Effect of the substitution pattern of alkyl side chain in a benzodithiophene core π-system on intra and inter-molecular charge carrier mobility

Article abstract of DOI:10.1021/jp2036668

3,7-Didocecyl-2,6-di(5-phenylthiophen-2-yl)benzo[1,2-b:3,4-b′] dithiophene (1) and its 4,8-didodecyl isomer 2 were prepared as the representative soluble X- and cross-shaped π-conjugated oligomer systems to provide insight into the effect of the substitut

Full text of DOI:10.1021/jp2036668

ARTICLE
pubs.acs.org/JPCB
Effect of the Substitution Pattern of Alkyl Side Chain in a
Benzodithiophene Core π-System on Intra and Inter-Molecular
Charge Carrier Mobility
Jun Kumagai, Koji Hirano, Tetsuya Satoh, Shu Seki,* and Masahiro Miura*
Department of Applied Chemistry, Faculty of Engineering, Osaka University, Suita, Osaka 565-0871, Japan
S Supporting Information
b
ABSTRACT: 3,7-Didocecyl-2,6-di(5-phenylthiophen-2-yl)benzo[1,2-b:3,4-b0]dithio-
phene (1) and its 4,8-didodecyl isomer 2 were prepared as the representative soluble
X- and cross-shaped π-conjugated oligomer systems to provide insight into the effect of
the substitution pattern of the alkyl side chain on electronic properties. The absorption
and emission spectra as well as CV data showed the relatively longer effective
conjugation of cross-shaped 2. The intrinsic charge-carrier mobilities were then
estimated by flash-photolysis time-resolved microwave conductivity (FP-TRMC)
method and compared with their top contact FET properties. It was found that,
although the TRMC method showed the higher mobility of 2 than 1, the FET
performance of 1 after appropriate conditioning and thermal annealing was superior
to that of 2. The effective conjugation of cross-shaped 2 is well reflected in the
intramolecular mobility of positive holes estimated by FP-TRMC, showing striking
contrast to the rather higher mobility of X-shaped 1 observed by FET as well as TOF
measurements as the long-range translational motion of the carriers. This strongly suggests that the intermolecular packing of these
compounds plays a significant role in the range of hole mobility of <∼10ꢀ2 cm2 Vꢀ1 sꢀ1
.
’ INTRODUCTION
’ RESULTS AND DISCUSSION
Poly- and oligoaryl compounds involving thiophene units
have become increasingly important for the organic components
of electronic devices including organic field effect transistors
(OFETs), organic light-emitting diodes (OLEDs), and photovol-
talic cells. Thus, their synthesis and properties of thiophene-fused
aromatic compounds having benzothiophene and thienothio-
phene skeletons as well as oligothiophenes as the π-conjugated
moieties have recently been studied extensively.1 For solution-
processable OFET components, rod- or ladder-like oligomers
bearing two alkyl groups at their terminals, often the R-positions
of thiophene units, are usually designed, whereas the β-alkyl
derivatives are commonfor thiophene-based polymeric π-systems.
In this context, it appears to be interesting to investigate the
properties of β-alkyl derivatives of thiophene-based oligomers as
partial models of the corresponding polymers.2 Consequently,
we have undertaken to prepare such types of compounds:
[1,2-b:3,4-b0]benzodithiophene has been selected as the relatively
rigid central core3 and its 3,7-dialkylated and 2,6-di(5-phenyl-
thiophen-2-yl)-capped derivative 1 has been prepared as a new,
X-shaped oligomer (Figure 1). The 4,8-dialkyl isomer 2 as a
cross-shaped derivative has also been synthesized.4 Then, their
fundamental properties have been investigated. For the estimation
of their charge-carrier mobilities, flash-photolysis time-resolved
microwave conductivity (FP-TRMC) method, which allows us
to measure the intrinsic ability of a given sample, has been applied
and compared with those measured by top contact FET.
Our synthetic approaches to 1 and 2 are illustrated in
Scheme 1. The benzo[1,2-b:3,4-b0]dithiophene core in 1 was
constructed upon treatment of commercially available 1,4-phenyl-
enediacrylic acid (3) with thionyl chloride in the presence of
pyridine.5 The resulting acid chloride 4 was transformed to the
corresponding butyl ester 5 having substantial solubility. The
subsequent Sonogashira coupling with 1-dodecyne using a
catalyst system of PdCl2(PhCN)2/XPhos (XPhos = dicyclohexyl-
phosphino-20,40,60-triisopropylbiphenyl)6 followed by hydroge-
nation in the presence of Pd/C enabled the introduction of
dodecyl group to the 3- and 7-positions of the benzodithiophene
motif. The removal of ester function was achieved by hydrolysis
with aqueous KOH and Cu(0)-mediated decarboxylation to
furnish the desired 3,7-didodecylbenzo[1,2-b:3,4-b0]dithiophene
9. Finally, the regioselective dibromination with N-bromosucci-
nimide (NBS) and Stille coupling with tributyl(5-phenyl-
2-thienyl)tin led to 1 in good yield. On the other hand, the
preparation of 2, a target for comparison, was readily acces-
sible through the same cross-coupling strategy with the known
compound 11.3 All of the new compounds were fully char-
acterized by NMR and mass spectroscopy.
Received: April 19, 2011
Revised:
May 25, 2011
Published: May 26, 2011
r
2011 American Chemical Society
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Figure 1. Structures of X- and cross-shaped oligomers, 3,7-didocecyl-2,6-di(5-phenylthiophen-2-yl)benzo[1,2-b:3,4-b0]dithiophene (1) and 4,8-
didocecyl-2,6-di(5-phenylthiophen-2-yl)benzo[1,2-b:3,4-b0]dithiophene (2). R = n-C12H25.
Scheme 1. Synthetic Routes to Compounds 1 and 2
With the two desired oligomers in hand, we first investigated
their optical properties in CH2Cl2 solutions (Figure 2 and
Table 1). Although the absorption spectrum of X-shaped 1
exhibited the two major bands at λabs 296 and 390 nm, the
emission one showed two bands centered at λem 456 and 484 nm.
These peaks red-shifted in the cross-shaped oligomer 2, which is
indicative of relatively shorter effective conjugation length of 1.
On the contrary, the substitution pattern of the dodecyl group
gave little influence on the absolute fluorescence quantum yield
Φ. However, in the solid state, the Φ value of 1 was lower than
that of 2 (Figure 3), suggesting the importance of substituent
position for efficient self-assembly via intermolecular side-chain
alignment.7
consistent with their absorption and emission spectra. It is
worth noting that the outcome from the DFT calculations of
their HOMO and LUMO levels using the ethyl models (R = Et
for both 1 and 2, see Figure 5) also paralleled the observed
trends.
The optimized geometries of cross-shaped 1 and X-shaped 2
were also calculated by DFT method with B3LYP/6-31G(d)
basis sets using Gaussian 03 code.8 Unlike the complete planner
structures of compound 2 in steady state, compound 1 shows
apparently the twisted structures at both the ends of the
molecule. The symmetrical lobes of HOMO are observed on
the end-phenyl rings in compound 2, suggesting the effective
mixing of π-orbitals in the phenyl rings to the HOMO of the
compound. This may be the case given the slightly lower HOMO
level relative to that in compound 2.
Cyclic voltammetric measurement of both 1 and 2 in
CH2Cl2 solution displayed two reversible oxidation waves,
in which 1 provided a 0.07 eV higher ionization potential than
2, indicating a somewhat greater stability of 1 against oxidative
doping (Figure 4 and Table 2). These observations were
The thermal properties of 1 and 2 were also investigated by
differential scanning calorimetry (DSC) with the temperature
range of ꢀ20 to þ200 °C at a rate of 2 °C minꢀ1 (Figure 6).
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Figure 2. Absorption (left) and emission spectra (right) of compounds 1 and 2 in CH2Cl2 solution.
Table 1. Photophysical Data of Compounds 1 and 2 in
CH2Cl2 Solution
compd
λabs (nm)a
λem (nm)b
Eg (eV)c
log ε
Φd
1
2
296/390
456/484
468/499
2.8
2.7
4.6
4.5
0.63
0.58
303/396/420
a Absorption maximum in CH2Cl2 (5.0 ꢁ 10ꢀ6 M). b Emission max-
imum in CH2Cl2 (5.0 ꢁ 10ꢀ6 M). c Optical band gap. d Absolute
fluorescence quantum yield.
Figure 4. Cyclic voltammograms of 1 and 2 in CH2Cl2 containing
0.1 M Bu4NPF6 at a scan rate of 100 m V sꢀ1
.
Table 2. Cyclic Voltammogram Data and Optical Properties
of 1 and 2a
1/2
ox
compd
E
(V)a
EHOMO (eV)b
ELUMO (eV)c
Eg (eV)d
1
2
0.55/0.84
0.48/0.77
ꢀ5.35 (ꢀ5.00)e
ꢀ5.28 (ꢀ4.90)e
ꢀ2.55 (ꢀ1.88)e
ꢀ2.58 (ꢀ1.96)e
2.8
2.7
a Performed in Bu4NPF6/CH2Cl2 solution. ν = 100 mV/s, versus
Fe/Feþ. b HOMO = ꢀ4.8 ꢀ E . Estimated from HOMO, Eg: ELUMO
1/2 c
ox
= EHOMO þ Eg. d Optical band gap. Calculated by B3LYP/6-31G(d)
e
Figure 3. Emission spectra of compounds 1 and 2 in solid state: 1:
λem 531 nm, Φ = 0.20; 2: λem 537 nm, Φ = 0.36.
using the ethyl models (R = Et for both 1 and 2).
The phase transitions of 1 and 2 appeared at 99 and 157 °C,
respectively.
considerably in X-shaped 2, as plotted in Figure 7c. The higher
values of mobility in X-shaped 2 are responsible for the con-
tribution from second-order bulk recombination reactions in the
crystalline domains, whereas pseudo first-order recombination
kinetics is observed in Figure 7a for 1. Thus the second-order
bulk recombination analysis was performed for the transient
conductivity of X-shaped 2 as shown in Figure 7d, giving a clear
contribution from the recombination processes between positive
and negative charges.11
The major charge carrier species in the compounds were also
determined by time-of-flight (TOF) measurements. The current
transients were observed only under the positive bias mode
(positive bias applied at top electrodes), suggesting strongly
holes as the major charge carriers. Based on the TOF measure-
ment, the estimated yields of photocarrier generation upon
excitation at 355 nm are also listed in Table 3.
The higher transition temperature associated with the larger
enthalpy change (ΔH ∼ 3-hold in cross-shaped 2 relative to
X-shaped 1) is due to the effective π-stacking in the solid state of
cross-shaped 2, and this is also supported by the presumed planner
structureincross-shaped 2 leading to the densemolecular packing.
Prior to the investigation of the FET performance of 1 and 2,
the measurement by time-resolved microwave conductivity (TRMC)
method9,10 was carried out for the estimation of their intrinsic
charge mobility (Figure 7 and Table 3). The intrinsic mobility
Σμ of X-shaped 1 was calculated to be 1.6 ꢁ 10ꢀ2 cm2 Vꢀ1 sꢀ1
,
whereas that of cross-shaped 2 showed a higher value of 4.8 ꢁ
10ꢀ2 cm2 Vꢀ1 sꢀ1. This suggests that the local motion of charge
carriers in these compounds is in the same order with each other.
In the range of excitation photon density, cross-shaped 1
shows negligible dependence of the observed maximum values of
conductivity as shown in Figure 7a. However, the values decrease
The observed pseudosecond order rate constants in the TRMC
transients are increasing linearly from 0.055 to 0.11 μsꢀ1 upon
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Figure 5. Optimized geometries of compounds (a) 1 and (b) 2 calculated by DFT method with B3LYP/6-31G(d). Molecular orbitals calculated at
HOMO and LUMO levels are also presented for the respective compounds 1 and 2.
Figure 6. DSC thermograms of 1 (left) and 2 (right).
excitation photon density ranging from 5.2 ꢁ 1014 to 7.5 ꢁ 1016
photons cmꢀ2. On the assumption of the diffusion controlled
bulk recombination reactions as 1010 molꢀ1 dm3 sꢀ1, the pre-
dicted concentration of negative and positive charge carriers
are ∼100 nmol dmꢀ3 within 1 μs after the pulse exposure. In
contrast, the absorption coefficient of the solid state X-shaped 2 is
1.5 μmꢀ1 at 355 nm (based on the ε = 1.2 ꢁ 104 molꢀ1 dm3 cmꢀ1
at 355 nm, and the density of F ∼ 1 gcmꢀ3), leading to the
averaged initial charge carrier density of 20 to 2.9 ꢁ 103 nmol
dmꢀ3 under the excitation photon density range with ϕ = 6.2 ꢁ
10ꢀ3. This shows good agreement with the concentration in the
second-order recombination analysis, suggesting strongly the
adequacy of using ϕ values estimated by TOF measurements for
the mobility derived from TRMC analysis.
Figure 7e shows the values of mobility determined by the TOF
measurement as a long-range translational motion of holes in
these compounds. Cross-shaped 1 showed 1 order of magnitude
higher value of mobility than that of X-shaped 2. The small slopes
of the electric field dependence, especially in X-shaped 2, suggest
strongly the presence of the large hopping barriers in the
media.12 Thus the following FET device fabrications were tested
under varying surface structures and annealing conditions.
The top contact FET devices based on both 1 and 2 exhibited
typical p-type FET responses. The parameters extracted from
transfer characteristics are listed in Table 4. With neither surface
treatment nor annealing, the device based on cross-shaped 2 gave
a higher mobility than that with 1 (entries 1 and 4), consistent
with the preliminary investigation by the TRMC method. In
contrast, the surface modification of SiO2 with ODTS (octa-
decyltrichlorosilane) and an appropriate annealing to achieve
higher structural orders dramatically improved the FET perfor-
mance of 1 (entry 2). Since the attempt on ODTS modification
for 2 was failure due to difficulties in the deposition step, the
annealing alone was carried out (entry 5). However, the positive
effect was quite small, and its mobility was sluggishly improved.
The best performance was obtained when the device based on 1
was prepared with a longer source/drain pair was used (entry 3):
its mobility (μ), on/off current (Ion/Ioff), and threshold voltage
(Vth) were 3 ꢁ 10ꢀ3 cm2 Vꢀ1 sꢀ1, 10ꢀ4, and ꢀ36 V, respectively.
The transfer characteristics and output curves at different gate
voltages of some 1- and 2-based FET devices are shown in
Figure 8.13
’ CONCLUSION
We have synthesized 3,7-didocecyl-2,6-di(5-phenylthiophen-
2-yl)benzo[1,2-b:3,4-b0]dithiophene (1) and its 4,8-didodecyl
isomer 2 as the representative soluble X- and cross-shaped
oligomers bearing a benzodithiophene central core and com-
pared their hole mobilities as well as optical and thermal proper-
ties. Although the FP-TRMC method indicates the higher
charge-carrier mobility of 2, the 1-based device after appropriate
surface treatment and annealing proved to be superior to the
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Figure 7. Microwave conductivity transients of 1 (a) and 2 (b) upon 355 nm irradiation at excitation photon densities of (a) 2.1 ꢁ 1015 ꢀ 2.8 ꢁ 1016
and (b) 5.2 ꢁ 1014 ꢀ 7.5 ꢁ 1016 photons cmꢀ2, respectively. Observed maximum values of the transient conductivity of compound 2 were plotted vs
excitation photon density in panel c. (d) The second-order recombination analysis of the conductivity transient observed for compound 2 in the range of
excitation photon density. (e) The dependence of hole drift mobility determined by time-of-flight measurements on the applied electric field strength at
273 K. Circles and squares are the mobility observed in compounds 1 and 2, respectively.
Table 3. Results of Measurement by TRMC and TOF
Methods
Table 4. FET Performances of 1 and 2
surface
μ
Vth
entrya compd treatment
annealing
none
(cm2 Vꢀ1 sꢀ1) Ion/Ioff (V)
ϕmax Σμ
compd (cm2 Vꢀ1 sꢀ1
Σμ
μ
þ
(cm2 Vꢀ1 sꢀ1
)
(cm2 Vꢀ1 sꢀ1
)
a
)
ϕmax
1
1
1
1
2
2
bare
3.0 ꢁ 10ꢀ5
103 ꢀ40
104 ꢀ33
104 ꢀ36
103 ꢀ35
103 ꢀ35
ODTS 60 °C for 30 min 1.0 ꢁ 10ꢀ3
1
2
1.3 ꢁ 10ꢀ4
3.0 ꢁ 10ꢀ4
8.1 ꢁ 10ꢀ3
6.2 ꢁ 10ꢀ3
1.6 ꢁ 10ꢀ2
4.8 ꢁ 10ꢀ2
7 ꢁ 10ꢀ4, b
9 ꢁ 10ꢀ5, b
2
3b
ODTS 60 °C for 30 min 3.0 ꢁ 10ꢀ3
a Estimated by time-of-flight (TOF) method. b Hole drift mobility
determined by TOF measurement at 5.2 ꢁ 105 V cmꢀ1 (compd 1)
and 1.6 ꢁ 105 V cmꢀ1 (compd 2), respectively.
4
bare
bare
none
2.0 ꢁ 10ꢀ4
5
60 °C for 30 min 3.0 ꢁ 10ꢀ4
a Channel width (W) and length (L): W ꢁ L = 2000 μm ꢁ 20 μm.
b W ꢁ L = 2000 μm ꢁ 50 μm.
2-based one. These results provide intriguing insight into the
substitution pattern of alkyl side chain on the intra- and inter-
molecular charge carrier mobility. Further application of the
X-shaped core to other π-conjugated poly- and oligothiophene
derivatives are now in progress.
supporting electrolyte, using a platinum button as the working
electrode, a platinum wire as the counter electrode, and Fe/Feþ
(ferrocene/ferrocenium) as the reference redox couple. Differ-
entialscanning calorimetry (DSC) measurements wereperformed
on a 2920 MDSC V2.6A. The TRMC measurements were im-
plemented by Nd:YAG laser (third harmonic generation THG
(355 nm) from Spectra Physics INDY-HG (fwhm 5ꢀ8 ns)).9
The FET devices were characterized with a Keithley 4200 probe
station under reduced pressure. X-ray diffraction (XRD) patterns
were obtained by using Rigaku RINT 2000.
Unless otherwise noted, materials obtained from commercial
suppliers were used without further purification. Dicyclohexyl-
phosphino-20,40,60-triisopropylbiphenyl (XPhos) was purchased
from Aldrich. Tetrakis(triphenylphosphine)palladium (Pd(PPh3)4)
’ EXPERIMENTAL SECTION
Instrumentation and Chemicals. 1H and 13C NMR spectra
were recorded at 400 and 100 MHz for CDCl3 or DMSO-d6
solutions, respectively. MS data were obtained by EI or FAB.
Silica gel (Wakogel 200 mesh) was used for column chromatog-
raphy. Abosorption and photoluminescence spectra were mea-
sured as described previously.14 Cyclic voltammetry (CV) was
carried out on a BAS CV-50W voltammetric analyzer at a scan
rate of 100 mV sꢀ1 in CH2Cl2 containing 0.1 M Bu4NPF6 as
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Figure 8. Transfer characteristics at ꢀ60 V drain voltage (top) and output curves at different gate voltages (bottom) of FETs based on 1 and 2. (a) and
(d): FET of 1 after ODTS treatment and annealing (entry 2 in Table 4), (b) and (e): FET of 1 with longer source/drain pair after ODTS treatment and
annealing (entry 3 in Table 4), (c) and (f): FET of 2 after annealing (entry 5 in Table 4).
was available from TCI. Pd/C (10%) was obtained from Wako
Pure Chemical Co. Tributyl(5-phenyl-2-thienyl)tin was pre-
pared from 2-phenylthiophene and tributyltin chloride according
to the literature.15
1H NMR (400 MHz, CDCl3) δ 7.66ꢀ7.58 (m, 4H), 7.48
(s, 2H), 7.44ꢀ7.34 (m, 4H), 7.33ꢀ7.26 (m, 6H), 3.11 (t, J =
7.6 Hz, 4H), 1.88ꢀ1.78 (m, 4H), 1.55ꢀ1.45 (m, 4H), 1.42ꢀ1.20
(m, 32H), 0.86 (t, J = 6.4 Hz, 6H); 13C NMR (100 MHz, CDCl3)
δ 144.5, 137.4, 137.3, 137.1, 136.6, 134.2, 129.2, 128.8, 128.0,
126.1, 125.9, 124.1, 117.9, 33.6, 32.2, 30.2, 29.96 (2C), 29.90,
29.84, 29.77 (2C), 29.59, 22.9, 14.3; HRMS (EI): m/z (Mþ)
calcd for C54H66S4: 842.4047, found: 842.4061.
Flash-Photolysis Time Resolved Microwave Conductivity
(FP-TRMC) Mearsurement. The nanosecond laser pulses from a
Nd: YAG laser (third harmonic generation, THG (355 nm) from
Spectra Physics, INDY-HG (fwhm 5ꢀ8 ns) have been used as
excitation sources. The power density of the laser was set at 5.2 ꢁ
1014 to 8.9 ꢁ 1016 photons cmꢀ2. For time-resolved microwave
conductivity (TRMC) measurement, the microwave frequency
and power were set at ∼9.1 GHz and 3 mW respectively. The
TRMC signal picked up by a diode (rise time <1 ns) is monitored
by a digital oscilloscope of Tektronix TDS3032B. All the above
experiments were carried out at room temperature. The transient
photoconductivity of the samples is propotional to the reflected
microwave power (ΔPr/Pr) and sum of the mobilities of charge
carriers via:
Synthesis of 1. 3,7-Dibromobenzodithiophene 10 (200 mg,
0.29 mmol) (see Scheme 1 and the Supporting Information for the
preparation), tributyl(5-phenyl-2-thienyl)tin (300 mg, 0.67 mmol),
Pd(PPh3)4 (24 mg, 0.020 mmol), DMF (3 mdm3), and toluene
(3 mdm3) were placed in a 20 mdm3 two-necked reaction flask.
The solution was stirred at 85 °C for 24 h. The mixture was
quenched with water, extracted with toluene, and evaporated in
vacuo. The residue was purified by column chromatography with
toluene followed by washing with acetone afforded the X-shaped
oligomer, 3,7-didocecyl-2,6-di(5-phenylthiophen-2-yl)benzo-
[1,2-b:3,4-b0]dithiophene (1, 200 mg, 0.23 mmol) in 81% yield as
1
a yellow solid. H NMR (400 MHz, CDCl3) δ 7.68ꢀ7.62
(m, 6H), 7.45ꢀ7.37 (m, 4H), 7.44ꢀ7.34 (m, 4H), 7.26ꢀ7.23
(m, 2H), 3.10 (t, J = 7.6 Hz, 4H), 1.81ꢀ1.70 (m, 4H), 1.55ꢀ1.45
(m, 4H), 1.42ꢀ1.20 (m, 32H), 0.88 (t, J = 6.8 Hz, 6H); 13C
NMR (100 MHz, CDCl3) δ 145.2, 138.7, 135.8, 134.7, 134.3,
132.2, 129.8, 129.2, 128.0 (2C), 126.0, 123.7, 119.3, 32.2, 30.4,
30.1, 29.93 (2C), 29.89, 29.88, 29.69, 29.59, 28.1, 22.9, 14.3;
HRMS (EI): m/z (Mþ) calcd for C54H66S4: 842.4047, found:
842.4055.
1 ΔPr
Synthesis of 2. Using 2,6-dibromo-4,7-didodecylbenzo-
[1,2-b:3,4-b0]dithiophene (11, 270 mg, 0.39 mmol), which was pre-
pared according to the literature,4 as the starting material, the
coupling with tributyl(5-phenyl-2-thienyl)tin (420 mg, 0.94 mmol)
was carried out under the same palladium catalysis as described in
the synthesis of 1. The cross-shaped oligomer, 4,8-didocecyl-2,
6-di(5-phenylthiophen-2-yl)benzo[1,2-b:3,4-b0]dithiophene
(2, 240 mg, 0.28 mmol, 73%) was isolated as an orange solid.
¼ eϕN
μ
ð1Þ
∑
A Pr
where A, e, ϕ, N, and Σμ are a sensitivity factor, elementary charge
of electron, photo carrier generation yield (quantum efficiency),
the number of absorbed photons per unit volume, and sum of
mobilities for negative and positive carriers, respectively. Polar-
ization of the laser pulses is isotropic.
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The compounds were dissolved in o-dichlorobenzene and
cast onto quartz substrate at 0.6ꢀ2.1 μm thick determined by
Dektak150 surface profiler. The films were heated up to 200 °C
under an Ar atmosphere and annealed at the temperature for
30minbeforethemeasurement. Thenumberofphotons absorbed
by the film is estimated by the direct measurement of transmitted
power of laser pulses Opher NOVA-display power meter.
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(1) Recent reviews:(a) Katz, H. E.; Bao, Z.; Gilat, S. L. Acc. Chem.
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2007, 36, 578. (c) Murphy, A. R.; Frꢀechet, J. M. J. Chem. Rev. 2007,
107, 1066. (d) Ong, B. S.; Wu, Y.; Li, Y.; Liu, P.; Pan, H. Chem.—Eur.
J. 2008, 14, 4766. (e) Allard, S.; Forster, M.; Souharce, B.; Thiem, H.;
Schert, U. Angew. Chem., Int. Ed. 2008, 120, 4138. (f) Chen, J.; Cao, A. Y.
Acc. Chem. Res. 2009, 42, 1709.
Time-of-Flight Mearsurement. The compounds 1 and 2
were overcoated onto an Al electrode from o-dichlorobenzene
solutions at 8.0 and 4.2 μm thick, respectively. The films were
annealed at 200 °C for 3 h under high vacuum. The film was sub-
sequently overcoated by an Au semitransparent electrode. The
measurement was carried out in an environmental chamber at
273 K, > 10ꢀ4 Pa. The nanosecond laser pulses from a Nd: YAG
laser (third harmonic generation, THG (355 nm) from Spectra
Physics, Quanta-Ray (fwhm 5ꢀ8 ns) have been used as excita-
tion sources. The power density of the laser was set at 2.0 ꢁ 1015
photons cmꢀ2. Current transients were recorded through a
terminate resistance of 3 kΩ by a Tektronic TDS 3052B digitizing
oscilloscope. The bias voltage was applied to the top electrode by
an ADVANTEST R8252 power source. Photogenerated charge
carriers are estimated by an integration of the current transient,
with simultaneous accumulation by a Keithley R6487 current
integrator. The other set of apparatus is described in elsewhere.16
FET Measurement. FET devices based on 1 and 2 as the
semiconductors were fabricated with top contact configuration
on silicon wafer in inert conditions. A heavily n-doped silicon
wafer with a 300-nm thermal silicon dioxide (SiO2) was used as
the substrate/gate electrode, with the top SiO2 layer serving as
the gate dielectric. Two types of wafer were prepared; one was
employed without any special pretreatments and the other
was immersed in a solution of octadecyltrichlorosilane (ODTS,
200 μdm3) in octane (50 mdm3) overnight prior to fabrication.
The active layers were deposited on top of SiO2 surface by spin-
coating a solution of 1 or 2 in toluene (0.5 wt %). Subsequently,
some wafers were annealed at 60 °C for 30 min. MoO3 (15 nm)
and Au (50 nm) were sequentially deposited by vacuum evapora-
tion through a shadow mask to create a source/drain pair. The
channel width (W) and length (L) were 2000 μm and 20 (or 50)
μm, respectively. The field-effectmobility (μ) was calculated inthe
saturation region at the Vd of ꢀ60 V using the following equation:
Id = μCi(VgꢀVth)2, where Ci is the capacitance of SiO2, and Vg
and Vth are gate voltage and threshold voltage, respectively.
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’ ASSOCIATED CONTENT
S
Supporting Information. Preparation of compound 10,
b
detailed characterization data of compounds 6ꢀ11, and XRD
patterns of some devices based on 1 and 2. This material is
available free of charge via the Internet at http://pubs.acs.org.
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diffraction (XRD) measurements.
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Int. Ed. 2008, 47, 4019. (b) Horita, A.; Tsurugi, H.; Funayama, A.; Satoh,
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’ AUTHOR INFORMATION
Corresponding Author
*E-mail: seki@chem.eng.osaka-u.ac.jp; miura@chem.eng.osaka-u.ac.jp.
(15) Pelter, A.; Jenkins, I.; Jones, D. E. Tetrahedron 1997, 53, 10357.
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Furukawa, K.; Fujiki, M.; Matsumoto, N. Phil. Mag. B 1999, 79, 1631.
(b) Acharya, A.; Seki, S.; Koizumi, Y.; Saeki, A.; Tagawa, S. J. Phys. Chem.
B 2005, 109, 20174.
’ ACKNOWLEDGMENT
This work was supported, in part, by Grants-in-Aid from the
Ministry of Education, Culture, Sports, Science, and Technology,
Japan, and by the cooperative research with Sumitomo Chemical
Co., Ltd.
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dx.doi.org/10.1021/jp2036668 |J. Phys. Chem. B 2011, 115, 8446–8452