A systematic theory for optical properties of phenylene-based polymers

A complete picture of phenylene-based polymers is developed which unifies f eatures of band and molecular exciton models. It incorporates major experim ental 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 Co ulomb excitons and their roles in optical absorption. We show that a low bi nding 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 as signments for the most disputed features; for oligomers we identify new one s as edge states. In applying our model to the poly(phenylene) (PPP) family , more progress is available due to analytical results covering not only sp ectra but also the oscillator strength. We conclude similar assignments as for PPV and emphasize the delocalized nature of basic features. An importan t and intriguing peculiarity of PPP- compared with PPV-based polymers is th e possible level inversion between excitons. Comparison is made to availabl e optical and electron energy loss spectroscopy (EELS) data. Our conclusion s are suggestive for future light-polarized experiments. (C) 1999 Elsevier Science S.A. All rights reserved.