Construction of Long-Wavelength Fluorescein Analogues and Their Application as Fluorescent Probes

: Born to dye: Five fluorescein analogues were synthesised (see scheme). One analogue was found to emit in the NIR region with a high quantum yield, excellent photostability and good permeability. Three derivatives were found to specifically stain mitochondria and one dye responds to thiols with a strong turn-on NIR fluorescence signal and colorimetric change, in vitro and in vivo.