Topology and Excited State Multiplicity as Controlling Factors in the Carbazole-Photosensitized CPD Formation and Repair

Photosensitized thymine<>thymine (Thy<>Thy)
formation and
repair can be mediated by carbazole (Cbz). The former occurs from
the Cbz triplet excited state via energy transfer, while the latter
takes place from the singlet excited state via electron transfer.
Here, fundamental insight is provided into the role of the topology
and excited state multiplicity, as factors governing the balance between
both processes. This has been achieved upon designing and synthesizing
different isomers of trifunctional systems containing one Cbz and
two Thy units covalently linked to the rigid skeleton of the natural
deoxycholic acid. The results shown here prove that the Cbz photosensitized
dimerization is not counterbalanced by repair when the latter, instead
of operating through-space, has to proceed through-bond.