A systematic theory for optical properties of phenylene-based polymers

A complete picture of phenylene-based polymers is developed which unifies features of band and molecular exciton models. It incorporates major experimental findings in direct and photoinduced optical absorption, in stimulated photoemission and photoconductivity. Our theoretical picture is based upon a band description for electronic states, while invoking corrections from Coulomb interactions. We demonstrate the existence of different types of Coulomb excitons and their roles in optical absorption. We show that a low binding energy-emitting exciton also dominates in the fundamental absorption. Contradictions in the current modeling state-of-the-art are displayed and discussed. For poly(phenylene vinylene) (PPV)-type polymers, we give new assignments for the most disputed features; for oligomers we identify new ones as edge states. In applying our model to the poly(phenylene) (PPP) family, more progress is available due to analytical results covering not only spectra but also the oscillator strength. We conclude similar assignments as for PPV and emphasize the delocalized nature of basic features. An important and intriguing peculiarity of PPP- compared with PPV-based polymers is the possible level inversion between excitons. Comparison is made to available optical and electron energy loss spectroscopy (EELS) data. Our conclusions are suggestive for future light-polarized experiments.