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Biscarbazolylmethane-based cyanine: a two-photon excited fluorescent probe for DNA and selective cell imaging.

Here, we have introduced a novel biscarbazolylmethane-based cyanine as a two-photon excited fluorescent probe, 6,6'-bis[2-(1-methylpyridinium)vinyl]-bis(9-methyl-carbazol-3-yl)methane diiodide, which has two vinylpyridinium carbazole moieties connected by a non-rigid methylene bridge. This molecule possesses a larger Stokes shift and enhanced two-photon absorption cross-section than the previously reported vinylpyridinium carbazole monocyanine, which is mainly attributed to the "through-space" type intramolecular charge transfer. The low fluorescence quantum yield and 30-fold fluorescence enhancement once binding with calf thymus DNA highlight this molecule as a promising fluorescence light-up probe for DNA. The obvious induced circular dichroism signals have proved that the molecule with soft-connected bis-cationic centers can specifically interact with various DNA structures. Cell viability study shows that the probe has very low cytotoxicity. The probe exhibits high staining selectivity for mitochondria in living HeLa cells. Its capability to stain nuclear DNA has been confirmed by fixed cell staining. Furthermore, the application for two-photon excited fluorescence imaging demonstrates high potential of the probe for nonlinear bioimaging with 3D resolution.