  We report the symmetrized density matrix renormalization group (DMRG) study
of neutral and doped oligomers of PPP system within an extended Hubbard model.
Model parameters are determined by comparing the existing results for
interacting small system. We compute a number of properties in the ground state
as well as in the one-photon, two-photon and triplet states to completely
characterize these states. Based on the relative positions of the one-photon
and two-photon states in long oligomers, we suggest that invoking interchain
interactions is essential for explaining strong fluorescence in this system.
Bond-order studies show that the lowest two-photon state corresponds to a
localized excitation while one-photon and triplet excitations are extended in
nature. The bipolaronic state shows clear evidence for charge separation and
disproportionation into two polarons. We find that the extended nature of
one-photon and triplet states of neutral system are very similar to those of
the bipolaronic ground states.
