Synthesis and characterization of heterocyclic conjugated polymers containing planar benzo[c]cinnoline and tetraazapyrene structures for organic field-effect transistor application

Article abstract of DOI:10.1016/j.orgel.2018.12.027

Heterocyclic conjugated polymers BZCTVT and TAPTVT containing benzo[c]cinnoline (BZC) and tetraazapyrene (TAP), respectively, were prepared by using Stille coupling polymerization in microwave reactor. BZC and TAP act as acceptors and thiophene-vinylene-t

Full text of DOI:10.1016/j.orgel.2018.12.027

Accepted Manuscript
Synthesis and characterization of heterocyclic conjugated polymers containing planar
Benzo[c]cinnoline and tetraazapyrene structures for organic field-effect transistor
application
Shih-Wei Lee, Shih-Hsuan Chien, Jyh-Chien Chen, Shih-Hao Wang, Lee-Yih Wang,
Bo-Han Lai, Chien-Lung Wang
PII:
S1566-1199(18)30669-4
DOI:
https://doi.org/10.1016/j.orgel.2018.12.027
ORGELE 5036
Reference:
To appear in: Organic Electronics
Received Date: 3 August 2018
Revised Date: 19 October 2018
Accepted Date: 17 December 2018
Please cite this article as: S.-W. Lee, S.-H. Chien, J.-C. Chen, S.-H. Wang, L.-Y. Wang, B.-H. Lai,
C.-L. Wang, Synthesis and characterization of heterocyclic conjugated polymers containing planar
Benzo[c]cinnoline and tetraazapyrene structures for organic field-effect transistor application, Organic
Electronics (2019), doi: https://doi.org/10.1016/j.orgel.2018.12.027.
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ACCEPTED MANUSCRIPT

ACCEPTED MANUSCRIPT
Synthesis and Characterization of Heterocyclic Conjugated
Polymers
Containing
Planar
Benzo[c]cinnoline
and
Tetraazapyrene Structures for Organic Field-Effect Transistor
Application
Shih-Wei Lee,a Shih-Hsuan Chien,a Jyh-Chien Chen,a,* Shih-Hao Wang,a Lee-Yih
Wang,b Bo-Han Lai,c Chien-Lung Wang.c
a
Department of Materials Science and Engineering, National Taiwan University of Science
and Technology, Taipei 10617, Taiwan
b Center for Condensed Matter Sciences, National Taiwan University, Taipei 10617, Taiwan
c Department of Applied Chemistry, National Chiao Tung University, Hsinchu 30010, Taiwan
* Corresponding author: Jyh-Chien Chen
Abstract
Heterocyclic conjugated polymers BZCTVT and TAPTVT containing tetraazapyrene
(TAP) and benzo[c]cinnoline (BZC), respectively, were prepared by using Stille coupling
polymerization in microwave reactor. TAP and BZC act as acceptors and
thiophene-vinylene-thiophene (TVT) acts as donors. Alkylated thiophene is the linker
between acceptor and donor. This is the first time that tetraazapyrene was incorporated into
conjugated polymers. The films of BZCTVT and TAPTVT showed maximum absorption
wavelength at 522 and 534 nm, respectively. From cyclic voltammertry measurement,
TAPTVT and BZCTVT exhibited low LUMO (-3.61 and -3.47 eV) and HOMO (-5.64 and
-5.63 eV) energy levels. Even though BZC and TAP were planar structures, BZCTVT and
TAPTVT showed large dihedral angles between alkylated thiophene and TVT (10.8-22.6o),
and between alkylated thiophene and BZC and TAP (22.2-30.3o) based on their simulated
energy-minimized geometry. The bottom-gate, top-contact OFETs based on TAPTVT and
BZCTVT exhibited only p-channel behavior with hole mobility of 4.63 × 10-4 and 1.04 ×
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10-4 cm2 V-1 s-1 and current on/off ratios greater than 104 after thermal annealing. Grazing
incidence X-ray diffraction (GIXRD) patterns suggested that thin films of TAPTVT and
BZCTVT preferred edge-on orientation.
1. Introduction
Conjugated polymers with alternating single and double bonds can reduce energy gap to
become semiconducting or conducting polymers. In the last two decades, conjugated
polymers have been explored in many applications, including organic field-effect transistors
(OFETs), organic photovoltaic cells and organic light-emitting diodes. They offer advantages
such as light weight, flexibility, solution-processability and molecular tailorability, and have
attracted intense research efforts to develop diverse monomers and polymers to meet the
requirements. [1-9]
Conjugated polymers as semiconducting layers were discovered to conduct hole (p-type)
in the earlier development because of the poor stability of n-doped conducting polymers in
nature.[3, 10] Among them, a representative electron-rich material, regioregular P3HT,
composed of alkylated thiophene exhibited a hole mobility of 0.05-0.12 cm2 V-1･s-1.[11-13]
In order to increase the performance, various monomers were designed to increase
conjugated length, enhance chain stacking and decrease the injection barrier. Recently, to
increase the conjugated length and polymer orientation, P-type conjugated polymers with
thiophene-vinyl-thiophene (TVT) structure were studied widely and get high performance
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because of the small angular torsion of the main chain. Kim et al. reported that conjugated
polymers combined with TVT and three fused thiophenes of dithienothiophene (DTT) units
demonstrated highly lamella-order edge-on packing which is a preferred orientation in
OFETs. That material resulted in high hole mobility of 2.81 cm2 V-1･s-1 and current on/off
ratio greater than 103 after annealing.[14] When selenophene-vinylene-selenophene and DPP
were polymerized and the branching point of alkyl side chain was adjusted away from main
chain, the formed conjugated polymer exhibited the record-high mobility of 12 cm2 V-1･
s-1.[15]
On the other hand, n-type conjugated polymers were also developed to conduct electrons
by introducing electron-withdrawing groups to improve the moisture/oxygen stability and
reduce the electron injection barrier between conjugated polymer and electrode. A first n-type
conjugated polymer with high electron mobility (0.1 cm2 V-1 s-1) was
poly(benzobisimidazo-benzophenanthroline) (BBL), but it only dissolved in methanesulfonic
acid and its derivatives exhibited worse mobility.[16, 17] An effect way to improve carrier
mobility and stability was the combination of electron-rich and electron-withdrawing groups
to form donor-acceptor (D-A) structures on the main chain.[18] Conjugated polymers
containing D-A structures were proved that their HOMO and LUMO energy levels can be
adjusted effectively, and the intra- or intermolecular charge transfer (ICT) effect formed by
stronger dipole-dipole interaction can facilitate main chain stacking and led to higher carrier
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mobility.[19-22] By this method, various conjugated polymers with D-A structure were
developed and exhibited high performance and stability at nitrogen atmosphere or ambient
condition, and some of them even displayed ambipolar properties to conduct both electron
and hole carriers at the same time.[18, 23]
Generally, N-containing heterocyclic structures were widely used as acceptor such as
naphthalenediimide (NDI),[24] diketopyrrolopyrrole (DPP),[25] isoindigo (ID),[26, 27]
dithienoketopyrrole (DTKP).[28, 29] N-alkyl side chains of those structures provide good
solubility. High hole and electron mobility up to 12 and 2 cm2 V-1 s-1, respectively, were
reported in some cases.[30, 31]
Benzo[c]cinnoline (BZC) is a planar N-containing heterocyclic structure which is
physiologically active and can form complexes with metal such as iron.[32-34] In our
previous reports, various conjugated polymers containing benzo[c]cinnoline structures were
synthesized and their optical and electrochemistry properties were investigated. [35-37] In
addition, conjugated benzo[c]cinnoline polymers containing fluorene structures showed
linear relationship between UV doses and photoluminance (PL) quantum yields in dilute THF
solution after UV irradiation. It exhibited good potential for UV-sensing applications.[36]
In this study, benzo[c]cinnoline and a new N-containing heterocyclic tetraazapyrene
(TAP) as acceptors are polymerized with thiophene-vinylene-thiophene (TVT) as donor and
alkylated thiophene as linker to prepare conjugated polymers, BZCTVT and TAPTVT,
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respectively. Conjugated polymers containing tetraazapyrene structures are reported for the
first time. The optical, electrochemical properties and chain packing are investigated. The
OFET characteristics of these conjugated are also evaluated.
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2. Experimental
2.1 Materials
2,6-Dinitroanline (98.0%) was purchased from Alfa Aesar and used directly.
3,8-dibromobenzo[c]cinnoline (9) was synthesized according to the procedures described in
our previous report.[38] Tributyl(4-(2-octyldodecyl)thiophen-2-yl)-stannane (6) and
(E)-1,2-bis(5-(trimethylstannyl)thiophen-2-yl)ethane (12) were prepared following literature
procedures.[25, 39] Tetrabutylammonium perchlorate (TBAP) used in cyclic voltammetric
o
measurements was recrystallized with ethyl acetate and dried at 120 C under vacuum
overnight. Anhydrous toluene from Sigma-Aldrich was used as received. THF and ethyl ether
were dried with sodium and distilled prior to use. Other reagents and solvents were purchased
from commercial sources and used as received.
Synthesis of 4-bromo-2,6-dinitroanline (1)
To a 100-mL, three-necked and round-bottomed flask equipped with a condenser was
added 2,6-dinitroaniline (5.00 g, 27.3 mmol) in acetic acid (50.0 mL). After bromine (1.5 mL,
o
29 mmol) was slowly added by syringe, the reaction mixture was heated to 120 C for 2 h.
The reaction mixture was poured into water after cooled to room temperature. The precipitate
was collected by filtration and then extracted with ethyl acetate. The combined organic
solution was collected and dried with anhydrous magnesium sulfate. After the solvent was
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evaporated, the crude product was dried at 70 C under reduced pressure to afford 6.88 g
(95% yield) of orange crystals; mp: 159-161 °C. 1H NMR (600 MHz, CDCl3, δ, ppm): 8.64 (s,
2H, Ar-H), 8.45 (s, 2H, NH2). 13C NMR (150 MHz, CDCl3, δ, ppm): 140.47, 136.43, 135.59,
104.75.
Synthesis of 4-bromo-2,6-dinitroiodobenzne (2)
A nitrite solution was prepared by adding NaNO2 (1.65 g, 24.0 mmol) in sulfuric acid
(18 ml) at room temperature. Then, the nitrite solution was added dropwisely to a 100-mL
beaker containing 4-bromo-2,6-dinitroanline (1) (6.50 g, 24.8 mmol) in acetic acid (44 mL) at
50 oC. The mixture solution was stirred at room temperature for 30 min and added dropwisely
to a 250-mL beaker containing potassium iodide (3.98 g, 23.9 ml) in water (40 mL) at 70 oC.
o
After the solution was stirred at 70 C for 15 min, the solution was cooled to room
temperature. The solution was poured into water (500 mL). The precipitate was collected by
filtration and recrystallized in ethanol (100 mL) to afford 5.64 g (61% yield) of brown
crystals; mp: 154-155 °C. 1H NMR (600 MHz, CDCl3, δ, ppm): 7.95 (s, 2H, Ar-H). 13C NMR
(150 MHz, CDCl3, δ, ppm): 156.38, 129.87, 123.52, 78.37. EIMS (m/z): Calcd. for
C6H2BrIN2O4, 371.8; Found: 371.8 [M]+.
Synthesis of 4,4’-dibromo-2,2’,6,6’-tetranitrobiphenyl (3)
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To a 100-mL, three-necked and round-bottomed flask equipped with a condenser were
added 4-bromo-2,6-dinitroiodobenzene (2) (4.00 g, 10.7 mmol), xylene (30 mL) and
activated copper powder (1.00 g). The mixture was stirred at 130 oC for 24 h under nitrogen
atmosphere. After the reaction mixture was cooled to room temperature, n-hexane was added.
The precipitates that formed were collected and washed several times with n-hexane.
Dichloromethane (DCM) was then used to wash the precipitates. The filtrate was collected.
After the solvent was removed under reduced pressure, the crude solid was purified by
column chromatography with 2/1 (v/v) DCM/n-hexane as the eluent to afford 1.63 g (62%
yield) of yellow crystals. mp: 166-171 °C. 1H NMR (600 MHz, CDCl3, δ, ppm): 8.62 (s, 4H,
13
Ar-H). C NMR (150 MHz, CDCl3, δ, ppm): 148.38, 132.84, 124.44, 122.59. EIMS (m/z):
Calcd. for C12H4Br2N4O8, 491.8; Found: 491.9 [M]+.
Synthesis of 4,4’-dibromo-2,2’,6,6’-tetraaminobiphenyl (4)
To a 250-mL, three-necked and round-bottomed flask equipped with a condenser were
added 4,4’-dibromo-2,2’,6,6’-tetranitrobiphenyl (3) (4.89 g, 9.93 mmol), hydrogen chloride
(40 mL, 37%) and ethanol (100 mL). After reaction solution became homogeneous, tin
powder (9.48 g, 79.8 mmol) was slowly added. The reaction mixture was heated at reflux for
1 h. The reaction mixture was cooled to room temperature, then poured into cool water and
neutralized with sodium hydroxide solution (10%) to pH > 8. The solution was extracted with
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ethyl ether several times and the combined organic solution was dried with anhydrous
o
magnesium sulfate. After the solution was evaporated, the product was dried at 80 C under
1
reduced pressure to afford 3.51 g (95% yield) of brown powder; mp: 186-191 °C. H NMR
(600 MHz, CDCl3, δ, ppm): 6.37 (s, 4H, Ar-H), 3.62 (s, 8H, NH2). 13C NMR (150 MHz,
CDCl3, δ, ppm): 147.01, 123.86, 108.27, 101.63. EIMS (m/z): Calcd. for C12H12Br2N4, 372.0;
Found: 372.0 [M]+.
Synthesis of 2,7-dibromo-4,5,9,10-tetraazapyrene (5)
To
a
100-mL, three-necked and round-bottomed flask were added
4,4’-dibromo-2,2’,6,6’-tetraaminobiphenyl (4) (0.57 g, 1.53 mmol) and anhydrous DCM (10
mL) with a stirrer under nitrogen atmosphere. After the reaction solution was cooled to -78 oC
with dry ice/methanol bath, iodobenzene diacetate (1.48 g, 4.59 mmol) and potassium
carbonate (1.54 g, 11.2 mmol) were added slowly into the flask. The reaction mixture was
allowed to warm to room temperature and stirred for additional 12 h. It was then filtered and
washed with DCM several times. The filtrate was concentrated under reduced pressure and
o
purified by column chromatography using DCM as eluent. The product was dried at 80 C
under reduced pressure overnight to afford 0.12 g (22% yield) of red powder; mp > 300 °C.
13
1H NMR (600 MHz, CDCl3, δ, ppm): 9.47 (s, 4H, Ar-H). C NMR (150 MHz, CDCl3, δ,
ppm): 142.13, 134.79, 125.23, 104.03. EIMS (m/z): Calcd. for C12H4Br2N4, 363.9; Found:
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363.9 [M]+. Anal. Calcd for C12H4Br2N4: C, 39.60; H, 1.11; N, 15.39. Found: C, 39.50; H,
1.45; N, 15.11.
Synthesis of 2,7-bis(4-(2-octyldodecyl)thiophen-2-yl)tetraazapyrene (7)
To a 30-mL microwave tube were added 2,7-dibromo-4,5,9,10-tetraazapyrene (5) (0.24
g, 0.66 mmol), tributyl(4-(2-octyldodecyl)thiophen-2-yl)stannane (6) (1.40 g, 2.14 mmol),
Pd(PPh3)4 (0.023 g, 0.020 mmol) and anhydrous toluene (15 mL). The tube was sealed with
Teflon cap and placed into a microwave reactor. The temperature was raised to 30 oC within
2 min, then to 190 oC and kept at that temperature for 90 min. After the reaction mixture was
o
cooled to 50 C, the solvent was evaporated under reduced pressure. The crude product was
purified by column chromatography with 1/1 (v/v) chloroform/n-hexane as eluent to afford
1
0.34 g (55% yield) of red viscous liquid; H NMR (600 MHz, CDCl3, δ, ppm): 9.46 (s, 4H,
Ar-H), 7.63 (d, J = 1.0 Hz, 2H), 7.13 (d, J = 1.0 Hz, 2H), 2.67 (d, J = 6.6 Hz, 4H, Th-CH2),
1.76 (m, 2H, CH), 1.35 (m, 64H), 0.86 (m, 12H). 13C NMR (150 MHz, CDCl3, δ, ppm):
144.29, 142.28, 141.63, 138.15, 128.04, 127.93, 123.67, 104.53, 38.99, 35.19, 33.41, 31.91,
30.03, 29.70, 29.35, 26.72, 22.67, 14.09. EIMS (m/z): Calcd. for C60H90N4S2, 930.6; Found:
930.5 [M]+.
Synthesis of 2,7-bis(5-bromo-4-(2-octyldodecyl)thiophen-2-yl)tetraazapyrene (8)
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To
a
50-mL, single-necked and round-bottomed flask were added
2,7-bis(4-(2-octyldodecyl)thiophen-2-yl)tetraazapyrene (7) (0.28 g, 0.30 mmol) and
o
chloroform (20 mL). After the reaction mixture was stirred at 0 C for 10 min,
N-bromosuccinimide (0.16 g, 0.90 mmol) was added and the reactor was covered by
aluminum foil to avoid light exposure. The reaction mixture was stirred at 0 oC overnight and
then at room temperature for additional 24 h. After DI water (10 mL) was added, the reaction
mixture was extracted with ethyl ether and washed with water several times. The combined
organic solution was dried with anhydrous magnesium sulfate then evaporated under reduced
pressure. The crude product was purified by column chromatography with toluene as eluent
to afford 0.29 g (89% yield) of red crystals; mp: 133-140 °C. 1H NMR (600 MHz, CDCl3, δ,
ppm): 9.38 (s, 4H, Ar-H), 7.49 (s, 2H, Th-H), 2.63 (d, J = 7.2 Hz, 4H, Th-CH2), 1.80 (m, 2H,
13
CH), 1.64 (m, 64H), 0.87 (m, 12H). C NMR (150 MHz, CDCl3, δ, ppm): 143.73, 142.22,
141.11, 137.26, 127.66, 112.68, 104.53, 38.69, 34.51, 33.45, 31.91, 30.02, 29.68, 29.34,
26.62, 22.67, 14.09. EIMS (m/z): Calcd. for C60H88Br2N4S2, 1088.8; Found: 1088.8 [M]+.
Anal. Calcd for C60H88Br2N4S2: C, 66.16; H, 8.14; N, 5.14; S, 5.89. Found: C, 65.54; H, 7.49;
N, 4.74; S, 5.22.
Synthesis of 3,8-bis(4-(2-octyldodecyl)thiophen-2-yl)benzo[c]cinnoline (10)
To a 30-mL microwave tube were added 3,8-dibromobenzo[c]cinnoline (9) (0.50 g, 1.47
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mmol), tributyl(4-(2-octyldodecyl)thiophen-2-yl)stannane (6) (2.88 g, 4.41 mmol), Pd(PPh3)4
(0.050 g, 0.043 mmol) and anhydrous toluene (15 mL). The tube was sealed with Teflon cap
and placed into a microwave reactor. The temperature was raised to 30 oC within 2 min, then
to 190 oC and kept at that temperature for 90 min. After the reaction mixture was cooled to 50
oC, the solvent was evaporated under reduced pressure. The crude product was purified by
column chromatography with 1/1 (v/v) chloroform/n-hexane as eluent to afford 0.91 g (68%
yield) of yellow viscous liquid; 1H NMR (600 MHz, CDCl3, δ, ppm): 8.92 (d, J = 1.9 Hz, 2H),
8.51 (d, J = 8.5 Hz, 2H), 8.14 (dd, J1 = 8.5 Hz, J2 = 1.9 Hz, 2H), 7.41 (d, J = 1.2 Hz, 2H), 6.98
(d, J = 1.2 Hz, 2H), 2.62 (d, J = 6.8 Hz, 4H, Th-CH2), 1.64 (m, 2H, CH), 1.31 (m, 64H), 0.86
(m, 12H). 13C NMR (150 MHz, CDCl3, δ, ppm): 145.53, 143.70, 141.86, 135.84, 129.79,
126.66, 126.47, 122.21, 121.98, 119.87, 38.93, 35.17, 33.38, 31.93, 30.03, 29.69, 29.35,
26.66, 22.68, 14.11.
Synthesis of 3,8-bis(5-bromo-4-(2-octyldodecyl)thiophen-2-yl)benzo[c]cinnoline (11)
To
a
100-mL, single-necked and round-bottomed flask were added
3,8-bis(4-(2-octyldodecyl)thiophen-2-yl)benzo[c]cinnoline (10) (0.91 g, 0.86 mmol) and
o
chloroform (50 mL). After the solution was stirred at 0 C for 10 min, N-bromosuccinimide
(0.53 g, 2.97 mmol) was added and the reactor was covered by aluminum foil to avoid light
exposure. The reaction mixture was stirred at 0 oC overnight and then at room temperature for
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additional 24 h. After DI water (15 mL) was added, the reaction mixture was extracted with
ethyl ether and washed with water several times. The combined organic solution was dried
with anhydrous magnesium sulfate then evaporated under reduced pressure. The crude
product was purified by column chromatography with 2/1 (v/v) toluene/n-hexane as eluent to
1
afford 0.91 g (85% yield) of yellow powder; mp: 63-68 °C. H NMR (600 MHz, CDCl3, δ,
ppm): 8.74 (d, J = 1.8 Hz, 2H), 8.41 (d, J = 8.4 Hz, 2H), 7.97 (dd, J1 = 8.4 Hz, J2 = 1.8 Hz,
2H), 7.21 (s, 2H, Th-H), 2.54 (d, J = 7.2 Hz, 4H, Th-CH2), 1.73 (m, 2H, CH), 1.33 (m, 64H),
0.72 (m, 12H). 13C NMR (150 MHz, CDCl3, δ, ppm): 145.32, 143.05, 141.32, 134.87, 129.13,
126.14, 122.14, 119.79, 111.03, 38.59, 34.42, 33.40, 31.91, 30.01, 29.68, 29.35, 26.57, 22.67,
14.09. EIMS (m/z): Calcd. for C60H90Br2N2S2, 1062.5; Found: 1062.9 [M]+. Anal. Calcd for
C60H90Br2N2S2: C, 67.77; H, 8.53; N, 2.63; S, 6.03. Found: C, 67.81; H, 8.55; N, 2.78; S,
5.98.
Polymer synthesis
The typical procedure to synthesize the conjugated polymers (TAPTVT as the example)
by Stille coupling was described below. To a 10-mL microwave tube were added compound
(8) (0.313 g, 0.287 mmol), compound (12) (0.148 g, 0.287 mmol), Pd2(dba)3 (0.006 g, 0.007
mmol) and P(o-Tol)3 (0.008 g, 0.026 mmol) in the glove box. After anhydrous toluene (3.5
mL) was added by syringe, the tube was sealed with Teflon cap and placed into a microwave
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o
reactor. The temperature was raised to 200 C within 2 min and kept at that temperature for
o
10 min then at 210 C for 120 min. After the reaction mixture was cooled to room
temperature, chloroform (5 mL) was added. The reaction mixture was then poured
dropwisely into methanol. The precipitated TAPTVT was collected and washed by Soxhlet
extraction using methanol, acetone, ethyl ether, n-hexane and chloroform, sequentially.
o
TAPTVT solution extracted from chloroform was evaporated and dried at 80 C under
reduced pressure overnight to afford 0.136 mg (42% yield) of yellow-green solid. In the case
o
of BZCTVT, solution extracted from n-hexane was evaporated and dried at 80 C under
reduced pressure overnight to afford yellow-green solid (71% yield).
2.2 Measurements
Melting points of compounds were measured on a Mel-Temp capillary melting point
13
apparatus. 1H NMR and C NMR spectra were recorded on a Bruker AVIII spectrometer at
600 MHz and 150 MHz, respectively. UV/Visible spectra were measured using a JASCO
V-670 UV-Vis/NIR spectrophotometer. Thermal gravimetric analyses (TGA) were conducted
in nitrogen with a TA TGA Q500 thermogravimetric analyzer at a heating rate of 10 oC min-1.
Glass transition temperatures of polymers were measured on a PerkinElmer DSC 4000
o
analyzer under nitrogen atmosphere at a heating rate of 10 C min-1. Mass spectroscopies
were conducting on JEOL JMS-700 mass spectrometer. Molecular weights were analyzed on
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ACQUITY Advanced Polymer Chromatography (APC) System. Elemental analyses were
obtained with elementar Vario EL cube. Cyclic Voltammetry of polymer films were
performed using an electrolyte solution of 0.1 M tetrabutylammonium perchlorate (TBAP) in
acetonitrile. The potential windows for the oxidative and reductive scan were from 0 to +2.0
and from 0 to -2.0 V, respectively. Microwave reaction was operated on Anton Paar
Monowave 300. GIXRD pattern were investigated in National Synchrotron Radiation
Research Center (Hsinchu, Taiwan) (BL17A1) with Cu-Kα radiation (λ=1.5406 Å).
2.3 Fabrication of organic field-effect transistors
OFET devices were fabricated with a bottom-gate, top-contact configuration. A heavily
doped n+-Si/SiO2 (300 nm/200 nm) wafer was used as the substrate, wherein n+-Si and
thermally-grown SiO2 functioned as gate electrode and dielectric layer, respectively. The
capacity of dielectric layer was 11 nF/cm2. The dielectric layer was coated with a
self-assembly monolayer of PETS ((2-phenylethyl)trichlorosilane) to facilitate structure order
of semiconductor layer and prevent electrons from being trapped by the OH end groups of
SiO2.[40, 41] Polymer solution of BZCTVT (3 mg/mL) and TAPTVT (1 mg/mL) in
chlorobenzene were spin-coated with 700 rpm for 600s onto the modified SiO2/n+-Si subtract
at room temperature and dried at hot plate under nitrogen in a glove box. The source and
drain electrodes were deposited by thermally evaporating gold (Au) through a shadow mask.
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The channel length (L) and channel width (w) were 100 um and 1000 um, respectively. All of
the measurements of OFETs were conducted under nitrogen atmosphere.
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3. Results and discussion
3.1 Syntheses and characterization of monomers and polymers
O2N
NO2
Br
NO2
NO2
NO2
NH2
NO2
NH2
NO2
I
H2SO4, NaNO2
AcOH, KI
Br2
Cu
AcOH
n-xylene
Br
NO2
NO2
Br
NO2
Br
2
3
1
H2N
NH2
N
N
Br
Br
Sn
Iodobenzene diacetate
CH2Cl2, K2CO3
EtOH/HCl
NH2
NH2
N
Br
Br
N
4
5
Br
S
N
S
C8H17
N
S
Pd(PPh3)4
microwave
N
Bu3Sn
C10H21
N
C8H17
NBS
C10H21
N
+
5
CHCl3
Toluene
S
N
N
C10H21
S
N
C8H17
Br
C10H21
C8H17
C10H21
C8H17
6
7
Scheme 1. Synthetic routes of monomer (8).
Scheme 2. Synthetic routes of monomer (11).
Scheme 1 and Scheme 2 illustrate the synthetic routes of dibromide monomer (8) and
monomer (11), containing benzo[c]cinnoline and tetraazapyrene structures, respectively.
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2,6-Dintroanline as the starting material was first brominated, amino group was converted to
diazonium salt then iodide. Ullmann coupling reaction of compound (2) was used to form
biphenyl structure, which was further converted to tetraazapyrene structure (compound (5))
by reduction and oxidative cyclization. Compound (9) was synthesized from
2,5-dibromonitrobenzene and was reported in our previous publication.[38] Compound (7)
and (10) were prepared by using Stille coupling reaction between compound (5) and (9) with
compound (6), respectively. Compound (8) and (11) were obtained by brominating compound
(7) and (10). These compounds were characterized by NMR spectroscopy, mass spectrometry
and element analysis to confirm their chemical structures as shown in experimental section.
When brominating compound (7), two equivalents of NBS gave low yield (12%) of
compound (8). The product mixture contained mono-brominated compound and
non-brominated starting material. Since tetraazapyrene is an electron-deficient structure,
bromination could only occurs on thiophene ring at room temperature. Increasing the amount
of NBS to three equivalents gave a high yield (90%) of compound (8). Figure 1(a) shows the
proton NMR spectra of compound (8). Two singlet peaks at chemical shifts of 9.37 and 7.48
ppm with integral of 2:1 represent the protons on tetraazapyrene and thiophene, respectively.
Compound (11) was obtained from compound (10) by using also three equivalents of NBS to
afford high yield (85%). Figure 1(b) shows the proton NMR spectrum of compound (11).
The peaks at chemical shifts of 8.74, 8.41 and 7.97 ppm are assigned to protons on BZC ring.
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The singlet peak at chemical shift of 7.21 ppm is resulted from proton on thiophene ring.
Combined with the results from elemental analyses, 13C NMR and EI-Mass spectra, the
chemical structures of compound (8) and (11) are confirmed.
Br
S
N
N
C10H21
C8H17
Pd2(dba)3, P(o-Tol)3
Me3Sn
S
S
N
+
N
Br
toluene
microwave
S
SnMe3
C8H17
C10H21
12
8
N
N
N
S
S
S
n
S
N
C8H17
C8H17
C10H21
C10H21
TAPTVT
Br
S
N
N
C10H21
Me3Sn
C8H17
S
Pd2(dba)3, P(o-Tol)3
S
+
Br
toluene
microwave
S
SnMe3
C8H17
C10H21
11
12
N
N
S
S
S
n
S
C8H17
C8H17
C10H21
C10H21
BZCTVT
Scheme 3. Polymerization of TAPTVT and BZCTVT.
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As shown in Scheme 3, conjugated polymers TAPTVT and BZCTVT were synthesized
by Stille coupling reaction of compound (8) and compound (11) with compound (12),
respectively. However, due to their limited solubility in CDCl3, satisfactory and
1
high-resolution H-NMR spectra were not available. Their proton NMR spectra and
elemental analysis results were shown in supporting information (Figure S1 and Table S1).
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Figure 1. 1H NMR spectra of compound (8) and compound (11) in CDCl3.
3.2 Molecular weight and solubility
Table 1 shows the molecular weights and solubility of TAPTVT and BZCTVT. After
both polymers were washed by Soxhlet extraction from polar to nonpolar solvents, residual
oligomers and small molecules were removed. The yields of TAPTVT and BZCTVT were
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42% and 71%, respectively. The molecular weights of TAPTVT and BZCTVT were 38.3
and 51.1 kDa and the polydispersity of them were 1.34 and 1.49, respectively. It could be
possible that lower yield of TAPTVT was caused by premature precipitation during
polymerization. Both of them exhibited high enough molecular weights to prepare the thin
films for OFET devices.
Table 1. Molecular weights of TAPTVT and BZCTVT.
Mna
(kDa)
38.3
Mwa
(kDa)
50.1
Yield
(%)
42
Polymer
PDIb
1.34
1.49
TAPTVT
BZCTVT
51.1
76.5
71
a
Obtained from GPC using THF as solvent and calibrated with polystyrene
standards.
b PDI=Mw/Mn
The solubility of polymers is important for OFET solution process. In this study, long
branched alkyl chains were attached on thiophene rings to improve the solubility which could
be limited by planar benzo[c]cinnoline and tetraazapyrene structures. Table 2 shows the
solubility of BZCTVT and TAPTVT. The solubility was tested by using the same polymer
concentration (1 mg of polymer in 1mL of solvent) for device fabrication. BZCTVT could
dissolve in chloroform, tetrahydrofuran and chlorobenzene at room temperature, while
TAPTVT could only dissolve in chlorobenzene. It was supposed that TAPTVT exhibited
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better stacking and aggregation behavior than BZCTVT. Both polymers showed solubility
enough for OFET fabrication.
Table 2. Solubility of TAPTVT and BZCTVT.
Solventa
Polymer
n-Hexaneb
-
DCM
CF
+-
THF
+
CB
++
++
-
TAPTVT
BZCTVT
c
+
+
++
++
a Abbreviations: DCM: Dichloromethane; CF: Chloroform; THF: Tetrahydrofuran;
CB: Chlorobenzene.
b Qualitative solubility was determined using 1 mg of polymer in 1 mL of solvent.
c The symbols represent the following: ++: Soluble at room temperature; +:
Soluble on heating; +-: Partially soluble on heating; -: Insoluble even on heating.
3.3 Thermal properties of polymers
TGA curves of both polymers were shown in Figure 2. TAPTVT and BZCTVT
exhibited decomposition temperatures of 5 wt% weight loss (Td5%) at 410 oC and 406 oC, and
the residual weights at 800 oC were 56 and 38 wt%, respectively. Figure 3 shows DSC curves
o
of polymers. The glass transition temperatures of TAPTVT and BZCTVT were 239 C and
237 oC, respectively. The results show that both BZC and TAP structures are thermally stable
and exhibited similar thermal properties.
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Figure 2. TGA curves of TAPTVT and BZCTVT.
Figure 3. DSC curves of TAPTVT and BZCTVT.
3.4 Optical properties of polymers
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Figure 4. UV-Vis absorption spectra of (a) BZCTVT and (b) TAPTVT films and in
solutions (10-5 M).
Figure 4 shows the UV/Vis spectra of TAPTVT and BZCTVT and the peak data was
listed in Table 3. All of samples were measured by using a polymer concentration of 10-5 M
or their films. BZCTVT exhibited the maximum absorption wavelengths (λmax) in
chlorobenzene and THF both at 504 nm which were red-shifted to 522 nm and 529 nm when
chloroform and hexane were used, respectively (Figure 4a). It could be explained that
polymer segments aggregated in non-polar solvents and enhance π-π stacking.[42] This
phenomenon was also observed in polymer film, which exhibited a similar λmax at 522 nm.
The red shift in the maximum absorption wavelength was resulted not only from aggregation
but intramolecular charge transfer (ICT) of donor-acceptor conjugated polymers as well. In
our previous report, conjugated polymers containing BZC and MEH-PPV moieties had a λmax
at 440 nm.[37] Therefore, it is possible that BZC structure forms a better donor-acceptor
pair with thiophene-vinylene-thiophene (TVT) than with MEH-PPV. The optical energy band
gap of BZCTVT was 1.99 eV which was calculated from the onset absorption wavelength
(622 nm).
Table 3. Optical properties of BZCTVT and TAPTVT.
Solutiona
THF
Filmb
Polymer
CB
CF
Hexane
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λmax
(nm)
λmax
(nm)
λmax
(nm)
λmax
(nm)
529
λmax
(nm)
λonset Egopt
(nm) (eV)
BZCTVT
TAPTVT
504
504
522
522
622
654
1.99c
371/530
371/535
371/530
371/534
1.89
d
-
/563
/562
/563
/566
a Measured by UV-vis spectroscopy with polymer concentration of 10-5 M.
b Measured by UV-vis spectroscopy with polymer film.
c Optical energy band gap calculated from the absorption edge. Egopt = 1240 / λonset
.
d Not dissolved in hexane.
The maximum absorption wavelength of TAPTVT appeared at 530 nm and shoulder
peaks were present at 563 nm in chlorobenzene and chloroform (Figure 4b). In THF dilute
solution, the maximum absorption peak slightly shifted to 535 nm and a shoulder was also
observed at 562 nm. In the solid state (film), TAPTVT showed double absorption peaks at
534 and 566 nm. It indicated that TAPTVT exhibited better ICT effect with TVT than
BZCTVT.[26, 30, 43] TAPTVT in the tested solvents and in film state as well showed an
absorption peak at 371 nm which could be attributed to the absorption of π−π* transition. In
addition, because no obvious red-shift absorption was observed between solid and solution
states for TAPTVT, it suggested that the polymer showed pre-aggregation phenomenon in
three tested solvents.[44] Furthermore, TAPTVT exhibited longer absorption onset
wavelength (654 nm, 1.89 eV) than BZCTVT (622 nm, 1.99 eV), indicating more planar
structure of TAPTVT.
3.5 Electrochemical properties and energy levels of polymers
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The electrochemical properties were investigated by cyclic voltammetry, using a
reference electrode (Ag/Ag+) and a counter electrode (Pt-gauze). The working electrodes
prepared by coating polymer solution on ITO glass were used to measure redox potentials in
an electrolyte solution (0.1 M tetrabutylammonium perchlorate in acetonitrile). The absolute
redox energy of ferrocene/ferrocenium (Fc/Fc+) is assumed to be 4.8 eV below the vacuum
level. The external Fc/Fc+ redox standard E1/2 was 0.09 V, which was determined versus
Ag/Ag+ in acetonitrile by our CV measuring system. The HOMO and LUMO energy levels
were calculated from the onset potentials of oxidation and reduction, respectively. The results
were listed in Table 4.
Table 4. Electrochemical properties of BZCTVT and TAPTVT.
Polymer
ꢀ
(V) HOMO(eV)a
LUMO(eV)b Egel (eV)c
ꢁꢆ
ꢇꢄꢈ
ꢀ
(V)
ꢁꢂꢃꢄꢅ
ꢁꢂꢃꢄꢅ
0.92
0.93
-5.63
-5.64
-1.24
-1.10
-3.47
-3.61
2.16
2.03
BZCTVT
TAPTVT
a
ꢁꢆ
HOMO = - (ꢀ
LUMO = - (ꢀ
+ 4.8 - 0.09 eV).
+ 4.8 - 0.09 eV).
ꢁꢂꢃꢄꢅ
b
ꢇꢄꢈ
ꢁꢂꢃꢄꢅ
c Calculated by the equation：Egel = LUMO – HOMO.
The onset of oxidation potentials (ꢀꢁꢁꢂꢆꢃꢄꢅ) of BZCTVT and TAPTVT were 0.92 and
0.93 V, and the onset of reduction potentials (ꢀꢁꢇꢂꢄꢈꢃꢄꢅ) were -1.24 and -1.10 V, respectively.
Figure 5 shows the redox curves of BZCTVT and TAPTVT. The HOMO energy levels of
BZCTVT (-5.63 eV) and TAPTVT (-5.64 eV) were similar, but the later showed lower
LUMO energy level (-3.61 eV) than the former (-3.47 eV). The results were consistent with
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the calculated energy levels of benzo[c]cinnoline and tetraazapyrene molecules.[45] It
suggested that BZC and TAP structures could be used as acceptor unit in conjugated
polymers. On the other hand, TAPTVT also exhibited lower band gap (2.03 eV) than that of
BZCTVT (2.16 eV) by CV measurement. The trend in energy gap was also consistent with
that obtained from optical measurements.
Figure 5. Cyclic voltammetry curves of BZCTVT (a) and TAPTVT (b).
Figure 6 collects the energy level diagrams of BZC-based conjugated polymers which
we had reported previously.[36, 37] Compared to P50 and BF50 which were copolymers of
BZC with fluorene and MEH, respectively, BZCTVT and TAPTVT exhibited smaller
energy band gap and maintained HOMO energy level at around -5.63 eV, and TAPTVT had
the lowest LUMO energy level among them. In addition, BZCTVT and TAPTVT both had
low energy barrier between the work function of gold electrode (5.1 eV) and their HOMO
energy levels. It might suggest that both of them are p-type semiconductors.
28