Soluble 2,6-Bis(4-pentylphenylethynyl)anthracene as a High Hole Mobility Semiconductor for Organic Field-effect Transistors

Article abstract of DOI:10.1246/cl.160731

The balance between good solubility and high crystallinity is an advantageous characteristic of 2,6-bis(4-pentylphenyl-ethynyl)anthracene (1). Organic field-effect transistors featuring either a vacuum-deposited film or a simple drop-cast film of 1 both showed high hole mobilities of 0.94 and 0.63 cm² V^-1 s^-1, respectively.

Full text of DOI:10.1246/cl.160731

Received: August 8, 2016 | Accepted: September 15, 2016 | Web Released: November 16, 2016
CL-160731
Soluble 2,6-Bis(4-pentylphenylethynyl)anthracene as a High Hole Mobility Semiconductor
for Organic Field-effect Transistors
Yuta Takaki,1 Yutaka Wakayama,*2,3 Yasushi Ishiguro,2,3 Ryoma Hayakawa,2 Masakazu Yamagishi,4
Toshihiro Okamoto,4 Jun Takeya,4 Kenji Yoza,5 and Kenji Kobayashi*1
1Department of Chemistry, Faculty of Science, Shizuoka University, 836 Ohya, Suruga-ku, Shizuoka 422-8529
2International Center of Materials Nanoarchitectonics, National Institute for Materials Science,
1-1 Namiki, Tsukuba, Ibaraki 305-0044
3Department of Chemistry and Biochemistry, Faculty of Engineering, Kyushu University, 1-1 Namiki, Tuskuba, Ibaraki 305-0044
4Department of Advanced Materials Science, Graduate School of Frontier Sciences, The University of Tokyo,
5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8561
5Bruker axs, 3-9-B Moriya, Kanagawa-ku, Yokohama, Kanagawa 221-0022
(E-mail: kobayashi.kenji.a@shizuoka.ac.jp, WAKAYAMA.Yutaka@nims.go.jp)
The balance between good solubility and high crystallinity
is an advantageous characteristic of 2,6-bis(4-pentylphenyl-
ethynyl)anthracene (1). Organic field-effect transistors featuring
either a vacuum-deposited film or a simple drop-cast film of 1
¹1
both showed high hole mobilities of 0.94 and 0.63 cm2 V¹1 s
respectively.
,
Chart 1.
Keywords: Organic field-effect transistor (OFET)
Anthracene Good solubility
|
|
However, the high planarity of the conformationally rigid π-
conjugated structure of 2 greatly reduces the solubility of the
compound in common organic solvents, which makes the use of
2 as a solution-processable organic semiconductor difficult.
Based on these considerations, we have designed 2,6-bis(4-
pentylphenylethynyl)anthracene (1).3e,3g The replacement of the
double bonds in 2 by triple bonds in 1 allows for free rotation
of the p-pentylphenyl groups connected to the anthracene ring
in solution, which we anticipated would lead to significant
improvement of the solubility of 1 compared with that of 2.
It was also expected that the coplanar conformation between
the p-pentylphenyl group and anthracene ring in 1 would be
maintained so as to maximize the extended π-conjugation in the
solid state, with the assistance of crystal packing forces.9 Herein,
we report the crystal packing structure and OFET properties of
soluble 1. Notably, devices constructed with either a vacuum-
deposited film or a simple drop-cast film of 1 showed high hole
mobilities of 0.94 and 0.63 cm2 V¹1 s¹1, respectively.10
Compound 1 was synthesized in 66% yield by the
Sonogashira cross-coupling reaction of 2,6-dibromoanthracene3a
with p-pentylphenylacetylene (Scheme S1). In marked contrast
to 2, compound 1 is soluble at room temperature in a range of
organic solvents such as chloroform, toluene, and o-dichloro-
benzene.11 TG-DTA analysis indicates that 1 is thermally stable
at around 200 °C and has an endothermic peak at 165 °C arising
from phase transition (Figure S2).
p-Type organic semiconductors have been studied inten-
sively for their applications in high-performance organic field-
effect transistors (OFETs).1 To apply OFETs to flexible and
printed electronics such as electronic paper, it is a prerequisite
that the hole mobility of the OFETs is greater than
¹1
0.5 cm2 V¹1 s to exceed the mobility of amorphous silicon
semiconductors.1 Recent developments achieved on the basis
of excellent molecular designs have led to very high hole
¹1 1,2
mobilities of more than 1 cm2 V¹1 s
.
Air-stable organic
semiconductors are important for OFETs, and it is desirable for
such semiconductors to possess a large band gap and a HOMO
level deeper than ¹5.0 eV. In this regard, the use of anthracene
as a π-electronic core is more desirable than the use of
pentacene.3 The high solubility of organic semiconductors in
common organic solvents is also important for solution-
processable OFETs. In addition to spin-coating methods, several
elaborate methods for solution-processed single crystal OFETs
have been developed.1b,4 However, reports on the construction
of high-performance semiconductors by using only a simple
drop-casting method are rare.2a,5
A large transfer integral is an essential factor for high hole
mobility in organic semiconductors. This property is highly
dependent on the crystal packing arrangement,6 wherein a dense
herringbone packing with π-π overlap between adjacent mole-
cules (slipped π-stacking) or a brick-wall type of two-dimen-
sional lamellar π-stacked packing is desirable.1a,7 Meng and
co-workers reported the use of 2,6-bis[2-(4-pentylphenyl)vinyl]-
anthracene (2) (Chart 1) as an organic semiconductor for
OFETs.8 Although this was an anthracene-based derivative,
the vacuum-deposited film of 2 with a top-contact/bottom-gate
DFT calculations at the B3LYP/6-31G(d,p) level showed
that the HOMO energy levels (and HOMO-LUMO gaps) of 1,
2, and pentacene are ¹5.07 eV (gap = 3.04 eV), ¹4.91 eV
(2.99 eV), and ¹4.61 eV (2.21 eV), respectively (Figure S3).
This result suggests that 2 possesses slightly more extended π-
conjugation than 1, whereas 1 seems to be more air-stable than 2
because the HOMO energy level of 1 is 0.16 eV deeper than that
of 2. The HOMO level of 1 in o-dichlorobenzene was ¹5.63 eV,
determined by the peak of differential pulse voltammogram
(Figure S4). In the UV-vis absorption spectra (Figure S5a), -max
device configuration showed hole mobilities of up to 1.28
¹1 8a
cm2 V¹1 s
.
The high performance of the OFET containing
2 can be ascribed to the densely packed herringbone crystal
structure arising from the high planarity of π-conjugated 2.
Chem. Lett. 2016, 45, 1403–1405 | doi:10.1246/cl.160731
© 2016 The Chemical Society of Japan | 1403

Figure 2. (a) AFM image (10 ¯m © 10 ¯m) and (b) XRD
pattern of the vacuum-deposited film of 1.
whereas those for 2 were a = 5.85 ¡ and b = 7.23 ¡, and
a = 6.27 ¡ and b = 7.79 ¡ for pentacene.8a These results
indicate that 1 is densely packed in a manner similar to 2. By
using the Amsterdam Density Functional (ADF) program
package,15 intermolecular electronic couplings of HOMO (trans-
fer integral, t) based on the packing structure of 1 were estimated
as ta1 = 38 meV, ta2 = 34 meV, tp = 56 meV, and tq = 53 meV
(Figure 1e), which suggests that two-dimensional carrier trans-
port in the ab plane is facilitated. These values of 1 were
comparable to those of 2 (ta = 33 meV and tp = 59 meV)
(Figure S7), which suggests that 1 and 2 should exhibit similar
carrier transport ability.
AFM images of the vacuum-deposited film of 1 (Figures 2a
and S8) revealed that the thin film is composed of wide terrace
structures with each step of ca. 3.32 nm, which is consistent
with the interlayer distance of 3.32 nm elucidated by the X-ray
crystallographic analysis, as noted above. The XRD pattern of
the vacuum-deposited film of 1 on the bear Si/SiO2 substrate
(Figures 2b and S9a) showed that a series of peaks assignable to
(00l) reflections were clearly observed, which corresponds well
with the (00l) reflections from single-crystal data; the interlayer
distance (d-spacing) calculated from these reflections was 3.33-
3.36 nm. These results clearly indicate that the molecules of 1
in the vacuum-deposited film are aligned nearly perpendicular
to the substrate and form layer-by-layer structures; that is, the
crystallographic c-axis of 1 is oriented nearly perpendicular to
the substrate, and the ab plane (the conduction plane) is parallel
to the substrate. The XRD pattern of a simple drop-cast film of 1
was the same as that of the vacuum-deposited film (Figure S9c),
indicating that the packing structure of 1 in the drop-cast film is
the same as that of 1 in the vacuum-deposited film.
OFETs of 1 with a top-contact/bottom-gate configuration
were fabricated by vacuum deposition on bare Si/SiO2 substrates
at substrate deposition temperatures (Tsub) in the range of 50 to
80 °C. The performance of the device was evaluated under
ambient conditions in air. All of the OFETs based on 1
showed typical p-channel transistor responses (Table S2 and
Figure S11). The highest FET performance was observed for
the device at Tsub = 70 °C, which showed hole mobility ® =
0.94 cm2 V¹1 s¹1, threshold voltage Vth = ¹12.1 V, and on/off
current ratio Ion/Ioff = 1.6 © 107 (Figure 3a). These values were
comparable to those of OFETs based on 2.8a On HMDS-treated
Figure 1. X-ray crystal structure of 1.14 (a) Molecular
structures of 1 (two conformers). 3-D packing structure of 1:
(b) front view and (c) side view, wherein two conformers are
shown in red and black. (d) Herringbone packing of 1, wherein
the p-pentylphenylethynyl groups are omitted for clarity. (e)
Herringbone packing of 1 with transfer integral values (t) in the
ADF program.15
of 1 in the drop-cast film prepared from the o-dichlorobenzene
solution appeared at 441 nm, which was red-shifted by 24 nm
relative to that of 1 in o-dichlorobenzene. This result suggests
that, in the drop-cast film, 1 forms a herringbone packing
structure including J-aggregate and/or a coplanar conformation
between the p-pentylphenyl group and the anthracene ring,
leading to extended π-conjugation.9,12
Thin plate-like single crystals of 1 suitable for X-ray
diffraction analysis were obtained by slow diffusion of hexane
into a solution of 1 in benzene. The molecular and crystal
packing structures of 1 are shown in Figure 1.13 Two kinds of
molecules of 1, with slightly different conformations, are present
in the unit cell (Figure 1a).14 The anthracene ring and the phenyl
ring are almost coplanar, leading to efficiently extended π-
conjugation, although the triple bonds are bent at ca. 12° with
respect to the anthracene mean plane. The crystal packing
structure of 1 is similar to that of 2.8a Molecules of 1 assume a
typical lamella-like layer-by-layer structure consisting of alter-
nate alkyl and 2,6-bis(phenylethynyl)anthracene layers along
the c-axis (Figures 1b and 1c), wherein the interlayer distance of
1 was 33.22 ¡. In the 2,6-bis(phenylethynyl)anthracene layer,
the molecules adopt a herringbone packing arrangement in the
ab plane (Figure 1d). The intermolecular distances between the
anthracene core parts of 1 were a = 5.92 ¡ and b = 7.43 ¡,
substrates, the highest FET performance of 1 was observed for
¹1
the device at Tsub = 60 °C, which showed ® = 0.63 cm2 V¹1 s
,
Vth = ¹20.7 V, and Ion/Ioff = 6.8 © 106 (Figure S11f). Thus,
surface treatment of the Si/SiO2 substrates did not improve the
performance of the device based on 1. The balance between good
1404 | Chem. Lett. 2016, 45, 1403–1405 | doi:10.1246/cl.160731
© 2016 The Chemical Society of Japan

129, 15732. c) M. J. Kang, I. Doi, H. Mori, E. Miyazaki, K.
Takimiya, M. Ikeda, H. Kuwabara, Adv. Mater. 2011, 23, 1222. d) T.
Okamoto, C. Mitsui, M. Yamagishi, K. Nakahara, J. Soeda, Y.
Hirose, K. Miwa, H. Sato, A. Yamano, T. Matsushita, T. Uemura, J.
Takeya, Adv. Mater. 2013, 25, 6392.
3
a) K. Ito, T. Suzuki, Y. Sakamoto, D. Kubota, Y. Inoue, F. Sato, S.
Tokito, Angew. Chem., Int. Ed. 2003, 42, 1159. b) H. Meng, F.
Sun, M. B. Goldfinger, G. D. Jaycox, Z. Li, W. J. Marshall, G. S.
Blackman, J. Am. Chem. Soc. 2005, 127, 2406. c) K. Kashiwagi, T.
Yasuda, T. Tsutsui, Chem. Lett. 2007, 36, 1194. d) C. Wang, Y. Liu,
Z. Ji, E. Wang, R. Li, H. Jiang, Q. Tang, H. Li, W. Hu, Chem. Mater.
2009, 21, 2840. e) K. H. Jung, S. Y. Bae, K. H. Kim, M. J. Cho, K.
Lee, Z. H. Kim, D. H. Choi, D. H. Lee, D. S. Chung, C. E. Park,
Chem. Commun. 2009, 5290. f) F. Silvestri, A. Marrocchi, M. Seri,
C. Kim, T. J. Marks, A. Facchetti, A. Taticchi, J. Am. Chem. Soc.
2010, 132, 6108. g) J. A. Hur, S. Y. Bae, K. H. Kim, T. W. Lee, M. J.
Cho, D. H. Choi, Org. Lett. 2011, 13, 1948.
4
a) T. Uemura, Y. Hirose, M. Uno, K. Takimiya, J. Takeya, Appl.
Phys. Express 2009, 2, 111501. b) K. Nakayama, Y. Hirose, J. Soeda,
M. Yoshizumi, T. Uemura, M. Uno, W. Li, M. J. Kang, M.
Yamagishi, Y. Okada, E. Miyazaki, Y. Nakazawa, A. Nakao, K.
Takimiya, J. Takeya, Adv. Mater. 2011, 23, 1626. c) H. Minemawari,
T. Yamada, H. Matsui, J. Tsutsumi, S. Haas, R. Chiba, R. Kumai, T.
Hasegawa, Nature 2011, 475, 364. d) C. Xu, P. He, J. Liu, A. Cui, H.
Dong, Y. Zhen, W. Chen, W. Hu, Angew. Chem., Int. Ed. 2016, 55,
9519.
Figure 3. Output characteristics (left) and transfer character-
istics at VDS = ¹50 V (right) of the 1-based FET devices on
the bear Si/SiO2 substrates: (a) vacuum-deposited film (Tsub
=
70 °C) and (b) drop-cast film prepared from o-dichlorobenzene
solution.
5
6
H. Iino, T. Usui, J. Hanna, Nat. Commun. 2015, 6, 6828.
J. L. Brédas, J. P. Calbert, D. A. da Silva Filho, J. Cornil, Proc. Natl.
Acad. Sci. U.S.A. 2002, 99, 5804.
a) S. Allard, M. Forster, B. Souharce, H. Thiem, U. Scherf, Angew.
Chem., Int. Ed. 2008, 47, 4070. b) I. Yavuz, B. N. Martin, J. Park,
K. N. Houk, J. Am. Chem. Soc. 2015, 137, 2856.
a) H. Meng, F. Sun, M. B. Goldfinger, F. Gao, D. J. Londono, W. J.
Marshal, G. S. Blackman, K. D. Dobbs, D. E. Keys, J. Am. Chem.
Soc. 2006, 128, 9304. b) L. Jiang, W. Hu, Z. Wei, W. Xu, H. Meng,
Adv. Mater. 2009, 21, 3649.
solubility and high crystallinity is an important characteristic of
1. The solution-processed OFET devices of 1 with a top-contact/
bottom-gate configuration were fabricated by using a simple
drop-casting method, whereby a solution of 1 in o-dichloro-
benzene (3.9 mM, several μL) was dropped on the bare Si/SiO2
substrate and slowly dried in air at room temperature overnight
(Figure S10). The device without thermal annealing showed
® = 0.63 cm2 V¹1 s¹1, Vth = ¹22.1 V, and Ion/Ioff = 4.9 © 105
(Figure 3b). Notably, the mobility of the drop-cast film is
comparable to that of the vacuum-deposited film on OFETs
based on 1.
In summary, we have demonstrated that (1) the replacement
of the double bonds in 2 by triple bonds in 1 leads to significant
improvement of the solubility of 1, compared with that of 2,
while maintaining high crystallinity, (2) the crystal packing
structure including the dense herringbone packing arrangement
of 1 was similar to that of 2, and (3) the FET performance of the
vacuum-deposited film of 1 was comparable to that of 2. For
the OFET devices based on 1, it should be emphasized that
the mobility of the drop-cast film is comparable to that of the
vacuum-deposited film. These results suggest that the use of
triple bonds in place of double bonds in extended π-conjugated
aromatics is a promising approach that can be incorporated
into the molecular design of solution-processable organic
semiconductors.
7
8
9
M. Levitus, M. A. Garcia-Garibay, J. Phys. Chem. A 2000, 104, 8632.
10 S.-G. Lee et al. reported FET properties of 2,6-bis(4-hexylphenyl-
ethynyl)anthracene, wherein OFETs of this compound fabricated by
a spin-coating method on the OTS-treated Si/SiO2 substrates showed
® = 0.004 cm2 V¹1 s¹1. S.-H. Jang, H.-J. Kim, M.-J. Hwang, E.-B.
Jeong, H.-J. Yun, D.-H. Lee, Y.-H. Kim, C.-E. Park, Y.-J. Yoon, S.-K.
Kwon, S.-G. Lee, Bull. Korean Chem. Soc. 2012, 33, 541.
¹1
11 Solubility of 1 in organic solvents at room temperature: 1.8 mg mL
¹1
(3.5 mM) in CHCl3, 1.6 mg mL
(3.1 mM) in toluene, and
¹1
2.6 mg mL (5.0 mM) in o-dichlorobenzene.
12 Y. Hirumi, K. Tamaki, T. Namikawa, K. Kamada, M. Mitsui, K.
Suzuki, K. Kobayashi, Chem.®Asian J. 2014, 9, 1282.
13 An ORTEP view and crystal data are shown in Figure S6 and
Table S1, respectively. Crystallographic data reported in this manu-
script have been deposited with Cambridge Crystallographic Data
Centre as supplementary publication No. CCDC-1479834. Copies of
the data can be obtained free of charge via http://www.ccdc.cam.
ac.uk/conts/retrieving.html (or from the Cambridge Crystallographic
Data Centre, 12, Union Road, Cambridge, CB2 1EZ, U.K.; fax: +44
1223 336033; or deposit@ccdc.cam.ac.uk).
14 Carbon atoms C2-C5 of the pentyl chains of the two conformers of
1 were disordered. The occupancy factors were 0.725 and 0.275 for
one conformer and 0.657 and 0.343 for the other conformer. In
Figure 1, the pentyl chains with minor occupancy factors are omitted
for clarity.
Supporting Information is available on http://dx.doi.org/
10.1246/cl.160731.
References and Notes
1
a) C. Wang, H. Dong, W. Hu, Y. Liu, D. Zhu, Chem. Rev. 2012, 112,
2208. b) J. Mei, Y. Diao, A. L. Appleton, L. Fang, Z. Bao, J. Am.
Chem. Soc. 2013, 135, 6724, and references cited therein.
15 The transfer integrals were calculated based on PW91/TZP level in
Amsterdam Density Functional (ADF) program: Amsterdam Density
Functional (ADF), Users Guide, Release 1.1, Department of
Theoretical Chemistry, Free University, Amsterdam, The Nether-
lands, 1994.
2
a) S. K. Park, T. N. Jackson, J. E. Anthony, D. A. Mourey, Appl.
Phys. Lett. 2007, 91, 063514. b) H. Ebata, T. Izawa, E. Miyazaki, K.
Takimiya, M. Ikeda, H. Kuwabara, T. Yui, J. Am. Chem. Soc. 2007,
Chem. Lett. 2016, 45, 1403–1405 | doi:10.1246/cl.160731
© 2016 The Chemical Society of Japan | 1405