Density functional theory and time-dependent density functional theory calculation for a series of photophysical properties (absorption spectra, singlet and triplet excitation energies and spin–orbit matrix elements) have been performed on the sulfur-, selenium- and tellurium-substituted thymine. The heavy atoms have been substituted in 2 or 4 and in both 2,4 position of the thymine ring. Different pathways for the population of the lowest triplet state have been considered. We find that all the considered systems are potential UVA chemotherapeutic agents since the lowest triplet states lie above the energy required for the production of the highly cytotoxic 1 Δ g excited oxygen molecule and due to the possible and efficient intersystem crossings ensured by high spin–orbit coupling values.
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