Since the discovery of light emitting diodes based on PPV in 1990

PPV and its derivatives have been investigated widely with experiments or theories due to their electroluminescence properties

but the excited state and the interchain species of them remain controversial. To study the nature of the interchain species

the optical properties of the isolated MEH-PPV chains in the dilute solutions are investigated. It has been found that the fluorescence of conjugated polymers is heavily influenced by the conformations of the polymer chains. The polymer chains become more tightly coiled in the poor solvent to make more conjugated segments aggregated and then to form more interchain excitons under photoexcitation. With increasing the proportions of the poor solvent in the dilute solutions

the intensities of the highest PL peaks (P

1

) (emission from intrachain excitons) are reduced due to the increase of conformational defects in the polymer chains

and the relative intensities of the peaks at about 590 nm (P

2

) (emission from interchain excitons) are increased. From the absorption and photoexcitation spectra of the dilute solutions

it can be found that the intensities of absorption peaks and photoexcitation at long wavelength are increased with increasing the proportions of the poor solvent in the dilute solutions. By analyzing the absorption and the luminescence spectra of MEH-PPV dilute solutions

it can be concluded reasonably that the formation processes of interchain exciton are dependent on the photoexci-tation energies. For hν

>

E

a

a majority of intrachain excitons are relaxed by radiative recombination

but a few of intrachain excitons can be transferred to the aggregated segments along the polymer chains

then π electrons are delocalized over the aggregated segments to form interchain excitons. In other words

interchain excitons have same photoexcitation process as intrachain excitons. For hν

<

E

a

the aggregated segments in the single chain can absorb photon energies to form interchain excitons.