Polarons in 7r-conjugated polymers and oligomers

A linear polymer consists of the repetition along the direction of the polymer chain of a small monomeric subunit. As such, it can be viewed as a one dimensional lattice. Accordingly, the electron energy level can be obtained by using Bloch functions, which results in one dimensional energy bands along the direction of the polymer chain. 

The spatial extension of polarons has been calculated for various 7r-conjugated polymer [25-27]. In the case of polythiophene, it corresponds to five monomer units. In a short oligomer, the polaron can no longer be considered as a charge free to move along the polymer chain. Hence, the concept of polaron has to be changed to that of radial cation, in which the geometrical structure modification extends all over the molecule. 

Radical cations in oligothiophene have been extensively studied during the past years [28-36]. They are quite readily obtained in solution by adding an oxidizing agent such as ferric chloride [28-31], or by photochemical methods [33-35]. The 

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