Polarized ultraviolet absorption by a highly oriented dialkyl derivative of poly(paraphenylene vinylene)

We present polarized ultraviolet absorption spectra obtained from films of poly[2-butyl-5-(2' ethyl)-hexyl paraphenylene vinylene], BuEH-PPV, in polyethylene; the films were oriented by gel processing to high draw ratios. The spectra show three strong features between 2 and 6.2 eV. The lowest-energy absorption is peaked at 2.9 eV and is strongly polarized parallel to the draw axis. At 4.8 eV, there is a sharp onset for perpendicular absorption, indicative of a transition from a localized state to a delocalized state (or vice versa). Finally, there is a predominantly parallel-polarized absorption band at 5.8 eV that we attribute to excitations localized on the phenyl ring. Since the aliphatic side chains have only weak electronic interactions with the conjugated polymer backbone, the dialkyl-substituted PPV's are excellent model systems for studying the electronic structure of PPV.