3,3'''-dioctyltetrathiophene.

....... 13 c nmr



 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.

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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^-1corresponds 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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REFERENCES

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