Donor-acceptor-donor type conjugated polymers for electrochromic applications: Benzimidazole as the acceptor unit

Article abstract of DOI:10.1016/j.polymer.2010.10.045

Donor-acceptor-donor (DAD) type benzimidazole (BIm) and 3,4 ethylenedioxythiophene (EDOT) bearing monomers were synthesized and electrochemically polymerized. Pendant group at 2-C position of the imidazole ring was functionalized with phenyl (P1), EDOT (P

Full text of DOI:10.1016/j.polymer.2010.10.045

Polymer 51 (2010) 6123e6131
Contents lists available at ScienceDirect
Polymer
journal homepage: www.elsevier.com/locate/polymer
Donoreacceptoredonor type conjugated polymers for electrochromic
applications: benzimidazole as the acceptor unit
,b,c,
Hava Akpınar a, Abidin Balan a, Derya Baran a, Elif Köse Ünvera, Levent Toppare a
*
a Department of Chemistry, Middle East Technical University, 06531 Ankara, Turkey
b Department of Biotechnology, Middle East Technical University, 06531 Ankara, Turkey
c Department of Polymer Science and Technology, Middle East Technical University, 06531 Ankara, Turkey
a r t i c l e i n f o
a b s t r a c t
Article history:
Donoreacceptoredonor (DAD) type benzimidazole (BIm) and 3,4 ethylenedioxythiophene (EDOT)
bearing monomers were synthesized and electrochemically polymerized. Pendant group at 2-C position
of the imidazole ring was functionalized with phenyl (P1), EDOT (P2) and ferrocene (P3) in order to
observe substituent effect on electrochemical and electrochromic properties of corresponding polymers.
Spectroelectrochemical results showed that different pendant groups resulted in polymers with slightly
different optical band gaps (1.75, 1.69 and 1.77 eV respectively) and different number of achievable
Received 28 July 2010
Received in revised form
17 October 2010
Accepted 22 October 2010
Available online 30 October 2010
colored states. Optoelectronic performance and comparison of results with other well known
accepting benzazole bearing DAD type polymers were reported in detail.
p-
Keywords:
Benzimidazole
Donor-acceptor approach
Electrochromism
Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction
donoreacceptoredonor (DAD) method can be considered as the
most utilized one among all [7].
Electrically conducting, conjugated polymers (CPs) have been
attracting great deal of interest due to their suitability for numerous
applications including photovoltaics (PVs) [1], field effect transis-
tors (FETs) [2], light emitting diodes (LEDs) [3] and electrochromic
devices (ECDs) [4]. The use of CPs as materials which possess the
ability to change color reversibly by altering redox state has a fast
receiving attention. Synthetic availability of conducting polymers
allows fine tuning in electronic and optical properties [5]. The
great majority of CPs are colored in their neutral states since the
energy difference between their HOMO and LUMO levels, termed
as band gap (Eg), lies within the visible region. Through charge
carrier formation on polymer backbone upon oxidation, the
The DA type polymers consisting of alternating electron donors
and acceptors generally have lower band gaps and wider band-
widths than either of the corresponding homo-polymers when D
and A moieties match appropriately [8]. The basic idea here is the
contributions of donor and acceptor heterocycles to HOMO and
LUMO energy levels of the polymer [9]. Thus, mainly the polymer is
expected to have an ionization potential (IP) closer to the donor and
an electron affinity (EA) closer to the acceptor [10]. DAD type
polymers thus, can be classified as copolymers made up with donor
and acceptor units and match between these units designate the
resulting polymer’s properties. Comprehensive reports of Yama-
moto et al. on n-type
p-conjugated units, benzimidazole (BIm),
intensity of the
pe
p* transition decreases, and low-energy transi-
benzothiadiazole (BTd), benzotriazole (BTz) and benzoselenadia-
zole (BSe) which are potentially acceptor units, showed that their
electrochemical and optical characteristics are slightly different
since they possess different atoms on their isoelectronic structures
[11]. These heterocyclic benzazole derivatives exhibit high electron
transporting ability because of the electron withdrawing imine (C]
N) bonds on their backbones.
tions emerge to produce a second colored or a transmissive state.
By all means, band gap energy of these polymers is dominant
among factors affecting optical and electronic properties [6]. There
are plenty of synthetic routes for adjusting the band gap however,
Previously reported DAD type polymers obtained by coupling
benzazoles with electron donating EDOT resulted in fine tuned
electrochromic properties. BTd derivative DAD polymer was
synthesized [12] and later it was shown to be the first green to
transmissive electrochromic polymer [13]. BSe based polymer was
* Corresponding author. Middle East Technical University Department of Chemistry,
Department of Biotechnology, Department of Polymer Science and Technology,
Turkey. Tel.: þ90 312 2103251; fax: þ90 312 2101280.
E-mail address: toppare@metu.edu.tr (L. Toppare).
0032-3861/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.polymer.2010.10.045

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H. Akpınar et al. / Polymer 51 (2010) 6123e6131
also investigated as a green to transmissive polymer with red shifted
absorptions compared to BTd derivative [14]. Recently, introduction
ofBTzunitsonPEDOTresultedintheenhancementofelectrochromic
properties such as optical contrast, switching time and coloration
efficiencycompare toparentpolymerPEDOT[15]. Although, BTd, BSe
and BTz based polymers and their different copolymers have been
studied extensively in literature, DAD type electrochromic polymers
with BIm as the acceptor unit are still unexplored up to date. Instead,
benzimidazoles which are an important molecule in vitamin B12
have been widely used in biological applications. It was shown that
they have antibacterial, virucidal and antitumor properties [16].
Polybenzimidazoles on the other hand showed great thermal
stability and resistance to high temperatures [17].
In this report, we highlight the synthesis and optoelectronic
properties of BIm based DAD type polymers containing EDOT as the
donor unit. Substituent effect on electrochemical and optical
properties was investigated. Substitution of benzene and EDOT
units on 2-C position of BIm resulted in two homologue polymers.
Additionally, ferrocene (Fc) incorporation as pendant group was
expected to reveal different redox behaviors due to its reversible
and well defined electrochemical signal. Electrochromic properties
of polymers were also compared with their different acceptor unit
analogues where EDOT is the donor unit, to have a better under-
standing on acceptor unit effect.
2.2.2. 4-(2,3-Dihydrothieno[3,4-b][1,4]dioxin-5-yl)-7-(2,3-
dihydrothieno[3,4-b][1,4]dioxin-7-yl)-2-benzyl-1H-benzo[d]
imidazole (M1)
In a three necked round-bottom flask fitted with a condenser
and argon inlet, 4,7-dibromo-2-phenyl-1H-benzo[d]imidazole
(100 mg, 0.28 mmol) and tributyl(2,3-dihydrothieno[3,4-b][1,4]
dioxin-7-yl)stannane (490 mg, 1.14 mmol) were dissolved in 25 ml
of dry THF. After 30 min stirring under argon flow, dichlorobis
(triphenyl phosphine)palladium(II) (30 mg, 0.027 mmol) was
added at room temperature. The mixture was left to reflux for 18 h.
Solvent was evaporated under vacuum and the crude product was
purified by column chromatography over silica gel 1:2 (ethyl-
acetate:hexane) to obtain M1 as yellow solid (78 mg, yield: 58.7%).
1H (400 MHz, CDCl3,
(m, 3H), 7.42 (d, 1H), 6.52 (s, 1H), 6.44 (s, 1H), 4.48 (d, 2H), 4.38 (d,
4H), 4.29 (d, 2H). 13C NMR (100 MHz, CDCl3,
): 150.4, 142.0, 141.5,
d): 10.61 (s, 1H), 8.09 (d, 2H), 8.05 (d, 1H), 7.50
d
141.1, 139.5, 135.9, 131.5, 130.2, 129.9, 129.0, 128.2, 126.4, 125.3,
123.9, 121.3, 120.7, 116.7, 114.6, 114.5, 100.9, 99.2, 65.6, 64.9, 64.5,
64.4. MS (m/z): 475 [Mþ].
2.2.3. 2,4-Bis(2,3-dihydrothieno[3,4-b][1,4]dioxin-5-yl)-7-(2,3-
dihydrothieno[3,4-b][1,4]dioxin-7-yl)-1H-benzo[d]imidazole (M2)
This monomer was prepared with the same procedure
described for M1 using 4,7-dibromo-2-(2,3-dihydrothieno[3,4-b]
[1,4]dioxin-5-yl)-1H-benzo[d]imidazole (200 mg, 0.48 mmol),
tributyl(2,3-dihydrothieno[3,4-b][1,4]dioxin-7-yl)stannane
2. Experimental section
(829.1 mg, 1.92 mmol), and dichlorobis(triphenyl phosphine)
palladium(II) (50 mg, 0.045 mmol) in 35 ml of dry THF. After
removing the solvent on rotary evaporator, the residue was sub-
jected to column chromatography on silica gel, eluting with 1:1
(ethyl acetate:hexane) to obtain M2 as yellow solid (156 mg, yield:
2.1. General
All chemicals were purchased from commercial sources and
used without further purification. All reactions were carried out
under argon atmosphere unless otherwise mentioned. 4,7-
Dibromobenzothiadiazole [18], 3,6-dibromobenzene-1,2-diamine
[19], tributyl(2,3-dihydrothieno[3,4-b][1,4]dioxin-7-yl)stannane
[20], 2,3-dihydrothieno[3,4-b][1,4]dioxine-5-carbaldehyde [7],
4,7-dibromo-2-phenyl-1H-benzo[d]imidazole [11a] and 4,7-
dibromo-2-(2,3-dihydrothieno[3,4-b][1,4]dioxin-5-yl)-1H-benzo
[d]imidazole [11a] were synthesized according to previously
published procedures. Electrochemical studies were performed in
a three-electrode cell consisting of an Indium Tin Oxide doped
glass slide (ITO) as the working electrode, platinum wire as the
counter electrode, and Ag wire as the pseudo reference electrode
under ambient conditions using a Voltalab 50 potentiostat. 1H and
13C NMR spectra were recorded in CDCl3 on Bruker Spectrospin
Avance DPX-400 Spectrometer. Chemical shifts were given in ppm
downfield from tetramethylsilane. Varian Cary 5000 UVeVis
spectrophotometer was used to perform the spectroelec-
trochemical studies of polymers. Mass analysis was carried out on
a Bruker time-of flight (TOF) mass spectrometer with an electron
impact ionization source.
60.3%).1H (400 MHz, CDCl3,
d
): 10.86 (s, 1H), 8.02 (d, 1H), 7.38 (d,
1H), 6.50 (s, 1H), 6.49 (s, 1H), 6.43 (s, 1H), 4.36 (m, 12H). 13C NMR
(100 MHz, CDCl3, ): 145.2, 143.9, 140.9, 140.3, 139.7, 139.2, 135.5,
d
129.1, 127.3, 124.6, 120.5, 119.7, 114.9, 114.1, 110.3, 107.9, 101.9, 101.3,
98.7, 64.7, 64.6, 64.3, 63.3, 63.2. MS (m/z): 539 [Mþ].
2.2.4. 4-(2,3-Dihydrothieno[3,4-b][1,4]dioxin-5-yl)-7-(2,3-
dihydrothieno[3,4-b][1,4]dioxin-7-yl)-2-ferrocenyl-1H-benzo[d]
imidazole (M3)
This compound was also prepared with the same procedure
described above, using 4,7-dibromo-2-ferrocenyl-1H-benzo[d]
imidazole (500 mg, 1.08 mmol), tributyl(2,3-dihydrothieno[3,4-b]
[1,4]dioxin-7-yl)stannane (829.1 mg, 1.92 mmol), and dichlorobis
(triphenyl phosphine)palladium(II) (70 mg, 0.064 mmol) in 50 ml
of dry THF. In this case completion of the reaction was achieved by
an additional 30 h of reflux as monitored by TLC. Solvent was
removed under vacuum and the crude product was purified by
column chromatography over silica gel 1:3 (ethylacetate:hexane) to
obtain M3 as orange solid (430 mg, yield: 68%). 1H (400 MHz,
DMSO-d6,
(s, 1H), 5.22 (t, 2H), 4.48 (t, 2H), 4.39 (m, 4H), 4.28 (m, 4H), 4.14
(s, 5H). 13C NMR (100 MHz, DMSO-d6,
): 152.7, 142.2, 141.3, 140.1,
d): 11.69 (s, 1H), 7.92 (d,1H), 7.09 (d,1H), 6.75 (s,1H), 6.69
2.2. Synthesis
d
139.1, 138.4, 132.4, 124.3, 122.0, 121.5, 119.1, 113.8, 112.6, 100.7, 98.8,
79.3, 79.1, 78.9, 78.6, 74.2, 69.8, 69.4, 67.9, 67.8, 64.8, 64.6, 64.2, 64.0,
56.0. MS (m/z): 583 [Mþ].
2.2.1. 4,7-Dibromo-2-ferrocenyl-1H-benzo[d]imidazole (3c)
3,6-Dibromobenzene-1,2-diamine (500 mg, 1.88 mmol) and fer-
rocenecarboxaldehyde (483 mg, 2.41 mmol) were dissolved in 30 ml
methanol (MeOH). After clear solution was obtained, 20 mg iodine
were added to the mixture and stirred at room temperature over-
night. Subsequent precipitate was filtered and washed with cold
MeOH(3ꢀ 50ml). The product wasobtained asorangesolid (530mg,
yield: 77%) after re-crystallization in MeOH. 1H (400 MHz, DMSO-d6,
3. Results and discussions
3.1. Synthesis
Synthesis of electroactive donoreacceptoredonor (DAD) type
monomers was achieved as shown in Scheme 1. Bromination of
benzothiadiazole was performed with Br2 in HBr to give the 4,7-
dibromobenzothiadiazole (2) in very high yields (98%) [18].
d
): 12.80 (s,1H), 7.30 (m, 2H), 5.26 (t, 2H), 4.53 (t, 2H), 4.15 (s, 5H). 13
NMR (100 MHz, DMSO-d6, ): 155.4, 142.9, 134.8, 133.3, 132.1, 125.4,
125.3, 110.1, 105.6, 101.7, 101.6, 99.5, 79.1, 72.8, 70.3, 69.5, 68.0.
C
d

H. Akpınar et al. / Polymer 51 (2010) 6123e6131
6125
Scheme 1. Synthetic route and chemical structures for monomers M1, M2 and M3.

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H. Akpınar et al. / Polymer 51 (2010) 6123e6131
Fig. 1. Electrochemical polymerizations of monomers (left) and scan rate dependence of corresponding polymers (right); a) M1 and P1, b) M2 and P2, and c) M3 and P3.
Subsequent reduction of 2 was achieved using excess amount of
NaBH4 in ethanol (EtOH) [19]. In order to observe pendant group
effect on optoelectronic properties different aromatic aldehydes
were condensed with 3,6-dibromobenzene-1,2-diamine (3).
Commercially available benzaldehyde and easily synthesized 2,3-
dihydrothieno[3,4-b][1,4]dioxine-5-carbaldehyde [7] were used in
condensation reaction with catalytic amount of p-toluenesulfonic
acid(pTSA)inEtOHtogivecorrespondingbrominatedbenzimidazole
benzo[d]imidazole (3b) [11a]. Ferrocene functionalized benzimid-
azole (3c) was synthesized via different method with a good yield
(77%). 3a, 3b and 3c were then coupled with tributyl(2,3-dihy-
drothieno[3,4-b][1,4]dioxin-7-yl)stannane [20] in the presence of Pd
(PPh3)Cl2 as the catalyst in THF to give M1, M2 and M3, respectively.
3.2. Electrochemistry
derivatives
4,7-dibromo-2-phenyl-1H-benzo[d]imidazole
(3a)
M1, M2 and M3 were polymerized potentiodynamically on ITO
and 4,7-dibromo-2-(2,3-dihydrothieno[3,4-b][1,4]dioxin-5-yl)-1H-
coated glass slides in a mixture of dichloromethane (DCM) and

H. Akpınar et al. / Polymer 51 (2010) 6123e6131
6127
acetonitrile (ACN) (5/95, v/v) using 0.1 M tetrabutylammonium
hexafluorophosphate (TBAPF6) supporting electrolyte with
repeated scan intervals ꢁ300 mV/1200 mV, ꢁ300 mV/900 mV, and
ꢁ500 mV/1250 mV; respectively (Fig. 1). In the first cycle of vol-
tammograms, irreversible monomer oxidation peaks at 0.82 V (M1)
and 0.80 V (M2) appeared and reversible polymer redox peaks for
P1 were centered at 0.56 V and 0.38 V versus Ag wire pseudo
reference electrode. Polymer p-doping/dedoping potentials were
decreased for P2 (0.51 and 0.33 V) as benzene was replaced by
electron rich 3,4-ethylenedioxy thiophene (EDOT) unit (Fig. 1).
During stepwise oxidation of M3, first peak for Fc/Fcþ oxidation was
seen at 0.65 V and the monomer oxidation peak was appeared at
0.96 V. After monomer oxidation, a reduction appeared at 0.6 V.
This reduction leads to electrode surface passivation hampering to
observe the ferrocene reduction peak. Nonetheless, after repetitive
cycles of electropolymerization Fcþ/Fc reduction peak became
visible (as overlapped with the polymer reduction peak) since
polymer reduction peak shifted to less positive potentials. Big
difference between doping/dedoping (1.06/0.40 V) potentials for P3
confirms a multi-electron redox process due to electroactive and
bulky ferrocenyl substituents on BIm units. These bulky groups also
affect the insertion of the electrolyte anion to the polymer film and
the rate of quinoid to benzenoid transformation.
Among electron accepting benzazoles (BTd, BIm, BTz and BSe),
BSe was shown to be the best electron accepting unit. Although it is
expected to be the opposite, replacement of sulphur atom by
selenium results in a better electron donating character among the
donor units (thiophene, selenophene) [21]. Also the electron
accepting capacity of BSe is better than that of BTd. This trend can
be explained by the large polarizability and the electrochemical
amphotericity of selenium atom as stated by Yamamoto [11b] and
Suzuki [19]. In addition, the difference between CeXeC and
NeXeN bonds may play an important role (X ¼ Se or S) as well.
Since all above mentioned acceptor units differ by the atoms at 2-
positions (S, Se, C, N), electrochemical properties of the DAD
monomers are affected by the polarizability differences between
these atoms. In this context, BIm unit would be expected to have
electron accepting ability between BTd and BTz since it possesses
a C atom. However, since the DAD type polymers are the so called
copolymers, donor acceptor match is the point of importance to
achieve dual character with enhanced electronic and optical
properties. Thus, the comparison should be related to the resulting
Table 1
Chemical structures and optoelectronic properties of polymers containing different benzazoles as the acceptor units.
Em,a (V)
Ep,a (V)
Ep,c (V)
lmax (nm)
Eg (eV)a
Colorsb
Optical Contrasts
Switching
Times
Ref.
0.56
0.38
338
580
44.5% (580 nm)
75.3% (1800 nm)
0.4 s (580 nm)
0.6 s (1800 nm)
0.82
1.75 (580 nm)
Blue/Transparent Sky Blue
c
0.51
0.33
360
560
44.5% (560 nm)
71.4% (1800 nm)
0.4 s (560 nm)
0.5 s (1800 nm)
0.80
1.69 (560 nm)
Blue/Transparent Light Gray
c
1.06
0.40
332
513
10.0% (513 nm)
36.0% (1370 nm)
2.5 s (513 nm)
2.0 s (1800 nm)
0.96
0.97
1.77 (513 nm)
1.60 (618 nm)
Purple/Blue
c
0.23
ꢁ0.04
53%-618 nm
71%-1600 nm
618
Blue/Transparent
1.1 s (618 nm)
[15]
ꢁ0.06
ꢁ0.26
428
755
23%-755 nm
72%-1500 nm
0.4 s (755 nm)
∼1.0s (1500 nm)
0.95
0.85
1.19 (755 nm)
1.13 (796 nm)
Green/Transparent Sky Blue
Green/Transparent Sky Blue
[13]
[14]
343
448
796
ꢁ0.09
ꢁ1.24
27.3% (448 nm)
10.9% (796 nm)
2.1 s (448 nm)
1.7 s (796 nm)
a
b
c
Optical band gaps which were calculated from the onset of
Representing colors for neutral/oxidized states.
This work.
pe
p* transitions in UV.

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H. Akpınar et al. / Polymer 51 (2010) 6123e6131
Fig. 2. Electronic absorption spectra of P1 film in 0.1 M TABPF6/ACN solution between ꢁ0.5 V and 1.0 V with 0.05 V potential intervals.
DAD polymers rather than the corresponding homo-polymers. In
the series of benzazole bearing polymers (Table 1), only BIm based
polymers were not n-dopable which confirms a poor match
between BIm and EDOT unit in the context of DA approach.
The difference between the oxidation potentials of M1, M2 and
M3 can not be simply explained by the electron density and the
electron donating capacity of the subtituents, because all the three
substituents are expected to affect the planarity of the imidazole
ring differently. Corresponding polymers, P1, P2 and P3 were
subjected to scan rate alternation experiments in order to deter-
mine diffusion dependencies of thin films on the working elec-
trodes (ITO). Linear relationships between scan rates and current
intensities showed that all polymers were well adhered on ITO
surface and the electrochemical processes were not limited by
diffusion control [22].
3.3. Spectroelectrochemistry
Spectroelectrochemical studies of the polymers were performed
in order to examine their optical and structural responses upon
doping process which proves the evolution of the charge carries in
the structure. UVeviseNIR spectra for electrochemically coated
polymer films acquired during stepwise oxidation in a monomer
free, 0.1 M TBAPF6/ACN solution.
Although BIm based polymers exhibited two transitions due to
their donor-acceptor nature, the lowest energy transitions affected
the colors of neutral films dominantly since other transitions were
not in the visible region. The two transitions in DAD type polymers
are attributed to the transitions from the thiophene-based valence
band to its antibonding counterpart (high-energy transition) and to
the substituent-localized conduction band (low-energy transition)
Fig. 3. Electronic absorption spectra of P2 film in 0.1 M TABPF6/ACN solution between ꢁ0.5 V and 1.0 V with 0.05 V potential intervals.

H. Akpınar et al. / Polymer 51 (2010) 6123e6131
6129
Fig. 4. Electronic absorption spectra of P3 film in 0.1 M TABPF6/ACN solution between 0.0 V and 1.0 V with 0.05 V potential intervals.
hence, interactions between donor and acceptor units (their match)
determine the energy and intensity of these transitions. This
phenomenon is in agreement with the blue shifts and decreasing
intensity for series of polymers with decreasing acceptor strengths.
Neutral state absorption maxima for polymers were recorded as
580 nm for P1, 560 nm for P2 and 513 nm for P3. These absorptions
were also all blue shifted compared to other benzazole and EDOT
based polymers. This can be attributed to the non-decreased band
gap due to poor donor acceptor match between EDOT and BIm
which was also the case in BTz. However, for polymers with
stronger acceptors BTd and BSe have energy transitions at around
400 and 700 nm which resulted in green neutral polymers whereas
P1 and P2 were blue and P3 was purple in their neutral states.
Optical band gaps of the polymers were calculated as 1.75, 1.69 and
1.77 eV for P1, P2 and P3 respectively from the onset of
transitions in the visible region.
pe
p*
Upon oxidation of polymers (P1, P2 and P3), formation of charge
carriers such as polarons and bipolarons [23] led to new absorption
bands in NIR while absorptions for the neutral states were
decreasing. In-situ spectroelectrochemical studies for the polymer
films showed that the color of the film changed from saturated blue
to highly transmissive sky-blue for P1. On the other hand, its EDOT
substituted homologue P2, revealed different shade and tones as
the transition colors since polaronic absorption bands tailed more
into the visible region. Although both P1 and P2 showed identical
polaronic absorptions, these bands decreased less with increasing
Fig. 5. Square wave potential step chronoapsorptometry studies of P1 and P2 monitored at 580 nm and 560 nm between ꢁ0.5 V and 1.0 V (vs. Ag wire). Switching time intervals:
5 s, 3 s and 1 s.

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H. Akpınar et al. / Polymer 51 (2010) 6123e6131
Table 2
3.4. Kinetic studies
Optical contrasts and switching abilities of polymers P1, P2 and P3.
Optical contrasts were monitored as the function of time at the
maximum absorption of P1 and P2 in order to characterize their
response times and switching abilities. Electrochemically poly-
merized films were subjected to chronoabsorptometry studies in
a monomer free solution containing 0.1 M TBAPF6/ACN. As depicted
in Fig. 5, both polymers P1 and P2 revealed optical contrast values
of ca. 45% at their short wavelength maximum absorptions at
580 nm and 560 nm. Although for P1 a variation in electrochromic
contrast of 4% was observed as the stepping interval was changed
from 5 to 1 s, the latter (P2) revealed the robustness and the
stability of the material applications like electrochromic devices
and optical displays. Table 2 summarizes the switching abilities for
P1 and P2 at their dominant wavelengths in visible and NIR regions.
As seen, it does not matter how fast the polymer films were
switched, the time required to switch between their neutral and
oxidized states did not change significantly which also can be an
evidence for the durability of the polymer films.
P2 did not exhibit a difference in its optical contrast values
(44.5% ꢂ 43.4%) even though the switching time was changed to
1 s. 75.3% and 71.4% optical contrast values were determined for
P1 and P2 at 1800 nm which are quite impressive and eminent
to use these materials can be used in NIR applications [26]
(Fig. 6). Among others, when switched between its neutral and
oxidized states, P3 showed trapezoidal pulses which resulted in
longer switching times (2.5 s) and lower contrast values (10% at
513 nm). This property may stem from the difficulties in charge
accumulation due to large differences between polymer chains
[27].
Switching intervals
Tred
%
Tox
%
D
T%
tox (s)
P1
P1
P2
P2
5 s
3 s
1 s
5 s
3 s
1 s
5 s
3 s
1 s
5 s
3 s
1 s
5 s
25.2
26.0
27.9
17.2
18.5
18.8
23.2
23.6
24.0
17.1
17.3
17.4
22.3
69.7
69.5
68.9
92.5
89.4
83.8
67.9
67.7
67.4
88.5
88.4
87.6
32.5
44.5
43.5
41.0
75.3
70.9
65.0
44.5
44.1
43.4
71.4
71.1
70.2
10.2
0.42
0.60
0.40
0.62
0.60
0.52
0.48
0.40
0.42
0.54
0.36
0.30
2.5
lmax: 580 nm
lmax: 1800 nm
lmax: 560 nm
lmax: 1800 nm
P3
P3
lmax: 513 nm
lmax: 1800 nm
5 s
41.2
79.4
38.2
2.0
potential for P2 than those of P1 due to simultaneous formation of
polaronic and bipolaronic charge carriers (Figs. 2 and 3). Addi-
tionally, isosbestic point at around 650 nm for P2 confirmed the
coexistence of more than one charged species absorbing in the
visible region on the polymer chain. These different species
contribute differently to the resulting color of the film in the sense
that as if one mixes two different colored polymers [24]. As their
absorption bands react differently upon stepwise oxidation,
multicolored states are achieved. It can also be generalized that
polymers having isosbestic points usually result in multicolored
achievable states [25].
Color of P3 in its different redox states were affected by the
bulky and visible absorber Fc pendant group. Oxidation of P3 film
resulted in a similar behavior to P1 and P2 at its early doping stages,
but later a new absorption band was emerged at 625 nm which
stands for polaronic absorption bands (770 nm) with increasing
bias due to characteristic blue color of Fcþ ions. This visible
absorption of Fcþ groups conduced with a blue colored oxidized
state for P3 film. Moreover, partial oxidation of P3 allowed detec-
tion of a gray colored transition state as a consequence of mixing
different colored states of main chain and Fc groups (Fig. 4).
Coloration efficiency (CE (ƞ)) is defined as the relationship
between the injected/ejected charge density (Qd) and the change
in optical density (difference between colored and bleached
states),
DOD, at a specific dominant wavelength (lmax). The desired
material or device should exhibit a large optical contrast giving
rise to large CEs. Using the related formula, coloration efficiencies
for P1 and P2 were calculated as 352 cm2/C and 325 cm2/C,
respectively. Coloration efficiency results for P3 were not calcu-
lated since transmissive (bleached) color in its oxidized form was
not achieved.
Fig. 6. Square wave potential step chronoapsorptometry studies of P1 and P2 monitored at 1800 nm between ꢁ0.5 V and 1.0 V (vs. Ag wire reference electrode). Switch intervals:
5 s, 3 s and 1 s.

H. Akpınar et al. / Polymer 51 (2010) 6123e6131
6131
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4. Conclusion
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Three DAD type monomers bearing electron deficient BIm units
as the acceptor and electron rich EDOT units as the donor groups
were synthesized. Their electrochemically synthesized polymers on
ITO coated glass slides were characterized in terms of their elec-
trochemical and optical properties. Electrochemical studies showed
that donor acceptor match between BIm and EDOT units is not
strong as was the case in the other benzazole bearing DAD type
polymers. However, spectroelectrochemical results proved their
great candidacy for electrochromic display device applications.
Altering the pendant group affected the electrochromic behavior of
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substituted polymer switched between the different shades of the
same color. Ferrocene group was also incorporated into polymer
chain as a pendant group and its states also contributed to the
colors of the polymer. Additionally ferrocene substitution might
improve polymers’ potential use for biochemical applications
which will be investigated in due course. Reported results for all
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type polymers. The synthesis of different DAD analogues with
different donor units to achieve better donor acceptor match will
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