N -Cyanorhodamines: cell-permeant, photostable and bathochromically shifted analogues of fluoresceins.

Fluorescein and its analogues have found only limited use in biological imaging because of the poor photostability and cell membrane impermeability of their O -unprotected forms. Herein, we report rationally designed N -cyanorhodamines as orange- to red-emitting, photostable and cell-permeant fluorescent labels negatively charged at physiological pH values and thus devoid of off-targeting artifacts often observed for cationic fluorophores. In combination with well-established fluorescent labels, self-labelling protein (HaloTag, SNAP-tag) ligands derived from N -cyanorhodamines permit up to four-colour confocal and super-resolution STED imaging in living cells.