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 a ring system to the pentamethine cyanine ring system improves the quantum yield and extends the fluorescence lifetime. Here, we report an optimized synthesis of persulfonated variants that enable efficient labeling of nucleic acids and proteins. We demonstrate that a bifunctional sulfonated tertiary amide significantly improves the optical properties of the resulting bioconjugates. These new conformationally restricted cyanines are compared to the parent cyanine derivatives in a range of contexts. These include their use in the plasmonic hotspot of a DNA‐nanoantenna, in single‐molecule Förster‐resonance energy transfer (FRET) applications, far‐red fluorescence‐lifetime imaging microscopy (FLIM), and single‐molecule localization microscopy (SMLM). These efforts define contexts in which eliminating cyanine isomerization provides meaningful benefits to imaging performance. [ABSTRACT FROM AUTHOR]