Methyl Viologens of Bis-(4’-pyridylethynyl)arenes - Structures, Photophysical and Electrochemical Studies, and their Potential Application in Biology
Résumé
A series of bis-(4’-pyridylethynyl)arenes (arene = benzene, tetrafluorobenzene, and anthracene) were synthesized and their bis- N -methylpyridinium compounds were investigated as a class of π-extended methyl viologens. Their structures were determined by X-ray diffraction, and their photophysical and electrochemical properties as well as their interactions with DNA/RNA were investigated. The dications showed bathochromic shifts in emission compared to the neutral compounds. The neutral compounds showed very small Stokes shifts, which are a little larger for the dications. The compounds showed very short fluorescence lifetimes (< 4 ns). The neutral compound with an anthracene core has a quantum yield of almost unity. With stronger acceptors, the analogous dicationic compound showed a larger 2-photon absorption (TPA) cross-section than its neutral precursor. All of the dicationic compounds interact with DNA/RNA; while the compounds with benzene and tetrafluorobenzene cores bind in the grooves, the one with an anthracene core intercalates as a consequence of its large, aromatic linker moiety, and it aggregates within the polynucleotide when in excess over DNA/RNA. All cationic compounds showed highly specific CD spectra upon binding to ds-DNA/RNA, attributed to the rare case of forcing the planar, achiral molecule into a chiral rotamer, and negligible toxicity toward human cell lines at ≤ 10 µM concentrations. The anthracene-analogue exhibited intracellular accumulation within lysosomes, preventing its interaction with cellular DNA/RNA. However, cytotoxicity was evident at 1 µM concentration upon exposure to light, due to singlet oxygen generation within cells. These features, in combination with its TPA properties, suggest it to be a promising lead compound for development of light-activated theranostic agents.
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