Electronic and geometric structures of oligothiophenes studied by UPS and MNDO: π-band evolution and effect of disorder

Abstract

Ultraviolet photoelectron spectroscopy (UPS) has been applied to the investigation of the electronic structure of oligothiophenes with 4-8 thiophene ring. In a series of α-linked oligomers (αn, with the number of rings n = 4-8), a systematic evolution of the π band forming levels are observed in the region of 0.7-3eV below the Fermi level (EF) and the bandwidth becomes broader with n. The non-bonding π band is observed at 3.5eV below EF and its energy is almost independent of n. UPS spectra of α7 and α8 are fairly similar to the spectra of polythiophene, showing that these oligomers are good model compounds of the polymer. The effect of irregularity on the π-electron system was also studied by using oligomers which contain a β linkage or a vinylene group in the middle of the molecule. The UPS spectra showed that the β linkages significantly affect the electronic structure of polythiophene, but vinylene does not. In order to analyze the UPS spectra and to investigate the electronic structures of oligomers, the orbital energies and the geometry of these oligomers are calculated by the semi-empirical modified neglect of diatomic overlap self-consistent field molecular orbital (MNDO-SCF-MO) method. The calculated spectra using the obtained orbital energies agree well with the observed ones, particularly in the π region. It is shown from the optimized geometry that (1) αn's have planar structures and π electrons can be delocalized, (2) the oligomer with β-linkages have nonplanar structure leading to limited delocalization of π-electrons, and (3) the oligomers with a vinylene group are almost planar and the disturbance by the vinylene group on the delocalization is small.