CHAPTER 4: Triplet Channel Photochemistry in DNA

This chapter deals with the photochemistry of DNA triggered by the triplet excited state of its nucleobases. It is focused on the pyrimidine (mainly thymine) units and includes both the triplet population by intersystem crossing and the more efficient photosensitized processes. Emphasis is placed on recent findings related to mechanistic aspects such as insight from up-to-date theoretical calculations, direct detection of transient species by ultrafast spectroscopy, DNA lesions as potential photosensitizers, triplet energy migration along DNA, and biphotonic chemistry from upper triplet states. Cyclobutane pyrimidine dimerization is presented as the paradigmatic triplet-mediated chemical reaction; however, the possible formation of other types of dimers from the triplet excited state, such as (6-4) photoproducts and spore photoproducts, is also discussed.