Optical transitions in oligothiophene radical cations (positive polarons): a semi-empirical study

Oligothiophene radical cation geometries were optimised using the PM3 molecular orbital method. Electronic transition energies were calculated using ZINDO/S coupled with configuration interaction using configurations generated by multiple excitations within two occupied and two unoccupied orbitals. Experimental energies (two per polaron) were reproduced to within 0.1 eV. This represents an improvement relative to literature calculations where arbitrary energy shifts close to 1 eV were apparently necessary in order to reproduce experimental results.