| Abstract
| - The structural and electronic properties of pyrrole-, thiophene-, phosphole-, 3,4-ethylenedioxythiophene-, and3,4-ethylenedithiafurane-containing oligomers have been studied using density functional theory with theB3PW91 functional and the 6-31+G(d, p) basis set. Calculations were performed on systems containing two,four, six, and eight monomers, with the results being extrapolated to infinite chain length polymers. Thestudy on neutral oligomers indicated that polyphosphole presents the lowest energy gap (1.17 eV) and, inaddition, it can be oxidized and reduced easily, that is, it is p- and n-dopable. The energy gaps estimated forpoly(3,4-ethylenedioxythiophene) and poly(3,4-ethylenedithiafurane) are lower than those predicted forpolythiophene and polypyrrole. On the other hand, calculations on two positively charged oligomers showedthat the triplet electronic state is favored with respect to the singlet state when the number of monomers islong enough: the exact number of monomers required to satisfy this condition depends on the chemicalnature of the system. Thus, dicationic pyrrole-, thiophene-, phosphole-, 3,4-ethylenedioxythiophene-, and3,4-ethylenedithiafurane-containing oligomers with more than 8, 8, 11, 9 and 9 monomers, respectively, forma biradical with two separated polarons.
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