  We describe both pi-conjugated polymers and semiconducting single-walled
carbon nanotubes because of the remarkable similarities in their behavior under
photoexcitation. The common themes between these two seemingly different
classes of materials are pi-conjugation, quasi one-dimensionality, and strong
Coulomb interactions. Theoretical understanding of both systems requires going
beyond traditional band theory. We present our current understanding of the
electronic structures and excited state absorptions in both systems within a
common theoretical model. Our aim is to give physical interpretations of
experiments that have been performed and to give guidance to future
experimental work.
