Pyridine based polymers. Synthesis and characterization

Article abstract of DOI:10.4067/S0717-97072014000200014

Two new conjugated polymers, poly(2,3-di(thiophen-2-yl)pyrido[2,3-b]pyrazine) and poly(2,3-di(thiophen-2-yl)pyrido[3,4-b]pyrazine), based on thiophene and pyridine rings were chemically and electrochemically synthesized and characterized by IR, UV-Vis and

Full text of DOI:10.4067/S0717-97072014000200014

J. Chil. Chem. Soc., 59, Nº 2 (2014)
PYRIDINE BASED POLYMERS. SYNTHESIS AND CHARACTERIZATION
M. A. PARDO, J. M. PEREZ, M. A. del VALLE, M. A. GODOY, F.R. DIAZ*
Pontificia Universidad Católica de Chile, Facultad de Química, Av. V. Mackenna 4860, 7820436, Macul, Santiago, Chile.
ABSTRACT
Two new conjugated polymers, poly(2,3-di(thiophen-2-yl)pyrido[2,3-b]pyrazine) and poly(2,3-di(thiophen-2-yl)pyrido[3,4-b]pyrazine), based on thiophene
and pyridine rings were chemically and electrochemically synthesized and characterized by IR, UV-Vis and elemental analysis. Monomers were chemically
synthesized at high yield percentages and characterized by FTIR and NMR. Polymers showed p- and n-type doping as well as low bandgap, making them potential
candidates for the development of electronic devices.
Keywords: 2,3-di(thiophen-2-yl)pyrido[2,3-b]pyrazine, 2,3-di(thiophen-2-yl)pyrido[3,4-b]pyrazine, poly(2,3-di(thiophen-2-yl)pyrido[2,3-b]pyrazine) and
poly(2,3-di(thiophen-2-yl)pyrido[3,4-b]pyrazine).
In this context, our research group has developed different materials
based on thiophene rings, and nitrogen containing rings in its structure to be
employed for the development of photovoltaic devices 10, 11, e.g. the synthesis
of 2,3-di(thiophen-2-yl)quinoxaline monomer was achieved using a simple
method, with 98 % yield 12. Other authors have found new synthetic route for
INTRODUCTION
Thiophene based polymers have been widely investigated for the
development of new technologic applications 1, 2. On the other hand, thanks
to its structural diversity, nitrogen containing heterocycles have also attracted
considerable interest in recent years due to their unique properties 3-5. Thiophene
and quinoxaline copolymers have been employed as electronic carriers in
electroluminescent devices 6 and as material for organic light-emitting devices
7, among others.
13.
the synthesis of this compound, using Grignard reagents with a 90% yield
Moreover, it has also been reported the synthesis of pyrazine derivatives with
antimicrobial activity, with yields close to 40% 14
.
This work is the continuation of our study concerning to conjugated
pirazine-like structures with the insertion of a new nitrogen atom into the
monomer, from pyridine-based monomers synthesis, aimed to obtaining a
better performance using 2,3-di(thiophen-2-yl)quinoxaline as reference, Fig. 1.
Likewise, thiophene combined with other hetero-aromatic rings such as
pyrrole, pyridine, pyrazine, quinoxaline and aniline has been studied in order
to obtain a stable compound that has low enough reduction potential to be used
instead of fullerenes as n-type semiconductor in organic solar cells 8, 9
.
Figure 1. a) 2,3-di(thiophen-2-yl)quinoxaline (TQ), b) 2,3-di(thiophen-2-yl)pyrido[2,3-b]pyrazine, c) 2,3-di(thiophen-2-yl)pyrido[3,4-b]pyrazine.
spectra were obtained on a Analytik Jena Specord 40, using acetonitrile and
chloroform as solvent. The determination of melting point was carried out
on a Stuart Scientific Melting Point Apparatus (SMP3). Potentiodynamic and
potentiostatic measurements were acquired using a VoltammetricAnalyzer CV-
50W by BAS; electrochemical experiments were performed in a conventional
three electrodes cell, using anhydrous acetonitrile as solvent, and 0.1 mol·L-1
tetramethylammonium hexafluorophosphate (NH (CH3)4PF6, supplied by
Aldrich) as supporting electrolyte. It was employe4d a Ag|AgCl electrode as
reference in tetramethylammonium chloride with potential adjusted to that of
SCE. The working electrodes were platinum disc (0.1 cm2 of geometrical area).
As counter electrode it was used a Pt gauze of large geometrical area,
separated from the working electrode compartment by a fine frit glass. Prior
to all the experiments, solutions were purged with high purity argon and it was
maintained over the solution during the measurements. The conductivity of the
polymer was measured with a conductance meter Elchema, model CM-508, by
the four probe method. All experiments were carried out at room temperature
EXPERIMENTAL
Materials
All starting materials and reagents used were: acetonitrile (98 %,
Merck); ethyl acetate for liquid chromatography (99.8%,Merck); chloroform
(99.4%,Merck), anhydrous ferric chloride (Sigma); iodine for analysis
(99.8%, Merck), ethanol (98 %, Merck), o-phenylenediamine (99 %, Aldrich
Chemistry); silica gel 60 (Merck); 2,2’-thenil (98 %, Aldrich Chemistry);
2,3-diaminopiridine (Merck); 3,4-diaminopiridine (Merck); HCl concentrated
(37 %, Merck). All reagents were used without further purification.
Measurements
1H NMR spectra were collected on a Bruker 400 spectrometer using
chloroform-d as solvent and TMS as internal standard. FT-IR spectra were
recorded on a Brucker Vector-22 spectrophotometer using KBr pellets.
Elemental analysis was performed on a Fison 1108 microanalyser. UV-VIS
e-mail: fdiaz@uc.cl
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J. Chil. Chem. Soc., 59, Nº 2 (2014)
(20 °C).
a polymer as brow-red solid was obtained with a yield of 38 %. Elemental
analysis: found % (calculated %): C, 49.27 (60.99); N, 10.91 (14.23); H, 1.9
(3.07); S, 15.31 (21.71). FT-IR (cm-1): 3430.1, 3100.4, 3074.6, 1598, 1520.4,
827.8, 729.8. UV-Vis (nm): 257, 290 and 395.
From UV-Vis spectra of the doped polymers the forbidden energy band
was calculated, following the Born-Einstein energy relationship 6. The frontier
orbital energy HOMO (EHOMO) correlates linearly with the oxidation onset
potential (Eon)ox and it was calculated directly from the voltammograms 10
.
Electrochemical polymerization
Synthesis of monomers
Synthesis of 2,3-di(thiophen-2-yl)quinoxaline (TQ)
Electrochemical polymerizations were carried out following the procedure
previously reported 12
.
The synthetic route of 2,3-di(thiophen-2-yl)quinoxaline was performed
from the procedure previously reported, with 93 % yield 12. A mixture of
2,2’-thenil (200 mg; 0.9 mmol) and o-phenylenediamine (291 mg; 2.7 mmol)
in ethanol was refluxed for 3 h and cooled to room temperature 12. Elemental
analysis, found % (calculated %): C, 65.23 (65.30); H, 3.54 (3.40); N, 9.58
(9.52); S, 21.65 (21.77). Melting point: 146.1-146.6 °C. FTIR (cm-1): 3488.7,
Electropolymerization of 2,3-di(thiophen-2-yl)quinoxaline
The monomer electropolymerization was carried out by 20 successive
voltammetric scans. It was performed in the following conditions: 5.7·10-3
mol·L-1 monomer, 2.2·10-2 mol·L-1 supporting electrolyte (NH4(CH3)4PF ) in
anhydrous acetonitrile. The optimal potential range to polymerize was -60.50
to 1.80 V.
1
3075.5, 1515.9, 1450.9, 1417.5, 836.9, 762.5. H-NMR (CDCI3, 400 MHz,
ppm): 8.01-8.05 (m, 2H, 4,4’-H); 7.75-7.70 (m, 2H, 5,5’- H); 7.51-7.49 (m,
2H, 3,3’-H); 7.27-7.23 (m, 2H, 1,1’-H); 7,06-7,02 (m, 2H, 2,2’-H). UV-Vis
(nm): 257, 274 and 382.
Electropolymerization of 2,3-di(thiophen-2-yl)pyrido[2,3-b]pyrazine
P2,3BP electrosynthesis was carried out by 30 successive voltammetric
scans. It was performed in the following conditions: 5.7·10-3 mol·L-1 monomer,
2.2·10-2 mol·L-1 supporting electrolyte (NH4(CH )4PF6) in anhydrous
acetonitrile. The optimal potential range to polymerize3was -1,0 to 1.5 V.
Synthesis of 2,3-di(thiophen-2-yl)pyrido[2,3-b]pyrazine (2,3BP)
A mixture of 2,2’-thenil (500 mg; 2.26 mmol) and 2,3-diaminepyridine
(1.4 g, 0.128 mol) in ethanol was refluxed until a yellow solid was observed.
The monomer was purified using a silica gel column and hexane : ethyl acetate
mixture as eluent. Product was obtained in a 75 % yield. Elemental analysis,
found % (calculated %): C, 60.85 (60.99); N, 14.29 (14.23); H, 2.54 (3.07); S,
18.98 (21.71). Melting point: 165.3-165.5 °C. FT-IR (cm-1) 3430.8, 3088.4,
Electropolymerization of 2,3-di(thiophen-2-yl)pyrido[3,4-b]pyrazine
P3,4BP electrosynthesis was carried out by 30 successive voltammetric
scans. It was performed in the following conditions: 5.7·10-3 mol·L-1 monomer,
2.2·10-2 mol·L-1 supporting electrolyte (NH4(CH )4PF6) in anhydrous
acetonitrile. The optimal potential range to polymerize3was -1.0 to 1.5 V.
1
1516.3, 1416.7, 845.8, 714.9. H-NMR (CDCI3) 400 MHz, ppm): 9.5 (s, 1H,
6); 8.79 (s, 1H, 4); 7.91-7.89 (d, 1H, 5); 7.60-7.57 (m, 2H, 1,1´); 7.38-7.37(m,
2H, 3,3´); 7.10-7.06 (m, 2H, 2,2´). UV-Vis (nm): 253, 286 and 391.
RESULTS AND DISCUSSION
Synthesis of 2,3-di(thiophen-2-yl)pyrido[3,4-b]pyrazine (3,4BP)
3,4BP was synthetized following the same procedure described above. The
product was obtained in a 81 % of yield. Melting point: 163.9-164.8 °C. FT-IR
(cm-1) 3447.8, 3101.7, 1596.8, 1520.1-1410.7, 829.7, 728.7. 1H-NMR (CDCI )
400 MHz, ppm): 9.39 (s, 1H, 6); 8.68-8.67 (d, 1H, 4); 7.89-7.78 (d, 1H, 53);
7.49-7.47 (t, 2H, 1,1´); 7.28-7.27 (m, 2H, 3,3´); 7.0-6.96 (m, 2H, 2,2´). UV-Vis
(nm): 253, 283 and 390.
Synthesis and characterization of monomers
The synthetic route of all monomers was performed from the procedure
previously reported 12. After 3 h all compounds were quantitatively converted
to product, without by-products.
Chemical polymerization and characterization
All polymerizations afforded low reaction yields, ca. 30 %, considering
just the insoluble fraction or higher molecular weight. Elemental analysis
showed significant variation in the experimental and theoretical percentages,
considering that the polymer in its oxidized state has Fe2+ ions trapped in its
structure (2-3 atoms of Fe per monomer) which justifies the lower concentration
of elements in the actual sample.
The solution UV–Vis absorption spectra for all polymers showed two or
three distinct absorption bands: two or a single band around 250-300 nm can
be assigned to the π-π* transition while a long-wavelength absorption peaks,
around 390 nm, can be attributed to intramolecular charge transfer between
thiophene rings and acceptor moieties 15, 16. The polymers absorption spectra
are red shifted compared to the corresponding monomers spectra (Figure 2),
Chemical Polymerization
All of the polymers were synthesized in chloroform using methodology
previously reported 12. The corresponding polymers were named PTQ, P2,3BT
and P3,4BT, respectively. Each monomer and FeCl3 were dissolved in
chloroform to achieve a molar ratio of FeCl to monomer (4:1), while it was
being stirred at room temperature under a N3 flow during 72 h. The resulting
polymer was washed with water and ethan2ol, and the precipitated is dried
in a vacuum owen. The resulting polymers were compacted into pellets and
weighed. These pellets were then put into open weighing bottles, which were
placed in a small desiccator protected from light and charged with ground iodine
powder. Iodine uptake by the polymers was followed by mass differential.
Doped polymers were dissolved in DMSO to obtain UV-Vis absorption
spectra. This enabled the forbidden energy band of each material to be worked
out.
which can be attributed to an increase in the chain length 17
From UV-Vis spectrum of the doped polymers the forbidden energy band
was calculated, following the Born-Einstein energy relationship . Likewise,
the conductivity of doped polymers was measured; an increase of up to 6 orders
of magnitude compared to the neutral polymers was observed. Comparison
of P2,3BP and P3,4BP with PTQ showed a conductivity increase of about
10 times, which is probably related to the insertion of the nitrogen atom into
the aromatic ring that would confer higher planarity to the polymeric chain,
generating thus improved charge transfer between parallel chains, Table 1.
.
6
Synthesis of poly(2,3-di(2-thenil)quinoxaline) (PTQ)
PTQ was synthesized by following the same procedure explains above.
After purification a brown-red solid was obtained with a yield of 34%.
Elemental analysis, found % (calculated %): C, 54.71 (66,28); N, 14.29 (9,52);
H, 1.82 (3,42); S, 17.15 (21.78). FT-IR (cm-1) 3441.1, 3093.9, 1423.2, 1418,
851.9, 763.4. UV-Vis (nm): 254, 291, 384.
Table 1. Conductivity of the synthesized species and Eg of the doped
polymers.
Synthesis
(P2,3BP)
of
poly(2,3-di(thiophen-2-yl)pyrido[2,3-b]pyrazine)
conductivity (S·cm-1)
polymer
bandgap energy (eV)
P2,3BP was synthesized following the same procedure explains for
chemical polymerization. Additionally the solid was washed twice with ethyl
acetate. After purification a brown-red solid was obtained with a yield of 31
%. Elemental analysis: found % (calculated %): C, 54.71 (60.99); N, 10.91
(14.23); H, 1.9 (3.07); S, 15.31 (21.71). FT-IR (cm-1): 3423.9, 3084.6, 1520,
852.3, 715.3. UV-Vis (nm): 260, 394.
monomer polymer
PTQ
2.0·10-5
3.0·10-10
3.0·10-10
2.4·10-5
1.1·10-4
0.7·10-4
3.05
3.82
3.33
P2,3BP
P3,4BP
Synthesis
(P3,4BP)
of
poly(2,3-di(thiophen-2-yl)pyrido[3,4-b]pyrazine)
P3,4BP was synthesized by the same procedure explains for chemical
polymerization, and the solid was washed with acetonitrile. After purification
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J. Chil. Chem. Soc., 59, Nº 2 (2014)
Figure 2. a) P2,3BP UV-Vis absorption spectrum; b) P2,3BP IR spectrum.
Electrochemical polymerization and characterization
the oxidation of thiophene rings in the molecule 12. It is well know that the fact
that potential gives less positive values during successive voltammetric scans
is attributed to a high conjugation due to the increase of the chain length, as the
oligomerization and polymerization processes take place. The oxidation charge
is bigger than that of the corresponding reduction peak because in the first
are included polymerization and p-doping charges while the reduction process
involves only the p-undoping charge. The other monomers present the same
behavior showing oxidation potentials near 1.1 and 1.2 for P2,3BP and P3,4BP,
respectively (Table 2).
Figure 3 shows the potentiodynamic j/E profiles during the first 20
voltammetric cycles on Pt of each monomer in acetonitrile. Table 2 shows the
oxidation potential of all monomers on Pt. In TQ potentiodynamic i/ E profile
it can be observed that the oxidation potential of the electroactive species (near
1.4 V vs SCE) shifts continuously to smaller values when the number of cycles
increases, until it stabilized near 1,3 V while the corresponding reduction peak
remain practically constant in 0.35 V. This behavior was previously reported
under similar experimental conditions. Then, it could be inferred that the profile
observed in this potential range can be assigned to the polymerization through
Figure 3. Potentiodynamic i/E profiles in acetonitrile: a) 2,3-di(2-thiophen)quinoxaline; b) 2,3-di(2-thiophenyl)pyrido[2,3-b]pyrazine; c) potentiodynamic
2,3-di(2-thiophenyl)pyrido[3,4-b]pyrazine.
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J. Chil. Chem. Soc., 59, Nº 2 (2014)
Table 2. Electrical properties of the synthesized polymers.
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reduction
potential (eV) potential (eV)
oxidation
HOMO
(eV)
LUMO
(eV)
polymer
PTQ
0.35
-0.2
-0.9
1,35
1.10
1.20
-5.8
-5.5
-5.6
-2.75
-1.68
-2.27
P2,3BP
P3,4BP
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M. Makha, J. C. Bernède, International Journal of Electrochemical
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Cattin, G. Louarn, J. C. Bernède, International Journal of Electrochemical
Science 7, 8276, (2012).
CONCLUSIONS
The synthesis of the proposed monomers was possible, following the
same methodology previously implemented, with very good results. A new
series of polymers was synthesized and characterized by FT-IR and NMR.
Electrochemical characterization demonstrates that he polymers showed p- and
n-doping and low bandgap (~ 1.00 eV), making those prospective candidates
for the development of electronic devices.
12. F. R. Diaz, del M. A. del Valle, C. Núñez, A. Godoy, J. L. Mondaca, A.
Toro-Labbé, J. C. Bernède, Polymer Bulletin 56, 155, (2006).
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Andersson, Solar Energy Materials and Solar Cells 94, 1275, (2010).
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DEDICATION
This work is dedicated to Dr. Luis Núñez Vergara (RIP), to whom we
express our sincere admiration and respect.
ACKNOWLEDGEMENTS
15. T. Cai, Y. Zhou, E. Wang, S. Hellström, F. Zhang, S. Xu, O. Inganäs, M. R.
Andersson, Solar Energy Materials and Solar Cells 94, 1275 (2010).
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T. Pullerits, V. Sundström, The Journal of Chemical Physics 121, 12613,
(2004).
The authors acknowledge financial support from CONICYT through
FONDECYT project 1120055.
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