First tetrabutylanthradithiophene (TBADT) derivatives for solution-processed thin-film transistors

Article abstract of DOI:10.1055/s-0030-1261191

Three new solution-processable tetrabutylanthradithiophene (TBADT)-based organic semiconductors bearing two phenylethynyl, thiophen-2-ylethynyl, and thieno[3,2-b]thiophen-5-ylethynyl substituents have been synthesized and their thermal, optical, and elect

Full text of DOI:10.1055/s-0030-1261191

LETTER
2151
First Tetrabutylanthradithiophene (TBADT) Derivatives for
Solution-Processed Thin-Film Transistors
T
BADTDeriv
e
ativesfor
S
n
olution-Proc
g
essedThin-F
-
ilm
T
ran
Y
sistors i Huang,a Choongik Kim,*b Ming-Chou Chen*a
a
Department of Chemistry, National Central University, Jhong-Li, Taiwan 32054, P. R. of China
E-mail: mcchen@ncu.edu.tw
b
Department of Chemical and Biomolecular Engineering, Sogang University, 1 Shinsoo-Dong, Mapo-Gu, Seoul 121-742, Korea
E-mail: choongik@sogang.ac.kr
Received 30 April 2011
appended to the phenylethynyl group, showed good TFT
performance via drop-casting. All of these studies demon-
strated that substituents on the PEN/ADT cores signifi-
cantly affect the intermolecular stacking of the
Abstract: Three new solution-processable tetrabutylanthra-
dithiophene (TBADT)-based organic semiconductors bearing two
phenylethynyl, thiophen-2-ylethynyl, and thieno[3,2-b]thiophen-5-
ylethynyl substituents have been synthesized and their thermal, op-
tical, and electrochemical properties have been characterized. Pre- corresponding molecules and their electrical perfor-
mance.10
liminary tests of these compounds via drop-casting for thin-film
transistors showed p-channel TFT transport with hole mobilities as
high as 1.5×10–3 cm2/Vs and with a current on/off ratio of 104.
In general, ADT-based compounds exhibit superior de-
vice performance and photooxidative stability under am-
bient conditions compared to PEN derivatives.9 In our
previous study, soluble PEN or ADT derivatives (D–G)
accommodated the phenylethynyl substituents in the solid
state by adopting a herringbone packing with small dihe-
dral angles (ca. 10–20°) between the main core and the
phenyl substituents.9 To further decrease the dihedral an-
gle and thereby enhance the intermolecular orbital overlap
between the core and the substituents of the ADT deriva-
tive, thiophenyl (T-) and thieno[3,2-b]thiophenyl (TT)
substituents have been introduced for the first time in this
study. Compared to phenyl substituents, thiophenyl is less
crowded and therefore more likely to adopt a decreased
dihedral angle between the ADT core and the substituents.
Furthermore, more highly conjugated thieno[3,2-
b]thiophenyl substituents may enhance the molecular or-
bital overlap between the core and the substituents. Ac-
cordingly, phenylethynyl (PE), thiophen-2-ylethynyl
(TE), and thieno[3,2-b]thiophen-5-ylethynyl (TTE)
groups have been employed as core substituents for ADT
derivatives (Figure 2). 2,8-Dialkylsubstituted ADT deriv-
atives have been reported as thin-film transistors, but
these are typically limited to vacuum deposited films.4a In
order to further enhance the solubility of ADT derivatives
for expanded applicability in solution processes, com-
pounds with tetrabutyl groups at the 2-, 3-, 7-, and 8-posi-
tions of the ADT main core have been prepared for the
first time in this contribution.11 In brief, three new soluble
tetrabutylanthradithiophenes (TBADTs 1–3; Figure 2)
have been synthesized and their thermal, optical, and elec-
trochemical properties have been established. Thin films
of all three compounds have been fabricated via drop-
casting, and preliminary results indicate that these materi-
als all exhibit p-channel OTFT transport with hole mobil-
ities as high as ca. 1.5×10–3 cm2/Vs.
Key words: organic semiconductor, organic thin-film transistor,
anthradithiophene, solution process, pentacene
For the last decade, significant efforts have been focused
on developing new organic semiconductors as compo-
nents of organic thin-film transistors (OTFTs) for inex-
pensive, large-area electronics such as electronic papers,
sensors, and smart textiles.1,2 Pentacene (PEN),3 anthra-
dithiophene (ADT),4 and fused thiophene5 derivatives are
among the most widely developed organic semiconduc-
tors. These small-molecule semiconductors, however, are
not very soluble in common solvents, which limits their
applicability in many high-throughput solution processes.
Furthermore, photooxidation at the C-6/C-13 positions of
the pentacene limits long-term stability.6 Introducing sub-
stituents at the pentacene C-6/C-13 positions has proven
to be effective at enhancing the stability of the pentacene
framework, as well as improving carrier mobility.7 Triiso-
propylsilylethynyl (TIPS)-substituted pentacene (TIPS-
PEN; A; Figure 1) exhibited enhanced ambient stability
compared to unsubstituted pentacene,7b while 4-pen-
tylphenylethynyl-substituted pentacene (PPE-PEN; C)
showed carrier mobility as high as 0.52 cm2/Vs via ex-
tended p-electron delocalization between the pentacene
core and the substituents.7c Similar approaches have been
applied for ADT derivatives, and a solution processable p-
channel triethylsilylethynyl ADT derivative (TES-ADT;
B) with a mobility as high as 1.0 cm2/Vs has been report-
ed.8 Recently, four solution-processable PEN and ADT
derivatives (D–G) bearing phenylethynyl and triethylsi-
lylphenylethynyl substituents have been reported.9 Espe-
cially, compounds F and G, having peculiar 2-D ‘slipped-
crossed’ stacking with a bulky triethylsilyl (TES) moiety
The synthesis of three soluble TBADTs was achieved by
a modification of established synthetic approaches toward
PEN and ADT derivatives (Scheme 1).9 Starting from two
equivalents of 4,5-dibutylthiophene-2,3-dicarbaldehyde
SYNLETT 2011, No. 15, pp 2151–2156
Advanced online publication: 24.08.2011
DOI: 10.1055/s-0030-1261191; Art ID: W10711ST
© Georg Thieme Verlag Stuttgart · New York
x
x
.x
x
.2
0
1
1

2152
P.-Y. Huang et al.
LETTER
R1
R1
S
S
R1
R1
R2
R2
R2
TIPS-PEN; A
TES-ADT; B
(R2 = i-C3H7)
(R2 = C2H5)
Si
PPE-PEN; C
R2
(R2 = n-C5H11
)
R1
=
PE-PEN; D
PE-ADT; E
Si
TESPE-PEN; F
TESPE-ADT; G
Figure 1 Examples of soluble PEN and ADT semiconductors used in OTFT
R2
R1
(4; Scheme 2), treatment with one equivalent of 1,4-cy-
clohexanedione in the presence of KOH, led to isolation
of TBADT quinone in 98% yield. PE-substituted 1, TE-
substituted 2, and TTE-substituted 3 were obtained via the
corresponding ethynyl anion alkylation and SnCl2/HCl re-
duction of TBADT quinone, in overall yields of ca.
30%.4d Of particular note, when isopropyl magnesium
chloride was employed instead of n-butyllithium for the
deprotonation of phenylacetylene, PE-TBADT (1) was
obtained in only 9% yield.7c Compared to PE-ADT or PE-
PEN, without substituents at 2-, 3-, 7, and 8-position of
the core, TBADT 1–3 exhibited significantly enhanced
solubility in common organic solvents, such as benzene,
toluene, and CH2Cl2. For example, these three soluble
S
TBADT
R2
R2
(R2 = n-C4H9)
S
R2
R1
S
R1
=
S
S
TTE-TBADT
PE-TBADT
TE-TBADT
1
3
2
Figure 2 Chemical structures of new soluble tetrabutylanthra-
dithiophene (TBADT) derivatives employed in this study.
O
C4H9
S
OHC
S
C4H9
O
O
C4H9
C4H9
C4H9
S
KOH, EtOH
OHC
C4H9
O
4
TBADT quinone (98%)
R
C4H9
1.
R
Li
S
C4H9
C4H9
2. SnCl2, HCl
S
C4H9
R
1; PE-TBADT; R = C6H5 (30%)
2; TE-TBADT; R = C4H4S (26%)
3; TTE-TBADT; R = C6H3S2 (31%)
Scheme 1 Synthesis of TBADT 1–3
Synlett 2011, No. 15, 2151–2156 © Thieme Stuttgart · New York

LETTER
TBADT Derivatives for Solution-Processed Thin-Film Transistors
2153
TBADT could be purified by chromatography using 10% thiophene (5¢) has been reported recently via the reaction
benzene–hexanes as eluent.
of corresponding brominated thiophene (9¢) with Grinard
reagent (C6H13MgBr) catalyzed by Ni(dppp)Cl2 in a yield
of 64%.13 Our synthetic route provides a simpler alterna-
tive to the dialkyl-substituted thiophene with better yield.
The new tetrasubstituted thiophene, 4,5-dibutylthio-
phene-2,3-dicarbaldehyde (4), was prepared as shown in
Scheme 2. Bromination of 2,3-dibutylthiophene (5) with
NBS afforded 6, which was treated with LDA for the ‘2- 2-Ethynylthieno[3,2-b]thiophene (TTE) was prepared as
Br’ rearrangement,12 followed by direct formylation with shown in Scheme 4, by employing a similar synthetic pro-
N-formylpiperidine to give 7. Protection of the aldehyde cedure used for the synthesis of 2-ethynylthiophene
with ethylene glycol offered 8, which was then debromi- (TE).14,15 Via a Witting reaction with carbon tetrabromide
nated with n-butyllithium, formylated with N-formylpipe- and PPh3, 2-thieno[3,2-b]thiophenecarboxaldehyde was
ridine, and protonated to generate 4 in a yield of ca. 22% converted into 2-(2,2-dibromovinyl)thieno[3,2-b]thio-
for the seven-step sequence.
phene, which was then treated with n-butyllithium to give
TTE in an overall yield of 45%.
S
S
C4H9
OHC
C4H9
Weight loss of the three new soluble TBADTs was ob-
served only at a temperature >372 °C, as demonstrated by
thermogravimetric analysis (Table 1). Thermal stability
of these TBADTs was, however, lower than that of the
previously reported PE-ADT (E). From differential scan-
ning calorimetry (DSC) measurements, TBADTs 1 and 2
(decomposition temperatures of 215 °C and 210 °C, re-
spectively) exhibited higher thermal stabilities than the
analogous 3 (decomposition temperatures of 152 °C) and
TESPE-ADT (G, decomposition temperature of 168
°C).16 Note that compared to the PE and TE groups, TTE
substituents decreased the thermal stability of the corre-
sponding ADT. Concurrently, in order to improve the syn-
thetic yield for compound 3, a lower reaction temperature
was required compared to compounds 1 and 2; 0 °C for 3,
40 °C for 2, and 60 °C for 1.
C4H9
Br
C4H9
7 (51%)
5
NBS
S
Br
C4H9
N
ethylene glycol
C4H9
6 (78%)
O
S
S
OHC
C4H9
C4H9
1. n-BuLi
O
2. N-formylpiperidine
3. H+
OHC
C4H9
Br
C4H9
With arylethynyl substituents, TBADTs 1–3 exhibited
significantly red-shifted optical absorption spectra with
4 (61%)
8 (87%)
l
max values of >577 nm compared to ADT (lmax = 490 nm;
Scheme 2 Synthesis of 4,5-dibutylthiophene-2,3-dicarbaldehyde
(4)
Figure 3). As expected, TTE-TBADT (3) with more con-
jugated TTE substituents afforded highest absorption
maximum at l = 604 nm, demonstrating the most extend-
ed p-electron delocalization between the main core and
the arylethenyl substituents among three TBADTs. Calcu-
lated from the onset of the optical absorption, compound
Dibutyl-substituted thiophene 5 was prepared as shown in
Scheme 3. 3-Bromothiophene was deprotonated with
LDA and alkylated with butyl iodide to give 9. Lithium–
halogen exchange and alkylation with butyl iodide gave 5
in good yield (70%). For comparison, dihexyl-substituted
S
S
S
1. LDA
C4H9
C4H9
1. n-C4H9Li
2. C4H9I
2. C4H9I
Br
Br
9 (84%)
H9
C4
5 (70%)
S
S
C6H13
1. C6H13Br, Mg
2. Ni(dppp)Cl2
C6H13
Br
C6H13
5' (64%)
9'
Figure 3 Optical absorption spectra of TBADT 1–3 in chloroben-
zene solution
Scheme 3 Synthesis of 2,3-dibutylthiophene (5)
S
S
S
Br
n-BuLi
Br
3
CHO
S
S
S
Scheme 4 Synthesis of 2-ethynylthieno[3,2-b]thiophene (TTE)
Synlett 2011, No. 15, 2151–2156 © Thieme Stuttgart · New York

2154
P.-Y. Huang et al.
LETTER
Table 1 Thermal, Optical Absorption/Emission, and Electrochemical Data for TBADTs 1–3
Compound
Decomp temp TGA
Optical
max (nm)b
Reductive
Oxidative
DEgap (eV)
(Td; °C)
(°C; 5%)
l
potential (V)c
potential (V)c
(UV)b
2.06
2.05
2.05
2.01
1.95
(DPV)c
2.09
PE-ADTa
339
168
215
210
152
320
239
400
400
372
571
576
577
587
604
–1.14d
–1.14d
–1.21
–1.17
–1.12
0.948d
0.956d
0.84
TESPE-ADTa
PE-TBADT (1)
TE-TBADT (2)
TTE-TBADT (3)
2.10
2.05
0.83
2.00
0.82
1.94
a From ref. 9.
b In chlorobenzene.
c In 1,2-dichlorobenzene at 25 °C (Fc/Fc+ as an internal standard at +0.6 V).
d In 1,2-dichlorobenzene at 100 °C.
3 exhibited smaller HOMO–LUMO energy gaps than values than that of ADT (–1.67 V). Hence, the LUMO
compounds 1 and 2, consistent with the electrochemically levels of the new compounds lie significantly lower than
derived energy gap values (vide infra, Table 1).
that of the unsubstituted ADT. Note that TTE-TBADT
with more conjugation than the others afforded the highest
HOMO and the lowest LUMO, which can be attributed to
more extensive p-electron delocalization.21 Overall, the
electrochemically-derived HOMO–LUMO energy gaps
are ranked in the order of 3 < 2 < 1 < ADT (Figure 4), in
agreement with the values obtained from optical spectros-
copy (Table 1).
The photooxidative stability of compounds 1–3 was in-
vestigated by monitoring the absorbance decay at lmax in
aerated chlorobenzene solutions under the exposure of
white light (fluorescent lamp) at room temperature (Fig-
ure S1, see Supporting Information). Decomposition of
the corresponding compounds was observed for all three
TBADTs over the course of hours. Of particular note, the
compound with a lower LUMO level (vide infra) was
more stable than the others.17 TTE-substituted 3 exhibited
longer half-lives than 1 and 2, demonstrating enhanced
photooxidative stability via arylethynyl substitution, as
observed in the other PEN or ADT derivatives.18
To further investigate the decomposition mechanism
more detailed experiments were carried out. First, under
ambient conditions (O2 and H2O present) but in the ab-
sence of white light, no decomposition was observed for
all three compounds over the course of 24 hours.
Figure 4 Electrochemically derived HOMO and LUMO energy le-
vels of compounds 1–3 and ADT
Furthermore, under fluorescent light but without O2 (N2
degassed from the solution, trace water present), no de-
composition was observed over 24 hours. Therefore, the
three new TBADT derivatives investigated in this study
show promise as candidates for solution-processable or-
ganic semiconductors under ambient conditions in the ab-
sence of O2 or light. Note that unsubstituted PEN and
ADT suffer photooxidative decomposition.19
The newly synthesized compounds were also tested as or-
ganic semiconductors for organic thin-film transistors
(OTFT). Preliminary top-contact/bottom-gate OTFT were
fabricated by drop-casting films of compounds 1–3 onto
bare p++-Si/SiO2 (300 nm SiO2 as dielectric) substrates,
followed by Au deposition through a shadow mask to de-
fine the source and drain electrodes. OTFT characteriza-
tion was performed under ambient conditions, and the
corresponding device characteristics are summarized in
Table 2 (see also Supporting Information, Figure S3).
Differential pulse voltammograms (DPV; using fer-
rocene/ferrocenium as internal standard at +0.6 V) of 1–3
were recorded in dichlorobenzene at 25 °C, and the result-
ing oxidation and reduction potentials are summarized in
Table 1.20 As expected, the DPV spectra of compounds 1–
3, exhibited oxidative peaks at +0.82 V to +0.84 V, which
are lower than that determined for ADT (+0.94 V; see
Supporting Information, Figure S2). Therefore, the
HOMO levels of the TBADTs are slightly higher than that
of ADT (Figure 4; assuming ferrocene/ferrocenium oxi-
dation at 4.8 eV). By contrast, the reductive peaks of com-
pounds 1–3 at –1.12 V to –1.21 V are shifted to higher
The devices fabricated with compounds 1–3 all exhibited
TFT activity as p-channel semiconductors (see Support-
ing Information, Figure S3). All the compounds employed
in this study were sufficiently soluble in common solvents
(toluene, chlorobenzene, and dichlorobenzene) at room
temperature,22 and drop-casting of the corresponding so-
lutions was found to be a reliable semiconductor film-
growth procedure for OTFT. Preliminary devices from
Synlett 2011, No. 15, 2151–2156 © Thieme Stuttgart · New York

LETTER
TBADT Derivatives for Solution-Processed Thin-Film Transistors
2155
Table 2 Charge Carrier Mobility (m, cm2/Vs),a Current On/Off Ra-
tio (Ion/Ioff), and Threshold Voltage (VT, V) Data for OTFT Fabricated
with Compounds 1–3 on Bare p++-Si/SiO2 Substrates
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Compd Solvent
m
Ion/Ioff
104
VT
1
2
3
1,2-dichlorobenzene
1.5×10–3
5.0×10–4
1.0×10–3
– 8.0
+ 4.0
+13.0
1,2-dichlorobenzene
chlorobenzene
103
102
a Calculated in the saturation regime.
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drop-casted films showed similar carrier mobility values
of ca. 0.001 cm2/Vs. Compound 1 exhibited the highest
device performance with a carrier mobility of 1.5×10–3
cm2/Vs and a current on/off ratio of 104. Compounds 2
and 3 showed slightly lower device performance com-
pared to compound 1, with a carrier mobility of 5×10–4 to
1×10–3 cm2/Vs and a current on/off ratio of 102–103.
In summary, three new solution-processable tetrabutyl-
anthradithiophene (TBADT) derivatives have been syn-
thesized and characterized. The four butyl substituents
and the arylethynyl substituents are found to be effective
in increasing both the solubility and the environmental
stability of the corresponding semiconductors. Prelimi-
nary tests of the TBADT-based compounds for organic
thin-film transistors showed TFT activity as p-channel
semiconductors with carrier mobilities of ca. 0.001 cm2/
Vs. Further studies on device fabrication and characteriza-
tion under different conditions (e.g., solvent, concentra-
tion, substrate) using different solution process (e.g., spin-
coating, printing) are under way. These results provide
new information about the structural characteristics of
TBADT-based semiconductors and the benefits of using
these cores for the design and fabrication of solution-
processable OTFT materials.
Supporting Information for this article is available online at
http://www.thieme-connect.com/ejournals/toc/synlett. It contains
all experimental sections and Figures S1–S26.
Acknowledgment
This work was supported by National Science Council, Taiwan,
Republic of China (Grant Numbers NSC99-2628-M-008-005 and
NSC99-2627-E-006-003) and by the Sogang University Research
Grant of 2011, Republic of Korea.
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Jackson, T. N. J. Am. Chem. Soc. 2005, 127, 4986.
(b) Subramanian, S.; Park, S. K.; Parkin, S. R.; Podzorov,
V.; Jackson, T. N.; Anthony, J. E. J. Am. Chem. Soc. 2008,
130, 2706.
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P.-Y. Huang et al.
LETTER
(9) Kim, C.; Huang, P.-Y.; Jhuang, J.-W.; Chen, M.-C.; Ho,
J.-C.; Hu, T.-S.; Yan, J.-Y.; Chen, L.-H.; Lee, G.-H.;
Facchetti, A.; Marks, T. J. Org. Electron. 2010, 11, 1363.
(10) Anthony, J. E. Chem. Rev. 2006, 106, 5028.
(11) Since three new TBADT derivatives are very soluble in
common solvents, no crystal structures were obtained. Thus,
the elucidation of interrelationships between molecular
structure and the characterization of organic thin-film
transistors are not available so far.
(12) Reinecke, M. G.; Adickes, H. W.; Pyun, C. J. Org. Chem.
1971, 36, 2690.
(13) Zhao, C.; Zhang, Y.; Ng, M.-K. J. Org. Chem. 2007, 72,
6364.
loss from TGA, dimerization of the ADT is suspected, see:
Coppo, P.; Yeates, S. G. Adv. Mater. 2005, 17, 3001.
(17) Maliakal, A.; Raghavachari, K.; Katz, H. E.; Chandross, E.;
Siegrist, T. Chem. Mater. 2004, 16, 4980.
(18) It has been reported that alkynyl substitution lowers the
LUMO energy for pentacene derivatives as compared to that
of unsubstituted pentacene, which hinders their
photooxidation. See: Akhtaruzzaman, M.; Kamata, N.;
Nishida, J.; Ando, S.; Tada, H.; Tomura, M.; Yamashita, Y.
Chem. Commun. 2005, 3183; see also ref. 7c.
(19) Yamada, H.; Yamashita, Y.; Kikuchi, M.; Watanabe, H.;
Okujima, T.; Uno, H.; Ogawa, T.; Ohara, K.; Ono, N. Chem.
Eur. J. 2005, 11, 6212.
(14) Corey, E. J.; Fuchs, P. L. Tetrahedron Lett. 1972, 4831.
(15) Beny, J. P.; Dhawan, S. N.; Kagan, J.; Sundlass, S. J. Org.
Chem. 1982, 11, 2201.
(16) The endotherm was not observed in the second run of DSC
measurement. After performing the second run (samples
were heated above the decomposition temperature), the
residues in the DSC cell were monitored by 1H NMR and
were found to be different from the starting ADT. In addition
to the color change, the solubility of these ADT (residues)
was also dramatically decreased. Since there was no weight
(20) Measured with a Pt working electrode in an o-dichloro-
benzene solution using 0.1 mol dm–3 Bu4NPF6 as the
supporting electrolyte. See: Naraso Nishida, J.; Kumaki, D.;
Tokito, S.; Yamashita, Y. J. Am. Chem. Soc. 2006, 128,
9598.
(21) Similar trends are reported in: Wang, J.; Liu, K.; Liu, Y.-Y.;
Song, C.-L.; Shi, Z.-F.; Peng, J.-B.; Zhang, H.-L.; Cao,
X.-P. Org. Lett. 2009, 11, 2563.
(22) The solubility of TBADT derivatives was ca. 20 (for 3)
to 40 (for 1 and 2) mg/mL in CHCl3.
Synlett 2011, No. 15, 2151–2156 © Thieme Stuttgart · New York

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