Interaction of the Triplet State of Biscarbocyanine Dye with a Nitroxyl Radical
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Biscarbocyanine dyes involves two conjugated chromophores that causes the splitting of the excited singlet state level in these molecules as a result of dipole–dipole coupling and electron tunneling through the central heterocycle [1, 2]. The appearance of a new intense absorption band in the red and NIR spectral range coupled with a high quantum yield of intersystem crossing (ISC) opens the possibility to use biscarbocyanines as promising photosensitizers in photodynamic therapy (PDT) [3, 4, 5, 6]. The relatively high quantum yield of the triplet state distinguishes biscarbocyanines from many cyanine dyes, for which the low quantum yield of the triplet state is caused by efficient internal conversion (IC) via the mechanism of fast photoisomerization around the long polymethine chain . Recently, it has been shown  that the introduction of nitroxyl radical into the chromophore structure with partial cyclization of the polymethine chain increases the yield of the triplet state due...
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