Trimethine cyanine dyes with an indole nucleus: Synthesis and spectral properties studies

► Indole quinoline cyanine dyes were synthesized based on Cy3 and TO. ► The fluorescence intensities were enhanced compared to those of TO and Cy3. ► The fluorescence wavelength shifted red compared to those of TO and Cy3. ► The photostability is superior to that of TO and Cy3. ► The fluorescence of dyes labeled with BSA also showed fluorescence enhancement.