Dipyrrolo[2,3-b:2′,3′-e]pyrazine-2,6(1H,5H)-dione based conjugated polymers for ambipolar organic thin-film transistors

Article abstract of DOI:10.1039/c2cc37266e

We report two new low band-gap conjugated polymers containing dipyrrolo[2,3-b:2′,3′-e]pyrazine-2,6(1H,5H)-dione (PzDP), PPzDPDP-BT and PPzDPDP-TT, both showing ambipolar transport in organic thin film transistors (OTFTs), with hole mobility up to 0.066 cm

Full text of DOI:10.1039/c2cc37266e

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Dipyrrolo[2,3-b:2',3'-e]pyrazine-2,6(1H,5H)-dione based conjugated
polymers for ambipolar organic thin-film transistors
Wei Hong,a Bin Sun,a Chang Guo,a Jonathan Yuen,a Yuning Li,*a Shaofeng Lu,b Chun Huang,b Antonio
Facchetti*b
a Department of Chemical Engineering/Waterloo Institute for Nanotechnology (WIN), University of
Waterloo, 200 University Avenue West, Waterloo, Ontario, Canada N2L 3G1;
bPolyera Corporation, 8045 Lamon Avenue, Skokie, IL 60077, USA.
Contents
1. Materials and Characterization
2. Synthetic Procedures
3. OTFT Device Fabrication and Characterization
1
4. Additional data: H-NMR and 13C-NMR spectra, thermal gravimetric analysis (TGA) , differential
scanning calorimetry (DSC), ultraviolet and visible absorption (UV-Vis), cyclic voltammograms (CV),
X-ray diffraction (XRD) diagrams and atomic force microscopy (AFM) images
5. References
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1. Materials and Characterization
Reagents and solvents were purchased from commercial resources and used as received. Ethyl 2-(4-
1
2
bromophenyl)-2-oxoacetate, 1,4-diacetylpiperazine-2,5-dione and 2,2-bis((3,7-dimethyloctyl)oxy)
ethyl bromide (compound 3) 3 were synthesized according to the literature methods. NMR data were
recorded on a Bruker DPX 300 MHz spectrometer. The chemical shifts were reported relative to an
internal reference, tetramethylsilane (TMS, 0 ppm). High-temperature gel-permeation chromatography
(HT-GPC) measurements were performed on a Malvern 350 HT-GPC system using 1,2,4-
trichlorobenzene as an eluent with polystyrene as standards at 140 C. UV-Vis spectra were recorded on
a Thermo Scientific Genesys 10 UV instrument. Differential scanning calorimetry (DSC) measurements
were carried out on a TA Instruments DSC Q2000 at a scanning rate of 10 °Cmin-1 under nitrogen.
Thermal gravimetric analysis (TGA) was carried out using a TA Instruments SDT 2960 at a heating rate
of 10 °Cmin-1 under nitrogen. Cyclic voltammetry (CV) measurements were performed on an EPP-4000
potentiostat/galvanostat (Princeton Applied Research) using an Ag/AgCl reference electrode, a Pt wire
counter electrode, and a Pt foil working electrode. The working electrode was coated with a polymer
thin film by drop-casting a polymer solution in chloroform. CV measurements were recorded in 0.1 M
tetrabutylammonium hexafluorophosphate in anhydrous acetonitrile at a sweeping rate of 50 mVs-1
under nitrogen, using the ferrocene/ferrocenium (Fc/Fc+) couple (with a HOMO level of -4.8 eV) 4 as a
onset
reference. The HOMO energy levels were calculated using the equations of EHOMO (eV) = - (Eox
-
E
Fc/Fc+) - 4.8 eV, respectively, where Eoxonset is the onset potential for oxidation, relative to the Ag/AgCl
reference electrode. The LUMO energy levels were calculated from the HOMO levels and optical band
gaps (Eg), using the equation of ELUMO = EHOMO + Eg. X-ray diffraction (XRD) diagrams of polymer
thin films were obtained with a Bruker D8 Advance powder diffractometer using standard Bragg-
Brentano geometry with Cu Kα radiation (λ = 1.5406 Å). The polymer thin films (~100 nm) for the
XRD analysis were deposited by drop-casting polymer solutions in chloroform on
dodecyltrichlorosilane (DTS)-modified Si/SiO2 substrates. Atomic force microscopic (AFM) images
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were obtained from polymer thin films on DTS-modified SiO2/Si substrates using a Dimension 3100
Scanning Probe Microscope. The polymer thin films (~30-40 nm) for the AFM analysis were deposited
by spin-coating polymer solutions in chloroform.
2. Synthetic Procedures
Diethyl 2,2'-(3,6-dioxopiperazine-2,5-diylidene)bis(2-(4-bromophenyl)acetate)) (Compound 1)
Under argon protection, a mixture of ethyl 2-(4-bromophenyl)-2-oxoacetate (3.86 g, 15 mmol), 1,4-
diacetylpiperazine-2,5-dione (1.29 g, 6.5 mmol) and 2.53 g triethylamine was heated at 55-60 C for 8.5
h. After removing most of triethylamine under a reduced pressure, the residue was triturated and stirred
in 10 mL of methanol for 12 h at room temperature, then filtered and washed with cold methanol to give
compound 1 as an orange solid (mixture of cis-/trans- isomers) (3.2 g, 83.1 % yield). 1H-NMR (CDCl3,
300 MHz, ppm)  11.91-11.75 (s, s, 1+2.2H, NH), 8.05-6.96 (m, ~ 25H, aroumatic), 4.34-4.13 (m, ~ 12
H, CH2), 1.35-1.15 (m, ~18H, CH3).
3,7-Bis(4-bromophenyl)dipyrrolo[2,3-b:2',3'-e]pyrazine-2,6 (1H,5H)-dione) (Compound 2)
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Compound 1 (2.1 g, 3.55 mmol) was heated in formamide (10 mL) at 150 C for 4 h. After cooling
down to room temperature, 20 mL methanol was added into the mixture. The mixture was filtered and
washed with 20 mL of methanol and dried in a vacuum oven to give compound 2 as a dark brown solid
1
(0.405 g, 23 % yield), which was used in the next step without further purification. H-NMR (DMSO-
d6, 300 MHz, ppm) 11.44 (s, 1H, NH), 8.23 (d, 2H, aromatic), 7.72 (d, 2H, aromatic).
1,5-Bis(2,2-bis((3,7-dimethyloctyl)oxy)ethyl)-3,7-bis
(4-bromophenyl)dipyrrolo[2,3-b:2',3'-
e]pyrazine-2,6(1H,5H)-dione) (M1)
To a suspension of compound 2 (0.249 g. 0.5 mmol) and anhydrous K2CO3 (0.207 g, 1.5 mmol) in
anhydrous N,N-dimethylformamide (DMF) (9 ml) was added compound 3 (0.653 g, 1.55 mmol) in
portions, and the reaction mixture was stirred and heated at 130 °C for 8 hr. The reaction mixture was
allowed to cool down to room temperature, poured into water (80 mL), and stirred for 5 min. The
product was extracted with dichloromethane (40 mL  3), washed with water, and dried on anhydrous
MgSO4. Removal of the solvent afforded the crude product, which was further purified using
chromatography on silica gel (eluted with CH2Cl2 : hexane = 1 : 1 ~ 2 : 1, v : v) to give M1 as a deep red
1
solid (0.300 g, 50.8 % yield). H-NMR (CDCl3, 300 MHz, ppm) d, 4H, aromatic, J = 8.7 Hz),
7.57 (d, 4H, aromatic, J = 8.7 Hz), 4.95 (t, 2H, O-CH-O, J = 5.4 Hz), 3.90 (d, 4H, N-CH2, J = 5.4 Hz),
3.80-3.64 (m, 4H, OCH2-a), 3.60-3.44 (m, 4H, OCH2-b), 1.70-1.00 (m, 40H, CH and CH2), 0.82 (t, 36H,
CH3, J = 6.6 Hz). 13C-NMR (CDCl3, 75 MHz, ppm)  
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40.54, 39.39, 37.46, 36.95, 29.97, 29.86, 28.07, 24.82, 24.77, 22.81, 22.72,
19.71, 19.60.
PPzDPDP-BT (P1)
M1 (0.2712 g, 0.23 mmol) and 5,5’-bis(trimethylstannyl)-bithiophene (0.1131 g, 0.23 mmol) were
charged into a 25 mL flask. After degassing and refilling argon for 3 times, anhydrous toluene (12 mL)
and bis(triphenylphosphine)palladium(II) dichloride (3.9 mg) were added and the reaction mixture was
raised to 90 C and stirred for 36 hr at this temperature. The temperature of reaction was then raised to
110 C and stirred for 4 hr before bromobenzene (0.5 mL) was added. The mixture was further stirred at
110 C for 8 hr before cooling down to room temperature. The mixture was poured into 200 mL of
stirring methanol. The solid was filtered off, washed with methanol, and dried. The solid was then
further purified by Soxhlet extraction using acetone and hexane to remove oligmers. Subsequently
chloroform was used for extraction for 8 hr to give P1 as deep-blue solid films after removal of the
solvent (263 mg, 96.6 % yield). HT-GPC (in 1,2,4-trichlorobenzene at 140 C): Mn = 27,532; Mw =
abs
78,075; Mw/Mn = 2.84. λmax (~10-5 M in chloroform): 774 nm.
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PPzDPDP-TT (P2)
P2 was synthesized and purified similarly using the procedure described for P1 from M1 (0.2830 g, 0.24
mmol) and 2,5-bis(trimethylstannyl)thieno[3,2-b]thiophene (0.1118 g, 0.24 mmol). P2 was obtained as
deep-blue solid films (270 mg, 92.1 % yield). HT-GPC (in 1,2,4-trichlorobenzene at 140 C): Mn =
28,561; Mw = 73,094; Mw/Mn = 2.56. λmax (~10-5 M in chloroform): 788 nm.
3. OTFT Device fabrication and characterization
Bottom-gate, bottom-contact OTFT devices were fabricated on heavily n+-doped Si wafer with a
thermally grown silicon oxide (SiO2) top layer ( ~200 nm) having a capacitance of ~14 nF cm−2 . The
SiO2 layer served as the gate insulator, while the conductive silicon layer was used as the gate electrode.
The Si wafer substrate was pre-patterned with gold source-drain electrode pairs using the conventional
photolithography technique. Prior to device fabrication, the substrate was washed with acetone and
isopropanol, cleaned by O2 plasma and then merged in a solution of dodecyltrichlorosilane (DTS) (10
mg/mL) in toluene at 70 C for 20 min to form a monolayer of DTS on the surface of the SiO2 layer.
The substrate was then washed with toluene and dried under a nitrogen flow. A solution of P1 or P2 in
chloroform (1.5 mg /mL) was spin coated at 4500 rpm for 40 s to form a ~35 nm-thick polymer film on
the DTS modified SiO2/Si substrate. Then the polymer film was annealed at 150 °C for 15 min under
nitrogen. Finally, the devices were encapsulated with a ~500 nm-thick polymethyl methacrylate
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(PMMA) layer by spin coating a PMMA solution in butyl acetate (7 mg/mL) at 3000 rpm for 50 s,
followed by drying on a hotplate at 85 C for 10 min. OTFT devices have a channel length (L) of 30 μm
and a channel width (W) of 500 μm. The OTFT devices were characterized using an Agilent 4155C
Semiconductor Analyzer in air. The carrier mobility in the saturated regime, sat, is calculated according
to equation (1):
IDS = Ci sat (W/2L) (VG-VT)2
(1)
where IDS is the drain current, W and L are the semiconductor channel width and length, respectively, Ci
is the capacitance per unit area of the gate dielectric, VG is the gate voltage and VT is the threshold
voltage. VT was determined from the VG axis intercept of the linear extrapolation of the (IDS)1/2—VG in th
saturation regime at IDS = 0.
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4. Additional data:
Fig. S1 The 300 MHz 1H NMR spectrum of compound 1 measured in CDCl3 at room temperature.
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Fig. S2 The 300 MHz 1H NMR spectrum of compound 2 measured in DMSO-d6 at room temperature.
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O
Br
O
N
O
N
N
O
N
O
Br
O
M1
Fig. S3 The 300 MHz 1H NMR spectrum of monomer M1 measured in CDCl3 at room temperature.
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O
Br
O
N
O
N
N
O
N
O
Br
O
M1
Fig. S4 The 75 MHz 13C NMR spectrum of monomer M1 measured in CDCl3 at room temperature.
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O
O
S
O
N
S
N
N
N
O
O
O
n
P1
Fig. S5 The 300 MHz 1H NMR spectrum of polymer P1 measured in CDCl3 at 50 C. The broad peaks
and inaccurate integrals are due to the aggregation of polymer chains.5
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Fig. S6 The 300 MHz 1H NMR spectrum of polymer P2 measured in CDCl3 at 50 C. The broad peaks
and inaccurate integrals are due to the aggregation of polymer chains.5
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100
90
80
70
60
50
P1
P2
100 150 200 250 300 350 400 450
Temperature (oC)
8
P1
P2
6
4
2
0
-2
-50 0 50 100 150 200 250 300
Temperature (oC)
Fig. S7 Diagrams of thermal analysis of P1 and P2. Top: TGA curves with a heating rate of 10 °Cmin-1
under N2. Bottom: DSC curves with a heating rate of 10 °Cmin-1 under nitrogen.
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M1 sol.
M1 film
P1 sol.
P1 film
P2 sol.
P2 film
1.0
0.8
0.6
0.4
0.2
0.0
300 400 500 600 700 800 900 1000 1100
Wavelength (nm)
Fig. S8 Normalized UV-Vis absorption spectra of monomer M1, polymers P1 and P2 in ~10-5
M
solutions in chloroform and as thin films spin-coated from chloroform solutions on glass substrates.
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100
Fc
P1
80
60
40
20
0
P2
-20
-40
-1.2 -0.8 -0.4 0.0 0.4 0.8 1.2 1.6
Potential (V)
Fig. S9 Cyclic voltammograms of films of P1 and P2 in 0.1 M tetrabutylammonium
hexafluorophosphate in dry acetonitrile at a sweeping rate of 50 mV s-1 under nitrogen, using ferrocene
(Fc) as a standard.
12.0k
P1
rt
P2
rt
150oC
200oC
3k
2k
1k
0
10.0k
8.0k
6.0k
4.0k
2.0k
0.0
150 oC
200 oC
5
10
15
20
25
30
5
10
15
20
25
30
2
2
Fig. S10 XRD diagrams obtained from spin-coating P1 and P2 thin films on dodecyltrichlorosilane
(DTS)-modified SiO2 /Si substrates annealed at different temperatures under nitrogen.
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Fig. S11 AFM images (2 × 2 m) of polymer thin films (~30-40 nm) on DTS-modified Si/SiO2
substrates annealed at different temperatures.
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50
VG
VG
0 V
80 V
60 V
40 V
20 V
0 V
-20 V
40
-40 V
-60 V
-80 V
30
20
10
0
-80 -60 -40 -20
0
20
40
60
80
VSD (V)
1E-3
1E-3
VSD = -80 V
VSD = 80 V
e = 6.2E-3 cm2V-1s-1
h = 6.6E-2 cm2V-1s-1
1E-4
1E-4
1E-5
1E-6
1E-7
1E-8
1E-9
1E-10
1E-11
1E-5
1E-6
1E-7
1E-8
1E-9
1E-10
1E-11
(-)
-80
-40
0
40
0
40
80
VG (V)
Fig. S12 Output (top) and transfer (bottom) curves of a typical OTFT device with a P1 thin film
annealed at 150 C. Device dimensions: channel length L = 30 µm; channel width W = 500 µm.
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5. References
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22282.
4 B. W. D'Andrade, S. Datta, S. R. Forrest, P. Djurovich, E. Polikarpov and M. E. Thompson, Org.
Electron., 2005, 6, 11.
5 (a) Y. Li, P. Sonar, S. P. Singh, M. S. Soh, M. M. van and J. Tan, J. Am. Chem. Soc., 2011, 133, 2198.
(b) Y. Li, S. P. Singh and P. Sonar, Adv. Mater., 2010, 22, 4862. (c) C. Kanimozhi, N. Yaacobi-Gross,
K. W. Chou, A. Amassian, T. D. Anthopoulos and S. Patil, J. Am. Chem. Soc., 2012, 134, 16532.
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