Thymine plays a central role in DNA photochemistry. Indeed, direct absorption of UV-light leads to thymine dimerization, producing either cyclobutane pyrimidine dimers (CPD) or 6-4 photoproduct (64-PP) as the most common lesions. The former damages are also toxic when they accumulate, as they are either repair resistant or highly mutagenic and are correlated to the development of malignant skin cancers as the result of unprotected exposure to sunlight. Furthermore, thymine is also an hot spot in DNA photosensitization. . Indeed, its triplet state, the lowest among the natural nucleobases, can be populated via triplet-triplet energy transfer from the excited sensitizer. This process can also be facilitated by the additional stabilization of thymine triplet ( 3 T) due to ⇡ stacking, and the related multichromophoric coupling in the DNA helical structure. Thymine photochemistry has been explored as early as 1960s 11 , but the presence of 3 T 12 has triggered renewed attention, notably for delineating the mechanism of triplet-triplet energy transfer with ex-ternal photosensitizers , or with modified nucleobases and oxidative lesions recognized as Trojan Horses . It has also been shown that the population of T opens rather e cient photochemical channels for dimerization, and especially for the production of 64-PP. The energy of T in a DNA strand has been inferred experimentally by Bosca et al. by assessing the energy transfer e ciency of di↵erent photosensitizers, such as norfloxacin and its N(4 0 )acetyl derivative, whose energies are precisely known . The assumption being that only those sensitizers whose triplet state is higher than the one of T embedded in the DNA environment will be able to induce energy transfer. This approach has provided a reference value of 270 kJ.mol 1 , which confirms the stabilization of T in DNA and provides a macroscopic average in a complex environment. Yet this value encompasses a manifold of di↵erent nucleobases whose direct surroundings di↵ers largely. Indeed, the average value masks many crucial aspects that may modify the local energy alignment, and hence may influence the transfer process.These includes the nature of the adjacent bases and sequence e↵ects, the helical embedding and its structural variability, the di↵erential screening induced by the DNA backbone, the presence of additional charges due to salt concentration or other charged species, etc. Furthermore, in biological media the situation may be complicated even further by the presence of proteins interacting with DNA, which may induce structural and electronic constraints. As a matter of fact, sequence dependence is known to impact energy transfer in DNA oligonucleotides 18 . The presence of adjacent thymines may be particularly relevant in stabilizing 3 T due to the formation of excitonic or excimeric intermediates presenting a delocalization of the spin density over multiple bases. In turn, the presence of delocalized states, may alter the e ciency of triplet triplet energy transfer, and hence the photosensitization e ciency, but also the rate of 3 T ! 1 T transition, influencing the subsequent DNA photochemistry.