THEORETICAL INVESTIGATIONS ON POSITIVE BIPOLARONIC DEFECTS IN OLIGOTHIOPHENES - STRUCTURES AND VIBRATIONAL-SPECTRA OF DOUBLY-CHARGED 2,2'-BITHIOPHENE AND 2,2'/5',''2-TERTHIOPHENE

We report the results of ab initio self consistent field (SCF) investigations on the structure and on the vibrational spectra of the dications (positive bipolarons) of 2,2'-bithiophene and 2,2':5',2''-terthiophene. The optimized geometry parameters, the vibrational frequencies, the infrared (IR) intensities and the Raman activities of the charged oligomers are compared to the corresponding properties of the neutral oligothiophenes. The structures of the dications are characterized by a quinonoid carbon backbone as opposed to the benzenoid carbon backbone of the neutral oligomers. In both the IR and the Raman spectra of the dications, we predict significantly enhanced intensities. This intensity enhancement is particularly strong in the IR spectra.