Poly-n-alkylthiophenes

Polythiophene is a conjugated polyheterocycle in which backbone stmcture differs markedly firom those of PDA or Polyacetylene. In contrast to PDA these polymers are easily dopable. A common way for describing the doped state is to use the concept of 

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Since the discovery that Poly-n-alkylthiophenes can be solubilized (15), they have been the subject Of intensive studies. The stability of the polymer together with the possibility to study the doped polymer in solution are die origins of the interest Besides, poly-n alkylthiophene films exhibit drastic change as shown with the thermochromism which varies widi the size of the substituent (Salaneck et al, this book). Moreover poly-n alkylthiophene provides an unique oppcntunity to investigate the chain conformation of a conjugal polymer with a conjugated backbone maiicedly different from the substituted polydiacetylene or polyacetylene. 

Additional measurements are ne ed in order to assess whether the n electrons have a major influence upon the local correlations or not The first ones were obtained on poly-n-alkylthiophenes, where it was found that the statistical length of an isolated poly-Sbutylthi hene chain is equal to 45 A (see chapter 6). Obviously the conjugated backbone of polythiophene is markedly different ftom polydiacetylene or polyacetylene, so a direct comparison between the different statistical conformations may not be meaningful. Nevertheless, it remains that the observed flexible behavior of the poly-3butylthiophene is a first and direct evidence that a conjugated backbone structure does not necessarily mean a stiff chain behavior. 

In the field of soluble conducting polymers new data have been published on poly(3-alkylthiophenes " l They show that the solubility of undoped polymers increases with increasing chain length of the substituent in the order n-butyl > ethyl methyl. But, on the other hand, it has turned out that in the doped state the electro-chemically synthesized polymers cannot be dissolved in reasonable concentrations In a very recent paper Feldhues et al. have reported that some poly(3-alkoxythio-phenes) electropolymerized under special experimental conditions are completely soluble in dipolar aprotic solvents in both the undoped and doped states. The molecular weights were determined in the undoped state by a combination of gel-permeation chromatography (GPC), mass spectroscopy and UV/VIS spectroscopy. It was established that the usual chain length of soluble poly(3-methoxthythiophene) consists of six monomer units. 

S Nagamatsu, W Takashima, K Kaneto, Y Yoshida, N Tanigaki, K Yase, and K Omote, Backbone arrangement in friction-transferred regioregular poly(3-alkylthiophene)s, Macro molecules, 36 5252-5257, 2003. 

Despite the fact that optical applications require thin films of poly(3-alkylthiophene)s, the photochemistry of these materials has been characterized in solution but only scarcely in the solid state. The UV/Vis spectra of these films of poly(3-butylthiophene) show an absorption band in the visible range corresponding to a n—n transition whose energy depends on 7r-electron delocalization. 

Yamamoto, T. and Kokubo, H., Selective stacking of HT-poly(3-n-alkylthiophene-2,5 diyl)s in mixed systems, J. Polym. ScL, Part B Polym. Phys. 38, 84-87, 2000. 

In view of the above findings it is expected that H-H coupling defects in H-T coupled poly(3-alkylthio-phenes) may also lower the conjugation. This is indeed the case. In regioregular poly(3-alkylthiophene) the maximum of the n—n absorption peak is red shifted as compared to FeClj prepared, non-regioregular poly(3-alkylthiophene) (see Table 4.1). 

Yamamoto. T. Komamdin. D. Arai, M. Lee, B.-L. Suganurna. H. Asakawa. N. Inoue. Y. Kubota. K. Sasaki. S. Fukuda, T. Matsuda. H. Extensive studies on n-stacking of poly(3-alkylthiophene-2,5-diyl)s and poly(4-alkylthiazole-2,5-diyl)s by optical spectroscopy. NMR analysis, light scattering analysis, and x-ray crystallography. J. Am. Chem. Soc. 1998, 120. 2047-2058. 

Nagamatsu, S., W. Takashima, K. Kaneto, Y. Yoshida, N. Tanigaki, K. Yase, and K. Omote. 2003. Backbone arrangement in firiction-transferred regioregular poly(3-alkylthiophene)s. Macromolecules 36 5252. 

The electrochemical properties of chemically synthesized poly(3-alkylthiophene) compounds have been investigated [65,66]. Kawai et al studied various alkly groups (CnH2n + i, n = 4-12) and found that the diffusion coefficient value decreased with increasing alkyl chain length [66]. Poly(3-octylthiophene) showed good performance as an electrode in a 0.1 M... 

K. Yazawa, Y. Inoue, T. Yamamoto and N. Asakawa. Twist glass transition in regioregulated poly(3-alkylthiophene)s. Los Alamos Nat. Lab. Prepr. Arch. Condens. Matter, 1-12 (2006). 

H. Tachibana, N. Hosaka and Y. Tokura. Hysteretic thermochromism of regioregular poly(3-alkylthiophene) thin films. Macmttwlecules 34(6), 1823-1827 (2001). 

N. Camaioni, G. Ridolfi, G. Casalbore-MiceU, G. Possamai, M. Maggini, The effect of a mild thermal treatment on the performance of poly(3-alkylthiophene)/fnllerene solar cells, Adv. Mater., 14, 1735-1738 (2002). 

Evidently, conducting polymers with an ordered structure have the advantage of a combination of high conductivity with good solubility properties. The alkylated electroactive polymers such as poly(3-alkylthiophene)s and N-alkylated polyanilines can be compared with alkylated liquid crystalline polyesters and polyamides. 

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