Substituent and heteroatom effects on the electrochromic properties of similar systems

Article abstract of DOI:10.1002/pola.25047

Electrochromic polymers called poly(3,4-dihydro-3,3-bis ((naphthalen-1-yl)methyl)-2H-thieno[3,4-b][1,4]dioxepine) (PProDOT-Np ₂), poly(3,3-dibenzyl-3,4-dihydro-2H-selenopheno[3,4-b][1,4] dioxepine), and poly(3,3-dibenzyl-3,4-dihydro-2H-thieno[3

Full text of DOI:10.1002/pola.25047

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Substituent and Heteroatom Effects on the Electrochromic Properties
of Similar Systems
1,2
1
1
3
_
¨
¨
Merve Ic¸li-Ozkut, Jetmire Mersini, Ahmet M. Onal, Atilla Cihaner
1Department of Chemistry, Middle East Technical University, TR-06531 Ankara, Turkey
2Department of Chemistry, Yuzuncu Yıl University, 65080 Van, Turkey
¨
¨
¨
3Department of Chemical Engineering and Applied Chemistry, Atilim Optoelectronic Materials and Solar Energy Laboratory
(ATOMSEL), Atilim University, TR-06836 Ankara, Turkey
¨
Correspondence to: A. M. Onal (E-mail: aonal@metu.edu.tr) or A. Cihaner (E-mail: cihaner@atilim.edu.tr)
Received 11 August 2011; accepted 8 October 2011; published online 28 October 2011
DOI: 10.1002/pola.25047
ABSTRACT: Electrochromic polymers called poly(3,4-dihydro-
size (PProDOT-Np2) causes lower coloration efficiency (CE) as
3,3-bis ((naphthalen-1-yl)methyl)-2H-thieno[3,4-b][1,4]dioxepine)
well as lower optical contrast, the replacement of S atom by Se
(PProDOT-Np2),
poly(3,3-dibenzyl-3,4-dihydro-2H-selenopheno
atom resulted in a lower band gap polymer with a higher CE
C
V
[3,4-b][1,4]dioxepine), and poly(3,3-dibenzyl-3,4-dihydro-2H-thieno
[3,4-b][1,4]dioxepine) were synthesized electrochemically and the
effect of substituents and heteroatoms on the electrochromic
properties were investigated for the similar systems. All poly-
mers show electrochromism from a colored state when neutral-
ized to transmissive when oxidized. Although, increasing bulky
than its thiophene analog. 2011 Wiley Periodicals, Inc. J Polym
Sci Part A: Polym Chem 50: 615–621, 2012
KEYWORDS: conducting polymers; conjugated polymers; elec-
trochemistry; electrochromism; polyselenophenes; 3,4-propyle-
nedioxyselenophenes; ProDOS; redox polymers
INTRODUCTION Electrochromism is defined as a visible color
change under external applied potentials, which is one of the
most important properties of conjugated polymers and
thanks to easy synthesis and tunable intrinsic properties (i.e.,
electrical and optical), a lot of distinct and saturated colors
like blue,1–3 green,3–7 and black8–10 were reported in the neu-
tral state. These features make them promising candidates for
displays,11 smart windows,12,13 electrochromic devices,14–16
mirrors,17,18 and camouflage materials.19–21 Among organic
electrochromic polymers, polythiophenes and their alkylene-
dioxythiophene derivatives22 have attracted considerable
interest because of their multicolor changes in the same
structure, long cycle life, high optical contrast ratio, film form-
ing properties as well as fast switching time, and low opera-
tion voltage, when compared to inorganic materials and other
polymers. The optical and electrical properties of organic
electrochromic polymers are very crucial in determining their
use. Therefore, it is very important to control the optical and
electrical properties of the conjugated polymers and there
have been extensive research on this issue. Reynolds and co-
workers23 reported the effect of ring size and substituent on
electrochromic properties. They conjectured that increasing
the bridge size and rigidity of substituents on 3,4-alkylene-
doxythiophene-based polymers causes higher optical contrast
ratio (D%T), as the interchain separation improves the
charge/discharge ability of the counter anion during doping
process. On the basis of foregoing results, Kumar and co-
workers24 reported a new electrochrome based on dibenzyl
substituted poly(3,4-propylenedioxythiophene) (PProDOT-
Bz2) with 89% at kmax ¼ 632 nm (no information was given
about polymer thickness) of the highest optical contrast
reported to date. Because of the inspiration of the results and
the solubility problem, different aromatic and alkyl analogs
were synthesized by the same group25 and unfortunately, all
optical contrast values were found to be lower than 89%.
Interestingly, in the same study, the optical contrast of PPro-
DOT-Bz2 was given as 76%, which caused some questions in
mind about the optical contrast of PProDOT-Bz2. On the other
hand, Bendikov and coworkers recently reported a novel elec-
trochrome based on 3,4-ethylenedioxyselenophene. When
compared to poly(3,4-ethylenedioxythiophene), the replace-
ment of S atom by Se atom results in a lower band gap poly-
mer (PEDOS, poly(3,4-ethylenedioxythiophene), 1.4 eV) with
lower oxidation potential (1.4 V vs. Ag/AgCl) and somewhat
higher stability (it retained almost 83% of its electroactivity
after 5000 cycles).26 Unexpectedly, monohexyl substituted
PEDOS (poly(3,4-propylenedioxyselenophene)) polymer was
reported as the second polymer bearing the highest optical
contrast (89%) at 763 nm.27 It can be easily concluded that
the kind of heteroatom in the same system significantly
affects the intrinsic properties of the obtained polymer. Under
the light of these findings, recently, we reported blue to
Additional Supporting Information may be found in the online version of this article.
C
V
2011 Wiley Periodicals, Inc.
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SCHEME 1 Chemical structures of aryl and alkyl substituted ProDOT and ProDOS monomers: ProDOS-Bz2, ProDOT-Bz2, ProDOT-
Np2, 3,3-didecyl-3,4-dihydro-2H-thieno[3,4-b][1,4]dioxepine (ProDOT-C10), and ProDOS-C10
.
highly transmissive soluble electrochromes based on dialkyl
substituted poly(3,4-propylenedioxyselenophene)s (PPro-
DOSs).2,28 Although the contrast ratios of the polymers are
less than their thiophene analogs, they have lower band gaps,
higher coloration efficiencies, and higher redox stabilities.
sition bands completely diminished and the polymer films
exhibited different colors. PProDOS derivatives exhibit pure
blue color due to the absence of absorbance between 400
and 500 nm when neutralized and are transparent when oxi-
dized, whereas PProDOT derivatives, except for PProDOT-
Np2, have dark blue/violet color in their neutral states and
are transparent in their oxidized states. PProDOT-Np2 shows
a color change from purple to transmissive during p-doping.
Inspired by the results, in this study, we try to investigate
systematically the effect of substituents and heteroatoms on
the electrochromic properties in the similar systems. To
achieve this aim, a series of substituted PProDOS- and
poly(3,4-propylenedioxythiophene) (PProDOT)-based electro-
chromes were polymerized electrochemically (Scheme 1).
The transmittance change of the polymer films were monitored
at their absorption maxima for potential steps of 10 s (square-
wave potential steps of 10 s) and the change in percent trans-
mittance, DT%, (optical contrast) was calculated between col-
ored and bleached states (Table 1). Although the optical contrast
value of PProDOT-Bz2 is lower than reported in the literature
(89% at 632 nm),24 it is in well agreement with the value
reported by Kumar and his coworkers25 in their second publica-
tion (PProDOT-Bz2; 18 mC/cm2) as 78% at 634 nm elsewhere.
RESULTS AND DISCUSSION
From the viewpoint of device and high performance display
applications, spectroelectrochemical properties of the electro-
chromes should be manifested using the changes in electronic
absorption spectra under a voltage pulse. Therefore, UV–vis
spectra of poly(3,3-dibenzyl-3,4-dihydro-2H-selenopheno[3,4-b]
[1,4]dioxepine) (PProDOS-Bz2), PProDOT-Bz2, and poly (3,4-
dihydro-3,3-bis((naphthalen-1-yl)methyl)-2H-thieno[3,4-b][1,4]
dioxepine) (PProDOT-Np2) films electrodeposited on indium-
tin oxide (ITO) glass were monitored in an electrolyte solution
containing 0.1 M tetrabutylammonium hexafluorophosphate
(TBAH) in acetonitrile (ACN).
PProDOT-Bz2 (10.28 mC/cm2) has 75% of optical contrast
and this value decreases to 67% when the S atom was
replaced by Se atom (PProDOS-Bz2; 14.80 mC/cm2) (Fig. 2
and Table 1). In this study, we replaced benzyl groups in
ProDOT-Bz2 with naphtyl groups to increase the interchain
separation in its polymer, PProDOT-Np2, for easier dopant
ion transport, which is expected to give higher optical con-
trast as it was previously reported by Kumar and his co-
workers.24 Unfortunately, the increasing rigid and bulky sub-
stituent on ProDOT-based polymers gave lower optical
contrast as 63% when compared to PProDOT-Bz2. An inspec-
tion of Table 1 reveals that almost the same optical contrast
value (73%) can be obtained using flexible alkyl
chains, PProDOT-C10 (poly(3,3-didecyl-3,4-dihydro-2H-thieno-
[3,4-b][1,4]dioxephine), instead of bulky groups. As a result,
it can be safely concluded that it is not necessary to use
bulky substituent to increase the optical contrast value, as
flexible alkyl chains can also do the same job with additional
advantage of bringing solubility.
As shown in Figure 1, especially neutral PProDOS-Bz2 and
PProDOT-Bz2 films have similar optical spectra bearing some
vibronic coupling indicating highly regular polymer back-
bones25 or various conjugation lengths within the polymer
structure.29 The band gap (Eg) values were calculated from
the onset of the lowest energy end of the p–p* transitions
and found to be as 1.62 eV for PProDOS-Bz2, 1.86 eV for
PProDOT-Bz2, and 1.85 eV for PProDOT-Np2 (Table 1). As
shown in Table 1, the polymer films based on ProDOS mono-
mers distinctly have lower band gaps and also, it can be con-
cluded that there is no any appreciable change in the band
gap value by changing substituents.
Also, among the polymer films, PProDOS-Bz2 film exhibited the
largest coloration efficiency (CE, at 95% of the full contrast) as
992 cm2/C, and it can be concluded that PProDOS derivatives
with rigid bulky groups or flexible alkyl chains have generally
larger CE values when compared to PProDOT derivatives.
As expected from an electrochromic polymer, during oxida-
tion, p–p* transition bands start to disappear and a concomi-
tant increase in near infrared region is observed due to the
formation of charge carriers. On further oxidation, p–p* tran-
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scan, which indicates both an increasing thickness and the
formation of electroactive polymer films, namely PProDOS-
Bz2, PProDOT-Bz2, and PProDOT-Np2 (Fig. 3).
Redox behaviors of the polymers were investigated in the
monomer free electrolyte solution containing 0.1 TBAH and
ACN. During anodic scan reversible redox couples were
observed and the currents of these couples increase linearly
as a function of scan rate, indicating the well-adhered poly-
meric film on the electrode surface and nondiffusional redox
process (Supporting Information, Fig. S5).
All polymers have a robust structure under ambient condi-
tions, as they retain their electroactivity after thousands of
switching between their redox states via a square wave
potential method. Stability of the polymers was controlled
via cyclic voltammetry. As shown in Figure 4, ProDOS-Bz2,
ProDOT-Bz2, and ProDOT-Np2 retain 89%, 67%, and 63% of
their electroactivities, respectively, after 5000 switchings
(Fig. 4). On the other hand, poly(3,3-didecyl-3,4-dihydro-2H-
selenopheno[3,4-b][1,4]dioxepine) (PProDOS-C10
) exhibited
the highest electrochemical stability with 97% of its electro-
activity after 5000 switchings, whereas PProDOT-C10 retains
68% of its electroactivity after 5000 switchings. In the light
of these findings, one can conclude that the polymer films
based on ProDOS units are exceptionally more stable than
their ProDOT analogs under ambient conditions.
EXPERIMENTAL
All chemicals were purchased from Aldrich Chemical and
used as received unless otherwise noted. TBAH (0.1 M) dis-
solved in distilled dichloromethane (DCM, under CaH2 and
N2 atmosphere) was used as an electrolyte solution. Plati-
num disc (0.02 cm2) and platinum wire electrodes were
used as working electrode and counter electrode, respec-
tively. A Ag/AgCl electrode in 3 M NaCl (aq) solution was
used as a reference electrode. The polymer films were
obtained both by repetitive cycling and constant potential
electrolysis. After electropolymerization, the working elec-
trode was washed with DCM to remove the nonpolymerized
monomers and then the film was transferred into 0.1 M
TBAH/DCM electrolyte solution. In situ optical properties
were investigated using an ITO (Delta. Tech. 8–12 X, 0.7 ꢀ 5
cm2) electrode in a UV cuvette. A platinum wire and Ag wire
electrodes were used as counter electrode and pseudorefer-
ence electrode, respectively. The polymer films coated on
ITO had been switched between the neutral and oxidized
states several times to break-in the polymer films before
electrochemical and electro-optical analysis were applied.
Electroanalytical measurements were performed using a
Gamry PCI4/300 potentiostat–galvanostat and the electro-
optical spectra were monitored on a Hewlett–Packard 8453A
diode array spectrometer. 1H and 13C NMR spectra were
recorded on a Bruker Spectrospin Avance DPX-400 Spec-
trometer and chemical shifts were given relative to tetrame-
thylsilane as the internal standard. FTIR spectra were
recorded on Nicolet 510 FTIR with attenuated total reflec-
tance. Photographs of the polymer film were taken using a
Canon (PowerShot A75) digital camera. Colorimetric
FIGURE 1 Optical absorption spectra of (a) PProDOS-Bz2, (b)
PProDOT-Bz2, and (c) PProDOT-Np2 on ITO in 0.1 M TBAH/ACN
at various applied potentials.
The appearance of the new redox couples indicates the for-
mation of the new products on the electrode surface and
these redox couples were intensified after each successive
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TABLE 1 Electrochemical and Optical Properties of PProDOS and PProDOT Derivatives
measurements were recorded on Specord S600 (standard il-
luminator D65, field of width 10ꢁ observer) and color space
was given by the International Commission of Illumination
with luminance (L*), hue (a*), and intensity (b*). Platinum
cobalt DIN ISO 621, iodine DIN EN 1557, and Gardner DIN
ISO 6430 are the references of colorimetric measurements.
ter cooling to room temperature, the solvent was removed
under reduced pressure. Using the solvent ratio of 2 hexane:1
DCM, the crude product was purified via column chromatog-
raphy to give ProDOT-Bz2 as a light pink solid with 40%
yield (see Supporting Information, Figs. S1 and S2).
1H NMR (400 MHz, CDCl3, d(ppm)): 7.23–7.10 (m, 10H,Ar
H), 6.9 (s, 2H,Ar H), 3.75 (s, 4H), 2.77 (s, 4H); 13C NMR (100
MHz,CDCl3, d(ppm)): 151.4, 136.65, 130.7, 128.2, 126.5,
109.1, 45.2, 39.7.
Synthesis of 3,3-Dibenzyl-3,4-dihydro-2H-
selenopheno[3,4-b][1,4]dioxepine
Like ProDOT-Np2, synthesis of ProDOS-Bz2 was done by the
standard transetherification reaction between 3,4-dimethoxy-
selenophene26 (5) and 2,2-dibenzylpropane-1,3-diol24 (6) as
shown in Scheme 2. To an argon degassed solution of 3,
4-dimethoxyselenophene (200 mg, 1.05 mmol) and 2,2-diben-
zylpropane-1,3-diol (537 mg, 2.1 mmol) in 40 mL dry tolu-
ene, p-toluenesulfonic acid (PTSA, 20 mg, 0.105 mmol) were
added. The mixture was heated under reflux during 24 h. Af-
Synthesis of 2-((Naphthalen-2-yl)methyl)-
2-((naphthalen-3-yl)methyl)propane-1,3-diol (3)
Na (0.46 g; 20.08 mmol) was added in 30 mL ethanol and
mixed up to all Na was dissolved. Then, 1 g (6.18 mmol)
diethyl malonate (1) was added to this solution and reaction
mixture was heated. After the observation of the reflux, 5.2 g
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(22.56 mmol) of 2-(bromomethyl)naphthalene was added
and the mixture was heated under reflux during 24 h. Etha-
nol was removed under reduced pressure, cold water was
added and extracted with cold ether. The crude mixture was
chromatographed on silica gel by eluting with ethyl acetate:
hexane (1:20, v/v) to diethyl 2-((naphthalen-2-yl)methyl)-2-
((naphthalen-3-yl)methyl)malonate (2) as a white solid with
a 39% yield (Scheme 3). Then, 1.07 g of 2-((naphthalen-2-
FIGURE 3 Electropolymerization of 1.0 ꢀ 10ꢂ2 M (a) ProDOS-Bz2,
(b) ProDOT- Bz2, and (c) ProDOT-Np2 in 0.1 M TBAH/DCM at a
scan rate of 100 mV/s versus Ag/AgCl.
yl)methyl)-2-((naphthalen-3-yl)methyl)malonate (2) was dis-
solved in 100 mL THF (tetrahydrofuran) and 175 mg (4.3
mmol) lithium aluminum hydride was added to this solution
under inert atmosphere. The mixture was heated under
FIGURE 2 Chronoabsoptometry experiments for (a) PProDOS-
Bz2, (b) PProDOT-Bz2, and (c) PProDOT-Np2 films on ITO in 0.1
M TBAH/ACN when the polymers switched between redox
states.
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1H NMR (400 MHz, CDCl3, d(ppm)): 7.3–7.8 (m, 14H,Ar H),
4.13 (q, 4H, J ¼ 7.2 Hz), 3.44 (s, 4H), 1.26 (m, 6H); 13C NMR
(100 MHz,CDCl3, d(ppm)): 171.12, 134.02, 133.40, 132.52,
129.19, 128.36, 127.79, 127.72, 127.66, 126.09, 125.75,
61.42, 60.44, 39.61, 13.96.
1
3: H NMR (400 MHz, CDCl3, d(ppm)): 7.84–7.27 (m, 14H,Ar
H), 3.67 (s, 2H), 2.97 (s, 2H), 1.43 (br, 2H).
Synthesis of 3,4-Dihydro-3,3-bis((naphthalen-1-yl)
methyl)-2H-thieno[3,4-b][1,4]dioxepine
The ProDOT-Np2 was synthesized by the standard transe-
therification reaction between 3,4-dimethoxythiophene (4)
and 2,2-dinapthyllpropane-1,3-diol (3) as shown in Scheme 2.
To an argon degassed solution of 3,4-dimethoxythiophene
(182 mg, 1.25 mmol); 2,2-dinapthyllpropane-1,3-diol
(890 mg, 2.5 mmol) and PTSA (24 mg, 0.125 mmol) were
mixed in dry toluene (50 mL) and the mixture was heated
under reflux during 24 h. After cooling to room temperature,
the solvent was removed under reduced pressure. The crude
product was purified via column chromatography using the
solvent ratio of 3 hexane:1 DCM to give ProDOS-C10 as
yellow viscous liquid in 55% yield (see Supporting Informa-
tion, Figs. S3 and S4).
1H NMR (400 MHz, CDCl3, d(ppm)): 7.38–7.73 (m, 10H,Ar
H), 6.45 (s, 2H,Ar H), 3.89 (s, 4H), 2.99 (s, 4H); 13C NMR
(100 MHz,CDCl3, d(ppm)): 152.1, 135.8, 132.1, 133.4, 129.6,
127.8, 125.8, 125.2, 106.4, 105.2, 46.7, 41.2.
The rest of the monomers were synthesized in the same
manner and we ended up with five different monomers, as
shown in Scheme 1.
CONCLUSIONS
In this study, different derivatives of 3,4-propylenedioxythio-
phene/selenophene having different substituent groups were
synthesized electrochemically and the effect of substituents
and heteroatoms on the electrochromic and optical proper-
ties were investigated. The results of chronoabsoptometry
experiments indicated that increasing the bulkiness of the
substituent does not increase the optical contrast of the
obtained polymer.
Therefore, instead of using bulky substituents, flexible alkyl
substituents can also be used. Thus, using flexible alkyl
groups will not only increase the optical contrast value but
also will enhance the solubility of corresponding polymer.
FIGURE 4 Electrochemical stability of PProDOS-Bz2, PProDOT-
Bz2, and PProDOT-Np2 after 5000 switching by a square wave
potential method with a 2 s interval at each redox state. ipa
:
Anodic peak current, ipc: cathodic peak current, and Qa: anodic
charge deposited (calculation of stabilities had been done
according to Qa values).
reflux during 14 h. THF was removed under reduced pres-
sure, water was added and extracted with ether. The crude
product was used without further purifications. 2:
SCHEME 2 Synthesis of dibenzyl propylenedioxyselenophene,
ProDOS-Bz2.
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SCHEME 3 Synthesis of dinapthyl propylenedioxythiophene, ProDOT-Np2.
12 Pennisi, A.; Simone, F.; Barletta, G.; Di Marco G.; Lanza, L.
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The crucial point to mention is that the two different con-
trast values, 89% and 78%, for ProDOT-Bz2 were reported in
the literature.24,25 However, from our repetitive experiments
a contrast values of 75% was obtained for ProDOT-Bz2.
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The authors gratefully acknowledge the financial support from
the Scientific and Technical Research Council of Turkey (TUBI-
TAK-110T564), European Cooperation in Science and Technology
(COST-108T959), Atilim University (ATU-ALP-1011-02 and ATU-
BAP-1011-01), and The Turkish Academy of Sciences (TUBA).
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