Synthesis and physical properties of brominated hexacene and hole-transfer properties of thin-film transistors

Article abstract of DOI:10.1039/c7ra13632c

A halide-substituted higher acene, 2-bromohexacene, and its precursor with a carbonyl bridge moiety were synthesized. The precursor was synthesized through 7 steps in a total yield of 2.5%. The structure of precursor and thermally converted 2-bromohexacene were characterized by solid state NMR, IR, and absorption spectra, as well as by DFT computation analysis. It exhibited high stability in the solid state over 3 months, therefore can be utilized in the fabrication of opto-electronic devices. The organic thin-film transistors (OFETs) were fabricated by using 2-bromohexacene and parent hexacene through vaccum deposition method. The best film mobility of 2-bromohexacene was observed at 0.83 cm² V^-1 s^-1 with an on/off ratio of 5.0 × 10⁴ and a threshold of -52 V, while the best film mobility of hexacene was observed at 0.076 cm² V^-1 s^-1 with an on/off ratio of 2.4 × 10² and a threshold of -21 V. AFM measurement of 2-bromohexacene showed smooth film formation. The averaged mobility of 2-bromohexacene is 8 fold higher than the non-substituted hexacene.

Full text of DOI:10.1039/c7ra13632c

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PAPER
Synthesis and physical properties of brominated
hexacene and hole-transfer properties of thin-film
transistors†
Cite this: RSC Adv., 2018, 8, 13259
a
b
acd
Motonori Watanabe, * Takaaki Miyazaki, Toshinori Matsushima,*
a
e
f
abcdg
Junko Matsuda, Ching-Ting Chein, Masahiko Shibahara, Chihaya Adachi,
Shih-Sheng Sun, Tahsin J. Chow* and Tatsumi Ishihara
e
eh
ag
A halide-substituted higher acene, 2-bromohexacene, and its precursor with a carbonyl bridge moiety were
synthesized. The precursor was synthesized through 7 steps in a total yield of 2.5%. The structure of
precursor and thermally converted 2-bromohexacene were characterized by solid state NMR, IR, and
absorption spectra, as well as by DFT computation analysis. It exhibited high stability in the solid state
over 3 months, therefore can be utilized in the fabrication of opto-electronic devices. The organic thin-
film transistors (OFETs) were fabricated by using 2-bromohexacene and parent hexacene through
2
ꢀ1
vaccum deposition method. The best film mobility of 2-bromohexacene was observed at 0.83 cm
V
Received 25th December 2017
Accepted 13th March 2018
ꢀ1
4
s
with an on/off ratio of 5.0 ꢁ 10 and a threshold of ꢀ52 V, while the best film mobility of hexacene
2
ꢀ1 ꢀ1
2
was observed at 0.076 cm
V
s
with an on/off ratio of 2.4 ꢁ 10 and a threshold of ꢀ21 V. AFM
DOI: 10.1039/c7ra13632c
measurement of 2-bromohexacene showed smooth film formation. The averaged mobility of 2-
rsc.li/rsc-advances
bromohexacene is 8 fold higher than the non-substituted hexacene.
suitable for use in organic electronic devices such as organic
1
. Introduction
6–8

eld-effect transistors. The extended p-conjugation of higher
Acenes are amongst the most representative hydrocarbons for acenes also induce an interesting phenomenon of singlet
9
analysing the physical properties of polycyclic hydrocarbon ssion that can be used on light harvesting. Bulky substituents
1
materials. Along with the increase in the number of aromatic can enhance the thermal and photo-stability of acenes by
10
benzene rings, acenes exhibit a reduction of both the HOMO– lowering the radical characteristic in the ground state. The
2
LUMO gap and the reorganisation energy. The chemistry of isolation of higher acenes, from hexacene to nonacene and
acenes higher than pentacene, particularly their open-shell derivatives, has been achieved by applying this strategy. The
3
characteristics and high charge transport properties, has modication of physical properties of acenes in the solid state
attracted considerable attentions. Hence, these compounds and requires a crystal engineering approach; however, their isola-
4
5
their analogs such thieonoacene based semiconductor are tion steps are either difficult or tedious in order to obtain
qualied structures due to their high thermal and light sensi-
11,12
tivity in solutions.
To overcome the difficulty, stable
a
precursors of acenes are prepared rst, which can then be
International Institute for Carbon Neutral Energy Research, Kyushu University, 744
Motooka, Nishi-ku,, Fukuoka 819-0395, Japan. E-mail: mwata@i2cner.kyushu-u.ac.jp converted to the corresponding acenes quantitatively in
b
Education Center for Global Leaders in Molecular Systems for Devices, Kyushu demand through either a thermal or a photo-driven process.13,14
University, 744 Motooka Nishi-ku, Fukuoka 819-0395, Japan
The synthesis of nonacene derivatives has been achieved by this
c
Center for Organic Photonics and Electronics Research (OPERA), Kyushu University,
approach utilising a diketone precursor through photo-induced
7
44 Motooka, Nishi-ku, Fukuoka, Japan
15
transformation. Recently, the dimer structure of heptacene
was converted to heptacene via a thermal retro-cyclization
reaction, therefore showing its feasibility for further
d
Japan Science and Technology Agency (JST), ERATO, Adachi Molecular Exciton
Engineering Project, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan
e
Institute of Chemistry, Academia Sinica, No. 128, Academia Road Sec 2, Nankang,
16
Taipei 11529, Taiwan
processes. The precursor method can be used to produce
f
Division of Natural Sciences, Faculty of Science and Technology, Oita University, higher acenes in large quantity that is required to become
Dannoharu 700, Oita, Japan
usable materials. Acenes have certain valuable potential appli-
g
Department of Applied Chemistry, Faculty of Engineering, Kyushu University,
Nishi-ku, Fukuoka 819-0395, Japan
17
cations, such as organic semi-conductors, singlet ssion
18,19
15
materials,
and organic biradical sources.
h
Department of Chemistry, Tunghai University, Taichung 40704, Taiwan
Previously, our group has developed the method of
producing higher acene molecules from either monoketone
†
Electronic supplementary information (ESI) available: Absorption spectra, DFT
results, XRD, and NMR data. See DOI: 10.1039/c7ra13632c
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Fig. 1 The synthesis of 2-bromohexacene from its monoketone
precursor.
2
0
21
precursors or from diethylketomalonate precursors. Both
types of precursor can be cleanly converted to hexacene either
thermally or photo-chemically. In addition, our group has
Scheme 1 Synthetic route of 2a.
22
23
generated halide-substituted tetracene and pentacene from
their corresponding monoketone precursors. They were used
successfully as the semiconductors in electronic devices. In
these devices, single crystal bromopentacene device exhibits
2
with PhI(OAc) to give desired 2a in 31% yield. The total yield
was 2.5% in 7 steps.
2
ꢀ1 ꢀ1
The absorption spectrum of 2a in 1,2,4-trichlorobenzene
a signicant superior hole mobility (>5 cm
V
s
) to the
). Such a high performance is
comparable with other related materials, such as triisopro-
1
1
2
ꢀ1 ꢀ1
a
exhibited characteristic A– L transitions of the anthracene
parent pentacene (1.4 cm V
s
chromophore at 353, 371, 391 and 408 nm, with vibrionic
progressions, which were red-shied from the peaks of parent
2
ꢀ1
ꢀ1 24–26
pylsilylethynylpentacene (>1 cm
dinaphtho[2,3-b:2 ,3 -f]thieno[3,2-b]thiophenes (>10 cm
V
s
),
alkylated
0
0
2
ꢀ1 2b at 350, 368 and 391 nm in the 1,2,4-trichlorobenzene solu-
V
ꢀ
1
27,28
ꢂ
tion (Fig. S1†). When the solution of 2a was heated at 230 C, the
s
),
and the crystal of tetracene analogous of rubrene (>18
2
ꢀ1 ꢀ1 29,30
solution changed from colorless to pale-green, which exhibited
characteristic acene vibration absorption bands at 573, 623 and
679 nm. However, this colour changed to yellow within a few
cm V
s
).
In previous reports, the charge mobility of
hexacene and derivatives were measured in the devices either
21
made with single crystals, or with crystalline thin lms
22,31
minutes owing to the dimerization or oxidation of 1a.
prepared through solution method.
However, the physical
properties and transistor characteristics of hexacene thin lm
that is prepared by vacuum deposition method have not yet 2.2. Physical properties
been reported.
The absorption maximum in the visible range was observed at
79 nm, whereas parent hexacene 1b exhibited the peak
maximum at 675 nm in 1,2,4-trichlorobenzene (Fig. S1†). The
peaks were red-shied from those of parent hexacene 1b by
It is believed that the bromine substituent can provide
a suitable size to improve crystal packing. Judged by the past
high-performance of brominated tetracene and pentacene, it is
therefore in demand to explore the possibility of brominated
analogue of hexacene. In this regard, the brominated analogue
of hexacene, i.e., 2-bromohexacene (1a), is synthesized and its
charge-transport property is examined. Precursor 2a is cleanly
converted to 1a by thermal decomposition, and 1a exhibits high
thermal stability in the dark over 90 days (Fig. 1). This is the rst
example of the charge-transport property of a stable vacuum-
deposited thin-lm of hexacene and halogenated hexacene for
electronic devices.
6
4
nm, indicating the reduction of HOMO–LUMO gap of 1a by
the bromo substituent. This phenomenon has similarly been
observed in the pairs of tetracene (473 nm in THF) and 2-bro-
22
motetracene (477 nm in THF), as well as pentacene (575 nm in
23
THF) and 2-bromopentacene (578 nm in THF). To compare
with the reported hexacene analogues, the peak maximum was
shown to be red-shied from that of pentaceno[2,3-b]thiophene
30
(
640 nm in o-DCB) due to the increase of aromaticity. In
comparison with other substituent group effect, however, it
showed a blue shi from those of tricyclohexylsilylethynyl-
3
1
octauorohexacene (725 nm),
tri-tert-butylsilylethyny-
2
. Results and discussion
32,33
hexacene (738 nm),
825–842 nm in hexane) due to the electron-donating effect
The synthesis of 2a is shown in Scheme 1. Dimethylfulvene of the trialkylsilylethynyl acetylene units.
and trialkylsilylethynyl-azahexacene
2.1. Synthesis
34
(
derivative 3 and benzoquinone 4 were treated with s-tetrazine,
In the thermal gravimetric analysis (TGA) prole of 2a, the
affording diketone 5 as an endo–exo mixture, then the double rst weight loss (8.5%, calcd 6.5%) was observed at approxi-
ꢂ
bond of 5 was reduced using zinc in acetic acid, affording mately 200 C to generate 1a. The thermal weight loss prole
ꢂ
adduct 6. The aldol reaction of 6 with dialdehyde 7 afforded did not change up to 360 C. Then it was followed by the second
diketone 8 as an endo compound in 17% yield (from 3, three weight loss caused by the vaporisation as well as the decom-
ꢂ
steps). Pure endo-8 was crystallized probably due to steric position of 1a at temperatures greater than 400 C (Fig. 2a).
ꢂ
inuence by the bromo-substituent. It was reduced by NaBH4, Decarbonylation at 200 C was conrmed by infrared (IR)
and further treatment with POCl /pyridine afforded aromatic spectroscopy, which revealed the disappearance of the charac-
3
ꢀ
1
compound 9 in 48% yield (from 8, two steps). The exo-double teristic C]O stretching band at 1786 cm aer heating at
ꢂ
bond was treated with OsO
4
to give a diol, followed by treatment 230 C (Fig. 2b). The high-resolution FAB-MS spectrum revealed
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13
state NMR that maintained invariant over
1 month.
Comparing it with heptacene under a similar situation, the
16
latter dimerised slightly aer 1 month. To verify the thermal
1
3
stability of 1a, C CP-MAS NMR spectra were recorded for
monitoring the variation of the carbon skeleton. Compound 2a
exhibited three main peaks at 193.2 (bridge position of C]O),
137–114 (aromatic carbon atoms) and 54.4 ppm (bridgehead
tertiary carbon atom). Aer the conversion to 1a, the spectrum
exhibited aromatic carbon peaks at 125.7 and 122.9 ppm only,
indicating a quantitative transformation. Aer maintaining 1a
for 90 days in the dark under air atmosphere, no changes were
observed in the CP-MAS spectrum, indicative of the high
thermal stability of 1a (Fig. 2d). This high thermal stability can
be compared with the reported property of hexacene (>1 month
2
0
in dark)
and tricyclohexylsilylethynylhexacene (several
33
month). The lm of 1a was grown by vacuum sublimation and
exhibited a structure similar to the lm in Fig. 2c (Fig. S3†). The
Fig.
2 (a) TGA profile of 2a under nitrogen. Heating rate was
ꢂ
ꢀ1
ꢂ
ionisation potential (E ) and electron affinity (E ) of the lm 1a
ip ea
1
0 C min . (b) IR spectra of 2a (black) and after heating at 230 C for
conversion to 1a (red) under ambient conditions. (c) Reflectance were ꢀ5.24 and ꢀ3.30 eV, respectively, while those of 1b were
absorption spectra of drop-casted 2a on a quartz plate (black) and ꢀ4.81 and ꢀ2.70 eV, respectively (Fig. S4†). The E of thin lm
ip
ꢂ
after heating at 230 C for conversion to 1a (red) under ambient
1
b (ꢀ4.81 eV) was consistent with that of the crystalline powder
13
conditions. (d) Solid-state C CP-MAS spectra of 2a (black), 1a (blue)
and 1a after 90 days in the dark (red). The asterisk denotes the spinning
side band.
14
reported previously (ꢀ4.96 eV). The energy gap of 1a (1.94 eV)
was less than that of 1b (2.11 eV). Theoretical computation
results (DFT, B3LYP/6-31G(d) level) revealed the HOMO and
LUMO of 1b to be ꢀ4.68 and ꢀ2.90 eV, respectively, while the
corresponding values for 1a were ꢀ4.81 eV and ꢀ3.04 eV,
respectively. The HOMO and LUMO were lowered by bromina-
tion compared with those of 1a, indicative of the electron-
withdrawing effect by the bromo substituent. The HOMO–
LUMO gap of 1a was 1.77 eV, whereas that of 1b was 1.78 eV,
supporting the experimental results (Table 1).
a molecular ion signal at m/z 406.0387 (M+, calcd 406.03571,
error ¼ +7.4 ppm), which corresponded to 1a. The purity was
also conrmed by correct elemental analysis. The absorption
spectra of 2a, fabricated by drop-casting a saturated chloro-
benzene solution on a quartz substrate, exhibited a similar
vibration structure to that of 2a in solution, with characteristic
peaks at 403, 380 and 364 nm, indicative of the fabrication of
ꢂ
2.3. Charge transport properties

lm of 2a. In contrast, aer heating at 230 C under nitrogen,
a broad band was observed at 500–900 nm, and the peaks at The properties of organic eld-effect transistors (OFETs) made
60–400 nm, which are characteristic of the anthracene moiety, with the lms of 1a–b were examined. The OFET devices were
3
disappeared.
fabricated by vacuum sublimation of 1a–b under a pressure of 8
ꢀ
6
These structures were different from that of 1a in solution. ꢁ 10
2
torr to deposit the thin lms on an HMDS/SiO /Si
Film 1a exhibited characteristic peaks at 833 and 776 and at 708 substrate, followed by the deposition of gold electrodes on the
and 661 nm. These peaks exhibited the appearance of Davydov top of the lms. The lm thickness of 1a–b was 60 nm. The
20,21,35
splitting effect.
The same pattern appeared in other related channel dimension of the source/drain electrodes was 45 ꢁ
acene structures in the solid state, including parent 1b, at 2000 mm. The output parameters were measured on a selected
20
840 nm, 765 nm, 708 nm and 654 nm. The rst bands at 833 lm across a source–drain channel, followed by the plot of drain
and 775 nm corresponded to the 0–0 band, and those at 708 and current (I ) versus drain source voltage (VDS) at various gate
D
6
54 nm corresponded to the 0–1 band (Fig. 2c). The high voltages (V ). The corresponding transfer characteristics were
G
thermal stability of 1b in the solid state was conrmed by solid- plotted for log(I ) versus V at a V of ꢀ100 V and I versus V
D
G
DS
D
DS
Table 1 Physical properties of 1a–b
Ionization
potential
(Eip, eV)
Electron
affinity b
Energy gap
(eV)
Optical gap
(eV)
HOMO
LUMO
Energy gap
(eV)
Reorganization
energy (l , meV)
a
c
d
e
e
f
+
e
Sample
(Eea, eV)
H
(E , eV)
(E
L
, eV)
1
1
a
b
ꢀ5.24
ꢀ4.81
ꢀ3.30
ꢀ2.70
1.94
2.11
1.47
1.41
ꢀ4.81
ꢀ4.68
ꢀ3.04
ꢀ2.90
1.77
1.78
85
79
a
b
c
d
Estimated by photoelectron yield spectroscopy. Estimated by low-energy inverse photoemission spectroscopy.
E
ip ꢀ Eea
.
Estimated by the
e
f
edge of absorption spectra of thin-lm. B3LYP/6-31G(d).
E
H
L
ꢀ E .
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DFT computations revealed that the reorganisation energy
between the radical cation and ground state of 1a was 85 meV
(B3LYP/6-31Gd level). This value was quite close to that of 1b (79
meV), suggesting a similar energy loss during structure reor-
ganization in the hexacenes 1a and 1b.
2.4. TEM, XRD, and AFM measurements of hexacene lms
To investigate the morphology of thin-lm, we performed
a surface analysis on the lms 1a and 1b. Fig. 4 showed cross-
section of transmission electron microscope (TEM) image of
ca. 60 nm deposited lms 1a and 1b on HMDS/SiO /Si surface.
2
The ion milling method allows us to examine a cross-section of
substrate at the interface of 1a–b and HMDS/SiO /Si. The TEM
2
image show a good continuous growth of 1a and 1b lms on
HMDS/SiO /Si surface, suggesting both lms were deposited
2
uniformly on the substrate.
Fig. 3 Vacuum-deposited thin-film OFET made using 1a. (a) Output
The parent structure of hexacene 1b exhibited a herringbone
arrangement, where the face-to-edge stacking arrangement
characteristics, where D and S are the drain and source, respectively.
(
b) Transfer characteristics recorded at VDS ¼ ꢀ100 V, where G is the
gate, and the vacuum-deposited thin-film OFET was made using 1b. between adjacent molecules avoided the progress of dimeriza-
(
c) Output characteristics, where D and S are the drain and source,
14
tion and led to a high thermal stability in the solid state. The
respectively. (d) Transfer characteristics recorded at VDS ¼ ꢀ100 V,
vacuum-deposited lm of 1b exhibited out-of-plane X-ray
diffraction (XRD) peaks along (00l) direction and in-plane
XRD along (hk0) direction (Fig. S4†).
where G is the gate.
These patterns indicated that the molecules in the deposited
lm are oriented vertical to the surface along their long axis. In
in the saturation mode. The eld-effect hole mobility of 1a was

2
measured, and the mobility values ranged from 0.21 to 0.83 cm
addition, this lm orientation was consistent with the reported
out-of-plane pattern of heat-converted lm 1b from corre-
ꢀ
1
ꢀ1
V
s
with threshold voltages of ꢀ50 to ꢀ69.3 V. The averaged
2
ꢀ1 ꢀ1
performance of six independent devices was 0.52 cm V
s
21
sponding precursor compound. The (001) peak was observed
and a threshold of ꢀ56.3 V. The best mobility of bromo-
ꢂ
˚
at 4.84 corresponding to an interplanar distance of 18.3 A. This
2
ꢀ1 ꢀ1
hexacene 1a was observed at 0.83 cm V
s
with an on/off
value can be compared with the d-spacing in the single crystal of
4
ratio of 5.0 ꢁ 10 and a threshold of ꢀ52 V (Fig. 3a–b). To
compare the hole mobility, parent 1b was also tested. The
mobility of 1b in the saturation mode ranged from 0.072 to
˚
1
b, which has been estimated to be 16.4 A. It indicates that the
molecules in the crystalline lm 1b are tilted on the surface of
HMDS/SiO /Si substrate. The out-of-plane and in-plane XRD
2
ꢀ1 ꢀ1
2
0
.076 cm V
s
with a threshold voltage ranging from ꢀ19 to
peaks of vacuum-deposited lm 1a exhibited (00l) and (hk0)
patterns, indicating that molecules in lm 1a is oriented along
the long axis normal to the surface (Fig. S4†). Although lm 1a
ꢀ
22 V. The averaged performance of six independent devices
2
ꢀ1 ꢀ1
was 0.074 cm V
s
and a threshold of ꢀ20.7 V. The best lm
2
ꢀ1 ꢀ1
mobility of 1b was observed at 0.076 cm V
s
with an on/off
exhibited weaker XRD diffraction peaks compared with that of
2
ratio of 2.4 ꢁ 10 and a threshold of ꢀ21 V (Fig. 3c–d). Previ-
ously, the hole mobility of hexacene 1b has been reported in the
single-crystal phase and in the spin-coated thin-lm phase by
the precursor method. The best hole-transfer mobility by spin-
ꢂ
1
b, the 2q angle of 1a observed at a smaller angle of 4.43 on the
HMDS/SiO
2
/Si substrate. The d-spacing of bromohexacene
˚
molecules is estimated to be 20.0 A. The larger d-spacing
revealed that the molecules interact through the a–b axis.
To further study of nding the difference of mobility
between 1a and 1b, the atomic force microscope (AFM)
analysis was investigated. The lms 1b revealed a high
surface roughness of 12.01 nm (Fig. 5). In contrast, lm 1a
revealed a lower roughness of 6.48 nm. This smoother
2
ꢀ1 ꢀ1
coated 1b was 0.035 cm V
ment on SiO /Si substrate. Although our fabrication conditions
were not fully optimised, the mobility of the vacuum-deposited
lm was greater than that of the spin-coated one. It indicates
s , with a similar surface treat-
2

that better crystalline lms of 1b were formed by thermal
deposition. The mobility of lm 1a exhibited a larger range of
randomness compared to 1b. However, a higher hole-transfer
efficiency of 1a ranging 7- to 10-folds compared with that of
1b was observed in all tested devices. This result indicated that
the packing moiety and/or lm morphology possessing a better
charge-transfer pathway may account for the mobility. Previ-
ously, a single-crystal bromopentacene was found to exhibit a 4-
fold faster hole-transfer speed than that of non-substituted
pentacene, while their reorganization energies were estimated
23
to be 102 meV and 95 meV (B3LYP/6-31Gd level), respectively.
Fig. 4 TEM image of thin films 1a (a) and 1b (b). Top: structure of films
2
on HMDS/SiO /Si.
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saturation mobility (msat) was extracted from the slope of the
square root of the drain current plot vs. V from eqn (1).
G
W
2
I
D;sat ¼ 2
C
i
msatðV
G
ꢀ V
T
Þ
(1)
L
where ID,sat is the drain-to-source saturated current; W/L is the
channel width to length ratio; C is the capacitance of the
insulator per unit area, and the V and V are gate voltage and
i
G
T
threshold voltage, respectively. A heavily doped silicon (Si) wafer
was used for a back gate electrode, which was covered with a 300
Fig. 5 AFM images of thin films of 1a and 1b.
ꢀ
2
nm-thick thermally grown SiO (C ¼ 10.2 nF cm ) as the gate
2
i
surface in 1a achieved a small energy loss during the trans- insulator. Channel length (L) and width (W) were 2000 mm and
port of holes between the source and drain.
45 mm.
3. Materials and methods
3.4. TEM measurement
3
.1. General information
The organic lms were vacuum sublimation of 1a–b under
ꢀ6
1
13
a pressure of 8 ꢁ 10 torr to deposit the thin lms for 350 nm
The H and C NMR spectra were recorded in solutions on
a BrukerAV600 (600 MHz) spectrometer. The H and the
NMR chemical shis were reported as d values (ppm) relative to
external Me Si. The coupling constants (J) were given in hertz.
High resolution FAB mass spectra were recorded on a JMS-700
MStation spectrometer. FAB MS spectra were measured with
1
13
on a HMDS/SiO /Si substrate. Cross-sectional TEM samples for
2
C
all of the lms were prepared by mechanical thinning and ion
milling. The transmission electron microscope used in this
study was a JEOL JEM-2100F, which was operated at an accel-
erating voltage of 200 kV.
4
3-nitrobenzyl alcohol (NBA) as the matrix. Analytical thin layer
chromatography (TLC) was performed on silica gel 60 F254 3.5. Synthesis of materials
Merck. Column chromatography was performed on KANT-
3.5.1. Synthesis of (6 aS,14aR,15S)-10-bromo-17-(propan-2-
OSi60N (neutral). Absorption and reectance spectra were
recorded on a SHIMADZU UV-3600. IR spectra were performed
by SHIMADZU IRPrestige-21 spectrophotometer. AFM
measurements were tested by SHIMADZU SPM-9700. The
elemental analyses were recorded on a Yanaco CHN recorder
MT-6. THF was distilled from sodium benzophenon ketyl.
Toluene was distilled from CaH . Other solvents and reagents
2
were of reagent quality, purchased commercially, and used
without further purication.
ylidene)-6,6a,14a,15-tetrahydro-6,15-methanohexacene-7,14-
dione 8. A solution of 1R,4S-11(propan-2-ylidene)-1,4-dihydro-
1
4
1
,4-methanoanthracene 3 (232 mg, 1.00 mmol), benzoquinone
(108 mg, 1.00 mmol), and 3,6-bis(2-pyridyl)tetrazine (248 mg,
ꢂ
.05 mmol) in toluene (50 mL) was heated at 95 C for 24 h
under a nitrogen atmosphere. Aer the reaction, the mixture
was quenched with 30% H SO , extracted with CH Cl , and the
organic solution was dried over MgSO and concentrated in
vacuo. The crude product was puried by a silica gel chro-
matograph eluted with CH Cl to afford a mixture of endo and
2
4
2
2
4
2
2
1
3
3.2.
C CP/MAS NMR
exo geometrical isomers of 5 (163 mg) as yellow solids. The
mixture was used for next step without further purication.
The mixture of 5 (163 mg, 0.519 mmol) and zinc (600 mg)
and glacial acetic acid (50 mL) was sonicated for 30 min at room
temperature. Aer reaction, the suspension was ltered and the
solution was evaporated to give the crude product. Silica gel
chromatography of the crude product with CH Cl and treat-
2 2
ment with iced MeOH afforded dione 6 (114 mg). The mixture
was used for next step without further purication.
A mixture of diketone 6 (114 mg, 0.361 mmol) and 4-bro-
mophthalaldehyde 7 (76.1 mg, 0.361 mmol) was dissolved in
EtOH (50 mL). The EtOH solution was bubbled by nitrogen gas
for 20 min to remove oxygen. To the mixture was added 10 wt%
KOH/EtOH solution (2–3 drops) in the nitrogen atmosphere and
stirred 72 h at room temperature under nitrogen gas. The
mixture gradually became dark, and pale-yellow powder
precipitated. Aer reaction, the precipitate was ltered and
washed with EtOH and hexane to afford endo-8 (86.2 mg, 17% in
1
3
The C CP/MAS NMR spectra were acquired with a Bruker
Avance 400 MHz NMR spectrometer, equipped with a 4 mm
1
13
double resonance probe operating at the H and the C Larmor
frequencies of 400.13 and 100.63 MHz, respectively. For the
polarization transfer, the contact-time was set to 1.75 ms.
1
During data acquisition, H decoupling by spinal-64 was
applied. Powdered samples were packed in 4 mm zirconium
oxide MAS rotors with Kel-F cap. A sample spinning frequency
of 12 kHz was used and regulated by a spinning controller
within ꢃ1 Hz. All CP-MAS experiments were carried out at
1
3
ambient temperature. The C NMR chemical shis are refer-
enced to the methyl signal (¼17.36 ppm) of hexame-
thylbenzene, which was used as an external standard. All
measurements were performed in air, and the sample tube was
kept in the dark between measurements.
3.3. FET measurement
three steps). Pale yellow powder (EtOH). dH (CDCl , 600 MHz)
3
The electrical measurements were carried out in vacuum using 1.23 (d, J ¼ 4.8, 6H), 3.17 (s, 2H), 4.47 (s, 2H), 7.45 (dd, J ¼ 6.0,
a semiconductor parameter analyzer (B1500A, Keysight). The 3.0 Hz, 2H), 7.76 (J ¼ 9.0, 7.7 Hz, 1H), 7.78 (s, 2H), 7.82 (dd, J ¼
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6
8
1
1
.0, 2.4 Hz, 2H), 7.95 (d, J ¼ 8.4 Hz, 2H), 8.24 (s, 1H), 8.53 (s, 1H),
4
. Conclusions
.61 (s, 1H). d (CDCl , 125 MHz) 19.7, 51.3, 52.9, 117.9, 119.0,
C
3
24.1, 125.6, 127.5, 127.9, 128.6, 131.4, 131.9, 132.2, 132.8, A novel hexacene precursor was successfully synthesised, which
ꢂ
32.9, 133.5, 136.0, 142.3, 143.7, 197.0, 197.8.
.5.2. Synthesis of
can be quantitatively converted at around 200 C to the corre-
3
(6S,15R)-10-bromo-17-(propan-2- sponding 2-bromohexacene. It exhibited high thermal stability
ylidene)-6,15-dihydro-6,15-methanohexacene 9. To a solution over 3 months in the dark. The bromine atom affected the
of dione 8 (220 mg, 0.446 mmol) in MeOH (50 mL) and THF (50 hexacene crystal packing and decreased the HOMO–LUMO
mL) in an ice bath was added NaBH
h, the reaction mixture was quenched with water. The aqueous spin-coating and vacuum sublimation methods, and both lms
solution containing precipitates was extracted with CH Cl . The exhibited exciton coupling, indicative the presence of herring-
4
(74 mg, 1.65 mmol). Aer energy gap. The thin-lm of 1a was fabricated by using both
2
2
2
organic layer was washed with water, and dried over anhydrous bone arrangement in the polycrystalline lm. The lm of 1a
MgSO . Removal solvent gave the diol 9 (212 mg) as yellow exhibited a more efficient hole-transport property compared
4
solids. This crude compound was subjected to the next step with that of parent 1b. Hence, lm 1a exhibits a higher hole
2
ꢀ1 ꢀ1
2
ꢀ1 ꢀ1
without further purication. To a mixture of diol 9 and dried mobility of 0.83 cm V
s
than that of 1b (0.074 cm V
s ).
ꢂ
pyridine (10 mL) was added dropwise POCl
3
(0.9 mL) at 0 C. Although these hole mobility were lower than that of single
2
ꢀ1 ꢀ1 26
The resulting mixture was stirred at room temperature for 72 h, crystal hexacene (4.28 cm V
then at 80 C for another 30 min. The mixture was poured into reported value of solution-processed single crystal tricyclohex-
ice water and was extracted with CH Cl . The organic layer was ylsilylethynyloctauorohexacene (0.1 cm V
s ), it was comparable with the
ꢂ
2
ꢀ1 ꢀ1 31
2
2
s ). To the best
washed successfully with 3 N HCl and brine, then was dried over of our knowledge, this is the rst study on the charge-transport
MgSO . The crude product was puried by a silica gel chro- property of a stable vacuum-deposited thin-lm of hexacene for
4
matograph eluted with hexane/CH
aromatized compound 9 (98.8 mg, 48%) as pale yellow powder. hexacene are prepared, and their properties related to opto-
Physical data of 9: pale yellow powder (EtOH). d (CDCl , 600 electronic devices are being examined. The results will be re-
MHz) 1.72 (s, 6H), 5.06 (s, 2H), 7.35 (dd, J ¼ 6.0, 3.0, 2H), 7.43 ported in due course.
dd, J ¼ 9.0, 7.2 Hz, 1H), 7.70 (J ¼ 6.0, 3.0 Hz, 2H), 7.72 (s, 2H),
.77–7.79 (m, 3H), 8.08 (s, 1H), 8.12 (s, 1H), 8.19 (s, 1H). d
2 2
Cl (4 : 1) to give the electronic device. Currently, other derivatives of halogenated
H
3
(
7
C
(
1
1
CDCl , 125 MHz) 19.8, 51.4, 113.2, 118.3, 118.4, 118.9, 119.0, Author's contributions
3
19.02, 124.9, 125.5, 126.1, 127.7, 128.3, 129.6, 131.3, 131.7,
32.2, 132.4, 145.2, 145.3, 145.6, 153.8.
MW designed and performed the experiments and theoretical
calculations; TM, CTC, MS and SSS were synthesized, measured
and analysed the materials and physical properties; TM and CA
designed the devices and analysed the data; JM measured and
analysed the TEM; MW and TI were measured and analysed the
NMR data; MW, TM and TJC were co-wrote the manuscript; all
authors gave nal approval for publication.
3
.5.3. Synthesis of (6S,15R)-10-bromo-6,15-dihydro-6,15-
methanohexacen-17-one 2a. A mixture of olen 9 (200 mg,
.433 mmol) and N-methylmorpholine N-oxide (NMO) (50% in
O, 1.5 mL) in a mixed solvent of acetone (50 mL) and H O (1.5
0
H
2
2
mL) was stirred at room temperature until 9 was dissolved
completely. To the solution was added a few drops of OsO (4%
4
H O soln). The reaction was monitored by TLC until comple-
2
tion, then the mixture was quenched with 15% aqueous
Conflicts of interest
Na
over Na
a silica gel chromatograph eluted with CH
2
S
2
O
4
. The aqueous solution was extracted with EtOAc, dried
SO , and evaporated. The product was puried by
Cl to give diol 10 (79
2
4
There are no conicts to declare.
2
2
mg), which was directly used in the next step. The diol 10 (79
mg) and PhI(OAc) (120 mg) in benzene (100 mL) was stirred at
2
Acknowledgements
ꢂ
60 C for 12 h. Aer reaction, the mixture was cooled in an ice
bath, while white precipitates formed. The solids were collected This work was supported by a Grant-in-Aid for Science Research
by suction ltration to give compound 2a (59 mg, 31% in two (JP17H04888, JP17K19123) from the Ministry of Education,
ꢂ
steps) as a white powder. Physical data of 2a: m.p. 181 C. (TGA, Culture, Sports, Science and Technology (MEXT), Japan, and
ꢀ
1
13
decomp.); IR (KBr): n 1786 cm (s, C]O); C CP/MAS NMR was performed under the Cooperative Research Program of
12 000 rpm, 100 MHz): 54.0, 118.5, 120.6, 122.5, 123.9, 126.1, “Network Joint Research Centre for Materials and Devices
30.5, 133.2, 135.2, 192.7. (IMCE, Kyushu University)”. The computation was mainly
.5.4. Synthesis of 2-bromohexacene 1a. Proper amount of carried out using the computer facilities at Research Institute
(
1
3
precursor 2a was loaded in a glass container, and the powder for Information Technology, Kyushu University. MW and TM
ꢂ
was heated at 210 C for 5 min under a nitrogen atmosphere. acknowledge support from I2CNER, funded by the World
The color of 2a changed from white to green to give 2-bromo- Premier International Research Centre Initiative (WPI),
ꢂ
hexacene 1a in a quantitative yield. M.p. 340 C. (TGA, sublime); Ministry of Education, Culture, Sports, Science and Technology
1
3
C CP/MAS NMR (12 000 rpm, 100 MHz): 123.1, 126.0; HRMS: of Japan (MEXT), Japan. TJC and SSS are also grateful for the
m/z 406.0387 (M+, calcd 406.03571); EA: found, %: C, 76.2; H, supports from the Ministry of Science and Technology of Tai-
.7. For C26 15Br. Calculated, %: C, 76.1; H, 3.7. wan and Academia Sinica.
3
H
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