Synthesis of EDOT-containing polythiophenes and their properties in relation to the composition ratio of EDOT

Abstract

Polythiophenes composed of 3,4-ethylenedioxythiophene (EDOT) and 3-hexylthiophene with different composition ratios of EDOT in the repeat unit of the polymer backbone are synthesized by polycondensation reactions. The optical and electrochemical properties of the polymers are compared with those of poly(3,4-ethylenedioxythiophene) and poly(3-hexylthiophene), and they are found to be well correlated with the EDOT composition ratio. In addition, the charge transport properties of the polymer films, measured using the in situ conductivity technique, are discussed in terms of the EDOT composition ratio, regioregularity, and the doping level.

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Introduction

Since polyacetylene was successfully obtained as a film state and the bromine-doped film was found to show high electrical conductivities,¹ π-conjugated polymers with various kinds of chemical structures have attracted a great deal of attention from many researchers because these polymers exhibit not only metallic nature in the doped (oxidized or reduced) state, but also semiconducting nature in the neutral state.² While doped polymers were originally studied to understand the mechanism of electrical conduction from the fundamental aspects, neutral polymers have been well-studied over the last few decades from the viewpoint of their use in many types of applications in the plastic electronics field, such as organic light-emitting diodes (OLEDs), organic field-effect transistors (OFETs) and organic photovoltaics (OPVs) which enable low-cost, large-area and flexible devices. Since it was found that poly(3,4-ethylenedioxythiophene) (PEDOT), a derivative of polythiophene, shows high electrical conductivity by chemical or electrochemical doping and the doped PEDOT can be applied to transparent conductive materials³ and organic thermoelectrics,⁴ doped polymers have started to attract attention again in very recent years. However, PEDOT is not soluble in common organic solvents because of the rigid backbone, which restricts their industrial applications. To improve the processability, PEDOT and its related polymers having 3,4-alkylenedioxythiophenes in a repeat unit have been enthusiastically developed, but the chemical structures of most of the polymers synthesized by chemical or electrochemical oxidation of their corresponding monomers were not well-defined.^3b,5

Recently, we have succeeded in the synthesis of polythiophenes containing 3,4-ethylenedioxythipohene (EDOT) in a repeat unit, poly(3′,4′-ethylenedioxy-2,2′:5′,2′′-terthiophene) (polyTET) and poly(3,3′′′′-dihexyl-3′,4′,3′′′,4′′′-diethylenedioxy-2,2′:5′,2′′:5′′,2′′′:5′′′,2′′′′-quinquethiophene) (polyHE5T) (Fig. 1).⁶ These polymers were soluble in common organic solvents, so that their chemical structures could be characterized well by ¹H NMR and MS spectroscopies, and GPC analysis. Furthermore, the electrical conductivities of these polymers were found to be controllable from 10⁻⁷ to 10¹ S cm⁻¹ by changing the chemical structure and doping level.

Fig. 1 Chemical structures of (a) polyTET and (b) polyHE5T.

In this paper, three kinds of polythiophenes containing EDOT and 3-hexylthiophene (3HT) with different EDOT composition ratios were synthesized by polycondensation reactions using direct C–H arylation and Stille-coupling, and their optical, electrochemical, and electrical properties were discussed with those of PEDOT and poly(3-hexylthiophene) (P3HT). In particular, the charge transport properties of these polythiophene films were investigated in relation to the EDOT composition ratio, the primary structure (regioregularity) of the polymers, and the extent of oxidation of the polymer chain (doping level).

Experimental

Materials and instrumentation

n-Hexane, toluene, tetrahydrofuran (THF), acetic acid (AcOH), chloroform, dichloromethane, o-dichlorobenzene and acetonitrile were purified by standard methods and used immediately after purification. Tetraethylammonium perchlorate (TEAP) and N-bromosuccinimide (NBS) were purified by recrystallization from ethanol and benzene, respectively, and dried under vacuum. EDOT and 3-bromothiophene were purchased from Tokyo Chemical Industry and used without further purification. P3HT used in this study was regiorandom (regioregularity of 54% determined by ¹H NMR). 2-Tributylstannyl-3,4-ethylenedioxythiophene,^6b 3HT⁷ and 2-bromo-3-hexylthiophene⁸ were synthesized according to the literature.

Microwave reactions were conducted using a Monowave 300 (Anton Paar GmbH). ¹H NMR spectra were recorded by 500 MHz spectrometers (Varian Inc., NMR System 500) and by a 600 MHz spectrometer (JEOL Ltd., JNM-ECA600). The molecular weights of the resulting polymers were analyzed by a GPC coupled with an UV detector (Shimadzu Corp., SPD-10A). The combination of Shodex KF-801 (30 cm, exclusion limit: M_n = 1.5 × 10³, polystyrene), KF-802 (30 cm, exclusion limit: M_n = 5.0 × 10³, polystyrene) and KF-803L (30 cm, exclusion limit: M_n = 7.0 × 10⁴, polystyrene) columns (linear calibration down to M_n = 100) were used for molecular weight analysis with THF (1.0 dm³ min⁻¹) as an eluent. Measurements of mass spectroscopy and the elemental analysis were made using a LTQ Orbitrap XL™ Hybrid Ion Trap-Orbitrap Mass Spectrometer (Thermo Fisher Scientific Inc.) and an Elemental Analyzer (Perkin Elmer, 2400 Series II CHNS/O), respectively. The UV-Vis absorption spectra were measured by a spectrophotometer (Shimadzu Corp., UV-3150). Cyclic voltammetry and in situ conductivity measurements were carried out using a potentiostat/galvanostat (Hokuto Denko Corp., HAB-151 or HZ-3000) with an X-Y recorder (Riken Denshi Co., Ltd., F-57), a coulometer (homemade), and a function generator (Iwatsu Electric Co., Ltd., SG-4105).

Synthesis

The synthetic routes of the monomers and polymers are shown in Scheme 1 and the detailed synthetic processes are described below.

Scheme 1 Synthetic procedures of the monomers and polymers: (i) n-BuLi, dry THF, Bu₃SnCl; (ii) 2-bromo-3-hexylthiophene, Pd(PPh₃)₄, toluene; (iii) NBS, CHCl₃/AcOH; (iv) MeMgBr, dry THF; water; (v) NBS, THF; (vi) Pd(dba)₂, P(o-Tol)₃, Cs₂CO₃, pivalic acid, THF, and (vii) Pd(PPh₃)₄, toluene.

3,4-Ethylenedioxy-3′-hexyl-2,2′-bithiophene (ET). To a solution of EDOT (0.70 g, 4.9 mmol) in THF (15 mL) at −78 °C was added 1.8 mL (4.9 mmol) of n-butyllithium (2.7 M in n-hexane) by a syringe. The mixture was stirred at −78 °C for 1 h. Tri(n-butyl)tin chloride (1.8 g, 5.6 mmol) was added to the solution, and the resulting mixture was stirred at −78 °C for 30 min, warmed to room temperature, and stirred further for 1 h. The solvent was removed via rotary evaporation, and n-hexane was added to the residue. The soluble fraction was extracted by filtration. The filtrated solution was added to a solution of tetrakis(triphenylphosphine)palladium (0.27 g, 0.23 mmol) and 2-bromo-3-hexylthiophene (1.2 g, 4.9 mmol) in toluene (30 mL). The solution was stirred at 80 °C for 7 h. The mixture was poured into sat. aq. Na₂CO₃ and extracted with dichloromethane. The extract was then successively washed with water. After being dried over anhydrous Na₂SO₄, the solvent was evaporated and the crude product was purified by column chromatography on silica gel with a mixed solvent of n-hexane and toluene (v/v = 1/2) as an eluent to afford a yellow liquid (0.80 g, 2.6 mmol). Yield: 53%. ¹H NMR (500 MHz, (CD₃)₂CO, δ, ppm): 0.86 (t, J = 6.82 Hz, 3H, CH₃), 1.22–1.36 (m, 6H, thienyl-(CH₂)₂(CH₂)₃CH₃), 1.59 (tt, J = 7.80, 7.80 Hz, 2H, thienyl-CH₂CH₂), 2.68 (t, J = 7.80 Hz, 2H, thienyl-CH₂), 4.23–4.31 (m, 4H, OCH₂CH₂O), 6.51 (s, 1H, EDOT-H), 6.99 (d, J = 5.32 Hz, 1H, thienyl-H), 7.38 (d, J = 5.32 Hz, 1H, thienyl-H). HRMS (APCI) m/z calcd for C₁₆H₂₁O₂S₂ ([M + H]⁺) 309.0978, found 309.0983.

5,5′-Dibromo-3,4-ethylenedioxy-3′-hexyl-2,2′-bithiophene (DBrET). To a CHCl₃/AcOH (75 mL/75 mL) solution of ET (1.2 g, 4.0 mmol), a CHCl₃/AcOH (175 mL/175 mL) solution of NBS (1.6 g, 9.1 mmol) was slowly added in the dark at 0 °C. The reaction mixture stirred in the dark at 0 °C for 2 h was poured into water, and extracted two times with chloroform. The organic extracts were washed with sat. aq. Na₂CO₃ and water, and dried over anhydrous Na₂SO₄. The solvent was evaporated, and the residue was purified by column chromatography on silica gel with a mixed solvent of acetone and dichloromethane (v/v = 1/1) to afford a yellow viscous liquid (1.1 g, 2.3 mmol). Yield: 56%. ¹H NMR (500 MHz, (CD₃)₂CO, δ, ppm): 0.87 (t, J = 6.88 Hz, 3H, CH₃), 1.24–1.38 (m, 6H, thienyl-(CH₂)₂(CH₂)₃CH₃), 1.59 (tt, J = 7.83, 7.83 Hz, 2H, thienyl-CH₂CH₂), 2.65 (t, J = 7.83 Hz, 2H, thienyl-CH₂), 4.36 (s, 4H, OCH₂CH₂O), 7.07 (s, 1H, thienyl-H). HRMS (APCI) m/z calcd for C₁₆H₁₈O₂Br₂S₂ (M⁺) 463.9110, found 463.9110.

5′-Bromo-3,4-ethylenedioxy-3′-hexyl-2,2′-bithiophene (BrET). To a solution of DBrET (0.93 g, 2.0 mmol) in THF (10 mL) was added 2.0 mL (2.0 mmol) of methylmagnesium bromide (1.0 M in THF) by a syringe. The mixture was stirred at reflux temperature for 3.5 h. The solvent was removed in vacuo. Water and dichloromethane were added to the residue, and the organic phase was extracted and dried over anhydrous Na₂SO₄. After removal of the solvent, the crude product was purified by column chromatography on silica gel with a mixed solvent of n-hexane and toluene (v/v = 2/1) as an eluent to afford a yellow solid (0.60 g, 1.5 mmol). Yield: 75%. ¹H NMR (500 MHz, (CD₃)₂CO, δ, ppm): 0.87 (t, J = 6.97 Hz, 3H, CH₃), 1.24–1.38 (m, 6H, thienyl-(CH₂)₂(CH₂)₃CH₃), 1.59 (tt, J = 7.76, 7.76 Hz, 2H, thienyl-CH₂CH₂), 2.67 (t, J = 7.76 Hz, 2H, thienyl-CH₂), 4.24–4.34 (m, 4H, OCH₂CH₂O), 6.55 (s, 1H, EDOT-H), 7.04 (s, 1H, thienyl-H). HRMS (APCI) m/z calcd for C₁₆H₁₉O₂BrS₂ (M⁺) 386.0004, found 386.0001.

3,4′-Dihexyl-2,2′-bithiophene (TT). To a solution of 3HT (0.63 g, 3.7 mmol) in THF (15 mL) at −78 °C was added 1.5 mL (4.0 mmol) of n-butyllithium (2.7 M in n-hexane) by a syringe. The mixture was stirred at −78 °C for 1 h. Tri(n-butyl)tin chloride (1.6 g, 4.7 mmol) was added to the solution, and the resulting mixture was stirred at −78 °C for 30 min, warmed to room temperature, and stirred further for 1 h. The solvent was removed via evaporation, and n-hexane was added to the residue. The soluble fraction was extracted by filtration. The filtrated solution was added to a solution of tetrakis(triphenylphosphine)palladium (0.22 g, 0.19 mmol) and 2-bromo-3-hexylthiophene (0.93 g, 3.7 mmol) in toluene (30 mL). The solution was stirred at 90 °C for 12 h. The mixture was poured into sat. aq. Na₂CO₃ and extracted with chloroform. The extract was then successively washed with water. After being dried over anhydrous Na₂SO₄, the solvent was evaporated and the crude product was purified by column chromatography on silica gel with a mixed solvent of n-hexane and toluene (v/v = 30/1) as an eluent to afford a yellow liquid (0.55 g, 1.6 mmol). Yield: 44%. ¹H NMR (500 MHz, (CD₃)₂CO, δ, ppm): 0.87 (t, J = 7.15 Hz, 3H, CH₃), 0.88 (t, J = 7.15 Hz, 3H, CH₃), 1.26–1.42 (m, 12H, thienyl-(CH₂)₂(CH₂)₃CH₃), 1.63 (tt, J = 7.85, 7.85 Hz, 2H, thienyl-CH₂CH₂), 1.65 (tt, J = 7.64, 7.64 Hz, 2H, thienyl-CH₂CH₂), 2.63 (t, J = 7.64 Hz, 2H, thienyl-CH₂), 2.76 (t, J = 7.85 Hz, 2H, thienyl-CH₂), 7.01 (d, J = 5.26 Hz, 1H, thienyl-H), 7.03 (d, J = 1.46 Hz, 1H, thienyl-H), 7.09 (d, J = 1.46 Hz, 1H, thienyl-H), 7.32 (d, J = 5.26 Hz, 1H, thienyl-H). HRMS (APCI) m/z calcd for C₂₀H₃₁S₂ ([M + H]⁺) 335.1862, found 335.1866.

5,5′-Dibromo-3,4′-hexyl-2,2′-bithiophene (DBrTT). To a THF (6 mL) solution of TT (0.40 g, 1.2 mmol), a THF (6 mL) solution of NBS (0.46 g, 2.6 mmol) was slowly added in the dark at 0 °C. The reaction mixture was stirred in the dark at 0 °C for 3 h. The solvent was removed in vacuo. Water and chloroform were added to the residue, and the organic phase was extracted and dried over anhydrous Na₂SO₄. The solvent was evaporated, and the residue was purified by column chromatography on silica gel with n-hexane as an eluent to afford a pale yellow viscous liquid (0.48 g, 0.98 mmol). Yield: 81%. ¹H NMR (500 MHz, (CD₃)₂CO, δ, ppm): 0.87 (t, J = 7.09 Hz, 3H, CH₃), 0.88 (t, J = 7.09 Hz, 3H, CH₃), 1.24–1.42 (m, 12H, thienyl-(CH₂)₂(CH₂)₃CH₃), 1.61 (tt, J = 7.82, 7.82 Hz, 2H, thienyl-CH₂CH₂), 1.63 (tt, J = 7.64, 7.64 Hz, 2H, thienyl-CH₂CH₂), 2.60 (t, J = 7.64 Hz, 2H, thienyl-CH₂), 2.70 (t, J = 7.82 Hz, 2H, thienyl-CH₂), 6.99 (s, 1H, thienyl-H), 7.10 (s, 1H, thienyl-H). HRMS (APCI) m/z calcd for C₂₀H₂₉Br₂S₂ ([M + H]⁺) 491.0072, found 491.0079.

Poly(3,4-ethylenedioxy-3′,3′′-dihexyl-2,2′:5′,2′′-terthiophene) (pETT). Bis(dibenzylideneacetone)palladium (5.7 mg, 0.01 mmol), tris(o-methoxyphenyl)phosphine (3.5 mg, 0.01 mmol), Cs₂CO₃ (0.14 g, 0.43 mmol), and pivalic acid (30 mg, 0.30 mmol) were placed in a 10 mL microwave vessel with a magnetic stir bar. A THF (2 mL) solution of EDOT (28 mg, 0.20 mmol) and DBrTT (99 mg, 0.20 mmol) was added and the resulting solution was bubbled by dry nitrogen to remove the dissolved oxygen. The vessel was sealed and placed in the microwave reactor. The solution was heated at 100 °C for 1 h. The mixture was poured into methanol, and the formed precipitate was washed with methanol and acetone and extracted with chloroform and THF by Soxhlet extraction. After purification with a metal scavenger (SiliaMetS DMT, SiliCycle Inc.), a black powder (57 mg, 0.12 mmol (per repeating unit)) was obtained. Yield: 60%. ¹H NMR (600 MHz, THF-d₆, 50 °C, δ, ppm): 0.84–0.94 (br, 6H, CH₃), 1.22–1.48 (br, 16H, thienyl-CH₂(CH₂)₄CH₃), 2.73–2.86 (br, 4H, thienyl-CH₂), 4.26–4.45 (br, 4H, OCH₂CH₂O), 6.95–7.05 (br, 1H, thienyl-H), 7.05–7.15 (br, 1H, thienyl-H). Anal. calcd for C₂₆H₃₂O₂S₃: C, 66.06; H, 6.82; S, 20.35. Found: C, 64.65; H, 6.95; S, 20.45.

Poly(3,4-ethylenedioxy-3′-hexyl-2,2′-bithiophene) (pET-C) prepared by direct C–H arylation. BrET (77 mg, 0.20 mmol), bis(dibenzylideneacetone)palladium (5.7 mg, 0.01 mmol), tris(o-methoxyphenyl)phosphine (3.5 mg, 0.01 mmol), Cs₂CO₃ (0.14 g, 0.43 mmol), and pivalic acid (30 mg, 0.30 mmol) were placed in a 10 mL microwave vessel with a magnetic stir bar. THF (2 mL) was added and the resulting solution was bubbled by dry nitrogen to remove the dissolved oxygen. The vessel was sealed and placed in the microwave reactor. The solution was heated at 100 °C for 20 min. The mixture was poured into methanol, and the formed precipitate was washed with methanol and acetone and extracted with chloroform and THF by Soxhlet extraction. After purification with a metal scavenger (SiliaMetS DMT, SiliCycle Inc.), a black powder (21 mg, 0.069 mmol (per repeating unit)) was obtained. Yield: 34%. ¹H NMR (600 MHz, THF-d₆, 50 °C, δ, ppm): 0.86–0.94 (br, 3H, CH₃), 1.26–1.46 (br, 8H, thienyl-CH₂(CH₂)₄CH₃), 2.71–2.81 (br, 2H, thienyl-CH₂), 4.27–4.40 (br, 4H, OCH₂CH₂O), 7.06–7.13 (br, 1H, thienyl-H). Anal. calcd for C₁₆H₁₈O₂S₂: C, 62.71; H, 5.92; S, 20.93. Found: C, 62.86; H, 6.53; S, 16.74.

Poly(3,4-ethylenedioxy-3′-hexyl-2,2′-bithiophene) (pET-S) prepared by Stille coupling. To a solution of ET (0.37 g, 1.2 mmol) in THF (6 mL) at −78 °C was added 1.7 mL (4.6 mmol) of n-butyllithium (2.7 M in n-hexane) by a syringe. The mixture was stirred at −78 °C for 2 h. Tri(n-butyl)tin chloride (1.6 g, 4.9 mmol) was added to the solution, and the resulting mixture was stirred at −78 °C for 30 min, warmed to room temperature, and stirred further for 1 h. The solvent was removed via rotary evaporation, and n-hexane was added to the residue. The soluble fraction was extracted by filtration. The bis-stannylated ET (BSnET) obtained by the evaporation of the filtrated solution was added to a solution of tetrakis(triphenylphosphine)palladium (69 mg, 0.06 mmol) and DBrET (0.56 g, 1.2 mmol) in toluene (3.2 mL). The solution was stirred at 90 °C for 24 h. The mixture was poured into methanol, and the formed precipitate was washed with methanol and acetone and extracted with chloroform and THF by Soxhlet extraction. After purification with a metal scavenger (SiliaMetS DMT, SiliCycle Inc.), a black powder (230 mg, 0.75 mmol (per repeating unit)) was obtained. Yield: 63%. ¹H NMR (600 MHz, THF-d₆, 50 °C, δ, ppm): 0.86–0.94 (br, 3H, CH₃), 1.26–1.46 (br, 8H, thienyl-CH₂(CH₂)₄CH₃), 2.72–2.81 (br, 2H, thienyl-CH₂), 4.27–4.42 (br, 4H, OCH₂CH₂O), 7.00–7.12 (br, 1H, thienyl-H). Anal. calcd for C₁₆H₁₈O₂S₂: C, 62.71; H, 5.92; S, 20.93. Found: C, 60.99; H, 5.90; S, 19.55.

Poly(3,4-ethylenedioxythiophene) (PEDOT) prepared by electrolytic polymerization. PEDOT film was prepared by cycling a potential between −1.5 and 0.9 V vs. Ag/Ag⁺ at a sweep rate of 20 mV s⁻¹ in an acetonitrile solution containing TEAP (0.1 mol dm⁻³) and EDOT (0.01 mol dm⁻³). After polymerization, the polymer film was dedoped at 1.5 V vs. Ag/Ag⁺. Since the resulting polymer was insoluble in common organic solvents, the polymer was used as prepared.

Conductivity measurements

In situ conductivity measurements were made in an acetonitrile solution of TEAP (0.1 mol dm⁻³) by the two-probe method.⁹ A solution of each polymer in o-dichlorobenzene (15 mg mL⁻¹) was spin-coated (1000 rpm for 1 min) on a micro-array Pt electrode (ALS Co., Ltd., 65 lines, separation distance = 5 μm, total width = 260 mm) or a two-band Pt electrode (homemade, separation distance = 100 μm, width = 7 mm). The film thickness of the polymer was found to be around 50 nm by a 3D laser microscope (Keyence Corp., VK-9700). The polymer film on the micro-array or two-band Pt electrode was electrochemically oxidized with a Pt wire and Ag/AgClO₄ (0.01 mol dm⁻³ in acetonitrile) as the counter and reference electrodes, respectively. The amounts of charges during the electrochemical oxidation (doping) and reduction (dedoping) of the polymer film were measured with a coulometer by stepping a potential from −0.5 V vs. Ag/Ag⁺ to a desired potential and back to −0.5 V vs. Ag/Ag⁺, respectively. Doping levels, defined as the number of charges per thiophene ring, were estimated from the doping/dedoping charges, the weight of the polymer film, and the molecular weight of the repeating unit. The apparent mobilities (μ) of the charge carriers in the polymer film at various doping levels (electrode potentials) were calculated from the equation, μ = σ/ne, where σ, n, and e denote the electrical conductivity at an electrode potential, the density of the charge carriers estimated from the doping/dedoping charges, and the elementary electric charge, respectively.

Results and discussion

Synthesis of monomers

Scheme 1 shows the synthetic route of the monomers. ET and TT were synthesized by the Stille coupling reaction of 2-bromo-3-hexylthiophene with 2-(tributylstannyl)-3,4-ethylenedioxythiophene and 2-(tributylstannyl)-4-hexylthiophene prepared from EDOT and 3HT, respectively. Bromination of ET and TT with 2 eq. of NBS gave DBrET and DBrTT. The yield of the latter is reasonable (81%), while that of the former is not so high (56%). The plausible reason for the low yield is that ET or DBrET will be decomposed by Br₂ formed during the reaction.¹⁰ A Grignard reagent, 5-(3,4-ethylenedioxy-3′-hexyl-2,2′-bithienyl)magnesium bromide, was predominantly obtained by the metal–halogen exchange reaction between DBrET and 1 eq. of methylmagnesium bromide, which was confirmed by ¹H NMR spectroscopy. The obtained Grignard reagent was decomposed by water to give BrET.

Polycondensation

In our previous paper, we found that the polycondensation reaction using the direct C–H arylation reaction was an effective way to obtain EDOT-containing polythiophene with a moderately high molecular weight.^6b Using this method, pETT and pET-C were obtained by the copolymerization of EDOT and DBrTT, and by the homopolymerization of BrET, respectively. It was found that the utilization of microwave heating gave polymers with higher molecular weight. Polythiophene with the same composition ratio of EDOT to that of pET-C and with different regioregularity (pET-S) was synthesized by polycondensation using the Stille coupling reaction between BSnET and DBrET, although the direct C–H arylation polymerization of ET and DBrET could not proceed.

The molecular weights of the polymers were estimated by GPC and the results are summarized in Table 1. It was found that the molecular weights of pET-C and pET-S are almost the same although the polycondensation methods are different, whereas that of pETT is higher by one-order of magnitude when compared to those of pET-C and pET-S. This may be caused by the difference in solubility between the polymers.

Table 1 Characterization of polymers

Polymer	REDOT (%)	Molecular weight			Absorption maximum/nm		Eox/V vs. Ag/Ag^+
		Mn/kg mol^−1	Mw/kg mol^−1	Mw/Mn	λ^solmax^a	λ^filmmax^b
a Absorption in o-dichlorobenzene.b Absorption of films.c Unmeasurable due to low solubility of PEDOT in solvents.
PEDOT	100	—^c	—^c	—^c	—^c	597	−0.99
pET-S	50	3.9	6.4	1.64	496	515	−0.34
pET-C	50	4.2	7.3	1.75	485	512	−0.23
pETT	33	16.9	29.4	1.74	487	487	−0.03
P3HT	0	34.8	168	4.83	441	454	0.43

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Fig. 2 shows the ¹H NMR spectra of the aromatic protons in the polymers. While pET-C showed only one broad peak at 7.10 ppm, pET-S showed two peaks at 7.10 and 7.04 ppm, whose integral ratio is around 1 : 1. These peaks at 7.10 and 7.04 ppm can be ascribed to the protons in the two kinds of repeat structures, “-ET-ET-” and “-ET-TE-” (E: EDOT unit, T: 3HT unit), H^a and H^b, respectively, as shown in Fig. 3. Their characterization was supported by the estimation of the chemical shifts of the ¹H NMR spectra for the two types of tetramers of ET with the “-ET-ET-” and “-ET-TE-” structures as model compounds using the density functional theory (DFT) calculation with the Gaussian 09 software with the Becke three-parameter hybrid functional combined with the Lee–Yang–Parr correlation functional (B3LYP) and a polarized 6-31G(d) basis set.¹¹ These results suggest that the regioregularity of pET-C is almost 100%, while that of pET-S is around 50%. pETT exhibited two broad peaks at 7.01 and 7.09 ppm, which are ascribed to the two types of protons, H^c and H^d as shown in Fig. 3c. Although it is possible that pETT contains two different regiostructures as shown in Fig. 3c, these difference could not be distinguished from the spectrum. Reflecting that the molecular weights of pET-C and pET-S are lower than that of pETT, the aromatic protons of the terminal groups were clearly observed at 7.08 ppm for pET-C and at 7.08 and 7.02 ppm for pET-S, respectively. The integral ratio of the protons in the main chain and the terminal group did not contradict the molecular weights estimated by GPC.

Fig. 2 ¹H NMR spectra of (a) pET-C, (b) pET-S, and (c) pETT.

Fig. 3 Assignments of aromatic protons in (a) pET-C, (b) pET-S, and (c) pETT.

Optical properties

The electronic absorption spectra of the polymer films are depicted in Fig. 4, and the spectroscopic data of the polymers in the film state and in o-dichlorobenzene are included in Table 1. The spectra of the polymer films exhibit a single absorption band due to the π–π* transition at 454, 487, 512, 515, and 597 nm for P3HT, pETT, pET-C, pET-S, and PEDOT, respectively. As is clearly seen in the inset of Fig. 4, the peak wavelength is correlated well with the composition ratio of EDOT in the repeat unit of the polymer (R_EDOT). We note also that all of the film spectra show a broadening compared to the solution spectra (Fig. S1, ESI†), suggesting that the polymers are somewhat π-stacked in the film state.¹² Furthermore, in comparison with the solution spectra, the film absorption peak of pET-C is red-shifted by 28 nm, which is greater than those of P3HT (14 nm), pETT (0 nm) and pET-S (19 nm). This implies that the pET-C film is the most tightly packed among the four polymers, reflecting the highest regioregularity of pET-C.

Fig. 4 Electronic absorption spectra of polymer films (inset depicts correlation between R_EDOT and absorption maximum (λ_max)).

Electrochemical properties

Cyclic voltammetry of the polymer films was carried out (Fig. 5) and the onset potentials of anodic oxidation (E_oxs) are summarized in Table 1. It is known that the alkoxy group introduced at the β-position of the thiophene ring can stabilize the positive charge formed by the anodic oxidation reaction.¹³ Thus, with increasing R_EDOT, the E_oxs are shifted to the negative direction reflecting the enhanced electron-donating natures (inset of Fig. 5).

Fig. 5 Cyclic voltammograms of polymer films (inset depicts correlation between R_EDOT and E_ox).

Spectroelectrochemical properties

To characterize the redox states of the polymers, the spectroelectrochemistry was investigated. Fig. 6 depicts the difference spectra of the pETT film, which were obtained by subtracting the spectrum of the neutral polymer as a reference from the spectra of polymers electrochemically oxidized at different electrode potentials. When the pETT film was oxidized at 0 V vs. Ag/Ag⁺, the intensity of the absorption band at ca. 500 nm decreased and two new absorption bands appeared at around 800 and 1600 nm, and their intensities gradually increased by increasing the potential up to 0.3 V. These new absorption bands were ascribable to the one-electron oxidized species (polaron and/or π-dimer). On further oxidation of pETT, one broad absorption band appeared at around 1500 nm, which can be ascribed to the two-electron oxidized species (bipolaron). Other polymers showed spectra similar to those of pETT (Fig. S2, ESI†), except that the one-electron oxidized species began to appear at −1.2, −0.4 and 0.275 V for PEDOT, pET-S, and P3HT,¹⁴ respectively, reflecting their higher electron-donating nature.

Fig. 6 Spectroelectrochemistry of pETT film.

Electrical properties

The doping levels and electrical conductivities of the polymer films were measured by using the in situ electrochemical technique^6,7 in order to investigate the electrical properties of the polymers. Fig. 7a depicts semilogarithmic plots of the doping level against electrode potential for the polymer films. In concert with the electrochemical oxidation of the films, the doping levels gradually increased and finally reached around 20–30%, suggesting that one positive charge is formed on every three to five thiophene rings. The maximum value of the doping levels tends to increase with increasing R_EDOT, reflecting the stabilization of the positive charge introduced by the electrochemical oxidation due to the electron-donating ethylenedioxy group in the EDOT unit. The plot of log(doping level) vs. electrode potential fits a straight line in the low doping region, and its slope values are around 190, 310, 180 and 289, and 60 mV per decade for PEDOT, pET-C, pET-S, pETT, and P3HT,^14,15 respectively. The slope values, except for P3HT, are much larger than the 60 mV per decade expected for a common one-electron transfer process at room temperature. We have already clarified that the slope value is a measure of the distribution of effective π-conjugation length in conjugated oligomers and polymers, and wider distribution leads to a larger slope value.^9b,d,16 We also note that the slope value of pET-C is larger than that of pET-S. This can be explained by a more widely distributed conjugation length because of the higher polydispersity (M_w/M_n) of pET-C.

Fig. 7 (a) Doping levels and (b) electrical conductivities of polymer films as a function of electrode potential, measured in acetonitrile containing TEAP (0.1 mol dm⁻³). Plots for P3HT are reproduced from Fig. 2 and 3 in ref. 15.

The electrical conductivities of the polymers increased with increasing the electrode potentials, and showed maximum values of 320, 16, 2.4, 1.4, and 1.0 S cm⁻¹ for PEDOT, pET-S, pET-C, pETT, and P3HT,^14,15 respectively (Fig. 7b). This result suggests that the conductivities of the doped polymer films tend to increase with R_EDOT. Although the R_EDOTs in pET-S and pET-C are the same, the maximum conductivity of pET-S is 7 times higher than that of pET-C. We have already found that the electrical conductivities are affected by the disordered structure of π-conjugated polymers and oligomers.^14–16 In view of this, the more disordered structure of pET-C may lead to the reduction of the electrical conductivity.

To further discuss the charge transport mechanisms of the polymer films, the apparent mobilities of the charge carriers were estimated by combining the data of the doping level and conductivity shown in Fig. 7. The mobilities are plotted in Fig. 8 as a function of the doping level. At doping levels as low as or below 1%, where the interchain hopping transport of monocation radicals (polarons) is a principal route of charge transport, the mobilities for the P3HT, pET-C, and pET-S films are constant at around 10⁻⁶ cm² V⁻¹ s⁻¹.¹⁷ The mobility plots showed maxima at doping levels of 10–15%, where two-electron oxidized (diamagnetic) species predominantly prevail and the intrachain charge transport is a main conduction mechanism.¹⁷ The maximum mobility (μ_max) values are plotted against R_EDOT (Fig. 8, inset). It was found that the μ_max values increased with increasing R_EDOT. A plausible reason for this could be due to the electron donating nature of the EDOT unit in the polymer. That is, it becomes difficult for the positive charge to move along the polymer chains because of coulombic repulsion, but the degree of the coulombic repulsion will become smaller when the electron-donating alkoxy groups are introduced. Thus, the mobilities become higher with the increase of R_EDOT. It is worth noting here that the μ_max values for the two types of polymers having the same ET repeat unit, i.e., the same R_EDOT values, differ a lot from each other: 0.1 and 0.02 cm² V⁻¹ s⁻¹ for pET-S and pET-C, respectively. The μ_max value for pET-S is larger than that for pET-C and this appears to be strange when we remember that the former has a regioregularity lower than the latter. According to our previous study, however, the regioregular nature of the polymer film affects mainly the mobilities of the charge carriers at low doping levels where the hopping transport of polarons is the main charge transport mechanism. In view of this, the relatively low μ_max value for pET-C may be ascribed to the disordered structure of the pET-C film inferred from its greater M_w/M_n value, which is supported by the large slope value for pET-C compared with that for pET-S, as shown in Fig. 7a.

Fig. 8 Apparent mobilities of charge carriers in polymer films plotted against doping level (inset depicts correlation between R_EDOT and μ_max). Plot for P3HT is reproduced from Fig. 4 in ref. 15.

Conclusions

A new family of polythiophenes containing EDOT with different composition ratios were synthesized by polycondensation reactions of the corresponding monomers, and their optical, electrochemical, and electrical properties were investigated in relation to the polymer structures. The polymers showed a red-shift of the absorption band and a negative shift of the oxidation potential as the ratio of the EDOT unit in the polymers increased. It was found that the electrical conductivities of the polymers could to be successfully controlled by the EDOT composition ratio, the regioregularity, and the doping level. The highest electrical conductivities of the electrochemically-doped polymers were found to increase with increasing the ratio of EDOT in the polymers.

Acknowledgements

This work was supported in part by grants from the General Sekiyu Research & Development Encouragement & Assistance Foundation (I. I.), the TANAKA Holdings (I. I.) and a grant-in-aid for scientific research from the Japan Society for the Promotion of Science (JSPS) (no. 25288085, Y. H.). The authors are grateful to Drs Tomoko Amimoto, Daisuke Kajiya, Hitoshi Fujitaka, and Yutaka Mouri, the Natural Science Center for Basic Research and Development (N-BARD), Hiroshima University for the measurements of MS and NMR spectra and the elemental analysis.

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Footnote

† Electronic supplementary information (ESI) available: Electronic absorption spectra in o-dichlorobenzene and films, and spectroelectrochemistry of polymer films. See DOI: 10.1039/c5ra17235g

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