Organic field-effect transistors based on novel organic semiconductors containing diazaboroles

Article abstract of DOI:10.1039/c1jm00002k

New π-conjugated systems containing a diazaborole unit have been prepared by the reaction of the corresponding boronic acids with diamines. They are nearly colorless compounds with electron-donating properties. Single crystal X-ray structure analysis of t

Full text of DOI:10.1039/c1jm00002k

View Article Online / Journal Homepage / Table of Contents for this issue
Dynamic Article Links <
C
Journal of
Materials Chemistry
Cite this: J. Mater. Chem., 2011, 21, 6607
www.rsc.org/materials
PAPER
Organic field-effect transistors based on novel organic semiconductors
containing diazaboroles†
a
Takahiro Kojima, Daisuke Kumaki, Jun-ichi Nishida, Shizuo Tokito and Yoshiro Yamashita
b
a
b
a
*
Received 1st January 2011, Accepted 8th March 2011
DOI: 10.1039/c1jm00002k
New p-conjugated systems containing a diazaborole unit have been prepared by the reaction of the
corresponding boronic acids with diamines. They are nearly colorless compounds with electron-
donating properties. Single crystal X-ray structure analysis of two compounds revealed their nearly
planar structures and unique crystal structures. The OFET devices using the transparent organic
semiconductors as an active layer have been successfully fabricated and showed good p-type FET
behaviour. The FET device based on the derivative showed high air stability in spite of its high HOMO
level. The film structures were investigated with XRD and AFM measurements.
have been reported.5 However, the carrier transporting proper-
ties in the solid state of diazaboroles have been little studied.
Diazaboroles have the following advantages as organic semi-
conductors. First, they can be easily prepared by a one-step
reaction of boronic acids with diamino compounds. The HOMO
and LUMO levels can be readily tuned by modifying the diamino
compounds and boronic acids. According to Yamamoto et al.,
diazaborole units have lower electron-donating properties than
phenylenediamines, suggesting that the HOMO levels of
diamines can be lowered by introducing the boron atom.5
Second, they are much more stable than boronate esters. Third,
they have planar structures and can connect p-electronic
systems. We report here the preparation, properties and struc-
tures of p-conjugated diazaboroles, and the OFETs based on
them.
1. Introduction
Organic semiconductors have recently attracted much attention
for applications in organic electronics such as organic field-effect
transistors (OFETs), organic light-emitting diodes (OLEDs) and
organicphotovoltaiccells(OPVCs). Althoughmanyp- andn-type
organic semiconductors have already been reported, development
of new materials is still important for the progress in this field.1
On the other hand, boron is a very attractive atom since it has
a vacant 2p orbital, which leads to Lewis acid characteristics and
electron accepting properties. Boron-containing materials have
been used for chemical sensors, electroluminescence, non-linear
optics and hole or electron transporting materials.2 However,
compounds with tricoordinated boron atoms have not been used
as semiconductors for OFETs, although some tetracoordinated
boron-containing compounds exhibited FET behavior.3 This is
attributed to the fact that compounds with vacant 2p orbital of
boron atom are generally unstable and bulky substituents are
introduced for stabilization. Such bulky groups disturb inter-
molecular interactions necessary for effective charge carrier
transport. Therefore, we first focused on boronate esters and the
related derivatives, and succeeded in fabricating OFETs based
on them, although the hole mobilities are not so good.4 As an
extension of this work, we have now focused on diazaboroles.
There are many reports about diazaboroles, and their synthesis,
structure, electro chemical properties and efficient fluorescence
2. Experimental
2.1. General
1
Melting points were obtained on a SHIMADZU DSC-60. H-
NMR spectra were recorded on a JEOL JNM-ECP 300 spec-
trometer and referenced to the residual solvent proton resonance.
EI mass spectra were collected on a JEOL JMS-700 mass spec-
trometer. Elemental analyses were performed at the Tokyo
Institute of Technology, Chemical Resources Laboratory. UV-
vis spectra were recorded on a SHIMADZU MultiSpec-1500.
Differential pulse voltammograms were recorded on a HOKU-
TODENKO HZ-5000 containing tetrabutylammonium hexa-
fluorophosphate (TBAPF6) (0.1 mol dmꢀ3 in dry DMF). The Pt
disk, Pt wire and SCE were used as working, counter, and
reference electrodes, respectively. X-Ray diffraction (XRD)
measurements were carried out with a JEOL JDX-3530 X-ray
diffractometer system. XRD patterns were obtained using the
Bragg-Brentano geometry with CuKa radiation as an X-ray
aTokyo Institute of Technology, Interdisciplinary Graduate School of
Science and Engineering, Department of Electronic Chemistry, 4259
Nagatsuta, Midori-ku, Yokohama, 226-8502, Japan. E-mail: yoshiro@
echem.titech.ac.jp; Fax: +81-45-924-5489; Tel: +81-45-924-5571
bNHK Science & Technical Research Laboratories, 1-10-11 Kinuta,
Setagaya-ku, Tokyo, 157-8510, Japan
† Electronic supplementary information (ESI) available: FET
characteristics. CCDC reference numbers 767653 and 767654. For ESI
and crystallographic data in CIF or other electronic format see DOI:
10.1039/c1jm00002k
This journal is ª The Royal Society of Chemistry 2011
J. Mater. Chem., 2011, 21, 6607–6613 | 6607

View Article Online
source with an acceleration voltage of 40 kV and a beam current
of 30 mA. AFM experiments for films in tapping mode were
performed using an SII NanoTechnology SPA-400 (DFM)
instrument.
Anal. Calcd. for C18H16B2N4: C, 69.75; H, 5.20; N, 18.08.
Found: C, 69.75; H, 5.51; N, 18.20%. HR-MS/FAB m/z Calcd
for C18H16B2N4 310.1561. Found 310.1552.
Compound 6. 4-Biphenylboronic acid (1.27 g, 6.4 mmol) was
dissolved in 50 ml of toluene. 1,2,4,5-Tetraaminobenzene tetra-
hydrochloride (458 mg, 1.6 mmol) was added to the solution, and
the mixture was refluxed for 2 d, and the residue was filtered and
sublimed to give 6 as a yellow solid in 8% yield (57 mg,
0.12 mmol). mp > 400 ꢁC. HR-MS m/z (M+). Anal. Calcd for
C30H24B2N4 462.2187. Found 462.2196.
2.2. Synthesis
Compound 1. 4,40-Dibromobiphenyl (1.56 g, 5.0 mmol) was
dissolved in anhydrous THF (70 ml), and cooled to ꢀ78 ꢁC. tert-
Butyllithium (13 ml, 20 mmol, 1.57 M in pentane) was added
dropwise over 5 min to the solution. After stirring the mixture for
1 h, trimethyl borate (1.5 ml, 13.5 mmol) was added. The solution
ꢁ
was stirred for 1 h at ꢀ78 C and for 2 h at room temperature.
2.3. X-Ray analysis
The mixture was neutralized with 150 ml of 1 N HCl and
extracted with diethyl ether. The organic phase was dried over
Na2SO4, filtered and evaporated. The crude biphenyldiboronic
acid was dissolved in 100 ml of toluene, and o-phenylenediamine
(1.10 g, 10.2 mmol) and p-toluenesulfonic acid (one grain as
a dehydration catalyst) were added to the solution. The mixture
was refluxed for 24 h. The residue was filtered and sublimed to
give compound 1 as an off-white solid in 40% yield (153 mg,
0.40 mmol). Mp > 400 ꢁC. MS (EI) m/z 386 (M+). Anal. Calcd for
C24H20B2N4: C, 74.67; H, 5.22; N, 14.51. Found: C, 74.51; H,
5.10; N, 14.39%.
The X-ray measurements of boronate esters 2 and 4 were carried
out on a Rigaku RAXIS-RAPID Imaging Plate diffractometer
ꢀ
(Mo-Ka radiation, (l ¼ 0.71075 A)). The data were collected at
93 K and the structure was solved by the direct method and
refined by the full matrix least-squares method on F2 with
anisotropic temperature factors for non-hydrogen atoms.
Hydrogen atoms of 2 and 4 were placed in geometrically calcu-
lated positions. The absorption correction was applied using the
empirical procedure. All calculations were performed using the
CrystalStructure crystallographic software package except for
refinement, which was performed using SHELXL-97.
Compound 2. Biphenyl-3,30,4,40-tetraamine (535 mg, 2.5 mmol)
was dissolved in 70 ml of toluene. Phenylboronic acid (1.22 g,
10.0 mmol) was added to the solution, and the mixture was
refluxed for 2 d, and the residue was filtered and sublimed to give
2 as a colorless solid in 83% yield (803 mg, 2.1 mmol). Mp 338–
341 ꢁC. MS (EI) m/z 386 (M+). Anal. Calcd for C24H20B2N4: C,
74.67; H, 5.22; N, 14.51. Found: C, 74.78; H, 4.95; N, 14.52%.
̄
Compound 2. C24H20B2N4,
M
¼
386.07, triclinic P1,
ꢀ
a ¼ 9.3960(7), b ¼ 9.7337(7), c ¼ 22.0591(15) A, a ¼ 78.68(2), b ¼
ꢁ
3
ꢀ
78.5522(18), g ¼ 75.835(2) , V ¼ 1893(9) A , Z ¼ 4, 2qmax
¼
55.0ꢁ, Dcalcd ¼ 1.354 g cmꢀ3, F(000) ¼ 808.00, m ¼ 0.803 cmꢀ1
,
18728 reflections collected, 8631 independent (Rint ¼ 0.1736), R1 ¼
0.0707, wR2 ¼ 0.2296. The CCDC reference number is 767653.
Compound 3. Biphenyl-3,30,4,40-tetraamine (428 mg, 2.0 mmol)
was dissolved in 100 ml of toluene. Naphthalene-2-ylboronic acid
(1.07 g, 6.0 mmol) was added to the solution, and the mixture was
refluxed for 2 d, and the residue was filtered and sublimed to give
3 as a colorless solid in 25% yield (242 mg, 0.5 mmol). Mp 360–
363 ꢁC. HR-MS m/z (M+). Anal. Calcd for C32H24B2N4
486.2187. Found 486.2188.
Compound 4. C36H28B2N4, M ¼ 538.26, orthorhombic P21/n,
ꢀ
a ¼ 8.6051(4), b ¼ 5.5304(2), c ¼ 27.6946(11) A, b ¼ 97.8185
ꢁ
3
(16) , V ¼ 1305.72(9) A , Z ¼ 2, 2qmax ¼ 55.0ꢁ, Dcalcd ¼ 1.369 g
cmꢀ3, F(000) ¼ 564.00, m ¼ 0.801 cmꢀ1, 12 115 reflections
collected, 2992 independent (Rint ¼ 0.0480), R1 ¼ 0.0427, wR2 ¼
0.1227. The CCDC reference number is 767654.
ꢀ
2.4. Fabricating of OFETs
Compound 4. Biphenyl-3,30,4,40-tetraamine (857 mg, 4.0 mmol)
was dissolved in 50 ml of toluene. 4-Biphenylboronic acid (1.84 g,
9.3 mmol) was added to the solution, and the mixture was
refluxed for 2 d, and the residue was filtered and sublimed to give
4 as a pale yellow solid in 41% yield (878 mg, 1.6 mmol). Mp >
400 ꢁC. MS (EI) m/z 538 (M+). Anal. Calcd for C36H28B2N4: C,
80.33; H, 5.24; N, 10.41. Found: C, 80.28; H, 5.14; N, 10.43%.
Bottom-contact OFETs were constructed on the heavily doped
n-type silicon wafer covered with 300 nm thick thermally grown
silicon dioxide. The silicon dioxide acts as a gate dielectric layer
and the silicon wafer serves as a gate electrode. Cr (10 nm)/Au
(20 nm) were successively evaporated and photolithographically
delineated to obtain the source and drain electrodes. The channel
length and width (L/W) were 25 mm/6 ꢂ 49 mm, respectively.
Organic thin films (50 nm) were deposited on the SiO2 treated
with HMDS (hexamethyldisilazane) or bare substrates at
different substrate temperatures by vacuum evaporation (10ꢀ5
Pa). The FET measurements were carried out at room temper-
ature in a high vacuum chamber and ambient condition.
Compound 5. A mixture of 4-phenylboronic acid (370 mg, 3.1
mmol), 1,2,4,5-tetraaminobenzene tetrahydrochloride (285 mg,
1.0 mmol) and sodium hydrogen carbonate (257 mg, 3.1 mmol)
in 70 ml of toluene was refluxed for 1 d through a Soxhlet
extractor containing magnesium sulfate under Ar. A pale purple
solid (407 mg) was filtered and washed with hexane. Purification
by sublimation gave 5 as a yellow solid (92 mg) in 30% yield. Mp
> 360 ꢁC (decomp.). 1H NMR (300 MHz, DMSO-d6): d 8.70 (s,
4H, NH), 7.84 (d, J ¼ 6.0 Hz, 4H, arom. H), 7.46–7.30 (m, 6H,
arom. H), 6.82 (s, 2H, arom. H). MS (EI) m/z 310 (M+, 100).
Top-contact OFETs were constructed on heavily doped n-type
silicon wafers covered with 200 nm thick thermally grown silicon
dioxide. The silicon dioxide acts as a gate dielectric layer, and
the silicon wafer serves as a gate electrode. Organic com-
pounds were deposited on the SiO2 treated with HMDS
6608 | J. Mater. Chem., 2011, 21, 6607–6613
This journal is ª The Royal Society of Chemistry 2011

View Article Online
(hexamethyldisilazane) or OTS (octyltrichlorosilane) by vacuum
evaporation at a rate of 0.2–0.4 A sꢀ1 under pressure of 10ꢀ4 Pa.
were +0.62, +0.64 and +0.61 V vs. SCE, respectively. The effect
of end-capped aryl groups on the oxidation potentials was little.
Compounds 5 and 6 were oxidized more easily than the others,
because tetraaminobenzene has a higher HOMO level (ꢀ3.27 eV)
than those of o-phenylenediamine (ꢀ4.47 eV) and dia-
minobenzidine (ꢀ4.51 eV), which were estimated by DFT
calculations.6 The HOMO levels of 5 and 6 estimated from the
oxidation potentials were 4.5 eV.
ꢀ
The thickness of the semiconductor layer was 50 nm. During the
evaporation, the temperature of the substrate was maintained at
80 ꢁC by heating a copper block on which the substrate was
mounted. Gold was used as source and drain electrodes and
deposited on the organic semiconductor layer through a shadow
mask with a channel width (W) of 1000 mm and a channel length
(L) of 50 mm.
The FET measurements were carried out at room temperature
in a vacuum chamber (10ꢀ5 Pa) without exposure to air with
Hewlett-Packard 4140A and 4140B models.
3.3. X-Ray single crystal analysis
The single crystals of 2 and 4 suitable for the X-ray structure
analysis were obtained by sublimation. The crystallographic
parameters of 2 and 4 are shown in Table 2 and these ORTEP
and packing structures are shown in Fig. 2 and 3. The X-ray
analysis reveals their nearly planar molecular structures. In
crystal 2, there exist two crystallographically independent
molecules (molecules 1 and 2). In molecule 1, the dihedral angles
between the phenylene rings (C7–C12), (C19–C24) and the
neighboring benzodiazaborole rings are 6.3ꢁ and 5.2ꢁ, respec-
tively, and that of the bisbenzodiazaborole core is 8.8ꢁ. In
molecule 2, the dihedral angles between the phenylene rings
(C31–C36), (C43–C48) and the neighboring heterocycle are 11.8ꢁ
and 4.4ꢁ, respectively, and the bisdiazaborole core is twisted by
only 5.4ꢁ. These molecules are packed in a half-slipped herring-
bone manner. The molecule of 4 is also almost planar as expec-
Carrier mobilities (m) were calculated in the saturation region
by the relationship: msat ¼ (2IdL)/[WCox(Vg ꢀ Vth)2], where Id is
the source–drain saturation current, Cox is the oxide capacitance,
Vg is the gate voltage, and Vth is the threshold voltage. The latter
can be estimated as the intercept of the linear section of the plot
of Vg(Id)1/2
.
3. Results and discussion
3.1. Synthesis and characterization
The syntheses of compounds 1–6 are described in Scheme 1. A
convenient one-step reaction of the corresponding boronic acids
with diamines and tetraamines gave the diazaboroles in
approximately 25–40% yields, with the exception of 2 (83%), and
6 (6%). All of them were thermally stable and purified by subli-
mation. The structures were determined by the spectral data
along with elemental analysis or high resolution mass
spectrometry.
ted. These molecules are also packed in
a half-slipped
herringbone manner. Packing of these molecules in the crystal is
similar to 2. The dihedral angle between the phenylene rings (C7–
C12, C13–C18) is 4.1ꢁ and that between the phenylene and the
diazaborole rings is 3.2ꢁ. Interestingly, the molecules 2 and 4
have many short atomic contacts with the neighboring molecules
in the crystals. In addition to the CH–p interactions, the contacts
between the boron atoms and neighboring atoms exist, which
seem to be favorable to charge carrier transport.
3.2. Optical and electrochemical properties
The optical and electrochemical properties of 1–6 are summa-
rized in Table 1. The absorption spectra of 1–6 in solution are
shown in Fig. 1. It should be noted that compounds 1–4 have no
absorptions in the visible region. The absorptions of 5 and 6 are
a little red-shifted compared to those of 1–4. The spectra of 1–6 in
solution are red-shifted compared to those in the solid. In this
case, DMF was used for the measurement of the PL spectra. The
red-shifts may be attributed to the solvent DMF, which stabilizes
the polar excited states.
3.4. OFET characteristics
The FET measurements were carried out at room temperature in
a high vacuum chamber and ambient condition. The devices of 1
did not exhibit typical FET characteristics (Fig. 4). Instead, these
devices were found to have different conductive states (high and
low conductive states), and they showed unusual varistor-like
I–V curves. Thus, the source–drain current comes to flow
suddenly when the source–drain voltage reaches to a certain
threshold voltage. The resistance values of these devices could be
measured by a digital multimeter (ꢃMU). It seems that the high
conductive state is produced by accumulation of the electric
charge in the thin film when high voltage is applied without
flowing the current between the electrodes. This phenomenon
may have been caused by the electron acceptability of boron
atom with vacant 2p orbital. Although this high conductive state
was kept in a high vacuum, it disappeared in ambient conditions.
This fact suggests that the doped state is unstable in air. Addi-
tionally, these phenomena are observed in the similar device
using some dioxaborole derivatives as active layers.3 The detail is
still puzzling and under investigation. On the other hand, the
devices of 2–6 exhibited good p-type FET behavior. The transfer
and output characteristics of 2–6 are shown in Fig. 5 and S2–S6†,
The differential pulse voltammograms (DPVs) of 1 exhibited
the oxidation potentials at +0.88 V vs. SCE, while those of 2–4
Scheme 1 Synthesis of diazaboroles.
This journal is ª The Royal Society of Chemistry 2011
J. Mater. Chem., 2011, 21, 6607–6613 | 6609

View Article Online
Table 1 Optical properties and redox potentials of 1–6
Solutiona
Solid
Compound
labs/nm
lem/nm
lem/nm
Eoxb/V
Eredb/V
1
2
3
4
5
6
326
320
330, 339
339
355
445
392
490
481, 517
437
532
410, 434
382
417
428, 444
—
466, 486, 550, 599
+0.88
+0.62
+0.64
+0.61
+0.19, +1.21
+0.20
—
—
—
—
—
—
377
a
In DMF. b In DMF 0.1 M n-Bu4NPF6, Pt electrode, vs. SCE.
Fig. 1 UV-vis absorptions of 1–6 in DMF.
Table 2 Crystallographic parameters of 2 and 4
Compound 2
Compound 4
Formula
Habit
C24H20B2N4
Colorless needle
C36H28B2N4
Colorless needle
P21/n
Orthorhombic
8.6051(4)
5.5304(2)
̄
Space group
Crystal system
P1
Orthorhombic
9.3960(7)
9.7337(7)
22.0591(15)
78.68(2)
78.5522(18)
75.835(2)
1893.9(2)
4
ꢀ
a/A
ꢀ
b/A
ꢀ
c/A
27.6946(11)
Fig. 2 X-Ray structure of 2: (a) ORTEP drawing of the molecular
structure, (b) molecular arrangement along the a-axis and (c) packing
view.
a/deg
b/deg
g/deg
97.8185(16)
3
ꢀ
V/A
1305.72(9)
2
Z
Dcalcd/g cmꢀ3
m(Mo-Ka)/cm
No. of reflections
R1
1.354
0.803
8631
0.0707
0.2296
1.369
0.801
2992
0.0427
0.1227
Ru
and the hole mobilities, on/off ratios and threshold voltages are
summarized in Table 3. All of the devices showed the improved
FET performances by using the HMDS treated SiO2/Si
substrate. The performance of the device of 4 was further
improved by increasing the substrate temperature. The hole
mobility and on/off ratio of 2 deposited on the HMDS treated
ꢁ
substrate at 80 C increased compared to those fabricated at rt
although the threshold voltage was little changed. The devices of
5 and 6 also showed good p-type FET behavior. Additionally,
the device of 6 exhibited good FET performance in ambient
conditions even 20 d later, whereas the FET performance of 5
Fig. 3 X-Ray structure of 4: (a) ORTEP drawing of the molecular
structure, (b) molecular arrangement and (c) packing view.
6610 | J. Mater. Chem., 2011, 21, 6607–6613
This journal is ª The Royal Society of Chemistry 2011

View Article Online
Table 4 Air stability of the device based on compound 6a
Mobility/cm2 On/off
Compound Condition
Vacuum
Vꢀ1 sꢀ1
ratio
Threshold/V
6
3.9 ꢂ 10ꢀ3
1.3 ꢂ 105 ꢀ46
4.4 ꢂ 104 ꢀ45
9.1 ꢂ 105 ꢀ30
2.3 ꢂ 105 ꢀ33
In air, 10 min 1.5 ꢂ 10ꢀ3
In air, 4 d
In air, 20 d
6.2 ꢂ 10ꢀ4
1.1 ꢂ 10ꢀ3
a
Electrode : Cr/Au ¼ 10/20 nm, SiO2/Si HMDS treated substrate, SiO2:
300 nm, L/W ¼ 25/294000 mm.
Fig. 4 Varistor like I–V property of the device based on 1.
Table 5 Field-effect characteristics on top-contact geometrya
SiO2
Compound treatment Tsub/ꢁC Vꢀ1 sꢀ1
Mobility/cm2 On/off
ratio
Threshold/V
2
6
HMDS
OTS
HMDS
OTS
80
80
80
80
1.4 ꢂ 10ꢀ2
1.4 ꢂ 10ꢀ2
2.8 ꢂ 10ꢀ3
1.5 ꢂ 10ꢀ3
105
105
104
104
ꢀ44
ꢀ60
ꢀ28
ꢀ33
a
Electrode : Au ¼ 50 nm, SiO2/Si substrate, SiO2: 200 nm, L/W ¼ 50/
1000 mm.
Fig. 5 Output and transfer characteristics of the FET devices of 6 on the
HMDS treated substrate at 80 ꢁC (a) and the device of (a) in ambient
condition after standing in air for 20 d (b).
Table 3 Field-effect characteristics on bottom-contact geometrya
Fig. 6 Output and transfer characteristics of top-contact devices of 2
deposited on the HMDS treated SiO2 at 80 ꢁC (a) and the OTS treated
SiO2 at 80 ꢁC (b).
SiO2
Mobility/
On/off
Compound treatment Tsub/ꢁC cm2 Vꢀ1 sꢀ1 ratio
Threshold/V
2
HMDS
Bare
HMDS
Bare
HMDS
Bare
HMDS
Bare
HMDS
Bare
HMDS
Bare
HMDS
Bare
HMDS
Bare
HMDS
rt
1.9 ꢂ 10ꢀ3 5 ꢂ 104 ꢀ27
80
80
rt
rt
rt
1.1 ꢂ 10ꢀ4 3 ꢂ 104 ꢀ14
5.4 ꢂ 10ꢀ3 1 ꢂ 105 ꢀ25
4.9 ꢂ 10ꢀ5 7 ꢂ 103 ꢀ28
2.8 ꢂ 10ꢀ4 7 ꢂ 104 ꢀ45
8.8 ꢂ 10ꢀ7 1 ꢂ 102 ꢀ29
5.5 ꢂ 10ꢀ5 6 ꢂ 103 ꢀ43
1.5 ꢂ 10ꢀ4 1 ꢂ 104 ꢀ20
4.5 ꢂ 10ꢀ3 5 ꢂ 104 ꢀ26
2.3 ꢂ 10ꢀ5 8 ꢂ 103 ꢀ59
1.0 ꢂ 10ꢀ3 3 ꢂ 104 ꢀ43
3.8 ꢂ 10ꢀ5 4 ꢂ 103 ꢀ55
1.1 ꢂ 10ꢀ3 2 ꢂ 104 ꢀ41
1.3 ꢂ 10ꢀ3 7 ꢂ 104 ꢀ31
3.2 ꢂ 10ꢀ3 2 ꢂ 105 ꢀ39
1.2 ꢂ 10ꢀ3 4 ꢂ 104 ꢀ37
3.9 ꢂ 10ꢀ3 1 ꢂ 105 ꢀ46
3
4
became dropped in a minute in the same conditions (Table 4).
For the air-stability in OFETs, the HOMO levels should be lower
than the energy level of electrode (5.1 eV). Therefore, it is
unusual that the device of 6, which has a high HOMO level of
4.5 eV, showed good air stability.
rt
80
80
rt
5
6
To investigate the effect of the device structures on the FET
characteristics, the top-contact devices using 2 and 6 were
fabricated. The FET characteristics of 2 and 6 are summarized in
Table 5, and the transfer and output characteristics of 2 and 6
deposited on the OTS or HMDS treated SiO2 are shown in Fig. 6
and 7. The hole mobility of 2 was improved up to 1.4 ꢂ 10ꢀ2 cm2
Vꢀ1 sꢀ1, while the FET performance of the device of 6 was similar
to that of the bottom-contact device. The device based on 6 was
deteriorated in a minute on standing in air in contrast to the
rt
80
80
rt
rt
80
80
a
Electrode : Cr/Au ¼ 10/20 nm, SiO2/Si substrate, SiO2: 300 nm, L/W ¼
25/294000 mm.
This journal is ª The Royal Society of Chemistry 2011
J. Mater. Chem., 2011, 21, 6607–6613 | 6611

View Article Online
Fig. 9 X-Ray diffraction patterns for the film of 5 deposited on the bare
SiO2 at rt (a), the bare SiO2 at 80 ꢁC (b), the HMDS treated SiO2 at rt (c)
and the HMDS treated SiO2 at 80 ꢁC (d).
Fig. 7 Output and transfer characteristics of top-contact devices of 6
deposited on the HMDS treated SiO2 at 80 ꢁC (a) and the OTS treated
SiO2 at 80 ꢁC (b).
air-stable behavior observed in the bottom-contact device. This
fact suggests that the air-stability of the device of 6 is due to the
dense packing of molecules which protect the contact with air.
3.5. X-Ray diffractogram and AFM measurements
The organic thin films of 2, 5 and 6 deposited on the bare or
HMDS treated SiO2/Si substrates at different temperatures were
investigated by X-ray diffraction (XRD) in reflection mode and
atomic force microscope (AFM). The first reflection peak
observed in the film of 2 deposited on the HMDS treated substrate
was2q¼ 4.22ꢁ (d-spacing ¼ 2.09 nm). Since the molecular lengthis
2.12 nm, the molecules are considered to be arranged nearly
perpendicularly to the substrate. The AFM image of the film
deposited on the HMDS treated substrate at room temperature
showed a large grain which is estimated to be 0.2–0.4 mm and
Fig. 10 X-Ray diffraction patterns for the film of 6 deposited on the
bare SiO2 at rt (a), the bare SiO2 at 80 ꢁC (b), the HMDS treated SiO2 at
rt (c) and the HMDS treated SiO2 at 80 ꢁC (d).
a lamella structure (Fig. 8). In the case of the thin film of 5, the first
reflection peak was observed clearly at 2q ¼ 4.86ꢁ (d-spacing ¼
1.82 nm), indicating the high crystallinity (Fig. 9). Since the
molecular length estimated from the MOPAC PM6 calculations is
1.76 nm, the molecules stand on the substrate perpendicularly. In
the case of the thin film of 6, the first reflection peak was observed
at 2q ¼ 3.76ꢁ (d-spacing ¼ 2.34 nm) (Fig. 10). However, the peaks
were not clear, suggesting that the crystallinity of thin films of 6 is
not so high. This may be ascribed to the fact that the biphenyl unit
of the molecule is twisted because of the H–H repulsion between
the phenylene rings. Since the molecular length estimated from the
MOPAC PM6 calculations is 2.62 nm, the molecules are consi-
dered to be a little tilted to the substrate. The AFM measurement
shows many small grains in the films (Fig. 11). The grain size of 6
in the film became larger at higher temperatures and the grains are
densely packed. The biphenyl unit may play an important role in
the dense packing which leads to the good air stability in the
bottom-contact device.
Fig. 8 X-Ray diffraction patterns for the film of 2 deposited on the
HMDS treated SiO2 at rt (a), the bare SiO2 at 80 ꢁC (b) and the HMDS
treated SiO2 at 80 ꢁC (c) and the AFM image for the film of 2 deposited
on the HMDS treated SiO2 at rt (d).
6612 | J. Mater. Chem., 2011, 21, 6607–6613
This journal is ª The Royal Society of Chemistry 2011

View Article Online
1066; (c) J. Zaumseil and H. Sirringhaus, Chem. Rev., 2007, 108,
1296; (d) B. A. Jones, A. Facchetti, T. J. Marks and
M. R. Wasielewski, Chem. Mater., 2007, 19, 2703; (e) Y. Yamashita,
Sci. Technol. Adv. Mater., 2009, 10, 024313, OLEDs: (f) C. W. Tang
and S. A. VanSlyke, Appl. Phys. Lett., 1987, 51, 913; (g) C. Adachi,
M. A. Baldo, M. E. Thompson and S. R. Forrest, Appl. Phys. Lett.,
2001, 90, 5049; (h) L. S. Hung and C. H. Chen, Mater. Sci. Eng.,
2002, 39, 143; (i) E. Holder, B. M. W. Langeveld and
U. S. Schubert, Adv. Mater., 2005, 17, 1109; (j) A. C. Grimsdale,
K. L. Chan, R. E. Martin, P. G. Jokisz and A. B. Holmes, Chem.
Rev., 2009, 109, 897, OPVCs: (k) G. Yu, J. Gao, J. C. Hummelen,
F. Wudl and A. J. Heeger, Science, 1995, 270, 1789; (l) G. Li,
V. Shrotriya, J. Huang, Y. Yao, T. Moriarty, K. Emery and
Y. Yang, Nat. Mater., 2005, 4, 864; (m) S. H. Park, A. Roy,
S. Beaupre, S. Cho, N. Coates, J. S. Moon, D. Moses, M. Leclerc,
K. Lee and A. J. Heeger, Nat. Photonics, 2009, 3, 297; (n)
G. Dennler, M. C. Scharber and C. J. Brabec, Adv. Mater., 2009, 21,
1323; (o) B. C. Thompson and J. M. J. Frechet, Angew. Chem., Int.
Ed., 2008, 47, 58; (p) S. Gnes, H. Neugebauer and N. S. Sariciftci,
Chem. Rev., 2007, 107, 1324; (q) G. Li, V. Shrotria, Y. Yao,
J. S. Huang and Y. Yang, J. Mater. Chem., 2007, 17, 3126; (r)
Y. Liang, Z. Xu, J. Xia, S.-T. Tsai, Y. Wu, G. Li, C. Ray and
L. Yu, Adv. Mater., 2010, 22, E135.
2 (a) S. Shinkai and M. Takeuchi, TrAC, Trends Anal. Chem., 1996, 15,
188; (b) X. T. Tao, H. Suzuki, T. Wada, S. Miyata and H. Sasabe, J.
Am. Chem. Soc., 1999, 121, 9447; (c) S. Yamaguchi, T. Shirasaka
and K. Tamao, Org. Lett., 2000, 2, 4129; (d) S. Yamaguchi,
S. Akiyama and K. Tamao, J. Am. Chem. Soc., 2000, 122, 6335; (e)
S. Yamaguchi, S. Akiyama and K. Tamao, J. Am. Chem. Soc., 2001,
123, 11372; (f) M. E. Itkis, X. Chi, A. W. Cordes and R. C. Haddon,
Science, 2002, 296, 1443; (g) W. Niu, B. Rambo, M. D. Smith and
J. J. Lavigne, Chem. Commun., 2005, 5166; (h) T. Agou,
J. Kobayashi and T. Kawashima, Chem.–Eur. J., 2007, 13, 8051; (i)
A. Wakamiya, K. Mori and S. Yamaguchi, Angew. Chem., Int. Ed.,
2007, 46, 4273; (j) B. M. Rambo and J. J. Lavigne, Chem. Mater.,
2007, 19, 3732; (k) M. Elbing and G. C. Bazan, Angew. Chem., Int.
Fig. 11 AFM images for the films of 6 deposited on the bare SiO2 at rt
(a), the bare SiO2 at 80 ꢁC (b), the HMDS treated SiO2 at rt (c) and the
HMDS treated SiO2 at 80 ꢁC (d).
4. Conclusions
We developed new p-conjugated systems containing diazaborole
rings 1–6 and succeeded in fabricating the FET devices based on
them. Compounds 2–6 showed good p-type semiconducting
behavior with hole mobilities ranging from 10ꢀ7 to 10ꢀ2 cm2 Vꢀ1
sꢀ1, whereas 1 showed unusual varistor-like I–V curve. Although
compound 6 has a high HOMO level (4.5 eV), the bottom-
contact devices of 6 showed good air-stability. The film of 6 is
composed of densely packed grains, which was revealed by the
AFM measurement. This fact suggests that the morphology of
the film is strongly related to the air stability. Diazaborole
derivatives have no absorptions in the visible region, leading to
transparent semiconductors. These results indicate that such
diazaborole derivatives are promising candidates for unique
organic semiconductors.
€
Ed., 2008, 47, 834; (l) F. Jakle, Chem. Rev., 2010, 110, 3985; (m)
C. D. Entwistle and T. B. Marder, Angew. Chem., Int. Ed., 2002, 41,
2927.
3 (a) Y. Sun, D. Rohde, Y. Liu, L. Wan, Y. Wang, W. Wu, C. Di, D. Yu
and D. Zhu, J. Mater. Chem., 2006, 16, 4499; (b) K. Ono,
J. Hashizume, H. Yamaguchi, M. Tomura, J. Nishida and
Y. Yamashita, Org. Lett., 2009, 11, 4326.
4 T. Kojima, J. Nishida, S. Tokito and Y. Yamashita, Chem. Lett., 2008,
37, 1122.
5 (a) S. Maruyama and Y. Kawanishi, J. Mater. Chem., 2002, 12, 2245;
(b) L. Weber, I. Domke, C. Schmidt, T. Braum, H.-G. Stammler and
B. Neumann, Dalton Trans., 2006, 2127; (c) L. Weber, V. Werner,
I. Domke, H.-G. Stammler and B. Neumann, Dalton Trans., 2006,
3777; (d) I. Yamaguchi, B.-J. Choi, T. Koizumi, K. Kubota and
T. Yamamoto, Macromolecules, 2007, 40, 438; (e) L. Weber, Coord.
Chem. Rev., 2008, 252, 1; (f) L. Weber, V. Werner, M. A. Fox,
T. B. Marder, S. Schwedler, A. Brockhinke, H.-G. Stammlera and
B. Neumanna, Dalton Trans., 2009, 1339; (g) I. Yamaguchi,
T. Tominaga and M. Sato, Polym. Int., 2009, 58, 17.
Acknowledgements
This work is supported by Japan Society for the Promotion of
Science (JSPS) and the Global COE program ‘‘Education and
Research Center for Emergence of New Molecular Chemistry.
6 (a) A. A. Granovsky, http://classic.chem.msu.su/gran/gamess/index.
html; (b) M. W. Schmidt, K. K. Baldridge, J. A. Boatz, S. T. Elbert,
M. S. Gordon, J. J. Jensen, S. Koseki, N. Matsunaga,
K. A. Nguyen, S. Su, T. L. Windus, M. Dupuis and
J. A. Montgomery, J. Comput. Chem., 1993, 14, 1347.
References
1 OFETs: (a) C. R. Newman, C. D. Frisbie, D. A. da Silva Filho, J.-
ꢁ
L. Bredas, P. C. Ewbank and K. R. Mann, Chem. Mater., 2004, 16,
4436; (b) A. R. Murphy and J. M. J. Frechet, Chem. Rev., 2007, 108,
ꢁ
This journal is ª The Royal Society of Chemistry 2011
J. Mater. Chem., 2011, 21, 6607–6613 | 6613