Novel soluble pentacene and anthradithiophene derivatives for organic thin-film transistors

Article abstract of DOI:10.1016/j.orgel.2010.04.029

Four new solution-processable pentacene- (PEN) and anthradithiophene- (ADT) based organic semiconductors bearing two phenylethynyl (PE-) or triethylsilylphenylethynyl (TESPE-) substituents have been synthesized, characterized, and incorporated in thin-fil

Full text of DOI:10.1016/j.orgel.2010.04.029

Organic Electronics 11 (2010) 1363–1375
Contents lists available at ScienceDirect
Organic Electronics
journal homepage: www.elsevier.com/locate/orgel
Novel soluble pentacene and anthradithiophene derivatives
for organic thin-film transistors
c
Choongik Kim a, , Peng-Yi Huang b, , Jhe-Wei Jhuang b, Ming-Chou Chen b, , Jia-Chong Ho ,
*
Tarng-Shiang Hu c, Jing-Yi Yan c, Liang-Hsiang Chen c, Gene-Hsiang Lee d, Antonio Facchetti a,
,
*
Tobin J. Marks a,
*
a Department of Chemistry and The Materials Research Center, Northwestern University, 2145 Sheridan Rd., Evanston, IL 60208-3113, USA
b Department of Chemistry, National Central University, Jhong-Li 32054, Taiwan, ROC
c Industrial Technology Research Institute, Taiwan, ROC
d Instrumentation Center, National Taiwan University, Taipei 10660, Taiwan, ROC
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 4 March 2010
Received in revised form 27 April 2010
Accepted 30 April 2010
Available online 13 May 2010
Four new solution-processable pentacene- (PEN) and anthradithiophene- (ADT) based
organic semiconductors bearing two phenylethynyl (PE-) or triethylsilylphenylethynyl
(TESPE-) substituents have been synthesized, characterized, and incorporated in thin-film
transistors (TFTs). The molecular structures of these four materials have been determined
by single-crystal X-ray diffraction. Thin films of all four compounds have been fabricated
via drop-casting and exhibited p-channel OTFT transport with hole mobilities as high as
ꢀ0.01 cm2/V s. Compared to PEN derivatives, ADT-based compounds exhibited superior
device performance and photooxidative stability in ambient. The film morphologies and
microstructures of these compounds have been characterized by optical microscopy and
X-ray diffraction to rationalize device performance trends.
Keywords:
Organic thin-film transistor
Organic semiconductor
Solution-process
Pentacene
Ó 2010 Elsevier B.V. All rights reserved.
Anthradithiophene
1. Introduction
by photooxidation at the C-6/C-13 positions [3]. There
have been a number of reports addressing the limitations
Solution-processable organic semiconductors have at-
tracted extensive research attention as components in or-
ganic thin-film transistors (OTFTs) for disposable,
inexpensive, large-area electronics, such as electronic pa-
pers, sensors, and smart textiles [1,2]. Among the well-
developed organic semiconductor classes, pentacene
(PEN) [3], anthradithiophene (ADT) [4], and fused-thio-
phene [5] derivatives are archetypical small-molecule
semiconductors. These small molecules are not very solu-
ble in common solvents, which limits their applicability
in many high-throughput coating and printing processes.
Furthermore, pentacene suffers from oxidative instability
of small molecule organic semiconductors by pursuing
solution-processable compounds with enhanced oxidative
stability. Among these efforts, pentacene precursors A [6]
and pentacene derivatives B–D [7] are representative
examples. Although these strategies have not yet been
fully successful [8], introducing substituents at the penta-
cene C-6/C-13 positions has proven to be effective at
enhancing the stability of the pentacene framework and
derivatives with greatly improved carrier mobility has
been reported. For example, triisopropylsilylethynyl
(TIPS)-substituted pentacene (TIPS–PEN; B) exhibited en-
hanced ambient stability [7a], while 4-pentylphenylethy-
nyl-substituted pentacene (BPPE–PEN; C) showed carrier
mobility as high as 0.52 cm2/V s, attributable to the ex-
* Corresponding authors. Tel.: +886 34273253; fax: +886 34227664.
E-mail addresses: mcchen@ncu.edu.tw (M.-C. Chen), a-facchetti@
northwestern.edu (A. Facchetti), t-marks@northwestern.edu (T.J. Marks).
These authors contributed equally to this work.
tended
p-electron delocalization between the pentacene
core and the substituents [3g]. For ADT derivatives, similar
approaches have been investigated, and a solution-pro-
1566-1199/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.orgel.2010.04.029

1364
C. Kim et al. / Organic Electronics 11 (2010) 1363–1375
cessable p-channel triethylsilylethynyl ADT derivative
(TES–ADT; E) with a mobility as high as 1.0 cm2/V s was
reported by Jackson and co-workers [9]. Meanwhile, tri-
methoxyphenylethynyl-substituted PEN and ADT (D)
derivatives have also been reported with relatively low
carrier mobility from solution-processed films [10]. All of
these studies demonstrated that ethynyl/phenyl substitu-
ents on the PEN/ADT cores significantly influence the solid
state electrical performance [11]. Presumably, more conju-
gated phenyl groups connected to the ethynyl group might
be expected to further enhance self-assembly of the aro-
2. Experimental section
2.1. Materials and methods
All chemicals and solvents used in this work were of re-
agent grade and were obtained from Aldrich, Arco, or TCI
Chemical Co. Reaction solvents (toluene, benzene, ether,
and THF) were distilled under nitrogen from sodium/ben-
zophenone ketyl, and halogenated solvents were distilled
from CaH2. 1H and 13C NMR spectra were recorded on a Bru-
ker 500 or a Bruker DRX-200 instrument. Differential scan-
ning calorimetry (DSC) was carried out on a Mettler DSC
822 instrument at a scan rate of 10 K/min. Thermogravi-
metric analysis (TGA) was performed on a Perkin–Elmer
TGA-7 thermal analysis system using dry nitrogen as a car-
rier gas at a flow rate of 40 mL/min. The optical absorption
and fluorescence spectra were obtained using JASCO V-530
and Hitachi F-4500 spectrometers, respectively, and all
spectra were measured in a specified solvent at room
temperature. The IR spectra were obtained using a JASCO
FT/IR-4100 spectrometer. Differential pulse voltammetry
experiments were performed using a CH Instruments/
CHI621C Electrochemical Analyzer. All measurements were
carried out at the temperatures indicated with a conven-
tional three-electrode configuration consisting of a plati-
num disk working electrode, an auxiliary platinum wire
electrode, and a non-aqueous Ag reference electrode. The
supporting electrolyte was 0.1 M tetrabutylammonium
hexafluorophosphate (TBAPF6) in a specified dry solvent.
All potentials reported were referenced to an Fc+/Fc internal
matic moieties into closely
p-stacked arrays, therefore
improving intermolecular orbital overlap and the resulting
device performance. To prove this postulation, four solu-
tion-processable PEN and ADT derivatives (1–4) bearing
phenylethynyl and triethylsilylphenylethynyl substituents
were synthesized, characterized, and incorporated in
OTFTs. From the single-crystal X-ray diffractions and film
morphologies of these four compounds, it has been clearly
seen that both substituents did strongly affect the intermo-
lecular stacking of these new derivatives. With the bulky
triethylsilyl (TES) moiety appended to the phenylethynyl
group, the molecular packings of compounds 2 and 4 via
X-ray analysis were peculiarly engaged in 2-D ‘‘slipped-
crossed” stacking to accommodate the bulky TES groups,
which were very different from those of compounds 1
and 3, and therefore, substantially directed their electronic
performance fabricated via drop-casting (vide infra).
R1
MeO
MeO
MeO
Si
R1
R1
R1
R =
R2
R1
R
D
BBPE-PEN; C
(R1= n-C5H11)
TIPS-PEN; B
(R1= i-C3H7)
R
R
S
PEN Precursors; A
TES-ADT; E
(R1= C2H5)
S
R
Si
R=
R
R
PE-PEN; 1
PE-ADT; 3
TESPE-PEN; 2
R
S
TESPE-ADT; 4
S
R

C. Kim et al. / Organic Electronics 11 (2010) 1363–1375
1365
standard (at +0.6 V). Elemental analyses were performed
on a Heraeus CHN-O-Rapid elemental analyzer. Mass spec-
trometric data were obtained with a JMS-700 HRMS instru-
ment. Prime grade silicon wafers (p+-Si) with ꢀ300 nm
( 5%) of thermally grown oxide (from Montco Silicon) were
used as device substrates.
nally purified via chromatography (silica gel; hexane as
the eluent) giving 1.36 g of desired oily product (85%
yield). 1H NMR(CDCl3): d 7.46–7.45(m, 4H), 3.09(s, 1H, C–
H), 0.95(t, J = 8 Hz, 9H), 0.79(q, J = 8 Hz, 6H).
2.2.3. Synthesis of 6,13-(4-triethylsilylphenylethynyl)-
pentacene (TESPE–PEN; 2)
2.2. Synthesis
Under nitrogen, 2.5 mL of 2.5 M n-BuLi (in hexane,
6.25 mmol) was slowly added to a 20 mL THF solution of
4-triethylsilylphenylacetylene (1.24 g, 5.7 mmol) at 0 °C,
and the mixture was stirred at 60 °C for 3 h. The resulting
black-brown lithium phenyl acetylide solution was then
transferred to a dry THF (50 mL) solution of pentacen-
6,13-dione (0.3 g, 1.0 mmol) at 0 °C, and the mixture was
refluxed for 12 h. After cooling to room temperature,
5 mL of water was added into the reaction mixture and a
deoxygenated SnCl2 (3 g, 15.7 mmol)/HCl (5 mL, 10%) solu-
tion was added to the reaction and stirring was continued
at room temperature for 3 h. Water was added to the reac-
tion mixture and the organic layer was collected and puri-
fied by chromatography (silica gel; hexanes as the eluent)
and recrystallization from hexane, giving 39 mg of blue so-
lid (5.6% yield). 1H NMR (200 MHz, CDCl3): d 9.29(s, 4H),
8.04(m, 4H), 7.87(d, J = 8 Hz, 4H), 7.65(d, J = 8 Hz, 4H),
7.41(m, 4H), 0.91(t, J = 8.0 Hz, 18H), 0.85(q, J = 8.0 Hz,
12H). Deep green crystals suitable for X-ray diffraction
were crystallized by slow evaporation of an ether solution
of compound 2. The molecular structure of 2 was con-
firmed by X-ray diffraction and is shown in Fig. 6 below
(cif file of 2 is in SI).
2.2.1. Synthesis of 6,13-bis(phenylethynyl)pentacene (PE-
PEN; 1)
The procedure was reported in the literature [12]. 1H
NMR (200 MHz, CDCl3): d 9.32(s, 4H), 8.08(dd, J = 6 Hz,
3 Hz, 4H), 7.93(dd, J = 8 Hz, 1 Hz, 4H), 7.55–7.41(m, 10H).
Deep blue crystals suitable for X-ray diffraction were crys-
tallized by slow diffusion of hexane into a xylene solution
of compound 1. The molecular structure of 1 was con-
firmed by X-ray diffraction and is shown in Fig. 4 below
(cif file of 1 is in SI).
2.2.2. Synthesis of 4-triethylsilylphenyl acetylene (5) [13]
As shown in Scheme 1, under nitrogen, 35.6 mL of 2.5 M
n-BuLi (in hexane, 0.089 mol) was slowly added to a
100 mL dry ether solution of 1,4-dibromobenzene (20 g,
0.085 mol) at ꢁ50 to ꢁ30 °C, and the mixture was then
stirred at 0 °C for 1 h. Next, 15.1 mL of chlorotriethylsilane
(0.085 mol) was slowly added to the above mixture and
stirring was continued for 8 h. Water was then added to
the reaction mixture, and the organic layer was collected.
After solvent evaporation, the desired oily 1-bromo-4-tri-
ethylsilylbenzene was purified by distillation at 110 °C/
0.02 Torr giving 21.7 g of product (94% yield). 1H NMR
(CDCl3):
d
7.47(d, J = 8 Hz, 2H), 7.33(d, J = 8 Hz, 2H),
2.2.4. Synthesis of 5,11-bis(phenylethynyl)anthradithiophene
(PE-ADT; 3)
0.96(t, J = 8 Hz, 9H), 0.78(q, J = 8 Hz, 6H).
Under nitrogen, a mixture of dry 1-bromo-4-triethylsi-
lylbenzene (2.0 g, 7.4 mmol), Pd(PPh3)2Cl2 (0.103 g,
0.15 mmol), CuI (56 mg, 0.3 mmol), and 15 mL of dry
piperidine was stirred at 0 °C for 10 min, and then trimeth-
ylsilylacetylene (1.58 mL, 11 mmol) was added to the mix-
ture. The mixture was then stirred at 80 °C for 18 h. After
cooling to room temperature, the solvent was removed un-
der vacuum, and a methanol (20 mL) solution of K2CO3
(3.07 g, 22.2 mmol) was added to the mixture and stirring
was continued at room temperature for 2 h. The methanol
was then removed under vacuum, and the organic product
was extracted with hexane and then filtered through a Cel-
ite flash column. The concentrated crude product was fi-
Under nitrogen, 7.6 mL of 2.5 M n-BuLi (in hexane,
0.019 mol) was slowly added to a 20 mL dioxane solution
of phenylacetylene (1.95 g, 0.019 mol) at 0 °C, and the mix-
ture was stirred at room temperature for 1 h. The resulting
black-brown solution was transferred to a dry dioxane
solution (30 mL) of anthradithiophene-5,11-dione [4a]
(0.306 g, 0.96 mmol) at 0 °C, and then the mixture was re-
fluxed for 4 h. After cooling to room temperature, 0.5 M
NH4Cl solution (50 mL, 0.25 mol) was added to the reaction
mixture, and the organic layer was collected. Hexane was
then added to the collected solution to precipitate green-
brown 5,11-dihydroxy-5,11-bis(phenylethynyl) anthradi-
thiophene (0.28 g, 54% yield). 1H NMR (200 MHz, CDCl3):
H-C C-TMS
1. n-BuLi
Pd(PPh )Cl (cat.)
3
2
Br
Br
Br
TES
TES
2. TESCl
CuI (cat.) / piperidine
Sonogashira Coupling
6
K CO
2
3
TES
TMS
H
MeOH
7
5
Scheme 1. Synthesis of 4-triethylsilylphenyl acetylene 5.

1366
C. Kim et al. / Organic Electronics 11 (2010) 1363–1375
d
8.79(s, 2H), 8.73(s, 2H), 7.76–7.72(m, 4H), 7.56(d,
(5 mL, 10%) solution was added to the reaction mixture,
and stirring was continued at 60 °C for 3 h. Water was next
added to the reaction mixture, and the organic layer was
collected and purified by chromatography (silica gel; hex-
ane as the eluent) and recrystallization from ether, giving
160 mg of purple-red precipitate (36% yield). 1H NMR
J = 5 Hz, 2H), 7.47–7.35(m, 8H), 4.33(t, J = 5 Hz, 2H, OH).
Under nitrogen, a deoxygenated acetic acid (7.5 mL,
50%) solution of SnCl2 (8.4 g, 0.044 mol) was added to the
above dioxane solution (45 mL) of 5,11-dihydroxyl-5,11-
bis(phenylethynyl)anthradithiophene (0.28 g, 0.53 mmol),
and the mixture was stirred at room temperature for 2 h.
Next, water was added to the resulting pink-red solution
and the resulting purple precipitate was collected by filtra-
tion and washed with ether and hexane. The product was
further purified via recrystallization from p-xylene giving
0.12 g of the product (46% yield). 1H NMR (200 MHz,
CDCl3): d 9.23(s, 2H), 9.17(s, 2H), 7.87–7.84(m, 4H), 7.57–
7.47(m, 10H). MS (FAB) (m/z) calcd. for C34H18S2: 490
(M+), Found: 490. Purple-red crystals suitable for X-ray dif-
fraction were crystallized by slow diffusion of hexane into
a xylene solution of compound 3. The molecular structure
of 3 was confirmed by X-ray diffraction and is shown in
Fig. 5 below (cif file of 3 is in SI).
(300 MHz, CDCl3):
d 9.22(s, 2H), 9.16(s, 2H), 7.83(d,
J = 7.5 Hz, 4H), 7.63(d, J = 7.5 Hz, 4H), 7.56(d, J = 6 Hz, 2H),
7.48(d, J = 6 Hz, 2H), 1.02(t, J = 7.5 Hz, 18H), 0.88(t,
J = 7.5 Hz, 12H). Purple-red crystals suitable for X-ray dif-
fraction were crystallized by slow evaporation of an ether
solution of compound 4. The molecular structure of 4
was confirmed by X-ray diffraction and is shown in Fig. 7
below (cif file of 4 is in SI).
2.3. X-ray crystal structure determinations of compounds
1–4
The selected crystals were mounted on a glass fiber.
Data collection on compounds 1–3 and 4 were carried
out on a BRUKER SMART APEX CCD and a NONIUS Kappa
CCD diffractometer, respectively, with Mo radiation
(k = 0.71073 Å) at 150(2) K. After data collection, the
frames were integrated and absorption corrections were
applied. The initial crystal structure was solved by a direct
method, the structure solution was expanded through suc-
cessive least-squares cycles, and the final solution was
determined. All non-hydrogen atoms were refined aniso-
tropically. Hydrogen atoms attached to the carbons were
fixed at calculated positions and were refined using a rid-
ing mode. Crystal data, data collection, and refinement
parameters are summarized in Table 1.
2.2.5. Synthesis of 5,11-Bis(4-triethylsiliylphenylethynyl)-
anthradithiophene (TESPE–ADT; 4)
Under nitrogen, 2.0 mL of 2.0 M isopropyl magnesium
chloride (4.0 mmol) in THF was slowly added to a 20 mL
THF solution of 4-triethylsilylphenylacetylene (0.86 g,
3.95 mmol) at 0 °C, and the mixture was stirred at 60 °C
for 2 h. The resulting black-brown solution was then trans-
ferred to a dry THF solution (50 mL) of an anthradithioph-
ene-5,11-dione
(200 mg,
0.624 mmol)
at
room
temperature, and then the mixture was refluxed at 60 °C
overnight. After cooling to room temperature, 1.5 mL of
water and a deoxygenated SnCl2 (3.0 g, 15.7 mmol)/HCl
Table 1
Summary of crystal structure data for compounds 1ꢁ4.a
Compound
1
2
3
4
Empirical formula
Formula weight
Temperature (K)b
Crystal system
C
38H22
C50H50Si2
707.08
150(2)
Orthorhombic
Pca2(1)
C34H18S2
490.60
150(2)
Monoclinic
P2(1)/n
C46H46S2Si2
719.13
150(2)
478.56
150(2)
Triclinic
Triclinic
ꢀ
ꢀ
Space group
P1
P1
Unit cell dimensions (Å)
a = 10.1371(6)
a = 23.6356(11)
a = 12.019(3)
a = 7.4170(3)
a
= 72.515(1)°
a
= 90°
a
= 90°
a = 81.015(2)°
b = 12.9940(7)
b = 82.654(1)°
c = 19.8189(11)
b = 47.245(2)
b = 90°
c = 7.3483(3)
b = 5.3975(11)
b = 100.571(5)°
c = 18.780(4)
b = 12.5298(6)
b = 85.000(2)°
c = 22.1051(10)
c
= 78.759(1)°
c
= 90°
c
= 90°
c = 85.107(2)°
2016.01(16)
V (Å3)
2435.5(2)
8205.5(6)
1197.6(4)
Z
4
8
2
2
d (calc) (g/cm3)
Absorption coefficient (mmꢁ1
F(0 0 0)
1.305
0.074
1000
1.145
0.120
3024
1.361
0.245
508
1.185
0.222
764
)
Crystal size (mm)
Reflections collected
Independent reflections
Rint
0.70 ꢂ 0.10 ꢂ 0.07
0.70 ꢂ 0.10 ꢂ 0.10
0.70 ꢂ 0.07 ꢂ 0.01
0.60 ꢂ 0.10 ꢂ 0.10
26,841
70,259
5790
17,924
8571
0.0536
R1 = 0.0961
wR2 = 0.2501
R1 = 0.1489
wR2 = 0.2883
14,375
0.0940
R1 = 0.0809
wR2 = 0.1580
R1 = 0.1019
wR2 = 0.1694
2113
0.1229
R1 = 0.1674
wR2 = 0.3203
R1 = 0.2107
wR2 = 0.3432
7008
0.0544
R1 = 0.0636
wR2 = 0.1727
R1 = 0.1014
wR2 = 0.1949
Final R indices [I > 2
r
(I)]c
R indices (all data)
a
b
c
CCD area detector diffractometer; Mo K
Temperature for data collection.
a radiation; k = 0.71073 Å.
Refinement method: full-matrix least-squares on F2.

C. Kim et al. / Organic Electronics 11 (2010) 1363–1375
1367
2.4. Semiconductor film deposition and characterization
3. Results and discussion
All p+-Si/SiO2 substrates were cleaned via sonication in
absolute ethanol for 3 min and then by oxygen plasma
treatment for 5 min (20 W, 0.5 Torr). For an SiO2 coating
layer, –SiMe3 groups were introduced using hexamethyldi-
silazane (HMDS) and were deposited by placing the clean
SiO2 substrates in a N2-filled chamber saturated with
HMDS vapor for 48–72 h. For vacuum deposition, semicon-
ducting materials were thermally deposited at
ꢀ6 ꢂ 10ꢁ6 Torr (ꢀ500 Å, 0.2–0.3 Å/s) at 20 or 50 °C. For
drop-casting, a 0.1–0.2 wt.% solution (in toluene, tetrachlo-
romethane, chlorobenzene, or dichlorobenzene) of each
compound was placed on a clean substrate (1 cm ꢂ 1 cm)
on a hot plate at a preset temperature. The temperature
setting was ꢀ50 °C lower than the boiling point of the cor-
responding solvent. The substrates were kept under a sat-
urated vapor of solvent (the same solvent used to dissolve
the compound) until the solvent evaporated completely.
Films were then annealed in a vacuum oven to remove
the residual solvent. Thin films of the organic semiconduc-
tors were analyzed by optical microscopy and wide angle
h ꢁ 2h X-ray diffraction (XRD) (Rigaku ATXG) using
3.1. Synthesis
The synthesis of four soluble PEN- (1 and 2) and ADT-
(3 and 4) based compounds was achieved according to
Scheme 2 following known synthetic approaches for other
soluble PEN or ADT derivatives [9]. Starting from penta-
cene or ADT quinone, phenylethynyl-substituted 1 and 2
as well as triethylsilylphenylethynyl-substituted 3 and 4
were obtained via phenylethynyl anion alkylation and
SnCl2/HCl reduction, with overall yields of 50%, 6%, 46%,
and 36%, respectively. Compared to PEN or ADT, com-
pounds 1–4 exhibited significantly enhanced solubility in
common organic solvents. Noteworthy is that triethylsilyl-
phenylethynyl (TESPE-) substituted compounds 2 and 4
were very soluble in common organic solvents such as hex-
anes, ether, and benzene.
3.2. Thermal and optical properties
As summarized in Table 2, ADT derivatives exhibited
greater thermal stability than do the analogous PEN deriv-
atives. Thus, compounds 3 and 4 exhibited higher thermal
decomposition temperatures (339 and 168 °C, respec-
tively) than compounds 1 and 2 (255 and 86 °C, respec-
tively). Note that ADT derivative 3, the most thermally
stable compound prepared in this study, did not exhibit a
melting endotherm up to 339 °C in differential scanning
calorimetry (DSC) measurements and exhibited weight
loss only on heating above 320 °C, as demonstrated by
thermogravimetric analysis. Compared to the phenyleth-
ynyl groups, triethylsilylphenylethyl substituents signifi-
cantly decreased the thermal stability of the
corresponding arenes. In particular, pentacene derivative
2 decomposed at 86 °C, which might explain the low syn-
thetic yield of the corresponding compound. Pen and
ADT derivatives with the present core substituents exhib-
ited significantly red-shifted optical absorption spectra.
Pentacene derivatives 1 and 2 in C6H5Cl exhibited kmax val-
ues of ꢀ664 nm (vs. ꢀ579 nm for pentacene), while ADT
monochromated Cu K
a radiation. For TFT device fabrica-
tion, top-contact electrodes (ꢀ70 nm) were deposited by
evaporating gold (ꢀ1 ꢂ 10ꢁ6 Torr) through a shadow mask
with a channel length (L) of 100 lm and a channel width
(W) of 500–5000
lm.
2.5. Electrical Measurements
All OTFT measurements were carried out in ambient
conditions using a Keithly 6430 subfemtoammeter and a
Keithly 2400 source meter, operated by a local Labview
program and GPIB communication. The mobilities (l) were
calculated in the saturation regime using the relationship
[14]:
l
sat = (2IDSL)/[WCi(VG ꢁ VT)2], where IDS is the
source–drain saturation current, Ci is the gate dielectric
capacitance (per area), VG is the gate voltage, and VT is
the threshold voltage. The latter can be estimated as the
1/2
x intercept of the linear section of the plot of VG vs. (IDS
)
.
R
O
R
R
O
1; R = Ph
OH
OH
R
2; R = TES-Ph
SnCl2 / H+
R
Li
R
S
Or
O
O
S
S
S
S
3; R = Ph
4; R = TES-Ph
R
Scheme 2. Synthesis of soluble PEN and ADT derivatives 1–4.

1368
C. Kim et al. / Organic Electronics 11 (2010) 1363–1375
Table 2
Thermal, optical absorption/emission, and electrochemical data for compounds 1–4.
Comp.
Decomp. Td (°C)
TGA (°C; 5%)
Opticalb kmax (nm)
Reductive potential (V)c
Oxidative potential (V)c
DEgap (eV)
(UV)b
(DPV)c
1
2
PEN
3
4
255
86
423
382
297
320
239
306
662
666
579
571
576
490
ꢁ0.924
ꢁ0.864
ꢁ1.27
ꢁ1.14
ꢁ1.14
ꢁ1.63
0.876
0.836
0.820
0.948
0.956
0.940
1.76
1.73
2.07
2.06
2.05
2.47
1.80
1.70
2.09
2.09
2.10
2.57
373a
339
168
450a
ADT
a
b
c
Melting temperature.
In C6H5Cl.
In o-C6H4Cl2 at 100 °C (Fc/Fc+ as an internal standard at +0.6 V).
derivatives 3 and 4 exhibited kmax values of ꢀ574 nm (vs.
-elec-
tron delocalization from the molecular core to the phenyl-
ethenyl substituents. Calculated from the onset of the
optical absorption, pentacene derivatives 1 and 2 exhibited
smaller HOMOꢁLUMO energy gaps than do ADTs 3 and 4,
which was consistent with the energy gap values derived
electrochemically (vide infra).
The photooxidative stability of derivatives 1–4 was
investigated by monitoring the absorbance decay at kmax
in aerated C6H5Cl solutions while exposing to white light
(fluorescent lamp) at room temperature (Fig. 2) [15].
ꢀ490 nm for ADT; Fig. 1), demonstrating extended
p
ADT PE-ADT;3
PE-PEN;1
TESPE-ADT;4
PEN
TESPE-PEN;2
1.0
0.8
0.6
0.4
0.2
0
400
600
700
800
500
Wavelength (nm)
Fig. 1. Optical absorption spectra of compounds 1–4 in C6H5Cl solution.
1
PE-PEN;1
0.8
TESPE-PEN;2
ADT
PEN
0.6
0.4
0.2
TESPE-ADT;4
PE-ADT;3
5.0
10.0
2.5
7.5
12.5
Time (hour)
Fig. 2. Photostability of compounds 1–4 in a C6H5Cl solution, as monitored by optical spectroscopy under white light at room temperature.

C. Kim et al. / Organic Electronics 11 (2010) 1363–1375
1369
Decomposition was observed for all compounds under
these conditions over 24 h. Nevertheless, the core-substi-
tuted compounds in this study exhibited longer half-lives
than non-substituted PEN and ADT, demonstrating en-
hanced stability via core substitution, as observed for other
pentacene derivatives [16]. The phenylethynyl-substituted
compounds (1 and 3) exhibited enhanced photostability
compared to triethylsilylphenylethynyl-substituted deriv-
ative 2 and 4. Note that under ambient conditions (O2
and H2O present) without white light, no decomposition
was observed for these soluble ADTs and PENs, demon-
strating their potential as solution-processable organic
semiconductors.
values than those of PEN (+0.82 V and ꢁ1.27 V, respec-
tively). Similar shifts of both oxidative and reductive peaks
were observed for the ADT series. The HOMO levels of the
ADT derivatives were therefore slightly lower than those of
the pentacene derivatives (Fig. 3; assuming ferrocene/fer-
rocenium oxidation at 4.8 eV). In contrast, the LUMO levels
of these soluble compounds were significantly lower in
both series compared to unsubstituted PEN and ADT,
which can be attributed to more extensive p-electron delo-
calization [19]. Overall, the electrochemically-derived
HOMOꢁLUMO energy gaps were ranked in the order of
1 ꢃ 2 < PEN ꢃ 3 ꢃ 4 < ADT (Fig. 3), in agreement with the
values obtained from optical spectroscopy (Table 2).
3.3. Electrochemical characterization
3.4. Single-crystal structure analysis
Differential pulse voltammograms (DPVs; using ferro-
cene/ferrocenium as internal standard at +0.6 V) of com-
pounds 1–4 were recorded in dichlorobenzene at 100 °C
[17], and the resulting oxidation and reduction potentials
are summarized in Table 2 [18]. For the PEN series, the
DPV spectra of compounds 1 and 2 exhibited oxidative
peaks at +0.84 V and +0.88 V as well as reductive peaks
at ꢁ0.86 V and ꢁ0.92 V, respectively (using ferrocene/fer-
rocenium as the internal standard at +0.6 V); both oxida-
tive and reductive peaks were shifted to more positive
The diffraction-derived crystal structures of compounds
1–4 are shown in Figs. 4–7, respectively, and the corre-
sponding data are summarized in Table 1. The molecular
packings of phenylethynyl-substituted compounds 1 and
3 were similar to that reported for BBPE–PEN [3g]. As
shown in Figs. 4–7, the phenyl moieties attached to the
ethynyl group were not coplanar with the central PEN or
ADT cores, but had torsional angles of 11.8° and 22.5° (1;
Fig. 4D), 10.1° (3; Fig. 5D), 16.6° (2; Fig. 6D), and 18.1°
and 28.2° (4; Fig. 7D), respectively [20]. The molecular pac-
-2.0
-2.57
-2.93
-3.06
-3.06
-3.28
-3.0
-3.34
-4.0
2.57
2.09
2.09
1.80
1.70
2.09
-5.0
-6.0
-5.02
PEN
-5.04
-5.14
ADT
-5.15
PE-ADT
-5.16
-5.08
PE-PEN TESPE
TESPE
-PEN
-ADT
Fig. 3. Electrochemically-derived HOMO and LUMO energies of compounds 1–4, PEN, and ADT.
Fig. 4. Crystal structure of compound 1. (A) Top view (space filling model). (B) Front view. Each of the
p-stacks interweave with each another in a
herringbone packing at an angle of 95.7°. (C) Side view. (D) Front view. The core interlayer spacing (ꢀ3.40 Å) and C6H5 groups have torsional angles of about
11.8° and 22.5° with respect to the PEN plane. (E) Side view. (F) Top view. The PEN cores form 2-D slipped stacks.

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C. Kim et al. / Organic Electronics 11 (2010) 1363–1375
Fig. 5. Crystal structure of compound 3. (A) Top view (space filling model). (B) Front view. Each of the ADT
p-stacks interweave with each another in a
herringbone motif at an angle of 82°. (C) Side view. (D) Front view. The core interlayer spacing (ꢀ3.54 Å) and C6H5 groups have torsional angles of about
10.1° with respect to the ADT plane. (E) Side view. (F) Top view. The ADT cores form 2-D slipped stacks.
kings of 1 and 3 were similar, exhibiting a herringbone-
type packing with tilt angles of 95.7° and 82.0° (Figs. 4B
and 5B), respectively [20]. In each packing column, the
PEN or ADT cores formed 2-D slipped stacks and adopted
a nearly face-to-face packing motif (Figs. 4 and 5), with
an interplanar angle of ꢀ0° and interplanar distances of
Fig. 6. Crystal structure of compound 2. (A) Top view. The PEN cores form 2-D ‘‘slipped-crossed” stacks. (B) Front view. Each of the PEN p-stacks interweave
with one another in a herringbone motif at an angle of 130°. (C) Side view. (D) Front view. The core interlayer separation (ꢀ3.34 Å) and the C6H5 groups have
torsional angles of ꢀ16.6° with respect to the PEN plane. (E) Side view. (F) Top view. The ‘‘slipped-crossed”
p
-stacks of PENs (A0, B0, and C0 are space filling
models of A, B, and C, respectively).

C. Kim et al. / Organic Electronics 11 (2010) 1363–1375
1371
Fig. 7. Crystal structure packing of compound 4. (A) Top view. The ADT cores form 2-D ‘‘slipped-crossed” stacks. (B) Front view. Each of the ADT
p-stacks
interweave with each another. (C) Side view. (D) Front view. The core interlayer separation (ꢀ3.27 Å) and C6H5 groups have torsional angles of ꢀ18° and 28°
with respect to the ADT core. (E) Side view. (F) Top view. The ‘‘slipped-crossed”
p
-stacks of ADTs (A0, B0, and C0 are space filling models of A, B, and C,
respectively).
ꢀ3.40 Å and ꢀ3.54 Å (Figs. 4D and 5D), respectively [20].
Note that the PEN or ADT cores were not perfectly aligned
with each other but were slipped ꢀ3.63 Å (1) and ꢀ3.31 Å
(3) along the long axis as well as half a unit cell length
along the short axis (Figs. 4E and 5F), revealing a close-
interweaving angle of ꢀ180°) of compound 4 may explain
the highest electronic performance among all compounds
employed in this study (vide infra; [22]).
3.5. Thin-film transistor characterization
packed layered structure with intimate p–p overlap.
With the bulky triethylsilyl (TES) moiety appended to
the phenylethynyl group, the molecular packings of com-
pounds 2 and 4 were very different from compounds 1
and 3, as shown in Figs. 6 and 7, respectively. The substitu-
tions of relatively bulky TESPE groups into peripositions of
polyacenes did disrupt the herringbone structure of the
main Pen and ADT core [21]. Compounds 2 and 4 engaged
in 2-D ‘‘slipped-crossed” stacking to accommodate the
Top-contact/bottom-gate OTFTs were fabricated by
drop-casting [23] films of compounds 1–4 onto bare and
HMDS-treated p+-Si/SiO2 substrates (compound 3 was also
fabricated via vacuum deposition on both substrates) [24],
followed by Au deposition through a shadow mask to de-
fine the source and drain electrodes. OTFT characterization
Table 3
bulky TES groups. For compound 2, each of the PEN
p-
Charge carrier mobility (l,
cm2/V s)a, current on/off ratio (Ion:Ioff), and
stacks interweaved with one another in a herringbone
motif at an angle of 130° (Fig. 6B), while this interweaving
angle is nearly 180° for compound 4 (Fig. 7B and C). The
PEN or ADT cores stacked with interplanar separations of
ꢀ3.34 Å and ꢀ3.27 Å (Figs. 6E and 7E), respectively. Note
that this bulky TES substituent induced significant devia-
tions from planarity in the PEN core of compound 2, in
contrast to the other three compounds of this study
(Fig. 6D and E). For compound 4, the interplanar distance
is very comparable to those in TES–ADT (3.25 Å) [3a]. The
shortest interplanar distance and planarity (with core
threshold voltage (VT, V) data for OTFTs fabricated with compounds 1–4 on
bare Si/SiO2 substrates.
Compound
Solvent
l
Ion:Ioff
VT
1
2
C6H5Cl
C6H5Cl
C6H5Cl
0.001
0.0004
0.005
0.015
0.013
104
ꢁ1.0
104
ꢁ7.3
3
104
ꢁ10.0
ꢁ1.2
3b
4
103–4
104
Toluene
ꢁ15.0
a
Calculated in the saturation regime.
This device was fabricated on an HMDS-treated SiO2 substrate by
b
vacuum deposition.

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C. Kim et al. / Organic Electronics 11 (2010) 1363–1375
3.0
2.0
1.0
0.0
10-6
10-8
10-10
-100
-50
VG (V)
0
-100
-50
0
VDS (V)
Fig. 8. Transfer and output characteristics of a representative OTFT fabricated with compound 4. The films were drop-cast from toluene.
was performed in ambient conditions, and the correspond-
ing device characteristics are summarized in Table 3 and
Fig. 8 [25].
two months of storing under nitrogen (without being ex-
posed to light) and did not show any degradation in perfor-
mance (both mobility and current on/off ratios).
The devices fabricated with compounds 1–4 all exhib-
ited TFT activity as p-channel semiconductors. In general,
enhanced performance was observed: (a) in the ADT fam-
ilies versus the PENs, and (b) when chlorobenzene was
used as the drop-casting solvent. All the compounds em-
ployed in this study were sufficiently soluble in common
solvents (e.g., toluene, chlorobenzene, and dichloroben-
zene) at room temperature. As a preliminary fabrication
of the device via solution-process, we prepared for thin
films of each compound via drop-casting from the solution.
For PEN derivatives, the highest carrier mobility values of
0.001 and 0.0004 cm2/V s were observed for films of com-
pounds 1 and 2, respectively, drop-cast from chloroben-
zene. The corresponding Ion:Ioff and VT values were 104
and ꢁ1 to ꢁ7 V for these PEN-based materials. In contrast,
ADT-based devices fabricated by drop-casting exhibited
relatively high mobilities of 0.005 cm2/V s for compound
3 (from chlorobenzene), and 0.013 cm2/V s for compound
4 (from toluene). Similarly to the PEN series, Ion:Ioff and
VT values of 104 and ꢁ10 to ꢁ15 V were observed for the
ADT derivatives. All of the devices were characterized after
3.6. Thin-film microstructure
Film crystallinity and morphology are frequently
strongly correlated with OTFT performance. Film morphol-
ogies and microstructures for the present new class of
materials were studied by optical microscopy and X-ray
diffraction for drop-cast thin films. Optical microscopic
examination of the drop-cast films revealed pronounced
differences in morphology between the PEN and ADT
derivatives (Fig. 9). Polycrystalline platelet-like features
were observed for the ADT-based films, while relatively
fewer crystalline features were observed for drop-cast
films of the PEN derivatives, which is in accord with the
performance of the corresponding TFTs (vide supra).
With crystal structure in hand, we could simulate the
XRD powder pattern and assign the reflections observed in
the film XRD data (Fig. 10). Comparingthese data can be use-
ful to analyze the molecular ordering of the compounds.
Fig. 10 shows a graphical comparison of experimental and
simulated data, with a h ꢁ 2h scan for a drop-casted film of
Fig. 9. Optical micrographs of drop-cast films of compounds 1–4. All images were taken at 50ꢂ magnification.

C. Kim et al. / Organic Electronics 11 (2010) 1363–1375
1373
(001)
(011)
105
104
103
102
101
TESPE-ADT
103
102
PE-PEN
(003)
(022)
(002)
(033)
(006)
(004)
101
103
102
101
(120)
TESPE-PEN
10000
(001)
8000
6000
4000
2000
0
(360)
(003)
(006)
25
(002)
104
103
102
101
(100)
P1
PE-ADT
5
10
15
20
30
(200)
(300)
2 Theta (degrees)
P2
P1
P1
P2
Fig. 10. Comparison of the h ꢁ 2h XRD scans of drop-cast films of TESPE–
ADT on bare Si/SiO2 substrates (top) with the simulated powder pattern
(bottom).
P2
105
104
103
102
101
(001)
compound 4 on bare Si/SiO2 substrate, and a powder pattern
generated from the single-crystal data. As shown, (0 0 1)
reflection family from (0 0 1) to (0 0 6) was pronounced,
indicating predominant ordering of the molecules in c direc-
tion. For other compounds, the (0 1 1) reflections for com-
pound 1 and (1 2 0) reflections for compound 2 could be
assigned, indicating no predominant ordering in the direc-
tion of the unit cell axis for both compounds (Fig. S5). For
compound 3, powder pattern did not show (1 0 0) reflec-
tions which were present in the film XRD [26].
Overall, the thin film XRD microstructural analysis
showed features in agreement with the optical microscopic
analysis. Fig. 11 shows representative h/2h out-of-plane
XRD scans for drop-cast films on bare Si/SiO2 substrates.
As shown, the ADT derivatives exhibited more highly tex-
tured films having large peak intensities with multiple
Bragg reflections. Notably, compound 4 exhibited the high-
est film texturing with multiple Bragg reflections up to
(0 0 6), indicating high film crystallinity and in agreement
with the TFT performance (vide supra). The estimated d-
spacing value from the (1 0 0) reflection for compound 4
was 22.2 Å (which corresponds to the length of the c axis
of the unit cell in the crystal structure), indicating a crys-
talline polymorph where the majority of the molecules
have edge-on orientation on the substrate surface and with
TESPE-ADT
(003)
(002)
(004)
(006)
25
(005)
20
5
10
15
30
2 Theta (degrees)
Fig. 11. Representative h ꢁ 2h XRD scans (in logarithmic scale) of drop-
cast films of the indicated materials on bare Si/SiO2 substrates (P1 and P2
indicate two different phases). Peaks are assigned in comparison with the
powder pattern generated from the single crystal structures.
indicating similar molecular organization in the films.
Compound 2, with the lowest OTFT performance, exhibited
two Bragg reflections having modest peak intensities.
4. Conclusions
In summary, four solution-processable pentacene (PEN)
and anthradithiophene (ADT) derivatives have been syn-
thesized and found to exhibit OTFT p-channel transport.
The phenylethynyl substituents were found to be effective
in increasing the solubility and environmental stability
of the corresponding semiconductors. From the prelimin-
ary fabrication of the device via drop-casting, compared
to the PEN derivatives, the ADT-based compounds exhib-
ited higher device performance with respectable carrier
mobilities of 0.005–0.013 cm2/V s. The magnitudes of
the mobilities were strongly correlated with the film
the p–p stacking direction parallel to the substrate, there-
by facilitating in-plane charge transport from source to
drain. The diffraction patterns of compounds 1 and 3
showing relatively low carrier mobilities (0.001–
0.005 cm2/V s) exhibited multiple Bragg reflections (up to
three peaks) but with modest peak intensities, and with
the latter material evidencing two different phases or
growth orientations. Estimated d-spacings for these com-
pounds were in the narrow range of 11.6–12.0 Å, probably

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C. Kim et al. / Organic Electronics 11 (2010) 1363–1375
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new information about the structural characteristics of
PEN and ADT-based semiconductors and the attractions
of using these cores and substitutents for designing more
solution-processable OTFT materials. A further complete
and detailed device fabrication via other solution-process-
ing methods is underway [22].
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We thank the National Science Council, Taiwan, Repub-
lic of China (Grant Numbers NSC98-2628-M-008-003,
NSC97-2113-M-008-003, and NSC98-2627-E-006-003)
and Industrial Technology Research Institute of Taiwan
for support. Research at Northwestern was supported by
AFOSR (FA9550-08-1-0331) and by the NSF-MRSEC pro-
gram through the Northwestern Materials Research Center
(DMR-0520513).
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[17] Because Pen is not sufficiently soluble, the DPV was performed at
high temperature. There was no indication of thermal
decomposition under this condition (under N2).
[18] Measured with a Pt working electrode in an o-dichlorobenzene
solution using 0.1 mol dmꢁ3 Bu4NPF6 as the supporting electrolyte.
N. Jun-ichi Nishida, D. Kumaki, S. Tokito, Y. Yamashita, J. Am. Chem.
Soc. 128 (2006) 9598.
[19] Similar trends were reported in: J. Wang, K. Liu, Y.-Y. Liu, C.-L. Song,
Z.-F. Shi, J.-B. Peng, H.-L. Zhang, X.-P. Cao, Org. Lett. 11 (2009)
2563.
[20] The planar angle (ꢀ8.1°) and interweaving angle (82°) were smaller
for BBPE-Pen with an interplanar core distance of 3.44 Å. In

C. Kim et al. / Organic Electronics 11 (2010) 1363–1375
1375
comparison, both the interplanar distances of compound 1 (3.40 Å)
and 3 (3.34 Å) were shorter than BBPE-Pen.
performance. The other compounds are inactive or exhibit very
low performance (mobility <10ꢁ6 cm2/V s).
[21] J.E. Anthony, D.L. Eaton, S.R. Parkin, Org. Lett. 4 (2002) 15.
[22] Improved TFT performance for compound 4 was achieved by device
fabrication via spin-coating.
[23] Standard deviations for 5 devices are typically <20%.
[24] All the compounds were fabricated via vacuum deposition on
bare Si/SiO2 and HMDS-treated substrates at various temperatures.
But only one compound (compound 3) exhibited good device
[25] All the available data for all compounds are in the Supporting
Information (Tables S1–S2 and Figs. S1–S4).
[26] Except compound 4, the X-ray diffraction pattern of the
semiconductor films did not match the simulated powder pattern.
This is likely due to polymorph formation (which is clearly shown in
compound 3).