The results on comparing the peak structure of the density of unoccupied electronic states (DOUS) of ultrathin films of naphthalene anhydride-1,4,5,8-teracabonic acid (NTCDA) and naphthalene-1,8-dicarboxylic acid anhydride (NDCA) and of two types of phthalide-based films: 3,3-bis (phenyl) phthalide (DPP) and 3,3-bis (phenyl) phthalide-4 ′, 4′-dicarboxylic acid (DPP-DCA) are presented. The measurements of the structure of the unoccupied electronic states in the energy range from 5 eV to 20 eV above the Fermi level of the films studied having thickness of 8–10 nm were conducted using the total current spectroscopy (TCS) technique. Analysis of the experimental results was conducted using the model total current spectra and DOUS dependences generated using the calculated orbital energies of the studied molecules by means of the density functional theory (DFT) method at the B3LYP/6-31G(d) level. The difference in the DOUS spectra of NTCDA and NDCA films is characterized by the shift of the main DOUS maxima of the NTCDA film to lower energies by about 1 eV at energies less than 12.5 eV, and at higher energies the DOUS maxima are shifted by 1.5-2 eV. The energy positions of the maxima of the total current spectra of the DPP-DCA and DPP films practically do not change when using various substrates: highly ordered pyrolytic graphite (HOPG) and layer-by-layer deposited CdS. The relative intensities of the maxima differ when using different substrates. The characteristic shift of the maxima of the total current spectra of DPP-DCA films is about 1 eV at energies less than 12.5 eV above the Fermi level and 1.5-2 eV and at higher energies, compared with the position of the corresponding maxima of the DPP films.

The results of diagnostics of the atomic composition of a diphenylphthalide (DPP) film thermally precipitated in vacuum by the of X-ray photoelectric spectroscopy (XPS) method are presented. The results of examination of the unoccupied electronic states of the ultrathin DPP films with the thickness up to 10 nm on the surface of the highly oriented pyrolytic graphite (HOPG) by the total current spectroscopy (TCS) method in the energy range from 5 to 20 eV above E _F are presented. In this range, the main maxima in the total current spectra are identified. The analysis of the TCS results with consideration of the theoretical calculation results has shown that the low-energy maxima observed at the energies from 6 to 7.5 eV are induced predominately by π* electron orbitals of DPP films. The values of the energy E _vac in relation to E _F, i.e., of the electron work function in the DPP films at the film thickness of 5–10 nm, are found experimentally at a level of 4.3 ± 0.1 eV. The negative charge transfer from an organic film to the substrate corresponds to the formation of the HOPG/DPP boundary potential barrier during the thermal deposition of the DPP film.

The results of a study of the unoccupied electronic states of ultrathin films of bis-carboxyphenyl-phthalide (DCA-DPP) and bis-methylphenyl-phthalide (DM-DPP) up to 8 nm thick are presented. The studies were carried out by total current spectroscopy (TCS) technique in the energy range from 5 eV to 20 eV above EF during thermal vacuum deposition of these organic films on the surface of highly oriented pyrolytic graphite (HOPG). The energy Evac relative to EF, that is, the electronic work function of the DM-DPP films, at a film thickness of 5–8 nm was 4.3 ± 0.1 eV. The electronic work function of the DCA-DPP films was 3.7 ± 0.1 eV. The structure of the maxima of the unoccupied electronic states of DCA-DPP films and DM-DPP films in the studied energy range is determined. The properties determined of DCA-DPP and DM-DPP films are compared with the properties of films of unsubstituted diphenylphthalide (DPP). According to our analysis, –CH3 substitution of the DPP molecule practically did not affect the height of the potential barrier between the film and the HOPG surface, and –COOH substitution of the DPP molecule led to an increase in the height of the potential barrier between the film and the HOPG substrate surface by 0.5–0.6 eV. Substitution of DPP molecules with –COOH functional groups which represents formation of DCA-DPP molecules led to a shift of two peaks of the experimental total current spectra located at energies in the range from 5 eV to 8 eV above EF, by about 1 eV towards lower electron energies.