An ab initio investigation of the charge-transfer complexes of alkali atoms with oligo (α,α′) thiophenes and oligoparaphenylenes: A model calculation on polaronic and bipolaronic defect structures

Ab initio self-consistent field (SCF), second- and third-order Moøller-Plesset calculations on the charge-transfer complexes of one and two alkali atoms with oligothiophenes and oligophenylenes are reported. Complexes up to quaterthiophene and quaterphenyl with Li, Na, and Cs have been investigated. The mono-alkali complexes are related to polaron and the di-alkali complexes to bipolaron defects. Extensive basis set investigations have been performed. The quinoid structures which result from the interaction of the alkali atoms with the oligomers are well reproduced by the SCF method, whereas for accurate interaction energies electron correlation effects are very important. Our calculations show that the spatial extension of the polaronic and bipolaronic defects on the chain depends strongly on the fact whether counterions are explicitly taken into account or not. From previous quantum chemical calculations one finds that, e.g., in the case of oligothiophene dications the bipolaron defects extend over 9-11 thiophene units whereas our calculations under inclusion of the electron donating alkali atoms show that in this case the defects are much more localized. A very crude estimate of the energy balance between two polaron defects and one bipolaron gives about 15 kcal/mol in favor of the bipolaron structure. We also find a rather similar behavior along the alkali series as concerns interaction energies, defect structures, and charge distribution patterns.