LUMINESCENCE EXCITATION SPECTROSCOPY IN POLY(P-PHENYLENE-VINYLENE) DERIVATIVES - IMPLICATIONS FOR A SEMICONDUCTOR BAND VS EXCITON MODEL IN CONJUGATED POLYMERS

High-resolution luminescence excitation spectroscopy (site-selective f lourescence) results are presented for two soluble derivatives of poly (p-phenylene-vinylene): poly(2-methoxy-5-(2'-ethylhexyloxy)-p-phenylen e vinylene) (MEH-PPV), and poly(2,5-bis(cholestanoxy)-p-phenylene viny lene) (BCHA-PPV). Measurements have been performed on both thin films of these polymers, and dilute blends in polyethylene oriented by tensi le drawing. By examining the effects of oxygen exposure in cast films, and the energy-dependence of the luminescence anisotropy in the orien ted blends, we provide evidence that the electronic states in the abso rption tail arise from localized states in an amorphous semiconductor.