A Highly Photostable Near‐Infrared Labeling Agent Based on a Phospha‐rhodamine for Long‐Term and Deep Imaging

: Various fluorescence microscopy techniques require bright NIR-emitting fluorophores with high chemical and photostability. Now, the significant performance improvement of phosphorus-substituted rhodamine dyes (PORs) upon substitution at the 9-position with a 2,6-dimethoxyphenyl group is reported. The thus obtained dye PREX 710 was used to stain mitochondria in living cells, which allowed long-term and three-color imaging in the vis-NIR range. Moreover, the high fluorescence longevity of PREX 710 allows tracking a dye-labeled biomolecule by single-molecule microscopy under physiological conditions. Deep imaging of blood vessels in mice brain has also been achieved using the bright NIR-emitting PREX 710-dextran conjugate.