Optical properties of conjugated polymers

Electroluminescent conjugated polymers, particularly those of the PPV class, are promising candidates for light emitting display applications. In the solid state, the polymers have reduced quantum efficiency because of the formation of weakly emissive or nonemissive interchain photoexcitations. We have observed and quantified interchain emission in MEH-PPV for the first time. We conclude that excimers that are nearly nonemissive at ambient temperature can form with quantum yields approaching 50%. Excimers form because the alkoxy substutients on the MEH-PPV backbone splay out and allow cofacial approach of the polymer backbones giving rise to π - orbital overlap. We hypothesized that separation of polymer backbones in two dimensions is not sufficient to eliminate interchain interactions. To this end we have studied PPV with bulky, dendritic sidegroups that allows three-dimensional separation of polymer chains. We show that fewer interchain species are formed and consequently observe a four-fold increase in quantum efficiency.; Optical spectroscopic studies of the polymers in solution revealed the importance of morphology on the photophysics of the polymers. Systematic variations of the solvent environment by changing the temperature and quality of the solvent induced or restricted the motions of the polymers. These changes were reflected in the absorption and photoluminescence of the polymers. We were able to reproduce our data quantitatively by hypothesizing two distinct configurations of polymers. Each of these species has distinct absorption and photoluminescence spectra, luminescence decay dynamics and luminescence quantum yield. These data suggest one morphology where the polymer chains behave independently of the surrounding polymer and another where the polymer chains are aggregated. Polymers in the aggregated condition have analogous photophysics to the polymers in the solid state and reduced photoluminescence. The fourfold increase in quantum yield in the first generation dendritic PPV can be attributed to the presence of a significant amount of polymer in the isolated configuration.; The first generation dendritically substituted PPV has some liquid crystalline order and can be shear oriented. Selective excitation and observation of luminescence parallel and perpendicular to the backbones lead to behaviors distinct from the unoriented samples of the same polymer. We understand these differences in terms of selective excitation into distinct morphological regions of the polymer.