Using quantum mechanical methods, the ionization potentials of a number of polyacenes, phthalocyaninates, and composite materials were calculated. It was shown that, in accordance with Koopmans' theorem, the ionization potentials are well approximated by the highest occupied molecular orbital of the system. This assessment is relevant for determining the work function of composite materials consisting of many atoms, for which ionized states are difficult to calculate. Calculations were performed using the Hartree-Fock method and the electron density functional theory with the B3LYP hybrid potential, taking into account various basis sets. The obtained results are in good agreement with existing experimental data. It has been theoretically shown that applying carbon-containing structures to the metal surface reduces the electron work function of the heteromaterial surface. The proposed method can be applied to determining the ionization potentials and work functions of other nanostructures.