Heptacene: Synthesis and Its Hole-Transfer Property in Stable Thin Films

Article abstract of DOI:10.1002/chem.202100936

Heptacene (1) has been produced via a monoketone precursor, 2, which was prepared from 1,2,4,5-tetrabromobenzene in nine steps in a total yield of 10 %. Compound 2 was converted to 1 quantitatively by heating at 202 °C. Heptacene exhibited high thermal stability in the solid state without any observable change over two months. To investigate the potential value of 1 as a material for p-type organic field-effect transistors (OFETs), top-contact OFET devices were fabricated by vacuum deposition of 1 onto a hexamethyldisilazane (HMDS)/SiO₂/Si substrate. The best hole mobility performance was 2.2 cm² V^?1 s^?1. This is the first report of stable heptacene being used in an effective device and examined for its charge carrier properties.

Full text of DOI:10.1002/chem.202100936

Accepted Article
Accepted Article
Title: Synthesis of Heptacene and Its Hole-Transfer Property of Stable
Thin Films
Authors: Takaaki Miyazaki, Motonori Watanabe, Toshinori
Matsushima, Ching-Ting Chien, Chihaya Adachi, Shih-Sheng
Sun, Hiroyuki Furuta, and Tahsin J. Chow
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To be cited as: Chem. Eur. J. 10.1002/chem.202100936
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1/2020

Chemistry - A European Journal
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FULL PAPER
Synthesis of Heptacene and Its Hole-Transfer Property of Stable
Thin Films
Takaaki Miyazaki,*[a, b] Motonori Watanabe, Toshinori Matsushima,*
[c]
[c, d, e]
Ching-Ting Chien, Chihaya
[f]
Adachi,[b, c, d, e] Shih-Sheng Sun, Hiroyuki Furuta, and Tahsin J. Chow*
[f]
[b]
[f, g]
[
a]
Dr. T. Miyazaki
Department of Chemistry, Faculty of Science
Fukuoka University
Fukuoka 814-0180 (Japan)
t.miyazaki@fukuoka-u.ac.jp
[
[
b]
c]
Dr. T. Miyazaki, Prof. Dr. C. Adachi, Prof. Dr. H. Furuta
Department of Chemistry and Biochemistry, Graduate School of Engineering
Kyushu University
Fukuoka 819-0395 (Japan)
Prof. Dr. M. Watanabe, Prof. Dr. T. Matsushima, Prof. Dr. C. Adachi
International Institute for Carbon-Neutral Energy Research
Kyushu University
Fukuoka 819-0395 (Japan)
tmatusim@i2cner.kyushu-u.ac.jp
[d]
Prof. Dr. T. Matsushima, Prof. Dr. C. Adachi
Center for Organic Photonics and Electronics Research (OPERA)
Kyushu University
Fukuoka 819-0395 (Japan)
[
e]
f]
Prof. Dr. T. Matsushima, Prof. Dr. C. Adachi
Japan Science and Technology Agency (JST), ERATO, Adachi Molecular Exciton Engineering Project
Fukuoka 819-0395 (Japan)
Dr. C.-T. Chien, Prof. Dr. S.-S. Sun, Prof. Dr. T. J. Chow
Institute of Chemistry, Academia Sinica
Taipei 11529 (Taiwan)
[
chowtj@gate.sinica.edu.tw
[g]
Prof. Dr. T. J. Chow
Department of Chemistry
Tunghai University
Taichung 40704 (Taiwan)
Supporting information for this article is given via a link at the end of the document.
Abstract: Heptacene (1) was produced through a monoketone
precursor (2), which was prepared starting from 1,2,4,5-
tetrabromobenzene through 9 steps in a total yield of 10%. Compound
become smaller and the HOMO–LUMO (the highest occupied
and the lowest unoccupied molecular orbitals) gaps become
narrower, which leads to the tendency of an open-shell electronic
configuration.[1, 2] Acenes have been used on a variety of
2
was converted to 1 quantitatively by heating at 202 °C. Heptacene
[
3–5]
exhibited high thermal stability in the solid state without observable
change over two months. To investigate the potential value of 1 as a
material for p-type organic field-effect transistors (OFETs), the top-
contact OFET devices were fabricated by vacuum-deposition of 1 on
applications, such as organic photovoltaic devices,
organic
[
6–10]
light-emitting diodes (OLEDs),
light harvesting dyes by singlet
fission,[
11–17]
and organic field-effect transistors (OFETs).
[18–22]
The highly reactive nature of higher member of acenes renders
them difficult for synthesis. In order to obtain higher acenes, many
a
hexamethyldisilazane (HMDS)/SiO
2
/Si substrate. The best
2
–1 –1
performance of hole mobility was 2.2 cm V s . This is the first report
that stable heptacene was used in an effective device and examined
on its charge carrier property.
synthetic strategies have been used, for instance, by the
introduction of bulky groups to acene skeletons,[
23–38]
by the use
of -diketone precursors through photochemical reactions in
solutions or in solid matrixes,[
39–45]
by the use of monoketone
precursors through thermal or photo-conversions,[46–56] by the use
of dimeric structures through thermal retrocyclization in the solid
state,[57] or through an on-surface synthetic technique.[58–60] So far
Introduction
the highest member of non-substituted acene that has been
isolated in pure form is heptacene, which has been generated
through thermal conversion from precursors in the solid state.
Acenes are
a group of polycyclic aromatic hydrocarbons
comprising linearly fused benzene rings. Higher members of
acenes have attracted considerable interest among physical and
organic chemists, because of their unique properties. With the
increasing number of benzene rings, their reorganization energies
[55,
57]
The precursor that adopted by Bettinger et al. was a dimer of
heptacene,[57] while that used by Jancarik and Gourdon was a
monoketone derivative of heptacene (Figure 1).[55]
1
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In our previous reports, we have synthesized hexacene and
54% (2 steps).[62–64] Treatment of 4 with n-BuLi in toluene
2
-bromohexacene through the thermal conversion of its
generated a benzyne moiety again, which reacted with
monoketone precursors. These compounds were found stable in
the solid state over one month under an ambient environment.
Owing to their high stability, hexacene compounds are potentially
useful materials for OFETs. Their hole mobility of single crystals
and thin films has been measured by the fabrication of OFET
devices.[50, 54] We anticipated that heptacene may also possess
good stability in the solid state, and its hole carrier mobility is
comparable to or even better than that of hexacene. The
reorganization energy of heptacene is expected to be smaller than
that of hexacene.
dimethylfulvene to form 5. Aromatization of the rings of 5 was
achieved by dehydration reaction under an acidity condition, then
followed by oxidation with DDQ to give an anthracene-fused
dimethylfulvene 7 in 56% (3 steps). Cycloaddition of 7 with a
furan-fused naphthalene 8 in the presence of s-tetrazine afforded
a pair of stereoisomers, that is, the exo-exo isomer 9a (63% yield)
and the exo-endo isomer 9b (30% yield). To optimize the yield,
this reaction had to be carried out in two successive steps: a
mixture of compound 8 and s-tetrazine was allowed to react first,
after then compound 7 was added. Because both compounds 7
and 8 were reactive toward s-tetrazine, so that adding them all
together let to a variety of products. The stereo-configurations of
1
1
9
a and 9b were distinguished by H NMR spectra. In the H NMR
of 9a, the signals at 2.12, 4.11, and 5.43 ppm do not exhibit spin-
spin coupling (Figure S1). On the other hand, 9b shows the spin-
spin coupling between the signals at 2.83 and 5.45 ppm with ca.
4
.0 Hz coupling constant (Figure S2). The signals at 5.43 and 5.45
Figure 1. Heptacene 1 generated from thermal conversion of the monoketone
precursor 2.
ppm in 9a and 9b, respectively, were assigned to the protons at
the bridgehead carbon atoms adjacent to the oxygen atom.
Therefore, the stereo-configurations of 9a and 9b were assigned
to be exo-exo and exo-endo, respectively. The double bond of 9a
was epoxidized by m-CPBA, followed by an acid catalyzed
rearrangement reaction[47] to give diol 10 in 60% yield. The
reaction from 9b did not give the desired diol structure under the
same conditions. The oxidation of diol 10 by iodobenzene
For the synthesis of stable heptacene, the best method is to
go through the monoketone precursor 2.[61] The key step for the
preparation of 2 is to generate a carbonyl group at the 7th position
of a norbornane moiety. In our previous study in the synthesis of
pentacene, the carbonyl group was transformed from a double
bond, which was converted to a diol by treatments with m-
diacetate [PhI(OAc)
2
]
in toluene produced the desired
monoketone 2 in 90% yield. The total yield from 1,2,4,5-
tetrabromobenzene to 2 was 10% through 9 steps. In a high-
resolution MALDI-TOF mass spectrum of 2, the m/z value was
chloroperbenzoic acid (m-CPBA) and p-toluenesulfonic acid (p-
TsOH) (Scheme 1).[47] Following the same strategy, here we
report the synthesis of heptacene. Stable solid of heptacene is
obtained and its hole-transfer property is studied through the
successful fabrication of OFET devices.
3
78.1403, which corresponds to the mass of 1 that was formed
from 2 by releasing a unit of carbon monoxide (calcd. for C30
H
18
+
3
78.1409 [M−CO] ) (Figure S3).
To investigate the thermal conversion of 2 into 1, thermal
gravimetric analysis (TGA) of 2 was performed under a nitrogen
atmosphere (Figure 2a). The first weight loss, releasing carbon
monoxide to form 1 (7.4%, calcd. 6.9%), was observed at ca.
2
02 °C. The decomposition/vaporization of 1 did not occur at a
temperature up to ca. 400 °C. The ATR-FTIR spectra of 2 showed
–
1
the carbonyl stretching band at 1780 cm , which disappeared
after heating at 230 °C in a nitrogen-filled glove box (Figure 2b).
The observed behaviors by TGA and IR of 2 were consistent with
those previously described.[55, 65] In the absorption spectrum
Scheme 1. Synthetic strategy of the monoketone precursor of pentacene.[47]
(
Figure 2c) of the solid film of 2, prepared by drop-cast a saturated
THF solution, exhibited a peak maximum at 400 nm deriving from
anthracene. After heating, a pair of broad absorption bands at
Results and Discussion
1
023 and 841 nm were assigned to the 0–0 and 0–1 transitions
of 1, separately. Davydov splitting was not observed at room
temperature. The thin film of 1, prepared by deposition on a quartz
glass under vacuum, exhibited broad bands at ca. 650 to 1050
nm without splitting (Figure S4). The spectral feature agreed well
with that of heptacene film, deposited on sapphire at 298 K,
reported by Bettinger et al.[57] The stability of 1 was monitored by
solid-state cross-polarization magic angle spinning (CP-MAS)
NMR spectroscopy (Figure 2d). In 13C CP-MAS spectra of 2, three
separated peaks at 56.9 ppm (bridgehead carbon atoms), 120.1–
The stability of higher acenes relies critically on the herringbone
motif in crystalline packing. The most effective method of
preparation is to heat up the corresponding precursors in the solid
state.[50, 54, 61] As shown in Figure 1, heptacene 1 can be generated
through a thermal reaction from precursor 2 in the solid state. The
synthesis of compound 2 is shown in Scheme 2. The synthesis
started with a cyclization reaction between furan and benzyne,
which was generated from 1,2,4,5-tetrabromobenzene and n-
BuLi in toluene, to give 3. A subsequent Diels–Alder reaction was
carried out between 3 and 1,3-butadiene in toluene to yield 4 in
1
34.0 ppm (aromatic carbon atoms), and 195.6 ppm (carbonyl
2
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Scheme 2. Synthetic route of 2.
Figure 2. (a) TGA profile of 2 under a nitrogen atmosphere with a heating rate of 10 °C min–1. (b) FTIR spectra (ATR) of 1 (red) and 2 (blue). 2 was converted into
1
2
by heating at 230 °C for 15 min in a nitrogen-filled glove box. (c) Absorption spectra of drop-casted 1 and 2 on a quartz plate. 1 was generated from drop-casted
by heating at 230 °C for 15 min in a nitrogen-filled glove box. (d) Solid-state 13C CP-MAS spectra of 2 (blue), 1 (red), and 1 after two months in a nitrogen-filled
glove box (black). The asterisks denote the spinning side bands.
3
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Figure 3. OFET device performance of thin film 1 fabricated by vacuum deposition (drain, source, gate, and threshold termed D, S, G, and th, respectively). (a)
Output characteristics. (b) Transfer characteristics recorded at VDS = –100 V.
carbon atom) were shown. After converting into 1 by heating, the
peaks corresponding to the bridgehead and carbonyl carbon
atoms disappeared, and the multiplet deriving from aromatic
carbon atoms was narrowed down to a broad doublet at 125.2
and 128.3 ppm. The resulting NMR spectra did not exhibit any
significant change after two months stored in a nitrogen-filled
glove box, showing the high stability of 1. The spectral features
and changes induced by heating were in good agreement with the
reported data.[55, 57]
On the other hand, our previous OFETs of hexacene did not
appear to be inhibited by high contact resistance; thus, it was
predicted that the crystalline size also influences the performance.
To verify the hypothesis, the crystalline size in the thin film of both
1 and hexacene was measured by X-ray diffraction (XRD)
patterns and analyzed by the Scherrer’s equation. The contacting
pattern of the vacuum-deposited film of 1 on the surface of a
substrate exhibited out-of-plane XRD peaks along (00l) direction.
The (001) peak of 1 was observed at 4.09° (Figure S5), and the
calculated crystalline size in the thin film was estimated to be 17
nm, which is smaller than that of hexacene (25 nm). The XRD
pattern of hexacene has been reported previously.[54] It is
considered that not only the presence of the hole-injection barrier
but also the formation of small crystals in the thin film of 1 leads
to high contact resistance, as holes injected from top-contact Au
electrodes are more difficult to reach the channel region formed
To evaluate the hole-transfer property of 1, top-contact
OFET devices were fabricated. Vacuum evaporation of 1 was
carried out under 10–6 torr to deposit the thin film with a thickness
of 50 nm on a HMDS/SiO
evaporation rate of 1 was 0.1 nm s . The HMDS treatment on
SiO /Si substrates was done by spin-coating at 1000 rpm for 60
s, followed by heating at 110 °C for 10 min in air. On top of the
films, gold was vacuum-deposited as drain and source electrodes. near the SiO
2
(300 nm)/Si substrate. The vacuum
–
1
2
2
gate insulator surface across smaller crystals.
The thickness of the gold electrodes was 50 nm, and the channel
dimension was 45 m x 2 mm. The OFET performance was
evaluated using a semiconductor parameter analyzer (B1500A,
Keysight) in vacuum. The hole mobility of 1 estimated in the
saturation region was 1.8–2.2 cm V–1 s–1 with threshold voltages
ranging –39 ~ –57 V. The average hole mobility and threshold
voltage of seven independent devices were 2.0 cm V–1 s–1 and –
Although the smaller crystals decrease the carrier mobility along
the channel region, the hole mobility was higher in 1 than in
hexacene. The result shows that heptacene (1) has outstanding
potential as a hole-transfer material for OFET. The hole mobility
of OFET devices made with a single crystal of hexacene was
2
2
–1 –1 [50]
reported to be 4.28 cm V s , which is much higher than that
2
made with thin films of hexacene (0.076 cm V–1 s ) . One
reason for the lower mobility of thin films is the presence of grain
boundaries. Therefore, using a single crystal of heptacene (1) for
the OFET fabrication would lead to an even higher (true) mobility,
although its single crystal with large enough size is still awaited to
be prepared.
2
–1 [54]
5
2 V, respectively. The highest hole mobility was observed at 2.2
2
–1 –1
3
cm V s , with an on/off ratio of 5 x 10 and a threshold voltage
of –56 V (Figure 3). Hysteresis was not observed on the output
profiles. In our previous work, the hole mobility of hexacene was
measured under the same conditions, and the thin film of
hexacene showed the best hole mobility 0.076 cm V s–1 with a
2
–1
The potential energy of molecular orbitals was estimated
using both PYS and low energy inverse photoelectron
spectroscopy (LEIPS).[68] The HOMO–LUMO energy gaps of
heptacene, hexacene, and pentacene were found to be 2.04, 2.24,
and 2.75 eV, respectively. The trend agrees well with the results
of the corresponding optical gaps estimated from the absorption
onsets (1.14, 1.41, and 1.76 eV), and the HOMO–LUMO gap
values calculated at B3LYP/6-31+G* level (1.47, 1.78, and 2.19
eV, respectively) with the Gaussian 16 program package.[69] The
calculated reorganization energy supported the fact that 1 is a
better hole-transfer material than either hexacene or pentacene
(67, 77, and 92 meV for heptacene, hexacene, and pentacene,
respectively) (Table 1). These results reconfirmed the theoretical
2
[54]
threshold voltage –21 V and an on/off ratio 2.4 x 10 . The thin
film of heptacene (1) exhibited higher hole mobility than that of
hexacene. However, a bend was observed in the output
characteristics of the OFET devices composing in 1, indicating the
influence of the high contact resistance. The existence of contact
resistance inhibited the improvement of the threshold voltage and
the on/off ratio. The HOMO level of 1 estimated by photoemission
yield spectroscopy (PYS)[66] was –5.03 eV (Table S1). Therefore,
it is considered that there is a hole-injection barrier between the
HOMO level of 1 and the Fermi level of Au electrode (–4.76 eV),[67]
resulting in a high contact resistance.
4
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prediction, that is, the higher member an acene is, the smaller
energy gap and reorganization energy it possesses.
General Information: Chemicals and reagents were commercially
available and used without further purification. H and C NMR (500 MHz)
spectra were measured on a JEOL JNM-ECX500. Chemical shifts were
1
13
reported in parts per million (ppm) relative to residual CHCl
3
in CDCl
3
(7.26
ppm for H and 77.16 ppm for 13C) and CH
1
in CD
(5.32 ppm for H).
1
2
Cl
2
2
Cl
2
Table 1. Physical properties of pentacene, hexacene, and heptacene (1).
The coupling constants (J) were given in hertz (Hz). High resolution mass
HRMS) in fast atom bombardment (FAB) were measured using m-
(
Energy
gap (eV)
Optical
gap (eV)
HOMO–
Reorganization
energy (meV)
[a]
nitrobenzyl alcohol (NBA) as a matrix on a JEOL JMS-700 MStation.
MALDI-TOF mass was performed on a JEOL JMS-S3000. Analytical thin
LUMO gap
eV)[
a]
(
layer chromatography (TLC) was carried out with silica gel Merck 60 F254
Column chromatography was carried out on KANTO silica gel 60N, neutral,
0–50 m available from Kanto Chemical Co., Inc. Absorption spectra
.
Heptacene (1)
Hexacene
2.04
2.24
2.75
1.14
1.41
1.76
1.47
1.78
2.19
67
77
92
4
were recorded on a SHIMADZU UV-3600. IR spectra were measured on
a JASCO FT/IR 4200. The HOMO and LUMO levels of organic films were
measured with PYS (AC-3, Riken Keiki) and LEIPS (LE-1, ALS
Technology), respectively.
Pentacene
[
a] B3LYP/6-31+G*.
13C CP-MAS NMR for 2: The 13
C CP-MAS NMR measurements were
performed using a Bruker Avance III 400 MHz NMR spectrometer,
equipped with a 4 mm double resonance probe operating at the 1H and
13C Larmor frequencies of 400.13 and 100.63 MHz, respectively. For the
polarization transfer, the contact-time was set to 1.75 ms. During data
Through single crystal analysis, it was found that hexacene
is packed in a herringbone motif, which can inhibit the process of
dimerization. As mentioned earlier, the molecular packing is
responsible for the high thermal stability of hexacene in the solid
state.[50] Heptacene (1) showed a similar high thermal stability,
thus the same packing motif is expected. The (001) peak of the
vacuum-deposited film of 1 on the surface of a substrate indicated
an interplanar distance of 21.6 Å (2 = 4.09°). The calculated
molecular length along the long axis of 1 was 21.5 Å, and this
length is well consistent with the d-space, that is, the edge of short
axis contacts on the surface of the substrate and the molecules
are oriented vertical to the surface along their long axis. The
direction of the carrier transfer is the same as the orientation of
the -system in heptacene (1), indicating that the thin film of 1
exhibited high hole mobility.
1
acquisition, H decoupling was done by spinal-64. A powdered sample was
packed in a double-bearing 4-mm zirconium oxide MAS rotor. A sample
spinning frequency of 12 kHz was used and regulated by a spinning
controller within ± 1 Hz. The 13C NMR chemical shifts were referenced to
the methylene carbon signal of adamantane ( = 38.48 ppm) as an external
standard.
13C CP-MAS NMR for 1: The 13
C CP-MAS NMR measurements were
performed with Bruker Avance III 600 MHz NMR spectrometer, equipped
with a 3.2 mm double resonance probe operating at the H and 13C Larmor
frequencies of 600.13 and 150.92 MHz, respectively. For the polarization
transfer, the contact-time was set to 2.0 ms. During the data acquisition,
1
1
H decoupling was done by spinal-64. A powdered sample was packed in
a double-bearing 3.2-mm zirconium oxide MAS rotor. A sample spinning
frequency of 14 kHz was used and regulated by a spinning controller within
±
1 Hz. The 13C NMR chemical shifts were referenced to the methylene
carbon signal of adamantane ( = 38.48 ppm) as an external standard.
Conclusion
OFET measurement: The electrical measurements were conducted in a
vacuum using a semiconductor parameter analyzer (B1500A, Keysight).
The saturation-regime mobility (sat) was extracted from the slope of the
The synthesis of the monoketone heptacene precursor 2 was
achieved with 10% total yield in 9 steps. A quantitative thermal
conversion of 2 into heptacene 1 was observed at 202 °C. The
hole-transfer property of 1 was measured by fabricating the OFET
devices composing of the top-contact thin film of 1 on a
square root of the drain current plot vs V
G
from equation 1.
)ꢀ
퐶 휇푠푎푡(푉 −푉 (1)
푖 퐺 푇
푊
퐼
=
퐷,푠푎푡
2
퐿
Where ID,sat is the drain-to-source saturated current; W/L is the channel
width to length ratio; C
i
is the capacitance of the insulator per unit area (C
i
2
=
10.2 nF/cm ), and V
G
and V are gate voltage and threshold voltage,
T
HMDS/SiO
2
/Si substrate. The best performance of hole mobility
was observed at 2.2 cm2 V–1 s . The average hole mobility of
–1
respectively. Channel length (L) and width (W) were 2000 m and 45 m,
respectively.
seven independent devices was 2.0 cm V–1 s–1. The hole mobility
was higher in heptacene 1 than in hexacene. The HOMO–LUMO
energy gap estimated by PYS and LEIPS complies well with the
calculated values. A small HOMO–LUMO gap and a low
reorganization energy supported the result of hole-transfer nature.
The result of XRD measurement revealed that the 2 angle of 1
was 4.09°, which corresponded to a d-space of 21.6 Å. It indicates
that the molecular layers in crystalline film of 1 are oriented
2
Scherrer’s equation: The size of crystals in the thin film was estimated
by the Scherrer’s equation 2.
퐾휆
ꢁ
=
(2)
훽 푐표ꢂ휃
Where D is the size of the crystal. K is the Scherrer constant (K = 0.9), 
is the X-ray wavelength ( = 0.154 nm),  is the full width at half maximum
–
3
(
FWHM) at the peak of XRD (heptacene: 8.38 x 10 rad; hexacene: 5.59
–3
x 10 rad), and  is the Bragg angle (heptacene: 2 = 4.09°; hexacene:
vertically to the surface of a HMDS/SiO
2
/Si substrate along the
 = 4.84°[54]).
2
long axis. The potential usage of heptacene as an effective
charge carrier in p-type OFET devices was reported for the first
time.
Synthesis of 4. To a mixture of 1,2,4,5-tetrabromobenzene (11.82 g, 30.0
mmol), furan (6.5 mL) and toluene (300 mL) were slowly added 1.6 M
solution of n-BuLi in hexane (22.5 mL, 36.0 mmol) at –30 °C under a
nitrogen atmosphere. The mixture was stirred at –30 °C for 1 h, and then
at room temperature for 3 h. After adding water, the reaction mixture was
extracted with AcOEt, and the organic layer was dried with Na
and the filtrate was concentrated under reduced pressure. The residue
was purified by silica gel column chromatography (hexane/CH Cl , 6/4) to
2 4
SO , filtered,
Experimental Section
2
2
5
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give 3 (5.05 g) as a white powder. This compound was used for the next
reaction without further purification. 3 (5.05 g), 3-sulfolene (3.98 g, 33.7
mmol), K CO (1.0 g) and toluene (20 mL) were charged in a round-bottom
2 3
pressure flask, and the mixture was stirred at 140 °C for 22 h under a
nitrogen atmosphere. The reaction mixture was directly purified by silica
Synthesis of 10. To a mixture of 9a (226 mg, 0.50 mmol), sat. NaHCO
aq. (7.5 mL) and CH Cl (15 mL) was added 70% m-CPBA (186 mg, 1.54
mmol) at 0 °C, and the mixture was stirred vigorously at same temperature
for 1 h. The reaction mixture was washed with Na SO aq., and the organic
layer was dried by Na SO , filtered, and the filtrate was concentrated under
reduced pressure. The residue was dissolved in toluene (15 mL), and to
the toluene solution under reflux was added p-TsOH•H O (30 mg, 0.16
3
2
2
2
3
2
4
gel column chromatography (hexane/CH
2 2
Cl , 6/4) to give 4 (5.76 g, 16.2
mmol, 54% from 1,2,4,5-tetrabromobenzene) as a white powder.
[
62] 1
3
2
4
2

:
H NMR (500 MHz, CDCl
3
):  = 5.67 (s, 2H), 7.00 (s, 2H), 7.48 (s,
):  = 82.0, 120.8, 125.6, 142.9, 150.4.
):  = 1.91 (m, 2H), 2.01 (m, 2H), 2.48 (m,
2
13
H); C NMR (125 MHz, CDCl
[
3
mmol) under a nitrogen atmosphere. The mixture was refluxed for 25 min
and cooled down in ice-bath. The precipitate was filtered, and the residue
powder was dissolved in CH Cl . The solution was passed through thin
63] 1
: H NMR (500 MHz, CDCl
3
13
H), 4.95 (s, 2H), 5.93 (m, 2H), 7.47 (s, 2H); C NMR (125 MHz, CDCl
= 27.3, 42.3, 84.6, 122.4, 124.4, 129.1, 147.1.
3
):
2
2
silica gel pad eluting with CH Cl . The eluate was dried under reduced
2
2
pressure. To the residue was added toluene (10 mL) and the suspension
was refluxed for 15 min. After cooling, the precipitate was filtered and
washed with cold toluene and MeOH, and then dried under reduced
Synthesis of 7. To a mixture of 4 (5.34 g, 15.0 mmol), 6,6-dimethylfulvene
2.7 mL, 22.4 mmol) and toluene (150 mL) was slowly added 1.6 M solution
of n-BuLi in hexane (11.3 mL, 18.0 mmol) at –35 °C under a nitrogen
atmosphere. The mixture was stirred at –35 °C for 1 h, and then at room
temperature for 4 h. After adding water, the reaction mixture was extracted
(
1
pressure to give 10 (142 mg, 0.30 mmol, 60%) as a white powder. H NMR
(
7
500 MHz, CDCl
3
):  = 1.11 (s, 6H), 2.47 (s, 1H), 2.90 (s, 1H), 4.60 (s, 2H),
.41 (m, 4H), 7.82 (s, 2H), 7.93 (m, 6H), 8.26 (s, 2H), 8.27 (s, 2H); 13
C
with AcOEt, and the organic layer was dried with Na
filtrate was concentrated under reduced pressure. The residue was
purified by silica gel column chromatography (hexane/CH Cl , 6/4) to give
2 4
SO , filtered, and the
NMR (125 MHz, CDCl
3
):  = 27.4, 57.0, 74.6, 102.8, 120.0, 121.7, 125.4,
2
2
125.4, 126.2, 126.2 128.1, 128.1, 131.2, 131.4, 131.7, 131.9, 142.9, 143.3;
HRMS (FAB-TOF): m/z calcd for C34H26O : 466.1933 [M] found:
a mixture of stereoisomers 5 (3.04 g) as a white powder. This compound
was used for the next reaction without further purification. A mixture of 5
+
;
2
466.1931.
(3.04 g), conc. HCl aq. (7.0 mL) and EtOH (75 mL) was refluxed under a
nitrogen atmosphere for 1 h. The reaction mixture was poured into ice-
water, and the precipitate was filtered to afford crude 6, which was directly
used in the next reaction because of the instability in air. To a solution of
crude 6 in toluene (100 mL) was slowly added a solution of DDQ (2.72 g,
Synthesis of 2. To a suspension of 10 (141 mg, 0.30 mmol) in toluene (15
mL) was added PhI(OAc) (194 mg, 0.60 mmol), and the mixture was
refluxed for 30 min. After cooling, the precipitate was filtered, washed with
CH Cl , and dried under reduced pressure to give 2 (104 mg, 0.27 mmol,
0%) as a white powder. 1H NMR (500 MHz, CD
2
1
2.0 mmol) in toluene (50 mL), and then the mixture was stirred at room
temperature for 30 min. The reaction mixture was filtered through a Celite
pad, and the filtrate was extracted with AcOEt and washed with NaHSO
aq. The organic layer was dried by Na SO , filtered, and the filtrate was
concentrated under reduced pressure. The residue was purified by silica
2
2
9
7
1
2
Cl
2
):  = 5.02 (s, 2H),
3
~
.46 (m, 4H), 8.00 (m, 4H), 8.11 (s, 2H), 8.42 (s, 4H); FTIR (ATR): 
=
2
4
–1
780 cm (C=O); HRMS (MALDI-TOF): m/z calcd for C30
H
18: 378.1409 [M
+
gel column chromatography (hexane/CH
2
Cl
2
, 4/1) to give 7 (2.36 g, 8.35
−CO] ; found: 378.1403.
mmol, 56% from 4) as a white powder.
1
The mixture of stereoisomers 5: H NMR (500 MHz, CDCl
(
(
3
):  = 1.49, 1.55
Thermal conversion of 2 into 1. The powder or film of 2 was heated at
230 °C for 15 min under a nitrogen atmosphere in an electric furnace
placed in a glove box with a nitrogen atmosphere (< 1 ppm O and H O)
2 2
to give 1 as a green powder or film.
s, 6H), 1.88 (m, 2H), 2.01 (m, 2H), 2.45 (m, 2H), 4.31 (m, 2H), 4.88, 4.89
13
s, 2H), 5.91 (m, 2H), 6.87, 6.91 (t, J = 2.0 Hz, 2H), 7.09, 7.10 (s, 2H);
):  = 18.9, 19.1, 27.7, 42.6, 42.6, 50.8, 50.8, 85.2,
5.2, 101.7, 101.9, 112.6, 112.9, 129.3, 142.7, 142.7, 143.1, 143.4, 149.5,
49.6, 162.0, 162.1; HRMS (FAB-TOF): m/z calcd for C22 23O: 303.1749
C
NMR (125 MHz, CDCl
3
8
1
H
+
[M+H] ; found: 303.1746.
Evaluation of the stability of 1 in the solid state. Compound 1 was
charged in a solid-state NMR sample rotor and stored in a dark in a glove
box filled with nitrogen atmosphere (< 1ppm O and H O).
2 2
1
6
2
2
1
:
H NMR (500 MHz, CDCl
3
):  = 1.59 (s, 6H), 3.52 (s, 4H), 4.41 (t, J =
.0 Hz, 2H), 5.99 (m, 2H), 6.84 (t, J = 2.0 Hz, 2H), 7.44 (s, 2H), 7.46 (s,
):  = 19.3, 30.0, 50.1, 105.6, 117.9, 117.9,
25.3, 126.3, 126.3, 103.9, 132.5, 142.0, 146.3, 158.6; HRMS (FAB-TOF):
20: 284.1565 [M] ; found: 284.1584.
):  = 1.63 (s, 6H), 4.46 (t, J = 2.0 Hz, 2H),
.85 (t, J = 2.0 Hz, 2H), 7.41 (m, 2H), 7.67 (s, 2H), 7.94 (m, 2H), 8.22 (s,
H); 13C NMR (125 MHz, CDCl
):  = 19.5, 50.0, 107.4, 118.1, 124.9, 125.8,
1
3
H); C NMR (125 MHz, CDCl
3
+
m/z calcd for C22
H
1
7
6
2
1
:
H NMR (500 MHz, CDCl
3
Acknowledgements
3
28.1, 131.2, 131.7, 141.5, 146.2, 157.0; HRMS (FAB-TOF): m/z calcd for
This work was supported by JSPS KAKENHI Grant Number
JP16H04192, JP20K15262, and 20H02817 from the Ministry of
Education, Culture, Sports, Science and Technology (MEXT),
Japan and The Samco Foundation and was performed under the
Cooperative Research Program of "Network Joint Research
Center for Materials and Devices." The computations were
primarily performed using the computer facilities at the Research
Institute for Information Technology, Kyushu University. MW and
+
C
22
H
18: 282.1409 [M] ; found: 282.1421.
Synthesis of 9a and 9b. A mixture of 8 (876 mg, 4.51 mmol), 3,6-bis(2-
pyridyl)tertazine (1.06 g, 4.50 mmol) and toluene (90 mL) was stirred at
70 °C for 1 h under a nitrogen atmosphere. To the reaction mixture was
added 7 (847 mg, 3.00 mmol), and the mixture was refluxed for 1 day under
a nitrogen atmosphere. After cooling, the white precipitate was filtered, and
washed with toluene to give exo-endo isomer 9b. The filtrate was purified
2
by silica gel column chromatography (hexane/CH
2
Cl
2
, 1/1) to give exo-exo
TM acknowledge support from I CNER, funded by the World
isomer 9a (850 mg, 1.89 mmol, 63%) as a white powder and exo-endo
isomer 9b (total 410 mg, 0.91 mmol, 30%) as an off-white powder. 9a: H
Premier International Research Center Initiative (WPI), Ministry of
Education, Culture, Sports, Science and Technology of Japan
1
NMR (500 MHz, CDCl
3
):  = 1.78 (s, 6H), 2.12 (s, 2H), 4.11 (s, 2H), 5.43
(
MEXT), Japan. TJC and SSS are also grateful for the supports
(
s, 2H), 7.39 (m, 4H), 7.57 (s, 2H), 7.60 (s, 2H), 7.74 (m, 2H), 7.92 (m, 2H),
from the Ministry of Science and Technology of Taiwan and
Academia Sinica.
8
1
1
.23 (s, 2H); 13C NMR (125 MHz, CDCl
17.2, 117.4, 124.9, 125.8, 125.8, 128.0, 128.3, 131.4, 131.5, 132.8, 143.6,
3
):  = 20.8, 47.5, 51.7, 82.4, 115.0,
44.4, 145.5; HRMS (FAB-TOF): m/z calcd for C34
H
26O: 450.1984 [M]+;
1
found: 450.1993. 9b: H NMR (500 MHz, CDCl
3
):  = 0.67 (s, 6H), 2.84 (m,
Keywords: Heptacene • Hydrocarbons • p-type OFET •
Synthetic methods • Thermal conversion
2
7
H), 3.60 (s, 2H), 5.45 (m, 2H), 7.40 (m, 2H), 7.45 (m, 2H), 7.53 (s, 2H),
.64 (s, 2H), 7.79 (m, 2H), 7.93 (m, 2H), 8.23 (s, 2H); 13C NMR (125 MHz,
):  = 20.5, 44.5, 49.9, 81.5, 116.8, 116.9, 118.2, 125.0, 125.8, 125.9,
28.0, 128.2, 131.4, 131.6, 133.6, 141.9, 142.0, 146.6; HRMS (FAB-TOF):
CDCl
1
3
[
1]
2]
Y. Yang, E. R. Davidson, W. Yang, Proc. Natl. Acad. Sci. USA 2016, 113,
E5098–E5107.
+
m/z calcd for C34H26O: 450.1984 [M] ; found: 450.1986.
[
M. Bendikov, H. M. Duong, K. Starkey, K. N. Houk, E. A. Carter, F. Wudl,
J. Am. Chem. Soc. 2004, 126, 7416–7417.
6
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Chemistry - A European Journal
10.1002/chem.202100936
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Chemistry - A European Journal
10.1002/chem.202100936
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Heptacene was synthesized through 9 steps in a total yield of 10% and its hole-transfer property was measured by fabricating the
organic field-effect transistor (OFET) devices. The best performance of hole mobility was 2.2 cm V s–1 in the stable thin film of
2
–1
heptacene. The potential usage of heptacene as an effective charge carrier was reported for the first time.
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