Abstract :
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.
