A bright red fluorescent cyanine dye for live-cell nucleic acid imaging, with high photostability and a large Stokes shift.

Many probes for nucleic acids are available, but few of them satisfy multiple criteria, particularly high photostability to endure laser scanning. We report a cyanine dye TO3-CN for the first time, synthesised by introducing a CN group to the trimethine chain of the classical red emitting TO-3 dye to improve its photostability, as well as their spectral properties and interaction with nucleic acids. TO3-CN shows excellent light fastness and a large fluorescence Stokes shift (more than 40 nm). Because of its sensitive fluorescence response to nucleic acids with a large fluorescence quantum yield (more than 0.7) and low cytotoxicity, this dye may be a potential candidate for nucleic acid detection in vitro and intracellular fluorescence imaging.