Molecular aggregation-performance relationship in the design of novel cyclohexylethynyl end-capped quaterthiophenes for solution-processed organic transistors

Article abstract of DOI:10.1016/j.dyepig.2012.11.018

The synthesis and characterization of cyclohexylethenyl end-capped quaterthiophenes is reported. Additionally, an investigation of the performance of organic field-effect transistors based on these quaterthiophenes in view of the relationship between the solid-state (or aggregate) order and the electronic performance is described. UV-vis absorption measurements revealed that the quaterthiophene with an asymmetrically substituted cyclohexylethynyl end-group induced the formation of H-type aggregates, whereas the quaterthiophene with a symmetrically substituted cyclohexylethynyl end-groups favored the formation of J-type aggregates. Two-dimensional grazing-incidence wide-angle X-ray scattering studies were performed to support the molecular structure-dependent packing of films of the new quaterthiophenes. Solution-processed quaterthiophenes were tested as the active layers of p-type organic field-effect transistors with a bottom gate/top contact geometry. The field-effect mobility of devices that incorporated asymmetric quaterthiophene molecules was quite high, exceeding 0.02 cm²/V s, due to H-aggregation and good in-plane ordering. In contrast, the field-effect mobility of devices that incorporated symmetrical quaterthiophenes, was low, above 5 × 10^-4 cm²/(V s), due to the formation of J-aggregates and poor in-plane ordering. A comparison of the symmetrical and asymmetrical quaterthiophene derivatives revealed that the molecular aggregation-dependent packing, determined by the cyclohexylethynyl end groups, was responsible for influencing the organic field-effect transistor performance.

Full text of DOI:10.1016/j.dyepig.2012.11.018

Dyes and Pigments 96 (2013) 756e762
Contents lists available at SciVerse ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Molecular aggregationeperformance relationship in the design of novel
cyclohexylethynyl end-capped quaterthiophenes for solution-processed organic
transistors
,
,
Tae Kyu An a 1, Seung-Hoon Hahn b 1, Sooji Nam a, Hyojung Cha a, Yecheol Rho d, Dae Sung Chung a,
Moonhor Ree d, Moon Seong Kang b, Soon-Ki Kwon c , Yun-Hi Kim
, Chan Eon Park
,
b,
a,
*
**
***
a POSTECH Organic Electronics Laboratory, Polymer Research Institute, Department of Chemical Engineering, Pohang University of Science and Technology, Pohang 790-784,
Republic of Korea
b Department of Chemistry & RIGET, Gyeongsang National University, Jinju 660-701, Republic of Korea
c School of Materials Science & Engineering and Engineering Research Institute (ERI), Gyeongsang National University, Jinju 660-701, Republic of Korea
d Department of Chemistry, Pohang University of Science and Technology, Pohang 790-784, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 8 July 2012
Received in revised form
19 November 2012
Accepted 19 November 2012
Available online 29 November 2012
The synthesis and characterization of cyclohexylethenyl end-capped quaterthiophenes is reported.
Additionally, an investigation of the performance of organic field-effect transistors based on these
quaterthiophenes in view of the relationship between the solid-state (or aggregate) order and
the electronic performance is described. UVevis absorption measurements revealed that the quater-
thiophene with an asymmetrically substituted cyclohexylethynyl end-group induced the formation of
H-type aggregates, whereas the quaterthiophene with a symmetrically substituted cyclohexylethynyl
end-groups favored the formation of J-type aggregates. Two-dimensional grazing-incidence wide-angle
X-ray scattering studies were performed to support the molecular structure-dependent packing of
films of the new quaterthiophenes. Solution-processed quaterthiophenes were tested as the active
layers of p-type organic field-effect transistors with a bottom gate/top contact geometry. The field-effect
mobility of devices that incorporated asymmetric quaterthiophene molecules was quite high, exceeding
0.02 cm2/V s, due to H-aggregation and good in-plane ordering. In contrast, the field-effect mobility of
devices that incorporated symmetrical quaterthiophenes, was low, above 5 ꢀ 10ꢁ4 cm2/(V s), due to the
formation of J-aggregates and poor in-plane ordering. A comparison of the symmetrical and asym-
metrical quaterthiophene derivatives revealed that the molecular aggregation-dependent packing,
determined by the cyclohexylethynyl end groups, was responsible for influencing the organic field-effect
transistor performance.
Keywords:
Organic thin film transistor (OTFT)
Mixed solvent
Organic semiconductor
Morphology
Fused acene
Crystallinity
Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction
performance devices [1,2]. Organic semiconductors can potentially
replace inorganic counterparts in the fabrication of organic inte-
Organic semiconductors based on oligomers and polymers have
recently received considerable attention for use as active
channel materials in organic field-effect transistors (OFETs). Organic
semiconductors are compatible with plastic substrates and
can potentially be solution-processed using inexpensive fabrication
techniques, which would facilitate the commercialization of high-
grated circuits, sensors, low-cost memories, smart cards, and
driving circuits for large-area applications, such as paper-like
displays [3e10]. Among the various organic semiconductors devel-
oped thus far, thiophene-based oligomers provide an important
platform because of their high chemical and electrochemical
stability, ease of synthesis, tuneable electronic properties, and the
availability of well-developed/regioselective ringering coupling
methodologies [11,12]. In addition, solution-processable oligomeric
semiconductors can be deposited using low-cost solution processing
methods, such as spin-coating, inkjet printing, or gravure printing
[13e15]. Oligomer solubility and stability toward oxidation can be
* Corresponding author.
** Corresponding author.
*** Corresponding author.
E-mail addresses: skwon@gnu.ac.kr (S.-K. Kwon), ykim@gnu.ac.kr (Y.-H. Kim),
cep@postech.ac.kr (C.E. Park).
increased by functionalizing the
a- and u-ends of the conjugated
1
Tae Kyu An and Seung-Hoon Hahn contributed equally to this work.
thiophene ring systems [2]. Straight alkyl chain functionalization at
0143-7208/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.dyepig.2012.11.018

T.K. An et al. / Dyes and Pigments 96 (2013) 756e762
757
the
a- and
u-ends of the conjugated thiophene oligomers allows the
a special
p-conjugated site [24], extended the
p-system in the
thiophene rings to orient themselves vertically on a substrate,
thereby inducing good molecular ordering [16].
molecule, and improved the molecular rigidity [25]. UVevis
absorption and grazing-incidence wide-angle X-ray scattering
(GIWAXS) analysis revealed that CHE4T tended to undergo
H-aggregationwhereas BCHE4T tended to undergo J-aggregation. To
test the utility of the quaterthiophene derivatives as organic semi-
conductors, FETs of these derivatives were fabricated using a solu-
tion process. The field-effect mobility in the H-aggregated material
was much higher than in the J-aggregated material. An under-
standing of the mechanism by which molecular aggregation affected
the performance of a transistor is discussed, and the aggregation
type-dependent charge carrier mobility is described in detail.
In many classes of these oligomers, the molecular ordering and
crystallinity can significantly influence the performance of devices,
such as OFETs [2], because the hopping of charge carriers between
individual organic molecules depends strongly on the molecular
ordering and crystallinity [16e18]. Therefore, many research groups
have endeavored to control the molecular ordering and crystallinity
of structures in organic semiconductor thin films [17]. As a crystal-
line film forms, organic materials aggregate with various aggrega-
tion patterns by combination of H- and J-types. The aggregation
types are distinguished by the sign of the resonant electronic
(excitonic) coupling [19,20]. H-aggregation yields a positive sign for
the resonantelectronic (excitonic) coupling, which occurs in a pair of
molecules in a parallel orientation [21]. The UV absorption peak for
an H-aggregated film is blue-shifted compared to the absorption
peak in solution because of cancellation of the transition dipole
moments of the lower-lying state [19]. J-aggregation occurs when
molecules assume a head-to-tail arrangement. In J-aggregation,
couplings are negative, and the UV absorption peak of the film is red-
shifted compared to the absorption peak in solution [19,22].
Previous studies reported that many organic semiconductors show
J-aggregation [22,23]; however, H-aggregation may have several
advantages over J-aggregated molecules for high-performance
2. Experimental section
2.1. Measurements
The 1H NMR and 13C NMR spectra were recorded using a Bruker
AM-200 spectrometer. The FT-IR spectra were measured on
a Bomem Michelson series FT-IR spectrometer. The melting points
were determined using an Electrothermal Mode 1307 digital
analyzer. The thermal analysis was performed using a TA TGA 2100
thermogravimetric analyzer under a nitrogen atmosphere with
a heating rate of 20 ꢂC/min. Differential scanning calorimetry (DSC)
measurements were conducted under nitrogen using a TA instru-
ment 2100 DSC. The sample was heated with a rate of 20 ꢂC/min
from 30 ꢂC to 250 ꢂC. UVevis absorption spectra were measured
using a Perkin Elmer LAMBDA-900 UV/VIS/IR spectrophotometer.
OFETs in view of the area of
conductor molecules. H-aggregation includes a parallel adjacent
moleculararrangement that yields a large area of -overlap between
adjacent molecules. In contrast, J-aggregation exhibits a head-to-tail
arrangement [21] and does not build up a large area of -overlap
p-overlap among adjacent semi-
p
p
2.2. Materials and synthesis
between adjacent molecules. H-type aggregation can improve the
performance of organic semiconductors in OFETs.
2-Bromothiophene, N-bromosuccinimide(NBS), n-butyllithium,
2-isopropoxy-3,3,4,4-tetramethyl-1,3,2-dioxaborolane, 1,3-bis(dip-
henylphosphinopropane)dichloronickel, N,N-dimethylformamide
(DMF), copper(I), sodium carbonate, 2,20-bipyridine, 1,5-cyclooct-
adiene, bis(cyclooctadiene)nickel(0), dichloro-((bis-diphenylphos-
phino)ferrocenyl)palladium(II), cyclohexylacetylene, and tetrakis(-
triphenylphosphine)palladium(0) were purchased from Aldrich.
The present study describes a novel synthetic strategy designed
to uncover the relationship between molecular aggregation in a film
state and OFET performance. To this end, two new compounds were
synthesized as shown in Scheme 1: cyclohexylethynylquater-
thiophene (CHE4T) and bis-cyclohexylethynylquaterthiophene
(BCHE4T). The triple bond in the cyclohexylacetylene provided
Scheme 1. Synthetic scheme for preparing the quaterthiophene’s cyclohexylacetylene end-capping groups.

758
T.K. An et al. / Dyes and Pigments 96 (2013) 756e762
The synthesis of the cyclohexylacetylene-end capped oligomers was
carried out through a series of cross-coupling reactions that
involved Suzuki couplings. The procedure involved treatment of the
respective cyclohexylacetylene and 5,50-dibromo-2,20-bithiophene
starting materials with Pd(dppf)Cl2 and CuI at 55 ꢂC under Sono-
gashira reaction conditions. All oligomers were purified by subli-
mation under high vacuum.
J ¼ 1 Hz), 7.05 (m, 7H), 2.63 (m, 1H), 1.90 (t, 2H, J ¼ 6 Hz), 1.78 (q, 2H,
J ¼ 3 Hz), 1.53 (t, 4H, J ¼ 6 Hz), 1.44 (m, 2H). 13C NMR (75 MHz,
CDCl3): d137.6, 137.0, 136.5, 136.2, 135.9, 135.6, 132.1, 127.9, 124.6,
124.4, 124.3, 123.8, 123.4, 120.4, 96.3, 79.9, 26.9, 25.8, 22.2, 21.4. HR-
MS (FAB, C24H20S4): calculated, 436.0448; found, 436.0424.
2.2.6. Bis-cyclohexylethynylquaterthiophene (BCHE4T)
A
mixture of bis(1,5-cyclooctadiene) nickel(0) (Ni(COD)2,
2.2.1. 2,20-Bithiophene (1)
(3.28 g, 11.95 mmol), 2,20-bipyridine (1.86 g, 11.95 mmol), and 1,5-
cyclooctadiene (1.29 g, 11.95 mmol) in anhydrous DMF (15 mL)
was stirred at 80 ꢂC for 30 min. Compound 4 (1.4 g, 3.98 mmol) in
toluene (80 mL) was added in one portion. The reaction mixture
was stirred at 80 ꢂC for 48 h. After cooling to room temperature, the
mixture was poured into water, extracted with diethyl ether, and
dried over MgSO4. The solvent was removed by rotary evaporation
and purified by column chromatography using n-hexane as the
eluent. Yield: 0.6 g (55.5%). mp: 231 ꢂC, IR (KBr, cmꢁ1); 3073
(aromtic CeH), 2982 (aliphatic CeH), 2215 (C^C). 1H NMR
2,20-Bithiophene (1) was synthesized according to the proce-
dure reported in the literature. (1) Yield: 48 g (94%). mp: 32 ꢂC (lit
mp 32e33 ꢂC), 1H NMR (300 MHz, CDCl3, ppm):
J ¼ 3.9 Hz), 7.07 (t, 2H, J ¼ 4 Hz) [26].
d 7.21 (d, 4H,
2.2.2. 5,50-Dibromo-2,20-bithiophene (2)
5,50-Dibromo-2,20-bithiophene (2) was synthesized according to
the procedure reported in the literature. (2) Yield: 4.20 g (84%). mp:
144 ꢂC (lit mp 144e146 ꢂC), 1H NMR (300 MHz, CDCl3, ppm):
(d, 2H, J ¼ 3 Hz), 6.95 (d, 2H, J ¼ 3 Hz) [27].
d 6.84
(300 MHz, CDCl3, ppm):
d
7.07 (t, 4H, J ¼ 4 Hz), 7.02 (q, 4H, J ¼ 4 Hz),
2.63 (p, 2H, J ¼ 5 Hz), 1.90 (t, 4H, J ¼ 5 Hz), 1.78 (q, 4H, J ¼ 4 Hz), 1.53
2.2.3. 2-(2,20-Bithiophen-5-yl)-4,4,5,5-tetramethyl-1,3,2-dioxaboro-
lane (3)
(t, 8H, J ¼ 9 Hz),1.36 (s, 4H). 13C NMR (75 MHz, CDCl3):
d136.9,135.9,
131.7, 124.5, 124.3, 123.3, 99.9, 73.5, 32.4, 30.0, 25.8, 24.9. HR-MS
(FAB, C32H30S4): calculated, 542.1230; found, 542.1257.
Compound 1 (7 g, 42.1 mmol) in THF (150 mL) was added drop-
wise (18.52 mL, 46.31 mmol) to n-butyllithium (2.5 M solution in
hexane) at ꢁ78 ꢂC. After the mixture had been stirred at ꢁ78 ꢂC for
1 h, 2-isopropoxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (8.61 g,
46.31 mmol) was added. The resulting mixture was stirred at ꢁ78 ꢂC
for 1 h then warmed to room temperature and further stirred over-
night. The mixture was poured into water, extracted with diethyl
ether, and dried over MgSO4. The solvent was removed by column
chromatography using hexane as the eluent. Yield: 3 g (24%).
IR (KBr, cmꢁ1); 3071 (aromtic CeH), 2973 (aliphatic CeH), 1H
2.3. Fabrication and characterization of the OFET devices
The electrical properties of CHE4T and BCHE4T films were
characterized in a top-contact OFET configuration using a 300 nm
thick SiO2 dielectric on a highly doped n-Si substrate, which served
as the gate electrode. Cleaned substrates were modified with
hydrophobic octyltrichlorosilane (OTS) by dipping the substrates in
an OTS toluene solution for 60 min at room temperature. Solutions
of the organic semiconductors were spin-coated at 2000 rpm from
a 0.5 wt% solution (CHE4T: chloroform, BCHE4T: chlorobenzene) to
form thin films with a nominal thickness of 50 nm, confirmed using
a surface profiler (Alpha Step 500, Tencor). Gold source and drain
electrodes were evaporated on top of the semiconductor layers
(100 nm). For all measurements, the channel lengths (L) were
NMR (300 MHz, CDCl3, ppm):
(m, 3H) [28].
d 1.37 (s, 12H), 7.03 (t, 1H), 7.26
2.2.4. 5-Bromo-50-(cyclohexylethynyl)-2,20-bithiophene (4)
Diisopropylamine (150 mL), CuI (0.31 g, 1.63 mmol), Pd(dppf)Cl2
(0.44 g, 0.54 mmol) and 5,50-dibromo-2,20-bithiophene (8.82 g,
27.2 mmol) were placed in flask and degassed with nitrogen at 0 ꢂC
for 20 min. After cyclohexylacetylene (3 g, 27.7 mmol) was added,
the mixture was heated at 55 ꢂC for 16 h. The mixture was poured
into water, extracted with diethyl ether, and dried over MgSO4. The
solvent was removed by rotary evaporation. The crude product was
purified by column chromatography using n-hexane as the eluent.
Yield: 4.5 g (34%). mp: 64 ꢂC, IR (KBr, cmꢁ1); 3083 (aromtic CeH),
2970 (aliphatic CeH), 2217 (C^C) 1H NMR (300 MHz, CDCl3,
100 mm and the channel widths (W) were 1000 mm. The electrical
characteristics of the FETs were measured in air using Keithley
2400 and 236 source/measure units. Field-effect mobilities were
extracted in the saturation regime from the slope of the sourcee
drain current, wherein the slope of a plot of the square root of
the drain current versus the gate voltage (VG) was fit to the
following equation: IDS ¼ (WCi/2L)
m
(VG ꢁ Vth)2, where IDS is the
drain current,
m
is the carrier mobility, and Vth is the threshold
ppm):
d
7.00 (s, 1H), 6.97 (s, 1H), 6.94 (s, 1H), 6.89 (s, 1H), 2.63 (q,
138.4,
voltage. X-ray diffraction (XRD) studies were performed at the 4C2
beamline at the Pohang Accelerator Laboratory (PAL). The
measurements were carried out with a sample-to-detector distance
of 136 mm. Data were typically collected for ten seconds using an X-
1H), 1.82 (m, 5H), 1.46 (m, 5H). 13C NMR (75 MHz, CDCl3):
d
136.2, 131.6, 130.6, 123.8, 123.6, 123.5, 111.2, 100.0, 73.4, 32.4, 30.0,
25.8, 24.9. MS (m/z): [Mþ] ¼ 349.9.
ray radiation source of
l
¼ 0.138 nm with a 2D charge-coupled
2.2.5. Cyclohexylethynylquaterthiophene (CHE4T)
detector (CCD) (Roper Scientific, Trenton, NJ, USA). The samples
were mounted on a home-built z-axis goniometer equipped with
a vacuum chamber. The incidence angle ai for the X-ray beam was
set to 0.14ꢂ, which was intermediate between the critical angles of
the films and the substrate (ac,f and ac,s).
Toluene (40 mL), tetrahydrofuran (THF) (10 mL), compound 3
(2 g, 5.6 mmol) and compound 4 (2.1 g, 7.4 mmol) were added to
a 2 M aqueous solution (10 mL) of sodium carbonate. After the
mixture was bubbled with nitrogen for 30 min, Pd(PPh3)4 (0.19 g,
0.17 mmol) was added. The mixturewas heated under reflux for 24 h
under a nitrogen atmosphere. The reaction mixture was cooled to
room temperature and poured into a solution containing aqueous
2 N-HCl (200 mL). The mixture was poured into water, extracted
with diethyl ether, and dried over MgSO4. The solvent was removed
by rotary evaporation and purified by column chromatography
using n-hexane as the eluent. Yield: 1.18 g (47.5%). mp: 164 ꢂC, IR
(KBr, cmꢁ1); 3078 (aromtic CeH), 2980 (aliphatic CeH), 2221 (C^C).
3. Results and discussion
3.1. Thermal properties
The thermal stabilities of CHE4T and BCHE4T were investigated
using TGA and DSC techniques (Fig. 1). Good thermal stability is
important for device longevity [26]. The CHE4T and BCHE4T dis-
played excellent thermal stability, with decomposition
1H NMR (300 MHz, CDCl3, ppm):
d
7.24 (t, 1H, J ¼ 4 Hz), 7.19 (d, 1H,

T.K. An et al. / Dyes and Pigments 96 (2013) 756e762
759
Fig. 1. Differential scanning calorimetry curves for (a) CHE4T and (b) BCHE4T; and TGA of (c) CHE4T and (d) BCHE4T.
temperatures of 365 ꢂC and 406 ꢂC, and melting temperatures of
164 ꢂC and 231 ꢂC, respectively.
H- or J-aggregation usually involves excitonic coupling or
dipoleedipole interactions between adjacent molecules in a self-
assembled film [32,33]. The potential energy of interaction
between two dipoles is a complex function of their relative orien-
tation [34]. The potential energy of interaction between two
dipoles can be described as follows:
3.2. UVevis absorption spectroscopy
Generally, intermolecular interactions between adjacent mole-
cules in the solid state can be characterized by UVevis absorption
spectroscopy. UVevisible spectroscopy is most widely used to
study molecular energy levels or molecular aggregation effects.
UVevis absorption spectroscopy can be used to characterize the
electronic structure profiles of a thin film [29]. The optical
absorption properties for CHE4T and BCHE4T in chloroform and
thin films are summarized in Table 1 and Fig. 2.
À
Á
m1m2 1 ꢁ cos2
q
m1
$
m2
ð
m
1$rÞð
m
2$rÞ
V ¼
ꢁ 3
¼
;
(1)
p
4
p
3
0r3
4
3
0r5
4
p
0r3
3
because
m
1$r ¼ m1rcos
q
m
2$r ¼ m2rcos
q
, as shown in Fig. 3 [22].
Equation (1) indicates that a blue shift (H-aggregation) in the UV
absorption corresponds to an increase in energy of the excitonic
A comparison of the peaks of the UVevis spectra of CHE4T and
BCHE4T in a dilute solution revealed that the maximum peak of
CHE4T was lower than that of BCHE4T. This result was observed
because BCHE4T included longer conjugated segments that low-
ered the
quasi-one-dimensional, so the ground-to-excited-state energy gaps
of their electrons scale inversely with the square of the conju-
p / p* transition energy gap [30]. CHE4T and BCHE4T are
p
gated length of the molecule [31].
The main absorption peaks of the CHE4T films were blue-shifted
by greater than 180 nm relative to the peaks of a dilute solution,
indicating that the CHE4T molecules formed H-aggregate struc-
tures [19,22]. In contrast, the main absorption peaks of the BCHE4T
film were red-shifted by greater than 50 nm. In addition, the
absorption peak of BCHE4T was broadened compared with that of
the dilute solution. This observation could be explained by the
formation of J-aggregated molecules [20].
Table 1
Optical properties of compounds CHE4T and BCHE4T.
a
Labs [nm]
Labs film [nm]
CHE4T
BCHE4T
407
421
225,273
397,447,483
Fig. 2. UVevis absorption spectra of (a) CHE4T and (b) BCHE4T in chloroform and in
the solid state.
a
CHCl3 solution.

760
T.K. An et al. / Dyes and Pigments 96 (2013) 756e762
Fig. 3. Arrangement of the molecular dipoles separated by a distance r (the reader is
referred to Ref. [22]).
energy states for
gation) in the absorption corresponds to a decrease in the energy
for
approaching 0ꢂ [33]. Such spectral shifts arise from changes in
q
approaching 90ꢂ, whereas a red shift (J-aggre-
q
the energy levels due to the molecular packing geometry [35,36].
The UV absorption results and Equation (1) suggest that the H-
aggregated CHE4T tended to stack in a parallel arrangement and
the J-aggregated BCHE4T tended to stack in
arrangement [19,22].
a head-to-tail
The molecular packing structure of the designed semi-
conductors was examined by GIWAXS. As shown in Fig. 4, the
CHE4T and BCHE4T diffraction patterns were observed along the
out-of-plane directions. These results indicated that CHE4T and
BCHE4T were vertically oriented on the substrate and grew along
the out-of-plane direction [37]. The diffraction patterns of CHE4T
were also observed along the in-plane direction, but the diffraction
patterns of BCHE4T were not obvious along the in-plane direction.
In place of a peak in the in-plane direction, diagonal BCHE4T
diffraction patterns were observed, suggesting that BCHE4Tcrystals
were diagonally packed on the substrate [38]. From data of GIWAXS
and UV-spectrum of CHE4T and BCHE4T, we can estimate the
molecular configuration that CHE4T forms an orthorhombic crystal
and BCHE4T forms monoclinic or triclinic crystal. These packing
trends were similar to those reported previously. The differences in
the packing trends were caused by the substituted side groups. The
cyclohexylacetylene group generated steric hindrance at the
periphery of the molecule. That is, the substituted side groups
favored packing of adjacent molecules so that the bulky side groups
avoided proximity and minimized steric hindrance [20].
The degree of
p-overlap and the molecular ordering among
molecules along the in-plane direction is important to high-
performance OFETs because charge carriers in an organic medium
Fig. 5. Typical transfer curves for (a) CHE4T and (b) BCHE4T devices; and an output
curve for (c) CHE4T.
move along
The UVevis absorption data demonstrate that CHE4T assumed
a high degree of -overlap. The GIWAXS data verified that molec-
p-overlapping areas along the in-plane direction [39].
p
3.3. Field effect transistor (FET) properties
ular ordering of CHE4T along the in-plane direction was better than
that of BCHE4T. Therefore, the performance of an OFET based on
a CHE4T thin film was expected to be higher than the performance
of an OFET based on a BCHE4T thin film.
The potential of the synthesized quaterthiophene derivatives for
use as organic semiconductors was tested by fabricating FETs of
these derivatives using a solution process. As shown in Fig. 5 and
Fig. 4. GIWAXS scattering results for (a) CHE4T and (b) BCHE4T.

T.K. An et al. / Dyes and Pigments 96 (2013) 756e762
761
Table 2
(H- or J-type aggregation) and packing structures were determined
by differences in the molecular structures. The bulky cyclo-
hexylacetylene end-capping side groups were found to generate
steric hindrance that pushed the molecules apart. The films formed
from asymmetrically substituted side groups (CHE4T) tended to
stack in such a way that adjacent molecules were parallel, which
minimized repulsion. In contrast, films based on symmetrically
substituted side groups (BCHE4T) formed with head-to-tail
arrangements. These two modes led to H- or J-aggregation,
respectively. The GIWAXS analysis showed that the H-aggregated
Device performance of solution-processed OFETs on CHE4T and BCHE4T (average
mobility values are given in parentheses).
Mobility (cm2/V s)
On/off
Vth (V)
SS (V/decade)
CHE4T
BCHE4T
3.21 ꢀ 10ꢁ2 (2.51 ꢀ 10ꢁ2
)
)
1.15 ꢀ 106
ꢁ7.98
ꢁ4.11
0.680
12.7
5.91 ꢀ 10ꢁ4 (4.76 ꢀ 10ꢁ4
3.11 ꢀ 102
CHE4T included more effective intermolecular
p-stacking than the
J-aggregated BCHE4T along the in-plane direction. We also found
that the field-effect mobility of a CHE4T-based device was two
orders of magnitude higher than that of a BCHE4T-based device.
The results of our study revealed that H-type aggregation assumed
a packing structure that resulted in a better OFET performance than
was observed for OFETs based on J-type aggregation.
Acknowledgments
This work was supported by a grant from the Korea Science and
Engineering Foundation (KOSEF), funded by the Korean Govern-
ment (MEST) (2012-0000127) and supported by a grant (F0004010-
2011-34) from the Information Display R&D Center, one of the 21st
Century Frontier R&D Programs, funded by the Ministry of
Knowledge Economy. This work was also supported by Nano
Material Technology Development Program through the National
Research Foundation of Korea (NRF) funded by the Ministry of
Education, Science and Technology (2012-049647). Seung-Hoon
Hahn thanks the support by MKE and KIAT through the Work-
force Development Program in Strategic Technology.
Fig. 6. Schematic diagram showing OFETs based on (a) CHE4T or (b) BCHE4T.
Table 2, FET devices composed of CHE4T and BCHE4T showed
p-channel transfer characteristics, and the output curve of the
CHE4T showed excellent saturation behavior under ambient
conditions. The average field-effect mobilities of the devices based
on CHE4T and BCHE4T were 2.51 ꢀ 10ꢁ2 cm2/(V s) and
4.76 ꢀ 10ꢁ4 cm2/(V s) in the saturation regime. The structural
analysis and UVevis absorption spectra revealed a clear correlation
between H-aggregation (CHE4T) and a high charge carrier mobility,
and between J-aggregation (BCHE4T) and a low mobility. Previously
reports of high-performance organic semiconductors, such as 6,13-
bis(triisopropylsilylethynyl)pentacene and its derivatives, usually
described J-aggregation with a head-to-tail molecular stacking
References
[1] Sonar P, Singh SP, Li Y, Ooi ZE, Ha TJ, Wong I, et al. High mobility organic thin
film transistor and efficient photovoltaic devices using versatile donore
acceptor polymer semiconductor by molecular design. Energy Environ Sci
2011;4:2288e96.
[2] Anthony JE. Functionalized acenes and heteroacenes for organic electronics.
Chem Rev 2006;106:5028e48.
[3] Braga D, Horowitz G. High-performance organic field-effect transistors. Adv
Mater 2009;21:1473e86.
[4] Saeki A, Seki S, Takenobu T, Iwasa Y, Tagawa S. Mobility and dynamics of
charge carriers in rubrene single crystals studied by flash-photolysis micro-
wave conductivity and optical spectroscopy. Adv Mater 2008;20:920e3.
[5] Usta H, Risko C, Wang Z, Huang H, Deliomeroglu MK, Zhukhovitskiy A, et al.
Design, synthesis, and characterization of ladder-type molecules and poly-
mers. Air-stable, solution-processable n-channel and ambipolar semi-
conductors for thin-film transistors via experiment and theory. J Am Chem Soc
2009;131:5586e608.
[6] Kim DH, Lee DY, Lee HS, Lee WH, Kim YH, Han JI, et al. High-mobility
organic transistors based on single-crystalline microribbons of triisopro-
pylsilylethynyl pentacene via solution-phase self-assembly. Adv Mater
2007;19:678e82.
[22,23]. However, large-area p-stacking among adjacent molecules
is accomplished within H-aggregation and not J-aggregation. As the
number of molecules stacked in parallel increased to produce a large
area of
improved [18], thereby producing a higher field-effect mobility in
the devices. The area of -overlap between adjacent molecules was
p-overlap, the hole mobility among adjacent molecules
p
higher in the H-aggregated materials such that they yielded better
charge carrier mobilities than the J-aggregated materials.
The GIWAXS studies confirmed that the asymmetric CHE4T
[7] Bao Z, Lovinger AJ. Soluble regioregular polythiophene derivatives as sem-
iconducting materials for field-effect transistors. Chem Mater 1999;11:
2607e12.
molecules were vertically aligned in
a face-to-face packing
structure, and the symmetric BCHE4T molecules were diagonally
oriented normal to the substrate, as shown in Fig. 6 [37,38].
Parallel packing and good ordering along the in-plane direction in
the H-type aggregate material (CHE4T) produced a higher FET
mobility than was observed in the J-type aggregate material
(BCHE4T).
[8] Li Y, Yang H, Cheng Y, Pomerantz M. Poly(alky1 thiophene-3-carboxylates).
Synthesis and characterization of polythiophenes with
a carbonyl group
directly attached to the ring. Macromolecules 1995;28:5706e8.
[9] Yan H, Chen Z, Zheng Y, Newman C, Quinn JR, Dötz F, et al. A high-mobility
electron-transporting polymer for printed transistors. Nature 2009;457:
679e86.
[10] Wang S, Tang JC, Zhao LH, Png RQ, Wong LY, Chia PJ, et al. Solvent effects and
multiple aggregate states in high-mobility organic field-effect transistors
based on poly(bithiophene-alt-thienothiophene). Appl Phys Lett 2008;93:
162103.
4. Conclusions
[11] Nogues C, Lang P, Desbat B, Buffeteau T, Leiserowitz L. Two-dimensional
crystal structure of a quaterthiophene-alkanethiol self-assembled monolayer
on gold. Langmuir 2008;24:8458e64.
[12] Wang JZ, Zheng ZH, Li HW, Huck WTS, Sirringhaus H. Dewetting of con-
ducting polymer inkjet droplets on patterned surfaces. Nat Mater 2004;3:
171e6.
The relationship between molecular aggregation and OFET
device performance using designed two end-capped quaterthio-
phene derivatives for use in solution-processed organic transistors
was studied. It was shown that the molecular aggregation type

762
T.K. An et al. / Dyes and Pigments 96 (2013) 756e762
[13] Park JH, Chung DS, Park JW, Ahn T, Kong H, Jung YK, et al. Soluble and easily
crystallized anthracene derivatives: precursors of solution-processable semi-
conducting molecules. Org Lett 2007;9:2573e6.
[26] Xia GM, Fang Q, Xu XG, Xu GB, Liu ZQ, Wang W. Synthesis and linear,
nonlinear fluorescence of bithiophene derivatives. Acta Chim Sin 2003;61:
976e82.
[14] Tang W, Singh SP, Ong KH, Chen ZK. Synthesis of thieno[3,2-b]thiophene
derived conjugated oligomers for field-effect transistors applications. J Mater
Chem 2010;20:1497e505.
[15] Minemawari H, Yamada T, Matsui H, Tsutsumi J, Haas S, Chiba R, et al. Inkjet
printing of single-crystal films. Nature 2011;475:364e7.
[16] Locklin J, Li D, Mannsfeld SCB, Borkent EJ, Meng H, Advincula R, et al. Organic
thin film transistors based on cyclohexyl-substituted organic semiconductors.
Chem Mater 2005;17:3366e74.
[17] Kim SH, Jang M, Yang H, Park CE. Effect of pentacene-dielectric affinity on
pentacene thin film growth morphology in organic field-effect transistors.
J Mater Chem 2010;20:5612e20.
[18] Chung DS, Park JW, Park JH, Moon D, Kim GH, Lee HS, et al. High mobility
organic single crystal transistors based on soluble triisopropylsilylethynyl
anthracene derivatives. J Mater Chem 2010;20:524e30.
[19] Spano FC. The spectral signatures of Frenkel polarons in H- and J-aggregates.
Acc Chem Res 2010;43:429e39.
[27] Bäuerle P, Würthner F, Götz G, Effenberger F. Selective synthesis of
a-
substituted oligothiophenes. Synthesis 1993;11:1099e103.
[28] Lee MW, Cha SB, Yang SJ, Park SW, Kim K, Park NG, et al. Synthesis of organic
dyes with linkers between 9.9-dimethylfluorenyl terminal and -cyanoacrylic
a
acid anchor, effect of the linkers on UVevis absorption spectra, and photo-
voltaic properties in dye-sensitized solar cells. Bull Korean Chem Soc 2009;30:
2269e79.
[29] Jin J, Li LS, Zhang YJ, Tian YQ, Jiang S, Zhao Y, et al. Characterization and
structure of side-on azo copolymers in LangmuireBlodgett films. Langmuir
1998;14:5231e6.
[30] Schwartz BJ. Conjugated polymers as molecular materials: how chain
conformation and film morphology influence energy transfer and interchain
interactions. Annu Rev Phys Chem 2003;54:141e72.
[31] Schwartz BJ. Conjugated polymers: what makes a chromophore? Nat Mater
2008;7:427e8.
[32] Curtis MD, Cao J, Kampf JW. Solid-state packing of conjugated oligomers: from
[20] Spano FC. Excitions in conjugated oligomer aggregates, films, and crystals.
Annu Rev Phys Chem 2006;57:217e43.
p-stacks to the herringbone structure. J Am Chem Soc 2004;126:4318e28.
[33] Meinardi F, Cerminara M, Sassella A, Bonifacio R, Tubino R. Superradiance in
molecular H aggregates. Phys Rev Lett 2003;91:247401e4.
[34] Atkins P, Paula J. Atkins’ physical chemistry. 7th ed. New York: Oxford
University Press; 2002.
[35] Schroeder BC, Nielsen CB, Kim YJ, Smith J, Huang Z, Durrant J, et al. Benzo-
trithiophene co-polymers with high charge carrier mobilities in field-effect
transistors. Chem Mater 2011;23:4025e31.
[36] Kim BJ, Park SY, Choi DH. Effect of molecular aggregation on the photo-
induced anisotropy in amorphous polymethacrylate bearing an amino-
nitroazobenzene moiety. Bull Korean Chem Soc 2001;22:271e5.
[37] Lee B, Park I, Yoon J, Park S, Kim J, Kim KW, et al. Role of the n-alkyl end of
bristles in governing liquid crystal alignment at rubbed films of brush poly-
mer rods. Macromolecules 2005;38:4311e8.
[21] Menzel H, Weichart B, Schmidt A, Paul S, Knoll W, Stumpe J, et al. Small-angle
X-ray scattering and ultravioletevisible spectroscopy studies on the structure
and structural changes in LangmuireBlodgett films of polyglutamates with
azobenzene moieties tethered by alkyl spacers of different length. Langmuir
1994;10:1926e33.
[22] Kim SO, An TK, Chen J, Kang I, Kang SH, Chung DS, et al. H-aggregation
strategy in the design of molecular semiconductors for highly reliable organic
thin film transistors. Adv Funct Mater 2011;21:1616e23.
[23] Park JH, Lee DH, Kong H, Park MJ, Jung IH, Park CE, et al. Organic thin-film
transistor properties and the structural relationships between various
aromatic end-capped triisopropylsilylethynyl anthracene derivatives. Org
Electron 2010;11:820e30.
[24] Hu W, Nakashima H, Furukawa K, Kashimura Y, Ajito K, Torimitsu K. Self-
assembled rigid conjugated polymer nanojunction and its nonlinear currente
voltage characteristics at room temperature. Appl Phys Lett 2004;85:115e7.
[25] Samori P, Severin N, Mullen K, Rabe JP. Nanoribbons from conjugated
macromolecules on amorphous substrates observed by SFM and TEM.
Nanotechnology 1999;10:77e80.
[38] Shin TJ, Yang H, Ling MM, Locklin J, Yang L, Lee B, et al. Tunable thin-film
crystalline structures and field-effect mobility of oligofluoreneethiophene
derivatives. Chem Mater 2007;19:5882e9.
[39] Schoonveld WA, Vrijmoeth J, Klapwijk TM. Intrinsic charge transport prop-
erties of an organic single crystal determined using a multiterminal thin-film
transistor. Appl Phys Lett 1998;73:3884e6.