A Versatile Route to Red‐Emitting Carbopyronine Dyes for Optical Microscopy and Nanoscopy

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 ( k isc ≈ 0.5 × 10 6 s –1 ) 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.