Synthesis and characterization of three thienopyridazine-based copolymers and their application in OFET

Article abstract of DOI:10.1016/j.tetlet.2016.02.049

Three low band gap thienopyridazine-based donor-acceptor conjugated polymers, PDTTPTT, PDTTPBT, and PDTTPBDT were synthesized by the Stille copolymerization of thienopyridazine with thienothiophene, benzodithiophene, and bithiophene, respectively. The optical band gaps of PDTTPTT, PDTTPBT, and PDTTPBDT polymers were determined as 1.41 eV, 1.43 eV, and 1.58 eV, respectively by UV absorption. The deep HOMO level of the three polymers was determined by cyclic voltammetry. The three polymers were fabricated to form organic field-effect transistors and their performance as p-type semiconductors were studied.

Full text of DOI:10.1016/j.tetlet.2016.02.049

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Accepted Manuscript
Synthesis and characterization of three thienopyridazine-based copolymers and
their application in OFET
Chia Juan Lim, Lu Li, Yanlian Lei, Feng Zhou, Bo Wu, Xuyao Liu, Furong Zhu,
Beng S. Ong, Xiao Hu, Haibin Su, Siu-Choon Ng
PII:
S0040-4039(16)30158-7
DOI:
http://dx.doi.org/10.1016/j.tetlet.2016.02.049
TETL 47324
Reference:
Tetrahedron Letters
To appear in:
Received Date:
Revised Date:
Accepted Date:
5 July 2015
7 February 2016
12 February 2016
Please cite this article as: Lim, C.J., Li, L., Lei, Y., Zhou, F., Wu, B., Liu, X., Zhu, F., Ong, B.S., Hu, X., Su, H.,
Ng, S-C., Synthesis and characterization of three thienopyridazine-based copolymers and their application in OFET,
Tetrahedron Letters (2016), doi: http://dx.doi.org/10.1016/j.tetlet.2016.02.049
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1
Tetrahedron Letters
Synthesis and characterization of three thienopyridazine-based copolymers and their
application in OFET
Chia Juan Lima,ɸ, Lu Lia,ɸ, Yanlian Leib, Feng Zhoud, Bo Wub, Xuyao Liua, Furong Zhub, Beng S. Ongc*,
Xiao Hud, Haibin Sud, Siu-Choon Nga*
aNanyang Technological University, School of Chemical & Biomedical Engineering, Division of Chemical and Biomolecular Engineering, 62 Nanyang Drive,
Singapore 637459.
bHong Kong Baptist University, Department of Physics, Kowloon Tong, Kowloon, Hong Kong SAR.
c Hong Kong Baptist University, Department of Chemistry, Research Centre of Excellence for Organic Electronics, and Institute of Creativity, Kowloon Tong,
Kowloon, Hong Kong SAR.
dNanyang Technological University, School of Material Science Engineering, Nanyang Avenue, Singapore 639798.
*Corresponding Author: Email: ngsc@ntu.edu.sg; (S.C. Ng)
ɸBoth authors contributed equally to this work.
ARTICLE INFO
ABSTRACT
Article history:
Received
Received in revised form
Accepted
Three low bandgap thienopyridazine-based donor-acceptor conjugated polymers,
PDTTPTT, PDTTPBT and PDTTPBDT were synthesized by Stille
copolymerization of thienopyridazine with thienothiophene, benzodithiophene
and bithiophene respectively. The optical band gaps of PDTTPTT, PDTTPBT and
PDTTPBDT polymers were determined as 1.41 eV, 1.43 eV and 1.58 eV
respectively by UV absorption. The deep HOMO level of the three polymers were
determined by cyclic voltammetry. The three polymers were fabricated to form
organic field-effect transistors and their performance as p-type semiconductors
were studied.
Available online
Keywords:
Pyridazine
OFET
Stille Coupling
Conjugated polymers
DFT
2009 Elsevier Ltd. All rights reserved.
allowed a significant increase in PCE by having a large D-A
interfacial area through an efficient interpenetrating network.
This concept marked a major breakthrough in this field and has
since been widely exploited.
1. Introduction
Organic photovoltaics have attracted tremendous interest in both
academia and industry due to the need for renewable and clean
energy.1 In particular, conjugated polymers have shown
enormous potential in various optoelectronic applications, such
as polymer solar cells (PSC), organic field-effect transistors
(OFET) as well as polymer light-emitting diodes (PLED).2
Polymeric semiconducting materials exhibit several advantages
in comparison to inorganic materials (e.g. Si) from the
perspective of low-cost, large scale processing, light weight and
chemical flexibility for modifications.3
During the development of organic photovoltaics (OPV) devices
with high power conversion efficiencies (PCE), Tang first
studied the concept of bi-layers which contained separate donor
and acceptor layers.4 However, this system displayed the
limitation of a small charge-generating interfacial area between
the donor-acceptor layers.5 Heeger and co-workers later reported
the construction of bulk-heterojunction solar cells, in which the
active layer was fabricated from a blended solution of donor and
acceptor materials using simple spin-coating.6 This system
An efficient OPV or OFET polymer should possess the following
physical and chemical properties: (1) high molecular weight; (2)
low band gap; (3) desirable HOMO and LUMO energy levels
(HOMO and LUMO should be tuned for considerations such as
stability, band gap, open circuit voltage and electron transfer); (4)
high charge carrier mobility; (5) favorable blend morphology; (6)
good solubility and processibility.7 The application of alternating
donor-acceptor systems (D-A polymer) has been an efficient
approach to tailor the HOMO-LUMO levels and band gap of
copolymers.8 To this end, a large amount of study has been
focused on developing and combining various building blocks to
prepare high performance D-A polymers.9-12
Acceptors based on thieno[3,4-b]pyrazine have emerged as an
important class of building blocks for the construction of low
band gap (LBG) polymers. Compared to poly(thianaphthene),
thienopyrazine exhibits more effective π-conjugation by having
two sp2-hybridized nitrogens to reduce steric strain.13

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2
Tetrahedron
Furthermore, the presence of two imine nitrogen atoms
The isomers were covalently linked and the Mulliken charge was
calculated. It was found that, the thienopyridazine subunit (right)
beared more negative charge than the thienopyrazine subunit
(left) (Figure 1). This meant that the thienopyridazine moiety has
a higher electron affinity than thienopyrazine.
provides its electron-withdrawing nature which is essential for
determining the LUMO level of a D-A copolymer. The
thienopyrazine unit is usually connected to two thiophene rings at
both sides for improved backbone flexibility and mobility. While
the thionopyrazine unit (Figure 1) has been extensively used for
constructing LBG polymers, its isomer, pyridazine has not been
studied. We report herein our studies on the synthesis and
characterization of new thienopyridazine-based copolymers and
their application in OFET.
Figure 1. Structure of thienopyrazine (left) and thienopyridazine
(right)
2. Results and Discussion
DFT calculations were carried out to study the electron density
and affinity of the thienopyrazine and thienopyridazine moieties.
Figure 3. Calculated HOMO and LUMO (TD-DFT)
On the other hand, TD-DFT calculations were employed to study
the HOMO-LUMO levels of the two isomers. Lower LUMO
levels can be expected for moieties with higher electron affinity.
The HOMO eigenvalues predicted by hybrid functionals are
generally more trustworthy. However, LUMO eigenvalues of
hybrid functionals fail to predict the electron affinity (EA) to the
extent that they include HF exchange. It has been reported that
TD-DFT with all functionals accurately predicts the HOMO-
LUMO gaps.14 As shown in Figure 3, the thienopyridazine
moiety had a LUMO with 0.287 eV lower energy than that of the
thienopyrazine moiety, indicating that thienopyridazine possesses
higher electron affinity than thienopyrazine.
Figure 2. Mulliken charge analysis for covalently linked
thienopyrizine-thienopyridazine
Scheme 1. (i) EtOH, H2SO4, reflux, overnight, 61%; (ii) Br2, AcOH, reflux, overnight, 40%; (iii) 9, Pd2dba3 (5 mol%), P(o-tolyl)3 (10
o
mol%), K2CO3, EtOH, H2O, toluene, 110 C, overnight, 84%; (iv) LiAlH4, Et2O, 2 h, 81%; (v) PCC, MgSO4, CH2Cl2, overnight, 86%;
(vi) NH2NH2•H2O, CH2Cl2, MeOH, 2 h, 67%; (vii) NBS, THF, overnight, 55%; (viii) n-BuLi then 2-isopropoxy-4,4,5,5-tetramethyl-
1,3,2-dioxaborolane, -78 oC to rt, overnight, 77%.
The synthesis of monomer 7 started with Fischer’s esterification
of dicarboxylic acid 1 with EtOH in the presence of H2SO4 as a
catalyst to afford diester 2,15 which underwent subsequent
electrophilic substitution with bromine to give dibromide 3.16
Dibromide 3 underwent Suzuki coupling with boronic ester 9 to
afford diester 4, which was reduced by LiAlH4 to diol 5 in good
yield (81%).17 Diol 5 was oxidized to the corresponding
dialdehyde using PCC.17 The dialdehyde
subsequently

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3
underwent double condensation with hydrazine to afford
pyridazine 6,18 which was brominated with NBS to provide
dibromide 7. Boronic ester 9 was synthesized from thiophene 8,
via deprotonation with n-BuLi at -78 oC, followed by the
addition of 2-isopropoxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane
(Scheme 1).
Scheme 2. (i) Pd(PPh3)4 (5 mol%), toluene, 100 o C, 1 h, 97% (PDTTPTT), 92% (PDTTPBT), 76% (PDTTPBDT)
underwent Stille-copolymerization with three
Monomer
7
commercially available organotin compounds 10, 11, and 12 to
afford PDTTPTT, PDTTPBT and PDTTPBDT polymers
respectively in good to excellent yields.
100
PDTTPTT
PDTTPBT
PDTTPBDT
The molecular weights of all three polymers were determined by
gel permeation chromatography (GPC) in chloroform using
polystyrene as a calibration standard. All three polymers had high
molecular weights and low PDI (Table 1). The onset of the
weight lost curve was employed to determine the decomposition
temperature (Td) (Figure 4). High thermal stability is crucial for
organic electronic materials such as OFET and OPV. All three
polymers had high thermal stability, (decomposition temperature,
80
60
40
o
Td> 300 C), when subjected to TGA under nitrogen. However,
100
200
300
400
C
500
600
Temperature/ o
substantial weight loss of roughly 8-10% was observed before
the onset. Minor degradation might have occurred due to
cleavage of the alkyl chain. No glass transition was observed
when the three polymers were subjected to differential scanning
calorimetry (DSC) experiments, suggesting that all three
polymers were amorphous.
Figure 4. TGA plots at a heating rate of 10 oC min-1 under
nitrogen
Copolymer
Mn
Mw
PDI
Td (oC)
398
911770
2835000 1.81
PDTTPTT
45754
71970
84835
2.05
1.61
396
323
PDTTPBT
110930
PDTTPBDT
Table 1. Molecular Weights and Thermal Properties of the
copolymers