The synthesis of 2 6-dialkylphenyldithieno[3 2-b:2' 3'-d]thiophene derivatives and their applications in organic field-effect transistors

Article abstract of DOI:10.1016/j.dyepig.2013.02.007

2,6-Dialkylphenyldithieno[3,2-b:2',3'-d]thiophene derivatives (DPCn-DTT) were synthesized and characterized. Effect of alkyl groups on optical characteristics, electrochemical properties, film-forming ability, and field-effect performance was studied. The

Full text of DOI:10.1016/j.dyepig.2013.02.007

Dyes and Pigments 98 (2013) 17e24
Contents lists available at SciVerse ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
The synthesis of 2,6-dialkylphenyldithieno[3,2-b:20,30-d]thiophene
derivatives and their applications in organic field-effect transistors
,
,
a
a
a
Minliang Zhu a, Hao Luo a b, Liping Wang b , Yunlong Guo , Weifeng Zhang , Yunqi Liu ,
**
,
Gui Yu a
*
a Beijing National Laboratory for Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, PR China
b School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, PR China
a r t i c l e i n f o
a b s t r a c t
2,6-Dialkylphenyldithieno[3,2-b:20,30-d]thiophene derivatives (DPCneDTT) were synthesized and char-
acterized. Effect of alkyl groups on optical characteristics, electrochemical properties, film-forming
ability, and field-effect performance was studied. The four compounds DPCneDTT show almost the
same energy levels of the highest occupied molecular orbits and optical energy gaps, but they exhibit
different charge carrier transport characteristics. The thin film transistors based on DPC1eDTT with the
Article history:
Received 27 December 2012
Received in revised form
6 February 2013
Accepted 8 February 2013
Available online 16 February 2013
shortest alkyl groups (methyl groups) show the highest mobility of 0.54 cm2 Vꢀ1 ꢀ1. Substrate tem-
s
perature and surface modification of the SiO2 insulators have a remarkable effect on field-effect per-
formance. High-quality microribbons of DPC8eDTT with octyl groups were prepared by a solution-phase
self-assembly process. Single crystal field-effect transistors based on an individual DPC8eDTT micro-
ribbon exhibit a high mobility of 1.1 cm2 Vꢀ1 sꢀ1 with a current on/off ratio of 6.5 ꢁ 104.
Ó 2013 Elsevier Ltd. All rights reserved.
Keywords:
Organic field-effect transistors
Single crystal microribbons
Organic semiconductors
Mobility
Dithieno[3,2-b:20,30-d]thiophene derivatives
Long alkyl groups
1. Introduction
semiconducting core to pack tightly, and thereby enhance carrier
mobility. This is exemplified by Kazuo Takimiya who has reported a
In the last two decades, substantial progress has been made in
organic field-effect transistors (OFETs). A large number of organic
semiconductors have been developed and investigated owing to
their potential applications in low-cost processes. Although prepa-
ration technology of the OFET devices is in continuous development
to enhance device performance, the key point determining high
performance of the OFET devices is still governed by organic semi-
conductors [1,2] For p-channel OFET semiconductors, pentacene
derivatives [3e5], fused thiophenes [6,7], and thienoacene de-
rivatives [8] are the major materials which show good performance
series fused rings with long-chain alkyl moieties (CneDNTT) as p-
channel organic semiconductors for vapor-processed OFET devices
[13]. The compounds CneDNTTshowed mobilities higher than those
of the compound without long-chain alkyl moieties [14]. Moreover,
long alkyl groups could also lead a facile formation of micro/nano
ribbons and single crystals. One-dimensional micro- and nano-
structures from semiconducting materials are promising objects in
terms of miniaturizing electronic devices in the field of micro- and
nanoelectronics. Organic single-crystal field-effect transistors could
offer an opportunity to explore the intrinsic charge transport prop-
erties of organic semiconductors. Dithieno[3,2-b:20,30-d]thiophene
(DTT) is an important building block to synthesize fused thiophene
due to their linear fused rings with CH/p interaction, and/or S/S
interactions, namely, enhanced orbital coupling [9,10]. Organic
molecules with long alkyl groups often show the nature of self-
organizing which is another effective approach to enhance orbital
coupling. Owing to the van der Waals intermolecular interaction
between the alkyl groups, or the so-called molecular fastener effect
[11,12] where long alkyl groups can act as a driving force for molec-
ular ordering arrangement in the solid state and render
derivatives and
p-conjugated polymers which have been widely
used as organic-semiconductor materials in OFETs [15e18]. The thin
film and micro/nano ribbon OFET devices based on long-chain
alkylated DTT derivatives were barely reported before.
Based on the above mentioned molecular design strategies for
enhancing OFET performance, we designed and synthesized a se-
ries of diphenyldithieno[3,2-b:20,30-d]thiophene derivatives with
different alkyl groups (DPCneDTT) shown in Scheme 1. We studied
optical, electrochemical, and field-effect performance of the com-
pounds DPCneDTT and analyzed the effect of different alkyl groups
on these properties. The compound DPC1eDTT with methyl groups
* Corresponding author. Tel.: þ86 10 62613253; fax: þ86 10 62559373.
** Corresponding author. Tel./fax: þ86 10 62334505.
E-mail addresses: lpwang@mater.ustb.edu.cn (L. Wang), yugui@iccas.ac.cn
(G. Yu).
0143-7208/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.dyepig.2013.02.007

18
M. Zhu et al. / Dyes and Pigments 98 (2013) 17e24
Scheme 1. Synthetic routes of the DPCnLDTT.
exhibits an excellent field-effect performance with a high mobility
of 0.54 cm2 Vꢀ1 sꢀ1 and a large on/off ratio of up to 2 ꢁ 106.
Furthermore, we prepared micro/nano ribbons of the compound
DPC8eDTT with two octyl groups by solvent diffusion method. The
single crystal transistor based on the individual DPC8eDTT
before solution of n-BuLi (2.4 M) in hexane (3.4 mL, 8.16 mmol) was
added dropwise. The reaction mixture was stirred for 10 min
at ꢀ78 ꢂC. 2-Isopropoxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane
(1.9 g, 10.2 mmol) was added then. The reaction mixture was
allowed to warm up to room temperature and stirred overnight
before it was poured into ice water. The solution was extracted with
100 mL dichloromethane (CH2Cl2), the organic layer washed with
70 mL brine and dried with Na2SO4 before the solvent was removed.
The crude product was purified by column chromatography on silica
gel with petroleum ether/CH2Cl2 (10:1, v/v) as an eluent to give a
microribbon shows a maximum mobility of 1.1 cm2 Vꢀ1 sꢀ1
.
2. Experimental section
2.1. Characterization of materials
white solid (1.1 g, 85% yield). 1H NMR (400 MHz, CDCl3, ppm)
d: 1.32
Nuclear magnetic resonance (NMR) spectra were recorded on a
Bruker Avance 400 spectrometer. Chemical shifts were reported as
dvalues [ppm] relative tointernaltetramethylsilane(TMS). Elemental
analyses were carried out using a Carlo Erba model 1160 elemental
analyzer. Electron-impact mass spectra (EI-MS) were collected on a
GCIeMS micromass (UK) spectrometer. UVevis absorption spectra
were measured on a Hitachi U-3010 spectrophotometer. Cyclic
voltammetric measurements were performed using a computer-
controlled EG&G Potentiostat/Galvanostat5 model 283. Thermogra-
vimetric analysis (TGA) measurements were recorded on a Perkine
Elmer series 7 thermal analysis system under N2 at a heating rate of
10 ꢂC minꢀ1. Atomic force microscopy (AFM) measurements were
carried out with a Nanoscope V instrument. X-ray diffraction (XRD) of
thin films was performed in the reflection mode at 40 kV and 200 mA
with CuKa radiation using a 2 kW Rigaku X-ray diffractometer. The
scanning electron microscopy (SEM) and transmission electron mi-
croscopy (TEM)imageswererecorded on aHitachi Se4800 SEMand a
TETEM Tecnai G2 F20 UeTWIN, respectively.
(s,12H), 2.36 (s, 3H), 7.20 (d, 2H, J ¼ 7.6 Hz), 7.73 (d, 2H, J ¼ 7.6 Hz). 13C
NMR (100 MHz, CDCl3, ppm) d: 21.72, 24.86, 83.64, 128.55, 134.26,
141.50. EI-MS m/z (Mþ): Calcd. for C13H19BO2, 218; found, 218.
2.2.3. 2-(4-Butylphenyl)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (3)
The compound 3 was synthesized according to the procedure
described for 2 using 1-bromo-4-butylbenzene and 2-isopropoxy-
4,4,5,5-tetramethyl-1,3,2-dioxaborolane. A white production was
obtained with a yield of 90%. 1H NMR (400 MHz, CDCl3, ppm)
d: 0.88
(t, 3H, J ¼ 7.0 Hz), 1.18e1.26 (m, 4H), 1.31 (s, 12H), 1.65(m, 2H), 2.62
(t, 2H, J ¼ 7.6 Hz), 7.22 (d, 2H, J ¼ 7.6 Hz), 7.75 (d, 2H, J ¼ 7.6 Hz). 13C
NMR (100 MHz, CDCl3, ppm) d: 14.11, 22.58, 24.90, 34.51, 35.84,
83.56, 128.16, 134.20, 144.54. EI-MS m/z (Mþ): Calcd. for C16H25BO2,
260; found, 260.
2.2.4. 2-(4-Hexylphenyl)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (4)
The compound 4 was synthesized according to the procedure
described for 2 using 1-bromo-4-hexylbenzene and 2-isopropoxy-
4,4,5,5-tetramethyl-1,3,2-dioxaborolane. A white product was ob-
2.2. Synthesis of materials
tained with a yield of 83%. 1H NMR (400 MHz, CDCl3)
d: d (ppm):
0.86 (t, 3H, J ¼ 7.6 Hz), 1.18e1.26 (m, 8H), 1.33 (s, 12H), 1.67 (m, 2H),
All chemicals and solvents are of reagent grade unless otherwise
indicated. Tetrahydrofuran (THF) was purified with a standard
distillation procedure prior to use. Dithieno[3,2-b:20,30-d]thiophene
was synthesized as previously reported [17].
2.62 (t, 2H, J ¼ 7.8 Hz), 7.20 (d, 2H, J ¼ 7.8 Hz), 7.73 (d, 2H, J ¼ 7.8 Hz).
13C NMR (100 MHz, CDCl3, ppm)
d: 14.09, 22.65, 25.06, 29.11, 31.88,
34.51, 35.94, 83.62, 128.26, 134.61, 144.83. EI-MS m/z (Mþ): Calcd.
for C18H29BO2, 288; found, 288.
2.2.1. 2,6-Dibromodithieno[3,2-b:20,30-d]thiophene (1)
2.2.5. 2-(4-Octylphenyl)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (5)
The compound 5 was synthesized according to the procedure
described for 2 using 1-bromo-4-octylbenzene and 2-isopropoxy-
4,4,5,5-tetramethyl-1,3,2-dioxaborolane. A white product was ob-
Dithieno[3,2-b:20,30-d]thiophene (0.98 g, 5.0 mmol) in chloro-
form (20 mL) was reacted with N-bromosuccinimide (NBS, 2.0 g,
11.2 mmol) at room temperature overnight. Batch-wise addition of
NBS is necessary. The light yellow suspension was treated with
water and filtered off. After silica column chromatography with
petroleum ether, pale needle crystals were obtained (1.8 g, 90%
tained with a yield of 93%. 1H NMR (400 MHz, CDCl3, ppm)
d: 0.89
(t, 3H), 1.18e1.26 (m, 10H), 1.33 (s, 12H), 21.67(m, 2H), 2.62 (t, 2H),
7.21 (d, 2H, J ¼ 8.0 Hz), 7.74 (d, 2H, J ¼ 8.0 Hz). 13C NMR (100 MHz,
yield). 1H NMR (400 MHz, CD2Cl2, ppm)
d
: 7.27 (s, 2H). 13C NMR
CDCl3, ppm) d: 14.23, 22.80, 24.99, 29.40, 29.45, 29.61, 29.86, 31.49,
(100 MHz, CDCl3, ppm)
d
: 112.32, 123.17, 130.82, 139.05. EI-MS m/z
32.02, 36.34, 83.69, 128.01, 134.95, 146.52. EI-MS m/z (Mþ): Calcd.
(Mþ): Calcd. for C8H2Br2S3, 352; found, 352.
for C20H33BO2, 316; Found, 316.
2.2.2. 2-(4-Tolyl)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (2)
1-Bromo-4-methylbenzene (1.0 g, 5.88 mmol) was dissolved in
30mL anhydrous THFunderargon. The solutionwas cooled to ꢀ78 ꢂC
2.2.6. 2,6-Bis(4-tolyl)dithieno[3,2-b:20,30-d]thiophene (DPC1eDTT)
The compounds 1 (500 mg, 1.42 mmol) and 2 (774 mg,
3.55 mmol) were dissolved in toluene/H2O (3:1, v/v). To the mixture

M. Zhu et al. / Dyes and Pigments 98 (2013) 17e24
19
Fig. 1. UVevis absorption spectra of DPCnLDTT in (a) CH2Cl2 solution (10ꢀ5 M) and (b) thin films. PL spectra of DPCneDTT in (c) CH2Cl2 solution (10ꢀ5 M) and (d) thin films.
was added 164 mg (0.14 mmol) Pd(PPh3)4 and then the mixture was
stirred for 5 min under argon. Finally, 590 mg (4.28 mmol) K2CO3
was added to the mixture. The reaction mixture was refluxed for
36 h under argon. After the reaction solution was cooled down to
room temperature, the solution was extracted with CH2Cl2. The
organic layer was washed with brine and dried with Na2SO4. The
solvent was evaporated under a reduced pressure and the product
was extracted with CH2Cl2. The extraction solution was washed
with brine and H2O, and then was dried with anhydrous MgSO4.
The crude product was purified by column chromatography on
silica gel using a mixture of petroleum ether/CH2Cl2 (10:1, v/v) as an
eluent to provide a yellow solid (410 mg, 77% yield). 1H NMR
460. Elemental analysis (%) Calcd for C28H28S3: C 72.99, H 6.13, S
20.88; found: C 72.85, H 6.01, S 21.06.
2.2.8. 2,6-Bis(4-hexylphenyl)dithieno[3,2-b:20,30-d]thiophene
(DPC6eDTT)
The compound DPC6eDTT was synthesized according to the
procedure described for DPC1eDTT using 4, 1, K2CO3, and
Pd(PPh3)4. The crude product was purified by column chroma-
tography on silica gel using a mixture of petroleum ether/CH2Cl2
(400 MHz, CDCl3, ppm)
d
: 2.39 (s, 6H), 7.22 (d, 4H, J ¼ 7.8 Hz), 7.46
(s, 2H), 7.53 (d, 4H, J ¼ 7.8 Hz). 13C NMR (100 MHz, CDCl3, ppm)
d:
21.54, 115.92, 125.51, 129.01, 129.87, 131.91, 141.37, 141.48,142.83. EI-
MS m/z (Mþ): Calcd. for C22H16S3, 376; found, 376. Elemental
analysis (%) calcd for C22H16S3: C 70.17, H 4.28, S 25.55; found: C
69.90, H 4.27, S 25.65.
2.2.7. 2,6-Bis(4-butylphenyl)dithieno[3,2-b:20,30-d]thiophene
(DPC4eDTT)
The compound DPC4eDTT was synthesized according to the
procedure described for DPC1eDTT using 3, 1, K2CO3, and
Pd(PPh3)4. The crude product was purified by column chromatog-
raphy on silica gel using a mixture of petroleum ether/CH2Cl2 (10:1,
v/v) as an eluent to provide a yellow solid in 66% yield. 1H NMR
(400 MHz, CDCl3, ppm)
d
: 0.95 (t, 6H, J ¼ 7.3 Hz), 1.39 (m, 4H), 1.63
(m, 4H), 2.64 (t, 4H, J ¼ 7.6 Hz), 7.22 (d, 4H, J ¼ 7.8 Hz), 7.46 (s, 2H),
7.55 (d, 4H, J ¼ 7.8 Hz); 13C NMR (100 MHz, CDCl3, ppm)
d: 13.98,
22.37, 33.55, 35.82, 116.02, 125.61, 129.12, 129.98, 132.00, 141.48,
141.59, 142.93. EI-MS m/z (Mþ): Calcd. for C28H28S3, 460; found,
Fig. 2. Cyclic voltammograms of the DPCneDTT thin films (Scan rate: 0.1 V/s).

20
M. Zhu et al. / Dyes and Pigments 98 (2013) 17e24
NMR (400 MHz, CDCl3, ppm)
d
: 0.86 (t, 6H, J ¼ 6.8 Hz), 1.26e1.30
(m, 16H), 1.64 (m, 4H), 2.63 (t, 4H, J ¼ 8.0 Hz), 7.22 (d, 4H,
J ¼ 7.8 Hz), 7.46 (s, 2H), 7.55 (d, 4H, J ¼ 7.8 Hz). 13C NMR (100 MHz,
CDCl3, ppm) d: 14.28, 22.79, 29.17, 31.11, 31.61, 31.90, 32.15, 35.84,
116.16, 126.00, 129.35, 130.21, 132.33, 141.95, 143.30, 145.51. EI-MS
m/z (Mþ): Calcd. For C36H44S3, 572; found, 572. Elemental analysis
(%) calcd for C36H44S3: C 75.47, H 7.74, S 16.79; found: C 75.42, H
7.56, S 16.97.
2.3. Fabrication of organic thin film transistor devices and
measurement
Prior to the deposition of organic thin films, all compounds
DPCneDTT were purified twice by vacuum sublimation (<10ꢀ3 Pa).
Top-contact bottom gate OFET devices were fabricated by vacuum
deposition on bare or octadecyltrichlorosilane (OTS)-treated SiO2/
Si substrates at preset substrate temperatures (Tsub). Organic
ꢀ
semiconductors were deposited 10 nm at a rate of 5 A/min, then
ꢀ
deposited 30 nm at the rate of 30 A/min. Gold source and drain
contacts were patterned 50 nm by thermal evaporation using
shadow masks. The channel length (L) and width (W) were 80 and
8800 mm, respectively. The characteristics of the OFET devices were
determined using a Keithley 4200 SCS semiconductor parameter
analyzer at room temperature in air. The mobility of the devices
was calculated in the saturation regime. The equation is listed as
follows:
IDS ¼ ðW=2LÞCimðVGS ꢀ VthÞ2
Fig. 3. (a) Output and (b) transfer curves of the DPC1eDTT based OFETs on bare SiO2
insulator deposited at substrate temperature of 100 ꢂC.
where W/L is the channel width/length, Ci is the insulator capaci-
tance per unit area, and VGS and Vth are the gate voltage and
threshold voltage, respectively.
(10:1, v/v) as an eluent to provide a yellow solid in 72% yield. 1H
NMR (400 MHz, CDCl3, ppm)
d
: 0.89 (t, 6H, J ¼ 6.4 Hz), 1.30e1.35
(m, 12H), 1.64 (m, 4H), 2.64 (t, 4H, J ¼ 7.8 Hz), 7.22 (d, 4H,
2.4. Preparation of micro/nano ribbons and their OFET devices
J ¼ 7.8 Hz), 7.46 (s, 2H), 7.55 (d, 4H, J ¼ 7.8 Hz). 13C NMR (100 MHz,
CDCl3, ppm)
d: 14.25, 22.77, 29.14, 31.51, 31.88, 35.84, 116.15,
Into a 5 mL cylinder bottle was added 1 mL solution of DPC8e
DTT in CHCl3 (10ꢀ4 mol/L), and then 12.5 mL ethanol was slowly
dropped along the walls of bottle into the solution. After standing
for about 6 days, micro/nano ribbons were formed and suspended
in the mixed solvent. OFET devices with a top-contact/bottom-gate
configuration were fabricated using an individual DPC8eDTT
125.75, 129.24, 130.12, 132.13, 141.63, 143.11, 145.34. EI-MS m/z
(Mþ): Calcd. for C32H36S3, 516; found, 516. Elemental analysis (%)
calcd for C32H36S3: C 74.37, H 7.02, S 18.61; found: C 74.03, H 6.96,
S 18.95.
2.2.9. 2,6-Bis(4-octylphenyl)dithieno[3,2-b:20,30-d]thiophene
(DPC8eDTT)
The compound DPC8eDTT was synthesized according to the
procedure described for DPC1eDTT using 5, 1, K2CO3, and
Pd(PPh3)4. The crude product was purified by column chroma-
tography on silica gel using a mixture of petroleum ether/CH2Cl2
(10:1, v/v) as an eluent to provide a yellow solid in 78% yield. 1H
microribbon precipitated from chloroform/alcohol solution.
A
heavily doped n-type Si substrate and a 300 nm SiO2 layer with a
capacitance per unit area of 11.5 nF cmꢀ2 were used as a gate
electrode and gate dielectric layer, respectively. The microribbon
was transferred onto an OTS-treated SiO2/Si substrate. Then Gold
shadow masks were used to deposit the gold source/drain elec-
trodes. For the top-contact/bottom-gate configuration, the
Table 1
The device characteristics of OFETs fabricated at various substrate temperatures.
Compound
Tsub (ꢂC)
Bare SiO2
(cm2 Vꢀ1 sꢀ1
OTS-treated SiO2
m
)
Ion/Ioff
Vth (V)
m
(cm2 Vꢀ1 sꢀ1
)
Ion/Ioff
Vth (V)
DPC1eDTT
25
70
100
25
70
100
25
70
100
25
70
100
0.20
0.21
0.54
0.25
0.12
0.10
0.14
0.09
0.12
0.12
0.11
0.08
7.8 ꢁ 103
1.4 ꢁ 106
2.2 ꢁ 106
8.3 ꢁ 105
1.0 ꢁ 106
1.5 ꢁ 105
2.8 ꢁ 105
2.8 ꢁ 104
5.2 ꢁ 104
4.2 ꢁ 103
5.5 ꢁ 105
1.5 ꢁ 105
0
8
16
1
5
9
ꢀ17
ꢀ22
ꢀ16
3
0.15
0.20
0.16
0.15
0.08
0.05
0.08
0.12
0.18
0.10
0.08
0.06
7.3 ꢁ 104
2.5 ꢁ 106
1.1 ꢁ 106
3.5 ꢁ 106
3.5 ꢁ 105
4.4 ꢁ 105
1.6 ꢁ 105
1.3 ꢁ 105
1.1 ꢁ 104
5.1 ꢁ 104
5.5 ꢁ 104
6.6 ꢁ 103
ꢀ9
2
3
DPC4eDTT
DPC6eDTT
DPC8eDTT
ꢀ11
ꢀ8
ꢀ7
ꢀ7
ꢀ14
ꢀ22
ꢀ16
6
ꢀ1
16
ꢀ3

M. Zhu et al. / Dyes and Pigments 98 (2013) 17e24
21
Fig. 4. AFM topography (5 ꢁ 5
mm) of the DPC1eDTT films.
thickness of Au is about 40 nm. SEM measurements of an individual
DPC8eDTT microribbon and Au electrodes for the OFET devices
based on the individual DPC8eDTT microribbon were performed
on a Hitachi S-4800 SEM.
maximum absorption of DPC1eDTT appears in longer wavelength
than those of the other three compounds. Additionally, the DPC1e
DTT thin film has a red shift of 5 nm compared with the DPeDTT
(363 nm) [18], suggesting a stronger intermolecular interaction in
the DPC1eDTT thin film. Optical band gaps (Egap) of DPCneDTT
were estimated from the absorption edges of the absorption
3. Results and discussion
3.1. Synthesis of materials
The synthetic routes of the compounds DPCnLDTT are shown
in Scheme 1. The synthesis of dithieno[3,2-b:20,30-d]thiophene was
carried out as previously reported [17]. Dithieno[3,2-b:20,30-d]
thiophene reacts with NBS to give the compound 1 with a high yield
of 90%. The lithiation of 1-bromo-4-methylbenzene with an
excess amount of n-BuLi at ꢀ78 ꢂC, followed by treatment with
2-isopropoxy-4,4,5,5-tetramethyl-[1,3,2]dioxaborolane, gave the
boronic ester 2. Compounds 3ꢀ5 were synthesized in a similar way.
The Pd catalyzed Suzuki coupling reaction of compound 1 with the
boronic ester 2 was performed to obtain the compound DPC1eDTT
with a 77% yield. The obtained diboromo derivative 1 treated with
the boronic esters 3ꢀ5, respectively, gives the corresponding
products DPC4eDTT, DPC6eDTT, and DPC8eDTT in modest yields
(66e78%) by a similar coupling reaction. All the synthesized com-
pounds were further purified twice by vacuum sublimations,
whose chemical structures were determined by 1H NMR, 13C NMR,
EI-MS, and elemental analysis. The compound DPCneDTT exhibits
good solubility in common organic solvents such as CH2Cl2, THF,
and o-dichlorobenzene.
3.2. Optical and electrochemical properties
Fig. 1 shows absorption and photoluminescent (PL) spectra of
the four compounds DPCneDTT in CH2Cl2 solution and thin films.
All compounds exhibit absorption maxima (lmax) at 360ꢀ372 nm
both in solution and thin films. In CH2Cl2 solution, the four com-
pounds exhibit similar absorption spectra. DPC1eDTT in solution
has a 4 nm blue shift compared with those of the other three
compounds which show almost the same absorption maxima.
DPC1eDTT exhibits the same lmax in solution and thin film as well.
However, different results are obtained for the compounds DPC4e
DTT, DPC6eDTT, and DPC8eDTT. The absorption peaks of the three
compounds in thin films demonstrate a blue shift of 8e12 nm
compared with those in solutions. The modest blue-shifts in solid
state could be attributed to H-aggregates [19]. In the film state, the
alkyl groups have an obvious effect on absorption peak. The
Fig. 5. (a) XRD of the DPCneDTT thin films on the bare SiO2 insulators deposited at
100 ꢂC and (b) XRD of DPC1eDTT on the bare SiO2 insulators at different deposited
temperature.

22
M. Zhu et al. / Dyes and Pigments 98 (2013) 17e24
spectra in thin films. Egap is larger than 3.0 eV for the four com-
pounds. The DPCneDTT thin films emit blue-greenish light with an
emission peak at 480ꢀ540 nm. The PL spectra in thin films shift to
longer wavelength compared with those in solution. A large red
shift of 70 nm was observed, which can contribute to the molecular
vibration and/or stronger intermolecular interaction [20].
Cyclic voltammetry measurements were conducted using an
Ag/AgCl reference and a glassy carbon working electrode with 0.1 M
[n-Bu4N][PF6] as a supporting electrolyte, in degassed CH2Cl2 solu-
tionpurged with nitrogen. Fig. 2 shows cyclic voltammograms of the
compounds DPCneDTT. The four compounds show fully reversible
of the DPC1eDTT based OFET devices fabricated on the bare SiO2
insulators. Device characteristics of OTFTs based on the DPCneDTT
thin films prepared at different substrate temperature (Tsub) are
summarized in Table 1. For the bare SiO2 insulator and at
Tsub ¼ 25 ꢂC, the OFET devices based on DPC1eDTT or DPC4eDTT
exhibit relatively high mobilities of 0.17e0.18 cm2 Vꢀ1 sꢀ1 which
are about twice higher than those of the corresponding devices
based on the compound DPC6eDTT or DPC8eDTT. Tsub has an
obvious effect on carrier charge mobility. On the bare SiO2 insulator,
only DPC1eDTT shows raised performance along with Tsub rising
from 30 to 100 ꢂC. The highest mobility of 0.54 cm2 Vꢀ1 sꢀ1 was
obtained at Tsub ¼ 100 ꢂC. An opposite effect of Tsub on field-effect
performance was obtained for the other three compounds.
Increasing Tsub value decreases their mobilities. Highest mobility
was obtained at Tsub ¼ 25 ꢂC for the compounds DPC4eDTT, DPC6e
DTT, or DPC8eDTT, respectively. For the OFETs based on the OTS-
treated SiO2 insulators, we found that OTS-treatment on the SiO2
surface does not enhance device characteristics. For the OTS-
treated SiO2 insulator and at Tsub ¼ 25 ꢂC, the DPC1eDTT based
oxidation behavior with two oxidation peaks. No obvious difference
onset
exists for the onset oxidation potentials ðE
Þ of the compounds
ox
DPCneDTT. The similar energy levels of the highest occupied
molecular orbits (HOMO) for the four compounds were evaluated to
onset
be ꢀ5.5 eV, according to the equation EHOMO ¼ ꢀðE
þ 4:4Þ eV.
ox
The energy levels of the lowest unoccupied molecular orbits (LUMO)
were estimated as ꢀ2.5 eV by combining the HOMO energy level
with the Egap. The thermal properties of the compounds DPCneDTT
investigated by means of thermal gravimetric analysis show that the
four compounds exhibit very good thermal stabilities. The decom-
position temperatures with 5% weight loss are 318, 336, 340, and
350 ꢂC for the DPC1eDTT, DPC4eDTT, DPC6eDTT, and DPC8eDTT,
respectively.
OFETs exhibit a mobility of 0.14 cm2 Vꢀ1 ꢀ1, which is higher than
s
those of the corresponding devices based on the compound DPC4e
DTT, DPC6eDTT, or DPC8eDTT. The OFETs based on the DPC1eDTT
thin films deposited on the OTS-treated SiO2/Si substrates at
different Tsub values from 25 to 100 ꢂC show almost the same mo-
bilities of 0.12e0.20 cm2 Vꢀ1 sꢀ1 with the current on/off ratios of
104ꢀ106. For the DPC4eDTT or DPC8eDTT based OFET devices
fabricated on the OTS-treated SiO2 insulators, mobility decreases
with increase of the Tsub value and the highest mobility was ob-
tained at 25 ꢂC. However, different result was observed for the
DPC6eDTT based OFET devices fabricated on the OTS-treated SiO2
insulators and the highest mobility 0.13 cm2 Vꢀ1 sꢀ1 was obtained
at 100 ꢂC.
3.3. OFET performance and film morphology
OFET devices were fabricated in a top-contact/bottom gate
configuration with DPCneDTT thin films deposited onto bare SiO2/
Si or OTS-treated SiO2/Si substrates as active semiconductor layers.
All the OFET devices exhibit p-channel transistor characteristics in
ambient conditions. Fig. 3 shows output and transfer characteristics
Fig. 6. (a), (b) SEM images, (c) TEM image, and (d) corresponding SAED pattern of a DPC8eDTT microribbon.

M. Zhu et al. / Dyes and Pigments 98 (2013) 17e24
23
In fact, AFM images (Fig. 4) of the evaporated thin films support
the OFET measurements. When deposited at 100 ꢂC on the bare
SiO2 insulators (Fig. 4c), the DPC1eDTT thin films show a better
uniformity than those deposited at other temperatures. While with
the OTS-treated SiO2 substrates, the grain size of the DPC1eDTT
films becomes larger as the Tsub increases, as generally observed
for films of organic semiconductor materials [21]. When the DPC1e
DTT thin film was deposited at the same temperature, the grain
boundary is deeper on the OTS-treated SiO2 insulator than that on
the bare SiO2 insulator, leading to a decreased mobility. The X-ray
diffraction (XRD) (Fig. 5) of the DPCneDTT thin films also coincides
with the OFET test. The DPC1eDTT thin films deposited at 100 ꢂC
on the bare SiO2 insulators show high order Bragg peaks indicating
excellent crystalline ordering in films [22], which corresponds to
the highest mobility. At different substrate temperatures, the
DPC1eDTT thin films on the bare SiO2 insulators exhibit the same
XRD peaks order with different intensity, which represents the
same crystal arrangement but different degrees of crystallinity.
Along with the shortening of alkyl chain, compounds DPCneDTT
on the bare SiO2 insulators at 100 ꢂC exhibit stronger XRD peak
intensities and multiple reflections, consistent with raising of field-
heterogeneity, and more electron trapping states for the thin film
on OTS-treated substrate [23,24].
3.4. DPC8eDTT micro/nano ribbons and their OFET devices
The DPC8eDTT micro/nano ribbons were prepared by a solvent-
diffusion method in solution phase. Ethanol is slowly injected into
chloroform solution of DPC8eDTT and the mixed solution sits to
form micro/nano structures. The DPC8eDTT molecules readily self-
assemble into micro/nano ribbons because long alkyl groups could
act as a driving force for molecular ordering in the solid state and
render semiconducting core to pack tightly. As shown in scanning
electron microscope (SEM) (Fig. 6a and b), micro/nano structure
based on DPC8eDTT exhibits a good linearity. The length of the
micro/nano ribbon ranged from 150 mm to 0.5 mm with a high
aspect ratio. Transmission electron microscopy (TEM) images and
the corresponding selected area electron diffraction (SAED) pat-
terns were collected from an individual micro/nano ribbon (Fig. 6c
and d). The TEM image of the DPC8eDTT micro/nano ribbons
shows very orderly diffraction point. Moreover, no difference was
observed in different areas of the individual micro/nano ribbon,
indicating that the whole ribbon is a single crystal. After an indi-
vidual DPC8eDTT microribbon was transferred onto the OTS-
treated SiO2/Si substrate, the gold source/drain electrodes were
prepared using a gold silk as a shadow mask. Device structure and
SEM image of single crystal OFETs based on an individual DPC8e
DTT microribbon are showed in Fig. 7a and b. Fig. 7c and d shows
the output and transfer characteristics of the single crystal OFET
devices. The transistors exhibit a p-channel behavior with a highest
effect mobility. According Bragg equation 2dsin
q
¼
l, the d space
was 22.82 and 26.30 Ǻ for compound DPC1eDTT and DPC4eDTT,
respectively. The diffraction peaks around 21.2 and 23.6ꢂ reflect
the intermolecular pep and S/S interactions, respectively. It is also
found that mobilities are relatively higher on the bare SiO2 in-
sulators than those on the OTS-treated SiO2 insulators. The AFM
images reveal much smaller crystallite grains compared to the
untreated substrates, which would suggest more grain boundaries,
Fig. 7. Field-effect characteristics of single crystal OFETs based on an individual DPC8eDTT microribbon. (a) Device structure. (b) SEM image of the OFET devices. (c) Output and
(d) transfer characteristics.

24
M. Zhu et al. / Dyes and Pigments 98 (2013) 17e24
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Among the four organic semiconductors, DPC1eDTT with the
shortest alkyl groups (methyl groups) exhibits an excellent field-
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This work was supported by the National Natural Science Foun-
dation of China (20825208 and 21021091), the Major State Basic
Research DevelopmentProgram (2011CB808403 and 2011CB932303).
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