Air-stable n-type organic field-effect transistors based on carbonyl-bridged bithiazole derivatives

Article abstract of DOI:10.1002/adfm.200901803

An electroneptive conjugated compound composed of a newly designed carbonyl-bridged bithiazole unit and trifluoroacetyl terminal groups is synthesized as a candidate for air-stable n-type organic field-effect transistor (OFET) materials. Cyclic voltammetry measurements reveal that carbonylbridging contributes both to lowering the lowest unoccupied molecular orbital energy level and to stabilizing the anionic species. X-ray crystallographic analysis of the compound shows a planar molecular geometry and a dense molecular packing, which is advantageous to electron transport. Through these appropriate electrochemical properties and structures for n-type semiconductor materials, OFET devices based on this compound show electron mobilities as high as 0.06 cm² V^-1 s^-1 with on/off ratios of 10 ⁶ and threshold volages of 20V under vacuum condition, Furthermore, these devices show the same order of electron mobility under ambient conditions.

Full text of DOI:10.1002/adfm.200901803

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Air-Stable n-Type Organic Field-Effect Transistors Based
on Carbonyl-Bridged Bithiazole Derivatives
By Yutaka Ie,* Masashi Nitani, Makoto Karakawa, Hirokazu Tada, and
Yoshio Aso*
Recently,wehavereportedthatthepresence
of trifluoroacetyl groups at the terminal
positions of a p-conjugated oligomer is
effective in both lowering the lowest
unoccupied molecular orbital (LUMO)
energy level and invoking strong intermo-
lecular interactions.[26] We have also
reported that this terminal group combined
with 5,50-diphenyl-2,20-bithiophene exhi-
bits good n-channel characteristics when
used in an OFET device.[27] On the basis of
these findings, we anticipated that if an
electron-accepting p-conjugated unit is
utilized instead of the electron-donating
bithiophene, then the n-type character
would be further enhanced, thus contribut-
ing to the stabilization of device operation
under ambient conditions. On the other
An electronegative conjugated compound composed of a newly designed
carbonyl-bridged bithiazole unit and trifluoroacetyl terminal groups is
synthesized as a candidate for air-stable n-type organic field-effect transistor
(OFET) materials. Cyclic voltammetry measurements reveal that carbonyl-
bridging contributes both to lowering the lowest unoccupied molecular orbital
energy level and to stabilizing the anionic species. X-ray crystallographic
analysis of the compound shows a planar molecular geometry and a dense
molecular packing, which is advantageous to electron transport. Through
these appropriate electrochemical properties and structures for n-type
semiconductor materials, OFET devices based on this compound show
electron mobilities as high as 0.06 cm2 Vꢀ1 sꢀ1 with on/off ratios of 106 and
threshold voltages of 20 V under vacuum conditions. Furthermore, these
devices show the same order of electron mobility under ambient conditions.
hand, Yamashita et al. have demonstrated that p-conjugated
oligomers containing electron-accepting thiazole rings exhibit a
high performance as n-type OFETmaterials.[6] However, the OFET
characteristics of these oligomers were not observed under
ambient conditions. With these studies in mind, we have focused
on the introduction of carbonyl bridging, because carbonyl
bridging of biaryls is known to cause a decrease in the LUMO
energy level.[9ad,11,13] Therefore, in this study, we have designed a
carbonyl-bridged bithiazole as a new electronegative unit intended
for combining with trifluoroacetyl terminal groups. We first
calculated the molecular orbitals of carbonyl-bridged conjugated
compoundC-BTzandnonbridgedBTz bydensityfunctionaltheory
(DFT) at the B3LYP/6-31G(d,p) level. As depicted in Figure 1, the
carbonyl group more strongly participates in the LUMO of C-BTz
than its highest occupied molecular orbital (HOMO), and as a
consequence C-BTz possesses a notably lower LUMO energy level
1. Introduction
Considerable efforts have been devoted to developing new p-
conjugated systems as active materials for organic field-effect
transistors (OFETs) because of their potential application in low-
cost, large-area, and flexible electronic devices.[1] Significant
progress has recently been made in the development of p-type
OFETmaterials.[2] However, limited studies have been conducted
on the development of n-type OFETmaterials.[3–14] This is because
of the insufficiency of their molecular design in stabilizing
electron-injected species and arranging the molecular ordering for
efficient electron transport in the solid state. Moreover, although
an improvement in device stability under ambient conditions is
highly desirable for practical applications in OFETs, only a few p-
conjugatedsystems, suchashalogenatedphthalocyanine,[15] acene
carboxylic diimide,[16–22] dicyanomethylene-substituted conju-
gated compounds,[23,24] and perfluoroalkyl-substituted fuller-
enes,[25] are known to operate as air-stable n-type OFETmaterials.
[*] Dr. Y. Ie, Prof. Y. Aso, M. Nitani, Dr. M. Karakawa
The Institute of Scientific and Industrial Research
Osaka University
8-1, Mihogaoka, Ibaraki, Osaka 567-0047 (Japan)
E-mail: yutakaie@sanken.osaka-u.ac.jp; aso@sanken.osaka-u.ac.jp
Prof. H. Tada
The Graduate School of Engineering Science
Osaka University
1-3, Machikaneyama, Toyonaka, Osaka 560-8531 (Japan)
Figure 1. Chemical structures and HOMO and LUMO energies calculated
using DFT at the B3LYP/6-31G(d,p) level.
DOI: 10.1002/adfm.200901803
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than nonbridged BTz. In this article, we report on the synthesis,
properties, and n-type OFETperformance of C-BTz. Furthermore,
the effect of carbonyl bridging is ascertained by comparing the
properties of C-BTz with those of nonbridged BTz.
2. Results and Discussion
2.1. Synthesis, Properties, and Structures
Figure 2. a) Cyclic voltammograms of C-BTz (red) and BTz (black) in
fluorobenzene containing 0.1 M TBAPF6. b) UV–vis absorption spectra of
C-BTz (red) and BTz (black) in THF.
As shown in Scheme 1, carbonyl-bridged bis(triisopropylsilyl)-
bithiazole 2 was synthesized by lithiation of 1 with lithium
diisopropylamide (LDA) and subsequent treatment with ethyl
1-piperidine carboxylate, after which it was converted into a
estimated to be ꢀ3.64 eV,[29] which is fairly consistent with the
calculated value (ꢀ3.73 eV) (Fig. 1).
stannylated bithiazole derivative
4
via acetal protection[28]
of the carbonyl group. The newly designed target compound
C-BTz was prepared by a Stille-coupling reaction of 4 with
40-bromo-2,2,2-trifluoroacetophenone and subsequent acidic
acetal deprotection. A similar cross-coupling procedure was
applied to the synthesis of reference compound BTz. Both the final
compounds were purified by gradient sublimation under a high
vacuum and unambiguously characterized by mass spectrometry
(MS), nuclear magnetic resonance (NMR) spectroscopy, and
elemental analysis (see Experimental section and Supporting
Information (SI)).
The electrochemical properties of the obtained conjugated
compounds were investigated by cyclic voltammetry (CV)
measurements, which were carried out at a scan rate of 100 mV
sꢀ1 in fluorobenzene containing 0.1 M tetrabutylammonium
hexafluorophosphate (TBAPF6) as the supporting electrolyte. A
platinum disk was used as the working electrode, a platinum wire
asthecounterelectrode, andAg/AgNO3 asthereferenceelectrode.
As shown in Figure 2a, carbonyl bridging caused a large positive
shift of 0.47 V in the first reduction potential. Furthermore, in
As shown in Figure 2b, the UV–vis absorption spectrum of BTz
in tetrahydrofuran (THF) exhibited an absorption maximum
related to the HOMO–LUMO transition at 405 nm. Interestingly,
introduction of carbonyl bridging leads to the appearance of a
shoulder at around 550 nm.[26] According to time-dependent DFT
calculations at the B3LYP/6-31G(d,p) level, this shoulder can be
mainlyascribedtotheHOMO–LUMOtransitionwithanoscillator
strength ( f ) of 0.29. This result indicates that the optical HOMO–
LUMO energy gap of C-BTz is smaller than that of BTz, which can
beexplainedbytheloweringoftheLUMOenergylevel(Fig. 1). The
intense absorption band of C-BTz at 405 nm is similarly related to
the HOMO–LUMOþ1 transition ( f ¼ 1.18).
To better understand the molecular structure and packing
diagram in the solid state, X-ray crystallographic analysis of C-BTz
was carried out. Single crystals were grown from a CHCl3 solution
byslowevaporation. The X-ray crystallographic analysisconfirmed
that the C-BTz molecule exhibits a planar geometry, with dihedral
angles between the thiazole rings and between the thiazole and
phenyl rings of around 0.58 and 128, respectively (Fig. 3a). As
expected, both the carbonyl groups in the molecule have a
significant influence on the molecular arrangement. As shown by
the dashed lines in Fig. 3b, the carbonyl-bridged bithiazole and
terminal trifluoroacetyl groups induce several short contacts
between adjacent molecules, resulting in a dense p-stacked
packing along the b-axis with interplanar distances of 3.40 and
contrast to the irreversible reduction wave for BTz, C-BTz showed
red
1=2
two reversible reduction waves with half-wave potentials E
¼
ꢀ1.16 and ꢀ1.53 V. These phenomena clearly indicate that
carbonyl bridging contributes considerably to not only an increase
in electron affinity but also to kinetically stabilizing of the anionic
species. From the first reduction potential of C-BTz and under the
premise that the energy level of ferrocene/ferrocenium (Fc/Fcþ) is
ꢀ4.8 eVbelowthevacuumlevel,theLUMOenergylevelofC-BTzis
˚
3.52 A. On the basis of this packing structure, the intermolecular
transfer integrals (t) of the HOMOs and LUMOs between
Scheme 1. Synthesis of C-BTz and BTz.
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p-doped silicon substrate. Source and drain gold electrodes with a
channel width/channel length (W/L) of 38 mm/5 mm were
prepatterned on a 300 nm layer of SiO2 dielectrics for OFETs
with bottom-contact geometry. Films of C-BTz or BTz were
vacuum-deposited after octadecyltrichlorosilane (ODTS) treat-
ment of the substrate. OFET devices with top-contact geometry
were fabricated by patterning the gold electrodes over organic thin
films using a shadow mask to achieve a W/L of 5 mm/20 mm. The
deposition of the organic films was carried out at different
substrate temperatures and various annealing temperatures were
investigated and optimized for all the devices. FET parameters
obtained from measurements under vacuum conditions are
summarized in Table 1. All the devices showed typical n-channel
characteristics, and among them, the C-BTz-based device
displayed a superior FET performance with a maximum electron
mobility of 0.06 cm2 Vꢀ1 ꢀ1, which is almost two orders of
s
magnitude higher than that of BTz (runs 1 and 7). Its output and
transfer characteristics are shown in Figure 4.[31] It is important to
notethattheC-BTz-baseddeviceshowedthesameorderofelectron
mobility irrespective of the device geometry, indicating the
potential versatility for practical device fabrication (runs 1, 2).
To examine the stability of the C-BTz-based OFETunder the biased
conditions, the top-contact device was repeatedly biased by
sweeping the gate voltage (VGS) from ꢀ20 to 50 V under a fixed
source–drain voltage (VDS) of 50 V, and its source–drain current
(IDS) was monitored. As shown in Figure 5, the transfer curves
showed little hysteresis under dual sweep conditions and an
almost unchanged electrical performance even after 500 cycles,
demonstrating a high stability under operating conditions. Thus,
Figure 3. a) Molecular structure and b) packing diagram of C-BTz. Dashed
lines indicate contacts shorter than the sum of van der Waals radii.
c) Estimated transfer integrals of HOMO and LUMO energy levels between
neighboring molecules viewed along the long molecular axis.
¨
neighboring molecules were estimated by an extended Huckel
molecular orbital calculation.[30] The calculations were indepen-
dently carried out for the HOMOs and LUMOs of the molecules.
Asshown in Fig. 3c, the calculated transfer integrals of theLUMOs
are one magnitude larger than those of the HOMOs, and large
overlaps were observed along the p-stacked direction. This result
impliesthatefficientelectrontransportalongtheb-axisisfavorable
for the C-BTz crystals.
C-BTz is a promising semiconductor for practical OFET
applications.
The morphology and structural order of the C-BTz and BTz thin
films on SiO2 substrates fabricated under the conditions of runs 1
and 7 in Table 1, respectively, were investigated by atomic force
microscopy(AFM)andX-raydiffraction (XRD). TheAFMimageof
the C-BTz film showed uniform grains that are tightly connected to
eachother(Fig.6a),whiletheBTz filmrevealedcomparativelylarge
crystalline grains of several micrometers in size and clearly
observable grain boundaries (Fig. 6b). In contrast to their different
morphologies, the XRD measurements of both the films exhibited
sharp reflections (Fig. 6c,d), indicating thecrystallinity of both thin
films. However, the peak intensities of the BTz film were weaker
than thoseof the C-BTz film and, togetherwith a series of peaks at a
2.2. Field-Effect Properties and Morphologies of the Thin Films
Thesemiconductorperformancesoftheseconjugatedcompounds
were evaluated in OFETs with bottom-contact and top-contact
geometries. OFETdeviceswerefabricatedonthegateelectrodeofa
˚
d-spacing of around 14.4 A, weak peaks corresponding to different
Table 1. Field-effect characteristics of C-BTz and BTz in vacuum and in air.
Run
Compound
TD [8C] [a]
TA [8C] [b]
Measurement conditions
m [cm2Vꢀ1sꢀ1
]
Ion/Ioff
Vth [V]
1
2
3
4
5
6
7
8
9
C-BTz[c]
C-BTz[d]
C-BTz[d]
C-BTz[d]
C-BTz[d]
C-BTz[d]
BTz[c]
110
110
110
110
110
110
90
210
130
130
130
130
130
150
110
110
Under vacuum
Under vacuum
In air
0.06
0.03
106
103
104
106
10
105
104
104
20
33
44
68
61
73
15
57
0.014
In air
0.013[e]
In air
6.1 ꢁ 10ꢀ5[f]
4.6 ꢁ 10ꢀ3[f]
7.1 ꢁ 10ꢀ4
2.0 ꢁ 10ꢀ3
No FET
Under vacuum
Under vacuum
Under vacuum
In air
BTz[d]
90
BTz[d]
90
[a] Substrate deposition temperature. [b] Annealing temperature. [c] Bottom-contact geometry. [d] Top-contact geometry. [e] After 24 h. [f] After 1 year.
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Figure 6. a,b) AFM images of a C-BTz film (a) and a BTz film (b) on SiO2.
c,d) XRD data of c) a C-BTz film and d) a BTz film on SiO2.
Finally, the device stability of the C-BTz- and BTz-based OFETs
with top-contact geometry was investigated under ambient atmos-
phericconditions. Asshowninruns2and3inTable1andFigure7,
the C-BTz-based device showed n-channel characteristics with an
electron mobility of the same order of magnitude as that measured
under vacuum conditions, and its performance was maintained
even after storage under ambient conditions for 24 h (runs 3 and 4,
Table 1). In contrast, the FET response of the BTz-based device
completely disappeared (run 9, Table 1). There have been several
argumentationsabouttheairstabilityofn-channelOFETsinterms
of the LUMO energy levels of the semiconductors and their formal
redoxpotentialforH2O,andithasbeensuggestedthatcompounds
having a lower LUMO level than ꢀ4.0 eV should be advantageous
for gaining air stability.[24b,33,34] Despite the higher estimated
LUMO energy level (ꢀ3.64 eV) of C-BTz, OFET devices based on
C-BTz show a fairly high stability under ambient conditions as
described above. This air stability of C-BTz-based devices is
attributable to both the kinetic stability of the anionic species and
the high crystallinity with dense favorable arrangements, which
apparently originates from the presence of carbonyl bridging as
well as the trifluoroacetyl terminal groups. After storage of the
device under ambient atmospheric conditions for 1 year the
Figure 4. a) Output characteristics of a bottom-contact OFET using C-BTz.
b) Transfer characteristics of the same device at drain voltage of 100 V. IDS
DS, and VGS denote source–drain current, source–drain voltage, and gate
voltage, respectively.
,
V
d-spacings were also observed (inset in Fig. 6d), indicating that BTz
crystals and/or molecules are randomly orientated between the
grains.[32] In addition to the relatively high LUMO energy level
compared to that of C-BTz, this observation as well as the large
grain boundaries might be accountable for the moderate electron
mobility of the BTz-based devices. On the other hand, the film of C-
BTz showed well distinguishable reflection peaks up to the third
˚
order corresponding to a d-spacing of 18.0 A, which isalmost equal
˚
to the length (17.7 A) of the crystallographic c-axis. XRD
simulations calculated from crystallographic data of C-BTz were
very similar (Fig. S3 (SI)) to the observed XRD patterns. These
results suggest that the molecules possess a stacking orientation
parallel to the SiO2 surface, which certainly contributes to the high
field-effect electron mobility of the C-BTz films.
Figure 5. Transfer characteristics of a top-contact OFET using C-BTz with
repeated biases for up to 500 cycles at a source–drain voltage of 50 V.
Figure 7. Transfer characteristics of a top-contact OFET using C-BTz
measured in air (solid line) and after 24 h of storage in air (dashed line).
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were obtained in spectrograde solvents. Cyclic voltammetry was carried out
on a BAS CV-620C voltammetric analyzer. Elemental analyses were
performed on a PerkinElmer LS-50B by the Elemental Analysis Section of
the Comprehensive Analysis Center (CAC) of The Institute of Scientific and
Industrial Research (ISIR), Osaka University. The surface structure of the
deposited organic film was observed by AFM (Shimadzu, SPM9600), and
the film crystallinity was evaluated by XRD (Rigaku, RINT2500). XRD
patterns were obtained using a Bragg–Brentano geometry with Cu Ka
radiation as an X-ray source with an acceleration voltage of 50 kV and a
beam current of 200 mA. The XRD patterns were carried out in the scanning
mode using u–2u scans between 2.58 and 408 with scanning step of 0.028.
Materials: All reactions were carried out under a nitrogen atmosphere.
Solvents of the highest purity grade were used as received. Unless stated
otherwise, all reagents were purchased from commercial sources and used
without purification. 5-Bromo-2-(triisopropylsilyl)thiazole was prepared by
the reported procedure [36]. The 1H NMR data of 5-bromo-2-(triisopro-
pylsilyl)thiazole was in agreement with that previously reported.
electron mobility and on/off current ratio measured in air
decreased to 6.1 ꢁ 10ꢀ5 cm2 Vꢀ1 sꢀ1 and 10, respectively (run 5 in
Table 1, and Fig. S4a (SI)). However, the electron mobility could be
improved by two orders of magnitude and an initial on/off current
ratioof105 couldbeobtainedwhencarryingoutthemeasurements
under a reduced pressure of 10ꢀ6 Pa (run 6 in Table 1, and Fig. S4b
(SI)). However, the threshold voltage (Vth) was much higher
(positive shift) with increasing storage period in air and could
not be recovered even under vacuum conditions.[35] These
results indicate that, although reversibly, physically adsorbed
oxygenand/orwaterdodegradethedeviceperformance, theC-BTz
molecule does have an intrinsic air stability when used as an active
material for n-channel OFETs. The positive shift of Vth
accompanied by a change in the electron mobility and on/off
current ratio might be ascribable to the formation of deep traps
at the semiconductor/insulator interface and/or at the grain
boundaries.[3a]
Synthesis of 1: 5-Bromo-2-(triisopropylsilyl)thiazole (8.60 g, 26.8 mmol),
5-(tri-n-butylstannyl)-2-(triisopropylsilyl)thiazole (15.0 g, 28.3 mmol), and
tetrakis(triphenylphosphine)palladium(0) (930 mg, 0.839 mmol) were
placed in a 200 mL round-bottomed flask and dissolved in toluene
(90 mL). The reaction mixture was stirred at 120 8C for 17 h. After being
cooled to room temperature, the reaction mixture was filtered over celite.
After removal of the solvent under reduced pressure, the residue was
purified by column chromatography on alumina (20:1 hexane/EtOAc) to
give 1 (11.6 g, 90%) as a white solid: m.p. 138–139 8C; 1H NMR (400 MHz,
3. Conclusions
We have successfully designed and synthesized carbonyl-bridged
bithiazole as a new electronegative unit and developed a carbonyl-
bridged-bithiazole-based conjugated molecule C-BTz. Results of
spectroscopic and electrochemical measurements and X-ray
analysis of C-BTz revealed that the combination of carbonyl-
bridging and electron-accepting thiazole rings contributes to a low
LUMO energy level and an appropriate molecular structure and
arrangement for charge-carrier transport. Because of this low
LUMO energy level and high transfer integrals, the C-BTz-based
OFET devices exhibited one of the highest levels of n-type
performances and air stability. Our study showed that this simple
modification of molecules could not only facilitate the improve-
ment in device performance but also increase the device stability
under ambient conditions, which will provide possibilities for
further development of a new category of n-type semiconductor
materials. We are currently investigating further improvements in
the performance of C-BTz-based OFET devices by optimizing the
device structure, metal electrodes, and gate dielectrics, as well as
using surface treatments.
CDCl3) d [ppm]: 8.19 (s, 2H), 1.46 (m, 6H), 1.16 (d, 36H, J ¼ 7.5 Hz); 13
C
NMR (150 MHz, CDCl3) d [ppm]: 170.6, 143.3, 131.0, 18.4, 11.7; MS (EI)
m/z 480 [Mþ]. Anal. calcd. for C24H44N2S2 Si2: C 59.94, H 9.22, N 5.82;
found: C 60.11, H 9.42, N, 5.74.
Synthesis of 2: Diisopropylamine (0.8mL, 58 mmol) and THF (1 mL)
were placed in a 50mL round-bottomed flask. To the mixture was added
dropwise n-BuLi (1.58M, 3.2 mL, 5.1 mmol) at ꢀ78 8C, and the resulting
mixture was allowed to warm up to 08C for 30min. After cooling the mixture
back down to ꢀ40 8C, 1 (385mg, 0.801mmol) in THF (20mL) was added
dropwise to it. After stirring for 1h at ꢀ40 8C, ethyl-1-piperidinecarboxylate
(200mg, 1.27 mmol) in THF (8 mL) was added. After further stirring for 1h at
ꢀ40 8C, the reaction was quenched by addition of H2O (5mL). The aqueous
layer was extracted with EtOAc, and the combined organic layers were washed
with water and dried over MgSO4. After removal of the solvent under reduced
pressure, the residue was purified by column chromatography on silica gel
(hexane) to give 2 (401 mg, 99%) as a red solid: m.p. 162–163 8C; 1H NMR
(400MHz, CDCl3) d [ppm]: 1.45 (m, 6H), 1.15 (d, 36H, J ¼ 7.5 Hz); 13C NMR
(150MHz, CDCl3) d [ppm]: 179.1, 174.0, 158.1, 145.3, 18.5, 11.5; MS (EI) m/z
506 [Mþ]. Anal. calcd. for C25H42N2OS2Si2: C 59.23, H 8.35, N 5.53; found:
C 59.27, H 8.64, N 5.25.
Synthesis of 3: To
a solution of 2 (3.00 g, 5.91 mmol) and 2-
chloroethanol (1.90 g, 23.6 mmol) in DMF (160 mL) and THF (80 mL)
was added t-BuOK (1.33 g, 11.9 mmol) at ꢀ78 8C, and the mixture was
stirred at ꢀ78 8C for 2 h. The reaction was quenched by the addition of 1 M
NH4Cl (50 mL). The aqueous layer was extracted with EtOAc, and the
combined organic layers were washed with water and dried over MgSO4.
After removal of the solvent under reduced pressure, the residue was
purified by column chromatography on silica gel (20:1 hexane/EtOAc) to
give A as a pale yellow solid: 1H NMR (400 MHz, CDCl3) d [ppm]: 4.54 (s,
4H), 1.44 (m, 6H), 1.14 (d, 36H, J ¼ 7.5 Hz); MS (EI) m/z 550 [Mþ].
A (2.74 g, 4.96 mmol) was placed in a 100 mL round-bottomed flask and
dissolved in THF (50 mL). To the mixture was added tetrabutylammonium
fluoride (1 M, 12.5 mL, 12.5 mmol) at 0 8C. After stirring for 1 h at 0 8C, the
reaction was quenched by adding H2O. The aqueous layer was extracted
with EtOAc, and the combined organic layers were washed with water and
dried over MgSO4. After removal of the solvent under reduced pressure,
the residue was purified by column chromatography on silica gel
4. Experimental
General Information: Column chromatography was performed on silica
gel (KANTO Chemical silica gel 60N, 40–50 mm) or neutral alumina (Merck
aluminum oxide 90 standardized). Thin-layer chromatography (TLC) plates
were visualized with UV. Preparative gel-permeation chromatography
(GPC) was performed on a Japan Analytical Industry LC-908 equipped with
a JAI-GEL 1H/2H. Melting points are reported uncorrected. 1H NMR and
13C NMR spectra were recorded on a JEOL LA-400 and LA-600 in CDCl3
with tetramethylsilane as an internal standard. Data are reported as follows:
chemical shift in ppm (d), multiplicity (s ¼ singlet, d ¼ doublet, t ¼ triplet,
m ¼ multiplet), coupling constant (Hz), and integration. Mass spectra
were obtained on a Shimadzu GCMS-QP-5050. UV–vis spectra were
recorded on a Shimadzu UV-3100PC. Fluorescence spectra were recorded
using a Fluoromax-2 spectrometer in the photocounting mode equipped
with a Hamamatsu R928 photomultiplier. The bandpass for the emission
spectra was 1.0 nm. Fluorescence quantum efficiencies were measured
using diphenylanthracene (Ff ¼ 0.90 in cyclohexane) as a standard. The
concentrations of solutions were adjusted to yield an absorptivity of A < 0.1
in the absorption spectrum for any fluorescence experiments. All spectra
(2:1 hexane/ EtOAc) to give
3 (677 mg, 48%) as a pale yellow
solid: m.p. 186–187 8C; 1H NMR (400 MHz, CDCl3) d [ppm]: 8.65 (s,
2H), 4.52 (s, 4H); 13C NMR (150 MHz, CDCl3) d [ppm]: 164.0, 153.2, 130.3,
103.6, 66.0; MS (EI) m/z 238 [Mþ]. Anal. calcd. for C9H6N2S2O2: C 45.36, H
2.54, N 11.76; found: C 45.19, H 2.27, N 11.57.
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Synthesis of 4: 3 (124 mg, 0.520 mmol) was placed in a 50 mL round-
bottomed flask and dissolved in THF (10 mL). To the mixture was added
dropwise n-BuLi (1.58 M, 0.7 mL) at ꢀ78 8C. After stirring for 1 h at ꢀ78 8C,
SnBu3Cl (352 mg, 1.08 mmol) was added. After stirring for 0.5 h at room
temperature, the reaction was quenched by the addition of H2O (2 mL).
The aqueous layer was extracted with EtOAc, and the combined organic
layers were washed with water and dried over MgSO4. After removal of the
solvent under reduced pressure, the residue was purified by column
chromatography on alumina (hexane) followed by further purification with
GPC (CHCl3) to give 4 (417 mg, 98%) as a yellow oil: 1H NMR (400 MHz,
CDCl3) d [ppm]: 4.55 (s, 4H), 1.58 (m, 12H), 1.25 (m, 24H), 0.90 (t, 18H,
J ¼ 7.5 Hz); 13C NMR (150 MHz, CDCl3) d [ppm]: 175.6, 166.0, 133.8,
103.4, 65.5, 28.8, 27.2, 13.7, 11.5; MS (EI) m/z 816 [Mþ].
X-ray Information: The diffraction data of C-BTz were collected on a
Rigaku RAXIS-RAPID Imaging Plate with monochromated Cu Ka
˚
(l ¼ 1.54187A) radiation. The structures were determined by a direct
method (SHELX97). The nonhydrogen atoms were refined anisotropically.
Crystallographic Data for C-BTz : C23H8F6N2O3S2, M ¼ 538.43, triclinic,
˚
space group P1 (No. 2); a ¼ 6.4444(3), b ¼ 9.6862(4), c ¼ 17.7387(8) A;
3
˚
a ¼ 98.685(2)8, b ¼ 93.765(2)8, g ¼ 105.438(2)8; V ¼ 1048.56(8) A ; Z ¼ 2,
Dcalcd ¼ 1.705 g cmꢀ3, F(000) ¼ 540, 11 150 reflections measured, 3711
unique, R ¼ 0.1027 for I > 2s(I), wR ¼ 0.1154 for all data.
Crystallographic data (excluding structure factors) for the structure
reported in this paper have been deposited with the Cambridge
Crystallographic Data Centre as supplementary publication no. CCDC-
746339.
Synthesis of C-BTz: 4 (240 mg, 0.29 mmol), 40-bromo-2,2,2-trifluoro-
acetophenone (223 mg, 0.88 mmol), and tetrakis(triphenylphosphine)pal-
ladium(0) (34 mg, 0.029 mmol) were placed in a test tube and dissolved
with toluene (6 mL). The reaction mixture was stirred at 120 8C for 13 h.
After removal of the solvent under reduced pressure, the residue was
washed with methanol and diethyl ether to give B (150 mg, 88%) as a red
solid: 1H NMR (400 MHz, CDCl3) d [ppm]: 8.15 (m, 8H), 4.62 (s, 4H); MS
(EI) m/z 582 [Mþ].
Bottom-Contact Device: The field-effect mobility of C-BTz and BTz was
measured using a bottom-contact, thin-film, field-effect transistor (FET)
geometry. The p-doped silicon substrate functioned as the gate electrode. A
thermally grown silicon oxide dielectric layer on the gate substrate was
300 nm thick and a capacitance of 10.0 nF cmꢀ2 was used. Interdigital
source and drain electrodes were constructed with a gold (30 nm) layer
formed on the SiO2 layer. The channel width (W) and channel length (L)
were 38 mm and 5 mm, respectively. The silicon oxide surface was first
washed with acetone and 2-propanol. The silicon oxide surface was then
activated by ozone treatment and pretreated with octadecyltrichlorosilane
(ODTS). The semiconductor layer was vacuum-deposited on the Si/SiO2
B (140 mg, 0.240 mmol) was placed in a 100 mL round-bottomed flask
and dissolved with AcOH (50 mL). To the mixture was added HCl aq. (12 M,
3 mL) at 100 8C. The reaction mixture was stirred at 100 8C for 2 h. After
being cooled to room temperature, the reaction was quenched by the
addition of H2O. The solid was collected and washed with H2O, methanol,
and diethyl ether followed by purification under gradient sublimation under
a high vacuum (10ꢀ2 Pa) to give pure C-BTz (102 mg, 79%) as a dark green
solid: m.p. > 300 8C; 1H NMR (400 MHz, CDCl3) d [ppm]: 8.09 (d, 4H,
J ¼ 8.7 Hz), 8.07 (d, 4H, J ¼ 8.7 Hz); MS (EI) m/z 538 [Mþ]; UV–vis (THF):
substrate at a rate of 0.2 A sꢀ1 under a pressure of 10ꢀ6 Pa to a thickness of
˚
10 nm determined by a quartz crystal monitor. The characteristics of the
OFET devices were measured at room temperature under a pressure of
10ꢀ6 Pa carefully avoiding exposure to air after fabricating of the active
layer. The field-effect mobility (m) was calculated in the saturated region at a
VDS of 80 V by the following equation:
abs
ems
l
(e) ¼ 404 nm (50 000); fluorescence spectra (THF): l ¼ 523 nm
max
max
(Ff 0.02). Anal. calcd. for C23H8F6N2 ꢂ O3S2: C 51.30, H 1.50, N 5.20; found:
W
2
IDS
¼
CimðVGS ꢀ VthÞ
(1)
C 51.04, H 1.41, N 5.09.
2L
Synthesis of 5: 1 (1.18 g, 2.46 mmol) was placed in a 200 mL round-
bottomed flask and dissolved in THF (50 mL). To the mixture was added
tetrabutylammonium fluoride (1 M, 9.0 mL, 9.0 mmol) at 0 8C. After stirring
for 19 h at 0 8C, the reaction was quenched by the addition of H2O. The
aqueous layer was extracted with EtOAc, and the combined organic layers
were washed with water and dried over MgSO4. After removal of the solvent
under reduced pressure, the residue was purified by column chromato-
graphy on silica gel (3:1 hexane/EtOAc) to give 5 (384 mg, 93%) as a white
solid: m.p. 91–92 8C; 1H NMR (400 MHz, CDCl3) d [ppm]: 8.79 (s, 2H),
8.01 (s, 2H); MS (EI) m/z 168 [Mþ].
The current on/off ratio was determined from the IDS at VGS ¼ 0 V (Ioff
)
and VGS ¼ 100 V (Ion
)
Top-Contact Device: The field-effect mobility of C-BTz and BTz was also
measured using a top-contact thin-film field-effect transistor (FET)
geometry. The n-doped silicon substrate functioned as the gate electrode.
A thermally grown silicon oxide dielectric layer on the gate substrate was
300 nm thick and a capacitance of 10.0 nF cmꢀ2 was used. The silicon oxide
surface was first washed with acetone and 2-propanol, and then activated
by ozone treatment, and pretreated with ODTS. The substrate was washed
again with toluene, acetone, and 2-propanol. The semiconductor layer was
Synthesis of 6: 5 (221 mg, 1.31 mmol) was placed in a 50 mL round-
bottomed flask and dissolved in THF (13 mL). To the mixture was added
dropwise n-BuLi (1.58 M, 2.5 mL, 3.95 mmol) at ꢀ78 8C. After stirring for 1 h
at ꢀ78 8C, SnBu3Cl (1.02 g, 3.13 mmol) was added. After stirring for 0.5 h at
room temperature, the reaction was quenched by the addition of H2O
(2 mL). The aqueous layer was extracted with EtOAc, and the combined
organic layers were washed with water and dried over MgSO4. After
removal of the solvent under reduced pressure, the residue was purified by
column chromatography on alumina (hexane) followed by further
purification with GPC (CHCl3) to give 6 (221 mg, 24%) as a brown oil:
1H NMR (400 MHz, CDCl3) d [ppm]: 8.14 (s, 2H), 1.60 (m, 12H), 1.35 (m,
12H), 1.23 (m, 12H), 0.90 (t, 18H, J ¼ 7.5 Hz); MS (EI) m/z 746 [Mþ].
Synthesis of BTz: 6 (221 mg, 0.30 mmol), 40-bromo-2,2,2-trifluoroace-
tophenone (223 mg, 0.88 mmol), and tetrakis(triphenylphosphine)palla-
dium(0) (34 mg, 0.029 mmol) were placed in a test tube and dissolved with
toluene (3 mL). The reaction mixture was stirred at 120 8C for 16 h. After
being cooled to room temperature, the solvent was removed under
reduced pressure. The crude solid was collected and washed with methanol
and diethyl ether followed by purification under gradient sublimation at a
high vacuum (10ꢀ2 Pa) to give pure BTz (35 mg, 23%) as a yellow
solid: m.p. > 300 8C; 1H NMR (400 MHz, CDCl3) d [ppm]: 8.19 (d, 4H,
J ¼ 8.5 Hz), 8.15 (d, 4H, J ¼ 8.5 Hz), 8.10 (s, 2H); MS (EI) m/z 512 [Mþ];
vacuum-deposited on the Si/SiO2 at a rate of 0.2 A sꢀ1 under a pressure of
˚
10ꢀ6 Pa to a thickness of 10 nm determined by a quartz crystal monitor.
On the top of the semiconductor layer, the gold source and drain
electrodes (20 nm) were deposited by using shadow masks with a
channel width of 5 mm and a channel length of 20 mm. The characteristics
of the OFET devices were measured at room temperature under a pressure
of 10ꢀ2 Pa or under atmospheric pressure in air. The field-effect
mobility was calculated in the saturated region at a VDS of 80 V. The
current on/off ratio was determined from the IDS at VGS ¼ 0 V (Ioff) and
VGS ¼ 100 V (Ion).
Acknowledgements
This work was supported by the Industrial Technology Research Grant
Program 2008 from the New Energy and Industrial Technology
Development Organization (NEDO) of Japan and a Grants-in-Aid for
Scientific Research from the Ministry of Education, Culture, Sports,
Science, and Technology, Japan, and by cooperative Research with
Sumitomo Chemical Co., Ltd. YI is thankful to the Advanced Technology
Institute Foundation and the Kinki Invention Center (Foundation). We
abs
ems
l
(e) ¼ 401 nm (55 000); fluorescence spectra (THF): l ¼ 491 nm
max
max
(Ff ¼ 0.39). Anal. calcd. for C22H10F6N2 ꢂ O2S2: C 51.56, H 1.97, N 5.47;
¨
acknowledge Masakazu Yamagishi for his assistance with the Huckel
found: C 51.32, H 1.98, N 5.70.
912
ß 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Funct. Mater. 2010, 20, 907–913

www.afm-journal.de
www.MaterialsViews.com
calculations. Thanks are extended to the CAC and ISIR for their assistance
in obtaining the elemental analyses. Supporting Information is available
online from Wiley InterScience or from the author.
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913