Theoretical simulations of UV-Vis and UP spectra for conjugated systems

Due to their unique electro-optical properties, there has been a great deal of scientific interest in electronic structure of conjugated systems. In order to reveal the complete map of their electronic structure, several experimental investigations are done using UV-Vis and ultraviolet photoelectron spectroscopy (UPS). The experimental findings are usually interpreted by the results of quantum chemical calculations. In this study, we present the theoretical simulations of UV-Vis and UP spectra of conjugated systems by using density functional theory (DFT). In UV-Vis simulations, we investigated the excited states of oligothiophene anions and cations and almost identical UV spectra were obtained for these systems. This similarity in excitation energies are explained by the resemblance in energy levels and nature of excited states in anions and cations. In UPS simulations, the energy levels of conjugated systems were calculated by using ∆SCF/TDDFT and DFT orbital eigenvalues. It is shown that there is a good agreement between ∆SCF/TDDFT and experiment, especially for the investigated oligomers. In contrast, DFT orbital energies are considerably lower than the experiment. However, spacing of energy levels is consistent with both experiment and ∆SCF/TDDFT.