Reexamination of assignments of electronic absorptions due to polarons and bipolarons in conducting polymers

Optical absorption spectroscopy is used for the identification of polarons and bipolarons in conducting polymers. We will reexamine the electronic absorptions due to polarons and bipolarons within the framework of a one-electron picture by using the data of the radical cations and dications of /spl alpha/-oligothiophenes and those of the radical anions and dianions of p-oligophenyls. For a positive polaron two localized electronic levels, bonding and antibonding, are formed in the band gap and an electron is removed from the polaron bonding level. A positive polaron is expected to have three transitions: w/sub 1/, polaron bonding level /spl larr/ valence band; w/sub 2/, polaron antibonding level /spl larr/ polaron bonding level; w/sub 3/, polaron antibonding level /spl larr/ valence band. For a positive bipolaron another electron is removed from the bonding level. Thus, a positive bipolaron is expected to have two transitions: w/sub 1´/, bipolaron bonding level /spl larr/ valence band; w/sub 3´/, bipolaron antibonding level /spl larr/ valence band. Similarly, a negative polaron is expected to have three transitions and a negative bipolaron two transitions. On this basis, three-band-patterns in observed electronic spectra have been usually assigned to polarons and two-band patterns to bipolarons. On the other hand, the radical cation and dication of /spl alpha/-sexithiophcne, which are viewed as models of a positive polaron and a positive bipolaron, respectively, show two absorption bands and one band, respectively, in the visible-near infrared region. The radical anion and dianion of p-sexiphenyl also shows two bands and one band, respectively. In these model systems, the w/sub 3/ and w/sub 3´/ transitions are symmetry forbidden. The results of the model systems indicate that the w/sub 3/ and w/sub 3´/ transitions are very weak in contrast to the others. We will propose new assignments of electronic absorption spectra induced by doping: two-band patterns to pol- rons and one-band patterns to bipolarons.