to reflux for 3
h and then cooled to room temperature. Subsequent filtration gave the
Wittig reagent 7 (2.39 g, 20 mmol, 98%).
Synthesis of 2,2′-[1,2-Phenylenedi(1Z)-2,1-ethenediyl]bis(4-
bromo)anisole (9) via a Wittig Reaction. To a solution of
phosphonium salt 7 (2.39 g, 4.4 mmol) in THF/H2O (10/1) was
added tBuOK (740 mg, 6.6 mmol) at 0 °C. After 10 min, a solution of
o-phthaldehyde (8; 134 mg, 1.0 mmol) in THF was added dropwise
over 10 min. The reaction mixture was stirred at room temperature for
8 h and then quenched with water. After extraction with ethyl acetate
(s, 6H), 6.17 (q, J = 8.7 Hz, 2H), 6.36 (s, 2H), 6.57 (q, J = 7.5 Hz,
2H), 6.78 (d, J = 12 Hz, 2H), 7.23 (s, 2H), 7.29 (s, 2H). 13C{1H}
NMR (CDCl3, 75 MHz, room temperature): δ 55.0, 55.4, 98.1, 104.1,
119.0, 123.5, 128.5, 128.9, 131.9, 137.9, 157.9, 160.3. MS (EI, m/z
(relative intensity)): 471 (M+, 7), 470 (26), 332 (12), 151 (100), 121
(16). HRMS (EI): calcd for C26H24Cl2O4 470.1052, found 470.1043.
1,4-Dichloro-2,3-bis[(1Z)-2-(4-methoxyphenyl)ethenyl]benzene
(3c). Yellow oil. Yield: 59%. FT-IR (neat, cm−1): 3007 (m), 2955 (m),
2835 (m), 2355 (m), 1606 (s), 1512 (s), 1303 (m), 1251 (s), 1177
(m), 1034 (m), 839 (s), 754 (m), 505 (m). 1H NMR showed a
complex mixture of various stereoisomers. MS (EI, m/z (relative
intensity)): 410 (M+, 5), 304 (4), 302 (6), 289 (4), 227 (12), 189 (3),
131 (2), 126 (4), 122 (9), 121 (100), 113 (2), 77 (3). HRMS (FAB):
calcd for C24H20Cl2O2 410.0840, found 410.0849.
General Procedure for Synthesis of Picene Derivatives via
the Pd-Catalyzed Intramolecular Annulation. Synthesis of 4d.
A 20 mL Schlenk tube equipped with a magnetic stirring bar was
charged with PCy3 (11.5 mg, 0.04 mmol, 20 mol %), PdCl2(NCPh)2
(7.6 mg, 0.02 mmol,10 mol %), and DMA (1 mL) under an argon
atmosphere. After the reaction mixture was stirred for 10 min, Cs2CO3
(130 mg, 0.4 mmol, 2.0 equiv), PivOH (8.3 mg, 0.08 mmol, 40 mol
%), and substrate 3d (70.2 mg, 0.2 mmol, 1 equiv) were added at
room temperature. The tube was placed in a preheated hot box at 150
°C for 24 h. The reaction mixture was cooled to room temperature,
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(3 × 10 mL), the organic layers were washed with brine and dried over
MgSO4. The solvents were removed under vacuum to give the crude
product, which was purified by silica gel column chromatography using
ethyl acetate/hexane (1/5) to give 9 (258 mg, 0.52 mmol, 52%) as a
yellow oil. FT-IR (neat, cm−1): 3055 (m), 3011 (s), 2965 (s), 2936
(s), 2836 (s), 1591 (s), 1287 (s), 1236 (s), 1175 (s), 1055 (m), 1017
Field-effect mobilities were calculated in the saturation regime of the
ID using the equation
ID = (WCi/2L)μ(VG − V )2
th
where Ci is the capacitance of the dielectric layer. ID is the source-drain
current, and VD, VG, and Vth are the source/drain, gate, and threshold
voltages, respectively.
1
(s), 962 (m), 802 (s), 600 (s). H NMR (CDCl3, 600 MHz, room
temperature): δ 3.45 (s, 2H), 6.47 (d, J = 2.8 Hz, 2H), 6.56−6.59 (m,
2H), 6.65−6.77 (m, 4H), 7.00−7.12 (m, 4H), 7.41 (d, J = 8.8 Hz,
2H). 13C{1H} NMR (CDCl3, 150 MHz, room temperature): δ 55.1,
114.6, 115.1, 115.7, 127.1, 129.5, 130.1, 130.5, 133.1, 135.5, 137.9,
158.1. MS could not be obtained because of the high molecular weight.
Anal. Calcd for C24H20Br2O2: C, 57.63; H, 4.03. Found: C, 57.48, H,
3.78.
ASSOCIATED CONTENT
* Supporting Information
■
S
Tables, figures, and CIF files giving experimental procedures,
characterization data for all compounds, X-ray data for 4b,c,e,
details on MO calculations, Cartesian coordinates of calculated
structures, and transfer integrals of the parent picene. This
material is available free of charge via the Internet at http://
pubs.acs.org.
Synthesis of 3,10-Dimethoxypicene (4e) via the Pd-
Catalyzed Intramolecular Annulation. In a 20 mL Schlenk tube
equipped with a magnetic stirring bar were placed PCy3 (11.5 mg, 0.04
mmol, 20 mol %), PdCl2(NCPh)2 (7.6 mg, 0.02 mmol, 10 mol %),
and DMA (1 mL) under an argon atmosphere. After the mixture was
stirred for 10 min, Cs2CO3 (130 mg, 0.4 mmol), PivOH (8.3 mg, 0.08
mmol, 40 mol %), and 9 (100 mg, 0.2 mmol) were added at room
temperature. Then, the solution was heated to 150 °C, maintained at
this temperature for 24 h, and then quenched with 1 M HCl.
Subsequent filtration and washing with water gave a solid, which was
isolated by silica gel column chromatography using CHCl3 to afford 4e
in 10% yield (6.8 mg, 0.02 mmol) as a white solid. Mp: >300 °C. FT-
IR (neat, cm−1): 2955 (m), 2930 (m), 1732 (m), 1622 (s), 1603 (s),
1526 (m), 1468 (s), 1456 (s), 1435 (s), 1362 (m), 1253 (s), 1177 (s),
AUTHOR INFORMATION
Corresponding Author
*E-mail for Y.N.: ynishiha@okayama-u.ac.jp.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
1
1020 (s), 858 (s), 810 (s), 799 (s), 750 (m). H NMR (CDCl3, 600
This work was partially supported by the MEXT program for
promoting the enhancement of research universities. The
authors gratefully thank Dr. Masayuki Iwasaki (Okayama
University) for HRMS measurements, Profs. Koichi Mitsudo
and Seiji Suga (Okayama University) for calculation of transfer
integrals with ADF and CV measurements, Ms. Megumi
Kosaka and Mr. Motonari Kobayashi (Department of
Instrumental Analysis, Advanced Science Research Center,
Okayama University) for the measurements of elemental
analyses, and the SC-NMR Laboratory of Okayama University
for NMR measurements.
MHz, room temperature): δ 4.01 (s, 6H), 7.35 (s, 2H), 7.37 (d, J = 7.8
Hz, 2H), 7.94 (d, J = 9.3 Hz, 2H), 8.74 (d, J = 9.0 Hz, 2H), 8.83 (s,
2H). 13C{1H} NMR was not observed due to poor solubility. MS (EI,
m/z (relative intensity)): 338 (M+, 100), 295 (32), 252 (34), 250
(12), 207 (13), 131 (16), 130 (10), 126 (15), 125 (18), 113 (10).
HRMS (EI): calcd for C24H18O2 338.1307, found 338.1286.
X-ray Structural Analyses. CCDC-976699 (4b), -976700 (4c),
and -976701 (4e) contain supplementary crystallographic data for this
paper. These data can be obtained free of charge from The Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/
cif., by e-mailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12, Union Road, Cambridge
CB2 1EZ, U.K. (fax (+44) 1223-336-033).
Instrumentation and Theoretical Calculations. A Pt electrode
(surface area A = 0.071 cm2, BAS), an Ag/Ag+ electrode (Ag wire in
0.01 M AgNO3/0.1 M TBAP/CH2Cl2), and a Pt-wire electrode were
used as the working, reference, and counter electrodes, respectively. All
of the potentials were calibrated with the standard ferrocene/
ferrocenium redox couple (measured under identical conditions).
Geometry optimizations and normal-mode calculations were per-
formed at the B3LYP/6-31G(d) level using the Gaussian 09, Revision
A.02, program package. Reorganization energies in the hole-transport
system were also calculated using the same program package at the
same level as the prescribed procedures.14 Calculations of
intermolecular transfer integrals were performed with the PW91
functional and Slater-type triple-ζ plus polarization (TZP) basis sets
using the ADF (Amsterdam Density Functional) package. Out-of-
plane and in-plane X-ray diffraction patterns of 4c,e thin films
deposited on hexamethyldisilazane (HMDS) treated Si/SiO2 sub-
strates were recorded using an X-ray diffractometer (RIGAKU Smart
Lab-Pro) with a Cu Kα source (λ = 1.541 Å).
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Device Fabrication and Characterization. Top-contact, bot-
tom-gate FET devices were fabricated with thin films of 4c,e formed
by thermal evaporation. An SiO2 (400 nm) insulator was used as the
gate dielectric. The SiO2 surface was treated with hexamethyldisilazane
(HMDS). Au source/drain electrodes of 50 nm thickness were
deposited on the 4c,e thin films. 2,3,5,6-Tetrafluoro-7,7,8,8-tetracya-
noquinodimethane (F4-TCNQ) of 3 nm thickness was inserted
between the 4c,e thin films and source/drain electrodes to decrease
the contact resistance. The capacitance (Ci) was 8.35 nF cm−2 for
HMDS-treated SiO2. All measurements were performed at 295 K.
(6) Review: Kubozono, Y.; Mitamura, H.; Lee, X.; He, X.; Yamanari,
Y.; Takahashi, Y.; Suzuki, Y.; Kaji, Y.; Eguchi, R.; Akaike, K.; Kambe,
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