Effect of the planar center moiety for a donor-acceptor polymeric electrochrome

Article abstract of DOI:10.1016/j.polymer.2016.12.016

In this work, phenanthrene centered donor-acceptor monomer; 5,10-bis(4-hexyl-2-thienyl)phenantro[9,10-c][1,2,5]thiodiazole; MES16 was synthesized and then its electrochromic application of corresponding polymer was carried out. 4,7-di(thiophen-2-yl)-2,1,3-benzothiadiazole (TST) was also synthesized as a standard molecule to determine the effect of the planar phenanthrene center on electrochromic performance. Electrochemical properties were elucidated by cyclic voltammetry and then HOMO-LUMO band gaps were calculated by using their oxidation and reduction onset potentials. Finally, electrochromic properties of poly-(MES16) and poly-(TST) were investigated via spectro-electrochemical measurements. DFT calculations exhibit that poly-(MES16) has a high planarity and an intramolecular charge transfer from the polymer backbone-donor to thiadiazole-acceptor subunit. Because of high solid state π?π interactions of the poly-(MES16), a good quality film was obtained corresponding to self-assembly property planar phenantrene center. Owing to this property, electrochromic performance of poly-(MES16) such as response time, stability and ΔT% were better than the standard poly-(TST).

Full text of DOI:10.1016/j.polymer.2016.12.016

Accepted Manuscript
Effect of the planar center moiety for a donor-acceptor polymeric electrochrome
Ceylan Doyranlı, Betül Çolak, Gözde Lacinel, Mustafa Can, Fatma Baycan Koyuncu,
Sermet Koyuncu
PII:
S0032-3861(16)31103-X
10.1016/j.polymer.2016.12.016
JPOL 19252
DOI:
Reference:
To appear in: Polymer
Received Date: 14 September 2016
Revised Date: 29 November 2016
Accepted Date: 6 December 2016
Please cite this article as: Doyranlı C, Çolak B, Lacinel G, Can M, Koyuncu FB, Koyuncu S, Effect of
the planar center moiety for a donor-acceptor polymeric electrochrome, Polymer (2017), doi: 10.1016/
j.polymer.2016.12.016.
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ACCEPTED MANUSCRIPT
Graphical Abstract
Effect of the planar center moiety for a Donor-Acceptor polymeric electrochrome
Ceylan Doyranlı, Betül Çolak, Gözde Lacinel, Mustafa Can,
Fatma Baycan Koyuncu, and Sermet Koyuncu
1

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Effect of the planar center moiety for a Donor-Acceptor
polymeric electrochrome
Ceylan Doyranlıa, Betül Çolakb, Gözde Lacinela, Mustafa Canc, Fatma Baycan
Koyuncu*a,b, and Sermet Koyuncu*b,d
aDepartment of Chemistry, Faculty of Sciences and Arts, Çanakkale Onsekiz Mart University,
17020 Çanakkale, Turkey
bDepartment of Energy Resources and Management, Çanakkale Onsekiz Mart University,
17020 Çanakkale, Turkey
cDepartment of Engineering Sciences, Faculty of Engineering, Izmir Katip Celebi University,
Cigli, 35620 Izmir, Turkey
dDepartment of Chemical Engineering, Çanakkale Onsekiz Mart University, Canakkale,
Turkey
*Corresponding authors:
Sermet Koyuncu, Fatma Baycan Koyuncu
Fax: +90-286-2180533; Tel: +90-286-2180018; e-mail: fatmabaycan@hotmail.com and
skoyuncu@comu.edu.tr
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Abstract
In this work, phenanthrene centered donor-acceptor monomer; 5,10-bis(4-hexyl-2-
thienyl)phenantro[9,10-c][1,2,5]thiodiazole; MES16 was synthesized and then its
electrochromic application of corresponding polymer was carried out. 4,7-di(thiophen-2-yl)-
2,1,3-benzothiadiazole (TST) was also synthesized as a standard molecule to determine the
effect of the planar phenanthrene center on electrochromic performance. Electrochemical
properties were elucidated by cyclic voltammetry and then HOMO-LUMO band gaps were
calculated by using their oxidation and reduction onset potentials. Finally, electrochromic
properties of poly-(MES16) and poly-(TST) were investigated via spectro-electrochemical
measurements. DFT calculations exhibit that poly-(MES16) has a high planarity and an
intramolecular charge transfer from the polymer backbone-donor to thiadiazole-acceptor
subunit. Because of high solid state π−π interactions of the poly-(MES16), a good quality
film was obtained corresponding to self-assembly property planar phenantrene center. Owing
to this property, electrochromic performance of poly-(MES16) such as response time,
stability and ∆T% were better than the standard poly-(TST).
Keywords: Phenantrene, Electrochromic polymers, Spectroelectrochemistry
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1. Introduction
Electrochromic materials can be defined as a reversible change in the absorption, reflection,
or transmission of optical radiation upon the material is involved in an electrochemical
reductive or oxidative process in the visible, ultraviolet (UV), infrared (IR), and also
microwave regimes of the electromagnetic spectrum.[1] First, inorganic semiconductors such
as tungsten trioxide (WO3)[2, 3] and iridium dioxide (IrO2)[4, 5] were used as electrochromic
material. In comparisons to the inorganic semiconductors, conjugated polymers have some
great advantage in electrochromic applications such as high coloration efficiency, low redox
potential, high optical contrast, the ability large-area device manufacturing by the solution
procesability.[6-8] Besides, the HOMO-LUMO band gap can be adjustment with
modification of the chemical structures by means of easily tuning of colors.[9] Due to their
advantages, conjugated electrochromic polymers have important applications in technology
such as smart-windows,[10] electrochromic mirrors,[11] anti-glare screens,[12]
electrochromic display devices.[13]
Donor acceptor conjugated polymers have a great importance in electrochromic applications
due to their good film forming properties, multi-electrochromic behavior, extended switching
stability, fast response time, and low operation potential.[14-16] In 2004, Fred Wudl’s group
reported the first neutral state green polymer from the thiophene-benzothiadiazole based
donor acceptor sturucture.[17] Due to charge tranfer from thiophene-donor to
benzothiadizaole-acceptor, a dual absorbance occurs in the visible spectrum to complete the
RGB colors. Further, Reynolds’ and Toppare’s groups improve the electrochomic
performance of the system by the chemical modification the donor-acceptor polymeric
sturucture.[17-20] Finally, Reynolds et al. has been completed to all CMYK color scale when
obtained the yellow colored polymer.[21] All neutral-state RGB and CMYK colored
polymers having a single chromophore demonstrated excellent transmissivity at the oxidized
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state.[22] Recently, electrochromic performance lifetime and processibility of the polymers
can be enhanced by the side chain modification of the polymers.[23-25]
The polycyclic aromatic hydrocarbons (PAH) such as phenantrene, anthracene, perylene
pyrene, chrysene etc. have significant attention for the optoelectronic technology because of
structural rigidity and strong π-stacking interactions on the film surface.[26, 27] These PAH
derivatives have high chemical stability against environmental conditions and excellent
mechanical strength due to consisting of a two-dimensional (2D) sheets of sp2-hybridized
carbon atoms.[27] Because of these useful properties, the PAH have found broad applications
in organic light-emitting diodes (OLEDs),[28] organic field effect transistors (OFETs),[29]
optical sensors,[30] liquid crystals[31]. Furhermore, the PAH have great importance for the
organic photovoltaic devices (OPVs)[32, 33] and electrochromic materials (ECDs)[34] to
improve the morphology on active area of the devices. On the other hand, PAH seems to be a
good solution for the stability problem of the optoelectronic devices.[35]
Herein, we report the planar phenantrene center effect on the electrochemical, optical, and
also electrochromic performance of a donor-acceptor polymer structure. The optical band gap
of poly-(5,10-bis(4-hexyl-2-thienyl)phenantro[9,10-c][1,2,5]thiodiazole) (poly-MES16) and
poly-(4,7-di(thiophen-2-yl)-2,1,3-benzothiadiazole) (poly-TST) were measured as 2.51 and
1.54 eV, respectively. According to theoretical DFT calculations, a good charge separation
was observed at HOMO and LUMO levels of MES16 sturucture. The lamellar morphology
could be obtained on the electrochemically prepared polymer film surface due to the solid
state π-π* interactions and also self-assembling properties of the poly-(MES16) containing
planar phenantrene center. On the other hand, the neutral state color of the polymer was
changed from the violet to yellow upon insertion of the phenantrene center instead of
phenylene moiety between thiophene-donor and benzazole-acceptor. In comparison to TST
standard polymer, poly-(MES16) has much better electrochromic performance in terms of
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high resistance to over oxidation and high contrast ratio at the visible regime as well as
multielectrochromic behavior under applied potential.
2. Experimental
2.1. Materials
All chemicals used without further purification were purchased from commercial suppliers.
TST was synthesized according to previously published procedures[36]. On the other hand,
initial compounds of MES16 (1,2,3 and 4) was prepared according to slightly modified
procedure from the literature (Scheme 1).[32]
HO
N
N
OH
O
O
H2N
NH2
b
a
(2)
(1)
c
S
S
N
N
N
N
d
I
I
(3)
(4)
Br
S
e
S
S
N
N
N
N
N
N
S
S
e
S
S
Br
TST
MES16
CH3
H3C
CH3
H3C
a) NH2OH.HCl pyridine/EtOH, 130
°C, 3h ; b)EtOH, Pd/C, N2H4OH, 80
°C, 24h.; c) DMF, S2Cl2, 0
°C, 2h.; d)
NIS, H2SO4, Ag2SO4, rt. 1h; e) 2-(Tri-n-butylstannyl)-4-hexylthiophene, Toluene, Pd(PPh4), 110 °C, 24h.
Scheme 1. Synthetic route to MES16 monomer
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2.2. Synthesis of 5,10-bis(4-hexyl-2-thienyl)phenantro[9,10-c][1,2,5]thiodiazole (MES16)
4 (0,25 g, 0.5 mmol), 2-(Tri-n-butylstannyl)-4-hexylthiophene (0,7 g, 1.5 mmol) and 15 mL
toluene were added in a 100 ml flask and stirred under argon atmosphere, for 30 minutes.
After adding the Pd(PPh3)4 (5 mol %) as a catalyst, the reaction mixture was refluxed at
o
110 C for 24 hours. Finally, the reaction mixture was cooled and precipitated in 500 mL of
ethyl alcohol. Column chromatography [silica gel, CHCl3:Hexane (1:1)] was carried out for
the final purification. The pure yellow product (MES16) was obtained and then dried at 40°C
in vacuum oven (Yield: 44% )
FT-IR (cm-1): (C–H aromatic) 3077, 3046; (C–H aliphatic) 2952, 2917, 2851; (C=N) 1611;
(C=C phenyl) 1575, 1545, 1567; (C-S thiophene) 741; 1H-NMR (CHCl3-d):
δ ppm, , 8.61
(s, 2H, Ar-Hc); 7.71 (s, 2H, Ar-He); 7.62 (d, 2H, Ar-Hd); 7.21 (d, 2H, Ar-Ha); 6.63 (d,2H, Ar-
Hb); 2.58 (t, 4H, Hf); 1.61 (m, 8H, -C2H4-); 1.22 (m, 8H, -C2H4-); 0.88 (t, 6H, -CH3).
S
N
N
c
S
d
S
e
b
a
f
CH3
H3C
2.3. Instrumentation
Fourier transform infrared spectra (FT-IR) were recorded on a Perkin Elmer FT-IR Spectrum
1
One by using an attenuated total reflectance (ATR) module (4000–650 cm−1). H-NMR
spectra were recorded on a Bruker Advance DPX-400 at 25°C in deuterated chloroform
solutions with tetramethylsilane (TMS) as internal standard. Electrochemical analyses were
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performed by Biologic SP50 potentiostat–galvanostat system with a platinum disk (0.02 cm2)
as working electrode (WE), Ag wire as reference electrode (RE) and Pt wire as counter
electrode (CE). Electrochemical polymerization of MES16 and standard compound TST were
realized in a dichloromethane solution consist of 2.0×10-3 M monomer and 0.1M TBAPF6 at
100 mV/s scan rate. The polymer was coated on platinum disc (0.02 cm2) or ITO/glass
electrode (8–12
Ω, 0.8 cm×5 cm). After the repeating scan in the system, the MES16 and TST
polymer thin films were washed with DCM and ACN to eliminate from the electrolyte salt
and other impurities. Electrochemical HOMO-LUMO band gap calculated from the
oxidation-reduction onset potentials were calibrated against ferrocene redox couple
E°(Fc/Fc+) = +0.30 V. UV-vis absorption spectra were recorded by an Analytic Jena Speecord
S-600 diode-array spectrophotometer.[37] The optical band gaps (Eg) of polymers were
calculated from their absorption onsets.[38] The fluorescence emission spectra were taken by
a PTI QM1 fluorescence spectrophotometer. Spectroelectrochemical measurements were
carried out to consider absorption spectra of polymer films under applied potential.[39] The
color coordinates are determined by three characteristics; luminance (L), hue (a), and
saturation (b) in Commission Internationale de l’Elcairage (CIE) system.[40] Analytic Jena
UV–vis spectrophotometer with chromameter module was used to determine these
characteristics. Surface morphology of the polymer films were investigated at ambient
condition and room temperature using a Nanosurf Naio-AFM. To take topographic and phase
images, the non-contact mode was used in this system. 20 µm scanner equipped with silicon
tips with 15 nm tip-curvature and ITO coated glass substrate was used for measurements. The
system was protected with acoustic chamber to avoid electromagnetic noises.
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3. Results and discussion
3.1. Synthesis and Characterization
Initial compounds (1, 2, 4 and 4) were obtained according to the literature.[32] Synthesis of
MES16 and TST electroactive monomer was obtained by Stille-coupling reaction between
2-(Tri-n-butylstannyl)-4-hexylthiophene and dibromo-thiadizole compounds. Then, chemical
sturucture of MES16 and TST were clarified by FT-IR, and 1H NMR (Fig 1).
Figure 1. 1H NMR spevtrum of MES16
Repeated scan electrochemical deposition behavior of a solution of 2.0×10-3 M of MES16 and
TST was investigated in 0.1M TBAPF6/CH2Cl2 electrolyte and 100 mV/s scan rate. Repetitive
cycling deposition of MES16 and TST were carried out at 0 – 1.4 V by potentiodynamic
electrochemical process. New semi-reversible oxidation waves were detected at
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ox
p,a
ox
p,c
ox
p,1/ 2
ox
p,a
ox
p,c
ox
p,1/ 2
E
= 1.10 V, E =1.22 V and E
= 1.16 V and E = 0.53 V, E =1.03 V and E
=
0.78 V in the anodic scan of MES16 and TST, respectively. The peak intensities amplified
after each successive cycle during the formation of polymer on the ITO/glass working
electrode surface (Fig. 2). Then, poly-(MES16) and TST thin film were washed with ACN
and DCM to eliminate the unreacted monomers and/or oligomeric species.
Figure 2. Electrochemical polymerization of MES16 (a) and TST (b) in 0.1 M TBAPF6 in
CH2Cl2, scan rate 100 mV/s.
3.2. Optical Properties
The optical behaviors of MES16 monomer and TST standard compound were examined by
UV-Vis absorption and fluorescence measurements recorded in CH2Cl2 (liquid phase). Figure
2 exhibits the absorption spectra of TST, MES16 and their corresponding polymers.
Absorption spectrum of TST shows a characteristic peak at λmax = 310 attributed to π-π*
transition of the conjugated system. Besides, resulting from the charge transfer from
thiophene-donor to benzathiadiazole-acceptor moieties, a charge transfer band was also
observed at λmax = 450. Characteristic π-π* transition band of MES16 was observed at higher
wavelength (λmax = 360 nm) due to extended conjugation in the phenanthrene moiety than that
of phenylene. Further, the charge transfer band of MES16 was observed at lower wavelength
and lower intensity than that of TST because of the electron localization of the condensed
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phenentrene center. On the other hand, the absorption bands of poly-(MES16) and poly-(TST)
bathochromicaly shifted at about 50-100 nm due to increasing π conjugation on the main
chain of the structures. The onset wavelength of poly-(TST) was observed at 790 nm
corresponding to 260 nm red-shift. MES16 has ring-inner electron localization due to having
condensed phenylene rings on the center of the sturucture. Therefore, only a 85 nm red-shift
was occurred for poly-(MES16) when compared the MES16 monomer. The optical band gaps
were found to be 2.51 eV and 1.54 eV poly-(MES16) and poly-(TST), respectively (Fig. 3).
Figure 3. UV-vis absorption spectra of MES16 and TST (a) and poly(MES16) and poly(TST)
(b) in CH2Cl2.
The fluorescence spectrum of TST exhibited that emission maxima (λmax: 454 nm) shifted 130
nm according to MES16 (λmax: 584 nm) because of a red-shift in absorbance of charge transfer
band (Fig 3). Because of this, the orange color emission turned into cyan upon attaching the
phenantrene center unit instead of phenylene (Fig 3).
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Figure 4. Fluorescence spectra of MES16 and TST in CH2Cl2. Insets: the corresponding photos
of compounds with UV light (366 nm).
3.3. Electrochemical properties
The electrochemical behaviors of TST and MES16 monomers and the corresponding
polymers poly-(TST) and poly-(MES16) were examined by cyclic voltammetry (CV). MES16
ox
exhibited an irreversible oxidation peak at E =1.60 V vs. Ag wire in the anodic regime
m,a
(Fig. 4). However, a semi-reversible oxidation peaks were observed at lower potential for
TST standard compound (Eomx,a = 1.12 V and E ox = 0.78 V; Eomx,1/2 =0.95 V). This result
m,c
indicated that MES16 has ring-inner electron localization due to having triple phenylene
rings. After the polymerization process, reversible electrochemical oxidation at lower
ox
p,a
ox
p,c
ox
p,1/2
potential was observed in the poly(MES16) (E = 1.27 and E = 1.19 V; E
= 1.23 V)
ox
p,a
ox
p,c
ox
p,1/2
and poly(TST) (E = 1.25 and E = 1.13 V; E
= 1.19 V).
Due to reduction process of thiadiazole acceptor moiety in the cathodic regime, TST standard
red
red
molecule exhibits characteristic reversible reduction peaks at E =-1.38 and E =-1.32 V ;
m,c
m,a
red
m,1/2
E
=-1.35 V (Fig. 4). The reduction peaks of MES16 were observed at more negative
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red
m,c
red
m,a
red
potential (E =-1.57 and E =-1.51 V; E =-1.54V). These slightly higher negative peak
m,1/2
potentials for MES16 as the phenanthrene based monomer can be attributed to more localized
electron density on the thiadiazole center when compared to TST standard molecule. Hence,
MES16 needs a more negative potential for the electron addition into the system. Besides,
because of the increasing π-π* conjugation on the polymer backbone at the end of the
polymerization, more negative shift was observed. Finally, a reversible reduction peak was
observed with a half wave potential (E rpe,da = -1.66 V and E red = -1.74 V; E red = -1.70 V for
p,c
p,1/2
the poly-(MES16). In addition, HOMO-LUMO energy levels and electrochemical band gaps
of MES16, TST and corresponding polymers were given in the Table-1.
Figure 5. Cyclic voltamogram of (a) TST, (b) MES16, (c) poly-(TST), (d) poly-(MES16) in
0.1M TBAPF6/DCM, scan rate 100 mV/s.
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Table 1. HOMO and LUMO energy levels, electrochemical and optical band gaps of TST,
MES16, poly-(TST) and poly-(MES16).
Oxidation
potential
(V)
Reduction
potential
Optical Electrochemical
HOMO
(eV)
LUMO
(eV)
Molecules
MES16
band gap
(eV)
band gap
(eV)
(V)
= -1.57
red
m1,a
E
E
E
ox
m,a
E
= 1.60
-5.66
-5.41
-5.44
-5.49
-3.02
-2.92
-3.07
-3.20
2.93
2.51
2.34
1.54
2.64
2.49
2.37
2.29
red
m1,c
= -1.51
ox
p,a
red
p,a
E
= 1.27
= -1.74
Poly(MES16)
TST
Eopx,c = 1.19
Erpe,dc = -1.66
ox
m,a
red
E
= 1.12
E
= -1.38
m1,a
Eomx,c = 0.78 E rme1d,c = -1.32
ox
p,a
red
p,a
E
E
= 1.25
= 1.13
E
E
= -1.38
= -1.16
Poly(TST)
ox
p,c
red
p,c
3.4. Surface Morphology
Surface morphologies of poly-(TST) and poly-(MES16) films were examined by atomic force
microscopy (AFM) (Fig. 5). Polymer films were obtained by potenthiodynamic
electrochemical polymerization process. After the coating process, the polymer films were
obtained at 100 nm in thickness. Owing to the planar structure of phenanthrene center, solid
state π-π* interaction is increased and the lamellar morphology could be obtained due to the
self-assembling properties of poly-(MES16) (Fig. 5) Besides poly-(MES16) film has more
smooth surface than that of Poly-(TST) (Fig. 5). The roughness values of the polymer films
were calculated to the 6 nm for poly-(MES16) and 14 nm for poly-(TST), respectively.
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Figure 6. AFM images of a) poly-(MES16) and b) poly-(TST) films; i) height image,
ii) phase image, and iii) topography image.
3.5. DFT Calculations
Frontier molecular orbitals have been investigated by Density Functional Theory (DFT)
calculations with Spartan10 program at the parameters of B3LYP and 6-31+G* basis set to
get a better understanding of the charge distribution effect on the planar center moiety of
donor-acceptor structure. The charge distribution in the frontier molecular orbitals and
theoretical calculated HOMO-LUMO levels is shown in Table 2. As displayed in
Table-2, HOMO orbitals (HOMO and HOMO-1) are delocalized on the electron-donating
main chain thiophenes and neighboring benzene as well as phenantrene. On the other hand,
LUMO orbitals (LUMO and LUMO+1) are more localized on the electron-accepting benzo-
or phenantro-thiadiazole core. The geometry optimized structure exhibited that the MES16
molecule has a higher planarity than that of TST as expected. Because of this property, the
charges are more localized at both HOMO and LUMO on the MES16 structure. The HOMO-
LUMO band gap calculated by DFT was observed to increase from 2.77 eV to 3.43 eV upon
attached phenantrene moiety between 3-hexylthiophene donor and thiadiazole acceptor. On
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the other hand, all theoretical calculated HOMO-LUMO values are lower than experimental
values from the CV and UV-vis measurements.
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Table 2. Molecular orbital diagrams of the HOMO and LUMO levels of MES16 and TST.
MES16
TST
LUMO+1
LUMO
HOMO
HOMO-1
Theoretical
Band Gap
3.43 eV
2.77 eV
(Eg)
3.6. Spectro-electrochemical properties
The electrochromic behavior of poly-(TST) and poly-(MES16) films were investigated by
spectro-electrochemical measurements using a diode-array spectrophotometer under applied
potential.[39] During to anodic scan of poly-(TST) film (0 - 1.6 V), the valance band-
conduction bands (π→π* transition) at about 325 and 525 nm started to decrease, whereas a
new absorption band at 825 nm intensified in the NIR region by the increased positive
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potential. The purple color (L*: 38.9; a: 7.1; b: -14,9) of the film was bleached and converted
to light blue (L*: 64.2; a: -0.4; b: -15.2) during the oxidation process (Fig 7a).
It is observed that the neutral state color of the polymer film converted from purple to yellow
upon attaching the phenantrene moiety instead of phenylene on the conjugated polymer main
chain. Upon oxidation of poly-(MES16), an absorption band at 370 nm corresponds to
valence band-conduction band (π→π* transition) started to decrease and the new absorption
bands intensified at 600 nm which indicated the formation of polarons and bipolarons on the
polymer backbone. Consequently, yellow color of the poly-(MES16) film (L*:83.9; a: -7.6; b:
33.8) at the neutral state turned to many different tones of green color (L*:72.3 ; a: -11.6; b:
63.1---L*:72.3 ; a: -11.6; b: 63.1---L*:52.6 ; a: -9.7; b: 41.1) by intensified the absorption
bands in the visible regime. (Fig. 7b)
Figure 7. UV-Vis spectra and the color changes during the applied potential of poly-(TST)
(a) and poly-(MES16) (b) films on ITO/glass surface in 0.1 M TBAPF6/ACN.
Kinetic performances of poly-(MES16) and poly-(TST) were investigated by the square-wave
voltammetry which is observed difference of transmittance at the point of absorption maxima
with against to time while applying potential between redox states of polymers with a
residence time of 10 s (Fig. 8). There are a few parameters to determine the electrochromic
performance of the polymer films such as stability and percentage transmittance change
(∆T%) against to applied potential as well as response time. ∆T% and response time of
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poly-(MES16) and poly-(TST) were determined between 0-1.6 V with switching time of 10s.
As a result of these measurements, ∆T% values of poly(MES16) (at 600 nm) and poly-(TST)
(at 900 nm) were measured as 47% and 40%, respectively (Fig. 8). According the stability test
under applied potential after about 5000 cycles, poly-(MES16) and poly-(TST) preserve the
optical activity as 98% and 71%, respectively. The oxidation and reduction response times
were also calculated as 3.2 and 3.4 s for poly-(MES16), 2.1 and 2.8 s for poly-(TST),
respectively. This phenomenon can be explained that slower charge exchange between
electrolyte solution and polymer chain was occurred during the oxidation process due to the
electron localization in the phenanthrene ring at the oxidized state.
Figure 8. Electronic absorption spectra for poly-(MES16) (a) and poly-(TST) (b) films upon
p doping between 0 and 1.6 V in a monomer-free 0.1 M TBAPF6/ACN solution.
Coloration efficiency (CE) is another important parameter for the electrochromic applications.
CE was calculated by this equation: CE = ∆OD/Qd and ∆OD = log (Tcolored/Tbleached
)
[Tbleached: Transmittance at neutral state, Tcolored:Transmittance at oxidized state and
Qd: Injected/ejected charge between redox states.[41] According to this equation, CE values
of poly-(MES16) and poly-(TST) were calculated as 228 and 132 cm2 C–1, respectively. It can
be clearly observed that CE value of poly-(MES16) is approximately two times greater than
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that of poly-(TST). Considering all data, electrochromic performance of the poly-(MES16)
was better in comparison to poly-(TST).
4. Conclusion
Herein, we have reported the effect of planar center unit on the electrochromic performance of
the conjugated polymers. It has been observed that thin film morphologies, optical and the
electrochemical properties were greatly influenced from the presence of the phenantrene
center unit on the conjugated structure. According to DFT calculations, the charges are more
localized at both HOMO and LUMO on the MES16 when compared with the TST standart
molecule. MES16 electroactive monomer was electrochemically polymerized onto transparent
ITO/glass surface to give a quite stable multi-electrochromic polymer with a high contrast
ratio in the visible regime (∆T = 47% at 600 nm). In comparison to poly-(TST) as a standard
polymer, poly-(MES16) has much better electrochromic performance. Owing to a high
contrast ratio in the visible regime and an impressive electrochemical stability against to
applied redox potentials, poly-(MES16) is a possible good candidate for the electrochromic
applications.
Acknowledgements
We also gratefully acknowledge to Scientific and Technical Research Council of Turkey
(TUBITAK, project #: 113Z581) for financial support.
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Highlights
► We report the planar phenantrene center effect on the electrochemical,
optical, and also electrochromic performance of a donor-acceptor polymer
structure.
► The neutral state color converted the violet to yellow upon insertion of the
phenantrene center instead of phenylene moiety.
► In comparison to TST standard polymer, poly-(MES16) has much better
electrochromic performance especially high resistance to over oxidation.