Introduction

Common conjugated polymers generally consist of sp- and/or sp2-carbon frameworks, and π-electrons are delocalized throughout the polymer backbone. Such conjugated polymers have attracted considerable attention because of their good processability and their readily tunable electronic and optical properties.1, 2, 3 Recently, a new type of conjugated polymer that exhibits through-space interactions of π-electron systems has been developed. Poly(dibenzofulvene)s,4, 5, 6, 7, 8, 9, 10, 11 7,7-diarylnorbornane-containing conjugated polymers,12, 13 cyclophane-containing polymers14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35 and face-to-face ferrocene polymers36, 37, 38 have been synthesized, and their optical and electrochemical properties have been studied in detail. Polymers with layered aromatic rings and π-electron systems have also been investigated;39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53 for example, Chen et al.53 recently reported the synthesis and formation of a two-dimensional assembly of polymeric ladder phanes containing face-to-face π-electron systems. These polymers have potential applications in optoelectronic devices and single-molecular devices such as single-molecular wires.

Cofacially aligned π-electron systems have an important role in biochemistry and material chemistry; DNA has face-to-face base pairs stacked in the double-stranded main chains,54 and the performance of organic optoelectronic devices strongly depends on the arrangement of the π-electron systems.55 We have recently reported the synthesis of aromatic-ring-layered polymers using xanthene compounds as scaffolds.40, 41, 42, 43, 44, 45, 46, 47 [2.2] Paracyclophane-layered polymers, which mimic multilayered cyclophanes,56, 57, 58, 59 exhibited fluorescence resonance energy transfer from the layered paracyclophanes to the end-capping groups. The use of xanthene as a scaffold enabled us to introduce various aromatic compounds, such as phenylenes,44 carbazoles,45 thiophenes46 and anthracene,47 into the polymers, with the distance between the layered aromatic units being ∼4.5 Å. Our next target is to construct layered and π-stacked aromatic systems in proximity, less than the sum of the van der Waals radius (3.40 Å) of an sp2-carbon, in the polymer backbone.

Nakayama and co-workers60, 61, 62 and Iyoda and co-workers63, 64 have reported the synthesis of cofacially oriented naphthalene-based oligothiophenes, in which two or three bithiophenes were layered. In addition, Iyoda and co-workers have synthesized 2,2′-bithiophenophanes.63, 64 The layered bithiophenes in these compounds exhibit strong π–π interactions. However, to the best of our knowledge, there has been no report on the synthesis of oligothiophene-stacked polymers. In this paper, we report the synthesis, characterization and properties of π-stacked polymers with oligothiophene as the π-stacked aromatic unit and naphthalene as the scaffold; these polymers were synthesized by the Suzuki–Miyaura coupling polymerization.

Experimental procedure

General experimental details

1H and 13C nuclear magnetic resonance (NMR) spectra were recorded on a JEOL JNM-EX400 instrument (JEOL, Tokyo, Japan) at 400 and 100 MHz, respectively. The chemical shift values were expressed relative to Me4Si as an internal standard. High-resolution mass spectra were obtained on a JEOL JMS-SX102A spectrometer. Matrix-assisted laser desorption ionization time-of-flight mass spectroscopy measurement was performed on Voyager DE-STR (Applied Biosystems, Carlsbad, CA, USA) using all-trans-retinoic acid as a matrix. Gel permeation chromatography was carried out on a TOSOH 8020 (TSKgel G3000HXL column; Tosoh, Tokyo, Japan) instrument using CHCl3 as an eluent after calibration with standard polystyrene samples. Recyclable preparative high-performance liquid chromatography was performed on a Japan Analytical Industry (Tokyo, Japan), Model 918R (JAIGEL-2.5H and 3H columns) using CHCl3 as an eluent. UV–vis absorption spectra were obtained on a SHIMADZU UV3600 spectrophotometer (Shimadzu, Kyoto, Japan). Photoluminescence spectra were obtained on a Horiba FluoroMax-4 luminescence spectrometer (Horiba, Kyoto, Japan). Cyclic voltammetry was carried out on a BAS CV-50W electrochemical analyzer (BAS, Tokyo, Japan) in CH2Cl2 containing 0.1 M of sample and 0.1 M of BuNClO4 with a glassy carbon working electrode, a Pt counter electrode, a Ag/AgCl (Ag/Ag+) reference electrode and a ferrocene/ferrocenium (Fc/Fc+) external reference. Analytical thin-layer chromatography was performed with silica gel 60 Merck F254 plates. Column chromatography was performed with Wakogel C-300 silica gel. Elemental analyses were performed at the Microanalytical Center of Kyoto University.

Materials

Tetrahydrofuran (THF) and Et2O were purchased and purified by passage through purification column under Ar pressure.65 Dehydrated grade CHCl3 was purchased and used without further purification. N,N,N′,N′-tetramethylethylenediamine (TMEDA) was purchased commercially, and purified by distillation with KOH. 3-Hexylthiophene (1), n-butyllithium (n-BuLi) (1.6 M in hexane), s-BuLi (1.0 M in cyclohexane and n-hexane), ZnBr2, I2, Pd(PPh3)4, Pd(OAc)2, 2-dicyclohexylphosphino-2′,6′-dimethoxybiphenyl (S-Phos), K3PO4, FeCl3, bis(pinacolato)diboron (M1), 2,5-thiophenediboronic acid (M2) and 2,2-bithiophene-5,5′-diboronic acid bis(pinacol) ester (M4) were obtained commercially, and used without further purification. 1,8-Diiodonaphthalene (2)66 and 2,5-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-thieno[3,2-b-]thiophene (M3)67 were synthesized according to the literature's procedure. All reactions were performed under Ar atmosphere.

1,8-Bis(4-hexylthien-2-yl)naphthalene (3)

1 (3.6 ml,. 20.0 mmol) in THF (2.5 ml) was slowly added drop wise to n-BuLi (1.6 M in hexane, 6.3 ml, 10.0 mmol) and TMEDA (2.2 ml, 15.0 mmol) in THF (5.0 ml) at −78 °C. After the solution was stirred at room temperature for 24 h, ZnBr2 (9.0 g, 40.0 mmol) in THF (20.0 ml) was transferred to this solution via a cannula at 0 °C. The reaction mixture was stirred at room temperature for 24 h. This solution was transferred via a cannula into a flask with 2 (0.76 g, 2.0 mmol) and Pd(PPh3)4 (0.20 g, 0.17 mmol). After the reaction mixture was stirred at reflux temperature (oil bath temperature at 70 °C) for 24 h, H2O (30 ml) and EtOH (15 ml) were added to quench the reaction. The aqueous layer was washed with CHCl3 (25 ml) three times, and the organic layer was dried over MgSO4. After removal of MgSO4, the solvent was evaporated. The residue was subjected to column chromatography on SiO2 with hexane as an eluent to obtain 3 (0.51 g, 1.1 mmol, 57%) as a pale yellow oil.

Rf 0.21 (hexane). 1H NMR (CDCl3, 400 MHz): δ=0.98 (t, J=6.4 Hz, –CH3, 6H), 1.38 (m, –CH2–, 12H), 1.54 (m, –CH2–, 4H), 2.40 (t, J=7.6 Hz, –CH2–, 4H), 6.36 (s, thiophene H, 2H), 6.65 (s, thiophene H, 2H), 7.54 (t, J=7.6 Hz, naphthalene H, 2H), 7.66 (d, J=7.6 Hz, naphthalene H, 2H), 7.92 (d, J=8.4 Hz, naphthalene H, 2H) p.p.m. 13C NMR (CDCl3, 100 MHz): δ=14.1, 22.6, 29.2, 29.9, 30.3, 31.7, 118.0, 125.0, 128.9, 129.0, 130.3, 132.0, 132.8, 135.4, 142.7, 143.4 p.p.m. High-resolution mass spectra (FAB): m/z calcd. for C30H36S2 (M+): 460.2258, found: 460.2254. Anal. calcd. for C30H36S2: C 78.21; H 7.88; S 13.92, found: C 78.33; H 8.09; S 14.11.

1,8-Bis(4-hexyl-5-iodothien-2-yl)naphthalene (4)

3 (0.64 g, 1.4 mmol) in Et2O (15.0 ml) was slowly added drop wise to s-BuLi (1.0 M in cyclohexane and n-hexane, 10.0 ml, 10.0 mmol) and TMEDA (3.3 ml, 22.0 mmol) at −78 °C. After the solution was stirred at room temperature for 24 h, I2 (2.8 g, 11.0 mmol) in Et2O (15.0 ml) was transferred to this solution via a cannula at −78 °C. The reaction mixture was stirred at room temperature for 24 h. Saturated Na2SO3 aqueous (30 ml) was added to quench the reaction. The aqueous layer was washed with CHCl3 three times, and the organic layer was dried over MgSO4. After removal of MgSO4, the solvent was evaporated. The residue was subjected to column chromatography on SiO2 with hexane as an eluent to obtain 4 (0.60 g, 0.80 mmol, 58%) as a pale yellow solid.

Rf 0.45 (hexane). 1H NMR (CDCl3, 400 MHz): δ=0.89 (t, J=6.4 Hz, –CH3, 6H), 1.32 (m, –CH2–, 12H), 1.46 (m, –CH2–, 4H), 2.32 (t, J=7.6 Hz, –CH2–, 4H), 6.12 (s, thiophene H, 2H), 7.54 (m, naphthalene H, 4H), 7.94 (m, naphthalene H, 2H) p.p.m. 13C NMR (CDCl3, 100 MHz): δ=14.3, 23.1, 29.6, 30.1, 32.0, 32.8, 72.4, 125.4, 128.9, 129.9, 131.9, 132.5, 147.6, 148.5 p.p.m. High-resolution mass spectra (FAB): m/z calcd. for C30H34I2S2 (M+): 712.0191, found: 712.0197. Anal. calcd. for C30H34I2S2: C 50.57; H 4.81; S 9.00, found: C 50.75; H 4.72; S 8.98.

1,8-Bis(2-thienyl)naphthalene (5)

This compound was synthesized according to the procedure63, 64 with some modifications. ZnBr2 was used instead of ZnCl2. The spectral data were matched with the literature's values.63, 64

Yield 60%. Rf 0.35 (hexane). High-resolution mass spectra (EI (electron impact)): m/z calcd. for C18H12S2 (M+): 292.0381, found: 292.0380. Anal. calcd. for C18H12S2: C 73.93; H 4.14; S 21.93, found: C 73.66; H 4.09; S 21.93.

Palladium-catalyzed coupling polymerization

A typical procedure68, 69, 70 is as follows. Monomers 4 (187.0 mg, 0.20 mmol), M2 (50.9 mg, 0.20 mmol), Pd(OAc)2 (4.5 mg, 0.02 mmol), S-Phos (8.2 mg, 0.02 mmol) and K3PO4 (169.8 mg, 0.30 mmol) in THF (4.0 ml) and H2O (1.0 ml) were refluxed for 48 h. After filtration by Celite, the solution was diluted in CHCl3 (20 ml) and washed twice with aqueous NH4OH (25 ml), brine (25 ml), and H2O (25 ml). The organic layer was dried over MgSO4. After filtration, the solvent was evaporated. The crude product was dissolved in a small amount of CHCl3 and precipitated from MeOH/acetone three times. And then precipitation was repeated twice with CHCl3 and acetone to obtain polymer P2 (52.3 mg, 0.09 mmol, 35%).

Polymer P1. Yield: 10%. 1H NMR (CD2Cl2, 400 MHz): δ=0.9 (br), 1.3 (br m), 2.3 (br), 6.3–6.8 (br m), 7.6 (br), 7.9 (br) p.p.m. 13C NMR (CD2Cl2, 100 MHz) δ=14.3, 23.1, 29–31 (m), 125 (m), 129–132 (m), 139.4, 141.8 p.p.m.

Polymer P2. Yield: 35%. 1H NMR (CDCl3, 400 MHz): δ=0.8 (br), 1.2 (br), 1.5 (br), 2.4 (br m), 6.4 (br), 7.0 (br), 7.6 (br m), 7.9 (br) p.p.m. 13C NMR (CDCl3, 100 MHz): δ=14.1, 22.6, 29–32 (m), 34.9, 130–132 (m), 136, 140, 142 p.p.m.

Polymer P3. Yield: 71%. 1H NMR (CD2Cl2, 400 MHz): δ=0.8 (br m), 1.2 (br m), 1.5 (br m), 2.5 (br), 6.4 (br m), 7.2 (br m), 7.4–8.0 (br m) p.p.m. 13C NMR (CD2Cl2, 100 MHz): δ=14.3, 23.1, 29–32 (m), 118, 126, 129–133 (m), 136–144 (m) p.p.m.

Polymer P4. Yield: 35%. 1H NMR (CDCl3, 400 MHz): δ=0.8 (br), 1.2 (br), 1.5 (br), 2.5 (br), 6.4 (br m), 6.9 (br), 7.1 (br), 7.6 (br m), 7.9 (br) p.p.m. 13C NMR (CDCl3, 100 MHz): δ=14.1, 22.7, 29–32 (m), 124 (m), 125 (m), 129–132 (m), 136 (m), 142, 143 p.p.m.

Oxidative coupling polymerization

Monomer 3 (0.46 g, 1.0 mmol) was added to a mixture of FeCl3 (0.65 g, 4.0 mmol) in CHCl3 (15 ml). The reaction mixture was stirred at reflux temperature (oil bath temperature at 70 °C) for 24 h and poured into a large amount of MeOH to obtain dark greenish-brown precipitates. The precipitates were washed with MeOH with a Soxhlet extractor for 48 h to yield a brown solid. The polymer was extracted from the precipitates with CHCl3 with the Soxhlet extractor for several hours to obtain a brown CHCl3 solution. The CHCl3 solution was washed with saturated aqueous Na2S2O3 and aqueous NH3. And then, the CHCl3 solution was stirred with aqueous H2NNH2 for 6 h. The CHCl3 solution was separated and dried over MgSO4. After removal of MgSO4, the solvent was evaporated to obtain a brown solid, which was dried in vacuo. The solid was dissolved in a small amount of CHCl3 and reprecipitated from a large amount of MeOH three times to obtain polymer P1 in the form of the brown powder (73.3 mg, 0.16 mmol, 16%).

Results and Discussion

Scheme 1 outlines the synthetic route to the naphthalene-based monomer 4. The treatment of 1 with n-BuLi and TMEDA afforded 2-(4-hexylthienyl)lithium as an intermediate. The addition of ZnBr2 afforded a 2-(4-hexylthienyl)zinc bromide intermediate, and the successive Negishi coupling reaction71 with 2 in the presence of a catalytic amount of Pd(PPh3)4 yielded 3 in 57% isolated yield. Monomer 4 was obtained in 58% isolated yield by lithiation of 3 with s-BuLi/TMEDA, followed by iodination.

Target polymers P1–P4 were synthesized by polymerization of 4 and comonomers M1–M4 by the modified Suzuki–Miyaura coupling reaction68, 69, 70 in the presence of a catalytic amount of Pd(OAc)2 and a bulky phosphine ligand, 2-dicyclohexylphosphino-2′,6′-dimethoxybiphenyl (S-Phos), with K3PO4 in THF/H2O at reflux temperature for 48 h, as shown in Scheme 2. Cyclic oligomers63, 64 were not detected under this reaction condition because of the steric hindrance of pinacolborane moieties. The polymerization results are listed in Table 1. Polymer P1 with the head-to-head structure72 was also obtained by the oxidative coupling reaction of compound 3 with FeCl3 in CHCl3 (Scheme 3). The reaction of 3 with FeCl3 in CHCl3 afforded crude polymer P1 as a dark greenish-brown solid. This solid was purified with MeOH using a Soxhlet extractor; it was then treated with saturated aqueous Na2S2O3, aqueous NH3, and aqueous H2NNH2 to remove iron species. The polymer solution was dried over MgSO4, and repeated reprecipitation was carried out to yield polymer P1 in 16% isolated yield as a brown solid. However, we found a small amount of contamination by iron species (∼1.3 wt%) in the polymer prepared by FeCl3-mediated oxidative coupling reaction (Supplementary Figure S13).

Table 1 Results of polymerization
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All polymers P1–P4 had good solubility in common organic solvents, such as cyclohexane, toluene, CH2Cl2, CHCl3 and THF. Polymer thin films were readily formed by spin coating or casting from toluene solutions. They were characterized by 1H and 13C NMR spectroscopy, which are shown in Supporting Information. Figure 1 shows the 1H NMR spectrum of P2 in CDCl3 as a representative example. The signals attributable to hexyl protons appeared at around 0.8, 1.2, 1.5 and 2.5 p.p.m. The signals for the terthiophene unit appeared at higher magnetic field at around 6.4–7.0 p.p.m, and those of the naphthalene unit were observed from 7.5 to 8.0 p.p.m. The layered terthiophenes are π-stacked with each other, which results in the upfield shift of thiophene protons to around 6.4 p.p.m, because of the ring current effect. Such an upfield shift was not observed in the xanthene-based oligothiophene-layered polymers.46 The shortest distance between oligothiophene units in the polymers is expected to be ∼3 Å based on the X-ray crystal structures of 1,8-bis(5,2′-bithiophen-2-yl)naphthalene61 and bithiophenophane.64, 73 We also obtained the X-ray crystal structures of 5, as shown in Figure 2 (the data are shown in Supporting Information). Thiophene rings adopted the face-to-face structure, and their shortest distance was estimated to be 2.988 Å in the solid state. This distance of ∼3 Å is much shorter than the sum of the van der Waals radii of an sp2-carbon (3.4 Å); thus π–π interactions between the layered oligothiophenes of the polymers in solution are expected.

Figure 1
figure 1

1H nuclear magnetic resonance spectrum of polymer P2 in CDCl3.

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Figure 2
figure 2

Structure of compound 5 and its X-ray crystal structure. Thermal ellipsoids are drawn at the 30% probability level.

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The number-average molecular weights (Mn) of the polymers were estimated by gel permeation chromatography (polystyrene standards); they are summarized in Table 1. They were calculated to be Mn=2200–3400, from which the number-average degrees of polymerization were estimated to be 5–6. The matrix-assisted laser desorption/ionization time-of-flight mass spectrum of polymer P1 was successfully obtained, as shown in Figure 3, although the spectrum was highly broadened. Polymer P1 exhibited a major series of peaks that were regularly separated by the molar mass of a repeating unit (m/z 458), which is in accordance with the proposed structure.

Figure 3
figure 3

Matrix-assisted laser desorption ionization time-of-flight mass spectrum of polymer P1 using all-trans-retinoic acid as a matrix.

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The optical properties of polymers P1–P4 were examined,74, 75, 76, 77 and the data are summarized in Table 2. Figure 4a shows their UV–vis absorption spectra in CHCl3 (1.0 × 10−5 M). An increase in the λabs,max value was observed with increasing thiophene ring number (Table 1 and Figure 4a). The absorption maximum (λabs,max) of P1 was observed at 331 nm (Entry 1 in Table 2 and Figure 4a); this value is almost identical to that of bithiophene-layered oligomers (λabs,max=330 nm).64 It is suggested that this absorption band is assigned to the S0 → S1 transition of the bithiophene-naphthalene moiety and the S1 state adopts a quinoidal-like structure of thiophene with naphthalene.75, 76 The bithiophenes in P1 as well as oligothiophenes in P2–P4 have difficulty in rotating freely because of the hexyl substituent; however, they can twist and have a degree of π-conjugation with the naphthalenes.

Table 2 Optical properties of the polymers
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Figure 4
figure 4

(a) UV–vis absorption spectra in CHCl3 (1.0 × 10–5 M) and (b) normalized fluorescence spectra in CHCl3 (1.0 × 10–7 M, excited at absorption maxima) of polymers P1–P4.

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Photoluminescence spectra of P1–P4 in a diluted CHCl3 solution (1.0 × 10−7 M) were recorded by the excitation at each absorption maximum (Figure 4b). We found bathochromic shifts of the spectra (Figure 4b) and an increase in the absolute photoluminescence quantum efficiencies (ΦPL) with increasing number of thiophene rings (Table 2).75, 76, 77 For polymer P3, which has a rigid fused-ring structure, the ΦPL value was 0.27, which was the same as that for P4 consisting of quaterthiophenes (Entries 3 and 4 in Table 2). The ΦPL values of P1, P2 and P4 were similar to the values found for naphthalene-based bithiophene, terthiophene and quaterthiophene oligomers,75, 76 respectively. Concentration quenching was not observed among the layered oligothiophenes for P1–P4, suggesting that the main quenching pathway is radiationless intersystem crossing from S1 to T1. The energy migration and conformational relaxation of the oligothiophene–napththalene moieties within ∼100 picoseconds should be considered for further studies to clarify these points, will be in progress.75, 76

The electrochemical behaviors of P1–P4 were studied by cyclic voltammetry in CH2Cl2 containing 0.1 M Bu4NClO4 with a Ag/AgCl (Ag/Ag+) reference and a ferrocene/ferrocenium (Fc/Fc+) external reference. Their cyclic voltammograms are shown in Figure 5. Irreversible and ambiguous peaks were observed, which impeded the determination of their half-wave potentials (E1/2). The onset potentials (Eonset) of P1–P4 ranged from ∼0.55 to 0.45 V (vs Fc/Fc+); the Eonset values decreased with increasing number of thiophene rings. The highest occupied molecular orbital (HOMO) energy levels of the polymers were estimated from the Eonset values and were found to be approximately −5.35 to −5.25 eV.78 Thus, the oligothiophene-stacked polymers can be potentially applied in electronic devices such as hole-transporting materials.

Figure 5
figure 5

Cyclic voltammograms of polymers P1–P4 in CH2Cl2 (0.1 M) containing Bu4NClO4 (0.1 M) using a glassy carbon working electrode, a Pt counter electrode, a Ag/Ag+ reference electrode, and ferrocene/ferrocenium as an external reference.

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In summary, we synthesized naphthalene-based oligothiophene-stacked polymers by the Suzuki–Miyaura coupling polymerization. The polymers consist of the stacked oligothiophenes in proximity of ∼3.0 Å from each other. The contribution of the quinoidal structure of the oligothiophene units involving the thiophene scaffolds in the excited state resulted in relatively high photoluminescence quantum efficiencies. The electrochemical behaviors of the polymers suggest that they can be potentially applied in electronic devices, such as hole-transporting materials and field-effect transistors. Further studies will focus on the control of their higher-ordered structure in the solid state and fabrication of such devices.

scheme 1

Synthesis of compounds 3 and 4.

scheme 2

Synthesis of polymers P1–P4.

scheme 3

Synthesis of polymer P1 by the oxidative coupling reaction.