Excited-state interactions in diastereomeric flurbiprofen-thymine dyads.

Excited-state interactions between (S)- or (R)-flurbiprofen ((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 (1)FBP*, but with a lower fluorescence quantum yield (φ(F)) and a shorter fluorescence lifetime (τ(F)) than when free in solution. These 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 φ(F) and τ(F) than its analogue 2, indicating that the relative spatial arrangement of the chromophores plays a significant role. The triplet quantum yields (φ(T)) of 1 and 2 are significantly higher than the expectations based solely on (1)FBP*-dThd intersystem crossing quantum yields (φ(ISC)), with φ(T) (1) > φ(T) (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 (τ(T)) of (3)FBP*-dThd and free (3)FBP* 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.