An ab initio study of the vibrational spectra of Li-doped thiophene, bithiophene, benzene and biphenyl as model systems for (bi)polaronic defects

Ab initio SCF investigations of the IR and Raman spectra of the charge-transfer complexes of Li atoms with thiophene, bithiophene, benzene and biphenyl are reported. Computed geometries show an aromatic → quinoid transition during the formation of the complexes. The force constants reflect this behavior. As a consequence, the vibrational frequencies are considerably rearranged. IR and Raman intensities are strongly enhanced in the complexes as compared to the undoped compounds. The inter-ring vibrational modes occur at frequencies close to 1600 cm^-1 both for di-Li-BT and di-Li BP. This suggests a reconsideration of the assignment of that mode in the experimental Raman spectrum of the p-oligophenyl dianions.