Relaxation Process of Photoexcited <i>meso</i>-Naphthylporphyrins while Interacting with DNA and Singlet Oxygen
Generation
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Abstract
Electron
donor-connecting cationic porphyrins <i>meso</i>-(1-naphthyl)-tris(<i>N</i>-methyl-<i>p</i>-pyridinio)porphyrin
(1-NapTMPyP) and <i>meso</i>-(2-naphthyl)-tris(<i>N</i>-methyl-<i>p</i>-pyridinio)porphyrin (2-NapTMPyP) were
designed and synthesized. DFT calculations speculate that the photoexcited
states of 1- and 2-NapTMPyPs can be deactivated via intramolecular
electron transfer from the naphthyl moiety to the porphyrin moiety.
However, the quenching effect through the intramolecular electron
transfer is insufficient, possibly due to the orthogonal position
of the electron donor and the porphyrin ring and the relatively small
driving force: Gibbs energies are 0.11 and 0.07 eV for 1- and 2-NapTMPyPs,
respectively. It was speculated that more than 0.3 eV of the driving
force is required to realize effective electron transfer in similar
electron-donor connecting porphyrin systems. These porphyrins aggregated
around the DNA strand, accelerating the deactivation of their excited
singlet state and decreasing their photosensitized singlet oxygen-generating
activities. In the presence of a sufficiently large concentration
of DNA, these porphyrins can bind to a DNA strand stably, leading
to an increased fluorescence quantum yield and lifetime. Singlet oxygen
generation was also suppressed by the aggregation of porphyrins around
DNA. Although the quantum yield of singlet oxygen generation was recovered
in the presence of sufficient DNA, the singlet oxygen generated by
DNA-binding porphyrins was significantly smaller than that without
DNA. These results suggest that DNA-binding drugs limit the generation
of photosensitized singlet oxygen by quenching the DNA strand