Excited-State Interactions in Diastereomeric Flurbiprofen-Thymine Dyads

Excited-state interactions between (S)- or (R) fluorescence (S)- or (R)-FBP) and thymidine (dThd) covalently linked in dyads 1 or 2 have been investigated. In both dyads, the only emitting species is (FBP)-F-1*, but with a lower fluorescence quantum yield (phi(F)) and a shorter fluorescene lifetime (tau(F)) than when free in solution. These an results indicate that dynamic quenching occurs either by electron transfer or via exciplex formation, with FBP as the charge-donating species. In acetonitrile, both mechanisms are favored, while in dioxane exciplex formation is predominating. Isomer 1 displays lower values of OF and rF than its analogue 2, indicating that the relative spatial arrangement of the chromophores plays a significant role. The triplet quantum yields (0-F) of 1 and 2 are significantly higher than the expectations based solely on 'FBP* dThd intersystem crossing quantum yields (Oisc), with OT (1) > OT (2). This can be explained in terms of intramolecular charge recombination at the radical ion pairs and/or the exciplexes, which would be again dependent on geometrical factors. The triplet lifetimes (IT) of 3FBP* dThd and free 3FBP* are similar, indicating the lack of excited-state interactions at this stage. The FBP dThd dyads could, in principle, constitute appropriate model systems for the elucidation of the excited-state interactions in noncovalent DNA ligand complexes.