A Direct Route to a Polybromothiophene as a Precursor for Functionalized Polythiophene by Electrooxidative Polymerization

Abstract

A reactive π-conjugated polymer, bromo-substituted polythiophene, was synthesized by constant potential electrooxidative polymerization of 3-bromo-4-dodecylthiophene in an acetonitrile solution of Bu₄NPF₆. The resulting polymer was applied as a reactive precursor for functional polythiophenes. The protonation via lithiation of the polybromothiophene proceeded quantitatively. The phenylation of the polybromothiophene by the Kumada-Tamao coupling reaction also proceeded with a 70 % efficiency. The changes in optical and electronic properties of the polymers by their reactions are discussed by the results of UV-vis absorption spectroscopy, photoluminescence spectroscopy, and cyclic voltammetric analysis. The emission colors of the bromo-substituted, protonated, and phenylated polymers were green, yellow, and blue, respectively, demonstrating the tunability of this approach.

1. Introduction

π-Conjugated polymers are applied for organic photovoltaics (OPVs),^1,2 organic field effect transistors (OFETs),³ organic light-emitting diodes (OLEDs),⁴ chemosensors,⁵ and so on. Since the optical and electronic features of π-conjugated polymers strongly depend on their primary structures, developing easier and more versatile synthetic methods to synthesize π-conjugated polymers is demanded. The most common route for π-conjugated polymers is transition-metal-catalyzed polycondensations, by which a wide variety of π-conjugated polymers has been fabricated.^6–8 However, polymers having functional groups sensitive to the catalytic reactions are not accessible by the polycondensation of the corresponding monomers. Polymer reactions can overcome this limitation by introducing these reactive groups into the polymers after polymerization,⁹ and various functional polymers can be prepared from one type of precursor polymer. As reactive π-conjugated polymers, halogenated polythiophenes were reported.^10–13 They can be modified to polythiophenes with various functional groups by polymer reactions based on lithiation and catalytic reactions. For example, the synthesis of polybromothiophene by bromination of poly(3-hexylthiophene-2,5-diyl) (P3HT) with N-bromosuccinimide (NBS) was reported.^10,11 Swager et al. reported the synthesis of P3HT bearing various side chains by lithiation of brominated P3HT followed by reactions with various electrophiles such as aldehydes, acid anhydrides, and so on.¹¹ Holdcroft et al. also reported the synthesis of polythiophenes bearing aromatic groups by the palladium-catalyzed Suzuki-Miyaura and Stille coupling reactions of brominated P3HT.¹² Inagi et al. reported the synthesis of polychlorothiophenes by electrochlorination of P3HT.¹³

However, the direct synthesis of halogenated polythiophenes from the polymerization of the corresponding halogenated monomers has not been reported due to the lack of tolerance of halogen moieties to catalytic reactions such as the Kumada-Tamao,^14,15 Suzuki-Miyaura,¹⁶ and Stille¹⁷ coupling reactions used for polycondensation. Halogen moieties behave as leaving groups of these catalytic reactions, and compounds with branched structures are formed. In fact, hyperbranched polythiophene was prepared by polycondensation based on the Kumada-Tamao coupling reaction of a thiophene monomer with two bromo moieties.¹⁸ To overcome this limitation, it is necessary to employ polymerization methods that are orthogonal to the halogen moieties present in the halogen-substituted thiophenes.

We focused on electrochemical reactions. Electrolytic oxidation polymerization is a method of polymerization of aromatic monomers such as pyrrole and thiophene by electrolytic oxidation on anode surface, and conductive polymer films can be easily obtained.¹⁹ This polymerization proceeds by coupling between radical cation species generated as reaction intermediates. Although the polymerization of aromatic monomers also proceeds by chemical oxidation polymerization by oxidants, the electrooxidation process is green and sustainable because harmful oxidants are not required. The electrolytic oxidation is inert to halogens, and we presumed that halogenated thiophene monomers can also be polymerized without the elimination of halogens. Herein, we describe the synthesis of soluble polybromothiophene by electrooxidative polymerization of 3-bromo-4-dodecylthiophene (1) and polymer reactions of the resulting polymer. The monomer 1 was designed to maintain the solubility by the dodecyl chain, to exhibit sufficient reactivity and stability by the moderately reactive bromo moieties, and to be polymerized selectively at the 2- and 5-positions.

2. Results and Discussion

First, the electrooxidation behavior of 1 was evaluated by linear sweep voltammetry (LSV) analysis in an acetonitrile (MeCN) solution of Bu₄NPF₆ (Fig. S1). The oxidation peak of 1 was observed at 2.4 V within the potential window of the electrolysis solution, indicating that 1 can be oxidized prior to the solvent and supporting electrolyte. Next, electrooxidative polymerization of 1 was carried out using a Pt plate (2 × 2 cm²) electrode in the MeCN solution of Bu₄NPF₆ at ambient temperature by constant potential mode at 2.4 V for 60 seconds (Scheme 1). A navy polymer film was deposited on the working electrode and collected by dissolving in CH₂Cl₂. The polymerization was repeated in the same electrolysis solution until no polymer film was deposited. The color of the resulting polymer was changed from navy to yellow by the de-doping through reprecipitation into MeOH, and poly(3-bromo-4-dodecylthiophene-2,5-diyl) (2) was obtained in a 43 mg (26 %) yield. The current efficiency of the polymerization was determined to be 56 % by the yield of the obtained polymer and theoretical yield calculated from passed electricity. The resulting polymer was soluble in common organic solvents such as dichloromethane (CH₂Cl₂), tetrahydrofuran (THF), toluene, and chloroform. The number-average molecular weight (M_n) and molecular weight distribution (M_w/M_n) of 2 were estimated to be 15000 and 2.0, respectively, by size exclusion chromatography (SEC). The structure of 2 was confirmed by ¹H NMR, ¹³C NMR, and energy dispersive X-ray (EDX) spectra. In the ¹H NMR spectrum of 2, peaks attributable to the aliphatic protons are observable at 2.70–0.87 ppm, and signals assignable to aromatic protons were not observed (Fig. 1). The presence of the Br group in the polymer was confirmed by the EDX measurement (Fig. S2). These data indicate that the polymerization of 1 proceeded without debromination. In the UV-vis absorption spectrum taken in CH₂Cl₂, the absorption maximum (λ_max) and absorption onset (λ_onset) of 2 were observed at 399 and 425 nm, respectively. The λ_max was bathochromically shifted 92 nm from that of 1 (Fig. S3, Table S1). The extension of effective π-conjugation along the polymer backbone was supported by this significant bathochromic shift.

Scheme 1.

Electrooxidative polymerization of 1 and polymer reactions of 2.

Figure 1.

¹H NMR spectra of (a) 2, (b) 3, and (c) 4 (CDCl₃, 400 MHz).

The precursor polymer 2 was transformed by the protonation via lithiation and phenylation by the Kumada-Tamao coupling reaction (Scheme 1). Lithiation of 2 by n-butyllithium (n-BuLi) was carried out in THF at −78 °C under N₂, and the lithiated 2 was protonated by the addition of MeOH. The protonated polymer (3) was obtained as an orange solid in a >99 % yield by reprecipitation into MeOH. The M_n and M_w/M_n of 3 were estimated to be 9200 and 2.2, respectively, by SEC. The protonation efficiency was calculated from the ¹H NMR spectrum of 3. The ratio of the intensities of the peaks of the aromatic protons at 7.0 ppm to those of the methylene protons adjacent to the thiophene ring at 2.5–2.9 ppm (1.0 : 2.0) accorded well with the theoretical ratio (1 : 2), which supported the quantitative protonation. The ¹H NMR spectrum of 3 shows peaks attributable to the thiophenylmethylene protons in the head-to-tail (HT: 2.7–2.8 ppm) and head-to-head (HH: 2.5–2.6 ppm) sequences.²⁰ The HT : HH ratio calculated from the intensity ratio of these peaks is 1.0 : 0.8 (Fig. 1). It is noted that electrooxidative polymerization of 3-dodecylthiophene without bromo moiety was also investigated as a comparison with 3. However, the resulting polymer was not soluble in any organic solvents. This result is presumably due to the formation of the cross-linked structure by polymerization not only at the 2- and 5-position carbons but also at the 4-position carbon.

The selectivity of the sequence is discussed by density functional theory (DFT) calculations at the B3LYP level. We chose 3-bromo-4-ethylthiophene (1′) with the smallest primary alkyl group as a model compound of the monomer for the DFT calculation. First, the selectivity on the position of oxidation was estimated from the highest occupied molecular orbital (HOMO) of 1′. The HOMO was delocalized on both 2- and 5-position carbons (Fig. 2a). This result indicates that both 2- and 5-position carbons are oxidized. Second, the reactivity of the radical cation species was investigated from the spin densities of the radical cation species of 1′. The spin densities at the 2- and 5-position carbons were comparable due to the delocalization of the radical cation (Fig. 2b). This calculation suggests that the reactivities of the 2- and 5-positions are almost identical, and the polymerization plausibly proceeds randomly. This presumption agrees with the comparable HT and HH compositions. The slightly lower HH composition is ascribable to the larger steric hindrance of the dodecyl chain than the bromo group impeding the homocoupling at the 5-position carbons.

Figure 2.

(a) HOMO (neutral molecule) and (b) spin densities (radical cation species) of 1′ calculated by DFT (B3LYP, 6-31G(d)).

Phenylation by the Kumada-Tamao coupling reaction was also carried out (Scheme 1). Phenylated polythiophene (4) was obtained in a >99 % yield by the reaction of 2 with phenylmagnesium bromide in the presence of a catalytic amount of [1,3-bis(diphenylphosphino)propane]nickel dichloride (Ni(dppp)Cl₂). In the ¹H NMR spectrum of 4, peaks attributable to the phenyl group were observed at 7.60–7.40 and 7.35–7.27 ppm (Fig. 1). The efficiency of phenylation of 2 was calculated to be 70 % from the ratio of the intensity of the peak of the o-protons of the phenyl ring (7.60–7.40 ppm) to that of the aliphatic (2.7–2.5 ppm) protons (measured ratio = 1.4 : 2.0, theoretical ratio = 2 : 2). This moderate phenylation efficiency is comparable to the efficiency of a reported phenylation of 3-bromothiophene under similar reaction conditions.²¹ The M_n and M_w/M_n of 4 were estimated to be 9000 and 2.4, respectively, by SEC.

Changes in the optical and electronic properties of 2 through the polymer reactions were evaluated by UV-vis absorption spectroscopy, photoluminescence (PL) spectroscopy, and cyclic voltammetry (CV) analysis (Table 1 and Figs. 3 and S4). In the UV-vis absorption spectra in CH₂Cl₂, the λ_max of 2, 3, and 4 were observed at 339, 401, and 339 nm, respectively. The optical band gaps (E_g) of 2, 3, and 4 calculated from their λ_onset were 425, 496, and 425 eV, respectively. This difference in E_g can be explained by the difference in the flatness of the π-conjugated main chain affected by the steric hindrance of the functional groups. The E_g of 3 substituted by the smallest proton was the narrowest among these polymers, probably due to the extended π-conjugation of the flat main chain, which was allowed by the smaller substituents. The E_g values of 2 and 4, wider than that of 3, probably owe to the twisted main chains originating from the steric hindrance of the bromo and phenyl groups. In the film states, all absorption peaks shifted bathochromically from those measured in solutions by the increased flatness of the main chains through π-π stacking interaction (Fig. 3). The length of shift in 4 was smaller than that of 2 because π-π stacking of the main chain was inhibited by steric hindrance of the phenyl group, which is bulkier than the bromo group. The HOMO was estimated from the onsets of oxidation peaks of polymers (Fig. S4). The lowest unoccupied molecular orbital (LUMO) energy levels were estimated from the HOMO energy levels and E_g of the polymer films. The HOMO of 3 was the highest of these polymers. This data also supports the extended π-conjugation of 3 by the flatness of the main chain. In the PL spectra of 2, 3, and 4 taken in CH₂Cl₂, their emission maxima were observed at 513, 535, and 501 nm, respectively (Fig. 3). The quantum yields (Φ) of 2, 3, and 4 were 0.06, 0.39, and 0.12. The emission colors of 2 changed from green to yellow and blue by the phenylation and protonation, respectively.

Table 1. Optical and electronic properties of 2, 3, and 4.

Polymer	λmax/nm		λonset/nm		Eg/eVa)		Emax /nmb)	Φc)	Emission color	HOMO /eVd)	LUMO /eVe)
	In CH2Cl2	Film	In CH2Cl2	Film	In CH2Cl2	Film
2	339	350	425	450	2.92	2.76	513	0.06	green	−5.76	−3.00
3	401	410	496	526	2.50	2.50	535	0.39	yellow	−5.26	−2.90
4	339	345	425	438	2.92	2.83	501	0.12	blue	−6.26	−3.42

a) Estimated from λ_onset. b) Emission maximum, irradiated at their λ_max. c) The quantum yields (Φ) were estimated at ambient temperature using quinine sulfate in 0.50 M sulfuric acid aqueous solution as a standard. d) HOMO = −(E_ox + 4.8), where E_ox is the onset of the potential of oxidation observed in the CV analyses.²² e) LUMO = E_g + HOMO.

Figure 3.

(a) Photo images of CH₂Cl₂ solutions, (b) UV-vis absorption spectra in CH₂Cl₂ and film, and (c) PL spectra in CH₂Cl₂ of 2, 3, and 4.

3. Conclusion

Polybromothiophene was synthesized by constant potential oxidative polymerization of 3-bromo-4-dodecylthiophene. The resulting polymer was soluble in organic solvents, and polymer reactions proceeded based on the reactivity of the bromo group. The protonation via lithiation proceeded quantitatively, which implies the extension of this approach to functionalization with various electrophiles. The phenylation by the Kumada-Tamao coupling also proceeded with an efficiency of 70 %. The emission colors of the polythiophene derivatives were tuned by the substituents regulating the planarity, and both hypsochromically shifted blue-emitting phenylated polythiophene and bathochromically shifted yellow-emitting protonated polythiophene were prepared from green-emitting polybromothiophene. A current challenge is the improvement of regioregularity, which often deteriorates optoelectronic properties,²³ by optimization of electrooxidation conditions. Furthermore, the design of functional π-conjugated polymer materials by various polymer reactions of the polybromothiophene is in progress.

Acknowledgments

This work was supported by the Kato Foundation for Promotion of Science.

Data Availability Statement

The data that support the findings of this study are openly available under the terms of the designated Creative Commons License in J-STAGE Data at https://doi.org/10.50892/data.electrochemistry.22108598.

• 1) A reactive π-conjugated polymer, bromo-substituted polythiophene, was synthesized by constant potential electrooxidative polymerization of 3-bromo-4-dodecylthiophene in an acetonitrile solution of Bu₄NPF₆. The resulting polymer was applied as a reactive precursor for functional polythiophenes. The protonation via lithiation of the polybromothiophene proceeded quantitatively. The phenylation of the polybromothiophene by the Kumada-Tamao coupling reaction also proceeded with a 70 % efficiency. The changes in optical and electronic properties of the polymers by their reactions are discussed by the results of UV-vis absorption spectroscopy, photoluminescence spectroscopy, and cyclic voltammetric analysis. The emission colors of the bromo-substituted, protonated, and phenylated polymers were green, yellow, and blue, respectively, demonstrating the tunability of this approach.

CRediT Authorship Contribution Statement

Hazuki Goto: Investigation (Lead), Writing – original draft (Lead)

Bungo Ochiai: Funding acquisition (Supporting), Investigation (Supporting), Supervision (Supporting), Writing – original draft (Supporting), Writing – review & editing (Equal)

Yoshimasa Matsumura: Funding acquisition (Lead), Investigation (Supporting), Supervision (Lead), Writing – original draft (Supporting), Writing – review & editing (Equal)

Conflict of Interest

The authors declare no conflict of interest in the manuscript.

Funding

Kato Foundation for Promotion of Science: KS-3424

Footnotes

Y. Matsumura: ECSJ Active Member

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