Optoelectronic properties of poly(2,5-dithienylpyrrole)s with fluorophore groups

Article abstract of DOI:10.1149/2.0311512jes

In this study two new, fluorophore anchored 2,5-dithienylpyrrole derivatives (SNS-Carb, SNS-Flo) were successfully synthesized via click chemistry. Both monomers were subjected to electrochemical polymerization and the corresponding polymers (PSNS-Carb and PSNS-Flo) were thoroughly characterized for their electrochromic properties. PSNS-Carb displayed yellow to blue coloration in 1.31 s with a coloration efficiency of 120 cm² C^-1 whereas PSNS-Flo revealed longer switching time (2.67 s) and lower coloration efficiency (78 cm² C^-1). Coexistence of 3,4-ethylenedioxythiophene (EDOT) with SNS-Carb or SNS-Flo in polymerization media resulted in the formation of novel copolymer films (P1 and P2, respectively) having entirely diverged multichromic, superior optoelectronic properties. P1 revealed a switching time of 1.33 s, with a coloration efficiency of 164 cm² C^-1, whereas P2 exhibited a slower response time (1.87 s) with a lower coloration efficiency (155 cm² C^-1), as in the case of their respective homopolymers. In general, P1 was shown to reveal higher ΔE values which indicate it's more noticeable and vivid color changing nature compared to P2. When the optoelectronic properties of homopolymers were compared with that of their respective copolymers, there was an explicit enhancement of the color pallet, switching time, optical contrast and coloration efficiency.

Full text of DOI:10.1149/2.0311512jes

Journal of The Electrochemical Society, 162 (12) H867-H876 (2015)
H867
0013-4651/2015/162(12)/H867/10/$33.00 © The Electrochemical Society
Optoelectronic Properties of Poly(2,5-dithienylpyrrole)s
with Fluorophore Groups
Nese Guven and Pinar Camurluz
Department of Chemistry, Akdeniz University, 07058 Antalya, Turkey
In this study two new, fluorophore anchored 2,5-dithienylpyrrole derivatives (SNS-Carb, SNS-Flo) were successfully synthesized
via click chemistry. Both monomers were subjected to electrochemical polymerization and the corresponding polymers (PSNS-Carb
and PSNS-Flo) were thoroughly characterized for their electrochromic properties. PSNS-Carb displayed yellow to blue coloration in
1.31 s with a coloration efficiency of 120 cm2 C−1 whereas PSNS-Flo revealed longer switching time (2.67 s) and lower coloration
efficiency (78 cm2 C−1). Coexistence of 3,4-ethylenedioxythiophene (EDOT) with SNS-Carb or SNS-Flo in polymerization media
resulted in the formation of novel copolymer films (P1 and P2, respectively) having entirely diverged multichromic, superior
optoelectronic properties. P1 revealed a switching time of 1.33 s, with a coloration efficiency of 164 cm2 C−1, whereas P2 exhibited
a slower response time (1.87 s) with a lower coloration efficiency (155 cm2 −1), as in the case of their respective homopolymers.
C
In general, P1 was shown to reveal higher ꢀE values which indicate it’s more noticeable and vivid color changing nature compared
to P2. When the optoelectronic properties of homopolymers were compared with that of their respective copolymers, there was an
explicit enhancement of the color pallet, switching time, optical contrast and coloration efficiency.
© 2015 The Electrochemical Society. [DOI: 10.1149/2.0311512jes] All rights reserved.
Manuscript submitted June 17, 2015; revised manuscript received July 29, 2015. Published September 1, 2015.
Electrochromism, which refers to the reversible color change of a
material upon electrochemical stimulus, have been the center of inter-
est due to its promising commercial use in smart mirrors and windows,
displays, active optical filters etc. Since the first discovery,1 synthesis
of new electrochromic materials and optimization of their optoelec-
tronic properties have developed into an active interdisciplinary re-
search area. Electrochromic materials could be basically subdivided
as metal oxides,2,3 molecular dyes4 and conducting polymers.5,6 Al-
though all the conducting polymers are potentially electrochromic,
most of them are not of practical utility, since the commercial appli-
cations mandate some limitations on properties such as the color tone,
switching time, optical contrast, coloration efficiency and stability.
Therefore, there is an ongoing research on assessment of the factors
affecting the optoelectronic properties of conducting polymers. Up
to date, significant progress has been achieved by critical control of
polymer chain structure through substitution of the repeating unit and
utilization of terarylene monomers, which combine two heteroaro-
matic building blocks with a central aromatic unit.
Among conducting polymers, polypyrrole (PPy) and polythio-
phene (PTh) derivatives are by far the most extensively studied poly-
mers due to their suitable oxidation potential, tolerable environmental
stability and well developed synthetic chemistry. Lately, tailoring of
the main chain of conducting polymers through association of thio-
phene and pyrrole units has been considered to be a rational, pow-
erful approach7 for achieving electrochromic materials with distinct
electronic and optical properties. Among these polymers, poly(2,5-
dithienylpyrrole) (PSNS) derivatives, which were generally synthe-
sized by electrochemical or chemical polymerization of a thiophene-
pyrrole-thiophene trimeric unit, were examined in detail. Studies in-
dicated that PSNS derivatives show low oxidation potential, relatively
high electrochemical stability, and chromatic variation upon substi-
tution through the central pyrrole unit.8–13 Hence, derivatization of
basic PSNS structure with various groups has been considered as an
exciting approach to manipulate the properties of these polymers. In
this context, recently our group proposed utilization of Huisgen 1,3-
dipolar cycloaddition click reaction as an effective route for effortless
synthesis of new monomers with substituents such as ferrocene,14
pyrene,15 and 1,8-naphthalimide.16 As a part of a broad survey, this
study focuses on synthesis and electrochemical polymerization of
two new SNS monomers (SNS-Flo, SNS-Carb) which are appended
with fluorophore groups such as fluorene and carbazole, respectively.
Moreover, fine tuning of optoelectronic properties of SNS-Flo and
SNS-Carb based polymers was accomplished via copolymerization
with 3,4-ethylenedioxythiophene (EDOT), where an impressive set
of multichromic polymers with enhanced electrochromic properties
was realized. The characterization and investigation of optoelectronic
properties of all polymers were elucidated via cyclic voltammetry
(CV), FTIR, spectroelectrochemistry, kinetic and colorimetry studies.
Experimental
General.— The chemicals used for the synthesis of the monomers
were purchased from Aldrich or Merck Chemicals as reagent grade.
Lithium perchlorate was used in electroanalytical grade. Acetonitrile
(ACN) was distilled over calcium hydride and it was kept on 4 Å
molecular sieves. 1 1-(2-azido-ethyl)-2,5-dithiophen-2-yl-1H-pyrrole
(SNS-N3) was synthesized according to literature.17
Equipments.— NMR spectra were recorded with a Bruker Avance
Spectrometer at 300 MHz for 1H NMR and at 75 MHz for 13C NMR,
with Bruker Spectrospin Avance DPX-400 spectrometer at 400 MHz
1
for H NMR and at 100 MHz for 13C NMR, with Varian Inova 500
Spectrometer at 500 MHz for 1H NMR and at 250 MHz for 13C
NMR. The FTIR spectra were recorded on a Brucker Tensor 27 spec-
trometer. Mass spectra (MS) were obtained on a Finnigan MAT 95
spectrometer. Varian Cary Eclipse was used for fluorescence spec-
troscopy studies. Electrochemical synthesis and cyclic voltammetry
studies were performed on Ivium stat potentiostat/ galvanostat under
argon atmosphere. A platinum wire was used as the counter electrode
and Ag/Ag+ electrode was used as the reference electrode. Thermo
Evolution Array UV-Visible spectrophotometer was utilized for spec-
troelectrochemistry and kinetic studies. Colorimetry measurements
were recorded on a Minolta CS-100A Chroma Meter in a proper box
having D-50 illumination. Measurements were performed with a 0/0
(normal/normal) viewing geometry as recommended by CIE. Pro-
vided L∗,a∗,b∗ data were calculated through conversion of Y,x,y val-
ues of each color and the standard illuminant (with a blank, bare ITO
proper electrolyte) to tristimulus values. ꢀE∗, which defines the dif-
ference between two colored states, were calculated according to the
ꢀ
following formula ꢀEa∗b
=
(L2∗ − L1∗)2 + (a2∗ − a1∗)2 + (b2∗ − b1∗)2.
Synthesis of alkyne functionalized fluorophores.— The mixture
of carbazole (4.175 g, 25 mmol) and metallic sodium (0.575 g, 25
mmol) in dioxane (20 ml) was refluxed under inert atmosphere until
metallic sodium was completely dissolved. Later, propargyl bromide
(3.38 ml, 39 mmol) was added into the mixture. The solution was
cooled to room temperature and stirred for 18 h. Finally, water was
added and the mixture was extracted with CH2CI2. The organic phase
was dried with NaSO4 and filtered. The solvent was removed un-
der vacuum and the residue was purified by column chromatography
zE-mail: pcamurlu@akdeniz.edu.tr
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Journal of The Electrochemical Society, 162 (12) H867-H876 (2015)
Scheme 1. Schematic representation of SNS-Carb, SNS-Flo and the related polymers.
(10:1 Hexane/Ethyl acetate) to yield 9-(prop-2-yn-1-yl)-9H-carbazole
(%76). Alkyne functionalized fluorene (2-ethynyl-9,9-dimethyl-9H-
fluorene) was synthesized according to the literature18 with a yield
electrodes. In either case the electrochemical polymerization was
achieved through potentiodynamic cycling between 0.0 V to 1.0 V
with a scan rate of 100 mV/s. P1 was synthesized in a mixture of
SNS-Carb (0.01 M) and EDOT (0.005 M) in 0.1 M LiCIO4/ACN me-
dia via potentiodynamic cycling between 0.0 V to 1.0 V with a scan
rate of 100 mV/s. Similar procedure was applied for the synthesis
of P2 where the electrolysis media contained SNS-Flo (0.01 M) and
EDOT(0.005 M).
1
of 75%. For 9-(prop-2-yn-1-yl)-9H-carbazole: H NMR (500 MHz,
CDCl3): δ 8.12 (d, J = 7.8 Hz, 2H), 7.49–7.54 (m, 4H), 7.28–7.31
(m, 2H), 5.04 (dd, J = 1.0, 2.4 Hz, 2H), 2.26 (t, J = 2.4 Hz, 1H).
APT NMR (125 MHz, CDCl3): δ 139.81 (Cquat.), 125.86 (CH), 123.24
(Cquat.), 120.41 (CH), 119.55 (CH), 108.69 (CH), 77.83 (Cquat.), 72.20
(CH∗, alkyne), 32.23 (CH2). For 2-ethynyl-9,9-dimethyl-9H-fluorene:
1H NMR (500 MHz, CDCl3): δ 7.72–7.71 (m, 1H), 7.67 (d, J = 7.8
Hz, 1H), 7.48–7.50 (m, 1H), 7.43–7.45 (m, 1H), 7.32–7.36 (m, 2H),
3.13 (s, 1H), 1.48 (s, 6H). APT NMR (125 MHz, CDCl3): δ 153.85
(Cquat.), 153.47 (Cquat.), 139.88 (Cquat.), 138.30 (Cquat.), 131.18 (CH),
127.76 (CH), 127.06 (CH), 126.41 (CH), 122.60 (CH), 120.39 (Cquat.),
120.31 (CH), 119.82 (CH), 84.43 (Cquat.), 77.00 (CH∗, alkyne), 46.79
(Cquat.), 26.92 (CH3).
Results and Discussion
Synthesis.— First
(9,9-dimethyl-9H-fluoren-2-yl)ethynyl)
trimethylsilane (1) was synthesized through Sonogashira cross-
coupling reaction of 2-bromo-9,9-dimethyl-9H-fluorene with
trimethylsilyl acetylene in the presence of PdCl2(PPh3)2 and CuI
and NEt3. Later, (1) was treated with KOH to afford 2-ethynyl-9,9-
dimethyl-9H-fluorene in 75% yield. 9-(prop-2-yn-1-yl)-9H-carbazole
(2), on the other hand, was synthesized through one pot reaction of
carbazole with propargyl bromide in the presence of metallic sodium.
Chemical structure of the products were verified by FTIR, 1H NMR
and 13C NMR analyses, where alkylenation reactions were followed
by presence of the typical acetylenic stretching around 2120 cm−1 in
Synthesis of SNS-Carb and SNS-Flo.— 0.5 mmol SNS-N3, 0.5
mmol alkyne functionalized fluorophore were dissolved in THF (5 ml)
and then water (5 ml), 1 M CuSO4.5H2O (0.1 ml), 1 M sodium ascor-
bate (0.17 ml) were added into the reaction media, consecutively. The
mixture was stirred at room temperature for 3 h. Later, THF was
evaporated and the reaction mixture was extracted with CH2CI2/H2O.
Finally, the residue was subjected to column chromatography (3:1
DCM/Hexane for SNS-Flo, and 5:1 DCM/Hexane for SNS-Carb) to
yield SNS-Flo (0.19 g, 77%) and SNS-Carb (0.20 g, 80%). For SNS-
Carb: 1H NMR (400 MHz, Chloroform-d) δ 8.06 (d, J = 7.7 Hz,
2H), 7.46 – 7.36 (m, 5H), 7.22 (dd, J = 7.8, 1.4 Hz, 2H), 7.09 (d,
J = 6.3 Hz, 2H), 6.50 (d, J = 4.6 Hz, 2H), 6.12 (s, 1H), 5.80 (s,
2H), 5.47 (s, 2H), 4.41 (t, J = 6.0 Hz, 2H), 4.09 (t, J = 6.0 Hz, 2H).
MS C29H23N5S2 (m/z): calculated 505.66, found 505. For SNS-Flo:
1H NMR (300 MHz, Chloroform-d) δ 7.95 – 7.86 (m, 1H), 7.79 –
7.71 (m, 2H), 7.58 (dd, J = 7.9, 1.5 Hz, 1H), 7.48 – 7.43 (m, 1H),
7.37 – 7.32 (m, 3H), 7.07 (dd, J = 5.2, 3.6 Hz, 2H), 6.98 – 6.92
(m, 3H), 6.41 (s, 2H), 4.71 (t, J = 6.1 Hz, 2H), 4.42 (t, J = 6.1
Hz, 2H), 1.54 (d, J = 3.4 Hz, 6H). 13C NMR (75 MHz, CDCl3)
δ 154.30, 153.90, 148.54, 139.26, 138.77, 133.57, 129.45, 128.89,
127.71, 127.40, 127.03, 126.71, 126.18, 124.76, 122.65, 120.27,
120.15, 120.05, 119.92, 111.97, 49.54, 46.99, 44.88, 27.17.
1
FTIR spectra and by the signals around 2.26–3.13 ppm in H NMR
spectra of (1) and (2). The final products (SNS-Carb and SNS-Flo)
were readily prepared by click reaction of alkyne functionalized
fluorophores (Scheme 1) with SNS-N3 with yields higher than 75%.
1H NMR spectra of both monomers displayed resonance signals of
2,5-dithienylpyrrole unit at around 7.3–6.4 ppm. In addition, 1H NMR
spectra of both SNS-Carb and SNS-Flo revealed typical resonance
characteristic of their respective fluorophore groups around 8.06–7.36
ppm and 7.91–7.35 ppm, respectively. The chemical structure of
the monomers was further supported via 13C NMR, FTIR and MS
studies.
The UV–visible absorption spectra and photoluminescence (PL)
emission spectra for the alkyne functionalized fluorophores, SNS-N3,
SNS-Flo and SNS-Carb were recorded in ACN (Figure 1). SNS-N3
revealed a single, broad π to π∗ absorption centered around 310
nm and its maximum emission wavelength was about 415 nm. The
fluorophores, on the other hand, revealed multiple absorption peaks,
which correspond to S0 to S1∗ transitions to different vibrational levels.
As seen in Figure 1c and Figure 1f, the absorption spectra of both
SNS-Carb and SNS-Flo are a linear combination of the spectra of their
corresponding fluorophore and SNS-N3. PL spectra of both monomers
Electrochemical synthesis.— PSNS-Carb and PSNS-Flo films
were synthesized by the electrochemical polymerization of their re-
spective SNS monomers (0.01 M) in 0.1 M LiCIO4/ACN on ITO
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Journal of The Electrochemical Society, 162 (12) H867-H876 (2015)
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Figure 1. Absorption and emission spectra of a) alkyne functionalized carbazole, b) SNS-N3, c) SNS-Carb, d) alkyne functionalized fluorene, e) SNS-N3 and
f) SNS-Flo in ACN (excitation 310 nm).
are dominated by the intense broad signal at 415 nm, which is in
accordance with the emission spectrum of SNS-N3.
Upon repetitive cycling, formation of a new, lower potential re-
versible redox couple (EP1/2 = 0.60 V, EP1/2 = 0.59 V, respectively)
was observed for both SNS-Carb and SNS-Flo, with a simultaneous
increase in the current intensities after each cycling. Such observation
indicated the enlargement of the surface area of the working electrode
due to grafting of a conducting polymer on the electrode. Despite col-
oration of polymerization medium, which presumably indicated the
leakage of oligomers, the thickness of the PSNS-Carb film gradually
enhanced with the increase in number of scans. However, in case of
potentiodynamic cycling of SNS-Flo, leakage of oligomers from the
electrode surface was more severe and the linear correlation between
the film thickness and number of scans was valid up to a lower extent.
The lower film forming ability of PSNS-Flo could be related with
higher solubility of its oligomers which might stem from the alkyl
substitution of the fluorene unit.
Electrochemistry of SNS derivatives.—Electrochemical polymeriza-
tion mechanism7,19 of 2,5-dithienylpyrrole derivatives was postulated
by considering the formation of radical-cation of the trimeric unit
(thiophene-pyrrole-thiophene), coupling of radical-cations and de-
protonation of the dimer. Continuous chain growth is achieved by
the further oxidation of the dimer which affords oligomers and so on.
Figures 2a and 2b depict the redox behavior of the monomers, which
were recorded in 0.1 M LiCIO4/ACN media via cyclic voltammetry.
As expected, due to their trimeric nature both monomers oxidized at
lower potentials than pristine pyrrole (1.1 V) and thiophene (1.8 V).
Comparison of the oxidation potentials of SNS-Carb (Em = 0.93
ox
V) and SNS-Flo (Em = 0.91 V) with SNS-N3 (Em = 0.87 V) re-
ox
ox
veals the limited influence of the substituents on monomer oxidation,
which could be ascribed to the minor deviations from coplanarity of
the trimeric molecule due to steric restrictions generated by the bulky
fluorophore groups.20
In order to elucidate the redox behaviors, the polymers were sub-
jected to linear sweep voltammetry in the monomer-free electrolyte
solution (Figures 2c, 2d). Both polymer films exhibited a reversible
redox couple (for PSNS-Carb Ep,a = 0.78 V and Ep,c = 0.51 V,
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Journal of The Electrochemical Society, 162 (12) H867-H876 (2015)
Figure 2. Cyclic voltammogram of a) SNS-Carb, b) SNS-Flo at 100 mV/s and c) PSNS-Carb, d) PSNS-Flo at various scan rates in 0.1 M LiCIO4/ACN, plot of
anodic and cathodic peak current density vs. scan rate for e) PSNS-Carb, f) PSNS-Flo.
for PSNS-Flo Ep,a = 0.82 V and Ep,c = 0.49 V). Intensity of the
redox couple revealed a linear proportionality with the scan rate. This
confirms the formation of an electro-active, surface confined material
with non-diffusional redox process.21
Electrochromic properties of PSNS-Carb and PSNS-Flo.—Elec-
trochromic response of a conducting polymer is based on reversible
creation and destruction of charge carriers (polarons and bipolarons),
which are signified by the evolution of new, lower energy absorption
bands. Spectroelectrochemistry, which allows probing of the spec-
tral variations as a function of electrochemical stimuli, is an emi-
nent technique for elucidating optoelectronic properties of conducting
polymers. Figure 3a represents the spectroelectrochemistry study of
PSNS-Carb in monomer free electrolyte media, where a series of spec-
tra were recorded while sequentially stepping the applied potential.
At −0.2 V (neutral state) PSNS-Carb revealed well-defined, multiple
absorptions (π-π∗ transition) at 330 nm and 345 nm with a shoulder
at 370 nm. On the other hand, PSNS-N3 (the PSNS derivative without
fluorophore groups) showed a broad, featureless absorption band at
345 nm at neutral state with a band gap of 2.49 eV.17 The polymer
displayed yellow to blue coloration upon doping. Band gap of PSNS-
Carb was calculated through the lower energy edge of its absorption
spectrum as 2.5 eV. Upon electrochemical oxidation of PSNS-Carb,
intensity of π–π∗ transition decreased, whereas new absorption bands
located around 615 nm and 950 nm emerged simultaneously. At this
state PSNS-Carb appeared as blue. Figure 3b represents the spectro-
electrochemistry studies of PSNS-Flo. In neutral state the polymer
had absorption bands (π-π∗ transition) at 323 and 375 nm (shoulder)
with a bandgap of 2.59 eV. Upon oxidation, the intensity of π-π∗
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Journal of The Electrochemical Society, 162 (12) H867-H876 (2015)
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jected/ejected charge and change in optical density (ꢀOD).23 Materi-
als which exhibit high optical contrast with a small amount of charge
are known to have high CE. To elucidate the CE of PSNS derivatives
the tandem chronoabsorptometry/chronocoulometry experiment was
used at 95% of the maximum change of transmittance, where quantity
of injected charge was attained through the integration of the current.
Coloration efficiencies of PSNS-Carb and PSNS-Flo were estimated
as 120 and 78 cm2 C−1, respectively.
CE (λ) = ꢀOD (λ) / (Q/A)
[1]
Electrochemical copolymerization.—Tuning of electrochromic prop-
erties of conducting polymers is not only limited with the controlled
polymerization of specifically designed monomers, but also it could
be accomplished through techniques such as copolymerization and
lamination. Electrochemical copolymerization, which amalgamates
the advantages of the electrochemical techniques and copolymeriza-
tion, is a robust, eminent technique for diversification of conducting
polymers. Hence, this part of the work details the electrochemical and
optoelectronic properties of SNS-Carb and SNS-Flo based copoly-
mer films which were synthesized in the presence of EDOT (readily
available, relatively cheap heteroaromatic compound).
Selection of a proper co-monomer with a suitable oxidation po-
tential is the key for a successful electrochemical copolymerization,
since its mechanism involves formation of radical cations of both
components. Thus, overlapping of the oxidation potentials of the
comonomers is mandatory for achieving true copolymers. The onset of
oxidation potential of EDOT was recorded as 0.85 V in LiCIO4/ACN.
Since the oxidation potentials of both SNS-Carb and SNS-Flo were
around 0.8 V, it is valid to assume formation of their radical cations
in the same potential window with EDOT. Figures 5a, 5b represents
the first anodic polarization curve of SNS-Carb and SNS-Flo in the
presence of EDOT, where the typical redox behavior of both SNS
derivatives and EDOT was observed. The well-defined redox peaks
around 0.8 V and a steady increase in current density beyond 0.85 V
for both SNS-Carb/EDOT and SNS-Flo/EDOT mixtures, signified the
formation of radical cations of SNS derivatives and EDOT, respec-
tively. Moreover, upon successive cycling between 0.0 V and 1.0 V,
evolution of a lower potential, quasi-reversible redox couple showing
a simultaneous increase in the current intensity after each cycling,
was observed for both systems, which indicated the formation of a
copolymer.
Figures 5c, 5d show the linear sweep voltammograms of P1 and P2
films in 0.1 M ACN/LiCIO4 at different scan rates, where the abbre-
viations P1 and P2 refers to the copolymers which were synthesized
in SNS-Carb/EDOT and SNS-Flo/EDOT mixtures, respectively. For
both copolymers a proportional increase in the current intensity was
observed with the increase in the scan rate. When the redox potentials
of the copolymers (EP,a = 0.65 V (P1), EP,c = 0.37 V (P1) and EP,a
= 0.65 V (P2), EP,c = 0.32 V (P2)) are compared with that of their
respective homopolymers (Figures 2c, 2d), there is a clear cathodic
shift. Taking into account the lower oxidation potential of pristine
PEDOT,24 cathodic shifting of redox potentials could be considered
as an indirect indication for the reaction between the SNS and EDOT
units, where the electron-rich EDOT unit could raise the HOMO of
PSNS derivatives.
Figure 3. Spectroelectrochemistry of a) PSNS-Carb and b) PSNS-Flo films
on an ITO coated glass slide in 0.1 M LiCIO4/ACN.
transition declined slightly while charge carrier bands located around
620 and 940 nm evolved. The polymer appeared in light yellow and
blue in neutral and oxidized states, respectively. However, it was not
possible to fully oxidize either polymer.
Although absorption spectrum of an electrochromic film (in visible
region) provides an objective measure for color absorption, it offers
little insight on how it is perceived by human eye. Since color appear-
ance depends on the light source, the sample size, the observer and
the surrounding colors, the field of colorimetry has been developed
for description of color in an objective manner.22 Hence, we have
utilized in-situ colorimetric analysis for quantitative examination of
PSNS derivatives at various states as reflected in Table I.
In order to explore the switching characteristics of PSNS-Carb, a
square wave potential step method coupled with simultaneous trans-
mittance probing was performed. In this experiment, the initial po-
tential was set at 0.0 V (according to spectroelectrochemistry study,
neutral state) for 10 s, and it was stepped to 1.0 V (ultimate oxidation)
for the same period of time. The percent transmittance change during
this repetitive doping and de-doping process was simultaneous mon-
itored at 900 nm and the switching time (t95), which is the response
time required to attain 95% of total the transmittance difference, was
calculated. Figure 4a represents the transmittance variations of PSNS-
Carb in 0.1 M LiCIO4/ACN. The polymer was found to reveal yellow
to blue coloration in 1.31 s with an optical contrast of 27.78%. Similar
to PSNS-Carb, PSNS-Flo was also subjected to kinetic studies while
the potential was consecutively stepped from the neutral, light yellow
state (0.0 V for 10 s), to blue state (1.0 V for 10 s). The switching time
(t95) of PSNS-Flo, was estimated as 2.67 s and the ultimate transmit-
tance variation was recorded as 5.53%. Such a low optical contrast is
valid with the spectroelectrochemistry studies, where PSNS-Flo re-
vealed a lower extent of decline in the intensity of the π-π∗ transition
upon oxidation compared to PSNS-Carb.
Electrochromic properties of P1 and P2.—Investigation of optoelec-
tronic properties of a copolymer not only provides a powerful insight
about its electrochromic properties but also offers a valid evidence for
effective copolymerization through comparison of the spectral behav-
iors of the copolymer with that of parent homopolymers. Figure 6a rep-
resents spectroelectrochemical analysis of P1 in 0.1 M ACN/LiCIO4.
It can be seen that at −0.2 V (neutral state) P1 revealed two sets of
absorption bands centered at 330–343 nm and 455 nm and appeared
in ruby color. PSNS-Carb, on the other hand, appeared in yellow with
a single set of absorption band with fine features (330–345 nm) at
this state. Meanwhile, PEDOT exhibited a single, broad, featureless
transition around 590 nm (Eg = 1.71 eV) and appeared in blue in neu-
tral state. P1 had a bandgap of 1.80 eV which is significantly lower
Another important, frequently used tool for evaluation of elec-
trochromic materials is coloration efficiency (CE) measurements.
As illustrated below, CE describes the relationship between the in-
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Journal of The Electrochemical Society, 162 (12) H867-H876 (2015)
Table I. Electrochromic properties of PSNS-Carb, PSNS-Flo, P1 and P2.
Material
λmax (nm)
Eg (eV)
t95
Optical Contrast ꢀT%
CE (cm2/C)
Colorimetry Data
PSNS-Carb
P1
PSNS-Flo
P2
330,345,370
330,343,455
3.49, 1.80
1.33
323,375
2.59
2.67
5.53
78
323,490
3.56, 1.81
1.87
31.26
155
2.50
1.31
27.78
120
x
34.75
164
x
Y
y
Y
y
Y
x
y
Y
x
y
At −0.2 V
At 0.0 V
At 0.1 V
At 0.2 V
At 0.3 V
At 0.4 V
At 0.5 V
At 0.6 V
At 0.7 V
At 0.8 V
At 0.9 V
At 1.0 V
At 1.1 V
At 1.2 V
793
841
826
854
868
835
737
606
529
478
435
370
300
254
0.371
0.375
0.376
0.377
0.378
0.376
0.364
0.348
0.341
0.345
0.349
0.352
0.355
0.360
0.399
0.402
0.403
0.404
0.405
0.403
0.394
0.378
0.364
0.360
0.359
0.361
0.364
0.366
597
631
683
727
785
714
688
598
531
508
482
415
339
298
0.426
0.426
0.423
0.413
0.398
0.378
0.360
0.344
0.339
0.342
0.343
0.345
0.347
0.350
0.405
0.410
0.416
0.422
0.423
0.417
0.400
0.381
0.369
0.365
0.364
0.366
0.370
0.371
1010
999
999
1030
1030
1020
1010
931
819
703
639
574
0.373
0.374
0.375
0.375
0.376
0.375
0.373
0.363
0.351
0.350
0.354
0.359
0.365
0.374
0.401
0.402
0.401
0.403
0.403
0.403
0.402
0.394
0.382
0.373
0.370
0.371
0.375
0.378
813
866
896
930
971
994
995
937
917
891
867
848
818
785
0.386
0.387
0.387
0.381
0.376
0.369
0.361
0.350
0.348
0.350
0.351
0.352
0.353
0.354
0.387
0.389
0.395
0.399
0.401
0.400
0.396
0.382
0.377
0.375
0.375
0.376
0.377
0.377
510
457
than that of PSNS-Carb (2.50 eV). The lowering of the band gap and
subsequent diversification of the color of the copolymer with respect
to PSNS-Carb could be attributed to the increase in the content of the
electron-rich heteroaromatic units (inclusion of EDOT units through
copolymerization) within the backbone of P1, which could to raise
the HOMO of the polymer and lower the bandgap. When the applied
potential increased, the two-band absorption of P1 (330–343 nm and
455 nm) in the visible region were simultaneously depleted with ap-
pearance of the polaronic charge transition bands around 800 nm. As
the applied potential further increased, polaronic transitions merged
Figure 4. T% variations of a) PSNS-Carb and b) PSNS-Flo monitored at 900 nm, charge density variations of c) PSNS-Carb and d) PSNS-Flo during repetitive
switching in 0.1 M LiCIO4/ACN.
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Journal of The Electrochemical Society, 162 (12) H867-H876 (2015)
H873
Figure 5. Cyclic voltammogram of a) SNS-Carb/EDOT, b) SNS-Flo/EDOT at 100 mV/s and c) P1, d) P2 at various scan rates in 0.1 M LiCIO4/ACN and plot of
anodic and cathodic peak current density vs. scan rate for e) P1, f) P2.
into the broader bipolaronic transition, which entail in to the visible
region. When fully oxidized, P1 film showed a dramatic absorption
within NIR region. Such impressive set of spectral variations resulted
in multichromic behavior where the polymer displayed vivid color
changes from ruby (−0.2 V) to various tones of orange (0.1 V), yel-
low (0.3 V), green (0.4 V), blue (0.7 V) to gray (1.2 V) upon oxidation.
To objectively elucidate the color of P1, colorimetric measurements
were performed, and photographs were obtained at each potential. Fig-
ure 7a represents the x,y data (CIE 1931) of P1 at various potentials
which are completely different from that of PSNS-Carb.
P1 at different potentials and the calculated ꢀE∗ values between each
states. As we could see the ꢀE values for ruby to orange, orange to
yellow, yellow to green, green to blue and ruby to blue transitions are
almost always higher than 7, which are well above the JND, indicating
the visually apparent, vivid color change of P1 upon amplification of
applied potential.
To evaluate the speed at which the color change takes place a
chronoabsorptometry/chronocoulometry study was performed on P1
under repeated cycling between 0.0 V and 1.0 V in square wave form at
900 nm. P1 displayed color variation in less than 1.33 s, which is close
to PSNS-Carb. As seen in Figure 8a, P1 exhibited higher transmittance
change (34.75%) and higher CE (164 cm2 C−1) compared to PSNS-
Carb. The substantial improvement of CE upon copolymerization
could be attributed to the inclusion of EDOT moieties within PSNS-
Carb chain backbone, which lowers the oxidation potential and might
provide higher accessibility to the doping sites.
∗
The divergence of color is objectively evaluated through ꢀEab
(color difference) which is the Euclidian distance between two points
in CIE 1976 color space. Basically, a large color difference (large ꢀE∗)
indicates the greater difference between the two colored states. ꢀEab
equal to 2.3 refers to the “just noticeable difference” (JND) threshold,
where the values less than 2.3 signifies that the difference between the
∗
two colors is perceptually indistinguishable. On the contrary, ꢀEab
To evaluate the electrochromic properties of P2, similar type of
studies were conducted. Figure 6b represents the spectroelectrochem-
istry study of P2 in 0.1 M ACN/LiCIO4 which was recorded during
values higher than 4 indicate that the two colors are noticeably differ-
ent from one another. Table II reflects the L∗a∗b∗ data (CIE-1976) of
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H874
Journal of The Electrochemical Society, 162 (12) H867-H876 (2015)
gradual amplification of applied potential from −0.2 V to 1.2 V. At
neutral state P2 displayed two maxima at 323 nm and 490 nm and the
polymer film appeared in boysenberry color. As the applied poten-
tial increased, evolution of polaronic charge carrier bands was clearly
observed at around 800 nm with a concurrent decrease in higher en-
ergy bands. Beyond 0.5 V a very broad, overwhelming bipolaronic
charge carrier band gradually intensified while the residual absorption
at 325 nm remained as of PSNS-Flo. In accordance, the copolymer re-
vealed multichromic behavior, displaying a wide range of colors from
boysenberry to various tones of orange, yellow, green, blue and gray,
which were objectively evaluated by the colorimetry studies (Table I).
When the ꢀE values for these color transitions were calculated, we
could state that all the color transitions are noticeable by an average
person, but not as obviously and vividly as P1. Figure 8b represents
the % transmittance change (900 nm) of P2 upon repeated stepping
of the polarization potential between 0.0 V and 1.0 V. P2 exhibited
an optical contrast of 31.26%, which was determined through the
transmittance difference between the two extreme redox states. The
current and charge responses accompanying the transmittance (color)
change were uniform. P2 had a switching time of 1.87 s and a CE of
155 cm2 C−1, which are significantly improved compared to that of
PSNS-Flo.
Among many other factors, the critical role of film morphology
on switching properties of the conducting polymers is generally ac-
knowledged in literature. In a general perspective the polymer films
having more open morphology are expected to provide smaller dif-
fusion distance for the dopant ions and thereby they allow effective
doping-dedoping with shorter switching times.25–28 Figure 9 repre-
sents the SEM images (under the same magnification) of P1, P2 and
Figure 6. Spectroelectrochemistry of a) P1 and b) P2 films on an ITO coated
glass slide in 0.1 M LiCIO4/ACN.
Table II. CIE 1976 data and calculated ꢀE values of PSNS-Carb, PSNS-Flo, P1 and P2 at various states.
PSNS-Carb
P1
PSNS-Flo
L∗ a∗ b∗ ꢀEab
P2
(V) Color L∗ a∗ b∗ ꢀEab
−0.2 V Yellow 84 −3 13
(V)
Color L∗ a∗ b∗ ꢀEab
(V)
−0.2 V Light Yellow 90 −2 12
Color
(V)
−0.2 V Boysen−berry 85
Color
L∗ a∗ b∗ ꢀEab
∗
∗
∗
∗
−0.2 V Ruby 78 15 27
8
6
2
9
15.30
7.55
7.00
8.60
5.39
4.47
3.16
0.6 V Green 76 −4
0
0.1 V Orange 82 11 32
0.6 V
0.7 V
1.1 V
Green
87 −4
6
0.1 V
0.2 V
0.3 V
0.7 V
1.2 V
Orange
Yellow
Green
Blue
88
13
13
7.81
12.45
10.44
27.93
15.81
0.7 V Blue 72 −1 −6
0.3 V Yellow 87
0 29
Blue
82 −4 −1
90
16.76
16.16
25.77
1.1 V Gray 56
3
−3
0.4 V Green 84 −6 21
0.7 V Blue 74 −4 −5
Gray
68
4
0
91 −1 13
89 −3 −3
16.25
5.92
1.2 V Gray 59
−0.2 V Ruby 78 15 27
0
−2
Gray
84
0
8
−2
−0.2 V Yellow 84 −3 13
−0.2 V Light Yellow 90 −2 12
−0.2 V Boysen−berry 85
9
32.80
37.78
13.64
1.1 V Gray 56
3
−3
1.2 V Gray 59
0
−2
1.1 V
Gray
68
4
0
1.2 V Gray 84
0
−2
Figure 7. x,y data of a) P1 and b) P2.
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Journal of The Electrochemical Society, 162 (12) H867-H876 (2015)
H875
Figure 8. T% variations of a) P1 and b) P2 monitored at 900 nm, current density variations of c) P1 and d) P2 and charge density of e) P1 and P2 during repetitive
switching in 0.1 M LiCIO4/ACN.
Figure 9. SEM micrographs of a) P1 (1000 X), b) P2 (1000 X) and c) PEDOT (1000 X).
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H876
Journal of The Electrochemical Society, 162 (12) H867-H876 (2015)
PEDOT which were synthesized under identical conditions. As we
could see P2 film showed a dense, uniform compact structure with
a lower surface roughness, whereas PEDOT film revealed a porous,
cauliflower type of morphology. P1, on the other hand, revealed globu-
lar morphology having less compact structure compared to P2, which
is in accordance with calculated switching times of the respective
copolymers.
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In this study we have synthesized two new SNS monomers (SNS-
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Acknowledgment
We are grateful to TUBITAK (Project No: 110T640) for the support
of this study.
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