Single-crystal organic field-effect transistors of naphthodifurans

Article abstract of DOI:10.1246/bcsj.20150033

Three types of naphthodifurans (NDFs) have been designed as active organic materials in organic field-effect transistors (OFETs), and one of them, naphtho[2,1-b:6,5-b′]difuran, has been found to show an excellent OFET mobility of up to 3.6 cm²V

Full text of DOI:10.1246/bcsj.20150033

BCSJ Award Article
Single-Crystal Organic Field-Effect Transistors of Naphthodifurans
Chikahiko Mitsui,1,2 Junshi Soeda,3 Kazumoto Miwa,3 Kazutaka Shoyama,1
Yoshinori Ota,1 Hayato Tsuji,*1,4 Jun Takeya,*2,3 and Eiichi Nakamura*1
1Department of Chemistry, School of Science, The University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113-0033
2Department of Frontier Sciences, The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8561
3ISIR, Osaka University, 8-1 Mihogaoka, Ibaraki, Osaka 567-0047
4Japan Science and Technology Agency, PRESTO, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012
E-mail: nakamura@chem.s.u-tokyo.ac.jp
Received: January 26, 2015; Accepted: February 25, 2015; Web Released: June 15, 2015
Three types of naphthodifurans (NDFs) have been designed as active organic materials in organic field-effect
transistors (OFETs), and one of them, naphtho[2,1-b:6,5-b¤]difuran, has been found to show an excellent OFET mobility of
up to 3.6 cm2 V¹1 s¹1. Based on the crystal-packing structure and theoretical analyses, we attribute the high carrier mobility
of the NDF derivatives to large intermolecular interactions and small reorganization energies. The NDF compounds show
very high thermal and electrochemical stability. Together with some previous studies, the present study indicates that fused
furan compounds show a performance that is equal to or better than that of the corresponding thiophene compounds.
Thiophene derivatives are popular organic semiconductor
materials for the fabrication of printed plastic electronics, and
research on organic field-effect transistors (OFETs) is no excep-
tion.1 In contrast, furan derivatives had rarely been exam-
ined for device applications2,3 until we explored the p-type
and ambipolar properties of benzodifuran (BDF) derivatives
(Figure 1)4-7 to find their utility in hetero- and homojunction
organic light-emitting diode (OLED) devices. Encouraged by
the results, we envisioned that fused furan are also useful for
single-crystal OFETs, because of the small van der Waals radius
of the oxygen atom that permits denser packing of the molecules
in a single crystal than seen for thiophenes. Considering strong
intermolecular electronic coupling caused by dense packing,
along with a small reorganization energy because of the effec-
tive π-electron conjugation over the fused aromatic framework,
the carrier mobility of the single crystals of these molecules may
be significantly enhanced.8 Herein we report the synthesis of
three classes of naphthodifuran derivatives (Figure 1), naph-
tho[2,1-b:6,5-b¤]difuran (NDF1),9 naphtho[1,2-b:5,6-b¤]difuran
(NDF2), and naphtho[2,3-b:6,7-b¤]difuran (NDF3), and also the
excellent charge-transporting properties of one of the NDF1
compounds in a single-crystal OFET10 device. We prepared
single crystals for the devices by a physical vapor transport
(PVT)11 method and by a solution process. The highest hole
mobility that we recorded was 3.6 cm2 V¹1 s¹1: a value among
the highest for solution-processed OFETs. These values are
much higher than those recently reported for nonfused furan
derivatives, such as oligofurans, which showed a mobility value
of less than 0.3 cm2 V¹1 s¹1 7q,12,13 and suggests that the fusion
,
of the furan rings to the rigid naphthalene structure is an
important structural feature of the new NDF materials.14
Experimental
All of the reactions dealing with air- or moisture-sensitive
compounds were carried out in a dry reaction vessel under a
positive pressure of nitrogen or argon. Air- and moisture-
sensitive liquids and solutions were transferred via a syringe or
a Teflon cannula. Flash column chromatography was performed
employing Kanto Silica gel 60 (spherical, neutral, 140-325
mesh).
Proton nuclear magnetic resonance (1H NMR) and carbon
nuclear magnetic resonance (13C NMR) spectra were recorded
on JEOL ECA-500 (500 MHz) and JEOL ECA-600 (600 MHz)
NMR spectrometers. High-vacuum train sublimation (pressure
O
O
O
O
O
O
BDFs
O
O
O
O
O
O
NDF2
NDF1
NDF3
Figure 1. Structures of benzodifurans (BDFs) and
naphthodifurans (NDFs).
776 | Bull. Chem. Soc. Jpn. 2015, 88, 776–783 | doi:10.1246/bcsj.20150033
© 2015 The Chemical Society of Japan

R
(a)
R
1) BuLi/hexane
THF
O
OH
0 °C to rt
HO
O
2) ZnCl2, toluene
120 °C
3) H2O
1a: R = H
1b: R = n-C8H17
DPNDF1: R = H (97%)
C8-DPNDF1: R = n-C8H17 (96%)
R
R
(b)
(c)
1) BuLi/hexane
THF
OH
O
0 °C to rt
2) ZnCl2, toluene
120 °C
3) H2O
O
OH
2
DPNDF2 (86%)
1) BuLi/hexane
THF
0 °C to rt
OH
O
O
2) ZnCl2, toluene
120 °C
3) H2O
HO
3
DPNDF3 (65%)
Scheme 1.
below 30 mTorr) was performed on a P-100HT instrument
(ALS Technology Co., Ltd.). Melting points of solid materials
were determined on a Mel-Temp II capillary melting-point
apparatus and are uncorrected. Mass spectra were obtained on a
JEOL Accu TOF JMS-T100LC instrument. Cyclic voltamme-
try (CV) measurements were performed on a HOKUTO
DENKO HZ-5000 voltammetric analyzer at room temperature
in a one-compartment cell under N2 gas, equipped with a
glassy-carbon working electrode, a platinum wire counter elec-
trode, and an Ag/Ag+ reference electrode. Thermogravimetry-
differential thermal analysis (TG-DTA) was performed on a
Rigaku Thermo plus 2 thermal analyzer (TG 8120).
(hexane:CHCl3 = 90:10) to afford the title compound (168
mg, 96%) as a white powder. Mp: 302-303 °C. H NMR (600
1
MHz, CDCl3): ¤ 0.89 (t, J = 7.2 Hz, 6H), 1.26-1.39 (m, 20H),
1.67 (q, J = 7.2 Hz, 4H), 2.65 (t, J = 7.2 Hz, 4H), 7.30 (d,
J = 8.4 Hz, 4H, C6H4), 7.50 (s, 2H), 7.79 (d, J = 9.0 Hz, 2H),
7.86 (d, J = 8.4 Hz, 4H), 8.04 (d, J = 9.0 Hz, 2H). 13C NMR
(150 MHz, CDCl3): ¤ 14.1, 22.7, 29.3, 29.4, 29.5, 31.4,
31.9, 35.9, 99.9, 112.0, 120.1, 124.2, 124.8, 125.9, 128.2,
128.9, 143.5, 151.8, 156.0. MS (APCI+): 585 (M + H+).
Anal. Calcd for C42H48O2: C, 86.26; H, 8.27%. Found: C,
86.34; H, 8.23%.
2,7-Diphenylnaphtho[2,1-b:6,5-b¤]difuran
(DPNDF1);
1
Synthesis. General Procedure for the Zinc-Mediated
White powder. Mp: 318-319 °C. H NMR (600 MHz, CDCl3):
¤ 7.37 (t, J = 7.2 Hz, 2H), 7.49 (dd, J = 7.2, 7.2 Hz, 4H), 7.57
(s, 2H), 7.81 (d, J = 7.8 Hz, 2H), 7.96 (d, J = 7.2 Hz, 4H), 8.07
(d, J = 7.8 Hz, 2H). 13C NMR (125 MHz, CDCl2CDCl2): ¤
99.7, 111.4, 119.6, 123.1, 123.9, 124.8, 127.7, 128.1, 129.5,
150.9, 154.7. MS (APCI+): 361 (M + H+). Anal. Calcd for
C26H16O2: C, 86.65; H, 4.47%. Found C, 86.35; H, 4.81%.
Double Cyclization Reaction:
2,7-Bis(4-octylphenyl)-
naphtho[2,1-b:6,5-b¤]difuran (C8-DPNDF1); The general
synthetic strategy is shown in Scheme 1 and discussed in the
following section.
A solution of butyllithium in hexane (0.37 mL, 1.62 mol L
¹1
,
0.60 mmol) was added to a solution of 1,5-bis[(4-octylphenyl)-
ethynyl]naphthalene-2,6-diol (1b, 175 mg, 0.300 mmol) in THF
(1.2 mL) at 0 °C. The resulting yellow suspension was allowed
to warm to ambient temperature and stirred for 30 min. After a
solution of zinc chloride in THF (0.60 mL, 1.0 mol L¹1, 0.60
mmol) was added, the volatiles were removed in vacuo, and
then toluene (1.2 mL) was added. The resulting mixture was
heated to 120 °C and stirred for 3 h at this temperature. After
cooling to ambient temperature, H2O was added to quench the
reaction. The organic layer was extracted with CHCl3 and dried
over MgSO4. After removal of solvent in vacuo, the crude
material was purified by silica gel column chromatography
2,7-Diphenylnaphtho[1,2-b:5,6-b¤]difuran
(DPNDF2);
1
White solid. Mp: 283-284 °C. H NMR (500 MHz, CDCl3): ¤
7.21 (s, 2H), 7.37 (t, J = 7.7 Hz, 2H), 7.50 (dd, J = 7.7, 7.7 Hz,
4H), 7.80 (d, J = 8.3 Hz, 2H), 7.97 (d, J = 7.7 Hz, 4H), 8.25
(d, J = 8.3 Hz, 2H). 13C NMR (125 MHz, CDCl3): ¤ 102.7,
116.0, 119.0, 120.1, 124.7, 124.8, 128.3, 128.9, 130.9, 151.2,
155.5. MS (TOF, APCI+): 361.13 (M + H+). Anal. Calcd for
C26H16O2: C, 86.65; H, 4.47%. Found: C, 86.53; H, 4.59%.
2,7-Diphenylnaphtho[2,3-b:6,7-b¤]difuran
(DPNDF3);
Yellow solid. Mp: Not available because of sublimation.
1H NMR (500 MHz, CDCl2CDCl2, 120 °C): ¤ 7.16 (s, 2H),
Bull. Chem. Soc. Jpn. 2015, 88, 776–783 | doi:10.1246/bcsj.20150033
© 2015 The Chemical Society of Japan | 777

(a) DPNDF1
(b) DPNDF2
(c) DPNDF3
/
/
/
/
/
/
Figure 2. Performances of OFET devices using NDFs. (a) DPNDF1, (b) DPNDF2, and (c) DPNDF3. Transfer characteristics are
shown in the left column (inset: photograph of device), and the output characteristics are shown in the right column.
7.44 (t, J = 7.6 Hz, 2H), 7.52 (dd, J = 7.6, 7.6 Hz, 4H), 7.98
(d, J = 7.6 Hz, 4H), 8.03 (s, 2H), 8.13 (s, 2H). 13C NMR could
not be measured because of the poor solubility. MS (TOF,
APCI+): 360.09 (M + H+). Anal. Calcd for C26H16O2: C,
86.65; H, 4.47%. Found: C, 86.42; H, 4.67%.
OFET Fabrication and Evaluation. Fabrication Proce-
dure of Laminated Single-Crystal OFET Device with
Bottom-Gate-Bottom-Contact Architecture (Device A):
This type of device was fabricated for DPNDF1 and DPNDF3
because these compounds afforded platelet single crystals by
the physical-vapor transport (PVT) method.
cleaned substrate was covered with perfluorotriethoxysilane (F-
SAM).15 On the surface-treated substrate, chromium and gold
were successively deposited under vacuum using a shadow
mask to thicknesses of 3 and 27 nm, respectively. A thin single
crystal of the PVT-grown DPNDF was attached electrostati-
cally to the prepared substrate to construct a bottom-contact
single-crystal transistor. A picture of a representative device is
given in Figure 2a. The doped-Si layer acts as a gate electrode
that has a dielectric constant of 3.9. Electrical characterization
was performed using a semiconductor parameter analyzer
(Keithley 4200). Field-effect mobility (®) values were esti-
mated in the saturation regime using eq 1:
A substrate that consists of a 500-nm thick thermally oxi-
dized SiO2 surface on n-doped Si was consecutively ultra-
sonicated with acetone and 2-propanol each for 5 min. The
2
Id ¼ ðWCi=2LÞ®ðVg ꢀ VthÞ
ð1Þ
778 | Bull. Chem. Soc. Jpn. 2015, 88, 776–783 | doi:10.1246/bcsj.20150033
© 2015 The Chemical Society of Japan

Table 1. OFET Characteristics and Properties of DPNDFs
®
Vth
/Vb)
Eox
/Vc)
Tm
/°Cd)
T95%
/°Ce)
Compound
Device type
Ion/Ioff ratio
¹1 a)
/cm2 V¹1 s
DPNDF1
A
C
C
B
A
0.8-1.3
0.6-1.0
1.5-3.6
0.1-0.2
0.4-0.6
¹10
¹50
¹100
0
104-105
104-105
104-105
103-104
104-105
0.68
302-303
322
C8-DPNDF1
DPNDF2
DPNDF3
0.62
0.70
0.63
318-319
283-284
ND (sub)f)
404
307
369
10
a) OFET mobility in the saturated regime. b) Threshold voltage of the OFET device. c) Oxidation potential (vs. Fc/Fc+). d) Melting
point. e) 5% weight loss temperature determined using TG-DTA. f) Not determined because of sublimation.
where Id is the source-drain current, W and L are the channel
width and length, respectively, Ci is the capacitance of the SiO2
insulator and Vth is the threshold voltage.
and 3, which were synthesized from α,α¤- and β,β¤-naphthalene-
diol, respectively (Schemes 1b and 1c). Details are shown in
Supporting Information.
Fabrication Procedure of a Single-Crystal OFET Device
with Top-Gate-Top-Contact Architecture Using a Parylene
OFETs Using PVT-Grown DPNDF Single Crystals
(Devices A and B). Performance of Single-Crystal OFETs:
We employed two types of device structure depending on the
shape of PVT-grown single crystals. As to the platelet and
flexible DPNDF1 and DPNDF3, the single crystals could
be manually laminated on the substrates with patterned gold
electrodes to form a bottom-gate-bottom-contact structure
(device A). On the other hand, DPNDF2, forming the sturdy
needle shaped crystal, is not suitable for FET evaluation in the
form of the device A, due to the imperfect contact on the sub-
strate. Thus, the single crystal transistors based on DPNDF2
were fabricated in top-gate-top-contact structure using carbon-
paste electrodes and a parylene gate dielectric (device B). The
OFET performances of the DPNDF-based devices are shown in
Figure 2 and summarized in Table 1. These devices showed
hole-transporting characteristics. The transfer characteristics of
DPNDF1 and DPNDF3 showed slight hysteresis (Figures 2a
and 2c), and that using DPNDF2 showed no hysteresis
(Figure 2b). This fact suggests that carrier injection from the
electrode to the organic material occurs without a significant
barrier. Among OFET devices using the three types of PVT-
grown DPNDFs, the bottom-gate-bottom-contact devices us-
ing platelet crystals of DPNDF1 showed the highest hole
mobility. The calculated hole mobility of the DPNDF1-based
Gate Insulator (Device B):
This type of device was
fabricated for DPNDF2, which afforded needle crystals by
PVT. On the top of the PVT-grown single crystals, source and
drain electrodes were drawn using carbon paste, and gold wire
was attached to each electrode. Parylene SR was deposited as a
dielectric with relative dielectric constant of 3.0 (800 nm thick-
ness) on the processed crystal. Afterward, carbon paste was
drawn as a gate electrode on the surface of the parylene-coated
single crystals. A picture of a representative device is given in
Figure 2b. The OFET performances were evaluated in a similar
manner to that used for device A.
Fabrication Procedure of a Solution-Crystallized OFET
Device with Bottom-Gate-Top-Contact Architecture
(Device C) by the Oriented Crystal-Growth Method:16
This type of device was fabricated for DPNDF1 and C8-
DPNDF1 because they have acceptable solubility. The fab-
rication process using C8-DPNDF1 is described as a represen-
tative example.
On the surface of the cleaned Si/SiO2 (same as the one used
for device A), decyltriethoxysilane (DTS) was vapor-deposited.
A droplet of 0.2-wt % solution of C8-DPNDF1 in chloro-
benzene was placed at room temperature at an edge of a
solution-sustaining piece on a slightly inclined substrate to grow
a crystalline domain in the direction of the inclination through
evaporation of the solvent. The source and drain gold electrodes
were then vacuum-deposited on the organic film by the use of
a shadow mask. The OFET performance was evaluated in a
manner similar to that used for device A.
¹1
device was up to 1.3 cm2 V¹1 s in the saturation regime with
an Ion/Ioff ratio of 105 and a threshold voltage of ¹10 V. The
DPNDF3-based devices having the same configuration showed
¹1
hole mobilities of 0.4-0.6 cm2 V¹1 s with Ion/Ioff ratios of
104-105 and a threshold voltage of 10 V. The top-gate-top-
contact devices using a needle crystal of DPNDF2 (type B)
showed a low threshold voltage (0 V) and somewhat lower
mobility (0.1-0.2 cm2 V¹1 s¹1) and Ion/Ioff ratios (103-104) than
the other NDFs.
Results and Discussion
Synthesis of NDFs. We synthesized the NDFs by a zinc-
mediated intramolecular cyclization reaction (Scheme 1).4,17
Thus, dialkynes 1a and 1b synthesized from β,β¤-naphthalene-
diol (syntheses of these compounds are shown in Supporting
Information) were deprotonated with butyllithium, converted
to the corresponding zinc salt by treatment with zinc chloride,
heated at 120 °C, and the reaction mixture was quenched
with water to obtain 2,7-diphenylnaphtho[2,1-b:6,5-b¤]difuran
(DPNDF1) and 2,7-bis(p-octylphenyl)naphtho[2,1-b:6,5-b¤]-
difuran (C8-DPNDF1), respectively, in excellent yield
(Scheme 1a). Other diphenyl-substituted NDFs, DPNDF2 and
DPNDF3, were prepared in the same manner from dialkynes 2
Packing Structures in Single Crystal and Theoretical
Calculations on Carrier-Transporting Abilities: To obtain
some insights into the difference in the mobility among three
compounds, we revealed the packing structures of DPNDFs by
means of X-ray single crystal structural analyses (Figure 3b).18
DPNDF1 and DPNDF3 were found to represent layer-by-
layer structures with the herringbone-packing motifs, which are
preferable for two-dimensional carrier transport. In contrast,
DPNDF2 stacks in slipped herringbone packing manner with
a large tilt angle of 129° and thus forms a one-dimensional
stacking arrangement. We then calculated and compared the
Bull. Chem. Soc. Jpn. 2015, 88, 776–783 | doi:10.1246/bcsj.20150033
© 2015 The Chemical Society of Japan | 779

(a)
(b)
(c)
Figure 3. Crystal structures of DPNDFs. (a) Molecular structures and reorganization energies (-), (b) Packing structures, (c)
Herringbone structures and calculated electronic coupling elements (V, in meV).
electronic coupling elements (V)19 among HOMOs in the
crystal, because their reorganization energies for hole forma-
tion (-) based on Marcus theory20 are small and comparable
(Figure 3a). In crystal, DPNDF1 and DPNDF3 exhibited a
herringbone-type packing with the different two-dimensional
interactions among the molecules as judged by the V values
(Figure 3c). DPNDF1 possesses well-balanced and moderate
V values of 27.7 and 39.9 meV for stacking and transverse
directions, respectively. On the other hand, DPNDF3 shows
anisotropic and smaller V values; only 6.5 meV for the stacking
direction and 40.3 and 20.8 meV for two transverse directions.
Thus, the OFET based on DPNDF1 recorded higher mobility
than that of DPNDF3. Here the reorganization energies for
hole formation (-) in DPNDF1 and DPNDF3 are also small,
as small as 178 and 199 meV, respectively, as calculated for a
single molecule using the B3LYP/6-31G* level of theory.21
The origin of these smaller values than those for α-oligofurans
(237-370 meV),22 can be ascribed to the effective π-electron
conjugation over the rigid fused aromatic framework, which
supports their high carrier transporting abilities. In contrast to
these compounds, DPNDF2 exhibits a one-dimensional col-
umnar packing in a single crystal, and shows highly anisotropic
V values of which one is very large (105 and 2.2 meV for the
stacking and the transverse directions, respectively). An OFET
using such a highly anisotropic crystal is known to be sensitive
to the quality of the specimen crystal,10 and the inhomogeneity
in a single crystal (e.g., impurity or defect in a crystal) creates
carrier traps, which often limit the measured mobility.7l This
may be one of the origins of the lower mobility of the OFETs
using needle crystals of DPNDF2.
Solution-Processed OFETs Using DPNDF1 and C8-
DPNDF1 (Device C). The solution-processed OFETs
(device C) using a solution of DPNDF1 or C8-DPNDF1 were
found to show high hole mobilities (Figure 4 for an OFET
using C8-DPNDF1. Data are summarized in Table 1). The hole
mobilities are 0.6-1.0 and 1.5-3.6 cm2 V¹1 s¹1, for DPNDF1
and C8-DPNDF1, respectively, in the saturation regime with a
high Ion/Ioff ratio of 104-105. These FET mobilities are among
the highest for solution-processed OFETs.
An effective intermolecular interaction in the crystal state as
well as the homogeneity of the solution-grown crystals of C8-
DPNDF1 was verified by X-ray diffraction (XRD) and atomic
force microscopy (AFM) analyses (Figure 5). The crystal pack-
ing of C8-DPNDF1 was characterized as a herringbone packing
with highly anisotropic electronic coupling elements of 59.3 and
0.3 meValong the stacking and the transverse directions, respec-
tively (Figure 5a). Transmission XRD measurements revealed
the molecular arrangement (See SI). By the out-of-plane XRD
measurement, diffraction peaks corresponding to (001) and
(002) were observed, suggesting that the C8-DPNDF1 stands
on the substrate and forms lamella-like layer-by-layer structure
and the carrier conduction a-b plane is parallel to the substrate.
On the basis of the in-plane XRD for a C8-DPNDF1 film, the
crystal-growth direction, along which the source and drain
channels were constructed, was not entirely parallel to the
molecular stacking direction (b axis) but tilts ca. 20-30° against
the b axis, that is, not the most deal orientation for efficient car-
rier transportation. We consider that the mobility data obtained
here (3.6 cm2 V¹1 s¹1) can be improved further in the future by
controlled installation of the source and drain channels.
780 | Bull. Chem. Soc. Jpn. 2015, 88, 776–783 | doi:10.1246/bcsj.20150033
© 2015 The Chemical Society of Japan

(a)
(b)
/
/
Figure 4. Performances of solution-processed OFET devices using C8-DPNDF1. (a) Transfer characteristics (inset: photograph of
device) and (b) Output characteristics.
(a)
(b)
0.3
59.3
~2.4 nm
c
a
(c)
(d)
0 nm
10.96 nm
2 µm
~2.4 nm
Distance/µm
Figure 5. Molecular orientation and homogeneity of a solution-grown single crystal of C8-DPNDF1. (a) Packing structure.
(b) Schematic illustration of the crystal structure. (c) Top view of the AFM image (inset: bird’s-eye view). (d) Cross-sectional
profile along the white dotted line in (c).
Such high apparent mobility of the C8-DPNDF1-based
OFET device can be attributed to the homogeneity of the C8-
DPNDF1 crystal as confirmed by AFM analyses (Figures 5b-
5d). The AFM image of the C8-DPNDF1 crystal showed a
molecularly flat area extending to a micrometer area with only
a few steps each with a height of ca. 2.4 nm, which corresponds
to a single molecular layer supported by the out-of-plane XRD
measurement. From the diffraction peak at 2ª = 4.825° (002),
the d spacing is calculated to be 2.37 nm. Considering that the
molecular length is 3.7 nm based on the X-ray single crystal
analysis, we concluded that the molecular long axis stands on
the substrate with a tilt angle of ca. 40°.
Thermal and Electrochemical Stabilities. Thermal and
electrochemical stability are also important factors for materi-
als. Indeed, DPNDFs are found to be stable (Table 1): both
the melting (Tm) and 5%-weight loss temperatures (T95%) are
higher than 300 °C (see Figure S1 in Supporting Information).
Unlike those of the nonfused furans,7q,12,13 cyclic voltammo-
graphic data for DPNDFs indicated reversible oxidation waves
(Figure S2). The oxidation potential of DPNDFs ranges from
0.62 to 0.70 V (vs. Fc/Fc+), from which we estimate the
HOMO energy level to be between ¹5.42 and ¹5.50 eV.23
These HOMO energy levels allow smooth hole injection from
the gold electrode to DPNDF, and work as p-type organic
Bull. Chem. Soc. Jpn. 2015, 88, 776–783 | doi:10.1246/bcsj.20150033
© 2015 The Chemical Society of Japan | 781

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semiconductors. We consider that the observed stability stems
from the rigid and extended π-conjugated framework of the
naphthalene-fused furan core.
Conclusion
In conclusion, we have designed and synthesized a new class
of p-type semiconducting materials: naphtho-fused furan com-
pounds (NDFs), and found that they show high OFET mobility
and high thermal and electrochemical stability. The hole
¹1
mobility data of up to 3.6 cm2 V¹1 s has been recorded for a
solution-grown single-crystal device using C8-DPNDF1. These
data rival those of the other known organic semiconductors.
The present OFET data suggest that solution-processable
devices can surpass the performance of amorphous silicon
devices. We consider that the present data provide further
support for our initial conjecture that furan compounds rival
the corresponding thiophene compounds for organic electronics
devices.
We thank MEXT for financial support (KAKENHI for H.T.,
No. 20685005; and the Global COE Program for Chemistry
Innovation). This work was partly supported by the Strategic
Promotion of Innovative R&D, JST (E.N.) and PRESTO, JST
(H.T.).
Supporting Information
Synthetic details and properties of the compounds. This
material is available electronically on J-STAGE.
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© 2015 The Chemical Society of Japan | 783