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Abstract

Biological microscopy favors photostable fluorescent markers with large fluorescence quantum yields, low dark triplet state population, good biocompatibility and absorption and emission maxima in the near-infrared, where cellular autofluorescence is minimized. In the present study, carbopyronines absorbing around 640 nm and emitting at around 660 nm, with a low intersystem crossing rate (kisc ≈ 0.5 e6/s) and excellent properties for cellular imaging were synthesized. A general synthetic route to carbopyronines with functional groups variable in the final steps of the synthesis or in the resulting fluorescent dye is presented. Possessing two 2-methoxyethyl groups, the parent dye is soluble in water and most organic solvents. Demethylation of the dye or its precursors is straightforward, clean, and furnishes compounds with one or two 2-hydroxyethyl groups, which can be used for further transformations. Modifications in the linker-containing carboxy group are also possible. A multistep synthesis of the dye starting from a simple precursor and utilizing a single temporary protective group is described. The presented approach may be further applied to the design of caged carbopyronines.

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