Theoretical investigation of excited states of oligothiophenes and of their monocations

Excitation energies of neutral thiophene oligomers with chain lengths of up to 25 rings and charged thiophene oligomers with chain lengths of up to 20 rings were calculated with time-dependent Hartree-Fock and time- dependent density functional theory (TDDFT). As recently for polyene cations, very good agreement is found between TDDFT and high-level ab initio calculations and with experimental results wherever data are available. For short thiophene oligomer cations, two sub-band transitions are predicted; for long chains, a third transition develops. Defects are found to be delocalized in bare cations; the inclusion of counterions induces localization. Despite the strong influence of counterions on the geometry, the influence of counterions on the spectra is small for the first two sub-band peaks. Since counterions are directly involved in the electron transitions contributing to the third sub-band peak of longer oligomers, the inclusion of counterions lowers the energy of this absorption peak. The agreement between theoretical spectra based on delocalized geometries and experimental spectra shows that defect localization (electron phonon coupling) is not the underlying cause of the two subband transitions. Investigation of the electronic configurations that contribute to the excited states does not confirm the nature of the transitions predicted with the polaron model.