ELECTRONIC-TRANSITIONS IN POLARONS AND BIPOLARONS OF POLY(P-PHENYLENE) OLIGOMERS

The electronic transitions in radical ions (polarons) and dications (b ipolarons) of oligomers of poly(p-phenylene) have been measured by las er flash photolysis and steady-state techniques. Plots of the electron ic absorption band energies for the polaron and bipolaron are found to be linear functions of inverse chain length. The predicted transition s in the polaron of poly(p-phenylene) (PPP), obtained from extrapolati ons of these plots to infinite chain length, are considerably lower in energy compared to those reported in doped PPP. Interestingly, the tr ansitions observed by us in the radical ions of p-quaterphenyl and p-s exiphenyl 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 t he polymer chain. On the basis of these results, we have concluded tha t diamagnetic pi-stacks of fairly localized polarons could be the cond ucting entities in doped PPP.