New oligothiophene-pentacene hybrids as highly stable and soluble organic semiconductors

Article abstract of DOI:10.1021/ol900838a

Two series of new oligothiophene-pentacene hybrid compounds were successfully synthesized and characterized, which consist of pentacene and anthradithiophene skeletons modified by different oligothienyl groups at 6,13 sites or 5,11 sites, respectively. Th

Full text of DOI:10.1021/ol900838a

ORGANIC
LETTERS
2009
Vol. 11, No. 12
2563-2566
New Oligothiophene-Pentacene Hybrids
as Highly Stable and Soluble Organic
Semiconductors
Jing Wang,† Ke Liu,† Yi-Yang Liu,† Cheng-Li Song,† Zi-Fa Shi,† Jun-Biao Peng,‡
Hao-Li Zhang,*,† and Xiao-Ping Cao*,†
State Key Laboratory of Applied Organic Chemistry (SKLAOC), College of Chemistry
and Chemical Engineering, Lanzhou UniVersity, Lanzhou, 730000, China, and Key
Laboratory of Specially Functional Materials and AdVanced Manufacturing
Technology, South China UniVersity of Technology, Guangzhou 510640, China
Haoli.zhang@lzu.edu.cn; Caoxplzu@163.com
Received April 16, 2009
ABSTRACT
Two series of new oligothiophene-pentacene hybrid compounds were successfully synthesized and characterized, which consist of pentacene
and anthradithiophene skeletons modified by different oligothienyl groups at 6,13 sites or 5,11 sites, respectively. Their optical, thermal, and
electrochemical properties show regular variations with the length change of the side groups. These materials exhibit much higher solubility
and significantly improved thermal and photooxidation stabilities compared with unmodified pentacene and anthradithiophene.
Organic field-effect transistors (OFETs) based on organic
π-conjugated materials have attracted tremendous interest
due to their potential applications in flexible and low cost
electronics. Pentacene is one of the most well studied organic
semiconductors,1,2 manifesting to the highest mobility.3
However, a singificant drawback of pentacene is its low
solubility in common solvents, which makes it incompatible
with low-cost solution deposition methods. Many soluble
pentacene derivatives have been reported, but most of them
are highly susceptible to photooxidation in solution (typical
half-life time under UV irradiation is around a few min-
utes),4-6 practically precluding them from solution fabrica-
tion applications under ambient conditions. Much efforts have
been made to tailor the properties of pentacene by changing
the substitution pattern on the main aromatic skeleton.5,7-9
For example, Anthony et al. reported that attaching various
trialkylsilylethynyl moieties on the center ring not only
imparted solubility to the pentacene derivatives but also
enhanced π-stacking in the solid state.10,11
† Lanzhou University.
‡ South China University of Technology.
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(1) Dimitrakopoulos, C. D.; Mascaro, D. J. AdV. Mater. 2002, 14, 99
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Haughn, C.; Martin, D. C.; Anthony, J. E. J. Mater. Chem. 2008, 18, 1961
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Chem. Soc. 2001, 123, 9482
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Lett. 2004, 6, 1609
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10.1021/ol900838a CCC: $40.75
Published on Web 05/18/2009
 2009 American Chemical Society

Meanwhile, oligothiophene and polythiophene derivatives
are also well-studied organic semiconductors and show
excellent p-type carrier properties. They are currently under
intense investigation for applications in electroluminescent
diodes,12 lasers,13 sensors,14 photovoltaic cells,15 and fluo-
rescent markers.16
We propose that joining thiophene moieties and pentacene
structures may bring about new functionalities that combine
the unique properties of the two excellent materials. This
purpose could be achieved via two strategies. The first
approach is to incorporate the thiophene units into the acene
skeletons, such as linear anthradithiophene (ADT), which is
structurally analogous to pentacene.17 Another approach is
to attach the oligothiophene structures to the acenes as side
groups.
systematic structural variation on their spectroscopic, elec-
trochemical and thermal properties.
The synthesis of six 6,13-oligothienyl-substituted penta-
cenes (1a-1f) and six 5,11-oligothienyl-substituted anthra-
dithiophenes (2a-2f) was presented in Scheme 1. Compound
3 reacted with 2-iodothiophene (4) or 5-iodo-2,2′-bithiophene
(6) by Suzuki cross coupling reaction produced 2,2′-
bithiophene (5) or 2,2′:5′,2′′-terthiophene (7) respectively.
Compounds 9 and 10 were synthesized from 2-hexylth-
iophene by the similar method. The 5-hexyl-2,2′-bithiophene
(9) and 5-hexyl-2,2′:5′,2′′-terthiophene (10) were successfully
obtained in the yields of 80% and 85%, respectively. 6,13-
Di(2′-thienyl)pentacene (1a), 2-thienylboronic acid (3),
5-iodo-2,2′-bithiophene (6) and sodium (2-hexylthiophene-
5-boronate) (8) were synthesized according to literature
methods.18-21 The target molecules were synthesized from
6,13-pentacenequinone or 5,11-anthradithiophenequinone by
reacting with the corresponding thienyl lithium followed by
reduction with 10% aqueous HCl/Tin (II) chloride, which
induced deoxygenation and produced the desired target
molecules.
To explore both of the above approaches, we designed
two series of oligothiophene-pentacene hybrids which con-
tained different thienyl groups as substituents at the 6,13
postitions or 5,11 positions for pentacenes and anthra-
dithiophenes, respectively (Scheme 1). Alkyl chains are
Introducing thienyl moieties and alkyl chains significantly
improved solubility, and the new materials generally have
solubilities one order higher than pentachene and ADT (S-
Table 1, Supporting Information). The ADT series is more
soluble than the pentacene series, suggesting that incorporat-
ing thiophene unit into the fused rings helps to improve the
solubility. In contrast, the number of thiophene units in the
side chain tends to decrease the solubility. Meanwhile, the
molecules with hexyl side chains have solubilities 2-3 times
higher than their alkyl free analogs.
Scheme 1
The UV-vis absorption bands of new hybrid materials in
toluene solution are similar to that of the parent pentacene
and ADT, but show significant red shifts (Table 1), indicating
an extended π-electron delocalization from the acene nucleus,
brought about by the oligothienyl substituents. In each set
of molecules with similar structures, the increase of thienyl
number induces a shift of the absorption maxima toward long
wavelength. This is consistent with the energy gaps between
HOMO and LUMO, determined by extrapolating the long-
wavelength absorption edges, decreases with the increase of
side chain length (Table 1). The absorption maxima in the
thin films of the compounds 1a-2f exhibit 20 to 40 nm red
shifts along with significant peak broadening (Table 1),
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M.; Lanzani, G.; Barbarella, G.; Favaretto, L. Appl. Phys. Lett. 2002, 81,
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S.; Sayama, K.; Arakawa, H. New J. Chem. 2003, 27, 783.
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Chem. Mater. 1996, 8, 2542.
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Steigerwald, M. L.; Nuckolls, C. J. Am. Chem. Soc. 2006, 128, 1340.
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2000, 37, 281.
incorporated to improve the solubility and tailor the molec-
ular packing in solid state. By comparing these new
compounds, we are aiming to understand the effects of
(21) Cammidge, A. N.; Goddard, V. H. M.; Gopee, H.; Harrison, N. L.;
Hughes, D. L.; Schubert, C. J.; Sutton, B. M.; Watts, G. L.; Whitehead,
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2564
Org. Lett., Vol. 11, No. 12, 2009

Table 1. Optical Absorption Maxima (Amax), Fluorescence Emission Maxima (λmax), Band Gap (Eg), Electrochemical Properties, and
Decomposition Temperature for Compounds 1a-2f
Amax (nm)
λmax (nm)
band gapa
b
c
b
b
c
Eg (eV) Eg (eV)
Eg
Eg, DFT
(eV)
EHOMO
(eV)
ELUMO
(eV)
EHOMO, DFT
dec temp
(°C)
half-life
timed (min)
compound film soln
soln
(film)
(soln)
(eV)
(eV)
1a
1b
1c
1d
1e
1f
622
625
633
633
673
686
604
609
611
607
611
612
577
515
520
523
516
521
522
488
620
635
639
628
638
643
-
531
550
557
535
552
562
-
1.91
1.80
1.77
1.86
1.69
1.68
1.85
2.22
2.20
2.07
2.27
2.06
2.04
1.97
1.97
1.94
1.93
1.96
1.93
1.92
2.1526
2.33
2.29
2.26
2.33
2.29
2.27
2.467
2.02
1.98
1.75
2.08
1.89
1.79
-
2.34
2.31
2.26
2.49
2.38
2.32
-
2.18
2.07
1.93
2.02
1.90
1.89
2.15
2.56
2.45
2.40
2.56
2.45
2.39
2.69
-5.26
-5.24
-5.16
-5.22
-5.16
-5.14
-
-5.61
-5.59
-5.57
-5.75
-5.69
-5.66
-
-3.24
-3.26
-3.41
-3.14
-3.27
-3.35
-
-3.27
-3.28
-3.31
-3.26
-3.31
-3.34
-
-5.01
-4.90
-4.76
-4.71
-4.65
-4.65
-4.82
-4.98
-5.01
-4.90
-4.87
-4.95
-4.81
-4.95
300
350
340
360
375
380
33027
352
365
250
375
385
370
-
12
75
54
39
97
50
5
pentacene 670
2a
2b
2c
2d
2e
529
535
541
524
531
552
560
363
432
470
260
415
741
-
2f
ADT
a Optical HOMO-LUMO gaps determined from the onset of lowest-energy visible absorption band. The onset is defined as the intersection between the
baseline and a tangent line that touches the point of inflection. b Eg ) ELUMO - EHOMO ) (E1/2red + 4.44) s (E1/2ox + 4.44). c HOMO and LUMO levels and
energy gap were obtained by DFT calculation at B3LYP/6-31G (d) level using Gaussian 03 package. d Half-life times were obtained by fitting the data in
Figure 1 according to a unimolecular first-order kinetics.
compared with the absorptions in solution, due to the
90 min (t1/2 ) 741 min), possessed the highest stability. The
observed stability order is: 2f > 2c > 2b > 2e > 2a > 2d >
1e > 1b > 1c > 1f > 1d > 1a > pentacene. It can be seen that
the ADT derivatives are more stable than pentacene deriva-
tives because the incorporation of fused thiophene units in
heteroacenes widens the HOMO-LUMO gap and stabilizes
the HOMO level due to the formation of kinked substruc-
tures.23,24 The formation of endoperoxide across the most
reactive central ring is the main reason for the photooxidation
of linear acenes.25 The thienyl groups on the new compounds
successfully blocked the most reactive sites on the center
ring, therefore the photooxidative resistances are also
influenced by both steric and electronic factors.28 The
increased stability is also attributed to their lower LUMO
orbital energy compared to that of pentacene (Table 1), as
the lower LUMO energy hinders photooxidation by reducing
the rate of electron transfer from photoexcited species to
oxygen.29
increased intermolecular π-π interactions in the solid state.22
The thermal stabilities were studied by thermogravimetric
analysis (TGA). All the new hybrid materials show higher
Figure 1. Photooxidative stability of 1a-2f monitored through
decrease in their absorption in THF solution upon exposure to
ambient light and air at 22 °C.
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Hegmann, F. A.; Parkin, S. R.; Anthony, J. E. J. Appl. Phys. 2005, 98,
33701.
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Tomura, M.; Nishidad, J.; Yamashitad, Y. Tetrahedron. 2007, 63, 9699.
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Ma, Y.; Cui, G.; Chen, S.; Zhan, X.; Yu, G.; Qin, J.; Hu, W.; Zhu, D.
J. Am. Chem. Soc. 2005, 127, 13283.
The photooxidative stabilities of 1a-2f were investigated
by monitoring the reduction in the absorbances at λmax in an
air-saturated 1.0 × 10-4 M THF solution under ambient light
at 22 °C. The half-life of “dilute” pentacene in o-dichlo-
robenzene under the identical photooxidative conditions was
found to be around 5 min. In contrast, the absorption of the
compounds 1a-2f are still detectable after more than one
hour exposure, showing much higher stabilities. The com-
pound 2f, whose absorption band diminished only 10% in
(27) Hong, W.; Wei, Z.; Xi, H.; Xu, W.; Hu, W.; Wang, Q.; Zhu, D. J.
Mater. Chem. 2008, 18, 4814.
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Pramanik, C.; McGruer, N. E.; Miller, G. P. J. Am. Chem. Soc. 2008, 130,
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Org. Lett., Vol. 11, No. 12, 2009
2565

decomposition temperatures than pentacene, except 2c (Table
1). The thermal stability of the hybrids appears to be related
to the substitutions in a complicated manner. With increasing
number of thienyl moieties, the decomposition temperature
first increases then decreases. Similar trend is also observed
in the photostability of the 1a-1f series but not on the 2a-2f
series. Meanwhile, the alkyl group is found to improve the
thermal stability.30
In the case of organic p-type semiconducting materials
where charge transport occurs predominantly by hopping
through HOMOs, the relative HOMO energetic positions are
crucial in determining hole injection from the electrodes to
the active layer, consequently affecting the threshold voltage
and contact resistance of the OFET devices.31,32 Cyclic
voltammetry (CV) was recorded in THF to investigate the
LUMO and HOMO positions (Table 1).33 The energy gaps
calculated from CVs are consistent with that obtained from
UV-vis spectroscopy. The HOMO levels of compounds
1a-1c or 1d-1f gradually increase with the lengthening of
thienyl chains, consistent with a more effective conjugation
with the acene cores.31 The ADT derivatives show the same
trend. The theoretical values of HOMO and LUMO have
been calculated by density functional theory (DFT) method
at B3LYP/6-31G (d) level. The calculation suggested that
the HOMO of the ADT derivatives are generally lower than
that of the pentacene derivatives, which is confirmed by the
CV results. The HOMO-LUMO gaps predicated by DFT
calculation are in good agreement with the experimental
results, showing a decreases of band gap with the increase
of π-electron delocalization.
Figure 2. X-ray crystal structure of compound 1d: (a) molecular
structure of 1d, (b) column stacking of 1d view along the c-axis,
and (c) face to face stacking of 1d in the crystal.
X-ray single-crystal analysis revealed the exact packing
structure of the planar 1d molecule (Figure 2). 1d forms a
triclinic unit cell that belongs to the P-1 space group with
the unit cell parameters a ) 7.7299 Å, b ) 9.2123 Å, c )
12.4220 Å, R ) 69.112°, ꢀ ) 81.845°, γ ) 85.655°. Figure
2b and c show the stacking pattern of molecule 1d in the
crystal. As expected, there is significant π-overlap with face-
to-face interactions of 3.410 Å between stacks.18
cantly improved thermal and photooxidation stabilities
compared with the unmodified pentacene and anthra-
dithiophene, and, therefore are promising for high perfor-
mance solution processed organic electronic applications.
Further investigations including OFET device fabrication
using the newly synthesized materials are under way.
Acknowledgment. This work is supported by Program
for New Century Excellent Talents in University (NCET),
National Natural Science Foundation of China (NSFC.
20621091, 20872055).
In conclusion, we have synthesized and characterized two
series of new oligothiophene-pentacene hybrid materials.
These materials exhibit much higher solubility and signifi-
Supporting Information Available: Experimental pro-
cedures, solubility data, spectroscopic data, 1H and 13C NMR
spectra for all new compounds. This material is available
free of charge via the Internet at http://pubs.acs.org.
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