Far-Red Emitting Fluorescent Dyes for Optical Nanoscopy: Fluorinated Silicon-Rhodamines (SiRF Dyes) and Phosphorylated Oxazines.

Far-red emitting fluorescent dyes for optical microscopy, stimulated emission depletion (STED), and ground-state depletion (GSDIM) super-resolution microscopy are presented. Fluorinated silicon-rhodamines (SiRF dyes) and phosphorylated oxazines have absorption and emission maxima at about λ≈660 and 680 nm, respectively, possess high photostability, and large fluorescence quantum yields in water. A high-yielding synthetic path to introduce three aromatic fluorine atoms and unconventional conjugation/solubilization spacers into the scaffold of a silicon-rhodamine is described. The bathochromic shift in SiRF dyes is achieved without additional fused rings or double bonds. As a result, the molecular size and molecular mass stay quite small (<600 Da). The use of the λ=800 nm STED beam instead of the commonly used one at λ=750-775 nm provides excellent imaging performance and suppresses re-excitation of SiRF and the oxazine dyes. The photophysical properties and immunofluorescence imaging performance of these new far-red emitting dyes (photobleaching, optical resolution, and switch-off behavior) are discussed in detail and compared with those of some well-established fluorophores with similar spectral properties.