The results of quantum chemical calculations on the electronic structure of chains of poly(2,5hyphen;thienylene), i.e., polythiophene, along which some of the agr;ndash;agr;rsquo;(2ndash;5rsquo;) linkages are replaced randomly by agr;ndash;bgr;rsquo;(2ndash;4rsquo;) linkages are presented. The randomness in the agr;ndash;bgr;rsquo;linkage positions is taken into account via the negative factor counting technique, on the basis of Fock matrices calculated by the valence effective Hamiltonian method. The eigenfunctions of the upper occupied levels are explicitly calculated using the inverse iteration technique and the degree of localization is determined via the inverse participation number approach. It is shown that a random distribution in the agr;ndash;bgr;rsquo;linkages leads to a localization of the upper occupied levels within segments containing exclusively agr;ndash;agr;rsquo;linkages; more extended levels start to appear some 0.3 eV below the HOMO level for agr;ndash;bgr;rsquo;linkage concentrations of about 6percnt;. On the contrary, when the agr;ndash;bgr;rsquo;linkages occur regularly, delocalization remains effective even for the upper occupied levels.
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