EPR study of positive holes on phenylene vinylene chains: from dimer to polymer

Isolated radical cations of substituted oligomers and polymers of phenylene vinylenes (PV) produced by irradiation in glassy toluene solutions were characterized by EPR. It was shown that the linewidth of the EPR signal from radical cations in oligomers decreased with increasing the number of repeat PV units, at least, up to n=8. This result implies extensive delocalization of hole on the isolated PV chain. In the case of polymer (MEH-PPV), the EPR signal linewidth is substantially smaller than the extrapolated value for oligomer radical cations, which was tentatively explained by the effect of substitution pattern.