TDDFT STUDY OF NUCLEOBASE THIOANALOGUES AND OXO-DERIVATIVES EXCITED STATES

Thioanalogues of nucleic-acid bases have been shown to be the place of preferential charge or energy localization when incorporated in the system of respective regular bases. Most evidence of such behavior is based on experiments using gamma-ray or other ionizing radiation. Here time dependent density functional theory (TDDFT) has been applied to investigate singlet and triplet excited state spectra of thioanalogues and some oxo-derivatives of nucleic-acid bases and compare the spectra to the calculated spectra of the regular bases as well as to the existing experimental data. It has been shown that thioanalogues have lower excitation singlet and triplet energies than are the respective energies in the regular bases. Energy localization on thioanalogues and their spectroscopic properties are related to the effect of sulfur substitution in such systems.