Ultraviolet photoemission and electron loss spectroscopy of oligothiophene films

Ultraviolet photoelectron spectroscopy (UPS) has been used to study the evolution of the valence electronic states as a function of conjugation length for thiophene, bithiophene, terthiophene, and sexithiophene films deposited in-vacuum on gold substrates at 130 K. The peaks due to localized thiophene ring electronic states are found to be at ca. 8 eV below the vacuum level, regardless of the number (n) of thiophene rings in the chain. Peaks due to orbitals delocalized along the oligomer backbone progress to lower binding energies as the chain length increases from n = 2 to 6. Electron loss spectroscopy (ELS) has also been performed using an electron gun typically employed for Auger electron spectroscopy, and it is found that the lowest energy loss feature shifts to lower energy as conjugation length increases. The results obtained are in general agreement with theoretical and higher resolution experimental studies, and the thresholds of the lowest energy ELS peaks provide estimates of the bandgaps of the films. These results show that the combination of UPS and ELS provides a convenient means of evaluating the conjugation length of short-chain oligothiophenes.