Coumarin Triazabutadienes for Fluorescent Labeling of Proteins

The use of small‐molecule fluorophores to label proteins with minimal perturbation in response to an external stimulus is a powerful tool to probe chemical and biochemical environments. Herein, we describe the use of a coumarin‐modified triazabutadiene that can deliver aryl diazonium ions to fluorescently label proteins by tyrosine‐selective modification. The labeling can be triggered by low‐pH‐induced liberation of the diazonium species, thus making the fluorophore especially useful in labeling biochemical surroundings such as those found within the late endosome. Additionally, we show that a variety of coumarin triazabutadienes might also be prone to releasing their diazonium cargo after irradiation with UV light.