Comparative theoretical study of heterocyclic conducting oligomers:: Neutral and oxidized forms

The structural and electronic properties of pyrrole-, thiophene-, phosphole-, 3,4-ethylenedioxythiophene-, and 3,4-ethylenedithiafurane-containing oligomers have been studied using density functional theory with the B3PW91 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. The study on neutral oligomers indicated that polyphosphole presents the lowest energy gap (1.17 eV) and, in addition, it can be oxidized and reduced easily, that is, it is p- and n-dopable. The energy gaps estimated for poly(3,4-ethylenedioxythiophene) and poly(3,4-ethylenedithiafurane) are lower than those predicted for polythiophene and polypyrrole. On the other hand, calculations on two positively charged oligomers showed that the triplet electronic state is favored with respect to the singlet state when the number of monomers is long enough: the exact number of monomers required to satisfy this condition depends on the chemical nature of the system. Thus, dicationic pyrrole-, thiophene-, phosphole-, 3,4-ethylenedioxythiophene-, and 3,4-ethylenedithiafurane-containing oligomers with more than 8, 8, 11, 9 and 9 monomers, respectively, form a biradical with two separated polarons.