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 ¹N-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 ¹N-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, ¹N-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. Pyrazole meso-substituted heptamethine cyanine dye showed pH-dependent fluorescence lifetime change within physiological useful range. This novel fluorescent probe is a good candidate for in vivo diagnostics using fluorescence lifetime imaging method.