Porphyrin–Coumarin Dyads: Investigation of Photophysical Properties and DNA Interactions

Two new nonconjugated porphyrin–coumarin dyads with different orientations with respect to donor–acceptor entities and their zinc complexes were synthesized. Single-crystal structures of the free-base porphyrin–coumarin dyads were successfully resolved. The absorption spectra of the dyads were linear combinations of the spectra of their corresponding monomers, indicating a negligible electronic communication between the coumarin and porphyrin moieties. However, the fluorescence emission of the coumarin entity in all of the dyads was quenched significantly compared to that of pristine coumarin, and this effect was attributed to intramolecular energy transfer from the coumarin to the porphyrin. The energy transfer kinetics from the coumarin to the porphyrin was shown to be fast (<i>k</i>_Förster = 1.13 × 10¹³ s^–1 for the ortho-isomer and 5.13 × 10¹¹ s^–1 for the para-isomer in DMF) and efficient (transfer efficiency ca. 96–97%). Transient absorption studies showed that the excited state decay process (S₂ → S₁*, S₁* → S₁, S₁ → S₀, and S₁ → T₁) of the para-isomer was faster than that of the ortho-isomer in DMF. All of the synthesized dyads were tested for their interactions with ct-DNA and photocleavage activity toward PBR322-DNA. The results revealed that all of the dyads interacted with ct-DNA via only an external groove-binding mode; the binding constants were calculated to be 3.24 × 10⁵ (<b>3a</b>), 3.05 × 10⁵ (<b>3b</b>), 3.04 × 10⁵ (<b>4a</b>), and 4.88 × 10⁵ (<b>4b</b>), and the photocleavage activity was in the order <b>4b</b> < <b>3b</b> < <b>4a</b> < <b>3a</b>. Furthermore, only the zinc complexes of the porphyrin–coumarin dyads could be absorbed by tumor cells (A549). These complexes were mainly localized in the cytoplasm, exhibited red fluorescence, and showed low cytotoxicity toward all of the tumor cell lines tested. The results showed that these zinc complexes of the porphyrin–coumarin dyads have potential applications in fluorescence imaging