Confined and delocalized polarons in ∏-conjugated oligomers and polymers: a study of the effective conjugation length

We studied optical absorption of polarons in ∏-conjugated oligomers and polymers using the photoinduced absorption technique as well as chemical doping spectra from the literature. We find that the photon energy of the polaron low-energy transition obeys a relationship that depends only on the oligomer-length in a wide class of unsubstituted and substituted oligomer solutions and films. Based on this observation, we show that polarons in polymer films can be either confined as in oligomers or quite delocalized, depending on the individual p-conjugated semiconductor. In high mobility polymer films polarons may be delocalized over several chains.