Cell permeable, fluorescent dye

The invention pertains to a near-infrared fluorescent dye that is cell permeable and can be attached to selected proteins in living cells. The dye has the general formula (I) or its or its corresponding spirolactone (II) wherein Y is chosen from the group consisting of Si, Ge and Sn R0 is -COO- or COOH R1, R2, R3, R4, R5, R6, R7, R8, R9, R10, R11, R12, R13, R14, R15 and R16 are substituents, including hydrogen, independently from each other. The dye (i) absorbs and emits light at wavelengths above 600 nm (ii) possesses high photostability (iii) has high extinction coefficients and high quantum yields (iv) can be derivatized with different molecules and (v) is membrane-permeable and shows mini not mal background binding to biomolecules and biomolecular structures.