Naphtho[2,3- b:6,7- b']dichalcogenophenes: Syntheses, characterizations, and chalcogene atom effects on organic field-effect transistor and organic photovoltaic devices

Article abstract of DOI:10.1021/cm202853b

New linear-shaped naphtho[2,3-b:6,7-b']-difurans (NDFs) and -selenophenes (NDSs) were synthesized selectively from 3,7-dibromo-2,6-dihydroxynaphthalene and evaluated as organic semiconductors in comparison to corresponding naphtho[2,3-b:6,7-b']dithiophene

Full text of DOI:10.1021/cm202853b

Article
pubs.acs.org/cm
Naphtho[2,3-b:6,7-b′]dichalcogenophenes: Syntheses,
Characterizations, and Chalcogene Atom Effects on Organic Field-
Effect Transistor and Organic Photovoltaic Devices
Masahiro Nakano,†,§ Hiroki Mori,†,§ Shoji Shinamura,† and Kazuo Takimiya*,†,‡
†Department of Applied Chemistry, Graduate School of Engineering, Hiroshima University, Higashi-Hiroshima 739-8527, Japan
‡Institute for Advanced Materials Research, Hiroshima University, Higashi-Hiroshima 739-8530, Japan
S
* Supporting Information
ABSTRACT: New linear-shaped naphtho[2,3-b:6,7-b′]-difurans (NDFs) and -sele-
nophenes (NDSs) were synthesized selectively from 3,7-dibromo-2,6-dihydroxynaph-
thalene and evaluated as organic semiconductors in comparison to corresponding
naphtho[2,3-b:6,7-b′]dithiophenes (NDTs). Evaluation of the electronic structures of
the parent compounds by means of electrochemical and optical measurements clearly
indicated that NDT and NDS are quite similar to each other, whereas only NDF has a
marked different electronic structure. Thin film devices, including organic field-effect
transistors (OFETs) and bilayer photovoltaics (OPVs) with C60 or C70 as an acceptor
layer, were fabricated with the diphenyl derivatives using vacuum deposition. The thin
films were found to be nicely crystalline with the edge-on molecular orientation both
on Si/SiO2 (for OFETs) and ITO substrates (for OPVs). The thin films acted as
active semiconducting layer in OFETs with mobility higher than 0.1 cm2 V−1 s−1 and
as a donor layer in OPVs with power conversion efficiencies of up to 2.0%, indicating
that the present naphthodichalcogenophenes are potential core structures for the development of new organic semiconductors.
KEYWORDS: naphthodichalcogenophene, electronic structure, organic semiconductor, organic field-effect transistor,
organic photovoltaic
INTRODUCTION
■
Recent interests in organic electronics are motivated by their
potential applications that can provide low-cost, large-area
electronic devices on arbitrary substrates.1 One of the key
materials in organic electronics is, needless to say, the organic
semiconductor, which has been recently developed signifi-
cantly; tuning the chemical structures and physical properties
and understanding the structural implications of electronic
properties have been intensively studied in the past decade.2
Among the numbers of molecular classes reported so far,
ladder-type π-conjugated molecules, often referred as acenes
and heteroacenes have been examined as promising organic
semiconductors, which, in fact, have realized high-performance
organic field-effect transistors (OFETs)3 and organic photo-
voltaics (OPVs).4
Acenedithiophenes (AcDTs), such as benzo[1,2-b:4,5-b′]-
dithiophene (BDT) and anthra[2,3-b:6,7(7,6)-b′]dithiophenes
(ADT) (Figure 1), are prototypical of such heteroacenes and
have been widely utilized as important π-cores for the
development of not only molecular organic semiconductors
but also polymer semiconductors.5 Recently, we reported on
the syntheses, characterization, and utilization of four isomeric
naphthodithiophenes (NDTs, Figure 1)6 as new members in
AcDTs and found that NDTs are very useful as superior π-
extended cores for molecular semiconductors as well as
polymer semiconductors.7 In particular, the linear-shaped
Figure 1. Molecular structures of linear-shaped acenedithiophenes
(AcDTs) and four isomeric naphthodithiophenes (NDTs).
naphtho[2,3-b:6,7-b′]dithiophene (anti-NDT, 1) is a promising
one, whose diphenyl derivative afforded OFETs with high
mobility of up to 1.5 cm2 V−1 s−1.6b Furthermore, very recently
Facchetti and co-workers have reported that its dialkyloxy
derivatives carrying two thiophene-capped diketopyrrolopyr-
roles (TDPP) showed remarkable results on bulk-hetero-
junction organic photovoltaics (OPVs) with power conversion
efficiencies higher than 4.0%.8
Received: September 21, 2011
Revised: November 24, 2011
Published: November 26, 2011
© 2011 American Chemical Society
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These promising results on the anti-NDT core have indeed
propelled us to study its homologues with different chalcogene
atoms,9,10 that is, naphtho[2,3-b:6,7-b′]difurans (NDFs, 5) and
naphtho[2,3-b:6,7-b′]diselenophenes (NDSs, 6) (Figure 2). In
diphenyl derivatives (5b and 6b) do not have solubilities
sufficient for the characterization by NMR spectroscopy and
thus were characterized by mass spectroscopy and combustion
elemental analyses.
Physicochemical Properties. Electrochemical and optical
behaviors of 5a and 6a along with NDT (1a)6b were compared
to evaluate the influence of the chalcogenophene rings on the
molecular properties. At the first glance, the cyclic voltammo-
grams (CVs) of three compounds (Figure 3a) present two
noticeable features; their similar oxidation onset potentials and
irreversibility of the 5a voltammogram in contrast to the
reversible voltammograms of 1a and 6a. Similar oxidation
onsets strongly imply that these compounds have similar
highest occupied molecular orbital (HOMO) energy levels.
Under the premise that the Fc/Fc+ redox couple corresponds
to 4.8 eV below the vacuum level,13 the HOMO energy levels
estimated from the oxidation onsets are 5.5, 5.3, and 5.3 eV for
5a, 1a, and 6a, respectively, which are also close to the formal
isoelectronic hydrocarbon, naphthacene (5.2 eV determined by
the same procedure). These experimentally estimated HOMO
energy levels qualitatively agree with those of theoretically
calculated ones (Figure 4),14 where a slightly low-lying HOMO
energy level for 5a compared to those of 1a and 6a is
reproduced. On the other hand, the irreversible redox wave of
5a is markedly contrasted to the reversible voltammograms of
1a and 6a (Figure 3a). Repeated redox cycles of 5a gradually
reduced the intensity of the oxidation peak without emergence
of new peaks in the voltamogram, indicating that the oxidized
species of 5a seems to be decomposed under the voltammetric
conditions. In fact, dark-colored insoluble material was
deposited on the surface of the working electrode.
Absorption spectra of 5a, 1a, and 6a depicted in Figure 3b
also demonstrate marked electronic-structure difference
between 5a and 1a/6a and similarity of 1a and 6a. These
difference and similarity can be qualitatively understood by
their aromatictiy and effectiveness of π-delocalization over the
naphthodichalcogenophene cores. Furan is known as the least
aromatic among the present three heteroaromatics,15 indicating
its poor conjugation nature. This can lead to the least effective
π-delocalization over the entire 18-π electron system of 5a core,
resulting in the blue-shifted absorption band among the three
compounds. In other words, the electronic structures of 1a and
6a are somehow similar to that of naphthacene, whereas that of
5a can be, to some extent, expressed as an oxygen-bridged
divinylnaphthalene. The theoretical TD-DFT calculations also
reproduce the blue-shifted absorption of 5a compared to those
Figure 2. Molecular structures of naphtho[2,3-b:6,7-b′]-
dichalcogenophenes (NDXs).
this paper, we report the facile synthesis and properties of these
new naphthodichalcogenophenes (NDXs) together with OFET
and OPV devices using their diphenyl derivatives (DPh-
NDXs).
RESULTS AND DISCUSSION
■
Synthesis. A key intermediate in the syntheses of both
NDFs (5) and NDSs (6) is 3,7-dibromo-2,6-dihydroxynaph-
thalene (7), conveniently prepared from 2,6-dihydroxynaph-
thalene via bromination and selective debromination.6b The
bromine and hydroxyl functionalities on 7, however, play
different roles in the respective synthesis. For the synthesis of
NDF derivatives, the bromine atoms were first replaced with
acetylene moieties by the Sonogashira reaction after acetylation
on the hydroxyl moieties. Then, the subsequent base-promoted
furan formation11 at the acetyl-protected o-ethynylphenol
substructures readily gave NDF derivatives in acceptable yields
for both the parent compound (5a) and the diphenyl derivative
(5b) (Scheme 1).
For the synthesis of NDSs, on the other hand, the hydroxyl
groups were utilized as the handle for introducing the acetylene
moieties via triflation and the subsequent chemo-selective
Sonogashira coupling to give 3,7-dibromo-2,6-diethynylnaph-
thalenes (11).6b Selenophene ring annulation was achieved by a
reaction with sodium selenide reagent, in situ generated from
sodium borohydride and selenium powder in ethanol, where
the first SNAr-type reaction on the bromine sites to afford o-
ethynylselenolate intermediates, which then cyclized intra-
molecularly to give the NDS derivatives.12 Parent 5a and 6a
were fully characterized by spectroscopic analyses, whereas the
Scheme 1. Synthesis of NDF (5) and NDS (6) Derivatives
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Figure 3. Cyclic voltammogram (a), absorption spectra (in dichloromethane) (b) and fluorescence spectra (λex = 254, 276, 282 nm, respectively) (c)
of 5a, 1a, and 6a.
reduced emission intensity compared to those of 5a and 1a,
which can be understood by considering the heavy atom
quenching effect of the incorporated selenium atom.16
Thin Film of Diphenyl Derivatives and Their
Applications to OFETs and OPVs. Thin Film Character-
izations. As the film-forming natures of parent 5a and 6a were
rather poor under typical vacuum deposition conditions, we
took the diphenyl derivatives (5b, 6b) to examine the utility of
NDF and NDS as an organic semiconducting core in
comparison to 1b previously reported.6b All the three
compounds afforded homogeneous thin films both on Si/
SiO2 and ITO substrates. Atomic force microscope (AFM)
images of the evaporated thin films of 5b, 1b, and 6b on the
ITO substrates shown in Figure 5 revealed that all the films
consist of crystalline grains with submicrometer in size as seen
in typical vapor-deposited thin-films of molecular semi-
conductors. Although the grain sizes were somewhat different,
the surface morphologies of the evaporated thin films on the
Si/SiO2 substrates were basically the same with those on the
ITO substrate (Supporting Information, Figure S1).
Figure 4. Calculated HOMOs and LUMOs of 5a, 1a, and 6a with the
DFT B3LYP/6-31(d) level. The electronic transition energies are also
calculated with the TD-DFT B3LYP/6-31(d) level to be 3.47 (5a),
3.17 (1a), and 3.10 eV (6a).
To evaluate the electronic properties of the thin films, we
first measured the ionization potentials (IP) of the evaporated
thin films by photoelectron spectroscopy in air (Figure 6a).
The IPs of 1b and 6b are almost the same (∼5.2 eV), whereas
that of 5b is slightly larger (5.4 eV), which can be explained by
the inherent difference of the HOMO energy levels of the core
parts (see Figure 3a, Figure 4, and Table 1). Although the IPs
are somewhat different, all these materials are expected to be
stable p-channel organic semiconductors in air owing to their
relatively low-lying HOMO energy levels.
of 1a and 6a; the calculated electronic transition energies for
5a, 1a, and 6a are 3.47, 3.17, and 3.10 eV, respectively
(Supporting Information, Table S1). Fluorescence spectra of
5a, 1a, and 6a are also depicted in Figure 3c. Similar to the
absorption spectra, the furan analogue (5a) shows the most
blue-shifted emission band among the three analogues. On the
other hand, the selenophene analogue (6a) has markedly
Figure 5. AFM images (2 × 2 μm) of evaporated thin films of 5b (a), 1b (b), and 6b (c) on ITO substrates.
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Figure 6. Photoelectron spectroscopy in air (a) and absorption spectra of evaporated thin films (b) of 5b, 1b, and 6b on ITO substrates.
Table 1. Electronic Properties of 5a, 1a, and 6a
a
b
c
d
e
compound
Eonset/V
HOMO/eV
λmax (λedge)/nm
Eg/eV
λem/nm
ϕem
0.18
0.10
8.3 × 10−3
5a
1a
6a
+1.14
+0.94
+0.94
−5.5
−5.3
−5.3
364 (370)
401 (410)
401 (410)
3.4
3.0
3.0
370, 390
410, 433
418, 444
a
b
V vs Ag/AgCl. All the potentials were calibrated with the Fc/Fc+ (E1/2 = +0.43 V measured under identical conditions). Estimated with a following
c
d
e
equation: EHOMO (eV) = −4.4 − Eonset. Calculated from λedge. Excited at 256, 277, and 282 nm for 5a, 1a, and 6a respectively. Emission quantum
yields were determined by the relative method using anthracene (ϕem 0.31, for 5a) and 9,10-diphenylanthoracene (ϕem 0.90, for 1a and 6a) as the
standards.
The absorption spectra of the thin films have a similar trend
with the solution absorption spectra of their heteroacene cores
(Figure 6b); very similar absorption bands for 1b and 6b and a
distinctly blue-shifted absorption for 5b are observed. These
similarity/difference between the thin films are well understood
in terms of the molecular electronic structure of the
heteroacene core as already discussed.
Both out-of-plane and in-plane X-ray diffraction (XRD)
patterns of the evaporated thin films (Figure 7) clearly indicate
that the films are nicely crystalline on both the Si/SiO2 and the
ITO substrates (Supporting Information, Figure S2). From the
out-of-plane XRD patterns, it can be determined that all three
material have edge-on molecular orientations with molecular
lamellar structures in the thin film state, and the interlayer
spacings (d-spacings, 20.2−20.8 Å) calculated from the out-of-
plane patterns are close to the molecular lengths expected from
theoretically optimized molecular geometries (Supporting
Information, Figure S3).
As all the peaks observed in the XRD patterns of the 1b thin
film are indexed by using the bulk single crystallographic cell
previously reported6b (Figure 7b), 1b must have basically the
same packing structure in both the bulk crystal and the thin
film. More precisely, the 1b crystallites in the thin film have a
preferred orientation, where the crystallographic c-axis stands
perpendicular to the substrate surface and the crystallographic
ab-plane lies on the substrate surface. This orientation can
afford a two-dimensional (2D) electronic structure on the
substrate surface, originating from the isotropic herringbone
molecular arrangement in the in-plane direction. It is well-
known that such structural type is desirable for effective carrier
transport in the OFET devices, as the 2D electronic structure
on the substrate surface enables isotropic and effective carrier
transport leading to high carrier mobility.17 In fact, this
consideration is consistent with the high carrier mobility (1.5
cm2 V−1 s−1) of 1b-based thin film OFET devices.6b
Although the single crystals suitable for X-ray structural
analyses were not obtained for 5b and 6b, it is likely expected,
judging from both the out-of-plane and the in-plane XRD
patterns (Figure 7a and 7c), that their molecular ordering
manner in the thin film state is similar to that of 1b with the
edge-on molecular orientation and good crystalline order in the
in-plane direction. Thus, the 5b and 6b thin films are also
expected as superior semiconducting channel in the OFETs
devices (vide infra).
Thin Film OFETs. OFETs of 5b and 6b were fabricated
using the thin films deposited on the Si/SiO2 substrates
modified with hexamethyldisilazane (HMDS) or octyltrichlor-
osilane (OTS). Although the transistor characteristics were
affected by the fabrication conditions such as the surface
treatment and substrate temperature during film deposition,
their OFETs in general showed typical p-channel transistor
characteristics with mobility higher than 0.1 cm2 V−1 s−1 in air
(Supporting Information, Table S2). Typical output and
transfer curves of the OFETs are depicted in Figure 8, and
optimized FET characteristics are summarized in Table 2. The
mobilities of the optimized OFETs of 5b and 6b are 0.6 and 0.9
cm2 V−1 s−1, respectively, which are slightly lower than those
previously reported for 1b-based OFETs (1.5 cm2 V−1 s−1).6b
Relatively low mobility of 5b- and 6b-based OFETs can be
attributed to the miss-oriented grains as observed in the XRD
patterns. In the case of 1b thin films, no detectable peaks
originated from the face-on orientation, that is, the in-plane
peaks (110, 120, and 020) in the out-of-plane XRD pattern
(Figure 7b) is observed, whereas the out-of-plane XRDs for
both 5b and 6b have noticeable peaks assignable to the in-plane
peaks designated with red asterisks (Figure 7a and 7c). These
miss-oriented crystal grains can be a cause for carrier traps that
limit experimental carrier mobility. Although the reasons for
such miss-orientation for the 5b and 6b thin films are not clear,
we speculated that the size of chalcogenophene rings attached
on the naphthalene can play an important role; the molecular
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brought about by selenium atoms are beneficial for effective
intermolecular orbital couplings in the solid state, which might
enhance carrier mobility.10,19 In the present study, however,
such positive effect is not the case, instead, perturbation to
packing perfection in the thin film state seems to limit the
carrier mobility in the actual thin film OFETs.
Bilayer OPVs. Standard donor/acceptor (D/A) bilayer solar
cells with a device structure of ITO/DPh-NDXs/C60/BCP/Al
were fabricated by using sequential vacuum depositions on
ITO-coated glass substrates.20 Current density−voltage (J-V)
characteristics of devices were measured under simulated AM
1.5 G solar illumination (100 mW cm−2) in air without
encapsulation. All the devices showed typical photovoltaic
responses as shown in Figure 9a. Reflecting the relatively
narrow absorption ranges of the thin films (Figure 6b), short-
circuit currents (Jsc's) are not that large (∼ 3 mA cm−2)
regardless of the donor layer: in fact external quantum
efficiency (EQE) spectra (Figure 9b) of the devices indicate
that there is almost no photocurrent response above 650 nm. In
sharp contrast to similar Jsc's, open circuit voltage (Voc) depends
largely on the donor material; as expected from the larger IP, in
other words, lower HOMO energy level, of 5b (5.4 eV) than
those of 1b and 6b (5.2 eV), the 5b-based devices afforded
larger Voc (0.69 V) by about 0.2 V. Another noticeable feature
of the current OPVs is fairly good fill factors (FFs, 0.59−0.64).
This could be partially attributed to slower charge-recombina-
tion (CR) rate,21 expected from the structure at the D/A
interface, where the perpendicular orientation of the donors
against C60 molecules originating from the edge-on structure of
the donor molecules on the substrate is most likely. As a result,
the 5b-based devices showed the best power conversion
efficiency (PCE) of 1.31%, which is relatively good for simple
bilayer solar cells based on small-molecular based OPVs.22
The deep HOMO energy levels of DPh-NDXs (∼5.4 eV)
affording large Voc's (∼0.69 V) and their large HOMO−LUMO
gaps limiting their photoharvesting ability in the visible range
leading to low Jsc's (∼3.16 mA cm−2) are indeed an unavoidable
two-sidedness as the donor materials for OPV applications. To
compensate the limited photoharvesting nature, we utilized C70
23
having a better light absorption property than that of C60 as
Figure 7. Out-of-plane and in-plane (insets) XRD patterns of
evaporated thin films of (a) 5b, (b) 1b, and (c) 6b on Si/SiO2
substrates.
the electron accepting layer in 5b-based OPVs. Figure 10a
shows the absorption and EQE spectra the 5b/C70 bilayer film,
which shows much enhanced light absorption and better EQE
in the visible range as expected. As a result, fairly enhanced Jsc
(∼4.59 mA) keeping high Voc (∼0.69 V) and FF (∼0.63) was
achieved, resulting in an overall PCE of 2.0% (Figure 10b).
shape of the present materials is almost straight and rectangular,
but the size of the chalcogenophene rings can slightly alter the
molecular shape that may affect overall packing effectiveness,
resulting in different crystallinity for the three compounds; the
oxygen atoms in furan rings in 5b could intrude from the edge
of the rectangular shaped molecule, whereas the selenium
atoms in 6b protrude vice versa.
Several furan-based materials, including α-oligofurans, furan-
thiophene co-oligomers, and furan-including donor−acceptor
type polymers, have been applied as an active semiconducting
material in OFET devices.18 Some of these materials show
promising device characteristics with high mobility,18c,e
indicating the great potential of furan as a substructure for
electronic materials. The present results on the 5b-based
OFETs can be another example of a superior furan-based
material, and thus, it can be said that the fused-furan structure is
also a promising substructure in the development of new
organic semiconductors for transistor applications. On the
other hand, selenophene-incorporated materials have been also
investigated with expectation that extended molecular orbitals
CONCLUSION
■
In summary, we have successfully synthesized a series of linear-
shaped naphthodichalcogenophenes and evaluated them as
organic semiconductors. In their synthesis, 3,7-dibromo-2,6-
dihydroxynaphthalene (7) was a versatile key intermediate for
both the NDF and the NDS derivatives, although the roles of
the bromine and hydroxyl groups are different. Comparison of
physicochemical properties of 5a, 1a, and 6a clearly showed
similarities of 1a and 6a, and differences between 5a and 1a/6a.
These trends in the molecular properties are basically
understood by the lower aromaticity of furan compared to
those of thiophene and selenophene. On the other hand, the
FET characteristics of the vapor-deposited DPh-NDXs devices
were almost similar to one another; the field-effect mobilties
extracted from the saturation regime are only slightly lower for
5b- and 6b-based devices than that of 1b-based one. The
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Figure 8. Output and transfer characteristics of FETs fabricated on OTS-treated Si/SiO2 substrate: (a) 5b (Tsub = 100 °C), (b) 6b (Tsub = 100 °C).
a
Table 2. Characterization of Evaporated Thin Films, OFET and OPV Devices of 5b, 1b, and 6b
b
c
d
compound
IP /eV
λmax (λedge)/nm
Eg /eV
μFET /cm2 V−1 s−1
Jsc/mA cm−2
Voc/V
FF
η/%
5b
1b
6b
5.4
5.2
5.2
440 (464)
480 (514)
482 (512)
2.67
2.41
2.42
0.6
2.98
3.16
3.03
0.69
0.52
0.46
0.64
0.60
0.59
1.31
0.98
0.83
e
1.5
0.9
a
Obtained by using typical bilayer OPVs with a device structure of ITO/donor/C60/BCP/Al under AM 1.5 G solar simulator (100 mW cm−2)
b
c
d
illumination. Determined by photoemission yield spectroscopy in air. Estimated from absorption edge. Extracted from the saturation regime for
the optimized devices. Reference 6b.
e
Figure 9. J-V characteristics under AM 1.5 G solar simulator (100 mW cm−2) illumination (a) and EQE spectra of ITO/donor/C60/BCP/Al solar
cell (b).
reduced mobilities for the former can mostly be attributed to
miss-oriented molecular ordering in the thin film state
elucidated by thin film XRD measurements. The DPh-NDX/
C60 bilayer films acted as a photoactive interface showing
typical OPV characteristics. Among them, the 5b-based OPV
showed the highest PCE of 1.31%, mainly owing to its large Voc
(0.69 V), originating from its low-lying HOMO energy level.
Although its large HOMO−LUMO gap makes its thin-films
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Figure 10. Absorption spectra of 5b/C70 bilayer film and EQE spectra of the 5b/C70/BCP solar cell (a) and J-V characteristics of the solar cell under
AM 1.5 G solar simulator (100 mW cm−2) illumination (b).
cm−1; Anal. Calcd for C14H10Br2O4: C, 41.84; H, 2.51%. Found: C,
42.14; H, 2.41%.
2,6-Diacetoxy-3,7-bis(trimethylsilylethynyl)naphthalene (9a). To
a deaerated solution of 8 (2.0 g, 5.0 mmol) and diisopropylamine (50
mL) in N,N-dimethylacetamide (50 mL) was added Pd(PPh3)2Cl2
(350 mg, 0.028 mmol, 10 mol %), CuI (190 mg, 0.056 mmol, 20 mol
%), and trimethylsilylacethylene (3.4 mL, 25.0 mmol). After the
transparent in the visible range resulting in limited Jsc in the
bilayer OPVs, its capability to afford relatively large Voc is
advantageous; the combination with C70 as an accepting layer
enables improved OPV performances with a PCE of 2.0%.
These results clearly indicated that fused-furan substructures
are also a useful structural motif in developing electronic
materials for opto/electronic applications such as OFETs and
OPVs. In particular, molecular modifications on the NDF core
that allow smaller HOMO−LUMO gap to improve photo
absorption property while keeping the low-lying HOMO
energy level are the next issue for making further use of the
advantages of the NDF core.
mixture was stirred for 24 h at 60 °C, water (1 mL) and hydrochloric
acid (1M, 100 mL) were subsequently added. The resulting mixture
was extracted with chloroform (100 mL × 2), and the combined
organic layer was dried (MgSO4) and concentrated in vacuo. The
residue was purified by silica-gel column chromatography (eluent,
dichloromethane-hexane, 1:1 v/v) and recrystallization from chloro-
form-ethyl acetate to give 9a (1.55 g, 71%) as a white solid. Mp
1
215.0−215.8 °C; H NMR (400 MHz, CDCl3) δ 0.27 (s, 18H), 2.37
(s, 6H), 7.46 (s, 2H), 7.93 (s, 2H); 13C NMR (100 MHz, CDCl3) δ
169.0, 148.9, 133.2, 131.1, 119.3, 118.1, 101.1, 99.7, 21.0, 0.2; EIMS
(70 eV) m/z = 436 (M+); IR (KBr) ν = 2154 (acetylene), 1769 (C
O) cm−1; Anal. Calcd for C24H28O4Si2: C, 66.02; H, 6.46%. Found: C,
65.91; H, 6.73%.
EXPERIMENTAL SECTION
■
Synthesis. General Procedures. All chemicals and solvents were
of reagent grade unless otherwise indicated. THF and NMP were
purified with a standard procedure prior to use. 3,7-Dibromo-2,6-
d i h y d r o x y n a p h t h a l e n e ( 7 ) , 3 , 7 - d i b r o m o - 2 , 6 - b i s -
(trifluoromethanesulfonyloxy)naphthalene (10), 2,6-dibromo-3,7-bis-
(trimethylsilylethynyl)naphthalene (11a), and 2,6-dibromo-3,7-bis-
(phenylethynyl)naphthalene (11b) were synthesized as reported.6b
Melting points were uncorrected. All reactions were carried out under
nitrogen atmosphere. Nuclear magnetic resonance spectra were
obtained in deuterated chloroform (CDCl3) with TMS as internal
reference unless otherwise stated; chemical shifts (δ) are reported in
parts per million. IR spectra were recorded using a KBr pellet for solid
samples. EI-MS spectra were obtained using an electron impact
ionization procedure (70 eV). The molecular ion peaks of the chlorine,
bromine, or selenium containing compounds showed a typical isotopic
pattern, and all the mass peaks are reported based on 79Br or 80Se,
respectively.
2,6-Diacetoxy-3,7-bis(phenylethynyl)naphthalene (9b). A similar
procedure as above using ethynylbenzene gave the title compound in
63% isolated yield. Mp 250.1−250.6 °C;1H NMR (400 MHz, CDCl3)
δ 2.43 (s, 6H), 7.37−7.40 (m, 6H), 7.52−7.55 (m, 4H), 7.56 (s, 2H),
8.03 (s, 2H); 13C NMR (100 MHz, CDCl3) δ 169.2, 148.7, 132.7,
131.8, 129.0, 128.7, 122.8, 119.4, 118.2, 95.3, 84.6, 21.1; EIMS (70 eV)
m/z = 444 (M+); IR (KBr) ν = 1768 (CO) cm−1; Anal; Calcd for
C30H24O4: C, 81.07; H, 4.54%. Found: C, 80.86; H, 4.30%.
Naphtho[2,3-b:6,7-b′]difuran (5a). To a solution of 9a (0.3 g,
0.688 mmol) in N,N-dimethylacetamide (10 mL) and water (2 mL)
was added Cs2CO3 (2.05 g, 6.88 mmol). After the mixture was stirred
at 80 °C for 24 h, water (20 mL) was added. The resulting mixture was
extracted with dichloromethane (50 mL × 2), and the combined
organic layer was dried (MgSO4) and concentrated in vacuo. The
residue was purified by silica-gel column chromatography (eluent,
hexane) and GPC to give 5a (145 mg, 45%) as a white solid. Mp
2,6-Diacetoxy-3,7-dibromonaphthalene (8). To a suspension of
2,6-dibromo-3,7-dihydroxynaphthalene (7, 31.8 g, 0.10 mol) and
pyridine (20 mL, 0.21 mol) in CH2Cl2 (300 mL) was added acetic
anhydride (50 mL, 0.60 mol) at room temperature (rt). After the
mixture was stirred for 15 h at the same temperature, diluted
hydrochloric acid (1M, 100 mL) was added. The resulting mixture was
separated, and the aqueous layer was extracted with dichloromethane
(100 mL × 2). The combined organic layer was dried (MgSO4) and
concentrated in vacuo, and the resulting residue was purified by
column chromatography on silica-gel eluted with chloroform (Rf 0.5)
to give 8 (39.2 g, 97%) as a white solid. Mp 266.5−267.1 °C;1H NMR
(400 MHz, CDCl3) δ 2.41, (s, 6H), 7.52 (s, 2H), 8.06 (s, 2H); 13C
NMR (100 MHz, CDCl3) δ 168.7, 146.1, 131.9, 131.6, 120.0, 116.8,
20.8; EIMS (70 eV) m/z = 400 (M+); IR (KBr) ν = 1747 (CO)
1
241.5−242.3 °C; H NMR (400 MHz, CDCl3) δ 6.87 (d, J = 2.0 Hz,
2H), 7.72 (d, J = 2.0 Hz, 2H), 8.01 (s, 2H), 8.14 (s, 2H); 13C NMR
(100 MHz, CDCl3) δ 153.2, 147.6, 128.8, 128.4, 118.5, 106.6, 106.2;
EIMS (70 eV) m/z = 208 (M+) ; Anal. Calcd for C14H8O2: C, 80.76;
H, 3.87%. Found: C, 80.58; H, 3.79%.
2,7-Diphenylnaphtho[2,3-b:6,7-b′]difuran (5b). To a solution of
9b (0.3 g, 0.675 mmol) in N,N-dimethylacetamide (10 mL) and water
(2 mL) was added Cs2CO3 (2.01 g, 6.75 mmol). The mixture was
stirred at 80 °C for 24 h and then diluted with water (20 mL). The
resulting precipitate was collected by filtration and washed
subsequently with water, methanol, and CHCl3 and dried in air. The
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Chemistry of Materials
Article
crude product was purified by vacuum sublimation (source temper-
ature: 240 °C at about 1.0 × 10−3 Pa) to give 5b (157 mg, 65%) as a
yellow solid. Mp >300 °C; EIMS (70 eV) m/z = 360 (M+); Anal.
Calcd for C26H16O2: C, 86.65; H, 4.47%. Found: C, 86.54; H, 4.23%.
Naphtho[2,3-b:6,7-b′]diselenophene (6a). To a suspension of
selenium powder (28 mg, 0.35 mmol) in ethanol (1 mL) was added
sodium borohydride (NaBH4, 14 mg, 0.35 mmol) at ice-bath
temperature. After the mixture was stirred for 40 min at the same
temperature, NMP (5 mL) and 2,6-dibromo-3,7-bis-
(trimethylsilylethynyl)naphthalene (11a, 50 mg, 0.1 mmol) was
added. The mixture was heated at 185 °C with distilling out ethanol
using a Dean−Stark condenser for 12 h. After cooling, the mixture was
poured into saturated aqueous ammonium chloride solution (10 mL).
The resulting precipitate was collected by filtration and purified by
vacuum sublimation (source temperature: 190 °C at about 1.0 × 10−3
Pa) to give 6a (10 mg, 30%) as a white solid. Mp >300 °C;1H NMR
(400 MHz, CDCl3) δ 7.66 (d, J = 5.9 Hz, 2H), 7.98 (d, J = 5.9 Hz,
2H), 8.37 (s, 2H), 8.52 (s, 2H); EIMS (70 eV) m/z = 334 (M+); Anal.
Calcd for C14H8Se2: C, 81.07; H, 4.54%. Found: C, 80.87; H, 4.30%.
subsequently deposited, and then Al (1 Å s−1, 100 nm) used as the
cathode was evaporated through a shadow mask with 2.0 mm diameter
openings. Current−voltage (J-V) characteristics of cells were measured
under simulated AM 1.5 G solar illumination (Asahi-Bunko Co., Ltd.)
in air without encapsulation using a Keithley 2400 source meter. ND
filters were used to adjust the light intensity measured with a calibrated
broadband optical power meter (Newport Instruments).
The power conversion efficiency (η) was calculated using
η = (J V FF)/P
oc
o
sc
where Voc is the open circuit voltage, Jsc is the short-circuit
current density, Po is the power of the incident light, and FF is
the fill factor.
ASSOCIATED CONTENT
* Supporting Information
■
S
Instrumentation for the characterization of compounds,
complete entry of reference 14, TD-DFT calculations for 5a,
1a, and 6a, MO calculations for 5b, 1b, and 6b, XRD patterns
of thin films of 5b, 1b, and 6b on ITO substrates, AFM images
of thin films of 5b, 1b, and 6b on Si/SiO2 substrates, FET
characteristics of 5b and 6b, NMR spectra of the synthetic
intermediates. This material is available free of charge via the
Internet at http://pubs.acs.org.
́
2,7-Diphenylnaphtho[2,3-b:6,7-b]diselenophene (6b). To a sus-
pension of selenium powder (282 m, 3.5 mmol) in ethanol (8 mL)
was added sodium borohydride (136 mg, 3.5 mmol) at ice-bath
temperature. After the mixture was stirred for 40 min at the same
temperature, NMP (40 mL) and 2,6-dibromo-3,7-bis(phenylethynyl)-
naphthalene (11b, 486 mg, 1.0 mmol) were added. Then, the mixture
was heated at 185 °C with distilling out ethanol using a Dean−Stark
trap for 12 h. After cooling, the resulting mixture was poured into a
saturated aqueous ammonium chloride solution (50 mL). The
resulting precipitate was collected by filtration, washed with water,
and dried. Continuous extraction from the solid with boiling
chloroform afforded 6b as a pale yellow solid (398 mg, 82%). For
device fabrication, 6b was further purified by vacuum sublimation
(source temperature: 340 °C at about 1.0 × 10−3 Pa). Mp >300 °C;
EIMS (70 eV) m/z = 486 (M+); Anal. Calcd for C26H16Se2: C, 64.21;
H, 3.32%. Found: C, 64.40; H, 3.11%.
Fabrication and Evaluation of FET Devices. OFETs were
fabricated in a “top-contact” configuration 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 octyltrichlorosilane (OTS) or
hexamethyldisilazane (HMDS) as reported previously.6b A thin film of
DPh-NDXs (5b and 6b) as the active layer was vacuum-deposited on
the Si/SiO2 substrates maintained at various temperatures (Tsub) at a
rate of 1 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 50 μm and 1.5 mm,
respectively. Characteristics of the OFET devices were measured at
room temperature under ambient conditions with a Keithley 4200
semiconducting parameter analyzer. Field-effect mobility (μFET) was
calculated in the saturation regime (Vd = −60 V) of the Id using the
following equation,
AUTHOR INFORMATION
Corresponding Author
*E-mail: ktakimi@hiroshima-u.ac.jp.
■
Author Contributions
§These two authors contributed equally to this work.
ACKNOWLEDGMENTS
■
This work was financially supported by Grants-in-Aid for
Scientific Research (No. 23245041) from MEXT, Japan and by
a Founding Program for World-Leading R&D on Science and
Technology (FIRST), Japan. One of the authors (S.S.) is
grateful for the research fellowship for young scientists from
JSPS.
REFERENCES
■
(1) (a) Rogers, J. A.; Bao, Z.; Katz, H. E.; Dodabalapur, A. In Thin-
Film Transistors; Kagan, C. R., Andry, P., Eds.; Marcel Dekker: New
York, 2003; p 377. (b) Organic Electronics, Manufacturing and
Applications; Klauk, H., Ed.; Wiley-VCH: Weinheim, Germany, 2006.
(c) Organic Field-Effect Transistors; Bao, Z., Locklin, J., Eds.; CRC
Press: Boca Raton, FL, 2007. (d) Facchetti, A. Mater. Today 2007, 10,
28−37.
2
I = C μ
(W/2L)(V − V )
d
i
g
th
(2) (a) Dimitrakopoulos, C. D.; Malenfant, P. R. L. Adv. Mater. 2002,
14, 99−117. (b) Murphy, A. R.; Frechet, J. M. J. Chem. Rev. 2007, 107,
1066−1096. (c) Klauk, H. Chem. Soc. Rev. 2010, 39, 2643−2666.
(d) Facchetti, A. Chem. Mater. 2011, 23, 733−758.
(3) (a) Anthony, J. E. Chem. Rev. 2006, 106, 5028−5048.
(b) Anthony, J. E. Angew. Chem., Int. Ed. 2008, 47, 452−483.
(c) Takimiya, K.; Shinamura, S.; Osaka, I.; Miyazaki, E. Adv. Mater.
2011, 23, 4347−4370.
(4) (a) Yoo, S.; Domercq, B.; Kippelen, B. Appl. Phys. Lett. 2004, 85,
5427−5429. (b) Lloyd, M. T.; Mayer, A. C.; Subramanian, S.; Mourey,
D. A.; Herman, D. J.; Bapat, A. V.; Anthony, J. E.; Malliaras, G. G. J.
Am. Chem. Soc. 2007, 129, 9144−9149. (c) Yang, J.; Nguyen, T.-Q.
Org. Electron. 2007, 8, 566−574.
(5) (a) Laquindanum, J. G.; Katz, H. E.; Lovinger, A. J.; Dodabalapur,
A. Adv. Mater. 1997, 8, 36−39. (b) Takimiya, K.; Kunugi, Y.; Konda,
Y.; Niihara, N.; Otsubo, T. J. Am. Chem. Soc. 2004, 126, 5084−5085.
(c) Takimiya, K.; Kunugi, Y.; Ebata, H.; Otsubo, T. Chem. Lett. 2006,
35, 1200−1201. (d) Kashiki, T.; Miyazaki, E.; Takimiya, K. Chem. Lett.
FET
where Ci is the capacitance of the SiO2 insulator, and Vg and Vth
are the gate and threshold voltages, respectively. Current on/off
ratio (Ion/Ioff) was determined from the Id at Vg = 0 V (Ioff) and
Vg = −60 V (Ion). The μFET data reported are typical values
from more than 10 different devices.
Fabrication and Evaluation of OPV Devices. Standard bilayer
solar cells were fabricated by using sequential vacuum depositions on
ITO-coated glass substrates.21 Twice vacuum sublimed DPh-NDXs
(5b, 1b or 6b), C60, and BCP in our laboratory were used. The ITO
substrates (110 nm thickness on glass, sheet registance <20 Ω
square−1) were successively washed by ultrasonication in a neutral
detergent, deionized water, distilled water, acetone, and isopropanol
for 10 min. Then, they were treated with UV ozone for 20 min
immediately prior to loading into a vacuum chamber (∼3 × 10−3 Pa).
Thin films of DPh-NDXs (deposition rate: 1 Å s−1, 40 nm), C60
(deposition rate: 2 Å s−1, 30 nm), and BCP (2 Å s−1, 10 nm) were
197
dx.doi.org/10.1021/cm202853b | Chem. Mater. 2012, 24, 190−198

Chemistry of Materials
Article
2008, 37, 284−285. (e) Kashiki, T.; Miyazaki, E.; Takimiya, K. Chem.
Lett. 2009, 38, 568−569. (f) Laquindanum, J. G.; Katz, H. E.;
Lovinger, A. J. J. Am. Chem. Soc. 1998, 120, 664−672. (g) Payne, M.
M.; Parkin, S. R.; Anthony, J. E.; Kuo, C.-C.; Jackson, T. N. J. Am.
Chem. Soc. 2005, 127, 4986−4987. (h) Dickey, K. C.; Anthony, J. E.;
Loo, Y. L. Adv. Mater. 2006, 18, 1721−1726. (i) Chen, M.-C.; Kim, C.;
Chen, S.-Y.; Chiang, Y.-J.; Chung, M.-C.; Facchetti, A.; Marks, T. J. J.
Mater. Chem. 2008, 18, 1029−1036.
(6) (a) Shinamura, S.; Miyazaki, E.; Takimiya, K. J. Org. Chem. 2010,
75, 1228−1234. (b) Shinamura, S.; Osaka, I.; Miyazaki, E.; Nakao, A.;
Yamagishi, M.; Takeya, J.; Takimiya, K. J. Am. Chem. Soc. 2011, 133,
5024−5035.
(21) Yi, Y.; Coropceanu, V.; Bredas, J.-L. J. Am. Chem. Soc. 2009, 131,
15777−15783.
(22) (a) Mutolo, K. L.; Mayo, E. I.; Rand, B. P.; Forrest, S. R.;
Thompson, M. E. J. Am. Chem. Soc. 2006, 128, 8108−8109.
(b) Kinoshita, Y.; Hasobe, T.; Murata, H. Appl. Phys. Lett. 2007, 91,
083518. (c) Perez, M. D.; Borek, C.; Forrest, S. R.; Thompson, M. E. J.
Am. Chem. Soc. 2009, 131, 9281−9286. (d) Mori, H.; Takimiya, K.
Appl. Phys. Express 2011, 4, 061602. (e) Brian, E. L.; Guodan, W.; Siyi,
W.; Jeramy, D. Z.; Viacheslav, V. D.; Mark, E. T.; Stephen, R. F. Appl.
Phys. Lett. 2011, 98, 243307.
(23) Sakai, J.; Taima, T.; Yamanari, T.; Saito, K. Sol. Energy Mater.
Sol. Cell 2009, 93, 1149−1153.
(7) (a) Osaka, I.; Abe, T.; Shinamura, S.; Miyazaki, E.; Takimiya, K. J.
Am. Chem. Soc. 2010, 132, 5000−5001. (b) Osaka, I.; Abe, T.;
Shinamura, S.; Takimiya, K. J. Am. Chem. Soc. 2011, 133, 6852−6860.
(8) Loser, S.; Bruns, C. J.; Miyauchi, H.; Ortiz, R. P.; Facchetti, A.;
Stupp, S. I.; Marks, T. J. J. Am. Chem. Soc. 2011, 133, 8142−8145.
(9) (a) Tsuji, H.; Mitsui, C.; Ilies, L.; Sato, Y.; Nakamura, E. J. Am.
Chem. Soc. 2007, 129, 11902−11903. (b) Li, H.; Jiang, P.; Yi, C.; Li,
C.; Liu, S.-X.; Tan, S.; Zhao, B.; Braun, J.; Meier, W.; Wandlowski, T.;
Decurtins, S. Macromolecules 2010, 43, 8058−8062.
(10) (a) Takimiya, K.; Kunugi, Y.; Konda, Y.; Ebata, H.; Toyoshima,
Y.; Otsubo, T. J. Am. Chem. Soc. 2006, 128, 3044−3050. (b) Izawa, T.;
Miyazaki, E.; Takimiya, K. Chem. Mater. 2009, 21, 903−912.
(c) Mishra, S. P.; Javier, A. E.; Zhang, R.; Liu, J.; Belot, J. A.; Osaka,
I.; McCullough, R. D. J. Mater. Chem. 2011, 21, 1551−1561.
(11) (a) Dai, W.-M.; Lai, K. W. Tetrahedron Lett. 2002, 43, 9377−
9380. (b) Hayashi, N.; Saito, Y.; Higuchi, H.; Suzuki, K. J. Phys. Chem.
A 2009, 113, 5342−5347.
(12) Kashiki, T.; Shinamura, S.; Kohara, M.; Miyazaki, E.; Takimiya,
K.; Ikeda, M.; Kuwabara, H. Org. Lett. 2009, 11, 2473−2475.
(13) (a) Pommerehne, J.; Vestweber, H.; Guss, W.; Mahrt, R. F.;
Bassler, H.; Porsch, M.; Daub, J. Adv. Mater. 1995, 7, 551−554.
̈
(b) Johansson, T.; Mammo, W.; Svensson, M.; Andersson, M. R.;
Inganas, O. J. Mater. Chem. 2003, 13, 1316−1323. (c) Cardona, C. M.;
Li, W.; Kaifer, A. E.; Stockdale, D.; Bazan, G. C. Adv. Mater. 2011, 23,
2367−2371.
(14) MO calculations were carried out with the DFT/TD-DFT
method at the B3LYP/6-31g(d) level using Gaussian 03 program
package. Frisch, M. J. et al. Gaussian 03, revision C.02; Gaussian, Inc.:
Wallingford, CT, 2004.
(15) (a) Cordell, F. R.; Boggs, J. E. J. Mol. Structure: THEOCHEM
1981, 85, 163−178. (b) Juric, A.; Sabljic, A.; Trinajstic, N. J. Heterocycl.
Chem. 1984, 21, 273−282.
(16) Yang, R.; Tian, R.; Hou, Q.; Zhang, Y.; Li, Y.; Yang, W.; Zhang,
C.; Cao, Y. J. Polym. Sci., Part A: Polym. Chem. 2005, 43, 823−836.
́
(17) (a) Bredas, J.-L.; Beljonne, D.; Coropceanu, V.; Cornil, J. Chem.
Rev. 2004, 104, 4971−5004. (b) Coropceanu, V.; Cornil, J.; da Silva
Filho, D. A.; Olivier, Y.; Silbey, R.; Bredas, J.-L. Chem. Rev. 2007, 107,
926−952.
́
(18) (a) Miyata, Y.; Terayama, M.; Minari, T.; Nishinaga, T.;
Nemoto, T.; Isoda, S.; Komatsu, K. Chem.Asian J. 2007, 2, 1492−
1504. (b) Bijleveld, J. C.; Karsten, B. P.; Mathijssen, S. G. J.; Wienk, M.
M.; de Leeuw, D. M.; Janssen, R. A. J. J. Mater. Chem. 2011, 21, 1600−
1606. (c) Li, Y.; Sonar, P.; Singh, S. P.; Zeng, W.; Soh, M. S. J. Mater.
2011, 21, 10829−10835. (d) Gidron, O.; Diskin-Posner, Y.; Bendikov,
M. J. Am. Chem. Soc. 2010, 132, 2148−2150. (e) Gidron, O.; Dadvand,
A.; Sheynin, Y.; Bendikov, M.; Perepichka, D. F. Chem. Commun.
2011, 47, 1976−1978.
(19) (a) Kunugi, Y.; Takimiya, K.; Yamane, K.; Yamashita, K.; Aso,
Y.; Otsubo, T. Chem. Mater. 2003, 15, 6−7. (b) Heeney, M.; Zhang,
W.; Crouch, D. J.; Chabinyc, M. L.; Gordeyev, S.; Hamilton, R.;
Higgins, S. J.; McCulloch, I.; Skabara, P. J.; Sparrowe, D.; Tierney, S.
Chem. Commun. 2007, 5061−5063. (c) Ha, J. S.; Kim, K. H.; Choi, D.
H. J. Am. Chem. Soc. 2011, 133, 10364−10367. (d) Al-Hashimi, M.;
Baklar, M. A.; Colleaux, F.; Watkins, S. E.; Anthopoulos, T. D.;
Stingelin, N.; Heeney, M. Macromolecules 2011, 44, 5194−5199.
(20) Tang, C. W. Appl. Phys. Lett. 1986, 48, 183−185.
198
dx.doi.org/10.1021/cm202853b | Chem. Mater. 2012, 24, 190−198