Pyrazole-substituted near-infrared cyanine dyes exhibit pH-dependent fluorescence lifetime properties.

Near-infrared heptamethine cyanine dye is functionalized with pyrazole derivatives at the meso-position to induce pH-dependent photophysical properties. The presence of pyrazole unsubstituted at (1) N-position is essential to induce pH-dependent fluorescence intensity and lifetime changes in these dyes. Replacement of meso-chloro group of cyanine dye IR820 with (1) N-unsubstituted pyrazole resulted in the pH-dependent fluorescence lifetime changes from 0.93 ns in neutral media to 1.27 ns in acidic media in DMSO. Time-resolved emission spectra (TRES) revealed that at lower pH, the pyrazole consists of fluorophores with two distinct lifetimes, which cor-responds to pH-sensitive and non-pH-sensitive species. In contrast, (1) N-substituted pyrazoles do not exhibit pH response, suggesting excited state electron transfer as the mechanism of pH-dependent fluorescence lifetime sensitivity for this class of compounds.