Fluorogenic Sydnone-Modified Coumarins Switched-On by Copper-Free Click Chemistry.

The synthesis, photophysical characterization, and biochemical application of sydnone-modified coumarins, a novel class of fluorogenic clickable reagents, are reported. The sydnone moiety, a stable aromatic 1,3-dipole, efficiently quenched the fluorescence of coumarin, which could be restored, with a 132-fold enhancement, upon cycloadditions with cyclooctynes, thereby expanding the fluorogenic click toolbox. TD-DFT calculations suggest that the fluorescence quenching of the sydnone-modified coumarins is likely due to the presence of an energetically low-lying nonemissive charge-separated state.