Syntheses and optoelectronic properties of quinoxaline polymers: The effect of donor unit

Article abstract of DOI:10.1002/pola.24849

Tuning the bandgap of electrochromic polymers is one of the important research topics in electrochromism. To understand clearly the effect of donor unit in donor-acceptor-donor-type polymers, 2,3-bis(4-tert-butylphenyl)-5,8- di(thiophen-2-yl)quinoxaline and 2,3-bis(4-tert-butylphenyl)-5-(2,3- dihydrothieno[3,4-b][1,4]dioxin- 5-yl)-8-(thiophen-2-yl)quinoxaline were synthesized and polymerized potentiodynamically. Their electrochemical and spectroelectrochemical studies were performed, and the results were compared with those of poly(2,3-bis(4-tert-butylphenyl)-5,8-bis(2,3-dihydrothieno[3,4-b] [1,4]dioxin-5-yl)quinoxaline) (Gunbas et al., Adv Mater 2008, 20, 691-695). A blue shift in the polymer π-π* transitions revealed that the bandgap of such polymers with the same acceptor unit is related to the electron density of donor units.

Full text of DOI:10.1002/pola.24849

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ARTICLE
Syntheses and Optoelectronic Properties of Quinoxaline Polymers: The
Effect of Donor Unit
Merve Sendur,1 Abidin Balan,2 Derya Baran,3 Levent Toppare1,4,5
1Department of Chemistry, Middle East Technical University, 06800 Ankara, Turkey
2Department of Chemical Engineering and Chemistry, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven,
The Netherlands
3Institute of Materials for Electronics and Energy, Friedrich Alexander University Erlangen-Nuremberg, Martensstrasse 7,
91058 Erlangen, Germany
4Department of Biotechnology, Middle East Technical University, 06800 Ankara, Turkey
5Department of Polymer Science and Technology, Middle East Technical University, 06800 Ankara, Turkey
Correspondence to: L. Toppare (E-mail: toppare@metu.edu.tr)
Received 13 May 2011; accepted 21 June 2011; published online 12 July 2011
DOI: 10.1002/pola.24849
ABSTRACT: Tuning the bandgap of electrochromic polymers is one
of the important research topics in electrochromism. To understand
clearly the effect of donor unit in donor–acceptor–donor-type poly-
thieno[3,4-b][1,4]dioxin-5-yl)quinoxaline) (Gunbas et al., Adv Mater
2008, 20, 691–695). A blue shift in the polymer p–p* transitions
revealed that the bandgap of such polymers with the same acceptor
C
V
mers,
2,3-bis(4-tert-butylphenyl)-5,8-di(thiophen-2-yl)quinoxaline
unit is related to the electron density of donor units. 2011 Wiley
and 2,3-bis(4-tert-butylphenyl)-5-(2,3-dihydrothieno[3,4-b][1,4]dioxin-
5-yl)-8-(thiophen-2-yl)quinoxaline were synthesized and polymerized
potentiodynamically. Their electrochemical and spectroelectro-
chemical studies were performed, and the results were compared
with those of poly(2,3-bis(4-tert-butylphenyl)-5,8-bis(2,3-dihydro-
Periodicals, Inc. J Polym Sci Part A: Polym Chem 49: 4065–4070, 2011
KEYWORDS: conducting polymers; conjugated polymers; do-
nor–acceptor theory; electrochemistry; electrochromism; p- and
n-doping; quinoxaline derivative; redox polymers
INTRODUCTION With the discovery of conducting polymers,1 a
new era had been started in macromolecular science. It was
found that they can be used in many areas, such as electrochro-
mic materials,2 organic-based solar cells,3,4 organic field effect
transistors,5,6 and organic light-emitting diodes.7 Because
bandgap8 of the polymer plays an important role in all these
areas, many approaches were used to tune the bandgap of the
conducting polymers. These are as follows: modifying the
bandgap of electrochromic polymers, planarity, bond length
alternation, resonance effects, interchain effects, and donor–
acceptor effects.9 The donor–acceptor phenomenon, also known
as push–pull mechanism, is based on lowering the bandgap via
the contribution of the HOMO level of the donor group to HOMO
level of the polymer and contribution of LUMO level of the
acceptor group to LUMO level of the polymer.10 Many of the elec-
trochromic polymers in the literature were designed using this
effect to complete the RGB color system.11–13 Also, some
approaches were followed to understand the mechanism of the
effect clearly. For example, instead of using two donor groups in
the polymer chain of a donor–acceptor–donor-type polymer,
benzotriazole polymers with a single donor unit in the polymer
chain were synthesized. Spectroelectrochemical studies revealed
that both types have a multichromic property, which is signifi-
cant in electrochromic materials.14,15
In this study, the effect of having two different donor groups,
thiophene and 3,4-ethylenedioxythiophene in
a donor–
acceptor–donor-type polymer bearing quinoxaline as the
acceptor unit, was investigated. For this aim, two new mono-
mers,
2,3-bis(4-tert-butylphenyl)-5,8-di(thiophen-2-yl)qui-
noxaline (TBPTQ) and 2,3-bis(4-tert-butylphenyl)-5-(2,3-
dihydrothieno[3,4-b][1,4]dioxin-5-yl)-8-(thiophen-2-yl)quinoxa-
line (TBPETQ), were synthesized and polymerized electro-
chemically. Electrochemical and spectroelectrochemical studies
of their polymers; PTBPTQ and PTBPETQ, were studied, and
the results were compared with those of 3,4-ethylenedioxy-
thiophene bearing poly(2,3-bis(4-tert-butylphenyl)-5,8-bis(2,3-
dihydrothieno[3,4-b][1,4]dioxin-5-yl)quinoxaline) (PTBPEQ).11
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SCHEME 1 Synthetic route for monomers TBPTQ and TBPETQ.
RESULTS AND DISCUSSION
Synthesis
The synthetic route to TBPTQ and TBPETQ is represented in
Scheme 1. 5,8-Dibromo-2,3-bis(4-tert-butylphenyl)quinoxa-
line,11 tributyl(thiophen-2-yl)stannane,16 tributyl(2,3-dihy-
drothieno[3,4-b][1,4]dioxin-5-yl)stannane,16 and 2,3-bis(4-
tert-butylphenyl)-5,8-bis(2,3-dihydrothieno[3,4-b][1,4]dioxin-
5-yl)quinoxaline11 were synthesized according to the litera-
ture procedures. All quinoxaline-based monomers were syn-
thesized using Stille cross-coupling reaction.
Electrochemistry
TBPTQ and TBPETQ were polymerized potentiodynamically with
applied potentials between 0 and 1.4 V and ꢀ0.2 and 1.3 V,
respectively, versus Ag wire pseudo-reference electrode in a 0.1 M
FIGURE 1 Electrochemical deposition of PTBPTQ (a) and PTBPETQ (b) on ITO-coated glass slide in a 0.1 M DCM/CH3CN/TBAPF6
solvent–electrolyte couple.
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FIGURE 2 Scan rate dependence of PTBPTQ (a) and PTBPETQ (b) film for p-doping in 0.1 M CH3CN/TBAPF6 solvent–electrolyte
couple at 100, 200, 300, and 400 mV/s. [Color figure can be viewed in the online issue, which is available at
wileyonlinelibrary.com.]
tetrabutylammonium hexafluorophosphate (TBAPF6) and 1 ꢁ
10ꢀ2 monomer solution in dichloromethane/acetonitrile (5:95,
v:v) onto indium tin oxide (ITO)-coated glass slides at a scan rate
of 100 mV/s. The repetitive cycles with increasing current density
reveal that both polymers were well adhered onto the ITO surface
(Fig. 1). The irreversible monomer oxidations were observed at
1.25 and 0.9 V for TBPTQ and TBPETQ, respectively. The potential
difference between the monomer oxidation peaks is related with
the higher electron density on 3,4-ethylenedioxythiophene group
than the thiophene group. Also, their monomer oxidations were
higher than the one for TBPEQ (0.8 V). This is due to the fact that
they have lower electron density than TBPEQ. Polymer redox cou-
ple was observed for PTBPTQ at 0.96 V/0.8 V and for PTBPETQ
at 0.7 V/0.6 V. Current response versus scan rate graphs showed
a linear relation that indicates both polymerization processes are
not diffusion limited and redox couples are reversible (Fig. 2). As
other quinoxaline derivative polymers,11,13,17 both PTBPTQ and
PTBPETQ have n-doping properties at ambient conditions, which
is a rare property in electrochromic polymers (Fig. 3).
bandgap energies. HOMO and LUMO energies were calcu-
lated against Fc/Fcþ reference electrode taking the value of
NHE as ꢀ4.75 eV versus vacuum.18 The HOMO/LUMO
energy levels of PTBPEQ were calculated from previously
reported data.11 As seen from Table 1, the LUMO levels of
three polymers are nearly the same because all contain qui-
noxaline as the acceptor unit. On the other hand, the HOMO
energy levels were increased when the donor strength
increased because of electron-rich 3,4-ethylenedioxy thio-
phene (EDOT) units incorporated into the polymer structure.
Calculated optical and electrochemical bandgap values of the
polymers (Table 1) indicate that this argument is valid as
proven by both techniques (CV and spectroelectrochemistry).
Spectroelectrochemistry
In their neutral state, PTBPTQ revealed maximum absorption
at 590 nm in the visible region, whereas its second transition
maximum is not in the visible region resulting in a deep blue
color for the polymer. On the other hand, both p–p* transitions
are red shifted for PTBPETQ, which have the maximum
absorptions at around 400 and 690 nm revealing greenish-
blue color in the neutral state of the polymer. To have a green
Because both polymers are p- and n-type dopable, oxidation
and reduction onset values were used to determine the
FIGURE 3 p- and n-type doping of PTBPTQ (a) and PTBPETQ (b) in 0.1 M CH3CN/TBAPF6. [Color figure can be viewed in the online
issue, which is available at wileyonlinelibrary.com.]
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TABLE 1 Monomer Oxidation, Corresponding Polymer Redox Potentials, Band Energies (HOMO/LUMO values), Optical and
Electronic Bandgap Values of PTBPTQ, PTBPETQ, and PTBPEQ1
Polymers
Eomx (V)
Eopx (V)
Erped (V)
Enpꢀdope (V)
Enpꢀdedope (V)
HOMO/LUMO (eV)
Eogp (eV)
Eegl (eV)
PTBPTQ
1.25
0.9
0.96
0.7
0.8
ꢀ1.43/-1.89
ꢀ1.75
ꢀ1.7
ꢀ1.17/-1.65
ꢀ1.4
ꢀ1.45
ꢀ5.25/-3.60
ꢀ4.98/-3.65
ꢀ4.75/-3.55
1.55
1.38
1.18
1.65
1.33
1.20
PTBPETQ
PTBPTEQ
0.6
0.8
0.27
0.08
color, there should be two transitions at around 400 and 700
nm in the visible region. The first p–p* transition of PTBPTQ
was seen in the UV region. On the other hand, the first p–p*
transition of PTBPETQ was seen at around 400 nm because of
3,4-ethylenedioxythiophene moiety. However, the second p–p*
transition is at around 690 nm; hence, the polymer reveals
greenish-blue color. The normalized monomer and polymer
film absorptions for PTBPTQ, PTBPETQ, and PTBPEQ (Fig. 4)
reveal that when thiophene groups were replaced by EDOT
groups, monomer absorbance values and also polymer film ab-
sorbances were shifted to higher wavelengths because of the
increase in electron density. Moreover, all dominant absorption
wavelengths were shifted to red region for polymers PTBPTQ,
PTBPETQ, and PTBPEQ compared to the absorption wave-
lengths for their monomers because of the increased conjugation
length (Fig. 5). Their optical bandgap values were calculated
from the onset of their second p–p* transitions as 1.55 eV for
PTBPTQ, 1.38 eV for PTBPETQ, and 1.18 eV for PTBPEQ.
bands, respectively. Multichromic property was seen for the
PTBPTQ as the tail of the polaron band is in the visible region.
Kinetic Studies
Polymers in thin film form were coated on ITO-coated glass.
Potential was swept between the neutral and fully oxidized states
to monitor the changes in transmittance as a function of time. The
transmittance differences between the fully oxidized and reduced
states of the polymers at different wavelengths were measured
using a UV–vis–NIR spectrophotometer as the applied potential
was switched between ꢀ0.3 and 1.3 V with a period of 5 s for
both polymers. PTBPTQ revealed 34% optical contrast at 590 nm,
38% at 830 nm, and 66% at 1500 nm (Fig. 7). The polymer film
switches between the two states in 1 s (590 nm), 0.9 s (830 nm),
and 0.3 s (1500 nm). PTBPTQ shows competitive optical results
compared with its EDOT-bearing homologs especially in NIR
region. Although, PTBPETQ exhibited lower optical contrast val-
ues in visible region compared to both PTBPTQ and PTBPEQ
homologs because of its extant absorption at around 400 nm, it
revealed promising optical contrast (64%) in the NIR region.
Spectral changes were recorded by UV–vis–NIR spectropho-
tometer in a monomer-free 0.1 M TBAPF6/CH3CN solution
while changing potential between ꢀ0.3 and þ1.5 V for
PTBPTQ and PTBPETQ. In their neutral form, both polymers
showed two transitions and no absorbance in NIR region.
Upon p-type doping, formation of free charge carriers leads to
new absorption bands at 830 and 1500 nm for PTBPTQ and
at 925 and 1800 nm for PTBPETQ, whereas absorptions for
the neutral state are decreasing. (Fig. 6). These new band for-
mations represent the formation of polaron and bipolaron
EXPERIMENTAL
General
All chemicals were purchased from Aldrich except THF (tet-
rahydrofuran), which was purchased from Acros. All reac-
tions were carried out under argon atmosphere unless other-
wise mentioned. All electrochemical studies were performed
under ambient conditions using a Voltalab 50 potentiostat.
Before each measurement, solutions were purged with
FIGURE 4 Electronic absorption spectra of monomers TBPTQ,
TBPETQ, and TBPEQ in CH2Cl2 and polymers PTBPTQ, PTBPETQ,
and PTBPEQ in thin film form. [Color figure can be viewed in the
online issue, which is available at wileyonlinelibrary.com.]
FIGURE 5 Chemical structures and colors of PTBPTQ and
PTBPETQ at different potentials.
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FIGURE 6 Electronic absorption spectra of PTBPTQ (a) and PTBPETQ (b) in 0.1 M CH3CN/TBAPF6 system upon p-type doping.
[Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
nitrogen for 5 min. Electropolymerizations were performed
in a three-electrode cell consisting of an indium tin oxide
(ITO)-doped glass slide as the working electrode, platinum
wire as the counter electrode, and a silver wire as the
pseudo-reference electrode. After each measurement, the ref-
erence electrode was calibrated against Fc/Fcþ.
Synthesis of 2,3-Bis(4-tert-butylphenyl)-5,8-di(thiophen-2-
yl)quinoxaline and 5-Bromo-2,3-bis(4-tert-butylphenyl)-8-
(thiophen-2-yl)quinoxaline
5,8-Dibromo-2,3-bis(4-tert-butylphenyl)quinoxaline (1.5 g, 2.72
mmol) and tributyl(thiophen-2-yl)stannane (3.04 g, 8.15 mmol)
were dissolved in dry THF (60 mL) and refluxed under argon
atmosphere. The catalyst, dichlorobis(triphenylphosphine)-palla-
dium(II) (60 mg, 0.085 ꢁ 10ꢀ3 mmol), was added, and the mix-
ture was stirred at 100 ꢂC under argon atmosphere for 16 h
while monitoring the reaction with TLC. At the end of 16 h, the
mixture was cooled and THF was removed from the mixture by
rotary evaporator. Then, the residue was subjected to column
chromatography (dichloromethane:hexane, 1:1) to afford a yel-
low-orange solid, 2,3-bis(4-tert-butylphenyl)-5,8-di(thiophen-2-
yl)quinoxaline and a yellow solid, 5-bromo-2,3-bis(4-tert-butyl-
phenyl)-8-(thiophen-2-yl)quinoxaline.
1H and 13C NMR spectra were recorded in CDCl3 on a Bruker
Spectrospin Avance DPX-400 Spectrometer. Chemical shifts
are given in ppm downfield from tetramethylsilane. HRMS
studies were done with a Waters SYNAPT MS system. A Var-
ian Cary 5000 UV–vis spectrophotometer was used to per-
form the spectroelectrochemical studies of the polymer at a
scan rate of 2000 nm/min. Column chromatography of all
products was performed using Merck Silica Gel 60 (particle
size: 0.040–0.063 mm, 230–400 mesh ASTM). Reactions
were monitored by thin layer chromatography using silica
gel–coated aluminum sheets. Solvents used for spectroscopy
experiments were of spectrophotometric grade.
1H NMR (for TBPTQ) (400 MHz, CDCl3): d (ppm) 1.27 (s,
18H), 7.08 (m, 2H), 7.31 (d, 4H), 7.42 (d, 2H), 7.63 (d, 4H),
FIGURE 7 Optical transmittance changes of PTBPTQ at 590, 830, and 1500 nm (a) and PTBPETQ at 690, 905, and 1800 nm (b) in 0.1
M AN/TBAPF6 system while switching the potentials between their neutral and oxidized states. [Color figure can be viewed in the
online issue, which is available at wileyonlinelibrary.com.]
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7.76 (d, 2H), 7.99 (s, 2H). 13C NMR (for TBPTQ) (100 MHz,
tion results of the polymers with a previously synthesized qui-
noxaline derivative polymer (PTBPEQ), it was concluded that ox-
idation potential of both monomers and polymers increases as
the electron density of the monomer/polymer decreases. Also,
this decrease in the monomers/polymers leads to a blue shift in
both monomer and polymer kmax values.
CDCl3):
d (ppm) 31.33, 34.78, 125.15, 126.38, 126.60,
126.81, 128.69, 130.17, 131.19, 135.94, 137.09, 138.94,
151.70, 152.19. HRMS-ESIþ (m/z): Calcd for C36H34N2S2
559.2242, found 559.2255.
1H NMR (for 5-bromo-2,3-bis(4-tert-butylphenyl)-8-(thiophen-
2-yl)quinoxaline) (400 MHz, CDCl3): d (ppm) 1.27 (s, 9H), 1.28
(s, 9H), 7.10 (m, 1H), 7.31 (d, 2H), 7.33 (d, 2H), 7.47 (d, 1H),
7.59 (d, 2H), 7.61 (d, 2H), 7.77 (d, 1H), 7.86 (d, 1H), 7.95 (d,
1H). 13C NMR (for 5-bromo-2,3-bis(4-tert-butylphenyl)-8-(thio-
phen-2-yl)quinoxaline) (100 MHz, CDCl3): d (ppm) 31.33, 31.89,
34.78, 35.02, 125.15, 125.76, 126.38, 126.95, 126.60, 126.81,
127.02, 127.51, 128.69, 129.03, 130.17, 131.19, 131.82, 135.94,
137.09, 138.94, 151.70, 152.07, 152.19, 153.08.
The authors thank TUBA for financial support.
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