Fig. 3. (a) Far-red emitting julolidine-derived amino-Si-pyronin dyes (ASiP^j) and their NIR-emitting derivatives (NIR-ASiP^j) that are stable inside cells. The corresponding Mulliken charges at C-10 of ASiP^a and ASiP^j are shown in atomic unit (a.u.). Structural changes of (b) SiR and (c) ASiP dyes depending on the polarity of the medium. (d) Changes of the normalized absorbance ratio of the enamine form (fluorescent) to the imine form (non-fluorescent) depending on the medium polarity (increasing water content in 1,4-dioxane), compared for ASiP^a2 and ASiP^j2.