Thermally, Operationally, and Environmentally Stable Organic Thin-Film Transistors Based on Bis[1]benzothieno[2,3- d:2′,3′- d ′]naphtho[2,3- b:6,7- b ′]dithiophene Derivatives: Effective Synthesis, Electronic Structures, and Structure-Property Relationshi

Article abstract of DOI:10.1021/acs.chemmater.5b01608

By developing an efficient synthetic route to the bis[1]benzothieno[2,3-d;2′,3′-d′]naphtho[2,3-b;6,7-b′]dithiophene (BBTNDT) framework, we have successfully synthesized new BBTNDT derivatives with phenyl (DPh-BBTNDT) or n-hexyl groups (C6-BBTNDT) at the 2

Full text of DOI:10.1021/acs.chemmater.5b01608

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Article
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Thermally, Operationally, and Environmentally Stable Organic Thin-
Film Transistors Based on
Bis[1]benzothieno[2,3‑d:2′,3′‑d′]naphtho[2,3‑b:6,7‑b′]dithiophene
Derivatives: Effective Synthesis, Electronic Structures, and Structure−
Property Relationship
Masahiro Abe,†,‡ Takamichi Mori,† Itaru Osaka,† Kunihisa Sugimoto,§ and Kazuo Takimiya*,†
†Emergent Molecular Function Research Group, RIKEN Center for Emergent Matter Science (CEMS), 2-1 Hirosawa, Wako, Saitama
351-0198, Japan
‡Center for Innovative Research (CIR), Research and Development Group, Nippon Kayaku Co., Ltd., 31-12 Shimo 3-Chome,
Kita-ku, Tokyo 115-8588, Japan
§Japan Synchrotron Radiation Research Institute (JASRI), 1-1-1, Kouto, Sayo-cho, Sayo-gun, Hyogo 679-5198, Japan
S
* Supporting Information
ABSTRACT: By developing an efficient synthetic route to the bis[1]benzothieno[2,3-
d;2′,3′-d′]naphtho[2,3-b;6,7-b′]dithiophene (BBTNDT) framework, we have success-
fully synthesized new BBTNDT derivatives with phenyl (DPh-BBTNDT) or n-hexyl
groups (C6-BBTNDT) at the 2 and 10 positions. Characterization of their vapor-
deposited thin films revealed that, depending on the substituents introduced, their
HOMO energy levels were slightly altered, and DPh-BBTNDT with the HOMO energy
level of ca. 5.3 eV was supposed to be a stable organic semiconductor under ambient
conditions. In fact, the DPh-BBTNDT-based OTFTs showed not only high mobility of
up to 7.0 cm2 V−1 s−1 under ambient conditions but also excellent operational and
thermal stabilities up to 300 °C, whereas the parent and the hexyl derivative were less
stable against the thermal treatments at high temperatures. The high mobility observed
for the DPh-BBTNDT-based OTFTs can be correlated to the interactive packing
structure in the bulk single crystal and thin film state of DPh-BBTNDT, which
corroborates the existence of the well-balanced two-dimensional electronic structure in
the solid state. With these excellent device characteristics, it can be concluded that DPh-BBTNDT is a promising and practical
vapor-processable organic semiconductor, which can afford thermally, operationally, and environmentally stable OTFTs as well
as high mobility.
INTRODUCTION
■
Organic thin-film transistors (OTFTs) have attracted great
interest for their potential use in electronic applications,
including active-matrix displays, electronic paper, and chemical
sensors.1−3 In the last decades, the performances of OTFTs
have been significantly improved,4−7 and in particular, p-
channel OTFTs have now realized many important achieve-
ments: high mobility (>3.0 cm2 V−1 s−1),8−12 solution
processability,13,14 air-stability, flexibility, and low-voltage
operation.15 For these achievements, the development of new
Figure 1. Molecular structures of BTBTs and related compounds.
superior organic semiconductors have contributed significantly.
Among recently developed organic semiconductors, [1]-
benzothieno[3,2-b][1]benzothiophene (BTBT) derivatives6−18
OTFTs, extension of π-conjugation system of organic semi-
conductors has been one of the most effective ways, as well
and its π-extended homologues, such as dinaphtho[2,3-b;2′,3′-
exemplified by the acene-based organic semiconductors.27,28
f]thieno[3,2-b]thiophene (DNTT),19−22 have been focused as
promising organic semiconductors capable of affording OTFTs
with high mobility and environmental stability (Figure 1),23
which can be further utilized in various sophisticated device
applications.24−26 In order to improve the performances of
Received: April 30, 2015
Revised: June 7, 2015
© XXXX American Chemical Society
A
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This is also the case for BTBT and related materials, and thus,
largely π-extended systems based on the BTBT framework has
recently been examined;29−31 for example, Yu and co-workers
have reported a BTBT covalently linked dimer, 2,2′-bi[1]-
benzothieno[3,2-b][1]benzothiophene (Bi-BTBT, Figure 1),
which affords vapor processed TFTs showing mobility as high
as 2.1 cm2 V−1 s−1 with excellent thermal stability up to 250 °C.
Much higher mobility up to 5.6 cm2 V−1 s−1 has been reported
for a BTBT-fused dimer, bis[1]benzothieno[2,3-d;2′,3′-d′]-
naphtho[2,3-b;6,7-b′]dithiophene (BBTNDT).32 It should be
noted that the mobility reported for the BBTNDT-based
OTFTs are among the highest for OTFTs based on
thienoacenes without substituents. On the other hand,
introduction of substituents, such as long alkyl groups and
aromatic rings has been another promising molecular
modification to develop high-performance organic semi-
conductors. This has been well exemplified by the superior
OTFTs with enhanced mobilities based on BTBT and DNTT
derivatives with alkyl or phenyl groups along the molecular long
axis direction. In particular, introduction of long alkyl groups on
BTBT or DNTT tends to help intermolecular interaction in the
condensed phase via intermolecular van deer Waals interaction
of the alkyl groups, which is believed to enhance mobilities of
their devices.17,18,33 On the other hand, the introduced phenyl
groups on DNTT can enhance thermal stability of not just the
compounds themselves but also the thin films used in the
OTFT devices, affording thermally stable OTFTs.34,35 With
these preceding results in mind, we have designed new
BBTNDT derivatives with alkyl or phenyl substituents at the
2,10-positions, which correspond to the molecular long axis
direction (Figure 1). We report here a new efficient synthesis of
BBTNDT derivatives, their electronic structures, single crystal
and thin film structures, and OTFT performances. OTFT
devices based on new BBTNDT derivatives demonstrated high
mobility of up to 7.0 cm2 V−1 s−1 with excellent thermal,
environmental, and operational stability.
disubstituted BBTNDT derivatives, however, this strategy can
potentially afford isomers because the precursor has two
structurally distinct reaction sites on each phenyl ring. To avoid
such undesired reaction, we have employed a thiophene
annulation reaction via the acid-mediated aryl-sulfide bond
formation reaction36−38 from naphthalene derivatives with two
benzo[b]thiophene- and methylsulfinyl- substituents (2)
(Route 2 in Scheme 1), which are supposed to selectively
afford 2,10-disubstituted BBTNDT derivatives.
According to the second route depicted in Scheme 1, we first
carried out the synthesis of parent BBTNDT (Scheme 2). The
Scheme 2. Syntheses of BBTNDT Derivatives
Suzuki−Miyaura coupling reaction between 2-(4,4,5,5-tetra-
methyl-1,3,2-dioxaborolan-2-yl)benzo[b]thiophene (4a)39 and
2,6-bis(methylthio)-3,7-bis(trifluoromethanesulfonyloxy)-
naphthalene (5)32 afforded 2,6-bis(benzo[b]thiophen-2-yl)-3,7-
bis(methylthio)naphthalene (6a), which was quantitatively
converted into the precursor, 2,6-bis(benzo[b]thiophen-2-yl)-
3,7-bis(methylsulfinyl)naphthalene (2a). The final cyclization
reaction and following purification by repeated vacuum
sublimation afford parent BBTNDT in a moderate yield
(33%). The total yield of BBTNDT from 5 by the present
approach was ca. 23% in three steps, which is better than the
previous approach (Route 1 with R = H in Scheme 1), where
the total yield of BBTNDT from 5 was 20−27% (four steps)
before purification.
For the syntheses of the BBTNDT derivatives, we employed
readily available 6-bromobenzo[b]thiophene,40 which can be
effectively converted into 6-phenyl- (3b) and 6-hexylbenzo[b]-
thiophene (3c). Borylation at the 2-position of 3b and 3c gave
4b and 4c, respectively, which were utilized in the Suzuki−
Miyaura coupling reaction with 5 to give corresponding 2,6-
bis(benzo[b]thiophen-2-yl)-3,7-bis(methylthio)naphthalenes
(6b and 6c) in good yields. Oxidation of the methylthio groups
on 6 proceeded smoothly to give the precursors (2b and 2c),
and the thiophene annulation reaction followed by sublimation
afforded the corresponding BBTNDT derivatives in moderate
yields.
RESULTS AND DISCUSSIONS
■
Synthetic Strategy and Actual Synthesis. The synthesis
of parent BBTNDT previously reported includes a thiophene
ring formation reaction via the Pd-catalyzed intramolecular
aryl−aryl coupling32 of the phenylthio groups at 3- and 8-
positions on naphtho[2,3-b:6,7-b′]dithiophene derivatives (1, R
= H) (Route 1 in Scheme 1). For the syntheses of 2,10-
Scheme 1. Synthetic Strategy of 2,10-Disubstituted
a
BBTNDT Derivatives
Electronic Properties. In order to evaluate the HOMO
energy levels of the BBTNDT derivatives, ionization potentials
(IPs) of their vacuum-deposited thin films on ITO substrates
were measured with photoemission yield spectroscopy in air
(Figure 2a). The IPs of BBTNDT, DPh-BBTNDT, and C6-
BBTNDT are 5.1, 5.3, and 5.0 eV, respectively, which are just
above or around of 5.0 eV, a borderline of air stability for p-
channel organic semiconductors.41−45 It should be noted that
the introduction of hexyl groups affords the slightly smaller IP
than the parent, whereas the phenyl derivative has the larger IP,
indicating that the latter is likely a stable organic semiconductor
(vide infra).
a
(a) Previously reported route via the intramolecular aryl−aryl
coupling reaction. Note that two possible reaction sites on the phenyl
moieties including undesirable positions marked with asterisk affording
isomeric byproducts. (b) Newly proposed route via the acid-mediated
aryl-sulfide bond formation reaction.
B
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Table 2. Transistor Characteristics of DPh-BBTNDT and
a
C6-BBTNDT Fabricated on ODTS-Treated Si/SiO2
Substrate
b
c
compound
Tsub/°C
μFET/cm2 V−1s−1
Ion/Ioff
Vth/V
DPh-BBTNDT
rt
0.98 (0.65)
3.9 (3.6)
105
106
106
107
105
106
106
106
107
−11.2
−2.5
−3.2
−5.2
2.8
100
150
200
rt
4.5 (4.1)
7.0 (6.3)
C6-BBTNDT
0.26 (0.20)
0.41 (0.25)
1.0 (0.54)
1.8 (1.2)
100
150
200
100
1.1
0.1
Figure 2. Photoemission yield spectroscopy in air (a) and UV−vis
absorption spectra (b) of BBTNDT, DPh-BBTNDT, and C6-
BBTNDT measured on evaporated thin films.
3.3
d
BBTNDT
5.6 (4.7)
−6.0
a
See Supporting Information Table S1 for device characteristics on the
b
OTS and HMDS treated substrate. Extracted from the saturated
UV−vis absorption spectra of the thin films on quartz
substrates are shown in Figure 2b. For the DPh-BBTNDT thin
film, the absorption peak at around 513 nm corresponds to the
π−π* transition shifted bathochromicaly by ca. 20 nm than that
of the BBTNDT thin film, which can be ascribed to additional
π-conjugation. On the other hand, C6-BBTNDT showed no
clear shift from the parent one. This is contrasted to the
tendency observed in the substituted DNTT derivatives
(Supporting Information Figure S1), where introduction of
the phenyl or alkyl groups on the molecular long axis direction
causes clear bathochromic shifts.33,34 Although the reasons for
the different effects caused by the alkyl substitution in the two
systems are not clear, the more π-extended BBTNDT core than
DNTT could be less sensitive in the electronic structure by
derivatization. These optical properties of three compounds are
summarized in Table 1 together with their HOMO energy
levels estimated from the IP values.
regime (Vd = Vg = −60 V). The values in parentheses are averaged
ones from more than ten devices Average value. See ref 32.
c
d
Table 1. Electronic Properties of BBTNDT Derivatives
a
b
b
c
compound
EHOMO/eV
λmax/nm
λedge/nm
Eg/eV
BBTNDT
−5.1
−5.3
−5.0
494
513
492
515
535
505
2.4
2.3
2.5
DPh-BBTNDT
C6-BBTNDT
a
Determined by photoemission yield spectroscopy in air. EHOMO was
b
defined as −(IP) in eV. From absorption spectra measured on
evaporated thin films. Calculated from λedge
c
.
OTFT Devices with Vapor-Deposited Thin Film. OTFT
devices of the new BBTNDT derivatives were fabricated on
alkyltrichlorosilane-treated Si/SiO2 substrates with a bottom
gate−top contact device configuration using their vapor-
deposited films and were evaluated under ambient conditions.
Table 2 summarizes the OTFT characteristics of the devices
fabricated on the octadecyltrichlorosilane (ODTS)-treated Si/
SiO2 substrate at different substrate temperatures during thin-
film deposition (Tsub). As depicted in Figure 3, the devices
fabricated on the ODTS-treated substrates (Tsub = 200 °C)
showed text-book like transfer and output characteristics with
relatively high hole mobilities. The DPh-BBTNDT-based
devices gave higher mobilities than those of C6-BBTNDT- or
BBTNDT-based devices. It is interesting to note that both
derivatives tend to afford better performances in the devices
fabricated at the higher substrate temperatures (Tsubs), and the
maximum mobilities were recorded for Tsubs = 200 °C for both
compounds. In particular, the best mobility of up to 7.0 cm2
V−1 s−1 was observed for the DPh-BBTNDT-based devices,
which is higher than that of the mobility of the parent
Figure 3. Transfer (a), output (b), and gate-voltage (Vg) dependence
of mobility (c) of DPh-BBTNDT (left) and C6-BBTNDT (right).
BBTNDT-based devices (5.6 cm2 V−1 s−1)32 and comparable
with the highest mobility so far reported for small-molecule-
based OFETs with polycrystalline thin films as the active
semiconducting channel.44−46 We also investigated gate-voltage
(Vg) dependence of the mobility (Figure 3c). In both the
derivatives, the mobility sharply rose up at the subthreshold
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regime and kept constant up to high gate voltage, indicating
that the mobility is independent of Vg. This suggests that there
is no overestimation of the mobilities.
devices compared with the parent BBTNDT devices thus can
be rationalized by better isotropic electronic structure.49
Evaluation of Vapor Deposited Thin Films. The above
discussion on the electronic structure based on the single
crystal analysis is consistent with the empirical high mobility of
the DPh-BBTNDT-based devices. On the other hand, the
device characteristics were evaluated with the vapor-deposited
thin films, and thus, we further characterized the thin films by
means of atomic force microscopy (AFM) and thin film X-ray
diffraction (XRD). Figure 5 shows the AFM images of the thin
Single Crystal X-ray Analysis of DPh-BBTNDT. It is
important to correlate the transport properties of the devices
and the packing structures of semiconducting molecules in the
condensed phase, which can rationalize the device perform-
ances from the structural point of view. Single crystals of DPh-
BBTNDT with suitable quality for single crystal X-ray analysis
were obtained by physical vapor transport,47 though all
attempts to prepare single crystals of C6-BBTNDT were failed.
Depicted in Figure 4 are the molecular and packing structures
Figure 5. AFM images (2 × 2 μm) of evaporated thin films on the Si/
SiO2 substrate affording the best device characteristics: (a) DPh-
BBTNDT, (b) C6-BBTNDT, and (c) BBTNDT on the ODTS-
treated substrate (Tsub = 200 °C for DPh- and C6-BBTNDT and Tsub
= 100 °C for BBTNDT).
films of DPh-BBTNDT and C6-BBTNDT evaporated at Tsub
=
200 °C on the ODTS-treated Si/SiO2 substrate, together with
that of parent BBTNDT (Tsub = 100 °C). All three compounds
gave similar surface morphologies with crystalline grains of
similar sizes. The grain sizes of these compounds depend on
Tsub as observed for many related organic semiconducting
materials (Supporting Information Figure S3), and apparently,
the larger grains at higher Tsubs contribute to the higher
mobility both for DPh-BBTNDT and C6-BBTNDT.
Figure 6 shows the out-of-plane and in-plane XRDs of DPh-
and C6-BBTNDT thin films deposited on the Si/SiO2
substrate, and the structural parameters both obtained from
the thin film XRDs and single crystal analysis are summarized
in Table 3 together with those of the parent BBTNDT.32
Obviously, from the out-of-plane XRDs, both molecules have
an edge-on orientation to the substrate surface, similar to the
parent BBTNDT thin film (Supporting Information Figure S4).
The interlayer spacing (d-spacing) of DPh-BBTNDT (28.9 Å)
is similar to or slightly longer than the molecular length
obtained from the single crystal X-ray analysis (27.6 Å) and the
optimized molecular structure by the DFT calculations (28.1
Å), corresponding to a model where the molecules stand
perpendicular to the substrate surface. In contrast, the d-spacing
of C6-BBTNDT, 32.5 Å, is shorter than one obtained from the
optimized molecular geometry (33.4 Å, see also Supporting
Information Figure S5), implying that C6-BBTNDT molecules
tilt to the substrate normal or the alkyl layers slightly
interdigitate, which anyway might cause less effective
intermolecular orbital coupling, in turn resulting in relatively
low mobility.
Figure 4. Structure of DPh-BBTNDT: (a) molecular structure, (b)
packing structure projected along the crystallographic a-axis, and (c)
herringbone packing structure in the crystallographic ab cell.
Calculated intermolecular transfer integrals of HOMOs are ta = 71
and tp = 43 meV.48
of DPh-BBTNDT. The molecule has an almost planar
BBTNDT core with the maximum deviation of 0.14 Å from
the mean plane and two phenyl groups with dihedral angles of
ca. 26.6° (Figure 4a). In the packing structure, the molecules
form a layered structure along the crystallographic c-axis
direction (Figure 4b), and in each layer, the molecular
arrangement is classified into the herringbone packing (Figure
4c) similar to that of parent BBTNDT32 and related BTBT and
DNTT derivatives.23 Calculated intermolecular transfer inte-
grals of HOMOs in the herringbone cell are 71 meV for the
stacking pairs along the a-axis (1 0 0) direction (ta) and 43 meV
for the side-by-side pairs along the (1 1 0) and (−1 1 0)
directions (tp) (Figure 4c).48 Although these values are almost
comparable with those calculated for the BBTNDT single
crystal, 75 meV for the stacking pairs and 32 and 41 meV for
the side-by-side pairs, the values for the two directions are
much balanced in DPh-BBTNDT, indicating that the electronic
structure of DPh-BBTNDT is more two-dimensional (2D)-like
and isotropic (Supporting Information Figure S2). The
enhanced mobility observed for the DPh-BBTNDT-based
In the in-plane XRDs of the DPh-BBTNDT thin film, three
characteristic peaks assignable to the herringbone ab cell (110,
020, and 120 reflections) were clearly observed and calculated
length of the a and b crystallographic axis from these three
peaks are well matched with those in the single crystal data
(Table 3). Similarly three peaks were also observed in the in-
plane XRD of the C6-BBTNDT thin film (Figure 6b, right).
Although the single crystal data of C6-BBTNDT could not be
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sues,25,34,35 and thus, we carried out various stability tests of
the BBTNDTs-based devices. We first examine continuous
operation of the devices by alternately applying Vg of 0 and −30
V at Vd = −30 V for 1000 cycles. As shown in Figure 7,
Figure 7. Operational stability test of (a) DPh-BBTNDT-, (b) C6-
BBTNDT-, and (c) BBTNDT-based devices by alternately applying
VG of 0 and −30 V at VDS = −30 V for 1000 cycles.
excellent operational stability of the devices, as confirmed by
the nearly negligible change of both on and off current, was
observed for all the devices based on three BBTNDT
derivatives.
Figure 6. Out-of-plane (left) and in-plane (right) XRD patterns of
evaporated film on the Si/SiO2 substrate: (a) DPh-BBTNDT and (b)
C6-BBTNDT on the ODTS-treated substrate. The indexes in panel a
are based on the bulk single crystal cell, whereas the indexes in the
parentheses in panel b are assigned based on the similarity to that of
DPh-BBTNDT.
To evaluate the thermal stability of the BBTNDTs-based
OTFTs, the device characteristics were measured after thermal
treatments for 30 min at gradually raised temperatures from
100 to 300 °C. As demonstrated in Figure 8, the devices with
different BBTNDT derivatives as the active semiconducting
layer showed distinct behaviors upon thermal treatments.
Among them, the most noticeable degradation was observed for
the C6-BBTNDT-based device, where a clear off-current
increase upon thermal treatment took place (Figure 8b). This
can be explained by its relatively smaller IP (5.0 eV) than those
of other BBTNDT derivatives; the thermal treatment may
accelerate oxidation of the semiconducting material by the
ambient oxygen. The parent BBTNDT-based devices also
showed gradual off-current increase and relatively large
threshold voltage (Vth) shift (ca. 15 V with the thermal
treatment at 300 °C, Figure 8c). On the other hand, the DPh-
BBTNDT-based devices was not very sensitive to the thermal
treatment; although a small Vth shift of ca. 5 V was observed,
the off current was still low even after the treatment at 300 °C,
keeping large on/off ratio over 107, which clearly demonstrates
its outstanding thermal stability (Figure 8a). The excellent
thermal stability of DPh-BBTNDT-based devices can be
rationalized by considering its relatively large IP (5.3 eV) and
tight packing structure assisted by the phenyl groups, which
intermolecularly interact with the phenyl moieties of adjacent
molecules via the CH−π hydrogen bond-like interactions
(Figure 4b and c).
available, the length of a and b axes are estimated from three
peaks in the in-plane XRD, provided that the unit cell is of
monoclinic or higher symmetry, that is, at least, α = γ = 90°,
similar to those of BBTNDT and DPh-BBTNDT. The
estimated a-axis length of C6-BBTNDT is ca. 6.0 Å, which is
similar to those of DPh-BBTNDT and parent BBTNDT. On
the other hand, the b-axis length, ca. 8.0 Å is apparently longer
than those of DPh-BBTNDT (ca. 7.6 Å) and the parent (ca. 7.8
Å). The crystallographic b axis can be related to the edge-to-
face intermolecular interaction (see Figure 4c), and the longer b
axis of C6-BBTNDT than those of others strongly imply its less
effective intermolecular orbital coupling in this directions.
Although the quantitative evaluation of the transfer integrals of
C6-BBTNDT was not possible, the present structural
characterization can qualitatively afford the reasons for
relatively lower mobility of C6-BBTNDT-based devices than
those of other BBDNDT derivatives.
Stability of OTFT Devices. In the recent advanced
applications of OFETs, stabilities of devices against various
factors, such as thermal treatment, repeated operations, and
environmental conditions, are regarded as important is-
Table 3. Structural Parameters of ab Cell Extracted from XRDs
a
b
c
d
compound
d-spacing /Å
molecular length (l)/Å
a(c)/Å
b/Å
DPh-BBTNDT
C6-BBTNDT
28.9
32.5
20.3
27.6 (28.1)
6.1280(5) (6.1)
−(6.0)
5.9894(3) (5.9)
7.581(2) (7.6)
−(8.0)
7.807(1) (7.8)
−(33.4)
19.3 (19.6)
e
d
BBTNDT
a
b
Calculated interlayer spacing from the 001 reflection for DPh- and C6-BBTNDTs and the 100 reflection for BBTNDT. Obtained from single
c
crystal X-ray analysis. Values in parentheses are obtained from molecular geometry optimized by the DFT calculations (B3LYP/6-31G*). Length of
the crystallographic axis in the stacking direction. The crystallographic a axis corresponds to the DPh-BBTNDT single crystal, whereas the c axis to
the parent BBTNDT. The values in parentheses are estimated from the in-plane XRD data. Length of crystallographic b axis (the side-by-side
direction). The values in parentheses are estimated from the in-plane XRD data. See ref 32.
d
e
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Figure 8. Transfer curves of (a) DPh-BBTNDT-, (b) C6-BBTNDT-, and (c) BBTNDT-based OFET devices upon annealing up to 300 °C
measured under ambient conditions at Vd = −60 V.
evaporated in vacuo. The residue was purified by column
CONCLUSION
In the present work, we have developed an effective and general
synthetic route to BBTNDT derivatives. With the new
synthetic route, two BBTNDT derivatives, DPh-BBTNDT
■
chromatography on silica gel eluted with hexane to afford 3b (6.95
1
g, 83%) as a white solid. Mp 51.5 °C. H NMR (400 MHz) δ 7.32−
7.38 (m, 1H), 7.34 (d, J = 5.2 Hz, 1H), 7.41−7.49 (m, 2H), 7.44 (d, J
= 5.2 Hz, 1H), 7.60 (dd, J = 1.6, 8.4 Hz, 1H), 7.63−7.67 (m, 2H), 7.86
(d, J = 8.4 Hz, 1H), 8.08 (s, 1H). 13C NMR (100 MHz) δ 120.9,
123.7, 123.9, 124.1, 126.8, 127.3, 127.5, 128.9, 137.7, 138.8, 140.6,
141.2. MS (EI) m/z = 210 (M+). Anal. calcd for C14H10S: C, 79.96; H,
4.79%. Found C, 79.95; H, 4.89%.
and C6-BBTNDT, were synthesized. Evaluation of their
electronic properties by using their vapor-deposited thin films
demonstrated that DPh-BBTNDT with two phenyl groups in
the molecular long axis direction has relatively large IP than
that of the parent, whereas C6-BBTNDT smaller IP. By
evaluating further the BBTNDT derivatives including the
parent one using the OTFT configuration, DPh-BBTNDT was
turned out to be the most superior one affording vapor-
processed OTFT devices showing very high hole mobility
(∼7.0 cm2 V−1 s−1) with excellent operational and thermal
stabilities (up to 300 °C). The high mobility of the devices can
be explained by its isotropic two-dimensional (2D) electronic
structure elucidated by XRDs and theoretical calculations. The
good thermal stability can be assisted by its relatively large IP
(5.3 eV) and the tight and interactive packing structure in the
thin film state. Thus, we believe that DPh-BBTNDT is a
promising and practical vapor-processable organic semiconduc-
tor, which can afford thermally, operationally, and environ-
mentally stable OTFTs showing very high mobility, and will be
utilized into sophisticated device applications in the future.
6-Hexylbenzo[b]thiophene (3c). A solution of 9-BBN (0.5 M
solution in THF, 98 mL, 49 mmol) and 1-hexene (4.12 g, 49 mmol)
were stirred for 15 h at room temperature. To the mixture was added
PdCl2(dppf) (41 mg, 0.05 mmol), 6-bromobenzo[b]thiophene (7.49
g, 35 mmoI) in deaerated solution of sodium hydroxide (1.96 g, 49
mmol) in water (20 mL) at rt, and the resulting mixture was refluxed
for 24 h. After cooling, the mixture was diluted with water (100 mL)
and extracted with chloroform (100 mL × 2). The combined extracts
were dried (MgSO4), concentrated in vacuo, and purified by column
chromatography on silica gel eluted with hexane to give 3c (7.11 g,
93%) as colorless oil. 1H NMR (400 MHz) δ 0.88(t, J = 7.2 Hz, 3H),
1.25−1.40 (m, 6H), 1.62−1.71 (m, 2H), 2.72 (t, J = 7.8 Hz, 2H), 7.19
(dd, J = 1.2, 8.0 Hz, 1H), 7.28 (dd, J = 0.8, 5.6 Hz, 1H), 7.35 (d, J =
5.6 Hz, 1H), 7.68 (d, J = 1.2 Hz, 1H), 7.72 (d, J = 8.4 Hz, 1H). 13C
NMR (100 MHz) δ 14.3, 22.8, 29.1, 31.8, 31.9, 36.2, 121.8, 123.3,
123.7, 125.3, 125.5, 137.7, 139.3, 140.1. MS (EI) m/z = 232 (M+).
Anal. Calcd for C14H18S: C, 77.01; H, 8.31%. Found C, 77.08; H,
8.30%.
6-Phenyl-2-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)benzo-
[b]thiophene (4b). To a solution of 3b (3.15 g, 15.0 mmol) in THF
(30 mL) was added 1.6 M hexane solution of n-BuLi (13.1 mL, 21.0
mmol) at −78 °C. After the mixture was stirred for 1 h at rt, 2-
isopropyloxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (4.18 g, 22.5
mmol) was added to the solution at −78 °C, and the resulting
mixture was stirred for 19 h at rt. The mixture was poured into water
(100 mL) and was extracted with ethyl acetate (100 mL × 2). The
combined extracts were washed with water (100 mL) and brine (100
mL × 2), dried (MgSO4), and concentrated in vacuo. The residue was
purified by column chromatography on silica gel eluted with
EXPERIMENTAL SECTION
■
General. All chemicals and solvents are of reagent grade unless
otherwise indicated. Tetrahydrofuran (THF), N,N-dimethylformamide
(DMF), dichloromethane, and toluene were purified with standard
distillation procedures prior to use. 3,7-Bis(methylthio)-2,6-bis-
(trifluoromethanesulfonyloxy)naphthalene (5),32 2-(4,4,5,5-tetrameth-
yl-1,3,2-dioxaborolan-2-yl)benzo[b]thiophene (4a),39 and 6-
bromobenzo[b]thiophene40 were prepared according to the literatures.
All reactions were carried out under nitrogen atmosphere. Melting
points were uncorrected. NMR spectra were obtained in deuterated
chloroform (CDCl3) with TMS as the internal reference; chemical
shifts (δ) are reported in parts per million, unless otherwise stated. EI-
MS spectra were obtained using an electron impact ionization
procedure (70 eV).
6-Phenylbenzo[b]thiophene (3b). A solution of 6-bromobenzo-
[b]thiophene (8.56 g, 40.0 mmol), phenylboronic acid (5.85 g, 48.0
mmol), and potassium carbonate (11.06 g, 80.0 mmol) in DMF (600
mL) and water (30 mL) was deaerated by argon stream for 30 min. To
the solution was added Pd(PPh3)4 (2.31 g, 2.0 mmol), and the mixture
was heated at 90 °C for 12h, cooled to rt, and poured into water (600
mL). The organic layer was separated and the aqueous layer was
extracted with ethyl acetate (300 mL × 2). The combined organic
layers were washed with brine (200 mL), dried (MgSO4), and
1
chloroform to give 4b (3.75 g, 74%) as yellow oil. H NMR (400
MHz) δ 1.39 (s, 12H), 7.33−7.39 (m, 1H), 7.43−7.49 (m, 2H), 7.60
(dd, J = 1.2, 8.4 Hz, 1H), 7.66−7.69 (m, 2H), 7.90 (d, J = 8.4 Hz, 1H),
7.90 (s, 1H), 8.11 (d, J = 0.8 Hz, 1H). 13C NMR (100 MHz) δ 24.9,
84.6, 120.9, 124.1, 124.6, 127.5(× 2), 128.9, 134.2, 138.7, 139.7, 141.1,
144.6. MS (EI) m/z = 358 (M+). Anal. calcd for C20H21BO2S: C,
71.44; H, 6.30%. Found C, 71.14; H, 6.34%.
6-Hexyl-2-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)benzo[b]-
thiophene (4c). To a solution of 3c (1.26 g, 5.8 mmol) in THF (15
mL) was added 1.6 M hexane solution of n-BuLi (5.0 mL, 8.0 mmol)
at −78 °C. After the mixture was stirred for 1 h at rt, 2-isopropoxy-
4,4,5,5-tetramethyl-1,3,2-dioxaborolane (1.61 g, 8.7 mmol) was added
to the solution at −78 °C, and the resulting mixture was stirred for 19
F
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Chem. Mater. XXXX, XXX, XXX−XXX

Chemistry of Materials
Article
h at rt. The mixture was poured into water (100 mL) and was
extracted with ethyl acetate (100 mL × 2). The combined extracts
were washed with water (100 mL) and brine (100 mL × 2), dried
(MgSO4), and concentrated in vacuo. The residue was purified by
column chromatography on silica gel eluted with chloroform to give 4c
(1.83 g, 92%) as yellow oil. 1H NMR (400 MHz): δ 0.88 (t, J = 7.2 Hz,
3H), 1.25−1.36 (m, 6H), 1.36 (s, 12H), 1.61−1.70 (m, 2H), 2.71 (t, J
= 8.0 Hz, 2H), 7.17 (dd, J = 1.6, 8.4 Hz, 1H), 7.69 (s, 1H), 7.74 (d, J =
8.4 Hz, 1H), 7.83 (s, 1H). 13C NMR (100 MHz) δ 14.2, 22.7, 24.9,
29.1, 31.7, 31.8, 36.2, 84.4, 121.7, 124.1, 125.6, 134.5, 138.6, 140.7,
144.2. MS (EI) m/z = 344 (M+). Anal. calcd for C20H29BO2S: C,
69.77; H, 8.49%. Found C, 69.63; H, 8.57%.
moiety affords two singlets (δ 2.55 and 2.56) in the 1H NMR
spectrum owing to the existence of diastereomers.
2,6-Bis(6-phenylbenzo[b]thiophen-2-yl)-3,7-bis(methylsulfinyl)-
naphthalene (2b). To a solution of 6b (0.318 g, 0.5 mmol) in
dichloromethane (150 mL) was added m-chloroperoxybenzoic acid
(0.173 g, 1.0 mmol) at 0 °C. After stirring for 13 h at rt, the mixture
was poured into aqueous K2CO3 solution (1 M, 20 mL), extracted
with dichloromethane (50 mL × 2). The combined extracts were
washed with aqueous K2CO3 solution (1 M, 40 mL), dried (MgSO4),
and evaporated to give 2b (0.315 g, 94%) as a pale yellow solid. Mp
1
273.1 °C. H NMR (400 MHz) δ 2.57, 2.59 (s, 6H), 7.38−7.43 (m,
2H), 7.47−7.54 (m, 4H), 7.61 (s, 2H), 7.68−7.72 (m, 4H), 7.70 (d, J
= 8.0 Hz, 2H), 7.94 (d, J = 8.0 Hz, 2H), 8.12 (s, 2H), 8.23 (s, 2H),
8.73 (s, 2H). 13C NMR (100 MHz) δ 42.1, 120.6, 124.6, 125.0, 127.5,
127.7, 129.1, 130.2, 131.2, 133.4, 138.5, 138.9, 139.1, 140.7, 141.2,
145.2. MS (EI) m/z = 668 (M+). HRMS (APCI) calcd for C40H29O2S4
[MH]+: 669.1050. Found: 669.1049. Note that the methylsulfinyl
moiety affords two singlets (δ 2.57 and 2.59) in the 1H NMR
spectrum owing to the existence of diastereomers.
2,6-Bis(benzo[b]thiophen-2-yl)-3,7-bis(methylthio)naphthalene
(6a). To a degassed solution of 2-(4,4,5,5-tetramethyl-1,3,2-dioxabor-
olan-2-yl)benzo[b]thiophene (4a, 5.00 g, 19.2 mmol), 3,7-bis-
(methylthio)-2,6-bis(trifluoromethanesulfonyloxy)naphthalene (5,
4.13g, 8.00 mmol), and K2CO3 (4.42 g, 32.0 mmol) in DMF (100
mL) and water (5.0 mL) was added Pd(PPh3)4 (0.924 g, 0.800 mmol).
The mixture was then heated at 90 °C for 12 h, cooled to rt, poured
into hydrochloric acid (1 M, 200 mL), filtered, and washed with water
and methanol to give 6a (2.73 g, 70%) as a yellow solid. Mp 295.9 °C.
1H NMR (400 MHz) δ 2.54 (s, 6H), 7.35−7.43 (m, 4H), 7.59 (s, 2H),
2,6-Bis(6-hexylbenzo[b]thiophen-2-yl)-3,7-di(methylsulfinyl)-
naphthalene (2c). To a solution of 6c (0.196 g, 0.3 mmol) in
dichloromethane (40 mL) was added m-chloroperoxybenzoic acid
(0.104 g, 0.6 mmol) at 0 °C. After stirring for 13 h at rt, the mixture
was poured into an aqueous K2CO3 solution (20 mL), extracted with
dichloromethane (50 mL × 2). The combined extracts were washed
with aqueous K2CO3 solution (1 M, 40 mL), dried over MgSO4, and
evaporated to give 2c (0.199 g, 97%) as a pale yellow solid. Mp 219.4
7.50 (s, 2H), 7.83−7.91 (m, 4H), 7.88 (s, 2H). 13C NMR (100 MHz)
δ 16.2, 122.2, 123.0, 124.0, 124.5, 124.6, 124.8, 129.0, 130.9, 132.7,
136.3, 140.0, 140.4, 140.9. MS (EI) m/z = 484 (M+). HRMS (APCI)
m/z calcd for C28H21S4 [MH]+: 485.0526. Found: 485.0533.
2,6-Bis(6-phenylbenzo[b]thiophen-2-yl)-3,7-bis(methylthio)-
naphthalene (6b). To a degassed solution of 4b (2.42g, 7.2 mmol), 5
(1.55 g, 3.0 mmol), and K2CO3 (1.66 g, 12.0 mmol) in DMF (70 mL)
and water (3.5 mL) was added Pd(PPh3)4 (0.924 g, 0.800 mmol). The
mixture was then heated at 90 °C for 18 h, cooled to rt, poured into
hydrochloric acid (1 M, 200 mL), filtered, and washed with water and
methanol to give 6b (1.58 g, 83%) as a yellow solid. Mp 287.4 °C. 1H
NMR (400 MHz) δ 2.56 (s, 6H), 7.36−7.41 (m, 2H), 7.46−7.52 (m,
4H), 7.61 (s, 2H), 7.63 (s, 2H), 7.66 (dd, J = 1.6, 8.0 Hz, 2H), 7.68−
7.72 (m, 4H), 7.91 (d, J = 8.0 Hz, 2H), 7.91 (s, 2H), 8.10 (t, J = 0.8
Hz, 2H). 13C NMR (100 MHz, 1,1,2,2-tetrachloroethane-d2 at 120
°C) δ 17.0, 120.5, 124.1, 124.3, 124.7, 125.2, 127.5, 129.0, 129.1,
131.4, 133.7, 136.7, 138.2, 139.4, 141.3, 141.6, 141.9. MS(EI) m/z =
636 (M+). Anal. calcd for C40H28S4: C, 75.43; H, 4.43%. Found C,
75.16; H, 4.50%.
2,6-Bis(6-hexylbenzo[b]thiophen-2-yl)-3,7-bis(methylthio)-
naphthalene (6c). To a degassed solution of 4c (2.93 g, 8.5 mmol), 5
(1.83g, 3.5 mmol), and K2CO3 (1.93 g, 14.0 mmol) in DMF (70 mL)
and water (3.5 mL) was added Pd(PPh3)4 (0.202 g, 0.175 mmol). The
mixture was then heated at 90 °C for 23 h, cooled to rt, poured into
hydrochloric acid (1 M, 200 mL), filtered, and washed with water and
methanol to give 6c (1.84 g, 80%) as a yellow solid. Mp 194.0 °C. 1H
NMR (400 MHz) δ 0.90 (t, J = 7.2 Hz, 6H), 1.28−1.42 (m, 12H),
1.64−1.73 (m, 4H), 2.52 (s, 6H), 2.75 (t, J = 7.8 Hz, 4H), 7.23 (dd, J
= 1.2, 8.0 Hz, 2H), 7.54 (s, 2H), 7.56 (s, 2H), 7.68 (s, 2H), 7.73 (d, J =
8.4 Hz, 2H), 7.86 (s, 2H); 13C NMR (100 MHz) δ 14.3, 16.2, 22.8,
29.1, 31.9, 36.2, 121.4, 123.0, 124.6, 125.7, 128.9, 130.8, 132.8, 136.3,
138.0, 139.8, 140.7; MS (EI) m/z = 652 (M+); Anal. Calcd for
C40H44S4: C, 73.57; H, 6.79%. Found C, 73.20; H, 6.79%.
2,6-Bis(benzo[b]thiophen-2-yl)-3,7-bis(methylsulfinyl)-
naphthalene (2a). To a solution of 6a (2.73 g, 5.63 mmol) in
dichloromethane (200 mL) was added m-chloroperoxybenzoic acid
(1.95 g, 11.3 mmol) at 0 °C. After stirring for 13 h at rt, the mixture
was poured into aqueous K2CO3 solution (1 M, 200 mL), extracted
with dichloromethane. The combined extracts were washed with
aqueous K2CO3 solution (1 M, 100 mL), dried over MgSO4, and
evaporated to give 2a (2.90 g, quant) as a yellow solid. Mp 294.4 °C.
MS (EI) m/z = 516 (M+). 1H NMR (400 MHz) δ 2.55, 2.56 (s, 6H),
7.40−7.48 (m, 4H), 7.58 (d, J = 1.6 Hz, 2H), 7.85−7.93 (m, 4H), 8.19
(d, J = 2.8 Hz, 2H), 8.70 (d, J = 1.6 Hz, 2H). 13C NMR (100 MHz) δ
42.0, 122.4, 124.4, 124.9, 125.2, 125.5, 130.2, 131.2, 133.3, 138.2,
138.3, 140.0, 140.4, 145.2. HRMS (APCI), m/z calcd for C28H20O2S4
[M]+: 516.0346. Found: 516.0341. Note that the methylsulfinyl
1
°C. H NMR (400 MHz) δ 0.90 (t, J = 6.8 Hz, 6H), 1.26−1.45 (m,
12H), 1.64−1.77 (m, 4H), 2.53, 2.55 (s, 6H), 2.76 (t, J = 7.6 Hz, 4H),
7.28 (d, J = 7.6 Hz, 2H), 7.52 (s, 2H), 7.69 (s, 2H), 7.77 (d, J = 8.4
Hz, 2H), 8.16 (d, J = 4.0 Hz, 2H), 8.68 (d, J = 4.0 Hz, 2H). 13C NMR
(100 MHz) δ 14.3, 22.7, 29.1, 31.7, 31.8, 36.2, 42.1, 121.5, 124.1,
124.7, 124.8, 126.4, 130.2, 131.1, 133.3, 137.1, 138.1, 140.8, 145.1.
HRMS (APCI) m/z calcd for C40H44NaO2S4 [MNa]+: 707.2122.
Found: 707.2128. Note that the methylsulfinyl moiety affords two
1
singlets (δ 2.53 and 2.55) in the H NMR spectrum owing to the
existence of diastereomers.
Bis[1]benzothieno[2,3-d;2′,3′-d′]naphtho[2,3-b;6,7-b′]-
dithiophene (BBTNDT). Under nitrogen atmosphere, 2a (2.82 g, 5.45
mmol) and P2O5 (0.387 g, 2.72 mmol) was added to trifluorometha-
nesulfonic acid (30 mL). The reaction mixture was stirred at rt for 72
h, poured into ice−water (50 mL), and filtered. The filtrate was
washed with water and added to pyridine (50 mL), and the resulting
mixture was refluxed for 24 h, poured into methanol, and filtrated. The
crude product was extracted with hot N-methylpyrrolidone (NMP,
150 mL, 180 °C) in 1 h. After cooling the solution to rt, resulting
precipitate was collected by filtration, washed with methanol, and
dried. Repeated vacuum sublimation with temperature gradient gave
analytical BBTNDT (0.75 g, 33%) as a yellow solid. Mp > 300 °C. MS
(EI) m/z = 452 (M+). Anal. calcd for C26H12S4: C, 68.99; H, 2.67%.
Found C, 68.87; H, 2.62%.
2,10-Diphenylbis[1]benzothieno[2,3-d;2′,3′-d′]naphtho[2,3-
b;6,7-b′]dithiophene (DPh-BBTNDT). Under nitrogen atmosphere, 2b
(0.308 g, 0.46 mmol) and P2O5 (0.033 g, 0.23 mmol) was added to
trifluoromethanesulfonic acid (10 mL). The reaction mixture was
stirred at rt for 70 h, poured into ice−water (50 mL), and filtered. The
filtrate was washed with water and added to pyridine (50 mL), refluxed
for 22 h, poured into methanol, and filtered. The crude product was
dissolved into NMP (50 mL) at 180 °C in 1 h. After cooling the
solution to rt, resulting precipitate was collected by filtration, washed
with methanol, and dried. Repeated vacuum sublimation with
temperature gradient gave analytical DPh-BBTNDT (0.090 g, 32%)
as an orange solid. Mp > 300 °C. Anal. calcd for C38H20S4: C, 75.46;
H, 3.33%. Found C, 75.47; H, 3.43%.
2,10-Dihexylbis[1]benzothieno[2,3-d;2′,3′-d′]naphtho[2,3-b;6,7-
b′]dithiophene (C6-BBTNDT). Under nitrogen atmosphere, 2c (0.199
g, 5.45 mmol) and P2O5 (0.021 g, 0.145 mmol) was added to
trifluoromethanesulfonic acid (5 mL). The reaction mixture was
stirred at rt for 68 h, poured into ice−water (50 mL), and filtered. The
filtrate was washed with water and dissolved into pyridine (15 mL),
G
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Chem. Mater. XXXX, XXX, XXX−XXX

Chemistry of Materials
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refluxed for 44 h, poured into methanol, and filtrated. The crude
product was dissolved into NMP (12 mL) at 180 °C in 1 h. After
cooling the solution to rt, resulting precipitate was collected by
filtration, washed with methanol, and dried. Repeated vacuum
sublimation with temperature gradient gave analytical C6-BBTNDT
(0.040 g, 22%) as an orange solid. Mp > 300 °C. 1H NMR (400 MHz,
1,1,2,2-tetrachloroethane-d2 at 120 °C) δ 0.97 (broad triplet, 6H),
1.35−1.55 (m, 12H), 1.75−1.85 (m, 4H), 2.84 (t, J = 8.4 Hz, 4H),
7.36 (d, J = 7.8 Hz, 2H), 7.80 (s, 2H), 7.82 (d, J = 7.8 Hz, 2H), 8.40
(s, 2H), 8.55 (s, 2H). Anal. calcd for C38H36S4: C, 73.50; H, 5.84%.
Found: C, 73.40; H, 5.85%. HRMS (APCI) m/z calcd for C38H36S4
[M]+: 620.1700. Found: 620.1712.
Device Fabrication and Characterization. Field-effect transistors
with a “bottom-gate, top-contact” configuration were fabricated in on a
heavily doped n+-Si (100) wafer with a 200 nm thermally grown SiO2
(Ci = 17.3 nF cm−2). The substrate surfaces were treated with HMDS,
OTS, or ODTS as reported previously.16 A thin film of the organic
semiconductors as the active layer was vacuum deposited on the Si/
SiO2 substrates maintained at various substrate temperatures (Tsub) at
a deposition rate of 1.0 Å s−1 under a pressure of ∼10−3 Pa. On top of
the organic thin film, gold films (80 nm) as drain and source
electrodes were deposited through a shadow mask. For a typical
device, the drain-source channel length (L) and width (W) are 40 μm
and 1.5 mm, respectively. Characteristics of the OFET devices were
measured at rt under ambient conditions with a Keithley 4200
semiconducting parameter analyzer. Field-effect mobility (μFET) was
calculated in the saturation regime (Vd = Vg = −60 V) of the Id using
the following equation
AUTHOR INFORMATION
Corresponding Author
*E-mail: takimiya@riken.jp.
Author Contributions
All authors have given approval to the final version of the
manuscript.
Notes
■
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was financially supported by Grants-in-Aid for
Scientific Research (Nos. 23245041 and 15H02196) from
MEXT, Japan. HRMSs were carried out at the Materials
Characterization Support Unit in RIKEN Advanced Technol-
ogy Support Division. The DFT calculations using the ADF
program were performed by using the RIKEN Integrated
Cluster of Clusters (RICC). The synchrotron radiation
experiments were performed at the BL02B1 of SPring-8 with
the approval of the Japan Synchrotron Radiation Research
Institute (JASRI) (Proposal No. 2014B1514).
REFERENCES
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Crystallographic Data for DPh-BBTNDT. C38H20S4 (604.78),
yellow plate, 0.08 × 0.05 × 0.003 mm3, monoclinic, space group, P21/a
(#14), a = 6.1280(15), b = 7.5805(19), c = 28.009(7) Å, β =
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all data.
ASSOCIATED CONTENT
■
S
* Supporting Information
NMR spectra, CIF for DPh-BBTNDT, and simulated
molecular structures optimized by the DFT calculations. The
Supporting Information is available free of charge on the ACS
Publications website at DOI: 10.1021/acs.chemmater.5b01608.
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H
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