7-Hydroxycoumarin, also known as umbelliferone, is a highly fluorescent, pH-sensitive fluorophore that emits in the blue region of the spectrum. However, its fluorescence is strongly quenched if the hydroxyl is alkylated or phosphorylated, making it useful in high-throughput screening for phosphatases and lipases. Interestingly, it was found that the 3-azido derivative (BCFA-047) is also highly quenched but, upon reaction with an alkyne in the presence of copper to form the triazole, the fluorescence is restored.1 The clicked coumarin emits at a lambda max of 480 nm and absorbs at 404 nm. Also, the fluorescence of the clicked coumarin is approximately 15 times higher than that of the coumarin azide.
This opens up the possibility to monitor a Copper-catalyzed Click reaction in real time and the detection of DNA in cells without requiring washing steps to remove the unreacted Coumarin Azide.2