Electron spectroscopic investigations of bithiophen and sexiphenyl as model systems for conducting polymers

I investigated the electronic (BT, 6P) and vibronic (BT) structure of Bithiophene (B I) and Sexiphenyl (6P) as model-systems for conducting polymers experimentally and theoretically. The occupied and unoccupied states of undoped and Caesium-doped BT thin-films, prepared in situ under ultra high vacuum (UHV) conditions, were measured with UPS and IPES. With Density Functional Theory (program: Wien 95) the bandstructure and density of states (DOS) of a doped and undoped BT- molecular crystal was calculated. The UPS spectrum and the calculated DOS agree very well. With DGAUSS (program) the geometry optimized minimum-energy configuration and a vibration-analysis was calculated. The constituting orbitals and the symmetries of the UPS/IPES-bands and their changes on doping are shown. The vibronic structure of doped and undoped BT was measured with high resolution electron loss spectroscopy (HREELS). The normal modes are visualized and assigned to HREELS-peaks; the doping induced changes in HREELS reflect the geometry alteration of BT on doping. Doped (Cs) and undoped 6P-films was probed with ARUPS, EELS and TDS. 6P monolayer was defined in UPS-followed heating-series and measured with UPS and EELS. Doping-series of 6P as a function of Cs and Cs as a function of 6P was measured with UPS and EELS. Main results for BT: LUMO of undoped BT becomes HOMO of doped BT - seen in experiments and calculation; measured gap: 4 eV; intermolecular interaction < 0.4 eV, formation of bipolarons at very low doping levels; dipolactiv peak at 205 meV (interring vibration with A-symmetry ) dominates doped HREELS-spectrum; shift from 196 meV (undoped) to 205 meV (doped) proves charge transfer to interring-bond on doping. Main results for 6P:6P bonds via π-system to Ni (flat lying); no long range order in mono- and multilayer; in high Cs-doping regime two bipolarons are formed per molecule as shown in UPS and EELS- experiments; model is confirmed by MNDO-calculation. (author)