Physico-Chemical nanomaterials science

Characteristics of charge carriers in 3,4-ethylenedioxythiophene oligomers (n=1–16)

Terebinska M.I., Filonenko O.V., Tkachuk O.I., Datsyuk A.M., Lobanov V.V.

Chuiko Institute of Surface Chemistry of the National Academy of Sciences of Ukraine, 17 Generala Naumova Street, Kyiv, 03164, Ukraine,
 email: terebinska@ukr.net

A number of oligomers of 3,4-ethylenedioxythiophene (n=1–16) in charge states +1, +2, +3, and +4 were studied by the density functional theory method (B3LYP/6-31+G(d,p). Optimization of the spatial structure and calculation of the electronic structure of the cations of 3,4-ethylenedioxythiophene (EDT) oligomers were carried out using methods with and without spin limitation. It has been found that the ground electronic state of short oligomeric cations (n-EDT)2+ and (n-EDT)4+ (n < 7) corresponds to a spin-limited singlet state. The charge carrier in these oligomeric cations has the characteristics of a bipolaron. In contrast to small oligomers, in (n-EDT)2+ with 7n14, the ground electronic state is also singlet, but unlimited in spin, and the charge carriers in them have the properties of a polaron pair. For the longest studied oligomers (n=15, 16) (n-EDT)2+ and (n-EDT)4+, as a result of singlet-triplet degeneracy, the ground electronic state is triplet, and the polaron pair becomes predominantly the charge carrier. For all considered cations with an odd charge (n-EDT)1+ and (n-EDT)3+, the ground electronic state turned out to be a doublet.
For n-EDT oligomers, the calculated C–C bond lengths between neighboring monomeric units are 1.433 Å, which is typical of the benzenoid phase. However, for (n-EDT)1+ cations, the lengths of these bonds decrease monotonically from the ends of the chain to its center, reaching a value of 1.417 Å on the central bond, which is characteristic of the quinoid phase. In (n-EDT)2+ dications, a similar dependence of bond lengths between monomer units is observed. For higher oxidation states (+3 and +4), the contribution of the quinoid phase increases and two separated polarons are formed at the ends of the oligomer chain.
Thus, the results obtained allow us to conclude that in oxidized EDT oligomers, at a high degree of doping, the conductivity is provided by the formation of two polarons at the ends of the chain.