Water-Soluble NIR-Absorbing Rylene Chromophores for Selective Staining of Cellular Organelles. 
Biocompatible organic dyes emitting in the near-infrared are highly desirable in fluorescence imaging techniques.  Herein we report a synthetic approach for building novel small peri-guanidine-fused naphthalene monoimide and perylene monoimide chromophores.  The presented structures possess near-infrared absorption and emission, high photostability, and good water solubility.  After a fast cellular uptake, they selectively stain mitochondria with a low background in live and fixed cells.  They can be additionally modified in a one-step reaction with functional groups for covalent labeling of proteins.  The low cytotoxicity allows a long time exposure of live cells to the dyes without the necessity of washing.  Successful application in localization super-resolution microscopy was demonstrated in phosphate-buffered saline without any reducing or oxidizing additives.