AN AB-INITIO INVESTIGATION OF THE CHARGE-TRANSFER COMPLEXES OF ALKALIATOMS WITH OLIGO (ALPHA,ALPHA') THIOPHENES AND OLIGOPARAPHENYLENES - A MODEL CALCULATION ON POLARONIC AND BIPOLARONIC DEFECT STRUCTURES

Ab initio self-consistent field (SCF), second- and third-order Moller- Plesset calculations on the charge-transfer complexes of one and two a lkali atoms with oligothiophenes and oligophenylenes are reported. Com plexes up to quaterthiophene and quaterphenyl with Li, Na, and Cs have been investigated. The mono-alkali complexes are related to polaron a nd the di-alkali complexes to bipolaron defects. Extensive basis set i nvestigations 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 energie s electron correlation effects are very important. Our calculations sh ow that the spatial extension of the polaronic and bipolaronic defects on the chain depends strongly on the fact whether counterions are exp licitly taken into account or not. From previous quantum chemical calc ulations one finds that, e.g., in the case of oligothiophene dications the bipolaron defects extend over 9-11 thiophene units whereas our ca lculations under inclusion of the electron donating alkali atoms show that in this case the defects are much more localized. A very crude es timate of the energy balance between two polaron defects and one bipol aron gives about 15 kcal/mol in favor of the bipolaron structure. We a lso find a rather similar behavior along the alkali series as concerns interaction energies, defect structures, and charge distribution patt erns. (C) 1995 American Institute of Physics.