Electronic transitions in polarons and bipolarons of poly(p-phenylene) oligomers

The electronic transitions in radical ions (polarons) and dications (bipolarons) of oligomers of poly(p-phenylene) have been measured by laser flash photolysis and steady-state techniques. Plots of the electronic absorption band energies for the polaron and bipolaron are found to be linear functions of inverse chain length. The predicted transitions in the polaron of poly(p-phenylene) (PPP), obtained from extrapolations of these plots to infinite chain length, are considerably lower in energy compared to those reported in doped PPP. Interestingly, the transitions observed by us in the radical ions of p-quaterphenyl and p-sexiphenyl are similar to those observed in highly conducting doped PPP. We have proposed that the polaron is localized to a short segment (n approximately 4-6, where n is the number of phenyl repeat units) of the polymer chain. On the basis of these results, we have concluded that diamagnetic pi-stacks of fairly localized polarons could be the conducting entities in doped PPP.