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

Oligothiophene radical cation geometries were optimised using the PM3 molec ular orbital method. Electronic transition energies were calculated using Z INDO/S coupled with configuration interaction using configurations generate d by multiple excitations within two occupied and two unoccupied orbitals. Experimental energies (two per polaron) were reproduced to within 0.1 eV. T his represents an improvement relative to literature calculations where arb itrary energy shifts close to 1 eV were apparently necessary in order to re produce experimental results.