Preparation and spectroscopic characterization of polarons and bipolarons of thiophene oligomers within the channels of pentasil zeolites: the evolution of organic radical ions into conducting polymers

Abstract

Pentasil zeolites such as ZSM-5 and Na-β can be used as supporting matrices in which short-chain oligomers of polythiophene can be prepared, oxidatively doped to the conducting state, stabilized, and finally spectroscopically characterized. For the first time the evolution of the electronic structure of doped polythiophene from monomer to polymer has been observed directly for chain lengths between two and nine. Plots of the electronic absorption band energies for the polaron and bipolaron are found to be linear functions of inverse chain length. These results are extrapolated to infinite chain length to predict the positions of heretofore unobserved electronic transitions of bulk polythiophene. These extrapolations suggest that the lowest energy polaron and bipolaron levels of doped polythiophene are remarkably close in energy, implying that transient formation of polarons from bipolarons in energetically feasible and that this process could play a role in interchain charge hopping in this material.