[Show abstract][Hide abstract] ABSTRACT: Molecular quantum chemical calculations were performed both at the ab initio and at the semi-empirical level to model the molecular conformations and electronic structure of oligomers of poly(phenylene vinylene) during the early stages of interface formation with Al, Ca and Mg. We found that the divalent metals, Mg and Ca, disrupt the conformation of the oligomers less than Al does. The highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) move into the energy gap both for Ca- and for Mg-doped systems, resulting in gap-state formation. This is consistent with the polaron/bipolaron picture. The electron density plots indicate that the de-localization of electrons is reduced more significantly by Al than it is by Ca and Mg. Our simulation results have been confirmed experimentally via XPS and NEXAFS.
Full-text · Article · Dec 1998 · Journal of Physics D Applied Physics
[Show abstract][Hide abstract] ABSTRACT: We have studied the radiation-induced photoluminescence (PL) quenching and the associated electronic structure modification of a light-emitting conjugated oligomer, namely, 1,4-bis[4-(3,5-di-tert-butylstyryl) styryl]benzene (4PV), in the absence of oxygen. We found that a short period of exposure to the radiation sources used in UV or x-ray photoelectron spectroscopies (UPS or XPS) resulted in total PL quenching in 4PV. Although there was no detectable change in the XPS C 1s core-level spectra UPS valence spectra indicated the disappearance of the vinylene peak upon exposure to UV radiation. This suggests the destruction of the vinylene groups, and the possible creation of radical species, which may serve as PL quenching sites. We also found that the modification in the electronic structure by Ca deposition masks that by radiation, although both quenches the PL effectively.
Full-text · Article · Jun 1997 · Physical Review B