EXPERIMENTAL AND THEORETICAL-STUDIES OF THE ELECTRONIC-STRUCTURE OF POLY(P-PHENYLENEVINYLENE) AND SOME RING-SUBSTITUTED DERIVATIVES

The electronic structure of a conjugated polymer of current interest i n organic LED's, poly(p-phenylenevinylene), or PPV, has been studied b y ultraviolet photoelectron spectroscopy and X-ray photoelectron spect roscopy. The focus of this work is on the nature of the pi-electronic band structure nearest the Fermi level and the physical influence of f inite torsion angles, the geometry of the polymer backbone, on the ele ctronic properties of the system. Details of the ct-electronic bands, as reflected in the associated density-of-states, are observed clearly in the spectra, from which some underlying geometrical details of the polymer system can be deduced. The experimental spectra have been ana lyzed theoretically using band structure calculations based upon the v alence effective Hamiltonian (VEH) model. In addition, in order to con trol the band structure, three ring-substituted derivatives of PPV, ea ch of which induces a different bonding geometry in the backbone, have been studied. The changes in the experimental results can be explaine d on the basis of both physical and chemical interactions of the subst ituents with the backbone, which lead to geometrical changes along the backbone, which influence the ct-bandwidths and contribute to differe nces in both the optical absorption threshold and the binding energy o f the valence band edge.