Synthesis and characterization of novel p-type alkyl bithiophene end-capped anthracene and naphthalene derivatives for organic thin-film transistors

Article abstract of DOI:10.1166/jnn.2012.5912

New semiconductors having naphthalene and anthracene cores with hexylated bithiophene side units, 2,6-bis(5'-hexylbithiophen-2'-yl)naphthalene (HBT-NA) and 2,6-bis(5'-hexylbithiophen-2'-yl)anthracene (HBT-AN), were synthesized. HBT-AN and HBT-NA were characterized using FT-IR, ¹H-NMR, Mass spectrum and elemental analysis. HBT-AN and HBT-NA showed well ordered crystalline with high thermal stabilities as evidenced by 5% weight loss at 447 °C for HBT-AN and 434 °C for HBT-NA. The closed packed structures between adjacent molecules were observed by studying UV-visible and photoluminescence (PL) in solution and film. The HOMO energy levels of HBT-NA and HBT-AN were found to be 5.47 eV and 5.42 eV, respectively. HBT-NA exhibits hole mobility of 8.4×10^-2 cm²/Vs and on/off ratio of 5.6×10⁵. HBT-AN shows 5.2×10^-2 cm ²/Vs and on/off ratio of 1.0×10⁵. Copyright

Full text of DOI:10.1166/jnn.2012.5912

Copyright © 2012 American Scientific Publishers
All rights reserved
Printed in the United States of America
Journal of
Nanoscience and Nanotechnology
Vol. 12, 4299–4304, 2012
Synthesis and Characterization of Novel p-Type Alkyl
Bithiophene End-Capped Anthracene and Naphthalene
Derivatives for Organic Thin-Film Transistors
Hye Jin Koh1, Sang Hun Jang1, Chul Young Kim2,
Sung Chul Shin2, Yun-Hi Kim2ꢀ∗, and Soon-Ki Kwon1ꢀ∗
1School of Materials Science and Engineering and Engineering Research Institute (ERI),
Gyeongsang National University, Jinju, 660-701, Republic of Korea
2Department of Chemistry and RINS, Gyeongsang National University, Jinju, 660-701, Republic of Korea
New semiconductors having naphthalene and anthracene cores with hexylated bithiophene
side units, 2,6-bis(5ꢀ-hexylbithiophen-2ꢀ-yl)naphthalene (HBT-NA) and 2,6-bis(5ꢀ-hexylbithiophen-2ꢀ-
yl)anthracene (HBT-AN), were synthesized. HBT-AN and HBT-NA were characterized using FT-IR,
1H-NMR, Mass spectrum and elemental analysis. HBT-AN and HBT-NA showed well ordered crys-
talline with high thermal stabilities as evidenced by 5% weight loss at 447 ꢁC for HBT-AN and 434 ꢁC
for HBT-NA. The closed packed structures between adjacent molecules were observed by studying
UV-visible and photoluminescence (PL) in solution and film. The HOMO energy levels of HBT-NA
and HBT-AN were found to be 5.47 eV and 5.42 eV, respectively. HBT-NA exhibits hole mobility of
8ꢀ4 × 10−2 cm2/Vs and on/off ratio of 5ꢀ6 × 105. HBT-AN shows 5ꢀ2 × 10−2 cm2/Vs and on/off ratio
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of 1ꢀ0×105.
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Copyright: American Scientific Publishers
Keywords: OTFT, Small Acenes, Hole Mobility.
1. INTRODUCTION
reported di(phenylvinyl)anthracene (DPVAnt) showed the
high mobility of 1.3 cm2/Vs and an on/off ratio of 107.11
Recently, we reported a new thienothiophene-end-capped
oligo-acene derivative, 2,6-bis(50-hexylthieno[3,2-b]thio-
phen-20-yl)anthracene (DH-TAT), which showed a hole
mobility of 0.14 cm2/Vs, and an on/off ratio of 6ꢁ3 ×
106.12 Recently, hybrid acenethiophene molecules were
reported.13–14 To obtain high carrier mobilities with-
out decreasing the chemical stability, it is crucial that
the organic molecules be arranged in a specific molec-
ular architecture.16–18 In this paper, a series of new
organic semiconductors, 2,6-bis(5ꢀ-hexylbithiophen-2ꢀ-
yl)naphthalene (HBT-AN), and 2,6-bis(5ꢀ-hexylbithiophen-
2ꢀ-yl)anthracene (HBT-NA), is designed with anthracene
or naphthalene core unit with alkylated bithiophene on
each side as symmetric side units. The electron-donating
hexyl groups were introduced in order to tune the elec-
tronic properties of the small fused acenes and to increase
the self-assembly stacking property. The conjugated side
chain system may guarantee strong interactions as a result
of the extension of electron acene group through the side
chain. The composed fused aromatics would be expected
to have a linear structure with a high degree of conju-
gation along the backbone of the molecules for efficient
ꢀ-conjugated oligomeric and polymeric semiconductors
have been the focus of intense research over the past few
decades as alternatives to inorganic semiconductors for
low-cost electronic applications such as organic thin-film
transistors (OTFTs).1–6 The best performing molecule as
the active material, pentacene has good TFT performance
(mobilities up to 3.0 cm2/Vs)7 due to its planar shape
facilitating crystal packing and the extended ꢀ-conjugated
system over molecules enabling the intermolecular overlap
of ꢀ-systems. However, chemical instability of pentacene
has been restricted in synthesis and thin film depo-
sition as well as commercial application despite their
excellent TFT characteristics. The oligomerization of a
small acene can lead to an effective means of extend-
ing the ꢀ-system.8 Frisbie and coworkers demonstrated
high mobility thiophene/acene hybrid semiconductors,9
reporting anthracene-end-capped oligomers with stable
device performance with a mobility of up to 0.12 cm2/Vs.
Values up to 0.40 cm2/Vs were reported for a naph-
thyl-end-capped quarterthiophene, 5,5ꢀꢀꢀ-bis(naphth-2-yl)-
2,2ꢀ:5,2ꢀꢀ:5ꢀꢀ,2ꢀꢀꢀ-quaterthiophene (NaT4).10 Klauk et al.
∗Authors to whom correspondence should be addressed.
J. Nanosci. Nanotechnol. 2012, Vol. 12, No. 5
1533-4880/2012/12/4299/006
doi:10.1166/jnn.2012.5912
4299

Synthesis and Characterization of Novel p-Type Alkyl Bithiophene End-Capped Anthracene and Naphthalene Derivatives Koh et al.
hole transport and have a suitable HOMO and solid-state
structure for use as a p-channel TFT.
13.39 mmol) and 2,6-dibromoanthracene (1.5 g,
4.46 mmol) dissolved in THF (5 mL) and toluene (30 mL)
were added an aqueous 2 M-K2CO3 (50 mL). The mixture
was bubbled with nitrogen for 30 min.ꢁPd(PPh3ꢂ4 was then
added. The mixture was heated to 90 C for 24 h under a
nitrogen atmosphere. The reaction mixture was poured into
aqueous 2 N-HCl and filtered off. The crude product was
purified by Soxhlet extraction with methanol and toluene,
then ꢁrecrystallized from toluene. Yield: 1.2 g (42%) mp:
329 C, MS(EI) (m/z): 674 (M+ꢂ, FT-IR (KBr, cm−1ꢂ:
3100 (aromatic C–H), 2849–2950 (aliphatic C–H), 1651
(C O). Anal. Cacld for C42H42S4: C, 74.73%; H, 6.27%.
Found: C, 74.68%; H, 6.21%.
2. EXPERIMENTAL DETAILS
2.1. Materials
Triphenyl phosphine, NaH, Magnesium, 2-bromothio-
phene, DMF, NBS, 1-bromohexane, Ni(dppp)Cl2 were
purchased from Aldrich. Tetrakis(triphenyl phosphine) pal-
ladium(o) were purchased from Lancaster. All reagents
purchased commercially were used without further
purification.
2.2. Device Fabrication
2.4. 2,6-Bis(5ꢀ-hexylbithiophen-2ꢀ-yl)
Anthracene (HBT-NA)
A organic semiconducting layer (60 nm) was deposited on
a SiO2 (300 nm, 10 nF/cm2ꢂ surface on a heavily doped
silicon wafer (gate electrode). The SiO2 surface were
pretreated with 1,1,1,3,3,3-hexamethyldisilazane (HMDS).
After evaporation, the OTFTs were completed by evaporat-
ing gold through a shadow mask to form source and drain
electrodes on the semiconducting thin films as a top con-
tact geometry. This device has a channel length and width
of 20 ꢃm and 5 mm, respectively. The TFTs characteristics
were measured with a KEITHLEY 4200 semiconductor
characterization system at an air atmosphere.
HBT-NA was prepared using the same method as
described for HBT-AN using 2,6-dibromonaphthalene
(1.0 g, 3.49 mmol) and 2-(5ꢀ-Hexyl-[2,2ꢀ]bithiophenyl-
5-yl)-4,4,5,5-tetramethyl-1,3,2 dioxaborolane (ꢁ3) (3.9 g,
10.49 mmol). Yield: 1.27 g (70%). mp: 349 C (DSC).
FT-IR (KBr, cm−1ꢂ: 3056 (aromatic C–H), 2848 (aliphatic
C–H), 800 (C–S). MS (EI) m/z: 622 (M+ꢂ. Anal. Cacld
for C38H40S4: C, 73.26%; H, 6.15%. Found: C, 73.09%;
H, 6.18%.
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2.3. 2,6-Bis(5ꢀ-hexylbithiophen-2ꢀ-yl)
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3. RESULTS AND DISCUSSION
Copyright: American Scientific Publishers
Naphthalene (HBT-AN)
The synthetic routes for HBT-AN and HBT-NA are
depicted in Scheme 1. The materials were easily syn-
thesized through Grignard reaction and Suzuki coupling
To a solution of 2-(5ꢀ-Hexyl-[2,2ꢀ]bithiophenyl-5-yl)-
4ꢄ4ꢄ5ꢄ5-tetramethyl-1ꢄ3ꢄ2 dioxaborolane (3) (5.4 g,
i) Mg/ether
i) n-BuLi/THF
S
S
S
S
Br
ii) 1-bromohexane
ii) Ni(dppp)Cl2
S
2-bromothiophene
(2)
(1)
O
S
B
n-BuLi/THF
O
O
S
B
O
(3)
O
S
Br
2M-K2CO3/toluene
Pd(pph3)4
S
(3)
S
Br
S
(HBT-AN)
S
Br
S
2M-K2CO3/toluene
Pd(pph3)4
(3)
S
Br
S
(HBT-NA)
Scheme 1. Synthesis routes for HBT-AN and HBT-NA.
4300
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Koh et al. Synthesis and Characterization of Novel p-Type Alkyl Bithiophene End-Capped Anthracene and Naphthalene Derivatives
Fig. 1. Optical absorption (UV) and emission (PL) spectra of (a) HBT-AN and (b) HBT-NA in o-Dichlorobenzene solution and film.
reaction. The obtained oligomers showed very poor solu-
bility in common organic solvents but they were soluble
only in hot aromatic solvents such as toluene, xylene and
chlorobenzene.
absorption and photoluminescence (PL) emission of a
solution (o-dichlorobenzene) and thin film. The absorption
maximum of HBT-AN was observed at 351, 359, 429 and
450 nm for solution and at 336, 380, 437, 480 and 496 nm
for film (Fig. 1(a)). In the case of HBT-NA, the absorp-
tion was observed at 405 nm for solution and at 310, 325
and 357 nm for film (Fig. 1(b)). The thin film of HBT-
AN and HBT-NA show a blue shift of the main absorption
peak and a red shift of the shoulder absorption compared
to those of solution spectra of oligomers. These phenom-
ena may be explained by H-aggregates or J-aggregates,
which are usually related to excitonic coupling between
adjacent molecules in a close-packed structure.15 More-
The structures of obtained The obtained materials were
purified by multiple Soxhlet extraction using methanol
and toluene. compounds were characterized by FT-
IR, mass spectrum and elemental analysis. In FT-IR
spectrum of oligomers, aliphatic C–H peak appeared
around 3100 cm−1 and aromatic C C peak appeared
around 2950–2850 cm−1. The elemental analysis was
well consistent with synthesized compounds. The ther-
mal properties of synthesized oligomers were evaluated
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by means of thermogravimetric analysis (TGA) and a
over, the PL maximum of the HBT-AN determined to be
repeated heating-cooling cycle using dCifofperyernigtihatl:sAcamnenrinicgan S4c8ie1ntaifnicd P5u07blinsmherfsor solution, and 546 and 586 nm for
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calorimetry (DSC). In the case of DSC of HBT-AN and
HBT-NA, two endothermic peaks were observed during
the heating trace. On the other hand, in the cooling
trace, only one exothermic peak was observed. It is sug-
gested that the transition at low temperature transition
may be a side chain melting during heating trace and
reversible transition at high temperature may be melt tran-
sition. HBT-AN containing anthracene main unit showed
higher transition temperature compared with HBT-NA
containing naphthalene main unit. The results may be
explained by the stronger intermolecular interaction. HBT-
the film. In the case of HBT-NA, the PL spectra deter-
mined to be 453 and 480 nm for solution, and 557 nm
for the film. In the PL spectra, the thin films of all com-
pounds showed a large red shift relative to their solution.
The existence of strong interactions in ꢀ-conjugated sys-
tem between neighboring molecules in the solid stated is
desirable for good TFT device performances. The electro-
chemical properties of the oilgomers were investigated by
cyclic voltammetry (CV). The CV measurement of the oil-
gomers in a 0.01 M solution of Bu4NClO4/acetonitrile was
carried out (Table II).
ꢁ
AN and HBT-NA showed weight loss of 5% at 447 C
HOMO and LUMO levels were calculated according
ꢁ
OX
OP
and 434 C, respectively. TGA analysis also showed that
to an empirical formula EHOMO = −ꢅE + 4ꢁ4) eV and
HBT-AN is more thermally stable than that of HBT-
NA, which may originate from the difference in ther-
mal stability between naphthalene and anthracene. From
the results, HBT-AN and HBT-NA are crystalline; well-
ordered thin films could be formed by thermal evap-
oration processes. Figure 1 shows normalized UV-vis
ELUMO = ꢅEHOMO −Egꢂ eV, Eg = (1240/UV edge) eV. The
HOMO energy level of oligomers are 5.42 eV for HBT-AN
and 5.47 eV for HBT-NA, respectively. From these results,
it is expected that the oilgomers will have high oxidative
stability due to the low HOMO level despite the narrow-
band gap. Moreover, because these HOMO energy values
Table I. Thermal properties of oligomers.
Table II. Electrochemical properties of oligomers.
Td (ꢁC)a
Tg (ꢁC)
Tm (ꢁC)
Tc (ꢁC)
UVedge
(nm)
HOMO
(eV)
LUMO
(eV)
Band gap
(eV)
Oxidation
HBT-AN
HBT-NA
447
434
234
156
334
279
330
275
HBT-AN
HBT-NA
1.02
1.07
517
490
5.42
5.47
3.02
2.94
2.40
2.53
Note. aDecomposition temperature at a 5% weight loss.
J. Nanosci. Nanotechnol. 12, 4299–4304, 2012
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Synthesis and Characterization of Novel p-Type Alkyl Bithiophene End-Capped Anthracene and Naphthalene Derivatives Koh et al.
Table III. Performance of TFTs based on HBT-AN and HBT-NA.
closely match the work function of gold (øf ∼ 5ꢁ4 eV), the
hole injection from a gold source electrode to a p-channel
organic thin-film transistor happens easily, which con-
tributes to the achievement of high mobility from an
OTFT. OTFTs using HBT-AN and HBT-NA were fabri-
cated (top-contact). The thin-film transistors exhibit type
p-channel TFT characteristics in the transfer and output
curves. The mobility (ꢃFET) and threshold voltage (VTHꢂ of
the OTFTs where obtained from saturation regimes using
IDsat = ꢅW/2LꢂꢃFETCiꢅVG–VTHꢂ2, where L is the channel
length, W is the channel width, and Ci is the capacitance
per unit area of the gate dielectric layer (Ci = 10 nF/cm2
for 300 nm thick SiO2ꢂ. The output characteristics showed
very good saturation behavior and clear saturation currents.
As shown in Figure 2, HBT-AN revealed typical transfer
curves, a hole mobility of 0.05 cm2/Vs, a threshold voltage
of −1ꢁ0 V, subthreshold swing (ss) of 0.6 V, off current of
2ꢁ3×10−11 and an on/off ratio of 1ꢁ0×105 from HMDS-
pretreated device. HBT-NA exhibited a hole mobility of
0.08 cm2/Vs, a threshold voltage of −0ꢁ9 V, subthresh-
old swing (ss) of 0.3 V, off current of 7ꢁ6×10−12 and an
on/off current ratio of 5ꢁ6×105. The mobilities and on/off
ratios are summarized in Table III. Usually, the oligomers
having anthracene core had showed higher mobilities
than those having naphthalene core because of rich
ꢀ electrons.
Tsub
[ꢁC]
ꢃFET
[cm2/Vs]
On/off
ratio
Vth
[V]
Treatment
HBT-AN
HBT-NA
90
120
90
HMDS
HMDS
HMDS
–
5ꢁ2×10−2
4ꢁ5×10−2
6ꢁ3×10−2
8ꢁ4×10−2
105
−1ꢁ0
−1ꢁ2
−1ꢁ2
−0ꢁ9
1ꢁ5×105
9ꢁ3×104
5ꢁ6×105
90
Notes: 60 nm SiO2; Channel length: 1000 nm; Width: 50 nm; Organic rate: 1 Å/s–
60 nm, Gold rate: 1 Å/s–50 nm.
ordering and film morphology of these oilgomers. AFM
images of the oligomers are shown in Figure 3.
The AFM image of HBT-NA showed fully covered sub-
strate with the materials and good network interconnec-
tions between the crystallites In the XRD as shown in
Figure 4, the sharper and stronger reflections up to the
second order for HBT-NA indicate a high degree of lamel-
lar ordering and crystallinity, compared with that of HBT-
AN. The d-spacing obtained from the first refraction peak
(2ꢆ = 2ꢁ5ꢁꢂ is 33.18 Å. As the molecular length of HBT-
NA obtained from theoretical calculations by PM3 geom-
etry optimization is 34.72 Å, the molecule of HBT-NA is
considered to be perpendicular to the substrate, which is
an ideal molecular arrangement for charge transport. From
the results of XRD and AFM, HBT-NA has full covering
However, theDmeloivbeilrietydobfyHPBuTb-NlisAhinisgmTuecchhhnioglhoegrythtoan: WesotfVthiregisnuiabsUtrantievewrsitihtyt/hHeemaalttherSiacliaCndtr gLoIbod network inter
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that of HBT-AN. To understand a higher performance connection between the crystallites are observed, which
Copyright: American Scientific Publishers
of HBT-NA than HBT-AN, the structural analysis by
likely account for the good charge carrier mobility mea-
AFM and XRD were studied to investigate the molecular
sured in the OTFT.
Fig. 2. Drain current (ID) versus drain–source voltage (VD) characteristics of (a) HBT-AN and (b) HBT-NA. Gate voltage (VG) versus drain current
(ID) characteristics of (c) HBT-AN and (d) HBT-NA.
4302
J. Nanosci. Nanotechnol. 12, 4299–4304, 2012

Koh et al. Synthesis and Characterization of Novel p-Type Alkyl Bithiophene End-Capped Anthracene and Naphthalene Derivatives
Fig. 3. AFM images of (a) HBT-AN and (b) HBT-NA.
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Copyright: American Scientific Publishers
Fig. 4. XRD images of (a) HBT-AN and (b) HBT-NA.
4. CONCLUSION
lamellar ordering, exhibited excellent mobility of 8ꢁ4 ×
10−2 cm2/Vs and on/off ratio of 5ꢁ6×105.
Acknowledgment: This work was supported by a grant
(F0004010-2011-34) from the Information Display R&D
Center, one of the 21st Century Frontier R&D Program
funded by the Ministry of Knowledge Economy and
Basic Science Research Program through the National
Research Foundation (NRF) funded by the Ministry of
The new hexylbithiophene end capped naphthalene and
anthracene semiconductors were prepared and character-
ized. The materials possess high ionization potentials
which lead to improved oxidation stability as well as
high crystalline structures which lead to increased inter-
molecular interaction. HBT-NA with good network inter
connection between the crystallites and high degree of
J. Nanosci. Nanotechnol. 12, 4299–4304, 2012
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Synthesis and Characterization of Novel p-Type Alkyl Bithiophene End-Capped Anthracene and Naphthalene Derivatives Koh et al.
Education, Science and Technology (2011-0027499 and
2010-0029084) and MKE and KIAT through the Work-
force Development Program in Stategic Technology.
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Received: 13 May 2010. Accepted: 29 July 2011.
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Copyright: American Scientific Publishers
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J. Nanosci. Nanotechnol. 12, 4299–4304, 2012