Targetable Conformationally Restricted Cyanines Enable Photon?Count?Limited Applications**

Cyanine dyes are exceptionally useful probes for a range of fluorescence-based applications, but their photon output can be limited by trans-to-cis photoisomerization. We recently demonstrated that appending ring system to the pentamethine cyanine improves quantum yield and extends fluorescence lifetime. Here, we report an optimized synthesis persulfonated variants enable efficient labeling nucleic acids proteins. demonstrate bifunctional sulfonated tertiary amide significantly optical properties resulting bioconjugates. These new conformationally restricted cyanines compared parent derivatives in contexts. include use plasmonic hotspot DNA-nanoantenna, single-molecule Förster-resonance energy transfer (FRET) far-red fluorescence-lifetime imaging microscopy (FLIM), localization (SMLM). efforts define contexts which eliminating isomerization provides meaningful benefits performance.