Unoccupied electronic states in polythiophene as probed by XAS and RAS

Ikeura, Hiromi*; Sekiguchi, Tetsuhiro  

Polythiophene is one of the typical conducting polymers. Unoccupied electronic states near the Fermi level of polythiophene polycrystalline powder were probed by S 1s X-ray absorption spectroscopy (XAS) and S, resonant Auger spectroscopy (RAS). Its overall XAS spectral shape is similar to those of 2,-2'-bithiophene and alpha-terthiophene polycrystalline powder. Any hybridization of the energy levels of thiophene unit was not observed in the -LUMO (the lowest unoccupied molecular orbital). It is found that the position of (C-C) resonance shifts to lower energy as increasing thiophene repeating units. This finding reflects the different bond lengths of C-C. Although and (S-C) states in XAS are not resolved due to their overlap of electronic transitions, those were clearly separated by plotting spectator Auger yields as a function of excitation energy. If the Auger spectator shifts of the corresponding states are different each other, such unresolved states in XAS can be distinguished by RAS technique. RAS also indicates that the -LUMO of polythiophene has localized character. The present study demonstrated that excitation energy-dependent RAS can overcome the limitation of conventional XAS. The obtained results suggest that polythiophene is poorly conjugated, where the delocalization is interrupted by the conformational disorder.