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Synthesis and Characterization of Thiophene and Thiazole Containing Polymers
Accepted Manuscript Title: Synthesis and Characterization of Thiophene and Thiazole Containing Polymers Author: B. Ustamehmeto˘glu PII: DOI: Reference:
S0013-4686(13)02583-8 http://dx.doi.org/doi:10.1016/j.electacta.2013.12.130 EA 21931
To appear in:
Electrochimica Acta
Received date: Revised date: Accepted date:
21-6-2013 2-12-2013 23-12-2013
Please cite this article as: B. Ustamehmeto˘glu, Synthesis and Characterization of Thiophene and Thiazole Containing Polymers, Electrochimica Acta (2014), http://dx.doi.org/10.1016/j.electacta.2013.12.130 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Synthesis and Characterization of Thiophene and Thiazole Containing Polymers B. Ustamehmetoğlu İstanbul Technical University, Department of Chemistry, 34469 Maslak, İstanbul, Turkey.
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[email protected](ISE member)
In
this
study,
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Abstract
bis(thiophene)-bis(ethylenedioxythiophene)-3,3’-dinonylthiazole
us
(ThEdNBTEdTh) comonomer was obtained by using organometallic polycondensation mediated by organotransition metal complexes. The properties of electrochemically obtained
an
poly[bis(thiophene)-bis(ethylenedioxythiophene)-3,3’-dinonylthiazole]
(P[ThEdNBTEdTh])
from this comonomer were characterized by electrochemical techniques such as voltammetry electrochemical
impedance
spectroscopy
M
and
(EIS).
Oxidative
polymerization
of
ThEdNBTEdTh was carried out in dichlorometane onto a platinum electrode. Effect of
were
further
characterized
by
te
films
d
polymerization charge on the redox behaviour of polymer films was investigated and these ATR-FTIR,
UV-Visible
and
fluorescence
Ac ce p
spectrophotometric measurements. EIS studies revealed that high capacitance values observed in the lower frequency region and proposed electrical equivalent circuit was applied to the experimental data to correlate the results, obtain the parameter of each element and explain the interface between the Pt/ P[ThEdNBTEdTh /Bu4NPF6 system. Results showed that polymer has low oxidation potential, low band gap (1.67 eV) and good stability in the oxidized state and it shows p- and n-dopable properties. n-doping was verified experimentally by ex-situ FT-IR spectroelectrochemical measurements. The measured capacitance values are quite promising for supercapacitor applications. Key words: Bis (thiophene)-bis (ethylenedioxythiophene)-3, 3’-dinonylthiazole, synthesis and electropolymerization, EIS, absorption and emission properties.
1
Page 1 of 43
1. Introduction There are a lot of application areas of conducting polymers from optical information displays to rechargeable batteries. A modification of electrically conducting polymers is a promising line
polymers
and
produce
polymeric
materials
with
new
ip t
of inquiry, which allows one to expand the range of physicochemical characteristics of properties.
Organometallic
cr
polycondensation reactions are raised great interest for the design of new classes of conjugated
us
materials with enhanced semiconducting properties [1-3]. Donor–acceptor–donor type molecules find different applications in electronic and optical devices [4-9].
an
Polythiophene (PTh) has rapidly become the subject of considerable interest [10-14]. The high environmental stability of both doped and undoped states of PTh together with its structural
materials, and organic semiconductors.
M
versatility has led to multiple developments aimed at applications such as conductors, electrode
d
After the development of new PTh derivative, poly (3, 4-ethylenedioxythiophene), (PEDOT)
te
during the second half of the 1980s, an exponential increase in the number of publications was
Ac ce p
observed [15-19]. PEDOT shows high conductivity, lower oxidation potential compared to PTh, high stability at doped state, low band gap and these properties make it attractive for many applications.
Recently thiazole-based conjugated polymers have received great interest [20-30]. Introduction of nitrogen atoms in addition to sulphur in the thiophene (Th) ring improve the properties of resulting polymers which are desirable for applications. In this study, nonylbithiazole (NBT), 3, 4-ethylenedioxythiophene (EDOT) and Th were chosen to obtain comonomer because of especially ! - staking and n-type transporters properties of BTh, superior electrical properties of EDOT and stability at both neutral and oxidized state for PTh. P [ThEdNBTEdTh] is expected to have useful properties for the design of a diode, transistor or capacitor. 2
Page 2 of 43
2. Experimental 2.1. Equipment and materials Cyclic voltammetry (CV) experiments, spectroelectrochemistry and electropolymerizations
ip t
were performed with a Gamry 600 model potentiostat/galvanostat interfaced to a PC. A Pt button electrode (0.022 cm2), a platinum wire and a silver wire were used as working, counter
cr
and pseudo reference electrodes respectively. The silver wire was calibrated externally using a
reported versus Ag/AgCl as suggested in literature [31].
us
5mM solution of ferrocene/ferrocenium (Fc/Fc+) couple in the electrolyte and the potentials are
an
Spectroelectrochemical data were recorded on a Schimadzu 160 A model UV–visible spectrophotometer. A three-electrode cell system was used where the working electrode was a
M
custom cut ITO-coated glass slide (8mm×50mm×1.1 mm, 30mm), counter electrode was a Pt wire and pseudo-reference electrode was Ag wire.
te
model Spectrophotometer.
d
The ATR-FTIR- spectra measurements were performed by using Perkin Elmer Spectrum one
Ac ce p
Voltammetry and EIS were used for electrochemical polymerization and characterization methods. Potentiodynamic polymerizations were done by using 10, 20, 35 and 50 cycles. Q1 and Q2 are the polymerization charges obtained in two different potential ranges (0.2-1.0 V and 0.2-1.35V) respectively by integration of CV curves for 10 (Q1(10) (Q2(10)) and 20 ((Q1(20)and Q2(20)) cycles.
The electrolyte was 0.1M tetraethylammonium hexafluorophosphate (TBAPF6) obtained from Fluka, dichloromethane (DCM) supplied by Merck. Ethylenedioxythiophene (EDOT), thiophene (Th), bromodecanon, dithiooxamide, bromine, trans-dicholorobistriphenylphosphine palladium (II) chloride, butyl lithium, trimetiltinchloride, 2-trimethylstannyl thiophene MgSO4 and ethanol were all analytical grade and used without further purification. During the synthesis of the comonomer, all preparations and reactions were performed under a nitrogen atmosphere; 3
Page 3 of 43
reagent grade solvents were dried and distilled before use. Tetrahydrofuran (THF) and toluene purchased from Carlo-Erba were distilled over sodium.
2.2. Comonomer Synthesis Methods
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Synthesis of bis (thiophene) – bis (ethylenedioxythiophene) - 3,3’-dinonylthiazole (ThEdNBTEdTh) was synthesized by Stille coupling reaction. Synthesis of the bis (3,4-
cr
ethylene-dioxythiophene)-(4,4-dinonyl-2,2-bithiazole) (ENBTE) comonomer and their bromine
us
and trimethylstannyl derivatives are similar to the literature procedures previously described for alkylbithiazole and its derivatives [8]. The mixture of 2’2’dibromoENBTE and 2-
an
trimethylstannyl thiophene was refluxed for 20 h. After cooling to room temperature the reaction mixture was poured into water, and dichloromethane, pentane and chloroform were
M
added. Extracted organic phase was washed with water and dried over MgSO4 and the solvent was rotary evaporated. The final product was re-crystallized in ethanol and obtained as reddish
te
Ac ce p
given in Scheme 1.
d
orange solid with the molecular weight of 806 g/mol. Yield was 45%. Synthetic route was
Scheme
1.
Synthetic
routes
for
bis
(thiophene)-bis
(ethylenedioxythiophene)-3,3’-
dinonylthiazole (ThEdNBTEdTh).
H-NMR spectrum was collected on Agilent VNMRS 500 MHz in CDCl3 and the absence of protons at about 6.3 ppm which belong to EDOT rings and the presence of protons at about 7.2 and 7.4 ppm which belongs to Th shows the coupling and proves the formation of comonomer (Fig.1).
Figure 1. 1H NMR of the ThEdNBTEdTh in CDCl3. ! (ppm) = 0.76-0.86 (m, 6H), 1.22-.151 (m, 24H), 2.10 (m, 4H), 3.37-3.47 (t, 4H), 5.23 (m, 8H), 7.20 (m, 2H), 7.37 (m, 4H). 4
Page 4 of 43
Optimized geometry of comonomer was obtained by DFT method and given in the Scheme 2.
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Scheme 2. Optimized geometry of ThEdNBTEdTh.
3. Results and Discussion
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3.1. Voltammetric Measurements
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Anodic polymerization curve of comonomer was obtained and the oxidation onset was obtained as 0.54 V. A conjugated polymer that shows both oxidation and reduction onsets gives
an
possibility to find the electrochemical band gap and estimate the ionization potential, IP and electron affinity, Ea with the same experiment. IP values are important for molecular design of
M
hole transport material for organic light-emitting diode applications. It is reported that IP values of flourene containing organic molecules can be changed from 5.42 to 5.80 eV, when the kinds
d
or positions of the substituents in the molecule are changed [32].
te
To tranpoze the measured redox behaviour into estimates the IP and Ea, it is necessary to relate the electrochemical potential to the vacuum level. Janietz et al [33] reported a relationship
Ac ce p
between IP and Ea values with the oxidation and reduction onset potentials of the same sample as suggested by Bredas [34];
IP = 4.4 + Eonset,ox (eV)
(1)
Ea = 4.4 + Eonset,red (eV)
(2)
where Eonset,ox, and Eonset,red are the onset potentials for the oxidation and reduction relative to Ag/AgCl reference electrode. Although Cardona et al [35] suggested the value of 4.4 eV vs SHE, in several studies 4.2-4.6 eV vs SCE was also reported [33,34]. In this study, in order to gain some ideas on the properties of ThEdNBTEdTh comonomer and its polymer, IP values were calculated by using the equation 1. Eonset,ox was obtained by extrapolating the electrochemical onset of the first oxidative scan of linear sweep data where 5
Page 5 of 43
the current starts to differ from the baseline and calculated IP values were compared with the value of starting monomers and polymers (Table 1). As it can be seen, the Eg of ThEdNBTEdTh is lower than the value of starting monomers and higher than the polymer as expected.
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Polymerization of ThEdNBTEdTh was performed in CH2CI2 containing 0.1M TBAPF6 at 10 cycles in the range of 0.2-1.0V (Fig. 2a) and resulting P [ThEdNBTEdTh] showed good redox
cr
behaviour even at very high scan rates (Fig. 2b). Scan limit was increased up to 1.35V during
us
polymerization (Fig. 3a) and obtained film was investigated at different scan rates (Fig. 3b). The reversible redox behaviour of this film was compared with the film obtained at the same
an
cycle number up to 1.0V scan limit and it can be seen that, since the polymer charge is high due to higher sweep potential limit, the higher current intensity was observed in the case of 1.35 V
M
(Fig.4a). Thicker polymeric film was obtained by applying 20 cycles and this film also shows reversible redox behaviour up to 500 mV/s scan rate in both potential ranges of 0.2-1.0V and
d
0.2-1.35V. The redox behaviours of two films obtained up to 1.0V and 1.35V scan limits, were
te
compared (Fig.4b) and it is observed that while the reversible redox behaviour was observed at
Ac ce p
lower potential limit, due to some degradation, it loses its reversibility as the potential limit was increased.
Figure 2. a)Polymerization of ThEdNBTEdTh in CH2CI2 containing 0.1M TBAPF6 at 10 cycles in the range of 0.2-1.0V(Q1(10) = 2.27! C) b)Scan rate dependence of P [ThEdNBTEdTh] in CH2CI2 containing 0.1 M TBAPF6. Figure 3. a)Polymerization of ThEdNBTEdTh in CH2CI2 containing 0.1M TBAPF6 at 10 cycles in the range of 0.2-1.35V. (Q2(10) = 54.8! C) b)-Redox behaviour of resulting polymer at different scan rates. Figure 4. Comparison of redox behaviour of polymeric films obtained at different charges by applying 10 cycles (a), 20 cycles (b). 6
Page 6 of 43
Polymerizations by applying further cycling such as 35 and 50 cycles in the range of 0.2-1.0 V were also performed and redox behaviours of resulting polymers at different scan rates were obtained. Scan rate dependence of all these polymeric films was tested according to scan rate
diffusion controlled
cr
behaviour when they obtained by applying 10, 20, 35 cycles and
ip t
and square root of scan rate (Figure 5 a,b) . It is found that polymeric films show thin film
behaviour was observed in the case of 50 cycles.
us
Figure 5. a) Ip vs ! 1/2 and b) Ip vs ! graphs of P[ThEdNBTEdTh] films.
an
The average of the anodic and cathodic peak potentials which is known as half-wave potential ( E1/2 ) was calculated with following equation for different scan rates and the average value
M
was used: E1/2 = 1/2(Epc+ Epa)
(3)
terthiophene(TTh),
EdNBTEd
and
ThEdNBTEdTh
were
te
bithiophene(BTh),
d
Calculated E1/2 values were summarized in Table 1. As it can be seen, when Th,
electropolymerized, the half-wave potential of resulting polymers (E1/2,pol) increase with the
Ac ce p
increase in the number of monomeric unit of starting monomer. This result indicates a decrease of their conjugation length. This might be explained by a well-known oligomeric approach [3639]. In these studies it is suggested that the rate constant of dimerization of oligomers and their coupling steps with the original monomers decrease with increasing chain length. n-doping behaviour of the film was also investigated by CVs of P [ThEdNBTEdTh] at different scan rates and given in Fig 6. In the consecutive scans, the cathodic potential was extended to the more negative values and this part of voltammogram was given as inset. The excursion in the negative potential range resulted in formation of a cathodic peak around -1.57 V and in the backward scan a reoxidation peak was observed at -1.55 V. This peak corresponds to inclusion of the Bu4N+ cations in to the polymer. The n-doping influences also on the shape of CV in the p-doping range. A new prepeak about -0.1 V which was not observed when the CV were 7
Page 7 of 43
performed only in p-doping range (Fig. 2b, 3b, 4) was appeared. The nature of the prepeak was reported in the literature and attributed to the charge trapping [40-43]. The electrochemical band gap of P [ThEdNBTEdTh] was calculated from the onset potentials of anodic and cathodic branches as suggested in literature [33, 44] and onsets were found as
ip t
Eonset,ox = 0.0 V and Eonset,red = -1.44 V which yield IPpol = 4.40 eV (Table 1) and
cr
Eg(electrochemical) = 1.44 V (Table 2). As it can be seen IPmon(5.04 eV) could be decreased
us
significantly due to increase in conjugation length.
scan rate of 100 mV/s. (Inset: n-doping range).
an
Figure 6. Cyclic voltammograms of P[ThEdNBTEdTh] film in 0.1M TBAPF6/DCM at the
M
Table 1. Comparison of oxidation onset potentials(Eonset,ox)and ionization potentials (IP) of monomers and polymer* and half-wave potentials (E1/2)of polymers.
d
Recently it is suggested that, the intense infrared active vibration (IRAV) bands grow both in
te
the low energy range (700-1600 cm-1) due to a strong electron–phonon coupling within the
Ac ce p
molecule and at higher energies (1600–8000 cm-1) with a broad electron absorption [45]. In this study in order to gain further information on n-doping behaviour, the ex- situ ATR-FTIR spectra of a P[ThEdNBTEdTh] film was measured during reduction (n-doping) between -1.0 and -2.0 V in 0.1 M Bu4NPF6/DCM (Figure 7). An enlargement of the spectra in the wavenumber region of 1500–4000 cm
-1
was shown as inset in Figure 7. During the reduction,
the electronic absorption related to the formation of free charge carriers in the polymer continuously increases. When the CV of the polymer film in the full range that shows both pand n-doping is concerned (Fig 6), from the observation of the lower current intensities for negative charge carriers than the positive ones, the slight increase in the FTIR absorption spectra during n-doping can be understand.
8
Page 8 of 43
Figure 7. Ex-situ FTIR-ATR spectra recorded during reduction (n-doping) of a P(ThEdNBTEdTh) film in the wavenumber region 4000–650 cm-1 and in the wavenumber region 4000-1500–cm-1(inset) . Stability of polymer films were checked by CV before and after 300 cycles in the potential
ip t
range of 0.2-1.0 V at scan rate of 100 mV/s (Figure 8). The charge (Qcv) was obtained by integration and differs by 1.8 % between 1st and 300th cycles for the film obtained at 10 cycles
cr
(Figure 8a). As the thickness of the film increases, difference in charges increases up to 8.7 %
us
(Figure 8b). If we obtained the film at 35 cycles, difference becomes 11.3% (Figure 8c). But for the thicker film obtained at 50 cycles an interesting behaviour was observed and the charge
an
increased after 100 cycles (Figure 8d) which might be due to increase in charged states of polymer chain. Further oxidation might be result formation of polaron and/or bipolaron and
M
consequently the increase in doping degree and conductivity [46]. Figure 8. Comparison of stability of the P[ThEdNBTEdTh] films obtained at different cycle
te
d
numbers; a)-10, b)-20,c)-35,d)-50 cycles in monomer free solution.
Ac ce p
3.2. In-situ Spectroelectrochemical Measurements In-situ spectroelectrochemical measurements were done on ITO working electrode by applying different potentials in the range of -1.0 – 1.0 V and the spectra were given in Fig.9a. Maximum absorbance value of monomer was observed at 435 nm which is higher than the starting material due to the increasing conjugation length. Isobestic point, maximum absorbances of neutral and doped polymer were observed at 650 nm, 560 nm and 812 nm respectively. While polaron and bipolaron formations were observed separately for starting material [43], here, they were observed together. When the change in absorbances at 560 nm and 812 nm, was considered together with CV, the intersection of change of two wavelengths with potential superimposed with the onset potential of the second peak in CV (Figure 9b). This result shows that bipolaron formation starts after 0.75 V. 9
Page 9 of 43
The band gaps (Eg) of polymer film (on ITO) and monomers in solution were estimated by extrapolation of the low energy edge of the absorption spectra to the baseline and were collected in Table 2. Eg value of P [ThEdNBTEdTh] was compared with PTh, PEDOT,
ip t
P[ThNBTTh] and P[EdNBTEd] as well as starting comonomer ThEdNBTEdTh.
cr
Figure 9. (a) UV–Visible absorption spectra of P[ThEdNBTEdTh] film in 0.1M TBAPF6 in DCM at different applied potentials from the range −1.0 to 1.0V, (b) The change of maximum
us
absorbances as a function of potential together with the cyclic voltammogram of P[ThEdNBTEdTh] film.
an
As it can be seen, P [ThEdNBTEdTh] has a band gap lower than the starting materials (P [ThNBTTh] [30], P [EdNBTEd] [8] and PTh [47]) due to increase in the conjugation length as
M
expected. These results suggest that by changing the donor segments of starting material with monomer having different electron-donating ability, the absorption spectra and band gaps of
d
the D-A polymers can be tuned, which is an advantage for the design of the photovoltaic
te
materials. These results are in agreement with the previous study on starting monomer
Ac ce p
EdNBTEd [8, 29, and 30]. The optical band gap of polymer was found different from the electrochemical band gap as reported previously by Janietz et al [33]. Furthermore, if electron affinity values of molecules were calculated by subtraction of IP and optical band gap values, onset potential for the reduction (Eonset,red) was obtained by using equation 2. The same difference between experimental (-1.44 V) and calculated (-1.67 V) values can be better understand. This difference might be due to the possible uncertainties of CV experiments as explained by Cardona et al recently [35]. Table 2. Optical band gap (Eg) values of polymers. 3.3. Optical measurements
10
Page 10 of 43
Presence of long alkyl chain can make the P [ThEdNBTEdTh] soluble. Solubility of polymer was tested in DMF, THF and CH3CI and it was found that it is soluble in all. Absorption and emission spectra of monomer and polymer in DMF solution and polymer film obtained by casting were given in Figure 10. Thiazole rings expected to show also fluorescence behaviour,
ip t
so emission spectra of monomer and polymer were taken in DMF. Comparison of absorption and emission spectra of ThEdNBTEdTh and P [ThEdNBTEdTh] were given in Figure 10a and
cr
b respectively. Monomer has absorptions at 270 and 436, and emissions at 337 and 436
us
respectively when it is exited at 436 nm. The emission maxima of P [ThEdNBTEdTh] have also a red shift as compared to monomer. Emission in the visible region makes
an
P[ThEdNBTEdTh] favourable for LED application. Absorption wavelength of polymer has a red shift according to monomer as expected, due to the increase in conjugation length. A red
M
shift in the absorption maximum of polymer in solution and polymer film indicates ! -stacking behaviour which is reported for thiozole moiety [21, 24, and 48].
d
Figure 10. Absorption and emission spectra of a)- 1x10-6 M ThEdNBTEdTh and b)- 1x10-6 M
te
P[ThEdNBTEdTh] in DMF. c)-Absorption spectra of ThEdNBTEdTh in solution and P
Ac ce p
[ThEdNBTEdTh] both in solution and as a film.
3.4. Electrochemical Impedance Spectroscopy Nyquist and Bode diagrams of P[ThEdNBTEdTh] obtained at 10 cycles in the range of 0.2-1.0 V at different applied potentials (Edc) were shown in Fig.11 a, b. As it can be seen from Bode diagrams, polymeric films show generally capacitive behaviour in the frequency range of 1.0! 10-2-1.0! 102 Hz (Fig 11b). When Edc is equal or higher than 0.6 V, this capacitive behaviour was destroyed, the line curved and become closer to Zreal axis in Nyquist diagram. At Edc =0.0V, 0.25 V and 0.6V, phase angles were observed as 85 ° at 2.0x10-2 Hz, 8.0x10-2 Hz and 8.0x10-2 Hz respectively. When Edc was increased to 0.9V, it decreases to 65° at 8 Hz due to some degradation of the film (Fig. 11 b). Same measurements were performed for the 11
Page 11 of 43
film obtained at 10 cycles in the range of 0.2-1.35V (Fig. 11c, d). In this case, since the polymerization is carried out at higher potential, degradation of polymeric film starts after 0.4V. If the polymer obtained with 20 cycles in the range of 0.2-1.0 V, this decomposition starts at lower potential, Edc= 0.25V. Increase in thickness might result increase in the
ip t
delamination rate of the polymeric film. The film obtained at 20 cycles at higher sweep potential range (0.2-1.35V), loses its capacitive behaviour in all applied potentials. Same
us
cr
decomposition was observed for the film obtained at 35 and 50 cycles.
Figure 11. Nyquist and Bode plots of P[ThEdNBTEdTh] film obtained potentiodynamically
an
at 10 cycles in the range of 0.2-1.0 V (a, b) and 0.2-1.35V (c, d) at different applied potentials
M
(Edc).
Capacitance values were obtained from anodic and cathodic peak currents in CV as suggested
d
in literature by Bobacka et al. [49].
te
Ccv = Ip / ! .
Ac ce p
The low frequency capacitance (CEIS) values (10 mHz and 1.0 Hz) from EIS were calculated from the slope of the imaginary component of impedance vs. the inverse of the frequency (f) [29,30]:
CEIS = (2! fZim)-1
In Figure 12, variation of the Ccv with scan rate and comparison with CEIS (a) and CEIS with potential (b) of the P [ThEdNBTEdTh] films obtained at different cycle numbers were given. It can be seen that the capacitance values obtained by CV are rate dependent for thicker films and decreases with increasing scan rate (Figure 12 a). Although it is expected that Ccv and CEIS values to be different from each other due to conformational changes during the oxidation of polymer, here they are very close and they increase with increasing cycle numbers since the amount of polymer increase with the polymerization charge. 12
Page 12 of 43
The rectangular shape of the voltammogram is expected geometry for electrochemical supercapacitors devised from conducting polymers [49]. While CV of polymer films that obtained by applying 10 and 20 cycles have rectangular shapes, thicker films do not have this shape (Figure 8). When the Ccv values compared with CEIS values (Figure 12a), although these
ip t
two values are in agreement with each other for thinner film, some deviation starts for the film obtained at 35 cycles and this deviation becomes higher for the film obtained at 50 cycles.
cr
This might be due to diffusion controlled behaviour of the thick film. However all results were
us
given for comparison.
When the changes in CEIS values of films obtained at different cycles plotted vs. Edc, the
an
optimum condition can be determined as 50 cycles (0.25V) (Figure 12 b). At higher Edc values, the film destroyed and lost its capacitive behaviour. So CEIS values were not obtained for
M
thicker films at higher Edc. If the stability measurements were concerned (Figure 8), it is better to choose 20 cycles (0.25V). The symmetry of the anodic and cathodic Ccv curves (Figure 12 a)
d
as well as rectangular shapes, of CV (Figure 8) suggests that this polymer has expected
te
geometry for electrochemical supercapacitors. This behaviour was also observed for the
Ac ce p
starting materials (P[EdBThEd]and PEDOT) [29, 49].
Figure 12. Variation of the (a) Ccv with scan rate and comparison with CEIS and (b) CEIS with potential of the P [ThEdNBTEdTh] films obtained at different cycle numbers.
The equivalent electrical analog model for coated metal was reported previously [50] and commonly used for conducting polymers [52, 53]. We modified this model according to our experimental results to obtain the highest “goodness of fit” (5x10-4) which is in the acceptable range of fitting parameters for such calculations [54]. An equivalent circuit model, [R(C(R(C(R))))] which is best fitted to the experimental impedance data was used in simulation of the impedance behaviour of the thin film (Figure 13a). The first component of this model is 13
Page 13 of 43
the bulk solution resistance of the polymer and the electrolyte (Rsoln), the second one is the capacitance of the coating (Cc). Pore resistance (Rpo) is the resistance of ion conducting path. The second capacitance is double layer capacitance (Ccor), and the third resistance is charge transfer resistance (Rcor). In this circuit, Cc and Ccor are two constant phase elements that take
ip t
into account the interfacial irregularities such as porosity, roughness, and geometry. The impedance of a constant phase element, Y0 that has shown as ! in the equivalent circuit model,
us
cr
has the form:
an
When this equation describes a capacitor, Y0 = C (the capacitance) and the exponent ! = 1. For a constant phase element, the exponent ! is less than one. In this study in the equivalent circuit
M
model the exponents m,n for Ccor and Cc respectively were obtained 1.0 and 0.923. Contribution from the ohmic (electronic) resistance of the oxidized P [ThEdNBTEdTh] film to
d
Rsoln causes to a high value (1.65! 103ohm) as well as very high Rpo (45! 103ohm) values.
te
Another reason of high values might be porous structure of the film while Rcor has
Ac ce p
comparatively lower value (1! 109 ohm). A quite well agreement between experimental and simulated data was obtained for both Nyquist and Bode plots (Figure 13 b and c). The results obtained from simulated data were summarized in Table 3. Coating capacitance values obtained from equivalent circuit model, nonideal capacitor value, (Cc =14µF), experimentally from EIS (CEIS =20 µF) and from CV measurements (Ccv = 21µF) are all in good agreement.
Figure 13. Electrical equivalent circuit model (a) Experimental (dot) and simulated (solid line) Nyquist plot (b) Bode plot (c) of P[ThEdNBTEdTh] film.
Table 3. Impedance parameters of P[ThEdNBTEdTh] film obtained from equivalent circuit model. 14
Page 14 of 43
ip t
4. Conclusion ThEdNBTEdTh comonomer was firstly synthesized by Stille coupling reaction and
cr
polymerized electrochemically by CV. Characterization was done by using CV and EIS. The
us
film obtained by applying 20 cycles shows the best reversible redox behaviour even at very low scan rates. Stability of polymer films were checked before and after 100 cycles and the highest
an
difference in charges between 1st an100th cycles was obtained as 11.3%. Capacitive behaviour of P [ThEdNBTEdTh] starts at very low potential. Capacitance values are decreasing slightly
M
with increasing scan rate for thicker films (35 and 50 cycles). The rectangular shape of voltammogram and the symmetry of the anodic and cathodic capacitance values (Ccv) of P
d
[ThEdNBTEdTh] suit the needs of supercapacitors. To use this polymer as a capacitor, one
te
should obtain a film at 20 cycles by applying 0.25V. Presence of long alkyl chain makes the P [ThEdNBTEdTh] soluble. Polymer shows emission in the visible region which makes it
Ac ce p
favourable for LED applications. Absorption spectra of monomer and polymer in solution and solid state are different, showing the intermolecular ! -! stacking. P [ThEdNBTEdTh] shows both p- and n- doping properties, and band gap was calculated as 1.44 eV which is lower than optical band gap, 1.67 eV. Results suggest that by changing donor segments with monomer having different electron-donating ability, band gap of the polymer can be tuned, which is useful for the design of the photovoltaic materials. Cc, CEIS and Ccv values were found very close to each other. The proposed electrical equivalent circuit is good enough to explain the behaviour of Pt/ P[ThEdNBTEdTh /Bu4NPF6 system. Acknowlegment: Autor thanks to Dr. Burak Ulgut from Gamry Instruments for his support during equivalent circuit model analysis. 15
Page 15 of 43
ip t
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Ac ce p
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ip t
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cr
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Ac ce p
poly(4,4'-dialkyl-2,2'-bithiazole-5,5'-diyl)s and their analogues comprised of electronaccepting five-membered rings. Preparation, regioregular structure, face-to-face stacking, and electrochemical and optical properties, Chem.Mater. 9(5) (1997) 1217.
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normal-doped and para-doped poly(thiophene) Chem. Mater. 3 (1991) 872. [41] O. Semenikhin, E.V. Ovsyannikova, M.R. Ehrenburg, N.M. Alpatova, V.E.Kazarinov,
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te
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Ac ce p
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20
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[47] J. Roncali, F.Garnier, M.Lemaire, R.Garreau, Poly Monotrithiophene, Bitrithiophene And Trithiophene - Effect Of Oligomer Chain-Length on The Polymer Properties, Synt. Met. 15 (1986) 323. [48] A.B. Koren, M.D. Curtis, A.H. Francis, J.W. Kampf, Intermolecular interactions in p-
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ip t
stacked conjugated molecules. Synthesis, structures and spectral characterization alkyl
cr
[49] J. Bobacka, A.Lewenstam, A.Ivaska, Electrochemical impedance spectroscopy of oxidized
us
poly(3,4-ethylenedioxythiophene) film electrodes in aqueous solutions, J.Electroanal Chem. 489 (2000) 17.
an
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M
[51] F. Mansfeld, Use of electrochemical impedance spectroscopy for the study of corrosion protection by polymer coatings, J. Appl. Electrochem., 25 (1995) 187.
d
[52] S. Sathiyanarayanan, R. Jeyaram, S. Muthukrishnan, and G. Venkatachari, Corrosion
Ac ce p
Soc.156 (2009) 127.
te
protection mechanism of polyaniline blended organic coating on steel, J. Electrochem.
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21
Page 21 of 43
Legends to Figures Scheme
1.
Synthetic
routes
for
bis
(thiophene)-bis
(ethylenedioxythiophene)-3,3’-
ip t
dinonylthiazole (ThEdNBTEdTh). Scheme 2. Optimized geometry of ThEdNBTEdTh.
cr
Figure 1. 1H NMR of the ThEdNBTEdTh in CDCl3. ! (ppm) = 0.76-0.86 (m, 6H), 1.22-.151
us
(m, 24H), 2.10 (m, 4H), 3.37-3.47 (t, 4H), 5.23 (m, 8H), 7.20 (m, 2H), 7.37 (m, 4H). Figure 2. a)Polymerization of ThEdNBTEdTh in CH2CI2 containing 0.1M TBAPF6 at 10
an
cycles in the range of 0.2-1.0V(Q1(10) = 2.27! C) b)Scan rate dependence of P [ThEdNBTEdTh] in CH2CI2 containing 0.1 M TBAPF6.
M
Figure 3. a)Polymerization of ThEdNBTEdTh in CH2CI2 containing 0.1M TBAPF6 at 10 cycles in the range of 0.2-1.35V. (Q2(10) = 54.8! C) b)-Redox behaviour of resulting polymer at
d
different scan rates.
te
Figure 4. Comparison of redox behaviour of polymeric films obtained at different charges by
Ac ce p
applying 10 cycles (a), 20 cycles (b).
Figure 5. a) Ip vs ! 1/2 and b) Ip vs ! graphs of P[ThEdNBTEdTh] films. Figure 6. Cyclic voltammograms of P[ThEdNBTEdTh] film in 0.1M TBAPF6/DCM at the scan rate of 100 mV/s. (Inset: n-doping range). Figure 7. Ex-situ FTIR-ATR spectra recorded during reduction (n-doping) of a P(ThEdNBTEdTh) film in the wavenumber region 4000–650 cm-1 and in the wavenumber region 4000-1500–cm-1(inset) .
Figure 8. Comparison of stability of the P[ThEdNBTEdTh] films obtained at different cycle numbers; a)-10, b)-20,c)-35,d)-50 cycles in monomer free solution.
23
Page 22 of 43
Figure 9. (a) UV–Visible absorption spectra of P[ThEdNBTEdTh] film in 0.1M TBAPF6 in DCM at different applied potentials from the range −1.0 to 1.0V, (b) The change of maximum absorbances as a function of potential together with the cyclic voltammogram of P[ThEdNBTEdTh] film.
ip t
Figure 10. Absorption and emission spectra of a)- 1x10-6 M ThEdNBTEdTh and b)- 1x10-6 M
cr
P[ThEdNBTEdTh] in DMF. c)-Absorption spectra of ThEdNBTEdTh in solution and P [ThEdNBTEdTh] both in solution and as a film.
us
Figure 12. Nyquist and Bode plots of P[ThEdNBTEdTh] film obtained potentiodynamically at 10 cycles in the range of 0.2-1.0 V (a, b) and 0.2-1.35V (c, d) at different applied potentials
an
(Edc).
Figure 13. Variation of the (a) Ccv with scan rate and comparison with CEIS and (b) CEIS with
M
potential of the P [ThEdNBTEdTh] films obtained at different cycle numbers. Figure 14. Electrical equivalent circuit model (a) Experimental (dot) and simulated (solid line)
Ac ce p
te
d
Nyquist plot (b) Bode plot (c) of P[ThEdNBTEdTh] film.
24
Page 23 of 43
Table 1. Comparison of oxidation onset potentials(Eonset,ox)and ionization potentials (IP) of monomers and polymer* and half-wave potentials (E1/2)of polymers.
1.50(2.02)
BTh
1.32
TTh 1.05
IP mon / eV
/V
47
5.90
0.70
ThEdNBTEdTh
0.54 (0.0*)
/V
1.0230
5.10
0.05
5.04 (4.40*)
0.36
47
47
47
36
te
d
M
36
1.04
5.50
an
EdNBTEd
0.97
5.45
47
1.1030
pol
1/2
0.80(0.95)
5.72
47
ThNBTTh
E
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mon onset,ox
cr
Th
E
us
Monomer
Table 2. Optical band gap and electrochemical*(Eg) values of monomer ** and polymers. E / eV
Ac ce p
Monomer/Polymer
g
8
1.75
30
2.53
P[EdNBTEd] P[ThNBTTh]
P[ThEdNBTEdTh]
1.67 (1.44*)
46
E / eV a
E
mon onset,red
2.73
-1.67
2.78
-1.72
/V
1.60
PEDOT 46
PTh
2.10
ThEdNBTEdTh**
2.26
25
Page 24 of 43
Table 3. Impedance parameters of P[ThEdNBTEdTh] film obtained from equivalent circuit
Rsoln/ohm
17! 102
Rcor/ohm
1! 109
Rpo/ohm
45! 103
Ccor /S sn
41! 10-7
cr
Value
an
us
Parameter
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model.
n
1.00
Cc /S sm
M
14! 10-6 0.923
5! 10-4
Ac ce p
te
Goodness of fit
d
m
26
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S0013-4686(13)02583-8 http://dx.doi.org/doi:10.1016/j.electacta.2013.12.130 EA 21931
To appear in:
Electrochimica Acta
Received date: Revised date: Accepted date:
21-6-2013 2-12-2013 23-12-2013
Please cite this article as: B. Ustamehmeto˘glu, Synthesis and Characterization of Thiophene and Thiazole Containing Polymers, Electrochimica Acta (2014), http://dx.doi.org/10.1016/j.electacta.2013.12.130 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Synthesis and Characterization of Thiophene and Thiazole Containing Polymers B. Ustamehmetoğlu İstanbul Technical University, Department of Chemistry, 34469 Maslak, İstanbul, Turkey.
ip t
[email protected](ISE member)
In
this
study,
cr
Abstract
bis(thiophene)-bis(ethylenedioxythiophene)-3,3’-dinonylthiazole
us
(ThEdNBTEdTh) comonomer was obtained by using organometallic polycondensation mediated by organotransition metal complexes. The properties of electrochemically obtained
an
poly[bis(thiophene)-bis(ethylenedioxythiophene)-3,3’-dinonylthiazole]
(P[ThEdNBTEdTh])
from this comonomer were characterized by electrochemical techniques such as voltammetry electrochemical
impedance
spectroscopy
M
and
(EIS).
Oxidative
polymerization
of
ThEdNBTEdTh was carried out in dichlorometane onto a platinum electrode. Effect of
were
further
characterized
by
te
films
d
polymerization charge on the redox behaviour of polymer films was investigated and these ATR-FTIR,
UV-Visible
and
fluorescence
Ac ce p
spectrophotometric measurements. EIS studies revealed that high capacitance values observed in the lower frequency region and proposed electrical equivalent circuit was applied to the experimental data to correlate the results, obtain the parameter of each element and explain the interface between the Pt/ P[ThEdNBTEdTh /Bu4NPF6 system. Results showed that polymer has low oxidation potential, low band gap (1.67 eV) and good stability in the oxidized state and it shows p- and n-dopable properties. n-doping was verified experimentally by ex-situ FT-IR spectroelectrochemical measurements. The measured capacitance values are quite promising for supercapacitor applications. Key words: Bis (thiophene)-bis (ethylenedioxythiophene)-3, 3’-dinonylthiazole, synthesis and electropolymerization, EIS, absorption and emission properties.
1
Page 1 of 43
1. Introduction There are a lot of application areas of conducting polymers from optical information displays to rechargeable batteries. A modification of electrically conducting polymers is a promising line
polymers
and
produce
polymeric
materials
with
new
ip t
of inquiry, which allows one to expand the range of physicochemical characteristics of properties.
Organometallic
cr
polycondensation reactions are raised great interest for the design of new classes of conjugated
us
materials with enhanced semiconducting properties [1-3]. Donor–acceptor–donor type molecules find different applications in electronic and optical devices [4-9].
an
Polythiophene (PTh) has rapidly become the subject of considerable interest [10-14]. The high environmental stability of both doped and undoped states of PTh together with its structural
materials, and organic semiconductors.
M
versatility has led to multiple developments aimed at applications such as conductors, electrode
d
After the development of new PTh derivative, poly (3, 4-ethylenedioxythiophene), (PEDOT)
te
during the second half of the 1980s, an exponential increase in the number of publications was
Ac ce p
observed [15-19]. PEDOT shows high conductivity, lower oxidation potential compared to PTh, high stability at doped state, low band gap and these properties make it attractive for many applications.
Recently thiazole-based conjugated polymers have received great interest [20-30]. Introduction of nitrogen atoms in addition to sulphur in the thiophene (Th) ring improve the properties of resulting polymers which are desirable for applications. In this study, nonylbithiazole (NBT), 3, 4-ethylenedioxythiophene (EDOT) and Th were chosen to obtain comonomer because of especially ! - staking and n-type transporters properties of BTh, superior electrical properties of EDOT and stability at both neutral and oxidized state for PTh. P [ThEdNBTEdTh] is expected to have useful properties for the design of a diode, transistor or capacitor. 2
Page 2 of 43
2. Experimental 2.1. Equipment and materials Cyclic voltammetry (CV) experiments, spectroelectrochemistry and electropolymerizations
ip t
were performed with a Gamry 600 model potentiostat/galvanostat interfaced to a PC. A Pt button electrode (0.022 cm2), a platinum wire and a silver wire were used as working, counter
cr
and pseudo reference electrodes respectively. The silver wire was calibrated externally using a
reported versus Ag/AgCl as suggested in literature [31].
us
5mM solution of ferrocene/ferrocenium (Fc/Fc+) couple in the electrolyte and the potentials are
an
Spectroelectrochemical data were recorded on a Schimadzu 160 A model UV–visible spectrophotometer. A three-electrode cell system was used where the working electrode was a
M
custom cut ITO-coated glass slide (8mm×50mm×1.1 mm, 30mm), counter electrode was a Pt wire and pseudo-reference electrode was Ag wire.
te
model Spectrophotometer.
d
The ATR-FTIR- spectra measurements were performed by using Perkin Elmer Spectrum one
Ac ce p
Voltammetry and EIS were used for electrochemical polymerization and characterization methods. Potentiodynamic polymerizations were done by using 10, 20, 35 and 50 cycles. Q1 and Q2 are the polymerization charges obtained in two different potential ranges (0.2-1.0 V and 0.2-1.35V) respectively by integration of CV curves for 10 (Q1(10) (Q2(10)) and 20 ((Q1(20)and Q2(20)) cycles.
The electrolyte was 0.1M tetraethylammonium hexafluorophosphate (TBAPF6) obtained from Fluka, dichloromethane (DCM) supplied by Merck. Ethylenedioxythiophene (EDOT), thiophene (Th), bromodecanon, dithiooxamide, bromine, trans-dicholorobistriphenylphosphine palladium (II) chloride, butyl lithium, trimetiltinchloride, 2-trimethylstannyl thiophene MgSO4 and ethanol were all analytical grade and used without further purification. During the synthesis of the comonomer, all preparations and reactions were performed under a nitrogen atmosphere; 3
Page 3 of 43
reagent grade solvents were dried and distilled before use. Tetrahydrofuran (THF) and toluene purchased from Carlo-Erba were distilled over sodium.
2.2. Comonomer Synthesis Methods
ip t
Synthesis of bis (thiophene) – bis (ethylenedioxythiophene) - 3,3’-dinonylthiazole (ThEdNBTEdTh) was synthesized by Stille coupling reaction. Synthesis of the bis (3,4-
cr
ethylene-dioxythiophene)-(4,4-dinonyl-2,2-bithiazole) (ENBTE) comonomer and their bromine
us
and trimethylstannyl derivatives are similar to the literature procedures previously described for alkylbithiazole and its derivatives [8]. The mixture of 2’2’dibromoENBTE and 2-
an
trimethylstannyl thiophene was refluxed for 20 h. After cooling to room temperature the reaction mixture was poured into water, and dichloromethane, pentane and chloroform were
M
added. Extracted organic phase was washed with water and dried over MgSO4 and the solvent was rotary evaporated. The final product was re-crystallized in ethanol and obtained as reddish
te
Ac ce p
given in Scheme 1.
d
orange solid with the molecular weight of 806 g/mol. Yield was 45%. Synthetic route was
Scheme
1.
Synthetic
routes
for
bis
(thiophene)-bis
(ethylenedioxythiophene)-3,3’-
dinonylthiazole (ThEdNBTEdTh).
H-NMR spectrum was collected on Agilent VNMRS 500 MHz in CDCl3 and the absence of protons at about 6.3 ppm which belong to EDOT rings and the presence of protons at about 7.2 and 7.4 ppm which belongs to Th shows the coupling and proves the formation of comonomer (Fig.1).
Figure 1. 1H NMR of the ThEdNBTEdTh in CDCl3. ! (ppm) = 0.76-0.86 (m, 6H), 1.22-.151 (m, 24H), 2.10 (m, 4H), 3.37-3.47 (t, 4H), 5.23 (m, 8H), 7.20 (m, 2H), 7.37 (m, 4H). 4
Page 4 of 43
Optimized geometry of comonomer was obtained by DFT method and given in the Scheme 2.
ip t
Scheme 2. Optimized geometry of ThEdNBTEdTh.
3. Results and Discussion
cr
3.1. Voltammetric Measurements
us
Anodic polymerization curve of comonomer was obtained and the oxidation onset was obtained as 0.54 V. A conjugated polymer that shows both oxidation and reduction onsets gives
an
possibility to find the electrochemical band gap and estimate the ionization potential, IP and electron affinity, Ea with the same experiment. IP values are important for molecular design of
M
hole transport material for organic light-emitting diode applications. It is reported that IP values of flourene containing organic molecules can be changed from 5.42 to 5.80 eV, when the kinds
d
or positions of the substituents in the molecule are changed [32].
te
To tranpoze the measured redox behaviour into estimates the IP and Ea, it is necessary to relate the electrochemical potential to the vacuum level. Janietz et al [33] reported a relationship
Ac ce p
between IP and Ea values with the oxidation and reduction onset potentials of the same sample as suggested by Bredas [34];
IP = 4.4 + Eonset,ox (eV)
(1)
Ea = 4.4 + Eonset,red (eV)
(2)
where Eonset,ox, and Eonset,red are the onset potentials for the oxidation and reduction relative to Ag/AgCl reference electrode. Although Cardona et al [35] suggested the value of 4.4 eV vs SHE, in several studies 4.2-4.6 eV vs SCE was also reported [33,34]. In this study, in order to gain some ideas on the properties of ThEdNBTEdTh comonomer and its polymer, IP values were calculated by using the equation 1. Eonset,ox was obtained by extrapolating the electrochemical onset of the first oxidative scan of linear sweep data where 5
Page 5 of 43
the current starts to differ from the baseline and calculated IP values were compared with the value of starting monomers and polymers (Table 1). As it can be seen, the Eg of ThEdNBTEdTh is lower than the value of starting monomers and higher than the polymer as expected.
ip t
Polymerization of ThEdNBTEdTh was performed in CH2CI2 containing 0.1M TBAPF6 at 10 cycles in the range of 0.2-1.0V (Fig. 2a) and resulting P [ThEdNBTEdTh] showed good redox
cr
behaviour even at very high scan rates (Fig. 2b). Scan limit was increased up to 1.35V during
us
polymerization (Fig. 3a) and obtained film was investigated at different scan rates (Fig. 3b). The reversible redox behaviour of this film was compared with the film obtained at the same
an
cycle number up to 1.0V scan limit and it can be seen that, since the polymer charge is high due to higher sweep potential limit, the higher current intensity was observed in the case of 1.35 V
M
(Fig.4a). Thicker polymeric film was obtained by applying 20 cycles and this film also shows reversible redox behaviour up to 500 mV/s scan rate in both potential ranges of 0.2-1.0V and
d
0.2-1.35V. The redox behaviours of two films obtained up to 1.0V and 1.35V scan limits, were
te
compared (Fig.4b) and it is observed that while the reversible redox behaviour was observed at
Ac ce p
lower potential limit, due to some degradation, it loses its reversibility as the potential limit was increased.
Figure 2. a)Polymerization of ThEdNBTEdTh in CH2CI2 containing 0.1M TBAPF6 at 10 cycles in the range of 0.2-1.0V(Q1(10) = 2.27! C) b)Scan rate dependence of P [ThEdNBTEdTh] in CH2CI2 containing 0.1 M TBAPF6. Figure 3. a)Polymerization of ThEdNBTEdTh in CH2CI2 containing 0.1M TBAPF6 at 10 cycles in the range of 0.2-1.35V. (Q2(10) = 54.8! C) b)-Redox behaviour of resulting polymer at different scan rates. Figure 4. Comparison of redox behaviour of polymeric films obtained at different charges by applying 10 cycles (a), 20 cycles (b). 6
Page 6 of 43
Polymerizations by applying further cycling such as 35 and 50 cycles in the range of 0.2-1.0 V were also performed and redox behaviours of resulting polymers at different scan rates were obtained. Scan rate dependence of all these polymeric films was tested according to scan rate
diffusion controlled
cr
behaviour when they obtained by applying 10, 20, 35 cycles and
ip t
and square root of scan rate (Figure 5 a,b) . It is found that polymeric films show thin film
behaviour was observed in the case of 50 cycles.
us
Figure 5. a) Ip vs ! 1/2 and b) Ip vs ! graphs of P[ThEdNBTEdTh] films.
an
The average of the anodic and cathodic peak potentials which is known as half-wave potential ( E1/2 ) was calculated with following equation for different scan rates and the average value
M
was used: E1/2 = 1/2(Epc+ Epa)
(3)
terthiophene(TTh),
EdNBTEd
and
ThEdNBTEdTh
were
te
bithiophene(BTh),
d
Calculated E1/2 values were summarized in Table 1. As it can be seen, when Th,
electropolymerized, the half-wave potential of resulting polymers (E1/2,pol) increase with the
Ac ce p
increase in the number of monomeric unit of starting monomer. This result indicates a decrease of their conjugation length. This might be explained by a well-known oligomeric approach [3639]. In these studies it is suggested that the rate constant of dimerization of oligomers and their coupling steps with the original monomers decrease with increasing chain length. n-doping behaviour of the film was also investigated by CVs of P [ThEdNBTEdTh] at different scan rates and given in Fig 6. In the consecutive scans, the cathodic potential was extended to the more negative values and this part of voltammogram was given as inset. The excursion in the negative potential range resulted in formation of a cathodic peak around -1.57 V and in the backward scan a reoxidation peak was observed at -1.55 V. This peak corresponds to inclusion of the Bu4N+ cations in to the polymer. The n-doping influences also on the shape of CV in the p-doping range. A new prepeak about -0.1 V which was not observed when the CV were 7
Page 7 of 43
performed only in p-doping range (Fig. 2b, 3b, 4) was appeared. The nature of the prepeak was reported in the literature and attributed to the charge trapping [40-43]. The electrochemical band gap of P [ThEdNBTEdTh] was calculated from the onset potentials of anodic and cathodic branches as suggested in literature [33, 44] and onsets were found as
ip t
Eonset,ox = 0.0 V and Eonset,red = -1.44 V which yield IPpol = 4.40 eV (Table 1) and
cr
Eg(electrochemical) = 1.44 V (Table 2). As it can be seen IPmon(5.04 eV) could be decreased
us
significantly due to increase in conjugation length.
scan rate of 100 mV/s. (Inset: n-doping range).
an
Figure 6. Cyclic voltammograms of P[ThEdNBTEdTh] film in 0.1M TBAPF6/DCM at the
M
Table 1. Comparison of oxidation onset potentials(Eonset,ox)and ionization potentials (IP) of monomers and polymer* and half-wave potentials (E1/2)of polymers.
d
Recently it is suggested that, the intense infrared active vibration (IRAV) bands grow both in
te
the low energy range (700-1600 cm-1) due to a strong electron–phonon coupling within the
Ac ce p
molecule and at higher energies (1600–8000 cm-1) with a broad electron absorption [45]. In this study in order to gain further information on n-doping behaviour, the ex- situ ATR-FTIR spectra of a P[ThEdNBTEdTh] film was measured during reduction (n-doping) between -1.0 and -2.0 V in 0.1 M Bu4NPF6/DCM (Figure 7). An enlargement of the spectra in the wavenumber region of 1500–4000 cm
-1
was shown as inset in Figure 7. During the reduction,
the electronic absorption related to the formation of free charge carriers in the polymer continuously increases. When the CV of the polymer film in the full range that shows both pand n-doping is concerned (Fig 6), from the observation of the lower current intensities for negative charge carriers than the positive ones, the slight increase in the FTIR absorption spectra during n-doping can be understand.
8
Page 8 of 43
Figure 7. Ex-situ FTIR-ATR spectra recorded during reduction (n-doping) of a P(ThEdNBTEdTh) film in the wavenumber region 4000–650 cm-1 and in the wavenumber region 4000-1500–cm-1(inset) . Stability of polymer films were checked by CV before and after 300 cycles in the potential
ip t
range of 0.2-1.0 V at scan rate of 100 mV/s (Figure 8). The charge (Qcv) was obtained by integration and differs by 1.8 % between 1st and 300th cycles for the film obtained at 10 cycles
cr
(Figure 8a). As the thickness of the film increases, difference in charges increases up to 8.7 %
us
(Figure 8b). If we obtained the film at 35 cycles, difference becomes 11.3% (Figure 8c). But for the thicker film obtained at 50 cycles an interesting behaviour was observed and the charge
an
increased after 100 cycles (Figure 8d) which might be due to increase in charged states of polymer chain. Further oxidation might be result formation of polaron and/or bipolaron and
M
consequently the increase in doping degree and conductivity [46]. Figure 8. Comparison of stability of the P[ThEdNBTEdTh] films obtained at different cycle
te
d
numbers; a)-10, b)-20,c)-35,d)-50 cycles in monomer free solution.
Ac ce p
3.2. In-situ Spectroelectrochemical Measurements In-situ spectroelectrochemical measurements were done on ITO working electrode by applying different potentials in the range of -1.0 – 1.0 V and the spectra were given in Fig.9a. Maximum absorbance value of monomer was observed at 435 nm which is higher than the starting material due to the increasing conjugation length. Isobestic point, maximum absorbances of neutral and doped polymer were observed at 650 nm, 560 nm and 812 nm respectively. While polaron and bipolaron formations were observed separately for starting material [43], here, they were observed together. When the change in absorbances at 560 nm and 812 nm, was considered together with CV, the intersection of change of two wavelengths with potential superimposed with the onset potential of the second peak in CV (Figure 9b). This result shows that bipolaron formation starts after 0.75 V. 9
Page 9 of 43
The band gaps (Eg) of polymer film (on ITO) and monomers in solution were estimated by extrapolation of the low energy edge of the absorption spectra to the baseline and were collected in Table 2. Eg value of P [ThEdNBTEdTh] was compared with PTh, PEDOT,
ip t
P[ThNBTTh] and P[EdNBTEd] as well as starting comonomer ThEdNBTEdTh.
cr
Figure 9. (a) UV–Visible absorption spectra of P[ThEdNBTEdTh] film in 0.1M TBAPF6 in DCM at different applied potentials from the range −1.0 to 1.0V, (b) The change of maximum
us
absorbances as a function of potential together with the cyclic voltammogram of P[ThEdNBTEdTh] film.
an
As it can be seen, P [ThEdNBTEdTh] has a band gap lower than the starting materials (P [ThNBTTh] [30], P [EdNBTEd] [8] and PTh [47]) due to increase in the conjugation length as
M
expected. These results suggest that by changing the donor segments of starting material with monomer having different electron-donating ability, the absorption spectra and band gaps of
d
the D-A polymers can be tuned, which is an advantage for the design of the photovoltaic
te
materials. These results are in agreement with the previous study on starting monomer
Ac ce p
EdNBTEd [8, 29, and 30]. The optical band gap of polymer was found different from the electrochemical band gap as reported previously by Janietz et al [33]. Furthermore, if electron affinity values of molecules were calculated by subtraction of IP and optical band gap values, onset potential for the reduction (Eonset,red) was obtained by using equation 2. The same difference between experimental (-1.44 V) and calculated (-1.67 V) values can be better understand. This difference might be due to the possible uncertainties of CV experiments as explained by Cardona et al recently [35]. Table 2. Optical band gap (Eg) values of polymers. 3.3. Optical measurements
10
Page 10 of 43
Presence of long alkyl chain can make the P [ThEdNBTEdTh] soluble. Solubility of polymer was tested in DMF, THF and CH3CI and it was found that it is soluble in all. Absorption and emission spectra of monomer and polymer in DMF solution and polymer film obtained by casting were given in Figure 10. Thiazole rings expected to show also fluorescence behaviour,
ip t
so emission spectra of monomer and polymer were taken in DMF. Comparison of absorption and emission spectra of ThEdNBTEdTh and P [ThEdNBTEdTh] were given in Figure 10a and
cr
b respectively. Monomer has absorptions at 270 and 436, and emissions at 337 and 436
us
respectively when it is exited at 436 nm. The emission maxima of P [ThEdNBTEdTh] have also a red shift as compared to monomer. Emission in the visible region makes
an
P[ThEdNBTEdTh] favourable for LED application. Absorption wavelength of polymer has a red shift according to monomer as expected, due to the increase in conjugation length. A red
M
shift in the absorption maximum of polymer in solution and polymer film indicates ! -stacking behaviour which is reported for thiozole moiety [21, 24, and 48].
d
Figure 10. Absorption and emission spectra of a)- 1x10-6 M ThEdNBTEdTh and b)- 1x10-6 M
te
P[ThEdNBTEdTh] in DMF. c)-Absorption spectra of ThEdNBTEdTh in solution and P
Ac ce p
[ThEdNBTEdTh] both in solution and as a film.
3.4. Electrochemical Impedance Spectroscopy Nyquist and Bode diagrams of P[ThEdNBTEdTh] obtained at 10 cycles in the range of 0.2-1.0 V at different applied potentials (Edc) were shown in Fig.11 a, b. As it can be seen from Bode diagrams, polymeric films show generally capacitive behaviour in the frequency range of 1.0! 10-2-1.0! 102 Hz (Fig 11b). When Edc is equal or higher than 0.6 V, this capacitive behaviour was destroyed, the line curved and become closer to Zreal axis in Nyquist diagram. At Edc =0.0V, 0.25 V and 0.6V, phase angles were observed as 85 ° at 2.0x10-2 Hz, 8.0x10-2 Hz and 8.0x10-2 Hz respectively. When Edc was increased to 0.9V, it decreases to 65° at 8 Hz due to some degradation of the film (Fig. 11 b). Same measurements were performed for the 11
Page 11 of 43
film obtained at 10 cycles in the range of 0.2-1.35V (Fig. 11c, d). In this case, since the polymerization is carried out at higher potential, degradation of polymeric film starts after 0.4V. If the polymer obtained with 20 cycles in the range of 0.2-1.0 V, this decomposition starts at lower potential, Edc= 0.25V. Increase in thickness might result increase in the
ip t
delamination rate of the polymeric film. The film obtained at 20 cycles at higher sweep potential range (0.2-1.35V), loses its capacitive behaviour in all applied potentials. Same
us
cr
decomposition was observed for the film obtained at 35 and 50 cycles.
Figure 11. Nyquist and Bode plots of P[ThEdNBTEdTh] film obtained potentiodynamically
an
at 10 cycles in the range of 0.2-1.0 V (a, b) and 0.2-1.35V (c, d) at different applied potentials
M
(Edc).
Capacitance values were obtained from anodic and cathodic peak currents in CV as suggested
d
in literature by Bobacka et al. [49].
te
Ccv = Ip / ! .
Ac ce p
The low frequency capacitance (CEIS) values (10 mHz and 1.0 Hz) from EIS were calculated from the slope of the imaginary component of impedance vs. the inverse of the frequency (f) [29,30]:
CEIS = (2! fZim)-1
In Figure 12, variation of the Ccv with scan rate and comparison with CEIS (a) and CEIS with potential (b) of the P [ThEdNBTEdTh] films obtained at different cycle numbers were given. It can be seen that the capacitance values obtained by CV are rate dependent for thicker films and decreases with increasing scan rate (Figure 12 a). Although it is expected that Ccv and CEIS values to be different from each other due to conformational changes during the oxidation of polymer, here they are very close and they increase with increasing cycle numbers since the amount of polymer increase with the polymerization charge. 12
Page 12 of 43
The rectangular shape of the voltammogram is expected geometry for electrochemical supercapacitors devised from conducting polymers [49]. While CV of polymer films that obtained by applying 10 and 20 cycles have rectangular shapes, thicker films do not have this shape (Figure 8). When the Ccv values compared with CEIS values (Figure 12a), although these
ip t
two values are in agreement with each other for thinner film, some deviation starts for the film obtained at 35 cycles and this deviation becomes higher for the film obtained at 50 cycles.
cr
This might be due to diffusion controlled behaviour of the thick film. However all results were
us
given for comparison.
When the changes in CEIS values of films obtained at different cycles plotted vs. Edc, the
an
optimum condition can be determined as 50 cycles (0.25V) (Figure 12 b). At higher Edc values, the film destroyed and lost its capacitive behaviour. So CEIS values were not obtained for
M
thicker films at higher Edc. If the stability measurements were concerned (Figure 8), it is better to choose 20 cycles (0.25V). The symmetry of the anodic and cathodic Ccv curves (Figure 12 a)
d
as well as rectangular shapes, of CV (Figure 8) suggests that this polymer has expected
te
geometry for electrochemical supercapacitors. This behaviour was also observed for the
Ac ce p
starting materials (P[EdBThEd]and PEDOT) [29, 49].
Figure 12. Variation of the (a) Ccv with scan rate and comparison with CEIS and (b) CEIS with potential of the P [ThEdNBTEdTh] films obtained at different cycle numbers.
The equivalent electrical analog model for coated metal was reported previously [50] and commonly used for conducting polymers [52, 53]. We modified this model according to our experimental results to obtain the highest “goodness of fit” (5x10-4) which is in the acceptable range of fitting parameters for such calculations [54]. An equivalent circuit model, [R(C(R(C(R))))] which is best fitted to the experimental impedance data was used in simulation of the impedance behaviour of the thin film (Figure 13a). The first component of this model is 13
Page 13 of 43
the bulk solution resistance of the polymer and the electrolyte (Rsoln), the second one is the capacitance of the coating (Cc). Pore resistance (Rpo) is the resistance of ion conducting path. The second capacitance is double layer capacitance (Ccor), and the third resistance is charge transfer resistance (Rcor). In this circuit, Cc and Ccor are two constant phase elements that take
ip t
into account the interfacial irregularities such as porosity, roughness, and geometry. The impedance of a constant phase element, Y0 that has shown as ! in the equivalent circuit model,
us
cr
has the form:
an
When this equation describes a capacitor, Y0 = C (the capacitance) and the exponent ! = 1. For a constant phase element, the exponent ! is less than one. In this study in the equivalent circuit
M
model the exponents m,n for Ccor and Cc respectively were obtained 1.0 and 0.923. Contribution from the ohmic (electronic) resistance of the oxidized P [ThEdNBTEdTh] film to
d
Rsoln causes to a high value (1.65! 103ohm) as well as very high Rpo (45! 103ohm) values.
te
Another reason of high values might be porous structure of the film while Rcor has
Ac ce p
comparatively lower value (1! 109 ohm). A quite well agreement between experimental and simulated data was obtained for both Nyquist and Bode plots (Figure 13 b and c). The results obtained from simulated data were summarized in Table 3. Coating capacitance values obtained from equivalent circuit model, nonideal capacitor value, (Cc =14µF), experimentally from EIS (CEIS =20 µF) and from CV measurements (Ccv = 21µF) are all in good agreement.
Figure 13. Electrical equivalent circuit model (a) Experimental (dot) and simulated (solid line) Nyquist plot (b) Bode plot (c) of P[ThEdNBTEdTh] film.
Table 3. Impedance parameters of P[ThEdNBTEdTh] film obtained from equivalent circuit model. 14
Page 14 of 43
ip t
4. Conclusion ThEdNBTEdTh comonomer was firstly synthesized by Stille coupling reaction and
cr
polymerized electrochemically by CV. Characterization was done by using CV and EIS. The
us
film obtained by applying 20 cycles shows the best reversible redox behaviour even at very low scan rates. Stability of polymer films were checked before and after 100 cycles and the highest
an
difference in charges between 1st an100th cycles was obtained as 11.3%. Capacitive behaviour of P [ThEdNBTEdTh] starts at very low potential. Capacitance values are decreasing slightly
M
with increasing scan rate for thicker films (35 and 50 cycles). The rectangular shape of voltammogram and the symmetry of the anodic and cathodic capacitance values (Ccv) of P
d
[ThEdNBTEdTh] suit the needs of supercapacitors. To use this polymer as a capacitor, one
te
should obtain a film at 20 cycles by applying 0.25V. Presence of long alkyl chain makes the P [ThEdNBTEdTh] soluble. Polymer shows emission in the visible region which makes it
Ac ce p
favourable for LED applications. Absorption spectra of monomer and polymer in solution and solid state are different, showing the intermolecular ! -! stacking. P [ThEdNBTEdTh] shows both p- and n- doping properties, and band gap was calculated as 1.44 eV which is lower than optical band gap, 1.67 eV. Results suggest that by changing donor segments with monomer having different electron-donating ability, band gap of the polymer can be tuned, which is useful for the design of the photovoltaic materials. Cc, CEIS and Ccv values were found very close to each other. The proposed electrical equivalent circuit is good enough to explain the behaviour of Pt/ P[ThEdNBTEdTh /Bu4NPF6 system. Acknowlegment: Autor thanks to Dr. Burak Ulgut from Gamry Instruments for his support during equivalent circuit model analysis. 15
Page 15 of 43
ip t
References [1] E. Sezer, A.S. Saraç, E.A. Parlak, Electrochemical synthesis of EDOT-ECz-EDOT
cr
copolymer on carbon fiber micro-electrodes, J. Appl. Electrochem. 33 (2003)1233.
us
[2] J.J. Li, G.W. Gribble, Palladium in Heterocyclic Chemistry, Pergamon, 2000.
[3] T. Yamamoto, A. Yamamoto, Novel type of polycondensation of polyhalogenated organic
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an
aromatic-compounds producinthermostable polyphenylene type polymers promoted by
M
[4] S. Yao, H.Y. Ahn, X.H. Wang, J. Fu, E.W. Van Stryland, D.J. Hagan, K.D. Belfield, Donor-acceptor-donor fluorene derivatives for two-photon fluorescence lysosomal
d
imaging, J. Org. Chem. 75(12) (2010) 3965.
te
[5] H.J. Ravindra, A.J. Kiran, K. Chandrasekharan, H.D. Shashikala, S.M. Dharmaprakash, Third order nonlinear optical properties and optical limiting in donor/acceptor substituted
Ac ce p
4'-methoxy chalcone derivatives, Appl. Phys. B-Lasers Opt. 88(1) (2007) 105.
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[7] M. Sendur, A. Balan, D. Baran, B. Karabay, L. Toppare, Combination of donor characters in a donor-acceptor-donor (DAD) type polymer containing benzothiadiazole as the acceptor unit, Org. Electron.11(12) (2010) 1877. [8] F.Ç. Cebeci, E. Sezer, A.S. Saraç, Synthesis and electrochemical characterization of bis(3,4-ethylene-dioxythiophene)-(4,4'-dinonyl-2,2'-bithiazole) comonomer, Electrochim. Acta 52(5) (2007) 2158. 16
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[9] C. Mallet, G. Savitha, M. Allain, V. Kozmik, J. Svoboda, P. Frere, J. Roncali, Synthesis and electronic properties of D-A-D triads based on 3-Alkoxy-4-cyanothiophene and benzothienothiophene blocks, J. Org. Chem. 77(4) (2012) 2041.
Chem. Rev. 92(4) (1992) 711.
ip t
[10] J. Roncali, Conjugated poly(thiophenes) - Synthesis, functionalization, and applications,
[11] X.H. Zhu, J. Peng, Y. Caoa, Solution-processable single-material molecular emitters for
cr
organic light-emitting devices, Chem. Soc. Rev. 40(7) (2011) 3509.
us
[12] R.J. Waltman, J. Bargon, Electrically conducting polymers: a review of the electropolymerization reaction, of the effects of chemical structure on polymer film
an
properties, and of applications towards technology, J. Can. J. Chem. 64 (1986) 76. [13] A.F. Diaz, J.C. Lacroix, Synthesis of electroactive conductive polymer-films -
M
electrooxidatıon of heteroaromatic-compounds, New J. Chem. 12(1988) 171.
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d
[14] Z. Adamcova, L. Demirova, Film-forming electropolymerization, Prog. Org. Coat. 16
te
[15] J. Roncali, P. Blanchard, P. Frere, 3,4-Ethylenedioxythiophene (EDOT) as a versatile
Ac ce p
building block for advanced functional p-conjugated systems, J. Mater. Chem. 15(16) (2005) 1589.
[16] B.L. Groenendaal, F. Jonas, D. Freitag, H. Pielartzik, J.R. Reynolds, Poly(3,4ethylenedioxythiophene) and its derivatives: Past, present and future., Adv. Mater. 12(7) (2000) 481.
[17] P.J. Shi, C.M. Amb, A.L. Dyer, J.R. Reynolds, Fast switching water processable electrochromic polymers, ACS Appl. Mater. and Inter. 4(12) (2012) 6512. [18] F. Jonas, W. Krafft, B. Muys, Poly(3,4-ethylenedioxythiophene): Conductive coatings, technical applications and properties, Macromol. Symposia 100(1) (1995) 169.
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[19] P. Çamurlu, T. Durak, A. Balan, L. Toppare, Electronic and optical properties of dibenzothiophen-S,S-dioxide and EDOT based conducting polymers, Synt. Met.161(1718) (2011) 1898. [20] T. Yamamoto, D. Komarudin, M. Arai, Extensive studies on pi-stacking of poly(3-
ip t
alkylthiophene-2,5-diyl)s and poly(4-alkylthiazole-2,5-diyl)s by optical spectroscopy, NMR analysis, light scattering analysis, and X-ray crystallography, J. Am.Chem. Soc.
cr
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poly(4,4'-dialkyl-2,2'-bithiazole-5,5'-diyl)s and their analogues comprised of electronaccepting five-membered rings. Preparation, regioregular structure, face-to-face stacking, and electrochemical and optical properties, Chem.Mater. 9(5) (1997) 1217.
[24] J.I. Nanos, J.W. Kampf, M.D. Curtis, Poly(alkylbithiazoles): A new class of variable-band gap, conjugated polymer, Chem. Mater. 7(12) (1995) 2232.
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ip t
Chem. Mater. 15(2) (2003) 404.
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[38] J. Heinze, Organic electrochemistry, in: H. Lund, O. Hammerich (Eds.), Ch. 32, Marcel Dekker, New York, 2000, p. 1309.
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normal-doped and para-doped poly(thiophene) Chem. Mater. 3 (1991) 872. [41] O. Semenikhin, E.V. Ovsyannikova, M.R. Ehrenburg, N.M. Alpatova, V.E.Kazarinov,
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stacked conjugated molecules. Synthesis, structures and spectral characterization alkyl
cr
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an
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Legends to Figures Scheme
1.
Synthetic
routes
for
bis
(thiophene)-bis
(ethylenedioxythiophene)-3,3’-
ip t
dinonylthiazole (ThEdNBTEdTh). Scheme 2. Optimized geometry of ThEdNBTEdTh.
cr
Figure 1. 1H NMR of the ThEdNBTEdTh in CDCl3. ! (ppm) = 0.76-0.86 (m, 6H), 1.22-.151
us
(m, 24H), 2.10 (m, 4H), 3.37-3.47 (t, 4H), 5.23 (m, 8H), 7.20 (m, 2H), 7.37 (m, 4H). Figure 2. a)Polymerization of ThEdNBTEdTh in CH2CI2 containing 0.1M TBAPF6 at 10
an
cycles in the range of 0.2-1.0V(Q1(10) = 2.27! C) b)Scan rate dependence of P [ThEdNBTEdTh] in CH2CI2 containing 0.1 M TBAPF6.
M
Figure 3. a)Polymerization of ThEdNBTEdTh in CH2CI2 containing 0.1M TBAPF6 at 10 cycles in the range of 0.2-1.35V. (Q2(10) = 54.8! C) b)-Redox behaviour of resulting polymer at
d
different scan rates.
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Figure 4. Comparison of redox behaviour of polymeric films obtained at different charges by
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applying 10 cycles (a), 20 cycles (b).
Figure 5. a) Ip vs ! 1/2 and b) Ip vs ! graphs of P[ThEdNBTEdTh] films. Figure 6. Cyclic voltammograms of P[ThEdNBTEdTh] film in 0.1M TBAPF6/DCM at the scan rate of 100 mV/s. (Inset: n-doping range). Figure 7. Ex-situ FTIR-ATR spectra recorded during reduction (n-doping) of a P(ThEdNBTEdTh) film in the wavenumber region 4000–650 cm-1 and in the wavenumber region 4000-1500–cm-1(inset) .
Figure 8. Comparison of stability of the P[ThEdNBTEdTh] films obtained at different cycle numbers; a)-10, b)-20,c)-35,d)-50 cycles in monomer free solution.
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Figure 9. (a) UV–Visible absorption spectra of P[ThEdNBTEdTh] film in 0.1M TBAPF6 in DCM at different applied potentials from the range −1.0 to 1.0V, (b) The change of maximum absorbances as a function of potential together with the cyclic voltammogram of P[ThEdNBTEdTh] film.
ip t
Figure 10. Absorption and emission spectra of a)- 1x10-6 M ThEdNBTEdTh and b)- 1x10-6 M
cr
P[ThEdNBTEdTh] in DMF. c)-Absorption spectra of ThEdNBTEdTh in solution and P [ThEdNBTEdTh] both in solution and as a film.
us
Figure 12. Nyquist and Bode plots of P[ThEdNBTEdTh] film obtained potentiodynamically at 10 cycles in the range of 0.2-1.0 V (a, b) and 0.2-1.35V (c, d) at different applied potentials
an
(Edc).
Figure 13. Variation of the (a) Ccv with scan rate and comparison with CEIS and (b) CEIS with
M
potential of the P [ThEdNBTEdTh] films obtained at different cycle numbers. Figure 14. Electrical equivalent circuit model (a) Experimental (dot) and simulated (solid line)
Ac ce p
te
d
Nyquist plot (b) Bode plot (c) of P[ThEdNBTEdTh] film.
24
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Table 1. Comparison of oxidation onset potentials(Eonset,ox)and ionization potentials (IP) of monomers and polymer* and half-wave potentials (E1/2)of polymers.
1.50(2.02)
BTh
1.32
TTh 1.05
IP mon / eV
/V
47
5.90
0.70
ThEdNBTEdTh
0.54 (0.0*)
/V
1.0230
5.10
0.05
5.04 (4.40*)
0.36
47
47
47
36
te
d
M
36
1.04
5.50
an
EdNBTEd
0.97
5.45
47
1.1030
pol
1/2
0.80(0.95)
5.72
47
ThNBTTh
E
ip t
mon onset,ox
cr
Th
E
us
Monomer
Table 2. Optical band gap and electrochemical*(Eg) values of monomer ** and polymers. E / eV
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Monomer/Polymer
g
8
1.75
30
2.53
P[EdNBTEd] P[ThNBTTh]
P[ThEdNBTEdTh]
1.67 (1.44*)
46
E / eV a
E
mon onset,red
2.73
-1.67
2.78
-1.72
/V
1.60
PEDOT 46
PTh
2.10
ThEdNBTEdTh**
2.26
25
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Table 3. Impedance parameters of P[ThEdNBTEdTh] film obtained from equivalent circuit
Rsoln/ohm
17! 102
Rcor/ohm
1! 109
Rpo/ohm
45! 103
Ccor /S sn
41! 10-7
cr
Value
an
us
Parameter
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model.
n
1.00
Cc /S sm
M
14! 10-6 0.923
5! 10-4
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Goodness of fit
d
m
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