Fluoro-substituted phenyleneethynylenes: Acetylenic n-type organic semiconductors

Article abstract of DOI:10.1246/cl.2010.1300

Fluoro-substituted phenyleneethynylenes are synthesized by Sonogashira coupling and acetylide-nucleophilic substitution of fluorobenzenes. Fluoro-substitution of benzenes enables deep LUMO potential, and CF ₃-substitution provides high electron

Full text of DOI:10.1246/cl.2010.1300

doi:10.1246/cl.2010.1300
1300
Published on the web November 11, 2010
Fluoro-substituted Phenyleneethynylenes: Acetylenic n-Type Organic Semiconductors
Daisuke Matsuo,1 Xin Yang,1 Akiko Hamada,2 Kyo Morimoto,2 Takuji Kato,2
Masayuki Yahiro,2 Chihaya Adachi,*2 Akihiro Orita,*1 and Junzo Otera*1
1Department of Applied Chemistry, Okayama University of Science, 1-1 Ridai-cho, Kita-ku, Okayama 700-0005
2Center for Organic Photonics and Electronics Research, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395
(Received October 4, 2010; CL-100848; E-mail: orita@high.ous.ac.jp, otera@high.ous.ac.jp)
R1
R1
R1
R1
R2
R2
R2
R2
Fluoro-substituted phenyleneethynylenes are synthesized by
Sonogashira coupling and acetylide-nucleophilic substitution of
fluorobenzenes. Fluoro-substitution of benzenes enables deep
LUMO potential, and CF3-substitution provides high electron
mobility in deposited film (® = 5.5 © 10¹2 cm2 V¹1 s¹1).
R1 = R2 = H (1)
R1 = R2 = F (9)
F3C
CF3
R1
R1
R1
R1
R1
R2
R1
R2
R2
R3
R2
R3
R2
R4
R2
R4
R1 = R2 = R3 = R4 = H (2)
R1 = F, R2 = R3 = R4 = H (3)
R2 = R3 = F, R1 = R4 = H (4)
R1 = R4 = F, R2 = R3 = H (5)
R1 = R2 = F, R3 = R4 = H (6)
R1 = R2 = R3 = F, R4 = H (7)
R1 = R2 = R3 = R4 = F (8)
R1
R4
R1
R1
R2
R2
R3
R3
R4
R4
A number of organic materials with highly expanded ³
systems have been developed for organic field-effect transistors1
(OFET) and organic light-emitting diodes2 (OLED). Fluoro-
and fluoroalkyl-substituted arenes have attracted great attention,
because they have low-energy LUMO and may serve as
electron-transporting materials.3 Although a number of n-type
OFET devices have been fabricated by using electron-trans-
porting materials,4 carrier mobilities observed in the devices are
insufficient for practical use, and further development of
organic semiconducting material with high mobility is still
necessary. We have been involved in synthesis of phenylene-
ethynylene derivatives5 and succeeded in application of CF3-
substituted phenyleneethynylene 1 (Figure 1) to n-type organic
semiconductor material by invoking the carrier-transporting
properties of phenyleneethynylene array and electron-with-
drawing effect of CF3 groups.6 We envisioned that fluoro-
substituted phenyleneethynylenes could serve more efficiently
as n-type semiconductors, because fluorines on benzenes would
give rise to deep HOMO and LUMO levels. We present herein
synthesis of 2-9, their cyclic voltammograms and preliminary
results of OFET properties using 9 as n-type semiconducting
material.
Figure 1. Structures of 1-9.
F
F
F
F
F
F
F
F
F
F
F
F
[Pd(PPh3)4] (5 mol%)
CuCl (1.2 equiv)
+
F
I
TMS
12
F
F
F
DMF, i-Pr2NH,
80 °C/ 15 h
F
F
F
F
55%
11
(1.2 equiv)
10
Li
4 60%
10
THF,
-78 °C - rt, 15 h
ClP(O)(OEt)2
LiHMDS(5.0 equiv)
I
+
OHC
I
SO2Ph
THF, 0 °C - rt, 6 h
15
TMS
16
14
K2CO3
[Pd(PPh3)4], CuI
TMS
THF, MeOH,
rt, 4 h
toluene, i-Pr2NH,
60 °C, 15 h
13
96%
82%
Li
13/BuLi
6
10 (3.0 equiv)
65%
THF,
0 °C - rt, 12 h
In Scheme 1 are shown representative synthetic processes
for 4, 6, and 9.7,8 Decafluorodiphenylethyne (10) was prepared
in 55% yield by coupling between 11 and 12 in the presence of
5 mol % of palladium catalyst and a stoichiometric amount of
copper(I) chloride. The target compound 4 was synthesized in
60% yield by substitution at 4- and 4¤-positions of 10 with
lithium phenylethynide. Similar substitution at the 4-position of
10 with lithium ethynide which was prepared by lithiation of 13
afforded nonafluoro-derivative 6 in 65% yield. In this substitu-
tion reaction, a large excess of 10 was required in order to
suppress formation of bis-adduct, and when only two equiv-
alents of 10 was used, the yield of 6 decreased to 18%. Terminal
ethyne 13 was provided by Sonogashira coupling between
trimethylsilylethyne and 14, followed by removal of the TMS
group, which had been obtained by one-shot double elimination
between benzyl sulfone 15 and iodobenzaldehyde (16). Iodina-
tion of 17 with I2/K3PO4 gave an iodide 18 in 55% yield, and
Sonogashira coupling of 18 with trimethylsilylethyne provides
an inseparable mixture of the desired product 19 and trimethyl-
silylethyne-homocoupling product 20 in 86% and 7% yield,
respectively. Treatment of a THF solution of 19 (containing 20)
TMS (1.2 equiv)
F
F
F
F
F
F
F
F
F
F
F
I2, K3PO4
[Pd(PPh3)4], CuI
F3C
F3C
I
F3C
TMS
86%
toluene, i-Pr2NH,
DMF,
130 °C, 2 h
19
80 °C, 20 h
F
17
18 55%
TMS
TMS
20 7%
TBAF (10 mol%)
19
10
+
9 27%
THF,
0 °C - rt, 12 h
Scheme 1. Synthetic processes for 4, 6, and 9.
and 10 with tetrabutylammonium fluoride afforded 9 in 27%
yield.
In order to assess the electronic effect of fluorine on HOMO
and LUMO potentials, cyclic voltammograms of 2-9 were
recorded in THF by using Ag/AgNO3 as a reference electrode,
and the half-wave reduction potentials Ered for 2-9 are summa-
rized in Table 1.8,9 Fluoro-substituted phenyleneethynylenes
3-9 undergo reversible electrochemical reduction at ¹1.20 to
¹2.02 V, while 2 does not. It is observed that reduction potential
Chem. Lett. 2010, 39, 1300-1302
© 2010 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/

1301
Table 1. Electrochemical properties and HOMO and LUMO
energy levels of 2-9
(b)
(a)
a,b
a,d
a,d
Compound
Ered
ELUMO
EHOMO
¦Ea,d,e
2
®
®
(¹2.10)
¹2.36
(¹2.26)
¹2.81
(¹2.64)
¹2.41
(¹2.37)
¹2.72
(¹2.56)
¹2.98
(¹2.77)
¹3.05
(¹2.88)
¹3.18
®
(¹5.47)
¹5.62
(¹5.58)
¹5.99
(¹5.90)
¹5.72
(¹5.72)
¹5.86
(¹5.75)
¹6.16
(¹6.01)
¹6.27
(¹6.18)
¹6.40
3.31
(3.37)
3.26
(3.32)
3.18
(3.26)
3.31
(3.35)
3.14
(3.19)
3.18
(3.24)
3.22
(3.30)
3.22
3
4
5
6
7
8
9
¹2.02
¹1.57
¹1.97
¹1.66
¹1.40
¹1.33c
¹1.20c
Figure 3. (a) SEM of 9. (b) SEM micrograph of 4.
Table 2. Field-effect transistor characteristics of 9-deposited
devices
T
Mobility ® On/off Threshold
Entry Surfacea
¹1
/°Cb /cm2 V¹1 s
ratio
/V
¹2
¹2
¹2
1
2
3
Si
H
H
25
25
60
5.5 © 10
5.2 © 10
5.2 © 10
105
106
106
45
30
40
aSi: SiO2, H: HMDS-treated SiO2. bTemperature of SiO2
substrate on vacuum deposition.
(¹3.05)
(¹6.34)
(3.29)
aV. bHalf-wave reduction potential (Ag/AgNO3, in THF
(1.0 © 10¹3 M for 3-7)). Concentration is unknown because
c
of poor solubility of 8 and 9. dCalculated results on B3LYP/6-
Terminal CF3 groups serve more efficiently as electron-with-
drawing groups than fluorine, and CF3-substituted derivative 9
exhibited by 0.13 V deeper ELUMO and EHOMO potentials in
comparison to those of F-substituted derivative 8.
e
31G(d) in parentheses. HOMO-LUMO energy gap.
Finally, we prepared vacuum-deposition films of 2-9 on
SiO2 at 25 °C and investigated their microstructures by using
scanning electron microscopy (SEM). As shown in Figure 3a,
the SEM micrograph of film of 9 demonstrates highly packed
polycrystalline texture with a number of protrusions, while
investigation of other fluoro derivatives 3-8 displays isolated
circular features or porous morphologies (micrograph of 4 is
shown in Figure 3b, representatively). Because substitution of
benzenes with CF3 groups enables highly ordered structure of 9
as well as deeper reduction potential, high electron transport
could be expected for OFET devices fabricated from 9. A field-
effect transistor device was fabricated from 9 using a top-
contact configuration: a 50-nm-thick fluorophenyleneethynylene
9 layer was deposited on Si/SiO2 or hexamethyldisilazane
(HMDS)-treated Si/SiO2 substrate at 25 or 60 °C under vacuum
(ca. 10¹5 Torr). The electrical measurements were performed in
a cryostat under vacuum at 25 °C because the device exhibited
unstable FET operation in air. Table 2 shows FET properties
such as mobility, on/off ratio, and threshold voltage, and
Figure 4 displays drain current (Id)-drain voltage (Vd) plots
operating at different gate voltages (Vg) and Id- and Id1/2-Vg
plots (representative, for Entry 2 in Table 2). The fluoro
derivative 9 exhibited n-type characteristics as expected, and
FET response was observed at high gate voltage (>30 V).
Equation 1 demonstrates rather high field-effect electron mobi-
lity of 9 (® = 5.2-5.5 © 10¹2 cm2 V¹1 s¹1), and this reveals that
fabrication conditions such as treatment of Si/SiO2 substrate and
temperature provide no influence on the mobility of 9.
Figure 2. (a) LUMO of 4. (b) LUMO of 5.
Ered gradually shifted to higher potential in accordance with the
number of fluorines on benzene rings: Ered = ¹2.02 V for 3,
¹1.97- ¹1.57 V for 4-6, ¹1.40 V for 7, ¹1.33 V for 8, and
¹1.20 V for 9. This shows that fluorine on benzene rings serves
as an electron-withdrawing group resulting in deep LUMO
potentials as we expected.
The ionization potentials (EHOMO) and electron affinities
(ELUMO) of 2-9 were estimated by their reduction potential Ered
and HOMO-LUMO energy gap which were calculated from
the corresponding wavenumber of UV-vis absorption edge
(Table 1).9,10
DFT calculations (B3LYP/6-31G(d)) were carried out for
planar conformers of 2-9 by taking solvent effect (¾THF = 7.43)
into consideration,11 and the calculated HOMO and LUMO level
potentials are shown in Table 1 as well. It is found that both
ELUMO and EHOMO potentials experimentally obtained are
consistent with those obtained from DFT calculation. Notably,
fluoro-substitution on internal benzene enables deeper ELUMO
potential than substitution on terminal benzene: Ered = ¹1.57 V
for 4 vs. ¹1.97 V for 5. The deeper ELUMO potential in 4 can
be explained by participation of whole fluorines on internal
benzenes in the LUMO. As shown in Figure 2, coefficients of
the LUMO are located on all fluorines in 4, and eight fluorine
atoms serve as electron-withdrawing group resulting in the deep
LUMO potential, while in 5, fluorines at 3- and 5-positions of
terminal benzenes are located on nodes of the LUMO giving rise
to participation of only four fluorine atoms in the LUMO.
2
Id ¼ W®CðVg ꢀ VthÞ =2L
ð1Þ
Id: drain current (¯A), W: channel width (mm), ®: mobility
¹8
(cm2 V¹1 s¹1), C: capacitance of SiO2 insulator (1.18 © 10
F cm¹2), Vg: gate voltage (V), Vth: threshold voltage (V), and
L: channel length (¯m).
Chem. Lett. 2010, 39, 1300-1302
© 2010 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/

1302
1.5 10-5
10-4
10-5
10-6
10-7
10-8
10-9
10-10
10-11
0.006
0.005
0.004
0.003
0.002
0.001
0
and Applications, ed. by K. Müllen, U. Scherf, Wiley-VCH,
Weinheim, 2006. b) J. H. Burroughes, D. D. C. Bradley,
A. R. Brown, R. N. Marks, K. Mackay, R. H. Friend, P. L.
Burns, A. B. Holms, Nature 1990, 347, 539.
µ
= 0.052 cm2 V−1 s−1
Vth = 30.1 V
ON/OFF = 106
V
g = 100 V
1 10-5
5 10-6
0 100
90 V
80 V
70 V
3
a) S. Ando, J. Nishida, H. Tada, Y. Inoue, S. Tokito, Y.
Yamashita, J. Am. Chem. Soc. 2005, 127, 5336. b) S. Ando,
R. Murakami, J. Nishida, H. Tada, Y. Inoue, S. Tokito, Y.
Yamashita, J. Am. Chem. Soc. 2005, 127, 14996. c) A.
Facchetti, M. Mushrush, M.-H. Yoon, G. R. Hutchison,
M. A. Ratner, T. J. Marks, J. Am. Chem. Soc. 2004, 126,
13859. d) A. Facchetti, M.-H. Yoon, C. L. Stern, G. R.
Hutchison, M. A. Ratner, T. J. Marks, J. Am. Chem. Soc.
2004, 126, 13480. e) S. B. Heidenhain, Y. Sakamoto, T.
Suzuki, A. Miura, H. Fujikawa, T. Mori, S. Tokito, Y. Taga,
J. Am. Chem. Soc. 2000, 122, 10240.
60 V
|
-20 V
-40 -20
0
20 40 60 80 100 120
Gate voltage/V
0
20
40
60
80
100
120
Drain voltage/V
Figure 4. FET properties of 9 film deposited on HMDS-
treated SiO2 (Entry 2 in Table 2). (a) Drain current (Id) versus
drain voltage (Vd) characteristics as a function of gate voltage
1/2
(Vg). (b) Id and Id versus Vg plots.
In summary, we established synthetic processes for a series
of fluoro-substituted phenyleneethynylenes and revealed that
substitution of benzenes with fluorine enabled low LUMO
potential both in terms of electrochemistry and ab initio
calculation. CF3-Substituted fluorophenyleneethynylene 9 ex-
hibited rather deep reduction potential in cyclic voltammetry and
provided a finely ³-stacked structure in vacuum-deposited film.
An FET device fabricated by use of 9 showed n-type properties
(® = 5.5 © 10¹2 cm2 V¹1 s¹1) as expected from its deep reduc-
tion potential. Further research in application of a series of
fluoro-substituted phenyleneethynylenes to n-type semiconduc-
tors is underway.
4
5
Recently a high electron mobility (1.5 cm2 V¹1 s¹1) has been
reported. T. Yamao, Y. Shimizu, H. Kuriki, T. Katagiri, S.
Hotta, Jpn. J. Appl. Phys. 2010, 49, 01AB01.
For syntheses of phenyleneethynylenes: a) T. Doi, A. Orita,
D. Matsuo, R. Saijo, J. Otera, Synlett 2008, 55. b) A. Orita,
H. Taniguchi, J. Otera, Chem. Asian J. 2006, 1, 430. c) A.
Orita, T. Nakano, T. Yokoyama, G. Babu, J. Otera, Chem.
Lett. 2004, 33, 1298. For applications of phenyleneethynyl-
enes to optoelectronic materials: d) G. Mao, A. Orita, D.
Matsuo, T. Hirate, T. Iwanaga, S. Toyota, J. Otera,
Tetrahedron Lett. 2009, 50, 2860. e) G. Mao, A. Orita, L.
Fenenko, M. Yahiro, C. Adachi, J. Otera, Mater. Chem.
Phys. 2009, 115, 378. f) C. Ding, G. Babu, A. Orita, T.
Hirate, J. Otera, Synlett 2007, 2559.
T. Oyamada, G. Shao, H. Uchiuzou, H. Nakanotani, A.
Orita, J. Otera, M. Yahiro, C. Adachi, Jpn. J. Appl. Phys.
2006, 45, L1331.
Representative synthetic procedure for 9 was shown in
Supporting Information, which is available electronically on
the CSJ-Journal Web site, http://www.csj.jp/journals/chem-
lett/index.html.
A.O. thanks Prof. Kan Wakamatsu (Okayama University of
Science) and Prof. Yutaka Ie (I.S.I.R, Osaka University) for
helpful discussion. This work was supported by a Grant-in-Aid
for Scientific Research on Innovative Areas Matching Fund
Subsidy for Private Universities from the Ministry of Education,
Culture, Sports, Science and Technology, Japan, the Funding
Program for World-Leading Innovative R&D on Science and
Technology and Okayama Prefecture Industrial Promotion
Foundation.
6
7
8
9
Syntheses, characterization, and spectral data of 2-9 and
procedure of preparation of FET devices and their properties
will be described fully in the following full paper.
CV profile of 9 was representative shown in Figure S1.7
References and Notes
1
a) Physical and Chemical Aspects of Organic Electronics:
From Fundamentals to Functioning Devices, ed. by C. Wöll,
Wiley-VCH, Weinheim, 2009. b) Organic Electronics:
Materials, Manufacturing and Applications, ed. by H.
Klauk, Willey-VCH, Weinheim, 2006. c) A. Facchetti,
Mater. Today 2007, 10, 28. d) A. R. Murphy, J. M. J. Fréchet,
Chem. Rev. 2007, 107, 1066.
10 a) Y. Li, J. Ding, M. Day, Y. Tao, J. Lu, M. D’iorio, Chem.
Mater. 2004, 16, 2165. b) D. M. Leeuw, M. M. J. Simenon,
A. R. Brown, R. E. F. Einerhand, Synth. Met. 1997, 87, 53. c)
Y. Cui, X. Zhang, S. A. Jenekhe, Macromolecules 1999, 32,
3824.
11 The theoretical calculations of 2-9 were performed with the
Gaussian 09 software package.
2
a) Organic Light-Emitting Devices: Synthesis, Properties,
Chem. Lett. 2010, 39, 1300-1302
© 2010 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/