Design and Synthesis of Coumarin-Based Pyrazolopyridines as Biocompatible Fluorescence Dyes for Live-Cell Imaging

An unprecedented strategy for the synthesis of highly fluorescent chromeno[4,3-b]pyrazolo[3,4-d]pyridin-4(3H)-ones via a 1,3-dipolar cycloaddition reaction/ring closure in the presence of NH4OAc/air oxidation in a one-pot procedure is described. The molecular structures were characterized by IR spectrophotometry, mass spectrometry, NMR spectroscopy, and single-crystal X-ray analysis. Furthermore, the photophysical properties of synthesized compounds were studied by absorption and fluorescence spectroscopy. All of the compounds exhibited almost similar spectra due to their resembling molecular structures. Therefore, one of the synthesized coumarin-based pyrazolopyridines was selected to evaluate its application as a live-cell imaging agent by an inverted phase-contrast microscope. The result reveals that synthesized chromeno[4,3-b]pyrazolo[3,4-d]pyridin-4(3H)-one has acceptable photostability, low cytotoxicity, satisfactory fluorescence efficiency (phi(f): 0.53), and excellent capability to penetrate into the intracellular space. Accordingly, it can be rendered as a promising dye in fluorescence bioimaging. Chemoselectivity, simple and available starting materials, operational simplicity, and good yields are other outstanding advantages of this reaction.