Influence of polymerization irregularities on the electronic properties of polythiophene

The results of quantum chemical calculations on the electronic structure of chains of poly (2,5-thienylene), i.e., polythiophene, along which some of the α-α′(2-5′) linkages are replaced randomly by α-β′ (2-4′) linkages are presented. The randomness in the α-β′ 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 α-β′ linkages leads to a localization of the upper occupied levels within segments containing exclusively α-α′ linkages; more extended levels start to appear some 0.3 eV below the HOMO level for α-β′ linkage concentrations of about 6%. On the contrary, when the α-β′ linkages occur regularly, delocalization remains effective even for the upper occupied levels.