C1'-Substituted Penta- and Heptamethine Cyanines: Synthetic Scope and Properties

Cyanine dyes are of particular importance in contemporary bioimaging and fluorescence-based applications. The structure of these dyes features a polymethine linker, which exerts a substantial influence on their spectroscopic properties. Despite the extensive use of cyanines, synthetic methods for the functionalization of their polymethine chain remain limited. This study presents a comprehensive methodology for the synthesis of penta- and heptamethine cyanines bearing diverse substituents in the C1' (chain) position. It is a follow-up to our recent report ( J. Org. Chem. 2025, 90, 17797), which explained why cyanines with bulky and electron-withdrawing groups in this position exhibit significantly altered absorption properties. The synthetic protocol demonstrates a high functional-group tolerance, moderate to excellent yields, and flexibility toward the reaction partners, thus establishing itself as a valuable tool for the preparation of new cyanine dyes. The steady-state absorption and emission spectra, singlet-oxygen production and photooxygenation efficiencies, and photostabilities of the synthesized derivatives are presented herein. Furthermore, our results indicate that the selected cyanines exhibit a high degree of sensitivity to pH variations and to nucleophilic addition, thus illustrating the potential of the novel cyanine derivatives to serve as sensors and probes in a variety of biological applications.