Spray-processable thiazolothiazole-based copolymers with altered donor groups and their electrochromic properties

Article abstract of DOI:10.1002/pola.26791

Two novel thiazolo[5,4-d]thiazole containing donor-acceptor type alternating copolymers, poly[2-(5-(2-decyl-2H-benzo[d][1,2,3]triazol-4-yl) thiophen-2-yl)-5-(thiophen-2-yl)thiazolo[5,4-d]thiazole] (BTzTh) and poly[2-(5-(2-decyl-2H-benzo[d][1,2,3]triazol-4-yl)furan-2-yl)-5-(furan-2-yl) thiazolo[5,4-d]thiazole] (BTzFr) were synthesized by Stille coupling polymerization and their electrochemical and electrochromic properties were explored. Electrochemical activities of the spray-casted polymer films were determined by cyclic voltammetry. To evaluate the effect of thiophene and furan moieties on the optical properties of the copolymers, spectroelectrochemistry studies were performed. To examine the switching abilities, copolymer films were subjected to a double potential step chronoamperometry in their local maximum absorptions. Both thiazolothiazole-containing copolymers showed multichromic properties with low band-gap values 1.7 and 1.9 eV for BTzTh and BTzFr, respectively. The decent electrochromical properties together with solution processability make them important candidates for electrochromic applications. Copyright

Full text of DOI:10.1002/pola.26791

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Spray-Processable Thiazolothiazole-Based Copolymers with Altered
Donor Groups and Their Electrochromic Properties
1
2
1,3,4,5
Hava Zekiye Akpinar, Yasemin Arslan Udum, Levent Toppare
1
Department of Polymer Science and Technology, Middle East Technical University, 06800 Ankara, Turkey
Institute of Science and Technology Department of Advanced Technologies, Gazi University, 06570 Ankara, Turkey
Department of Chemistry, Middle East Technical University, 06800 Ankara, Turkey
2
3
4
5
€
The Center for Solar Energy Research and Application (GUNAM), Middle East Technical University, 06800 Ankara, Turkey
Department of Biotechnology, Middle East Technical University, 06800 Ankara, Turkey
Correspondance to: Y. Arslan Udum (E-mail: y.udum@gazi.edu.tr)
Received 23 April 2013; accepted 18 May 2013; published online 14 June 2013
DOI: 10.1002/pola.26791
ABSTRACT: Two novel thiazolo[5,4-d]thiazole containing donor–
acceptor type alternating copolymers, poly[2-(5-(2-decyl-2H-
benzo[d][1,2,3]triazol-4-yl)thiophen-2-yl)-5-(thiophen-2-yl)thiazolo
subjected to a double potential step chronoamperometry in
their local maximum absorptions. Both thiazolothiazole-
containing copolymers showed multichromic properties with
low band-gap values 1.7 and 1.9 eV for BTzTh and BTzFr,
respectively. The decent electrochromical properties together
with solution processability make them important candidates for
[5,4-d]thiazole] (BTzTh) and poly[2-(5-(2-decyl-2H-benzo[d][1,2,3]-
triazol-4-yl)furan-2-yl)-5-(furan-2-yl)thiazolo[5,4-d]thiazole] (BTzFr)
were synthesized by Stille coupling polymerization and their
electrochemical and electrochromic properties were explored.
Electrochemical activities of the spray-casted polymer films
were determined by cyclic voltammetry. To evaluate the effect
of thiophene and furan moieties on the optical properties of the
copolymers, spectroelectrochemistry studies were performed.
To examine the switching abilities, copolymer films were
electrochromic applications. V
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KEYWORDS: benzotriazole; conducting polymers; conjugated
polymers; electrochemistry; electrochromism; multichromism;
thiazolothiazole; thin films
1
INTRODUCTION Invention of conducting polymers opened a
was used to modify the target molecules to design and
obtain novel structures for different purposes, such as liquid
new era in construction of organic electronics. The useful-
ness of this class of polymer originates since they can be tai-
lored to be used in different applications. As an example,
introducing alkyl chains to the polymer’s backbone gained to
obtain solubility, which is indispensable for developing low-
8
9
crystallines, binuclear heterocyclic ligands, biological activ-
1
0
11
12
ity, electroluminescence, and fluorescent chemosensors.
Because of high on/off ratio and stability against oxygen as
well as their p-stacked structure leading to strong intermo-
lecular interactions in solid state, TTz derivatives emerged
2
cost and large area optoelectronic devices.
1
3–23
The uncertainty in conductivity of polyfurans compare to
polythiophenes was interpreted in terms of unfavorable elec-
tronic effects of the oxygen atom. Additionally, due to the
difficulties in handling furan relative to other five-membered
heterocycles and its lower stability, it is less considered as a
building block than pyrrole and thiophene. However, biode-
gradable properties and availability of biorenewable sources
recently as promising semiconductors.
Additionally, TTz
derivatives appear to be very useful as active materials in
bulk heterojunction solar cells for effective charge transport.
Therefore, TTz containing small molecules and conjugated
copolymers were designed and synthesized for organic pho-
3
2
4–31
tovoltaics (OPVs).
4
,5
In obtaining a low band-gap-conjugated polymer, donor–
acceptor (DA) approach and planarity are the two important
parameters to be considered. The advantage of the absence
of the H-atoms in the core of TTz molecule should enhance
both stability and the planarity of the polymer when intro-
duced this molecule into a main chain. In literature, there
exists a review on thiazole-based organic semiconductors for
of furan-based materials could make them more fascinat-
6
,7
ing than current favorites of organic electronics.
Thiazolo[5,4-d]thiazole (TTz) molecule has a rigid and copla-
nar fused ring which ensures highly extended p-electron sys-
tem. Thanks to its quite straightforward preparation starting
from the corresponding aldehyde and dithiooxamide, TTz
V
C
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organic electronics which summarizes all small molecules
and polymers containing TTz molecule used in OFETs and
(eluent: DCM:hexane (1:1)) to obtain the product (yield:
65%).
3
2
OPVs. To our knowledge, electrochromic properties of TTz-
based polymers never explored so far. In this study, we were
interested to design and prepare two novel copolymers to
investigate their optoelectronic properties using thiophene and
furan as the electron rich and BTz and TTz as the electron
deficient moieties to obtain a DA configuration. Chemically
synthesized copolymers showed multichromic properties and
fast switching times and low band-gap values, which makes
them promising candidates for electrochromic device
applications.
1
3
3
H NMR (400 MHz, CDCl , d): 7.59 (d, J 5 3.6, 2H), 7.08 (d,
3
3
J 5 3.6, 2H), 1.51 (m, 12H), 1.28 (m, 12H), 1.08 (t, J 5 8,
2H), 0.83 (t, J 5 7.6–7.2, 18H). C NMR (100 MHz, CDCl3,
3
13
1
d): 162.3, 142.7, 142.6, 142.6, 136.1, 127.7, 28.9, 27.2, 13.6,
1.0.
1
Synthesis of 2,5-Bis(5-(tributylstannyl)furan-2-
yl)thiazolo[5,4-d]thiazole
The monomer was synthesized using the same procedure
described above with 2,5-bis(5-bromofuran-2-yl)thiazolo[5,4-
d]thiazole (1.33 g, 3.0 mmol), n-BuLi (3.0 mL, 4.73 mmol),
EXPERIMENTAL
and SnBu Cl (9.0 mL, 7.35 mmol) to obtain the monomer as
3
a dark orange oil (yield: 60%).
Instrumentation and Materials
1
3
3
Electrochemical studies were performed in a three-electrode
cell containing an indium tin oxide-doped glass slide (ITO,
H NMR (400 MHz, CDCl , d): 7.02 (d, J 5 3.6, 2H), 6.62 (d,
3
3
J 5 3.6, 2H), 1.54 (m, 12H), 1.28 (m, 12H), 1.08 (t, J 5 8,
2H), 0.84 (t, J 5 7.6–7.2, 18H). C NMR (100 MHz, CDCl3,
3
13
12 X-cm), a platinum wire, and an Ag wire as the working,
1
counter, and pseudo-reference electrodes, respectively with a
Voltalab PST 050 potentiostat. Spectroelectrochemical and
kinetic studies were conducted by Varian Cary 5000 UV–vis–
NIR spectrophotometer coupled with a Solartron 1285
Potentiostat. Minolta CS-100 spectrophotometer was used
for quantitative determination of the film colors coated on
d): 165.5, 158.9, 153.3, 150.6, 124.1, 109.8, 28.9, 27.2, 13.7,
0.4.
1
General Procedure for the Synthesis of the Copolymers
(BTzTh and BTzFr)
Polymers were synthesized according to the similar proce-
1
13
31
ITO slides. For structural characterization, H and C NMR
spectra were recorded in CDCl on Bruker Spectrospin
dures in literature
starting from respective monomers.
Equal equivalents of distannylated and dibrominated com-
pounds were dissolved in dry toluene. The mixture was
purged with argon for 15 min and Pd(PPh ) was added.
3 4
3
Avance DPX-400 Spectrometer. Chemical shifts are given in
ppm downfield from tetramethylsilane. Average molecular
weight was determined by gel permeation chromatography
Under argon flow, the reactants were refluxed for 36 h. After
removing the 90% of the solvent, the mixture was poured
into the cold MeOH and filtered through a Soxhlet thimble.
The polymers were washed with MeOH and hexane via Soxh-
(
GPC) using a Polymer Laboratories GPC 220 in tetrahydro-
furan (THF) and polystyrene as the standard.
1
All chemicals were purchased from Aldrich and Acros and used
as they received without further purification. THF was dried
before use in benzophenone/Na system. Polymerization reac-
tions were carried out under argon atmosphere. 4,7-Dibromo-2-
let and recovered with DCM. The H NMR and GPC data of
the polymers are listed below.
Poly[2-(5-(2-decyl-2H-benzo[d][1,2,3]triazol-4-yl)thiophen-2-yl)-
3
3
decyl-2H-benzo[d][1,2,3]triazole,
5,4-d]thiazole, 2,5-bis(5-bromothiophen-2-yl)thiazolo[5,4-d]thia-
zole, 2,5-di(furan-2-yl)thiazolo[5,4-d]thiazole, and 2,5-bis(5-
2,5-di(thiophen-2-yl)thiazolo
5-(thiophen-2-yl)thiazolo[5,4-d]thiazole] (BTzTh): M
w
: 17,889 g
: 2.2. H NMR (400 MHz,
, d: 7.91 (benzotriazole), 7.77–7.32 (thiophene), 4.70
(NACH ), 2.11 (CH ), 1.50–1.22 (alkyl chain), 0.80 (CH ).
2
1
21
1
[
mol ; M : 8288 g mol ; M /M
n w n
CDCl
3
14,29
bromofuran-2-yl)thiazolo[5,4-d]thiazole
according to the literature procedures.
were synthesized
2
2
3
Poly[2-(5-(2-decyl-2H-benzo[d][1,2,3]triazol-4-yl)furan-2-yl)-5-
furan-2-yl)thiazolo[5,4-d]thiazole] (BTzFr): 5962
mol ; M : 1516 g mol ; M /M : 3.9. H NMR (400 MHz,
(
M
w
:
g
Synthesis of 2,5-Bis(5-(tributylstannyl)thiophen-2-
yl)thiazolo[5,4-d]thiazole
21
21
1
n
w
n
CDCl3, d: 7.79 (benzotriazole), 7.30–6.84 (furan), 4.71
NACH ), 2.10 (CH ), 1.30–1.12 (alkyl chain), 0.79 (CH ).
This monomer was synthesized using a modified procedure
(
1
9
2
2
3
excerpted from the literature. 2,5-Bis(5-bromothiophen-2-
yl)thiazolo[5,4-d]thiazole (500 mg, 1.08 mmol) were dis-
solved in dry THF (50 mL) under argon atmosphere. After
RESULTS AND DISCUSSION
ꢀ
adjusting the temperature to 278 C, 2.5 M solution of n-
Monomer and Copolymer Synthesis
BuLi in hexane (1.07 mL, 2.68 mmol) were added dropwise
To synthesize the thiophene and furan-containing thiazolothia-
zole building blocks, corresponding aldehyde was condensed
with dithiooxamide in DMF. After bromination with NBS stan-
nylation reactions were performed using a strong base (n-BuLi)
in an inert atmosphere. To obtain solution processable copoly-
mers alkylated dibromobenzotriazole derivative coupled with
distannylated TTz derivatives in dry toluene. Reaction times
were kept as 36 h for both copolymers. After precipitating in
and stirred for 2 h. SnBu Cl (3.21 mL, 2.62 mmol) were
3
added in one portion to the mixture under same conditions,
and the mixture was warmed up to the room temperature.
Ethyl acetate (50 mL) and water (50 mL) were added.
Organic layer was separated and washed with water several
times and dried with MgSO . After removing the solvent, the
4
residue was purified via column chromatography over Al2O3
3
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TABLE 1 Electrochemical and Electrochromic Properties of Thi-
ophene and Furan Bridged Thiazolothiazole Containing
Copolymers
BTzTh
BTzFr
doped, ox
E
E
E
(V)
1.55
1.00
1.30
1.10
dedoped, ox
(V)
doped, ox
(onset) V
1.00
0.95
HOMO (eV)
25.32
25.27
a
LUMO (eV)
op
23.62
23.37
E
g
1.7
1.9
max
k
(nm)
460/630/1,230
10/40/65
2.5/0.5/0.5
440/700/1,280
–/35/25
–/0.6/1.8
Optical contrasts (%)
Switching times (s)
FIGURE 1 Cyclic voltammograms of the copolymers (a) BTzTh
a
LUMO energy levels calculated from subtracted optical band-gap val-
ues from HOMO levels.
and (b) BTzFr.
methanol, copolymers were purified by Soxhlet extraction. Both
copolymers exhibited good solubility in common organic sol-
vents, such as chloroform, dichloromethane, and toluene
are listed. HOMO levels are given relative to the vacuum
3
4
level, considering that the SCE is 4.7 eV versus vacuum
(Scheme 1).
3
5
and Fc/Fc1 is 0.35 V versus SCE.
Polymer Electrochemistry
All electrochemical experiments were performed in same
electrolyte solution (0.1 M TBAPF /ACN). A reversible redox
To investigate the redox potentials of the copolymers with
different repeat units, cyclic voltammetry (CV) was per-
formed. The spray-cast thin films on ITO slides were pre-
pared from the DCM solutions (5 mg mL ) to use as the
working electrode in combination with a platinum wire as
the counter and an Ag wire as the pseudo reference. Figure
6
couple was observed for copolymer BTzTh at 1.55 and 1.10
V. Furan incorporated copolymer showed a lower redox cou-
ple versus the same reference electrode at 1.30 and 1.10 V
in anodic region. As given in the literature, furan substitution
into a polymer backbone compare to thiophene, reduces the
oxidation potential since oxygen-containing furan is richer in
terms of electrons. In other words, when oxygen-containing
heterocycles used as the donor group in a polymer chain
2
1
1
shows a set of cyclic voltammograms performed in the
potential range between 0.0 and 1.6 V for BTzTh and BTzFr
at a scan rate of 100 mV s . In Table 1, electrochemical
results and HOMO/LUMO energy levels of the copolymers
2
1
3
6
they reduce the HOMO energy level.
SCHEME 1 Synthetic route of the copolymers BTzTh and BTzFr [(i) dithiooxamide, DMF, reflux, 5 h; (ii) NBS, DMF, reflux, 1 h;
iii) n-BuLi, tributyltin chloride, THF; (iv) Pd(PPh3)4, toluene, reflux, 36 h].
(
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HOMO levels of the BTzTh and BTzFr were calculated from
the onset oxidation potentials; 1.00 and 0.95 eV, respectively.
Upon replacing thiophene with furan moiety, a smaller E
onset value and a more destabilized HOMO level were found.
LUMO levels were also calculated subtracting the optical
band-gap values from the HOMO energy levels of the copoly-
mers (Table 1).
As shown in Figure 2(a), intensities of the newly generated
bands increased as the p–p band of BTzTh depleted. While
oxidizing the polymer BTzTh, three additional intermediate
film colors were observed at potentials 0.9, 1.2, and 1.3 V.
Upon fully oxidation a dark blue color was observed at 1.5 V.
However, for complete oxidation of BTzFr [Fig. 2(b)] a green
color was observed at 1.2 V. As known, to observe the green
color, presence of two bands is a must in the regions around
ox
Spectroelectrochemistry and Colorimetry
400 and 700 nm. During stepwise oxidation, the polaron
To figure out the relation between the redox processes and
UV–vis–NIR absorptions, spectroelectrochemistry experi-
ments were performed in the same electrolytic medium
band revealed at 700 nm of the BTzFr, and the intensity of
the neutral absorption band did not decrease. At potentials
1.0 and 1.1 V, two additional colors, which are different
(
TBAPF /ACN) with the same electrodes (ITO: working, plat-
6
shades of neutral and fully oxidized color tones, were deter-
mined. The more variety of the polymer film colors of BTzTh
than BTzFr may be explained as the polaron band of the
thiophene-containing polymer has a wider absorption range
in the visible region than that of furan containing polymer.
Bipolaron band of the both copolymers were generated in
the low energy region with a maximum peak values at 1230
and 1280 nm for BTzTh and BTzFr, respectively.
inum: counter and Ag wire: pseudo-reference) for both
copolymers. Thin films (160 nm) of the copolymers were
prepared utilizing the same procedure used for electrochem-
21
istry (5 mg mL in CHCl ). All copolymers showed multie-
3
lectrochromic nature since the absorption bands of oxidized
states lie mostly in visible region.
In Figure 2, a full characterization of all bands was shown
for related states via oxidation for both copolymers. UV–vis–
NIR spectra for polymer films were obtained first in their
fully reduced states at 0.0 V, which show different hues of
orange. Color discrepancies of the copolymers were clearly
characterized during sequentially stepping to higher oxidiz-
ing potentials with 0.1 V increments, which lead to the gen-
eration of polaronic and bipolaronic bands at longer
wavelengths. Because of intramolecular DA electronic inter-
actions of the neutral polymers, they have a single broad
absorption bands with a maximum peak value of 460 and
The optical band gaps of the copolymers were calculated
from their onset absorption wavelengths of the p–p* transi-
tions. A smaller band-gap value for thiophene containing
polymer (1.7 eV) was calculated than that of BTzFr (1.9 eV),
which are compatible with the electrochemical results.
According to both the optical and the electrochemical results,
both copolymers are promising candidates for the optoelec-
tronic device applications in organic solar cells. With the
help of optical band-gap values of the copolymers, LUMO
energy levels were calculated and reported in Table 1.
440 nm for BTzTh and BTzFr, respectively. BTzTh has a 20-
nm redshifted broad absorption band than BTzFr. This may
be attributed to thiophene having a wider absorption band
compare to furan and also the longer chain length of
BTzTh than BTzFr. As the polaron bands of both copolymers
were in the visible region, with a max value of 630 nm
To report the polymer film colors (Fig. 3) quantitatively in a
scientific manner, colorimetry studies were performed to
define three attributes “L” (brightness), and two color com-
ponents “a” (the axis extends from 2a: green to 1a: red)
and “b” (the axis extends from 2b: blue to 1b: yellow) as
3
7
0
(
BTzTh) and the 700 nm (BTzFr), they have a greater effect
defined by Commission Internationale de l’e Clairage (CIE).
on the colors of the polymers at oxidized states. In both
cases, the distinct increase in the intensities of the absorp-
tions was started to be seen beyond their E onset poten-
Neutral polymer film colors were given as L: 50.334, a:
40.684, and b: 61.332 for BTzTh and L: 60.516, a: 41.501,
and b: 101.426 for BTzFr. Upon oxidation of the BTzTh film,
color turns into blue with b: 230.056. During oxidation of
ox
tials as determined from CV.
FIGURE 2 Electronic absorption spectra of (a) BTzTh and (b) BTzFr in TBAPF6/ACN solution.
3
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FIGURE 3 L, a, and b values of the polymer films at different applied voltages (a) BTzTh and (b) BTzFr.
the BTzFr, a component had a more negative value, which
specifies the green region. L, a, and b components of all
states of the films of both copolymers were listed in Table 2.
chronoamperometry studies were performed between their
neutral and fully oxidized states using a UV–vis–NIR spectro-
photometer at their local maximum absorption wavelengths
(
Fig. 4). Experiments were carried out for BTzTh at its all
Switching Studies
absorption regions whereas for the BTzFr only in its polar-
onic and bipolaronic regions due to almost no change in the
p–p* absorption intensity. Switching times, listed in Table 1,
were calculated taking 95% of a full transition of neutral
fully doped states.
As explained in the spectroelectrochemistry section, both
copolymers showed multichromicity with prominent color
changes at different applied potentials. To define the switch-
ing speeds of the polymer films, double potential step
To evaluate the kinetic properties of the BTzTh and BTzFr,
polymer films were first spray-casted on ITO-coated glass
slides. Thiophene containing polymer films were consecu-
tively stepped between neutral (0.0 V) and oxidized (1.5 V)
TABLE 2 List of L, a, and b Values for Polymer Films Obtained
from BTzTh and BTzFr
L
a
b
(
vs. Ag wire) in 5-s time intervals. At 460 nm, only 10%
optical contrast was determined with a switching speed of
.5 s. In longer wavelengths, a faster switching time (0.5 s)
BTzTh (0.0 V)
BTzTh (0.9 V)
BTzTh (1.2 V)
BTzTh (1.3 V)
BTzTh (1.5 V)
BTzFr (0.0 V)
BTzFr (1.0 V)
BTzFr (1.1 V)
BTzFr (1.2 V)
50.334
38.199
38.199
43.360
38.021
60.516
58.696
69.184
66.213
40.684
19.435
61.332
65.215
19.262
9.427
2
20.631
211.234
28.762
41.561
was observed at both 630 and 1230 nm with increased opti-
cal contrast values of 40 and 65%, respectively. This result
shows charge transport is easier in its lower energy bands.
For the furan-containing polymer, smaller optical contrast
values (35% at 700 nm, 25% at 1280 nm) were observed
than the thiophene containing one. These results are also
consistent with their electrochemical studies. The charge
loaded (area under the curve of oxidative doping) on the
230.056
101.426
76.532
47.735
45.590
26.017
22.969
212.543
FIGURE 4 Double potential step chronoamperometry studies with 5-s time intervals at local absorption maximums (a) BTzTh (0.0
and 1.5 V) and (b) BTzFr (0.0 and 1.4 V).
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furan-containing polymer is lower with respect to the thio-
phene containing copolymer.
13 Q. Shi, H. Fan, Y. Liu, J. Chen, Z. Shuai, W. Hu, Y. Li, X.
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3
8
1
4 S. Ando, J. Nishida, E. Fujiwara, H. Tada, Y. Inoue, S.
All switching times and percent optical transmittance differ-
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1
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synthesized containing benzotriazole and thiazolothiazole
unit in the main chain via Stille coupling polymerization by
altering the thiophene and furan moieties. Electrochemical
and electrochromic properties of the copolymers were
reported. They showed low oxidation potentials and low
optical band-gap values. With the help of the spectroelectro-
chemistry technique, we defined the redox affected color
change of the polymer films. Copolymers showed multichro-
mic properties in a small potential range. It is known that
consequential variation in electrochromic performance can
be rationalized with small structural modifications in poly-
mer chain. As having suitable energy levels and multichromic
properties, electrochromic and solar cell application of a
branched chain-introduced BTzTh derivative is an issue in
our ongoing research.
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