The reaction pathway for the photochemical formation of thymine-thymine (6-4) dimers in DNA is explored using hybrid density functional theory techniques in gas and in bulk solvent. It is concluded that the photo-induced cycloaddition displays favorable energy barriers in the triplet excited state. The stepwise cycloaddition in the triplet excited state involves the initial formation of a diradical followed by ring closure via singlet-triplet interaction. The key geometric features and electron spin densities are also discussed. The difference in barriers of H3' transfer for the lowest-lying triplet and singlet states shows that the singlet oxetane intermediate could catch the second photon to accelerate the rate of proton transfer, leading to formation of the Dewar structure. The present results provide a rationale for the formation of thymine-thymine (6-4) dimers in the triplet excited states.
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