Polythiophenes thiophene-based monomers

The structure of polythiophene is based on monomer units coupling through their 2,5-positions (a,a ), which preserves the aromaticity in the polymer [192,260]. This is consistent with infrared spectra, in which absorption bands found in the monomer are observed in the polymer. Dopant ions are probably responsible for the formation of new bands in the spectra of doped polythiophene, since the frequencies of the new bands shift with different types of dopants. Furthermore, elec-trochemically synthesized polythiophene exhibits distinctly different absorption bands from chemically prepared poly-2,4-thiophene [261]. 

Polythiophene can be synthesized by electrochemical polymerization or chemical oxidation of the monomer. A large number of substituted polythiophenes have been prepared, with the properties of the polymer depending on the nature of the substituent group. Oligomers of polythiophene such as (a-sexithienyl thiophene) can be prepared by oxidative linking of smaller thiophene units (33). These oligomers can be sublimed in vacuum to create polymer thin films for use in organic-based transistors. 

Such polycondensation dehalogenation reactions remain a commonly employed route to poly thiophene, and a range of solvents, halogenothiophene substrates, and other metal-based catalysts have been examined, as recently reviewed.29 For example, the reaction of 2,5-dibromothiophene with Ni(cyclooctadiene)2 and triphenylphosphine in DMF leads to an almost quantitative yield of polythiophene.30 Solid-state 13C NMR studies confirm exclusive 2,5-coupling of the thiophene repeat units in the polymeric product. The 2,5-dichlorothiophene monomer is less active as a substrate in such reactions however, the corresponding 2,5-diiodothiophene is reported to be a good substrate.31... 

Finally, we will concentrate on the chemical reactivity of silyl derivatives of thiophene. The oxidative polymerization of various silyl monomers lead to polythiophene. The evaluation of this new polymerization reaction implies a precise characterization of the produced conjugated materials. Knowledge and the control of the pertinent parameters which direct the properties of the conjugated systems are essential. Also required is the development of methods which allow a precise characterization of the samples. The role of vibrational infrared and Raman spectroscopy is of fundamental importance in this field. Optical spectroscopy is one of the few tools for unravelling the structure of these materials and understanding their properties. First, new criteria based on infrared, Raman and photoluminescence spectroscopy which allow precise estimates of the conjugation properties will be reported. Then the synthesis and characterization of polythiophene samples arising from the oxidative polymerization of silyl thiophene will be presented. 

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