A small molecule composed of anthracene and thienothiophene devised for high-performance optoelectronic applications

Article abstract of DOI:10.1016/j.dyepig.2015.04.004

An asymmetric small molecule composed of anthracene and alkylated thienothiophene (2-(anthracen-2-yl)-5-hexylthieno[3,2-b]thiophene) was synthesized via the Suzuki coupling reaction. The thermal stability, photochemical properties, and morphological characteristics were investigated using thermogravimetric analysis, differential scanning calorimetry, cyclic voltammetry, UV-visible spectroscopy, and X-ray diffraction techniques. The compound demonstrated a good thermal stability of 5% at a decomposition temperature of 336 °C. The solution-processed single-crystal transistor, devised with 2-(anthracen-2-yl)-5-hexylthieno[3,2-b]thiophene, exhibited high performance with a hole mobility of 0.1 cm²/Vs. Furthermore, by utilizing the high mobility of 2-(anthracen-2-yl)-5-hexylthieno[3,2-b]thiophene, we demonstrated its unprecedented high photoresponsivity of 3370 A/W, a finding that can be attributed to the very efficient photoconductive behavior of a single crystal of 2-(anthracen-2-yl)-5-hexylthieno[3,2-b]thiophene.

Full text of DOI:10.1016/j.dyepig.2015.04.004

Dyes and Pigments 120 (2015) 30e36
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
A small molecule composed of anthracene and thienothiophene
devised for high-performance optoelectronic applications
Young Ju Jang a, 1, Byung Tack Lim , Soon Byung Yoon , Ho Jun Choi , Jae Un Ha ,
b
,
1
a
a
b
b
,
*
a, *
Dae Sung Chung , Sang-Gyeong Lee
Department of Chemistry, Research Institute of Natural Science, Gyeongsang National University, Jinju, 660-701, South Korea
a
b
School of Chemical Engineering and Material Science, Chung-Ang University, Seoul, 156e756, South Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
An asymmetric small molecule composed of anthracene and alkylated thienothiophene (2-(anthracen-2-
yl)-5-hexylthieno[3,2-b]thiophene) was synthesized via the Suzuki coupling reaction. The thermal sta-
bility, photochemical properties, and morphological characteristics were investigated using thermog-
ravimetric analysis, differential scanning calorimetry, cyclic voltammetry, UVevisible spectroscopy, and
X-ray diffraction techniques. The compound demonstrated a good thermal stability of 5% at a decom-
Received 26 February 2015
Accepted 1 April 2015
Available online 10 April 2015
Keywords:
Hole mobility
Solution processing
Organic thin-film transistor
Asymmetric compound and small molecule
Photoconductor
ꢀ
position temperature of 336 C. The solution-processed single-crystal transistor, devised with 2-
(
0
anthracen-2-yl)-5-hexylthieno[3,2-b]thiophene, exhibited high performance with a hole mobility of
.1 cm /Vs. Furthermore, by utilizing the high mobility of 2-(anthracen-2-yl)-5-hexylthieno[3,2-b]thio-
2
phene, we demonstrated its unprecedented high photoresponsivity of 3370 A/W, a finding that can be
attributed to the very efficient photoconductive behavior of a single crystal of 2-(anthracen-2-yl)-5-
hexylthieno[3,2-b]thiophene.
Anthracene
©
2015 Elsevier Ltd. All rights reserved.
1. Introduction
(BBT) [16], and arylene diimides [17] as key building blocks of po-
tential organic semiconductors.
During the last few decades, organic semiconductors have
emerged as viable alternatives to inorganic semiconductors due to
their advantages over their inorganic counterparts [1e5]. For
instance, extensive industrial applications of organic thin-film
transistors (OTFTs) are now relatively close to implementation
because of the great success of OTFTs in the field of flexible displays
Polymeric semiconductors for OTFTs have several advantages
over small molecules in terms of physical robustness, chemical
stability, and electrical reproducibility. These properties make them
a better choice than small molecules for OTFT applications. None-
theless, the simplicity of synthesis along with the ease of purifi-
cation of small molecules make them attractive candidates for
high-performance OTFTs, especially for low-cost organic elec-
tronics. Traditionally, linear acene derivatives have shown satis-
factory results in OTFT applications. Representative examples
include anthracene [18,19], naphthalene [20], and thienothiophene
[21] derivatives.
After considering the aforementioned issues, an asymmetric
small-molecule organic semiconductor containing an anthracene
attached to a thienothiophene was designed and synthesized.
The alkyl chain introduced on the thienothiophene facilitated the
solubility of the compound in common organic solvents. 2-
[6] as well as their cost-effective promise in the fields of radio-
frequency identification (RFID) tags [7,8] and sensors [9,10]. This
great success of organic semiconductors can be attributed to smart
molecular design, enabling not only solution processability in
various organic solvents [11,12] but also unprecedented high
charge-carrier mobility. There have been tremendous research ef-
forts expended to enhance the charge-carrier mobilities of OTFTs
using both small-molecule semiconductors and polymeric semi-
conductors, including using benzothiadiazole (BTD) [13], diketo-
pyrrolopyrrole (DPP) [14], isoindigo (IDG) [15], benzobisthiadiazole
(Anthracen-2-yl)-5-hexylthieno[3,2-b]thiophene (ASCTT) was
synthesized via the Suzuki coupling reaction between 2-
bromoanthracene (compound 2) and 2-(5-hexylthieno[3,2-b]thio-
phen-2-yl)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (compound 6).
*
Corresponding authors.
E-mail addresses: dchung@cau.ac.kr (D.S. Chung), leesang@gnu.ac.kr (S.-G. Lee).
Equally contributed as first authors.
1
http://dx.doi.org/10.1016/j.dyepig.2015.04.004
143-7208/© 2015 Elsevier Ltd. All rights reserved.
0

Y.J. Jang et al. / Dyes and Pigments 120 (2015) 30e36
31
The photophysical, electrochemical, structural, and electrical
characteristics of ASCTT were systematically investigated. Under
the single-crystal field-effect transistor (SCFET) geometry, ASCTT
2.1.5. Synthesis of 2-hexylthieno[3,2-b]thiophene (compound 5)
Compound 4 (4 g, 16.78 mmol) was added to a stirred mixture of
hydrazine (2.3 mL, 73.83 mmol), potassium hydroxide (3.77 g,
67.12 mmol), and triethylene glycol (24 mL). The mixture was
2
ꢁ1 ꢁ1
revealed a high hole mobility of 0.1 cm V
s . Moreover, we
ꢀ
demonstrated the very efficient, trap-limited photoconductive
stirred for 2 h at 210 C. The reaction mixture was then cooled to the
behavior of ASCTT, resulting in a photoresponsivity up to 3370 A/W.
ambient temperature. The reaction was quenched with cold water
and extracted with dichloromethane; the crude residue was puri-
fied via silica-gel chromatography with hexane to yield compound
2
2
2
. Experimental
ꢁ
1
5
as a pale yellow oil. Yield: 3.2 g (85%). IR (KBr, cm ): 2953e2852
3
1
(
1
sp CeH), 1462 (C]C); H-NMR (300 MHz, CDCl
H, J ¼ 5.4 Hz), 7.20 (dd, 1H, J ¼ 0.9 Hz, J ¼ 0.9 Hz), 6.97 (d, 1H,
J ¼ 0.9 Hz), 2.92e2.87 (m, 2H), 1.35e1.28 (m, 8H), 0.88 (t, 3H,
3
, ppm): d 7.29 (d,
.1. Materials and syntheses
.1.1. Materials
13
J ¼ 8.1 Hz); C-NMR (75 MHz, CDCl
3
, ppm): d 148.57, 138.74,137.33,
All reagents and chemicals were purchased from Sigma Aldrich
1
25.22, 119.43, 116.13, 37.14, 31.97, 30.08, 29.75, 22.73, 14.14; EI, MS
Co., TCI, or Alfa Aesar Co. Solvents such as tetrahydrofuran (THF),
diethyl ether, toluene, and methylene chloride were used after
distillation in the presence of sodium/benzophenone or calcium
hydride under nitrogen gas.
þ
m/z (%): 224 (100, M ).
2.1.6. Synthesis of 2-(5-hexylthieno[3,2-b]thiophen-2-yl)-4,4,5,5-
tetramethyl-1,3,2-dioxaborolane (compound 6)
ꢀ
In THF (24 mL) at ꢁ78 C, n-butyllithium (5.35 g, 13.37 mmol)
2
.1.2. Synthesis of 2-bromoanthraquinone (compound 1)
-Aminoanthraquinone (3 g, 13.44 mmol), tert-butyl nitrite
3.69 mL, 30.9 mmol), and copper (I) bromide (6.9 g, 30.9 mmol)
was added dropwise to a solution of compound 5 (2 g, 8.91 mmol).
2
ꢀ
After the mixture had been stirred at ꢁ78 C for 1 h, 2-isopropoxy-
(
4,4,5,5-tetramethyl-1,3,2-dioxaborolane (2.73 g, 13.37 mmol) was
were added to acetonitrile (50 mL). The mixture was stirred for 2 h
added to the mixture, and the resulting mixture was stirred
ꢀ
at 65 C. The reaction mixture was cooled to room temperature. The
ꢀ
at ꢁ78 C for 1 h and then warmed to room temperature and
reaction was quenched with 6 N HCl. The precipitate was filtered,
further stirred overnight. The mixture was poured into water,
washed with chloroform, and dried in a vacuum desiccator to give
4
extracted with diethyl ether, and then dried over MgSO . The sol-
vent was removed via rotary evaporation, and a yellow oil was
1
compound 1 as a pure brown solid. Yield: 2.4 g (62%). H-NMR
(
8
7
d
300 MHz, CDCl
3
, ppm):
d
8.42 (d,1H, J ¼ 2.1 Hz), 8.32e8.28 (m, 2H),
obtained after flash-column chromatography using hexane/ethyl
.18e8.15 (d, 1H, J ¼ 8.1 Hz), 7.94e7.90 (dd, 1H, J ¼ 2.1, 1.8 Hz),
ꢁ
1
acetate (10:1) as the eluent. Yield: 2.7 g (86%). IR (KBr, cm ):
13
.84e7.81 (q, 2H, J ¼ 3.3 Hz); C-NMR (75 MHz, CDCl
3
, ppm):
3
1
2
950e2849 (sp CeH), 1465 (C]C); H-NMR (300 MHz, CDCl
3
,
182.36, 137.16, 134.48, 134.36, 133.13, 130.02, 129.02, 127.38,
ppm):
d
7.69 (s, 1H), 6.98 (s, 1H), 2.89 (t, 2H, J ¼ 7.8 Hz), 1.75e1.68
þ
1
27.34; EI, MS m/z (%): 286 (100, M ).
13
(m, 2H), 1.35e1.25 (m, 18H), 0.88 (t, 3H, J ¼ 8.1 Hz); C-NMR
(75 MHz, CDCl , ppm): 151.54, 145.24, 140.08, 138.82, 129.13,
3
d
2.1.3. Synthesis of 2-bromoanthracene (compound 2)
1
16.35, 84.11, 82.81, 60.38, 34.66, 31.58, 31.33, 28.72, 26.21, 25.27,
Compound 1 (2 g, 6.9 mmol), hydroiodic acid (10.6 mL), and
þ
24.78, 24.73, 22.64, 21.00, 14.08; EI, MS m/z (%): 350 (100, M ).
hypophosphorous acid (6.4 mL) were dissolved in acetic acid
ꢀ
(
17 mL). The mixture was stirred for 96 h at 150 C. The reaction
2.1.7. Synthesis of 2-(anthracen-2-yl)-5-hexylthieno[3,2-b]
mixture was cooled to room temperature. The precipitate was
filtered and washed with ethanol. The solid was purified via soxhlet
thiophene (ASCTT)
To a solution of compound 2 (0.1 g, 0.33 mmol) and compound 6
extraction with toluene, yielding compound 2 as a pure yellow
(
1
0.12 g, 0.39 mmol) in toluene (4 mL), a 2 M K
.65 mmol) was added, and the mixture was degassed for 10 min.
Next, Pd(PPh (0.023 g, 0.02 mmol) was added and the resulting
2 3
CO solution (0.5 mL,
1
solid. Yield: 1.1 g (61%). H-NMR (300 MHz, CDCl
3
, ppm):
d
8
8.44e8.41 (t, 1H, J ¼ 6.9 Hz), 8.34e8.30 (d, 2H, J ¼ 10.2 Hz),
3 4
)
.19e7.99 (q, 2H, J ¼ 3.3 Hz), 7.91e7.87 (d, 1H, J ¼ 6.0 Hz), 7.91e7.73
mixture was stirred while being refluxed for 12 h under a nitrogen
atmosphere. The reaction mixture was quenched with water and
extracted with methylene chloride. The organic phase was dried
over anhydrous MgSO . The solvent was removed in vacuo, and the
4
crude product was purified via column chromatography using
13
(
dd, 1H, J ¼ 9.0, 9.3 Hz), 7.68e7.49 (m, 2H); C-NMR (75 MHz,
CDCl , ppm): 133.65, 129.89, 129.83, 129.65,128.87,128.23,128.19,
28.12, 126.58, 126.55, 126.05, 126.00, 125.78, 125.37; EI, MS m/z
3
d
1
(
þ
%): 256 (100, M ).
hexane as an eluent, yielding ASCTT as a yellow solid. Yield: (46%).
ꢁ
1
3
1
2
.1.4. Synthesis of 2-hexanoylthieno[3,2-b]thiophene (compound 4)
Hexanoyl chloride (4.0 mL, 28.55 mmol) was added to a solution
IR (KBr, cm ): 2850 (sp CeH), 1517 (C]C); H NMR (300 MHz,
CDCl , ppm):
3
d
8.44 (d, 2H), 8.21 (s,1H), 8.05e7.99 (t, 3H, J ¼ 4.5 Hz),
of compound 3 (2.6 g, 18.54 mmol) in CH
2
Cl
2
(70 mL). Aluminum
7.79e7.75 (dd, 1H, J ¼ 3.45 Hz), 7.60 (s, 1H), 7.49e7.46 (m, 2H), 7.00
chloride (3.7 g, 27.81 mmol) was added in portions over 10 min. The
mixture was stirred for 1 h at room temperature. Water was added
to the reaction solution to quench the reaction and then the solu-
tion was extracted with dichloromethane. The organic phase was
(s, 1H), 2.95e2.90 (m, 2H), 1.75e1.65 (m, 2H), 1.35e1.20 (m, 6H),
1
3
0.88 (t, 3H, J ¼ 8.1 Hz); C-NMR (75 MHz, CDCl
3
, ppm): d 143.20,
138.24, 133.36, 132.28, 131.72, 130.23, 128.16, 126.92, 125.14, 124.19,
116.29, 115.74, 31.58, 30.97, 29.73, 29.69, 22.60, 14.11; EI, MS m/z
þ
dried with anhydrous MgSO
4
and the solvent was removed under
(%): 400 (100, M ).
vacuum. The crude residue was purified via silica-gel chromatog-
raphy using hexane as the eluent to give compound 4 as a pale
yellow solid. Yield: 4.0 g (91%). Mp: 83 C; IR (KBr, cm ): 3091 (sp
2.2. Measurements
ꢀ
ꢁ1
2
3
1
1H and 13C NMR spectra were recorded on 300-MHz and 75-
MHz spectrometers, respectively. The chemical-shift values were
reported in d units (ppm). IR analysis was carried out using a
Mattson genesis series FT-IR spectrophotometer. Mass-spectral
analysis was carried on a JMS-700, JEOL. UVevisible absorption
CeH), 2947e2860 (sp CeH), 1655 (C]O), 1452 (C]C); H-NMR
300 MHz, CDCl , ppm):
7.92 (s,1H), 7.63 (d,1H, J ¼ 5.1 Hz), 7.31 (d,
H, J ¼ 5.1 Hz), 2.94 (t, 2H, J ¼ 7.4 Hz), 1.85e1.75 (m, 2H), 1.42e1.38
(
1
(
3
d
13
m, 4H), 0.93 (t, 3H, J ¼ 7.1 Hz); C-NMR (75 MHz, CDCl
3
, ppm):
d
194.21, 146.00, 144.73, 139.09, 132.32, 124.17, 120.06, 39.03, 31.54,
þ
2
4.68, 22.49, 13.93; EI, MS m/z (%): 238 (100, M ).
was obtained using
a
Shimadzu UV-1065PC UVeVis

32
Y.J. Jang et al. / Dyes and Pigments 120 (2015) 30e36
spectrophotometer. Cyclic-voltammogram measurements were
carried out on an epsilon E at room temperature in tetrabuty-
2.4. Fabrication and characterization of the photoconductor
3
lammonium perchlorite (0.1 M solution) in chloroform under a
nitrogen atmosphere at a scan rate of 50 mV/s. A Pt wire and Ag/
AgCl electrodes served as the counter and reference electrodes,
respectively. Melting points were measured using an Electro-
thermal Mode 1307 digital analyzer and were uncorrected. Dif-
ferential scanning calorimetry (DSC) measurements were carried
out under nitrogen, using a TA Instruments 2100 differential
For photoconductor fabrication, single crystals of ASCTT were
deposited onto the prepatterned gold electrode with a short
channel length (3 mm). The current density-voltage (J-V) charac-
teristics were measured using Keithley 2400 source/measure units
for two-terminal measurement under monochromatic illumination
from a 150-W xenon arc lamp assembled with a 1/8-m mono-
chromator. The photocurrent spectra were measured by synchro-
nizing the monochromator with the source meter, a 10-Hz
mechanical chopper, and a lock-in amplifier.
ꢀ
ꢀ
scanning calorimeter, by heating the samples from 0 C to 200 C
ꢀ
at a rate of 10 C/min. Thermogravimetric analysis (TGA) was
obtained under nitrogen, using a TA Instruments 2050 thermog-
ꢀ
ravimetric analyzer, by heating the samples at 10 C/min from
3
3. Results and discussion
ꢀ ꢀ
0 C to 800 C.
3.1. Synthesis and characterization
2.3. Fabrication and characterization of SCFETs
Scheme 1 outlines the synthesis of ASCTT. Briefly, the synthetic
procedure is as follows: 2-aminoanthraquinone, upon bromination
followed by reduction, resulted in 2-bromoanthracene (compound
Top-contact/bottom-gate OFETs were fabricated on a common
gate of highly n-doped silicon with a 300-nm-thick thermally
grown SiO dielectric layer. The dielectric layer was further modi-
2). Thienothiophene (compound 3) was synthesized by following
2
the procedure outlined in the literature [22]. On acylation, thie-
nothiophene produced compound 4, which on reduction followed
by borylation yielded 2-(5-hexylthieno[3,2-b]thiophen-2-yl)-
fied by an octyltrichlorosilane (OTS) self-assembled monolayer. The
solution containing the semiconductor was spin-coated at
2
000 rpm from 0.5 to 1 wt% solutions in chloroform to form a thin
4,4,5,5-tetramethyl-1,3,2-dioxaborolane (compound 6). The Suzuki
film with a nominal thickness of 30 nm, as confirmed using a sur-
coupling reaction between compounds 2 and 6 produced ASCTT.
face profiler (Alpha Step 500, Tencor). The film was annealed at
1
13
ꢀ
The compounds were characterized by IR, HNMR, CNMR, and
mass-spectroscopy analysis.
2
00 C for 10 min under a nitrogen atmosphere. Gold source and
drain electrodes were evaporated on top of the semiconductor layer
80 nm). For all measurements, the typical channel widths (W) and
lengths (L) were 1000 m and 100 m, respectively. The entire
(
m
m
3.2. Optical properties
measurement process was conducted using a home-built LabVIEW
program combined with Keithley electronic units under a nitrogen
atmosphere.
The optical properties of the synthesized compounds were
analyzed using UVevisible absorption spectra in a CHCl solution as
3
Scheme 1. Synthetic scheme of ASCTT.

Y.J. Jang et al. / Dyes and Pigments 120 (2015) 30e36
33
the compound was thermally stable. Fig. 2(b) represents the DSC
graph. For ASCTT, the endothermic melting temperature was
ꢀ
ꢀ
1
68 C, and the glass-transition temperature was 104 C. Table 1
lists the TGA and DSC data.
3.4. Electrochemical properties
The HOMO and LUMO energy levels of ASCTT were measured by
performing cyclic-voltammetry (CV) measurements. Fig. 3(a)
shows the CV graph of the compound. CV for the compound was
ꢁ3
measured at 1.0 ꢂ 10 M ASCTT in a chloroform solution con-
taining 0.1 M Bu NClO . The compound showed onset of oxidation
4
4
potential Eox at 1.22 V. The calculated HOMO and LUMO energy
levels were ꢁ5.64 eV and ꢁ2.76 eV, respectively. The HOMO and
LUMO values are listed in Table 1. From the value of the HOMO
energy level, we expect that a device prepared from this material
will demonstrate hole mobility.
Fig. 1. UVevis absorption spectra of ASCTT in chloroform and in film state.
3.5. OTFT characterization
well as on thin films. Fig. 1 represents the UVevisible absorption
spectrum of ASCTT. The absorption values are listed in Table 1. The
ASCTT exhibited absorption values at 323 and 337 nm in the solu-
tion state. The thin film showed a 26-nm redshift and an absorption
at 363 nm. The intermolecular interactions in the thin-film state
caused the redshift in the film state. In the thin film, ASCTT showed
2
The SCFET of ASCTT was fabricated on a Si/SiO substrate via the
solution-drying method with chloroform in a closed jar. As deter-
mined by X-ray diffraction (XRD, measured from 3C and 5A
beamline of Pohang accelerator laboratory) analysis on a single
crystal of ASCTT, the well-developed (l00) Bragg reflection peaks
appeared up to (400), revealing the purity of the single crystal. Gold
electrodes were used as source-drain electrodes after considering
the HOMO levels of ASCTT [Fig. 3(b)]. The alkyl chains present on
the thienothiophene group permitted reasonably good solubility of
ASCTT in chloroform; thus, the slow growth of the single crystal in
the solution phase was successful (~1 day). Fig. 4(a) and (b) shows
the transfer (VGS versus IDS) characteristics and representative
output (VDS versus IDS) of the SCFET. The device showed p-domi-
nant mobility, which is typical for linear acene derivatives. The
a
lonset at 430 nm in the UVevisible absorption spectrum; from this
measurement, we obtained an optical band gap of 2.88 eV.
3.3. Thermal properties
The thermal properties of ASCTT were analyzed by TGA and DSC
analysis. Fig. 2(a) represents the TGA graph. For ASCTT, decompo-
ꢀ
sition with 5% weight loss was observed at 336 C, indicating that
Table 1
Optical, electrochemical, thermal and OTFT properties of ASCTT.
abs film (nm) HOMO (eV)a LUMO (eV)b Eg (eV)c
T
(5%) ( C)d
ꢀ
T
ꢀ
( C)e
T
( C)e Mobility (cm . V
ꢀ
2
ꢁ1 ꢁ1
s
) On/Off
V
th (V)f
ꢁ5.44
Compound
l
abs sol (nm)
l
d
m
g
5
ASCTT
323, 337
363 ꢁ2.76 2.88
ꢁ5.64
336
168
104 0.1
1.5 ꢂ 10
a
Estimated from the onset of oxidation potential.
Calculated from (HOMO level e Band gap).
Calculated from the absorption edge wavelength.
Decomposition temperature at 5% weight loss.
b
c
d
e
f
T
m
g
, T represent endothermic melting point and glass transition temperatures, respectively.
Threshold voltage.
Fig. 2. (a) TGA curve of ASCTT. (b) DSC curve of ASCTT with two heating traces and one cooling trace.

34
Y.J. Jang et al. / Dyes and Pigments 120 (2015) 30e36
Fig. 3. (a) CV curve of ASCTT. (b) XRD curve of single crystalline ASCTT.
saturation-mode field-effect hole mobility, extracted from the
channel length. Therefore, obviously one can see that high-mobility
semiconductors can lead to high photoconductive gains.
1
/2
transconductance of the VGS versus IDS plot, reached up to 0.1
2
ꢁ1 ꢁ1
5
cm V
s
. The on-off ratio and threshold voltage were 1.3 ꢂ 10
Therefore, it seemed reasonable to fabricate a photoconductor of
ASCTT to test its potential photoconductive behavior. In addition, it
is noteworthy that there have not been many reports on the
photoconductive behaviors of acene derivatives, although their
high charge-carrier mobility can presumably lead to high photo-
conductive gains. The ASCTT single crystal for the photoconductor
was fabricated in the same manner as that of the SCFET but with a
different channel length. According to equation (1), a shorter
channel length yields a higher photoconductive gain therefore, we
and ꢁ5.44 V, respectively. These results are comparable to those of
previously reported widely known linear acenes [18e21].
3.6. Photoconductor behavior
Considering the reasonably high hole mobility of ASCTT, we
tried to determine its photoconductive behavior, because a pho-
toconductor is a gain-generating device governed by both charge
carrier mobility and charge carrier lifetime [23]. In other words, the
gain in a photoconductor is generated by the trapped minority
carriers, which can postpone the recombination of electron/hole
pairs and provide an opportunity for majority carriers to travel
along the device many times before final recombination. The
photoconductive gain is given as [24].
chose a much shorter channel length of 3 mm. Fig. 5 shows the
resulting IeV characteristics under darkness and illuminated light
with different intensities (at 350 nm). We chose a wavelength of
3
50 nm after considering the absorption spectra. Under the rela-
2
tively weak light intensity of 17.3
reached approximately 20, resulting in photoresponsivity up to
370 A/W. This value is among the best reported for any small-
mW/cm , the current on/off ratio
3
.
molecule photoconductor [25e27]. To elucidate the origin of such
high responsivity, we measured the light intensity-dependent
transient photocurrent, as summarized in Fig. 6. Clearly, one can
see that a higher intensity of incident light led to a faster transient
photoresponse, a finding that is typical for a trap-limited photo-
conductor because light energy can release the trapped charge
2
G ¼ ctmV L
(1)
where c is the fraction of excitons dissociated into holes and elec-
trons, is the lifetime of the trapped electrons, is the mobility of
the majority charge carriers, V is the applied voltage, and L is the
t
m
Fig. 4. The (a) transfer and (b) outputcharacteristics of ASCTT-based SCFET. (Inset: the optical microscopy image of the actual device).

Y.J. Jang et al. / Dyes and Pigments 120 (2015) 30e36
35
Acknowledgments
This research was supported by the Basic Science Research
Program through the National Research Foundation of Korea (NRF),
funded by the Ministry of Education, Science, and Technology
(Grant Number: 2010-0023775), and also supported by the Space
Core Technology Development Program through the NRF, funded
by the Ministry of Education, Science, and Technology (Grant
Number: NRF-2014M1A3A3A02034707).
References
[
1] Lee J, Cho H-J, Cho NS, Hwang D-H, Kang J-M, Lim E, et al. Enhanced efficiency
of polyfluorene derivatives: organiceinorganic hybrid polymer light-emitting
diodes. J Polym Sci Part A e Polym Chem 2006;44:2943e54.
[
2] Yang R, Tian R, Hou Q, Zhang Y, Li Y, Yang W, et al. Light-emitting copolymers
based on fluorene and selenophenedComparative studies with its sulfur
analogue: poly(fluorene-co-thiophene). J Polym Sci Part A e Polym Chem
2005;43:823e36.
Fig. 5. The IeV characteristics of the photoconductor based on ASCTT. (Inset: the
logged plot on y-axis).
[3] Lee J, Jung BJ, Lee SK, Lee JI, Cho H-J, Shim HK. Fluorene-based alternating
polymers containing electron-withdrawing bithiazole units: preparation
and device applications.
845e57.
[4] Brabec CJ, Sariciftci NS, Hummelen JC. Plastic solar cells. Adv Funct Mater
001;11:15e26.
J Polym Sci Part A e Polym Chem 2005;43:
1
carriers so that the charge carrier lifetime decreases [28]. Taken
together, we determined that the trap-limited photoconductor
mechanism works for ASCTT and, because of its rather high
mobility, a high responsivity up to 3370 A/W was obtained.
2
[
5] Katz HE, Bao Z, Gilat SL. Synthetic chemistry for ultrapure, processable, and
high-mobility organic transistor semiconductors. Acc Chem Res 2001;34:
359e69.
[6] Forrest SR. The path to ubiquitous and low-cost organic electronic appliances
on plastic. Nature 2004;428:911e8.
4
. Conclusions
[7] Rotzoll R, Mohapatra S, Olariu V, Wenz R, Grigas M, Dimmler K, et al. Radio
frequency rectifiers based on organic thin-film transistors. Appl Phys Lett
2
006;88:123502.
The asymmetric small molecule ASCTT, composed of an
anthracene attached to a thienothiophene, was synthesized via a
Suzuki coupling reaction between 2-bromoanthracene (compound
) and 2-(5-hexylthieno[3,2-b]thiophen-2-yl)-4,4,5,5-tetramethyl-
,3,2-dioxaborolane (compound 6). The compound exhibited good
[
[
8] Baude PF, Ender DA, Haase MA, Kelley TW, Muyres DV, Theiss SD. Pentacene-
based radio-frequency identification circuitry. Appl Phys Lett 2003;82:
3964e6.
9] Kato Y, Sekitani T, Noguchi Y, Yokota T, Takamiya M, Sakurai T, et al. Large-
area flexible ultrasonic imaging system with an organic transistor active
matrix. IEEE Trans Electron Dev 2010;57:995e1002.
2
1
thermal stability and decomposition, with 5% weight loss, was
observed at 336 C. The alkyl chains promoted good solubility of the
[10] Chang JB, Liu V, Subramanian V, Sivula K, Luscombe C, Murphy A, et al.
Printable polythiophene gas sensor array for low-cost electronic noses. J Appl
Phys 2006;100:014506.
ꢀ
compound in common organic solvents and further enabled the
fabrication of a solution-based SCFET of ASCTT. The ASCTT-based
[11] Zaumseil J, Sirringhaus H. Electron and ambipolar transport in organic field-
effect transistors. Chem Rev 2007;107:1296e323.
2
ꢁ1 ꢁ1
[12] Ma B, Woo CH, Miyamoto Y, Frechet JMJ. Solution processing of a small
molecule, subnaphthalocyanine, for efficient organic photovoltaic cells. Chem
Mater 2009;21:1413e7.
SCFET showed a reasonably high hole mobility of 0.1 cm V
s .
Furthermore, thanks to this high mobility, we could demonstrate
an efficient photoconductive mechanism from a single crystal of
the compound, resulting in an unprecedentedly high photo-
responsivity of 3370 A/W.
[13] Tsao HN, Cho DM, Park I, Hansen MR, Mavrinskiy A, Yoon DY, et al. Ultrahigh
mobility in polymer field-effect transistors by design.
011;133:2605e12.
J Am Chem Soc
2
[
[
14] Qu S, Tian H. Diketopyrrolopyrrole (DPP)-based materials for organic photo-
voltaics. Chem Commun 2012;48:3039e51.
15] Stalder R, Mei J, Graham KR, Estrada LA, Reynolds JR. Isoindigo, a versatile
electron-deficient unit for high-performance organic electronics. Chem Mater
2014;26:664e78.
[
[
[
[
[
[
[
16] Fan J, Yuen JD, Cui W, Seifter J, Mohebbi AR, Wang M, et al. High-hole-mobility
field-effect transistors based on co-benzobisthiadiazole-quaterthiophene. Adv
Mater 2012;24:6164e8.
17] Zhan X, Facchetti A, Barlow S, Marks TJ, Ratner MA, Wasielewski MR, et al.
Rylene and related diimides for organic electronics. Adv Mater 2011;23:
268e84.
18] Shaik B, Park JH, An TK, Noh YR, Yoon SB, Park CE, et al. Small asymmetric
anthracene-thiophene compounds as organic thin-film transistors. Tetrahe-
dron 2013;69:8191e8.
19] Jang SH, Chung DS, An JY, Kang I, Kim H, Park CE, et al. Symmetric long alkyl
chain end-capped anthracene derivatives for solution-processed organic thin-
film transistors. J Nanosci Nanotechnol 2012;12:4340e3.
20] An TK, Jang SH, Kim S-O, Jang J, Hwang J, Cha H, et al. Synthesis and transistor
properties of asymmetric oligothiophenes: relationship between molecular
structure and device performance. Chem Eur J 2013;19:14052e60.
21] Mazur L, Castiglione A, Ocytko K, Kameche F, Macabies R, Ainsebaa A, et al.
Charge carrier mobility study of a mesogenic thienothiophene derivative in
bulk and thin films. Org Electron 2014;15:943e53.
22] Fuller LS, Iddon B, Smith KA. Thienothiophenes. Thienothiophenes. Part 2.1
synthesis, metallation and bromine/lithium exchange reactions of thieno
[3,2-b]thiophene and its polybromo derivatives. J Chem Soc Perkin Trans
1997;1:3465e70.
[23] Chung DS, Kong H. Effect of solvent on detectivity of solution-processed
polymer photodetectors. Opt Lett 2013;38:2814e7.
[24] Campbell IH, Crone BK. Bulk photoconductive gain in poly(phenylene vinyl-
ene) based diodes. J Appl Phys 2007;101:024502.
Fig. 6. The transient photocurrent of the photoconductor based on ASCTT with
different light intensities.

36
Y.J. Jang et al. / Dyes and Pigments 120 (2015) 30e36
[
[
25] Zhang X, Jie J, Zhang W, Zhang C, Luo L, He Z, et al. Photoconductivity
of single small-molecule organic nanowire. Adv Mater 2008;20:
427e32.
26] Zhou Y, Wang L, Wang J, Pei J, Cao Y. Highly sensitive, air-stable photode-
tectors based on single organic sub-micrometer ribbons self-assembled
through solution processing. Adv Mater 2008;20:3745e9.
[27] Chung DS, Kim Y-H, Lee J-S. Achieving high sensitivity in hybrid photode-
tectors based on an organic single crystal and an inorganic nanocrystal array.
Nanotechnology 2014;25:035202.
[28] An TK, Park CE, Chung DS. Polymerenanocrystal hybrid photodetectors with
planar heterojunctions designed strategically to yield a high photoconductive
gain. Appl Phys Lett 2013;102:193306.
a
2