ELECTRONIC-STRUCTURE OF BIS[1,2,5]THIADIAZOLO-P-QUINOBIS (1,3-DITHIOLE) (BTQBT) STUDIED BY ULTRAVIOLET PHOTOEMISSION SPECTROSCOPY

Ultraviolet photoemission spectroscopy (UPS) has been used to investigate the electronic structure of the unusually high conductive organic compound, bis[1,2,5]thiazolo-p-quinobis(1,3-dithiole) (BTQBT). The observed spectral features were analyzed with the aid of the semiempirical MNDO-SCF-MO (modified neglect of diatomic overlap self-consistent field molecular orbital) method. BTQBT shows a low photoionization threshold of 4.57 eV in the solid state as compared with the gas phase value of 6.4 eV which is evaluated from the oxidation potential. This lowering from the ps phase can be explained with respect to the strong intermolecular interactions: if we take the calculated band width of 0.7 eV, the polarization energy and the other solid state effects of about 1.5 eV contribute to this lowering. The work function of BTQBT was found to be 4.3 eV. From this value and the ionization threshold energy, the energy difference between the Fermi level and the top of the valence band is determined to be 0.3 eV. This small value, in agreement with the observed activation energy in electric conduction, should be at least part of the origin of the high conductivity in BTQBT. These observations show that the strong intermolecular interaction in the solid state of BTQBT, suggested from the crystal structure of this compound, is essential for the high conductivity of this compound.