Dispersion of resonant Raman scattering in luminescent and non-luminescent conjugated polymers

We found that luminescent and non-luminescent pi-conjugated polymers can be distinguished by their remarkably different behavior of the phonon frequency dispersion measured by resonant Raman scattering (RRS). Non-luminescent conjugated polymers, like trans-polyacetylene, show strong phonon frequency dispersion, whereas strongly luminescent conjugated polymers, like PPV, show a weak RRS dispersion. We show that the RRS dispersion is not directly due to the commonly assumed variation of the optical gap with the conjugation length, but is mainly due to that of the first optically forbidden state. In luminescent conjugated polymers, the optically allowed state is the lowest lying excited state which varies with the conjugation length (as observed by the broad absorption spectrum), yet the RRS shows only a weak dispersion. In non-luminescent polymers, on the other hand, the lowest excited state is optically forbidden, and they show strong RRS dispersion.