Conformational Effects on the Photophysics of Conjugated Polymers:  A Two Species Model for MEH−PPV Spectroscopy and Dynamics

We report transient and steady state photoluminescence results along with absorption and NMR data to support the existence of two distinct morphological species in MEH−PPV solutions. NMR data provide evidence for the close packing of polymer chains, a consequence of solvent quality reduction. These data are correlated with optical properties of the aggregated species in poor solvents and the isolated chains in good solvents. We infer that steric hindrance of backbone motions increases effective conjugation length and leads to a spectral red shift in absorption and emission. At the same time, interchain excitations with negligible luminescence can be formed, leading to a dramatic reduction in photoluminescence quantum yield. While spectral changes are observed as packing is induced, we show that interchain state formation and its subsequent back-transfer to excitons are particularly sensitive to the interchain registry of the highly packed chains.