Baoyang Luab,
Shijie Zhenb,
Shouli Mingb,
Jingkun Xu*ab and
Guoqun Zhao*a
aKey Laboratory for Liquid-Solid Structural Evolution and Processing of Materials (Ministry of Education), Shandong University, Jinan, Shandong 250061, PR China
bSchool of Pharmacy, Jiangxi Science & Technology Normal University, Nanchang 330013, PR China. E-mail: xujingkun@tsinghua.org.cn; zhaogq@sdu.edu.cn; Fax: +86-791-83823320; Fax: +86-531-88392811; Tel: +86-791-88537967 Tel: +86-531-88393238
First published on 12th August 2015
Polyselenophenes exhibit several special properties and potential advantages over polythiophenes and have been extensively employed in organic electronics recently. Yet, electrosynthesized polyselenophene derivatives have attracted surprisingly scant attention. In this work, 3-methylselenophene (3MeS) was synthesized by a simple procedure, and its electropolymerization was comparatively investigated by employing different electrolyte systems, namely, CH2Cl2–Bu4NPF6, CH2Cl2–BFEE (boron trifluoride diethyl etherate), and ionic liquid BmimPF6. Further, the effect of electrolytes on the structure and morphology, electrochemical, electronic and optical properties, and the electrochromic performances of the as-obtained poly(3-methylselenophene) (P3MeS) films were minutely studied. Surprisingly, we find a very significant electrolyte effect on the electropolymerization behavior of 3MeS and also on the structure, morphology, redox activity and stability, and optoelectronic and electrochromic properties of the electrosynthesized P3MeS material. The 3MeS monomer could be successfully electropolymerized in all the electrolytes, mainly through the coupling at the α-sites of the selenophene ring, and the as-formed P3MeS films from all three electrolytes displayed several mutual characteristics, such as similar chain structures, insolubility in common solvents, electrical conductivity of 10−4 to 10−2 S cm−1, good redox activity and stability superior to polyselenophene, and electrochromic nature from yellow brown in the reduced form to dark gray upon oxidation, but with poor kinetic performances. We also show that the high intrinsic conductivity and viscosity of ionic liquid BmimPF6 provide milder polymerization conditions for 3MeS, leading to the facile electrodeposition of a homogeneous and continuous P3MeS film with less structural defects and better chain arrangement. Further, P3MeS from BmimPF6 exhibited uniform and compact morphology, higher electrical conductivity, better electroactivity and stability, and also a lower band gap of 1.83 eV.
In this context, great strides in polyselenophene research have been achieved over the last several years.1,2 Besides parent polyselenophene,3–8 several typical polyselenophenes with excellent performances have recently been reported, such as poly(3,4-ethylenedioxyselenophene),9 and its analogues and derivatives,10–13 thieno-/selenolo-fused polyselenophene,14 polyselenopheno[3,4-b]selenophene,15 poly(3-alkylselenophene),16 etc., along with several selenophene-based hybrid polymers,17–24 leading to the availability of promising polyselenophene materials. Our group has systematically investigated the effect of different monomeric precursors and electrolyte systems on the electropolymerization and properties of polyselenophene, and also designed a family of selenophene–EDOT conjugated systems and explored their optoelectronic properties.6,7,25,26
In spite of their short history, polyselenophenes have shown interesting and promising performances in several specialized applications, such as electrochromics,3,5,6,17,20,23 photovoltaic cells,15,21,27–33 field-effect transistors,8,28,33–38 and thermoelectrics.25 Recently, Bendikov et al. found that PEDOS derivatives displayed excellent electrochromic properties.11,12 PEDOS exhibits a high contrast ratio of 55% at 666 nm (λmax) and a coloration efficiency of 212 cm2 C−1, much better than PEDOT.12 Later results showed that PEDOS–Cn films revealed significantly improved electrochromic properties (high contrast ratio and high coloration efficiency with a low switching voltage, fast switching time, and remarkable stability) compared with unsubstituted PEDOS.11,12 Also, Lee et al. synthesized a series of novel thiophene- and selenophene-based low-bandgap polymers and fabricated bulk heterojunction solar cells based on these films, producing power conversion efficiencies (PCEs) of 0.64–3.18%. Saadeh et al. significantly improved selenophene-based polymers to give higher PCE values (6.87%), getting an 21% increase in PCE compared to the corresponding thiophene analogue (5.66%).15 Additionally, Gleason et al. obtained low band gap conformal polyselenophene thin films by oxidative chemical vapor deposition and found that these films had unique advantages for field-effect transistors.8 By designing selenophene–diketopyrrolopyrrole copolymers, Chung et al. realized a high charge carrier mobility of ∼2.8 cm2 V−1 s−1,35 while Heeney’s group synthesized low band gap selenophene–diketopyrrolopyrrole polymers and illustrated that high and balanced electron and hole mobilities in excess of 0.1 cm2 V−1 s−1 were observed in bottom-gate, bottom-contact devices.38 Most recently, our group also investigated thermoelectric performances of different types of polyselenophene and found that polyselenophene exhibited a very high Seebeck coefficient (>180 V K−1) and holded promise for thermoelectrics.25 However, despite these efforts devoted to this research field, the fundamental research and applications of polyselenophenes are far from maturity and still have significant scope for development.
Parent selenophene is commercially available; however, very few polyselenophene derivatives are known and only a very limited number of suitable monomeric precursors have been reported compared with thiophenes.1 The chemical instability of selenophenes makes the electrochemical preparation of high-quality polyselenophene materials very difficult and overoxidation always results in damage to the conjugated backbone of the polymer. In order to overcome this problem, substitution by electron-donating groups such as alkyl and alkoxyl groups at the 3-position would lead to a less energetic oxidation and could probably be an effective method, as in the case of polythiophenes.39 Poly(3-methylselenophene) (P3MeS), as an analogue of poly(3-methylthiophene) (P3MeT), which has been extensively researched in the conducting polymer family as one of the most important derivatives of polythiophene,39,40 has rarely been investigated. The excellent and promising optoelectronic properties of P3MeT suggest that its selenium analogue, P3MeS, should become an important member of the conducting polymer family. Indeed, given the similarity between thiophene and selenophene rings, and considering the unique properties of the Se atom (larger in size and more easily polarized than sulfur) and polyselenophene (intermolecular Se–Se interactions, lower oxidation and reduction potentials, lower band gap, etc.),1 P3MeS is even expected to have some advantages over P3MeT.
Yet, despite these potential advantages, P3MeS has attracted surprisingly scant attention except for a few reports more than twenty years ago. In 1986, Dian and coworkers first reported the electrosynthesis of P3MeS in CH3CN–LiClO4 (saturated)41,42 and its electrochemical behavior.43 Later, Tourillon et al. studied the electronic properties and orientation of P3MeS film together with other poly(3-alkylselenophenes), electrochemically deposited onto Pt, by Near Edge X-ray Absorption Fine Structure (NEXAFS) spectroscopy.44,45 The electron-energy-loss spectrum of 3MeS has been recorded, analyzed, and compared with that of the corresponding polymer.46 In addition, Bourahla et al. presented the electrical conductivity of electrochemically synthesized P3MeS pellets (reduced and doped with different counterions), which was determined to be in the range of 10−9 to 10−3 S cm−1.47 Overall, up to now, no systematic studies on the electrosynthesis and properties of P3MeS have been reported, and all previous reports indicate that the conductivities of doped P3MeS range from 10−4 to 10−3 S cm−1, which is significantly lower than the conductivity of doped P3MeT (up to 102 S cm−1).39,40 This low conductivity and the lack of a well-defined electrochemical response by P3MeS have prevented its study and application. The unavailability of synthetic methodologies for the synthesis of substituted selenophene-based monomeric precursors is another reason why it has remained practically unexplored. We presume that the low conductivity and poor electrochemical behavior of P3MeS result from its instability during electrochemical polymerization and that a polymer with higher conductivity and well-defined electrochemistry could be obtained if mild polymerization conditions could be found.
Our continuing interest in polyselenophenes prompts us to systematically investigate the electropolymerization of 3MeS and explore mild polymerization conditions to obtain P3MeS with better optoelectronic properties. Herein, we studied the electropolymerization performances of 3MeS in different electrolytes, namely conventional CH2Cl2–Bu4NPF6, CH2Cl2–BFEE (boron trifluoride diethyl etherate, Lewis acid) binary system, and ionic liquid BmimPF6 (Scheme 1). Different P3MeS samples were electrodeposited under optimum electrical conditions and their structure and properties, including electrochemical, spectroelectrochemical, electrochromic, and thermal properties were minutely explored and comparatively discussed.
The polymer film was grown potentiostatically under the optimized potentials in different electrolytes, and its thickness was controlled by the total charge passed through the cell, which was read directly from the current–time (I–t) curves by computer. After polymerization, the polymer film was washed repeatedly with anhydrous dichloromethane and neat water in order to remove the electrolyte, monomer and oligomers.
Spectroelectrochemical measurements were carried out to consider the absorption spectra of the polymer film under the applied potential. The spectroelectrochemical cell consists of a quartz cell, an Ag wire (RE), a Pt wire (CE), and ITO-coated glass as the transparent working electrode (WE). All measurements were carried out in monomer-free solvent–electrolyte systems. It should be noted here that a quartz cell filled with monomer-free electrolyte and ITO-coated glass without deposited film was used as the background for spectroelectrochemical measurements.
The optical density (ΔOD) at a specific wavelength (λmax) was determined from the %T values of electrochemically oxidized and reduced polymer films, using eqn (1):49
ΔOD = log(Tox/Tred) | (1) |
The coloration efficiency (CE) is defined as the relation between the injected/ejected charge as a function of electrode area (Qd) and the change in optical density (ΔOD) at a specific dominant wavelength (λmax), as illustrated by eqn (2):50
CE = ΔOD/Qd | (2) |
Using the Lewis acid boron trifluoride diethyl etherate (BFEE) as both the solvent and supporting electrolyte can remarkably decrease the onset oxidation potentials of aromatic monomers and readily improve the quality of conducting polymer films via electrochemical polymerization from small aromatic compounds,54 especially, from fused-ring aromatic compounds containing heteroatoms (nitrogen, sulfur and oxygen), and other fused-ring aromatic compounds.55 Since 1995, when Prof. Shi reported the facile electrosynthesis of a free-standing polythiophene film stronger than aluminum in BFEE in Science,56 a wide variety of high-quality conducting polymers electrochemically synthesized from aromatic heterocycles and hydrocarbon monomers (more than 100) have been successfully achieved in BFEE and more than 200 scientific papers have been published.54,55
Herein, to evaluate the effect of electrolytic media on the electropolymerization performances and the properties of the resulting polymer, we carried out the electrochemical tests of 3MeS in three typical electrolytes of different types, i.e., conventional CH2Cl2–Bu4NPF6, BFEE-based medium, and ionic liquid BmimPF6.
From the anodic oxidation behavior (see Fig. 1 and Table 1), the oxidation of 3MeS was initiated at around 1.17 V in CH2Cl2–Bu4NPF6, in good agreement with previous results in CH3CN–LiClO4 (saturated).29,30 By adding Lewis acid BFEE into the solution, the oxidation potential could be brought down to about 0.85 V, due to the fact that BFEE can interact with the aromatic selenophene ring of 3MeS, as in the case of thiophenes,39,40 which reduces the resonance stability of these monomers through the formation of π-complexes between them and BFEE, thus making electron loss from these monomers much easier. However, it should be noted here that after the addition of BFEE, the electrolyte solution containing 3MeS gradually changed to red, brown and then dark from colorless, probably owing to the chemical instability of 3MeS in BFEE, which probably led to a ring-opening reaction due to its moderately strong Lewis acidity and catalytic nature. When employing ionic liquid BmimPF6 as a green electrolyte, the onset oxidation potential of 3MeS (∼1.20 V) was similar to those of neutral organic electrolytes.
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Fig. 1 Anodic oxidation curves of 0.01 mol L−1 3MeS in CH2Cl2–Bu4NPF6 (0.10 mol L−1), CH2Cl2–5% BFEE, and ionic liquid BmimPF6 on a Pt working electrode. Potential scan rate: 50 mV s−1. |
Electrolytes | κ (298 K) (mS cm−1) | η (298 K) (mP s) | Eox,onset (V) | Polymerization potential (V) |
---|---|---|---|---|
CH2Cl2–Bu4NPF6 | 0.78 | 0.50 | 1.17 | 1.30 |
CH2Cl2–BFEE | 0.41 | 0.51 | 0.85 | 1.10 |
BmimPF6 | 1.46 | 275 | 1.19 | 1.35 |
Fig. 2 shows cyclic voltammograms (CVs) corresponding to the potentiodynamic electropolymerization of 3MeS, which have never been reported previously. Application of repetitive potential scans between −0.4 V and the foot of the oxidation wave of the precursor results in the development of a broad redox system in the region of 0–1.20 V, corresponding to the oxidation and reduction of the electrodeposited polymer. Visual inspection during CV experiments revealed the formation of P3MeS films on the electrode surface in all three electrolytes. The increase in the anodic and cathodic peak current densities in the CVs implied that the amount of the polymer film increased on the electrode surface. The broad redox waves of the polymer are probably ascribed to the wide distribution of polymer chain length or the conversion of conductive species on the polymer main chain from the neutral state to the metallic state. The potential shift in the current density peaks provided information about the increase in the electrical resistance of the polymer film and the overpotential needed to overcome this resistance. This behavior shows characteristic features of conducting polymers during potentiodynamic synthesis, also in full agreement with selenophene1,6,7 and thiophenes,39,40 such as 3-methylthiophene (3MeT).40
On the other hand, from these CV curves, 3MeS can be electropolymerized in neutral CH2Cl2–Bu4NPF6 (0.10 mol L−1) (A), but the redox waves were not so good and only a discontinuous thin film could be obtained on the electrode surface after CV scanning. After the introduction of Lewis acid BFEE, the redox behavior and the quality of the deposited product were apparently improved due to the decrease in the polymerization potential. Interestingly, although the polymerization potential in room temperature ionic liquid BmimPF6 was the highest among the three media, CV curves with well-defined redox waves and the largest increasing interval of the peak current densities could be observed, and visual inspection demonstrated that a homogeneous and continuous polymer film can be facilely deposited on the Pt electrode. This phenomenon could be attributed to its milder chemical conditions and higher intrinsic conductivity and viscosity for polymerization (Table 1). As we previously reported,57 BmimPF6 displayed a high intrinsic ionic conductivity (κ = 1.46 mS cm−1 at 25 °C), whereas that of CH2Cl2–Bu4NPF6 (0.10 mol L−1) was determined to be 0.78 mS cm−1 at 25 °C. The freshly distilled BFEE shows an ionic conductivity of ∼0.4 mS cm−1 at room temperature. Due to its water-sensitivity, its ionic conductivity varied significantly (0.4–1.1 mS cm−1) with prolonged time in air. In addition, BmimPF6 possesses much higher viscosity (η = 275 mP s at 25 °C, measured in our lab, and 305 mP s from ref. 58) than those of CH2Cl2–Bu4NPF6 (0.10 mol L−1) and CH2Cl2–5% BFEE (about 0.50 and 0.51 mP s at 25 °C, measured in our lab, respectively). This would contribute to slower ion/molecular transport kinetics and radical–radical coupling of monomers, and also to further oxidation of oligomers; thus the polymer deposited onto the electrode will be favorably accumulated under slow diffusion conditions.59
The current efficiency (f) of polymerization40 for P3MeS in different electrolytes was calculated to be about 65% in BmimPF6, ∼30% in CH2Cl2–Bu4NPF6 (0.10 mol L−1) and ∼10% in CH2Cl2–BFEE. The difference in the current efficiency for P3MeS obtained from these electrolytes also agrees well with the difficulty of electropolymerization. That is, in BmimPF6, P3MeS was easier to obtain than in the other two electrolytes. In addition, the determined f values are lower than P3MeT under the same conditions (30–70%),40 which is probably due to the instability of 3MeS and the formation of a significant amount of soluble oligomers during the process of electropolymerization. Overall, the electrolyte system has a very significant effect on the electropolymerization behavior of 3MeS.
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Fig. 4 FT-IR spectra of 3MeS (A) and P3MeS electrosynthesized in different media: CH2Cl2–5% BFEE (B), CH2Cl2–Bu4NPF6 (0.10 mol L−1) (C), and ionic liquid BmimPF6 (D). |
On the other hand, as can be seen from the figure, the absorption bands in the spectra of the polymers are obviously broadened in comparison with those of the monomers, similar to those of other conducting polymers reported previously.6,7,26,51–56 This incidence is probably due to the resulting product being composed of oligomers/polymers with wide chain dispersity. In more detail, the vibrational modes of the polymers with different degrees of polymerization show different IR shifts. These peaks overlap one another and produce broad bands with hyperstructures. Furthermore, the chemical defects on the polymer chains resulting from the overoxidation of the polymer and α–β, β–β connections also contribute to the band broadening of the IR spectra. However, it should be noted here that P3MeS from CH2Cl2–BFEE (Fig. 4B) and CH2Cl2–Bu4NPF6 (0.10 mol L−1) (Fig. 4C) exhibited fewer absorption bands in comparison with the sample from BmimPF6, especially in the range of 1650–1100 cm−1, probably due to the inevitable damage of the selenophene ring caused by overoxidation and ring opening reactions during the polymerization process.
After dedoping at a negative constant applied potential (−0.3 V), the morphologies of all the P3MeS films from different electrolytes were quite different from those of the doped polymer films. These differences between the doped and dedoped P3MeS films were mainly due to the migration of counterions out of the polymer film and also the gradual dissolution of oligomers/other species trapped in the polymer films from the electrode to the solution during the dedoping processes, which destroyed the original surface of the doped polymer films.
P3MeS films in the doped state were directly obtained via electropolymerization in these electrolytes. The electrical conductivity of the as-formed P3MeS film from CH2Cl2–Bu4NPF6 (0.10 mol L−1) was determined to be in the range of 10−4 to 10−3 S cm−1 by applying a conventional four-probe technique, similar to those values reported previously and also those of electrodeposited polyselenophene (10−4 to 10−2 S cm−1).1 Due to the difficulty in obtaining a sufficient amount of the polymer film, the electrical conductivity of the P3MeS film from CH2Cl2–5% BFEE cannot be accurately measured. The P3MeS film obtained from BmimPF6 displayed a higher electrical conductivity of 10−3 to 10−2 S cm−1. The use of BmimPF6 as the medium for the electrosynthesis of P3MeS results in an increase in electrical conductivity, which may be due to the fact that the concentration of dopants in BmimPF6 is much higher than that in conventional electrolytes. Generally, the larger the concentration of anions in the electrolyte, the higher the doping level is and thus the larger the electrical conductivity of the polymer is.39,40 Also, the mild polymerization conditions of BmimPF6 probably lead to less structural defects and better chain arrangement, thus improving the quality of the as-formed polymer films. However, these values are still much lower than P3MeT (101 to 102 S cm−1).
UV-vis spectra of both the monomer and the polymer films were examined, as shown in Fig. 6 and Table 2. The 3MeS monomer showed a maximum absorption at 253 nm in acetonitrile with the onset optical absorption at about 270 nm. In contrast, a characteristic π–π* transition peak at around 435 nm characterized the spectrum for P3MeS. Also, the overall absorption of the polymer tailed off to ∼700 nm due to the increase in the conjugated chain length. These spectral results confirmed the occurrence of electrochemical polymerization among the 3MeS monomers to form the conjugated polymer.
Electrolytes | Peaks and linear fitting | Redox stability | Absorption | Eg,opt (eV) | λem (nm) | |||||
---|---|---|---|---|---|---|---|---|---|---|
Eoxa (V) | Ereda (V) | Ran2 | Rcat2 | 1000th | 2000th | λonset (nm) | λmax (nm) | |||
a Under the scan rate of 50 mV s−1 (Fig. 7). | ||||||||||
CH2Cl2–Bu4NPF6 | 0.88 | 0.71 | 0.9995 | 0.9976 | 70.1% | — | 652 | 430 | 1.90 | 467 |
CH2Cl2–BFEE | 0.38 | 0.23 | 0.9997 | 0.9996 | 90.6% | 85.2% | 638 | 436 | 1.94 | 451 |
BmimPF6 | 0.78 | 0.56 | 0.9994 | 0.9990 | 84.8% | 67.1% | 677 | 510 | 1.83 | 498, 550 |
The fluorescence emission spectra of 3MeS and the dedoped polymers were also determined in acetonitrile (Fig. 6 and Table 2). In agreement with the UV-vis results, the emission spectra of the polymer displayed a significant red shift with the elongation of π-conjugated systems (maximal emission peaks from 341 nm to 450–550 nm). Overall, both the monomer and the polymer films show unsatisfactory emission properties, similar to most polyselenophenes and polythiophenes.1,2,39,54
All the peak current densities were proportional to the potential scanning rates (Fig. 7 and Table 2), indicating that the redox processes are non-diffusional and the electroactive materials adhere well to the working electrode surface. Furthermore, the CVs of all the polymers in monomer-free electrolytes (Fig. 7) showed an obvious hysteresis, i.e., an obvious difference between the anodic and cathodic peak potentials.60,61 The potential shift of the redox peaks among the CV curves for the conducting polymers is hardly explained by conventional kinetic limitations such as ion diffusion or interfacial charge transfer processes. The main reasons accounting for this phenomenon are usually as follows, slow heterogeneous electron transfer, local rearrangement effect of polymer chains, slow mutual transformation of various electronic species, electronic charging of interfacial exchange corresponding to the metal/polymer and polymer/solution interfaces, etc.60
It is well known that the good stability of conducting polymers is very significant for their applications in electronic devices.62 For example, the long-term stability upon switching and/or cycling plays a key role in the electrochromic performance of the devices and smart windows. For that reason, the long-term stability of P3MeS films, which were deposited on Pt electrodes, upon cycling was elaborated by potential scanning between the neutral and oxidized states in the corresponding monomer-free electrolytes at a potential scan rate of 150 mV s−1, as shown in Fig. 8.
It was noted that all the polymer films exhibited good stability, retaining at least 70% of the electroactivity even after 1000 cycles. The P3MeS films from CH2Cl2–BFEE and BmimPF6 displayed even better stability with the amount of exchange charge still remaining being ∼85% and 67% after sweeping 2000 cycles, respectively. However, the current density of redox waves for the P3MeS film from BmimPF6 was much higher than those from CH2Cl2–Bu4NPF6 (0.10 mol L−1) and CH2Cl2–5% BFEE. Note here that the stability tests were carried out under an air atmosphere and the imposed conditions were not very stringent. If they were sealed in fabricated devices, the long-term stability of these materials upon switching and/or cycling would be further increased. These results indicated excellent redox stability of the P3MeS materials, much better than polyselenophene1,6,7 and even competitive with P3MeT.39,40 Note here that from the FT-IR spectra peaks for P3MeS in different electrolytes after 1000 scans (Fig. S3†), their chain structures remained stable during the potential scan. However, the bands in the range of 1250–1000 cm−1 were obviously enhanced for all three samples. This is mainly due to the migration of doping ions into the polymer film (further doping), which was also observed for P3MeT.40 As stable electroactive materials, they would probably find applications in various fields, such as electrochromics, supercapacitors, and electrochemical (bio)sensors.
Similar to polyselenophene/P3MeT, a dominant absorption with the maximum at around 430 nm characterized the spectra for the neutral P3MeS film from CH2Cl2–Bu4NPF6 (0.10 mol L−1), whereas the maximal absorption of the neutral P3MeS film from BmimPF6 showed a significant red shift of about 80 nm to around 510 nm, indicating the more effective conjugated chain length and well-defined structure with less defects in the main chain of the as-formed P3MeS in BmimPF6. The optical band gaps (Eg) of P3MeS from CH2Cl2–Bu4NPF6 (0.10 mol L−1) and BmimPF6 were calculated from the onset of the low energy end of the π–π* transitions to be 1.90 (652 nm) and 1.83 eV (677 nm), respectively. These band gaps are similar to or just a little lower than polyselenophene (∼1.90 eV),1,6,7 indicating that the introduction of the electro-donating methyl group does not distinctly reduce the band gap of polyselenophene. This phenomenon could be attributed to structural randomness (head-to-head/head-to-tail couplings) and inevitable irregularity (α,β-coupling of selenophene rings) during electropolymerization.
Upon oxidation, the intensity of the absorption bands started to decrease simultaneously with a concomitant increase in the near-IR region, representing the formation of polaronic (at around 820 nm) and bipolaronic (more than 1000 nm) bands, similar to previous results for polyselenophene1 and polythiophenes.39 These changes in the absorption spectra were accompanied with obvious color changes from yellow brown in the reduced form to dark gray upon oxidation during the p-doping process.
In order to gain a deep insight into the electrochromic properties of P3MeS, the optical switching studies of the hybrid polymer films were carried out using a square wave potential step method coupled with optical spectroscopy, known as chronoabsorptometry, in monomer-free CH2Cl2–Bu4NPF6 (0.10 mol L−1) and BmimPF6 solution, respectively. The electrochromic parameters, such as optical contrast ratio (ΔT%) and response time, were investigated by the increment and decrement in the transmittance with respect to time at the specific absorption wavelengths, as shown in Fig. S4.† The potentials were switched alternatively between the reduced and oxidized states with a residence time of 10 s.
Contrary to the spectroelectrochemistry results, it is quite unexpected to find that all the P3MeS films displayed very low transmittance ratios in the range of 0.7–3% at different wavelengths, also leading to low coloration efficiency values. Unfortunately, the response required to attain 95% of total transmittance difference was found to be more than 5.0 s at both 470 and 870 nm for P3MeS from the reduced state to the oxidized state or from the oxidized state to the reduced state, respectively. These kinetic performances are much inferior to that of P3MeT, and even worse than polyselenophene. From all these results, P3MeS showed an electrochromic nature from reddish-brown to dark blue, but its kinetic performances (optical contrast ratio and response time) are relatively poor in comparison with other electrochromic conducting polymers.63
Footnote |
† Electronic supplementary information (ESI) available: 1H and 13C NMR spectra for 3-methylselenophene; detailed peak assignments of FT-IR spectra for both the monomer and the different polymer samples; FT-IR spectra of P3MeS electrosynthesized in different electrolytes after CV scanning for 1000 cycles in the corresponding monomer-free electrolytes; transmittance–time profiles of P3MeS films. See DOI: 10.1039/c5ra11849b |
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