Effect of electrolytes on the electropolymerization and optoelectronic properties of poly(3-methylselenophene)

Abstract

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.

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1. Introduction

In the conducting polymer family, in sharp contrast to oligo- and polythiophenes extensively employed in organic electronics, the research field of oligo-/polyselenophenes has only recently been established.^1,2 Compared with other types of conducting polymers, oligo-/polyselenophenes exhibit several special properties and advantages, such as lower band gap,^1–5 more planar backbone,^3–5 stronger intermolecular interactions (including Se–Se interactions),^3–5 greater degree of doping,^3–5 etc., which make them excellent candidates for applications in organic electronics.^1,2

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 (λ_max) and a coloration efficiency of 212 cm² C⁻¹, much better than PEDOT.¹² Later results showed that PEDOS–C_n 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.

Scheme 1 Synthetic procedure of 3-methylselenophene and its electropolymerization.

2. Experimental

2.1 Chemicals

Selenium powder (Se, 99+%; Energy Chemical), 2-methyl-1,3-butadiene (isoprene, 98%; J&K Scientific Ltd), nitric acid (HNO₃, 98%; J&K Chemical Ltd), and silicon dioxide (SiO₂, 99+%; Energy Chemical) were used directly without further purification. 1-Butyl-3-methylimidazoliumhexafluorophosphate (BmimPF₆, 98%; Energy Chemical) was dried under vacuum before use. Boron trifluoride diethyl etherate (BFEE, Beijing Changyang Chemical Plant) was distilled and stored at −20 °C before use. Dichloromethane (DCM, analytical grade; Shanghai Vita Chemical Plant, China) and N,N-dimethylformamide (DMF, analytical grade; Xilong Chemical) were purified by distillation with calcium hydride under a nitrogen atmosphere before use. Tetrabutylammonium hexafluorophosphate (Bu₄NPF₆, 99%; Energy Chemical) was dried under vacuum at 60 °C for 24 h before use. Tetrahydrofuran (THF, analytical grade; Shanghai Vita Chemical Plant, China) was distilled over Na/benzophenone prior to use. Other chemicals and reagents (analytical grade, >98%) were all purchased commercially from Shanghai Vita Chemical Plant (Shanghai, China) and were used directly without any further treatment.

2.2 Synthesis of 3MeS⁴⁸

In a two-neck 250 mL flask, the selenium power (5.0 g, 63.3 mmol) was mixed with silicon dioxide (50 g), which had already been cleaned with HNO₃, rinsed with water, and dried in a vacuum-drying oven. 2-Methyl-1,3-butadiene (12.94 g, 189.9 mmol) was added dropwise and the resulting mixture was heated to 400 °C. The mixture was refluxed for 4 h and cooled to ambient temperature. The organic phase was washed with water (20 mL) and then dried over anhydrous MgSO₄. The solvent was removed by rotary evaporation, and the product was purified by column chromatography to obtain 0.16 g of slightly yellow liquid (yield: 16%). ¹H NMR (400 MHz, CDCl₃, ppm): δ 7.81 (d, J = 7.8 Hz, 1H), 7.44 (s, 1H), 7.12 (d, J = 8.2 Hz, 1H), 2.22 (s, 3H); ¹³C NMR: 139.69, 132.47, 130.00, 124.69, 17.32.

2.3 Electrosynthesis and electrochemical tests

All electrochemical tests and polymerization were performed in a one-compartment cell with the use of a Model 263A potentiostat–galvanostat (EG&G Princeton Applied Research) under computer control. For electrochemical tests, the working and counter electrodes were typically both Pt wires with a diameter of 1 mm. To obtain a sufficient amount of the polymer films for characterization, Pt sheet or ITO-coated glass with a surface area of 3 cm × 2 cm was employed as the working electrode and another Pt sheet (3 cm × 2 cm) was used as the counter electrode. These aforementioned electrodes were polished carefully with 1500 mesh abrasive paper (for ITO: immersed in ethanol for 6 h and then cleaned ultrasonically for 15 min), cleaned with water and acetone successively, and then dried in air before each experiment. An Ag/AgCl electrode directly immersed in the solution served as the reference electrode, and it displayed sufficient stability during the experiments. Note here that all the electrolytic solutions were deaerated with a dry nitrogen stream (for more than 20 min) and maintained under a slight overpressure through all the experiments to avoid the effect of oxygen.

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.

2.4 Characterization

Electrochemical, spectroelectrochemical and kinetic studies were carried out on a Model 263A potentiostat–galvanostat (EG&G Princeton Applied Research) and a SPECORD PLUS UV-VIS Spectrophotometer (ANALYTIKJENA, Germany) under computer control. Infrared spectra were acquired with a Bruker Vertex 70 Fourier-transform infrared (FT-IR) spectrometer with samples in KBr pellets. UV-vis spectra of the monomer dissolved in acetonitrile and the polymer film electrodeposited on the indium-tin-oxide (ITO) coated glass were taken by using a SPECORD PLUS UV-VIS Spectrophotometer (ANALYTIKJENA, Germany). With an F-4500 fluorescence spectrophotometer (Hitachi), the fluorescence spectra of the monomer dissolved in acetonitrile and the polymer film electrodeposited on the ITO-coated glass were determined in acetonitrile. Scanning electron microscopy (SEM) measurements were made by using a VEGA II-LSU scanning electron microscope (Tescan) with the polymer deposited on the ITO-coated glass.

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 (λ_max) was determined from the %T values of electrochemically oxidized and reduced polymer films, using eqn (1):⁴⁹

ΔOD = log(T_ox/T_red) (1)

The coloration efficiency (CE) is defined as the relation between the injected/ejected charge as a function of electrode area (Q_d) and the change in optical density (ΔOD) at a specific dominant wavelength (λ_max), as illustrated by eqn (2):⁵⁰

CE = ΔOD/Q_d (2)

3. Results and discussion

3.1 Monomer synthesis

The monomer 3MeS is now commercially unavailable due to the lack of suitable synthetic methods. Herein, 3MeS was simply prepared by refluxing the mixture of 2-methyl-1,3-butadiene, selenium powder, and silicon dioxide at 400 °C for 4 h (Scheme 1), similar to the procedure reported previously but with a relatively lower yield of about 15–20%. However, it should be noted here that the 2-methyl-1,3-butadiene, the reaction bed (silicon dioxide) and the selenium that have not reacted can be reused and recycled for further syntheses without yields being adversely affected. ¹H and ¹³C NMR spectra of the target compound are displayed in Fig. S1 and S2.†

3.2 Electrochemical polymerization in different electrolytes

As is known, the electrolytic medium exerts a strong effect on the structure and properties of the deposited polymer films. Traditional electrolyte systems employed for electropolymerization generally consist of molecular solvents and certain supporting electrolytes.³⁹ The electrolytic medium of CH₂Cl₂–Bu₄NPF₆ is one of the most employed ones among the composed solvent–supporting electrolytes. In contrast to molecular solvent–supporting electrolyte media, ionic liquids have received increasing interest as novel electrosynthetic media because of their combined properties of non-flammability and low volatility, non-polluting nature, good intrinsic conductivity, wide potential window and extremely redox-robust character. In order to avoid the deleterious effect of water and improve the quality and optoelectronic properties of the as-formed conducting polymers, hydrophobic ionic liquids have been usually chosen as the electropolymerization media.^51–53 BmimPF₆ is a typical hydrophobic ionic liquid, and has been extensively employed for the electropolymerization of small heterocyclic compounds.^51–53

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.

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.

Table 1 Various experimental parameters of 3MeS in different electrolytes

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

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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 selenophene^1,6,7 and thiophenes,^39,40 such as 3-methylthiophene (3MeT).⁴⁰

Fig. 2 Cyclic voltammograms (CVs) of 0.01 mol L⁻¹ 3MeS in CH₂Cl₂–Bu₄NPF₆ (0.10 mol L⁻¹) (A), CH₂Cl₂–5% BFEE (B), and ionic liquid BmimPF₆ (C) on a Pt working electrode. Potential scan rate: 50 mV s⁻¹.

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.

3.3 Optimization of electrical conditions and preparation of polymers

Potentiostatic electrolysis was employed to prepare the P3MeS film for characterization. To optimize the applied potential for polymerization, a set of current transients during the electropolymerization in different media at different applied potentials were recorded, as shown in Fig. 3. Typically, for all three electrolytes at applied potentials below the onset oxidation potential of 3MeS, no polymer film was found on the electrode, indicating that polymerization did not occur on the electrode surface due to the low resultant current density. Once the applied potential reaches the threshold value, all the electrosynthetic current densities initially experience an increase and then a slow decrease and finally remain constant as a result of uniform deposition of the polymer film on the electrode surface. However, at relatively high potentials, the surfaces of the polymer films became rough, discontinuous, and heterogeneous due to significant overoxidation. Even worse, during/after the experiments, some film even fell down into the solution from the electrode surface. Considering the overall factors affecting the quality of the formed film, such as moderate polymerization rate, negligible overoxidation, regular morphology, and good adherence to the working electrode, the optimized polymerization potentials for 3MeS were 1.30 V vs. Ag/AgCl in CH₂Cl₂–Bu₄NPF₆ (0.10 mol L⁻¹), 1.10 V in CH₂Cl₂–5% BFEE, and 1.35 V in ionic liquid BmimPF₆ (Table 1). Therefore, the polymer films used for the characterization mentioned below were all prepared using the chronoamperometry method at these optimal potentials in these electrolytes.

Fig. 3 Chronoamperometric curves of 0.01 mol L⁻¹ 3MeS in CH₂Cl₂–Bu₄NPF₆ (0.10 mol L⁻¹) (A), CH₂Cl₂–5% BFEE (B), and ionic liquid BmimPF₆ (C) on a Pt working electrode at different applied potentials for 500 s.

3.4 Structural characterization

To interpret the polymerization mechanism of 3MeS and elucidate the structure of its polymer, FT-IR spectra of the 3MeS monomer and the corresponding neutral polymers from different media were recorded, as shown in Fig. 4 and Table S1† (detailed band assignment). From the figure, it can be seen that all the P3MeS films from the different electrolytes displayed similar absorption bands, although the peak shapes and intensity were variable. For 3MeS, the moderately strong bands at 2964 and 2916 cm⁻¹ were closely related to the stretching vibration of the –CH₃ group, which were shifted to ∼2968 and 2880 cm⁻¹ in all the polymer samples, indicating that the methyl group was not destroyed during the electropolymerization process. However, the peaks at 3078 and 3042 cm⁻¹ for the monomer, which could be assigned to the C–H vibration of the selenophene ring, were nearly absent or considerably weakened and shifted to higher wavenumbers (3172–3120 cm⁻¹), indicating the occurrence of electropolymerization at the selenophene rings. Due to the elongation in the conjugated chain length, the skeletal vibration and C–C vibration of the selenophene ring also shifted to higher wavenumbers by about 25 cm⁻¹. In the fingerprint region, the characteristic peaks at 887 and 750 cm⁻¹ for the monomer can be ascribed to the out-of-plane bending vibration of isolated C–H and adjacent C–H in the selenophene ring, respectively. In contrast, the emergence of new moderately strong peaks at around 843–841 cm⁻¹ (characteristic out-of-plane bending vibration of 2,3,5-trisubstituted selenophene/thiophene ring),^39,40 together with the significant reduction at ∼750 cm⁻¹, confirmed the polymerization at the α-positions of the selenophene ring, namely the C₍₂₎ and C₍₅₎ positions, in good agreement with the results for polyselenophene^6,7 and P3MeT.^39,40

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.

3.5 Morphology

The surface morphologies of conducting polymers are closely related to their properties, such as electrical conductivity, redox activity and stability, etc. Therefore, the surface of the as-formed P3MeS films deposited electrochemically on the ITO electrode was observed by scanning electron microscopy (SEM), as shown in Fig. 5. Macroscopically, P3MeS coated in CH₂Cl₂–Bu₄NPF₆ (0.10 mol L⁻¹) was thin and discontinuous on the ITO electrode, whereas in CH₂Cl₂–BFEE, contrary to the Pt wire electrode, the deposited P3MeS film was still discontinuous and was even worse; more precisely, a thin and uneven film covered only the lower edge of the ITO glass. In contrast, a homogeneous and continuous P3MeS film with significantly improved quality can be facilely deposited on the Pt electrode in ionic liquid BmimPF₆. Microscopically, at high magnifications (5000×), the surface of the doped polymer film (Fig. 5A) obtained from CH₂Cl₂–Bu₄NPF₆ (0.10 mol L⁻¹) seems smooth and continuous. The morphology of the P3MeS film from CH₂Cl₂–BFEE (Fig. 5C) was still discontinuous, like in plates. For the P3MeS film deposited from BmimPF₆ (Fig. 5E), the polymer film resembles compact and ordered arrangements of cauliflowers or corals. Further, this morphology facilitates the movement of dopants in and out of the polymer films during the doping and dedoping processes, well in accord with the good redox activity of P3MeS films in the monomer-free electrolytes.

Fig. 5 SEM images of P3MeS films deposited electrochemically on the ITO electrode in different electrolytes: CH₂Cl₂–Bu₄NPF₆ (0.10 mol L⁻¹) (A and B), CH₂Cl₂–5% BFEE (C and D), and ionic liquid BmimPF₆ (E and F). Magnification: 5000×. Left panel: doped P3MeS; right panel: dedoped P3MeS.

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.

3.6 Solubility, electrical conductivity, UV-vis and fluorescence spectra of 3MeS and polymers

Similar to electrosynthesized P3MeT films,^39,40 the as-obtained P3MeS films show very poor solubility in conventional organic solvents at room temperature, and even in strong polar solvents, such as dimethyl sulphoxide, N,N-dimethylformamide, tetrahydrofuran, etc. The poor solubility probably indicates their long polymer sequences and complex chain arrangement.

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.

Fig. 6 UV-vis spectra (A) and fluorescence emission spectra (B) of the 3MeS monomer (a) in acetonitrile and the dedoped polymer in the solid state: ionic liquid BmimPF₆ (b), CH₂Cl₂–Bu₄NPF₆ (0.10 mol L⁻¹) (c), and CH₂Cl₂–5% BFEE (d).

Table 2 The electrochemistry and optoelectronic parameters of P3MeS films from different electrolytes

Electrolytes	Peaks and linear fitting				Redox stability		Absorption		Eg,opt (eV)	λem (nm)
	Eox^a (V)	Ered^a (V)	Ran^2	Rcat^2	1000^th	2000^th	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

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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

3.7 Electrochemistry of P3MeS films

The electrochemical behaviors of the P3MeS-modified Pt electrodes from different electrolytes were studied by cyclic voltammetry in the corresponding monomer-free electrolytes to test their electroactivity and stability. The results are shown in Fig. 7 and the electrochemical parameters of the P3MeS films from different electrolytes, together with their optoelectronic parameters, are summarized in Table 2. The CVs of the polymers from all electrolytes under different potential scan rates displayed broad anodic and cathodic peaks, similar to those of polyselenophene^1,6,7,25 and P3MeT.^39,40

Fig. 7 Cyclic voltammograms (CVs, left column) of P3MeS electrodeposited from CH₂Cl₂–Bu₄NPF₆ (0.10 mol L⁻¹) (A), CH₂Cl₂–5% BFEE (B), and ionic liquid BmimPF₆ (C) on a Pt working electrode in the corresponding monomer-free solvent–electrolyte systems at potential scan rates of 300, 275, 250, 225, 200, 175, 150, 125, 100, 75, 50, and 25 mV s⁻¹. Right column: absolute values of redox peak current densities vs. potential scan rates. j_p is the absolute value of the peak current density, and j_p,a and j_p,c denote the absolute value of the anodic and cathodic peak current densities, respectively.

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.

Fig. 8 Long-term cyclic voltammograms of P3MeS electrodeposited from CH₂Cl₂–Bu₄NPF₆ (0.10 mol L⁻¹) (A), CH₂Cl₂–5% BFEE (B), and ionic liquid BmimPF₆ (C) on a Pt working electrode in the corresponding monomer-free electrolyte systems. Potential scan rate: 150 mV s⁻¹.

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 polyselenophene^1,6,7 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.

3.8 Spectroelectrochemistry

From the viewpoint of device and high-performance display applications, the spectroelectrochemical properties of the electrochromes should be manifested by using the changes in the optical absorption spectra under voltage pulses. Therefore, UV-vis spectra of different P3MeS films electrodeposited on ITO-coated glass slides were recorded in situ in corresponding monomer-free electrolytes after neutralization (Fig. 9). The potential applied to the polymer-coated electrode was initially in the fully reduced state and then sequentially increased to higher potentials to oxidize the polymer while monitoring the creation of the charge carriers. Note here that the spectroelectrochemistry for P3MeS obtained from CH₂Cl₂–5% BFEE failed due to the difficulty in obtaining a sufficient amount of the polymer film and also the significant structural defects caused by the ring opening reaction in the polymer films.

Fig. 9 Spectroelectrochemistry for P3MeS films on the ITO coated glass in corresponding monomer-free solutions. (A) P3MeS from CH₂Cl₂–Bu₄NPF₆ (0.10 mol L⁻¹) between −1.0 and 1.2 V (ΔE = 0.1 V); (B) P3MeS from BmimPF₆ between −1.0 and 1.8 V (ΔE = 0.1 V).

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 (E_g) 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

4. Conclusions

In summary, the synthesis and electropolymerization of 3MeS were investigated in different electrolyte systems, namely CH₂Cl₂–Bu₄NPF₆, CH₂Cl₂–BFEE, and ionic liquid BmimPF₆. The effect of electrolytes on the structure and morphology, electrochemical, electronic, and optical properties, and electrochromic performances of the as-obtained P3MeS films were systematically studied. Surprisingly, it is found that the electrolyte system has a very significant effect on the electropolymerization behavior of 3MeS and also on the structure, morphology, redox activity and stability, and optoelectronic and electrochromic properties of the P3MeS material. Overall, the monomer 3MeS could be successfully electropolymerized in all three electrolytes, mainly through coupling at the α-sites of the selenophene ring, and all the obtained P3MeS materials displayed the same chain structures, insolubility in common solvents, electrical conductivity in the range of 10⁻⁴ to 10⁻² S cm⁻¹, good redox activity and stability, superior to polyselenophene, and electrochromic nature from reddish-brown in the reduced form to dark blue upon oxidation, but poor kinetic performances. By comparison, we also show that the high intrinsic conductivity and viscosity of ionic liquid BmimPF₆ provided 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 results suggested that P3MeS from BmimPF₆ exhibited compact morphology, higher electrical conductivity, better electroactivity and stability, and a lower band gap of 1.83 eV.

Acknowledgements

We are grateful to the National Natural Science Foundation of China (grant number: 51303073, 51463008), Ganpo Outstanding Talents 555 projects (2013), the Training Plan for the Main Subject of Academic Leaders of Jiangxi Province (2011), the Science and Technology Landing Plan of Universities in Jiangxi province (KJLD12081), the Natural Science Foundation of Jiangxi Province (grant number: 20122BAB216011, 20142BAB206028, 20142BAB216029), and Scientific Research Projects of Jiangxi Science & Technology Normal University (2014QNBJRC003) for their financial support of this work.

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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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