Correlation between optical properties and chain-length in a quasi-one-dimensional electronic polymer

We study the molecular weight dependence of the photophysics of a soluble electroluminescent conjugated polymer DOO-PPV in dilute chloroform solution. The absorption and the fluorescence maxima red-shift progressively with increasing weight as the average chain length increases from approximately 9 to 30 monomer units. At the same time, the fluorescence yield of the polymer decreases even though the radiative rate is found to increase. This behavior is caused by a rapid increase in nonradiative decay as the optical gap of the polymer decreases, and we present a quantitative theory that satisfactorily accounts for the dominance of multiphonon relaxation via internal conversion. We also find that the polymer length at which the optical properties saturate is considerably longer than predicted by theoretical calculations of electronic coherence length or by modeling of absorption data using the Strickler-Berg relationship.