SYNTHESIS OF 3,3'''-DIOCTYLTETRATHIOPHENE OLIGOMER

FRANCISCO MARTINEZ ^*, GLORIA NECULQUEO AND MARIA E. VEAS

Depto. Química Básica, Facultad de Ciencias Físicas y Matemáticas, Universidad de Chile,
Casilla 2777, Santiago, Chile
(Received: August 23, 1999 - Accepted: November 10, 1999)

ABSTRACT

Synthesis and spectroscopical characterization of 3,3'''-dioctyltetrathiophene oligomer has been carried out. The dioctyl tetrathiophene was prepared by the nickel catalyzed cross-coupling reaction starting from 3,3'-diocytilbithiophene and 2-bromo-octylthiophene. The spectroscopical properties are reported. (UV, FT-IR, ¹H-NMR, ¹³C-NMR).

KEY WORDS: Alkylthiophene oligomer, thiophene tetramer, spectroscopic characterization.

RESUMEN

Se ha realizado la síntesis y caracterización espectroscópica del 3,3'''-dioctiltetratiofeno. El tetrámero se preparó utilizando la reacción de acoplamiento entre el 3,3'-dioctilbitiofeno y el 2-bromo-3-octiltiofeno catalizada por níquel. Las propiedades espectroscópicas de UV, FT-IR, ¹H-NMR, ¹³C-NMR son reportados.

PALABRAS CLAVES: Oligómero alquiltiofeno, tetrámero tiofeno, caracterización espectroscópica.

INTRODUCTION

The synthesis of alkylthiophene oligomers has received increasing attention due to the application on electronic devices such as transistors, light emiting diode or photovoltaic cells^1,2). The introduction of alkyl side chain improves solubility and builds up regio and regular structures for the modulation of the electrical and optical properties of these materials^3,4). On the other hand, the polythiophene derived from well defined oligomers, where a regio and stereochemical control is desirable, result in polymer with a significantly higher conductivity, due to regiospecific synthesis of ~100% head-to-tail polyalkylthiophene^5,6). The processability of these polyalkylthiophene is also highly improved due to the flexible side chains which reduce both the chain intractability and the b-coupling during polymerization.

By taking into account these considerations we have synthesized the 3,3'''-dioctyltetrathiophene oligomer which can be used as starting material for polymerization or as itself for applications in electro-optic devices. We have prepared the tetramer by using the cross coupling reaction, of dithienyl dibromides with the alkylthienyl magnesium bromide reagent catalyzed by nickel dichloride diphenyl propane phosphine⁷⁾, NiCl₂(dppp). Figure 1 shows the synthetic pathway of the reactions. The different compounds have been characterized by UV, FT-IR, ¹H-NMR, ¹³C-NMR.

FIG. 1. Synthesis of 3,3'''-dioctyltetrathiophene.

EXPERIMENTAL

The ¹H and ¹³C-NMR spectral data were obtained from Bruker WP-270 NMR spectrometer, the spectra were taken in CDCl₃. The ultraviolet-visible spectra were recorded using a Perkin Elmer Lambda 11 UV spectrophotometer. The infrared spectra were recorded from a Bruker, Vector 22 instrument.

Synthesis of 2-bromo-3-octylthiophene (1)
3-octylthiophene

Into a dry flask was placed 5.6 g (0.23 mmol) of magnesium turnings and iodine in diethyl ether (60 mL) which was purged with argon. Then 40 mL, (0.23 mol) of 1-bromooctane was slowly added maintaining reflux for 2 h. Then the octylmagnesium bromide obtained was added slowly to an ice-cooled solution of (18 mL, 0.19 mol) 3-bromothiophene and 100 mg (0.019 mol) of Ni (dppp) Cl₂ in 80 mL of diethyl ether The mixture was refluxed for 18 h, cooled to room temperature, and hydrolyzed with 1 M HCl. The organic phase was separated, washed with distilled water until pH neutral, dried over MgSO₄ for 3 h and then the solvent was removed by rotary evaporation. The product was purified by distillation under reduced pressure yielded 23.17 g, 62% of 3-octylthiophene obtained as a liquid colorless, l_max CHCl₃, 236.8 nm. ¹H-NMR (270 MHz, CDCl₃, ppm) 0.96 (t, 3H); 1.34-1.63 (m, 10H); 2.68 (t, 2H); 6.96 (d, 1H, thiophene ring, H-4); 6.97 (d, 1H, thiophene ring, H-5), 6.98 (s, 1H, thiophene ring, H-2). ¹³C-NMR (75 MHz, CDCl₃, ppm): Aromatics: 143.2 C(3); 128.3 C(4); 125.0 C(5): 119.5 C(2). Aliphatics: 31.89; 30.56; 30.27; 29.71; 29.44; 29.36; 22.68; 14.10.

2-bromo-3-octylthiophene

In the absence of light, to a solution of 16.81 g (0.085 mol) 3-octylthiophene in 35 mL of dimethylformamide (DMF) were slowly added 15.24 g (0.095 mol) of N-bromosuccinimie in 50 mL of DMF. the mixture was stirred for 40 h, and poured onto ice, and extracted several times with diethyl ether. The organic phase was washed with distilled water and dried over sodium sulfate. The solvent was extracted and the product was purified by vacuum fractional distillation. The product, 19 g of yellow liquid was obtained in 79% yield. l_max CHCl₃, 240.4 nm. ¹H-NMR (270 MHz, CDCl₃, ppm) 0.9 (t, 3H); 1.25-1.39 (m, 10H); 2.59 (t, 2H); 6.80 (d, 1H, thiophene ring, H-4); 7.19 (d, 1H, thiophene ring, H-5). ¹³C-NMR (75 MHz, CDCl₃, ppm). Aromatics: 141.86 C(3); 128.13 C(4); 125.03 C(5); 108.79 C(2). Aliphatics: 31.96; 31.90; 29.75; 29.39; 29.26; 29.25; 22.69; 14.11.

Synthesis of 2,2'-dibromobithiophene (2)

In the absence of light, 26.6 (0.15 mol) of NBS was added portionwise at 20°C to a solution of bithiophene (12.3 g, 0.074 mmol) in DMF (100 mL), stirred for 3 h, poured onto ide, and the precipitate was filtered and washed several times with water. The product was recrystallized from absolute ethanol. The yield was 21 g (88%), white solid, Mp 146°C, l_max CHCl₃, 340.8 nm.
¹H-NMR (270 MHz, CDCl₃, ppm), 6.84 (2H, d), 6.98 (2H, d).

Synthesis of 3,3''-dioctyltetrathiophene (3)

Into a dry round-bottom flask was placed 80 mL of diethyl ether, 0.44 g of magnesium and iodine. The solution was stirred and purged with argon. 5 g of octylthiophene monobromide (1) was added dropwise. After finished the reaction the solution was filtered and the 3-octylthiophene magnesium bromide was added dropwise to a solution at 0°C containing 2.9 g of bithiophene dibromide and 0.1 g of NiCl₂(dppp) in 80 mL of diethylether. The mixture was refluxed for about 20 h, hydrolyzed with 1N HCl, washed with distilled water, extracted with dichloromethane, dried with MgSO₄ and then evaporated the solvent. The product obtained was purified several times by chromatography on silicagel by elution with a mixture hexane/dichloromethane (10:1). Red-orange crystals, Mp. 55-56°C. Yield 19%.
¹H-NMR (270 MHz, CDCl ₃, ppm), 7.15 (2H, d), 7.10 (2H, d), 7.0 (2H, d), 7.0 (2H, d), 6.90 (2H, d), 2.78 (4H, t, a-CH₂), 1.65 (4H, m, b-CH₂), 1.24 (20H, m, CH₂), 0.88 (6H, t, CH₃). The UV max in CHCl₃ was 456.6 nm.

RESULTS AND DISCUSSIONS

The UV-visible absorption of the alkylthiophene tetramer (3), l_max = 456.6 nm (CHCl₃) is related with the conjugation length of the oligomer. For non-substituted thiophene oligomers this absorption increases from bithiophene to sexithiophene from 300 to 434 nm (CHCl₃), the tetrathiophene oligomer absorbs at 416 nm (CHCl₃)⁸⁾. The alkyl side chain lead to increased steric interaction between the conjugated backbones and thus reduces p-p interactions between the molecules in solid state. The presence of the side alkylchain increases the solubility alkylthiophene in aromatic and chlorinated hydrocarbons allowing a full characterization. As a result of the steric influence of the alkyl side chain, the substituted thiophene rings are rotated so the alkyl groups lie on opposite side facilitating the conjugation length through the rings and thus shifting the absorption to a lower energy.

Figures 2 and 3 show FT-IR, and ¹³C-NMR spectra of 3,3'''-dioctyltetrathiophene respectively. The infrared spectrum, Figure 2, shows three main regions, between 2850-2950 cm^-1 the C-H stretching vibration due to othe alkyin, between 1500-1400 cm^-1 which corresponds to the C=C stretching vibration and between 900-450 cm^-1, the -H bending (out of plane) vibrations. The absorption near 1490 cm^-1 (C=C) asymetric is characteristic of thiophene oligomers and increases in intensity from bithiophene to sexithiophene⁸⁾. The strong band at 798 cm^-1 corresponds to the C-H bending and is characteristic of 2,5-substituted thiophene ring⁹⁾.

The ¹³C-NMR spectrum is shown in Figure 3, a completed assignment of the aromatic carbon chemical shift has been achieved and is presented on the structure. The spectrum shows the 8 carbon signals due to the octyl alkyl group, betwen 13 and 31 ppm, which is normal for this region. Between 122 and 139 ppm are the absorption of the aromatic carbon of thiophene rings. The non-substituted tetrathiophene has absorptions between 123 and 134 ppm.⁸⁾

The a,a'-coupled carbon has at 136-137 ppm signals. The a carbons which are not coupled absorb at 122.9 ppm, while the b carbons present two different signals depending of their position. The ones which are on the extreme positions absorb at 129.1 ppm and the inner ones have signals between 124.5-125.5 ppm. The carbons carrying the alkyl chain absorb upfield at 138.9 ppm. This signal is observed at 143.2 ppm with the 3-octyl thiophene.

FIG. 2. FT-IR spectrum of 3,3''-dioctyltetrathiophene.

FIG. 3. ¹³C-NMR spectrum of 3,3''-dioctyltetrathiophene.
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*To whom correspondence should be addressed.

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