Conformation and Energy Transfer in a Single Luminescent Conjugated Polymer

The photophysics and conformation of single long-chain 2,5-dioctyloxy-p-phenylenevinylene (DOO-PPV) polymers are investigated. The fluorescence intensity−time trace of an individual polymer contains abrupt quantized intensity changes superimposed on small gradual changes. Under the same processing conditions, the size of abrupt changes varies from individual to individual, varying between 0 and 100% of the total intensity. Polarization modulation indicates considerable orientation of absorption dipoles within the polymer. Time-dependent measurements indicate that, in the majority of polymers, absorption dipoles in those regions of the polymer responsible for abrupt intensity changes are arranged less anisotropically than in regions responsible for the gradual changes. Spectroscopic measurements show a greater spectral variation accompanying jumps than that accompanying gradual decay. This can be explained by the coexistence of extended regions and a core region. The first is characterized by a relatively greater alignment of absorption dipoles and multiple emitters, while in the second, absorption dipoles are more isotropically distributed and energy is efficiently funneled to a few emitting excitons. Within a single processing batch, the ratio of these two regions varies from individual to individual.