Development of organic field-effect properties by introducing aryl-acetylene into benzodithiophene

Article abstract of DOI:10.1039/c0jm02895a

Ethynylene-containing benzo[1,2-b:4,5-b′]dithiophene derivatives 1a-c (BPEBDT, BTEBDT and BHPEBDT) were designed and synthesized. Their physicochemical properties were studied by absorption spectra and electrochemistry. 1a-c displayed high field-effect tr

Full text of DOI:10.1039/c0jm02895a

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PAPER
www.rsc.org/materials | Journal of Materials Chemistry
Development of organic field-effect properties by introducing aryl-acetylene
into benzodithiophene
a
Qing Meng,ac Lang Jiang,ac Zhongming Wei,ac Chengliang Wang,ac Huaping Zhao, Hongxiang Li, b Wei Xua
*
a
*
and Wenping Hu
Received 1st September 2010, Accepted 27th September 2010
DOI: 10.1039/c0jm02895a
Ethynylene-containing benzo[1,2-b:4,5-b0]dithiophene derivatives 1a–c (BPEBDT, BTEBDT and
BHPEBDT) were designed and synthesized. Their physicochemical properties were studied by
absorption spectra and electrochemistry. 1a–c displayed high field-effect transistors performance,
a mobility up to 1.17 cm2 Vꢀ1 sꢀ1 with on/off current ratio of 107 was achieved.
arylacetylenyl small molecular weight compound that displayed
hole mobility as high as 0.3 cm2 Vꢀ1 sꢀ1 11a
Yasuda et al. found
ethynylene-naphthalene derivatives that exhibited hole mobility
up to 0.12 cm2 Vꢀ1 sꢀ1 11c
And very recently, Silvestri et al.
Introduction
.
Small molecules used in organic field-effect transistors (OFETs)
have been extensively studied with the application progress of
organic electronics.1 The benchmark material pentacene2 and
fused-ring derivatives,3 which have rigid coplanar and p-conju-
gated structures, have been extensively researched, and some of
them have been reported to exhibit very high device perfor-
mance.4 However, these compounds are usually air-/photo-
sensitive or need complex synthetic steps with low yield. s-bond
linked small molecular weight compounds (s-SMWCs), in which
the aromatic/heteroaromatic functional groups are linked by s-
bonds, are another type of organic materials that are commonly
used in OFETs. They possess the merits of low-cost precursors,
efficient syntheses, convenient purification and tunable solu-
bility. Moreover, the properties of these can be optimized by
facile tailoring of the molecular structures. Though some
s-SMWCs, such as 5,50-bis(2-tetracenyl)-2,20-bithiophene,5 2,5-
bis[5-(4-trifluoromethylphenyl)thiophen-2-yl]-1,3-thiazolo[5,4-
.
characterized the transistors performance of a series of ethyny-
lene-containing anthracene and benzothiadiazole derivatives,
high hole mobility up to ꢁ0.1 cm2 Vꢀ1 sꢀ1 was observed.11i
Herein, we reported the syntheses and physicochemical prop-
erties of diethynyl-benzo[1,2-b:4,5-b0]dithiophene derivatives
(1a–c, Scheme 1). Field-effect transistor characteristics showed
that all compounds exhibited typical p-type transistor behavior
and high device performance. A mobility up to 1.17 cm2 Vꢀ1 sꢀ1
with on/off current ratio of 107 was achieved. To the best of our
knowledge, this is one of the highest mobility values reported for
ethynylene-containing small molecular weight compounds.
Experimental
General
d]-1,3-thiazole,6
and
2,7-diphenyl[1]benzothieno[3,2-b]
benzothiophene7 exhibited high mobility, most of the s-
SMWCs displayed lower device performance compared with
fused-ring compounds. And until now, only a few s-SMWCs
have been reported to have field-effect mobility (mFET) values
The NMR spectra were recorded on Varian 400 MHz and 600
MHz spectrometers in CDCl3 with TMS as internal standard.
The absorption spectra of the materials were determined by
a JLSCO V-570 UV/vis spectrometer. Cyclic voltammetry (CV)
was conducted using a CHI660C electrochemistry station with
tetrabutylammonium hexafluorophosphate (Bu4NPF6) as the
electrolyte, and Ag/AgCl as the reference electrode.
higher than 1.0 cm2 Vꢀ1 sꢀ1
.
Ethynylene, as an important p-electron linker, is usually used
in the main chain of polymers and oligomers to elongate the
conjugated system.8 Its special linear structure could eliminate
the steric repulsion between adjacent aromatic rings and lead to
conjugated planar configuration.9 Moreover, the electrons on
ethynylene are more localized, so the introduction of ethynylene
will not greatly affect the molecular HOMO–LUMO energy
gaps.9,10 Recently, some ethynylene-containing compounds have
been effectively used in OFETs.11 Roy et al. reported an
Thin films of 1a–c were deposited on OTS/SiO2/Si substrates in
a vacuum (ꢁ10ꢀ6 Torr). The AFM images of films were obtained
by using a Digital Instruments Nanoscope III atomic force
microscope in air. The XRD analyses were carried out in
aBeijing National Laboratory for Molecular Sciences, Key Laboratory of
Organic Solids, Institute of Chemistry, Chinese Academy of Sciences,
Beijing, 100190, P. R. China. E-mail: huwp@iccas.ac.cn; Tel:
+86-10-82615030
bShanghai Institute of Organic Chemistry, Chinese Academy of Sciences,
Shanghai, 200032, P. R. China. E-mail: lhx@mail.sioc.ac.cn; Tel: +86-
21-54925024
cGraduate School of Chinese Academy of Sciences, Beijing, 100039, P. R.
China
Scheme 1 Chemical structures of 1a–c.
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124.21, 116.49, 95.70, 83.07. MS (EI) m/z: 390 (M+). Anal. calcd
for C26H14S2 (%): C: 79.97, H: 3.61. Found: C: 80.11, H: 3.54.
2,6-Bis(thiophen-2-ylethynylene)benzo[1,2-b:4,5-b0]dithiophene,
1b. Compound 1b was synthesized as described for 1a. Yield,
ꢂ
1
69%. Mp 280 C. H NMR (400 MHz, CDCl3, d): 8.15 (s, 2H),
7.52 (s, 2H), 7.36 (dd, 4H), 7.05 (t, 2H). 13C NMR (150 MHz,
CDCl3, d): 137.98, 137.79, 132.77, 128.30, 127.89, 127.30, 123.85,
122.38, 116.56, 88.97, 86.70. MS (EI) m/z: 402 (M+). Anal. calcd
for C22H10S4 (%): C: 65.64, H: 2.50. Found: C: 65.40, H: 2.56.
2,6-Bis((4-hexylphenyl)ethynylene)benzo[1,2-b:4,5-b0]dithiophene,
1c. Compound 1c was synthesized as described for 1a. Yield, 65%.
Mp 276 ꢂC. 1H NMR (400 MHz, CDCl3, d): 8.13 (s, 2H), 7.48 (dd,
6H), 7.19 (d, 4H), 2.63 (t, 4H), 1.62 (m, 4H), 1.30 (m, 12H), 0.88 (t,
6H). 13C NMR (100 MHz, CDCl3, d): 144.38, 137.96, 131.71,
128.73, 127.63, 124.55, 119.71, 116.51, 96.18, 82.63, 53.55, 36.13,
31.83, 31.30, 29.06, 22.73, 14.21. MS (EI) m/z: 558 (M+). Anal.
calcd for C38H38S2 (%): C: 81.67, H: 6.85. Found: C: 81.66, H:
6.94.
Scheme 2 The synthetic route of 1a–c. Reagents: (i) C6H13MgBr,
Ni(dppp)Cl2, Et2O; (ii) K2CO3, THF/CH3OH; (iii) Pd(PPh3)2Cl2,
CuI, ^–R, Et3N, THF.
reflection mode at 40 kV and 200 mA with Cu Ka radiation as
the X-ray source.
OFET characteristics were recorded in air using a Keithley
4200 SCS and Micromanipulator 6150 probe station at room
temperature.
Results and discussion
Ethynylene-containing benzo[1,2-b:4,5-b0]dithiophene derivatives
1a–c were synthesized through palladium-catalyzed Sonogashira
coupling reactions of 2,6-diiodobenzo[1,2-b:4,5-b0]dithiophene13
and its corresponding acetylene derivatives (Scheme 2). All
compounds had moderate solubility in common solvents and
were fully characterized by MS, NMR and elemental analysis.
Fig. 1a illustrates the UV-vis absorption spectra of 1a–c in
dilute dichloromethane. The optical HOMO–LUMO energy
gaps (Eg) of 1a–c estimated from their initial absorptions were
3.07, 2.98 and 3.05 eV, respectively. UV-vis absorption spec-
troscopy of films 1a–c were also studied to ascertain the molec-
ular arrangement in the solid state. For 1a, the main absorptions
in the film were markedly red shifted compared to those of dilute
solution, suggesting J-aggregation was the dominant packing
mode in the solid state.14 Differently, the main absorptions of the
films of 1b–c exhibited blue shifts while compared with those of
dilute solutions. At the same time, a new shoulder at longer
wavelength appeared, which is a typical feature of Davydow
splitting. All these indicated that 1b–c adopted H-aggregation in
solid states. In order to examine the photo-stability of 1a–c,
a thin film of 1a as an example was reserved under ambient
conditions for 12 months. As shown in Fig. 1b, the comparison
of film absorption spectra before and after storing for 12 months
showed nearly no change, suggesting the good environmental
stability of 1a–c.
Synthesis
((4-Hexylphenyl)ethynylene)trimethylsilane, 3. To a solution of
212 (8.89 g, 35 mmol) and Ni(dppp)Cl2 (0.16 eq.) in anhydrous
Et2O (50 ml) was added C6H13MgBr (45 mmol) in Et2O. The
reaction mixture was refluxed for ꢁ24 h. After cooling to room
temperature, the mixture was quenched with an aqueous solution
of HCl and extracted with CH2Cl2. The organic layer was
collected and dried over anhydrous Na2SO4. Solvent was
removed under vacuum, and the residue was purified through
silica gel chromatography to give 3 as a colorless oil. Yield: 6.05 g
(67%). 1H NMR (400 MHz, CDCl3, d): 7.37 (d, 2H), 7.10 (d, 2H),
2.58 (t, 2H), 1.58 (m, 2H), 1.27 (m, 6H), 0.87 (t, 3H), 0.24 (s, 9H).
MS (EI) m/z: 258 (M+).
1-Ethynyl-4-hexylbenzene, 4. 3 (2.0 g, 7.75 mmol) was added to
a solution of K2CO3 (5.35 g, 38.8 mmol) in anhydrous CH3OH/
THF under nitrogen. The reaction mixture was stirred overnight
at room temperature. Solvent was removed under vacuum. The
residue was purified through silica gel chromatography, afford-
ing 4 as a colorless oil. Yield: 0.99 g (69%). 1H NMR (400 MHz,
CDCl3, d): 7.41 (d, 2H), 7.13 (d, 2H), 3.03 (s, 1H), 2.60 (t, 2H),
1.60 (m, 2H), 1.28 (m, 6H), 0.89 (t, 3H). MS (EI) m/z: 186 (M+).
2,6-Bis(phenylethynylene)benzo[1,2-b:4,5-b0]dithiophene, 1a. To
a solution of 2,6-diiodobenzo[1,2-b:4,5-b0]dithiophene13 (0.88 g,
2 mmol), CuI (0.046 g, 0.24 mmol) and PdCl2(PPh3)2 (0.084 g,
0.12 mmol) in THF (40 mL) was added ethynylbenzene
(0.49 g, 4.8 mmol) and triethylamine (10 mL) successively under
nitrogen. The reaction mixture was stirred at room temperature
for ꢁ12 h. The precipitate was collected through filtration and
purified by sublimation under vacuum, affording 1a as a yellow
Cyclic voltammetry of compound 1a–c were performed under
N2 in 0.1 M Bu4NPF6/CH2Cl2 solution by using ferrocene as the
internal standard. 1a–c exhibited similar electrochemical
behavior during cyclic voltammetry (CV) measurements
(Fig. 2a–c). Their oxidation potentials slightly shifted with the
change of end groups. The HOMO energy levels estimated for
1a–c were ꢀ5.66, ꢀ5.60 and ꢀ5.63 eV, respectively (Table 1).
These values are close to that of benzodithiophene (BDT,15 ꢀ5.71
eV, Fig. 2d), indicating that the introduction of aryl-acetylene
end groups just slightly affected the HOMO energy levels of
1a–c.
1
solid. Yield, 0.56 g (72%). Mp 296 ꢂC. H NMR (400 MHz,
CDCl3, d): 8.16 (s, 2H), 7.57 (dd, 4H), 7.53(s, 2H), 7.38 (t, 6H). 13
C
NMR (150 MHz, CDCl3, d): 131.64, 128.90, 128.46, 127.79,
10932 | J. Mater. Chem., 2010, 20, 10931–10935
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Table 1 Optical and electrochemical properties of 1a–c
Compound
labsa/nm
Eg/eV
Eoxb/V
EHOMO/eV
1a
1b
1c
356, 390
374
364, 398
3.07
2.98
3.05
1.44
1.37
1.41
ꢀ5.66
ꢀ5.60
ꢀ5.63
In CH2Cl2. b 0.1 M Bu4NPF6, Ag/AgCl as reference electrode.
a
Table 2 Field-effect characteristics of devices of 1a–c at different
substrate temperatures
1a
1b
1c
Tsub
ꢂC
/
m/cm2
Vꢀ1 sꢀ1
Ion
/
m/cm2
Ion
/
m/cm2
Ion/
Ioff
Ioff Vꢀ1 sꢀ1
Ioff Vꢀ1 sꢀ1
rt
50
100
0.44–0.72 107 0.014–0.018
0.45–0.58 107 0.054–0.070
106 0.049–0.089 107
106 0.016–0.28 107
0.24–0.38 107 (4.2–8.5) ꢃ 10ꢀ4 105 0.0055–0.20 107
100/50 0.79–1.17 107
prepared at different substrate temperatures (Tsub) were tested
under ambient conditions. All the devices displayed typical
p-type channel FET performance. Semiconductor 1a exhibited
high device performance with mobility up to 0.72 cm2 Vꢀ1 sꢀ1
and the mobility of 1b and 1c reached 0.07 and 0.28 cm2 Vꢀ1 sꢀ1
respectively (Table 2).
,
,
Fig. 1 (a) Absorption spectra of 1a–c in CH2Cl2 (10ꢀ5 M) and on films;
(b) absorption spectra of 1a film before and after storing in air for one
year.
The XRD results of films of 1a displayed a series of equidistant
peaks, indicating 1a in the films was high-ordered. As shown in
Fig. 3a, the order of the films was improved with the increase of
substrate temperatures. The AFM image of the 1a film exhibited
Thin films of 1a–c used in OFET devices were vacuum
deposited onto the octyltrichlorosilane (OTS)-treated substrates,
in which the thickness of the SiO2 layer was 300 nm. By using
a shadow mask, OFET devices were fabricated in top contact
geometry with Au as source and drain electrodes. Devices
ꢀ
terrace structures and the step height was estimated to be 22.41 A
(Fig. 3b,c). This value is very close to the simulated molecular
Fig. 3 XRD patterns of 1a (a), 1b (d) and 1c (e) films deposited at
different substrate temperatures; (b) AFM image of crystalline terrace
film of 1a; (c) step heights of terrace layers measured by section analysis
along the line marked in (b).
Fig. 2 Cyclic voltammograms of 1a (a), 1b (b), 1c (c) and BDT (d) in
CH2Cl2 solutions with Ag/AgCl as the reference electrode and ferrocene
as the internal standard.
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ꢀ
length (22.99 A) and the d-spacing determined from the first
ꢀ
X-ray diffraction peak (22.07 A). This result suggested that 1a
adopted a nearly perpendicular orientation on the substrate in
films. However, no peaks were observed in the XRD results of 1b
and 1c films even at elevated substrate temperatures (Fig. 3d,e),
indicating the crystallization of both films was poor.
Scanning electronic microscopy (SEM) was used to charac-
terize the film morphology of 1b due to the great height changes
of the surface. As shown in Fig. 4I–J, a lot of bent micro-belts
were inclined to the substrates, which caused great height
difference and poor crystallinity in films. Fig. 4K–L illustrates
the tapping mode AFM images of 1c films. Although the height
of the surface was average, the classical terrace structure based
on large grains was not observed. And the disordered structures
in long range further proved the poor crystalline property of 1c
films.
Fig. 5 The transistor output (left) and transfer (right) curves of device 1a
which was fabricated by the two-step technique at Tsub ¼ 100 ꢂC/50 ꢂC
with apparent thickness of 10 nm/50 nm.
In order to achieve this assumption, the deposition of 1a at
different substrate temperatures with different apparent thick-
nesses was carried out (Fig. 4E–H). AFM images showed that
high-ordered and continuous films were formed at Tsub ¼ 100 ꢂC/
Fig. 4A–D showed the AFM images of 1a deposited at
different substrate temperatures with the same apparent film
thickness (the film thickness was detected by a quartz monitor).
A great difference of film morphology was observed at different
substrate temperatures. And with the increase of substrate
temperatures the size of the grains increased remarkably, but the
continuity became worse. Considering the strong film
morphology dependence on substrate temperature, if 1a was first
deposited at high substrate temperature to obtain high-ordered
films with large grains and gaps, and then deposited again at
lower substrate temperature to fill the gaps, high quality films
would be obtained by this two-step deposition technique.
ꢂ
50 C with apparent thickness of 10 nm/50 nm (first deposited
1a with apparent film thickness of 10 nm at Tsub ¼ 100 ꢂC,
ꢂ
then decreased Tsub to 50 C and deposited 1a with apparent
film thickness of 50 nm, Fig. 4H). Based on the two-step
technique, high device performance with mobility up to 1.17
cm2 Vꢀ1 sꢀ1 and on/off current ratio of 107 was achieved. Fig. 5
gave an example of output and transfer curves of a device
fabricated by the two-step deposition technique. Though this
value still did not reveal the intrinsic property of 1a, it is one of
Fig. 4 AFM images of: (A–D) films of 1a deposited at different substrate temperatures; (E–H) films of 1a deposited by the two-step technique; (K–L)
films of 1c. (I–J) SEM images of films of 1b. (The thicknesses marked on the pictures are apparent thicknesses detected by a quartz monitor).
10934 | J. Mater. Chem., 2010, 20, 10931–10935
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the highest values reported for thin-film transistors of small
molecular weight organic semiconductors thus far. Remark-
ably, this mobility was nearly two orders of magnitude higher
than that of the ethynylene-absent counterpart of 1a (2,6-
diphenylbenzo[1,2-b:4,5-b0]dithiophene),16 which proved the
great contribution of ethynylene linker for high device
performance.
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Conclusions
In summary, ethynylene-containing benzodithiophene deriva-
tives BPEBDT (1a), BTEBDT (1b) and BHPEBDT (1c) have
been designed and synthesized. Optical and environmental
stabilities of these compounds were characterized by UV-vis
absorption results. Cyclic voltammetry measurements showed
that these compounds have high oxidation potentials. High
mobility up to 1.17 cm2 Vꢀ1 sꢀ1 with on/off current ratio of 107
was achieved based on 1a. This is one of the highest mobilities
reported for thin-film transistors of small molecular weight
organic semiconductors. All these results indicated that the field-
effect properties of benzodithiophene were effectively developed
by introducing aryl-acetylene into molecular backbones.
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This work was supported by National Nature Science Founda-
tion of China (60771031, 50873105, 60736004), the Ministry of
Science and Technology of China (2006CB806200,
2006CB932100) and Chinese Academy of Sciences.
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