Near-Infrared-Emitting Meso-Substituted Heptamethine Cyanine Dyes: From the Synthesis and Photophysics to Their Use in Bioimaging

Abstract

Heptamethine cyanine dyes were synthesized in good yields by the reaction between quaternary indoles and a pentamethinic salt, under mild reaction conditions minimizing photooxidation. These compounds were used as precursors to prepare meso-substituted derivatives. The cyanine dye precursors presented UV-Vis absorption, related to fully allowed electronic transitions and fluorescence emission in the NIR region, without any evidence of aggregation in both ground and excited states. The substitution at the meso position showed a fundamental role in their photophysics, with the main absorption in the green-orange region related to the monomeric species. Moreover, the excited state photophysics presented emission profiles dependent on the excitation wavelengths, complicating the correlation of spectroscopy and structure. Density Functional Theory and OO-SCS-MP2 calculations under different solvation conditions revealed the heavy impact of conjugation effects on ground and excited states’ geometries and electronic configurations of these compounds. Finally, the observed photophysical features of the meso-substituted heptamethine cyanine dyes were successfully used to explore their application as fluorescent probes in biological media, allowing stable staining in live and fixed cells.