Polarized ultraviolet absorption by an oriented derivative of poly(para-phenylene)

We present polarized ultraviolet (UV) absorption spectra of oriented poly (9-hexyl-9-(2'-ethyl-hexyl) -fluorene-2, 7-diyl). Samples oriented by gel processing in polyethylene show polarized photoluminescence and dichroism in the near and deep UV. The spectra consist of four distinct absorption bands with characteristic polarizations. The lowest energy band, polarized parallel to the draw axis, peaks at 3.3 eV. The second transition is at 5.3 eV and polarized primarily perpendicular to the draw axis. There are two strong absorption bands in the deep UV, one at 5.7 eV with polarization parallel to the chains and the other at 6.2 eV with off-axis polarization. Electronic-structure calculations based on dimers of the poly(para-phenylene) repeat unit with parameters estimated from the biphenyl spectra and supplemented by inclusion of the Coulomb attraction between electrons and holes successfully predict the energies and polarizations of these spectral features. Quantitative agreement of parameters that are extracted from the data indicates that the one-electron band structure is an appropriate starting point for calculating the electronic structure of phenylene-based polymers. [S0163-1829(99)08235-1].