Reexamination of the assignments of electronic absorption bands of polarons and bipolarons in conducting polymers

Electronic absorptions due to polarons and bipolarons in nondegenerate polymers have been reexamined on the basis of the data of the radical ions (models of polarons) and divalent ions (models of bipolarons) of α-oligothiophenes and p-oligophenyls. From the electronic absorption spectra of these model species, it has been concluded that two transitions are strong among the three expected for a polaron, whereas one transition is strong among the two expected for a bipolaron. Two bands observed in the electronic absorption spectra of doped polythiophene, poly(p-phenylene) and poly(p-phenylenevinylene), which were previously attributed to bipolarons, have been reassigned to polarons. These assignments are consistent with the results of Raman studies reported recently. The confinement parameters of these polymers have been estimated on the basis of the new assignments.