A rather large panel of experimental data are available to assess the decay channels of 3 T leading either to the population of the singlet ground state or to the formation of photodam-ages, for isolated nucleotides or in single-stranded DNA thymines . In parallel, several computational studies have considered 3 T embedded in B-DNA, to assess characteristic energies and lifetimes for triplet-triplet energy transfer, either with photosensitizers or along thymine stacks . In 2011, the characteristic time for triplet migration was estimated at 6.4 ns by semi-empirical quantum chemical methods coupled to classical molecular-dynamics simulations . The photochemistry of isolated thymine has also been intensively studied by molecular modeling using either static of dynamics approaches. Notably, it has allowed to rationalize the quantum yield of the population of the lowest triplet state or the evolution of Watson-Crick pairing . The evolution of the long-lived 3 (⇡ ! ⇡ ⇤ ) state has been shown to have a characteristic time which may encompass the nanosecond range, as it has also been confirmed by the study of the decay in DNA single strands , for thymine and their analogous. , Furthermore, the lifetime has also shown a dependency on the possible formation of excimeric states characterized by the delocalization of the triplet spin density over several stacked nucleobases. However the precise amount of such delocalization remains to be quantified.